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Sensible – DELIVERABLE

Protection and safety of storage systems

This project has received funding from the European Union's Horizon 2020 research and innovation programme under Grant Agreement No. 645963.

Deliverable number: D2.6

Due date: 31.12.2016

Nature1: R

Dissemination Level1: PU

Work Package: WP2

Lead Beneficiary: 11 - UoN

Contributing Beneficiaries: GPTech, Siemens, EDP, SIEMENS SA

Reviewer(s): Ricardo Jorge Santos, EDP DISTRIBUIÇÃO.

1 Nature: R = Report, P = Prototype, D = Demonstrator, O = Other Dissemination level PU = Public

PP = Restricted to other programme participants (including the Commission Services) RE = Restricted to a group specified by the consortium (including the Commission Services) CO = Confidential, only for members of the consortium (including the Commission Services) Restraint UE = Classified with the classification level "Restraint UE" according to Commission Deci-sion 2001/844 and amendments Confidential UE = Classified with the mention of the classification level "Confidential UE" according to Commission Decision 2001/844 and amendments Secret UE = Classified with the mention of the classification level "Secret UE" according to Commis-sion Decision 2001/844 and amendments

DOCUMENT HISTORY

D2.6_SENSIBLE_Deliverable_finalD2.6_SENSIBLE_Deliverable_final

Version Date Description

0.1 21/9/15 Initial Version – Description and Content Framework

0.2 29/06/16 Section 6 updated

0.3 29/9/16 Section 5 updated (UoN)

0.4 21/11/16 Deliverable ready for review Contribu-tions added, sent for minor corrections from contributors

0.5 16/12/16 Review done by Ricardo Jorge Santos (EDP DISTRIBUIÇÃO)

0.6 6/1/17 Sent to contributors to address review comments.

0.7 18/1/17 Review comments addressed by UoN

0.8 26/1/17 Review done by coordinator

0.9 20/2/17 Further adjustments by UoN

1.0 27/2/17 Approved by General Assembly - final

EXECUTIVE SUMMARY


1 Executive Summary 4

2 Introduction 6

2.1 Purpose and Scope of the Deliverable ........................................................ 6

2.2 References ................................................................................................... 6

2.3 Acronyms ..................................................................................................... 9

3 Electromechanical Storage 10

3.1 Electrical Safety ......................................................................................... 10

3.2 Physical Safety ( mechanical Safety) ..................................................... 10

3.3 Summary .................................................................................................... 18

3.4 Regulations ................................................................................................ 18

4 Electrochemical Storage 20

4.1 Protection Criteria for Energy Storage System (ESS) ............................... 20

4.2 Electrical Safety (Software protection). ...................................................... 21

4.3 Physical Safety (Hardware protection). ...................................................... 21

4.4 Recommendations for electrical protection. ............................................... 21

4.5 Regulations. ............................................................................................... 22

5 Evora Demonstrator Study 25

5.1 Microgrid Protection ................................................................................... 25

5.2 Évora network protection ........................................................................... 26

5.3 Summary .................................................................................................... 32

6 Review of UK regulations for grid connection of energy storage focussed on client protection 33

6.1 Electrical Regulations (UK) ........................................................................ 33

6.2 Low-voltage network protection ................................................................. 34

6.3 Protective device requirements .................................................................. 34

6.4 Additional protection required for the connection of DERs ........................ 35

6.5 Short-circuit current contributions from grid-connected energy storage .... 35

6.6 Other factors that may have an impact on protection ................................ 36

6.7 Known DER protection challenges ............................................................ 37

6.8 Responsibilities of consumers and DNOs .................................................. 39

6.9 Obligations as listed in ER G59/2 .............................................................. 40

6.10 Summary .................................................................................................... 45

7 Impact of integration of energy storage in to the local grid on existing electrical protection system 46

7.1 Introduction................................................................................................. 46

7.2 Test Setup .................................................................................................. 46

7.3 Results ....................................................................................................... 54

7.4 Summary and Conclusion .......................................................................... 67

8 Power Electronic Systems 69

8.1 Safety requirements for power electronics converters systems (PECS) used in renewable energy systems ............................................................ 69

8.2 Safety requirements for bidirectional power converters in grid connected applications ................................................................................................ 71

8.3 Safety of power converters for use in photovoltaic power systems ........... 72

8.4 Summary .................................................................................................... 73

EXECUTIVE SUMMARY


1 Executive Summary

Energy storage systems and their safe implementation – encompassing the energy stor-age element, the interface hardware and its impact on the grid it is connected to – are important considerations when implementing the types of community energy storage systems that are being considered as part of the SENSIBLE project.

Considerations for the following safety related aspects are reported in this deliverable: safety considerations of electrochemical and electromechanical energy storage, power electronics safety and grid connection. This work is focussed on the countries and regu-lations that relate to the two SENSIBLE living lab demonstrator sites: Portugal and the UK.

More specifically: The high speed, flywheel based electro-mechanical energy storage system considered, is characterised by high energy storage density resulting in high me-chanical stresses. The main safety requirement is the containment of mechanical parts under a failure of the system. Failure avoidance and mitigation methods and solutions are proposed and described in detail, e.g. material selection and critical component mon-itoring. The study was done as part of the design process for the flywheel system to be deployed in the SENSIBLE project.

For electrochemical storage, software and hardware based protection systems are con-sidered. Some ESS such as Li-ion based storage require a rapid response (ms) to over-currents, voltages and temperatures, which tends towards hardware based protection systems, e.g. fuses or electromagnetic devices while others need slower response times and software based schemes are suitable. These can isolate the ESS under cell failure and avoid damage to the interface hardware caused by high discharge e.g. short-circuit events.

Power electronic converters are a critical, enabling technology for most renewable en-ergy systems. As with all things, the safety of these components is of paramount im-portance, especially when these systems may be deployed in a domestic setting. The general safety requirements placed upon power electronic systems and the international standards presently used as guidelines are presented and salient points highlighted. It is noted that there is very few specific regulations presently available which apply to such systems.

A review of the current UK standards suggest that the fault current contribution of grid-connected energy storage is likely to be small, however there is evidence to suggest that transient fault currents could be higher than expected. Investigation of fault current contributions is required into the effect of multiple ES installations to determine whether there would be any adverse effect on the functionality of protective devices. A repre-sentative energy storage installation was created and tests performed to evaluate the impact of ESS on the current magnitude during over-load faults. The change in current magnitude could become significant if a large amount of ES was active on the system at the time of the fault and should be considered along with the fault level of the grid.

EXECUTIVE SUMMARY


This may impact the selection of protection device ratings employed and may also im-ply replacement / upgrade of grid protection systems if penetration of ESS continues to increase.

INTRODUCTION


2 Introduction

2.1 Purpose and Scope of the Deliverable

The deliverable presents elements of the work done during task 2.4 which revolves around the safety and protection aspects of energy storage systems.

The areas addressed are electrochemical storage systems, mechanical storage sys-tems, power electronic converters and connection of energy storage systems to the grid.

These topics are chosen as they are closely related to the two demonstrator sites to be implemented in the project. The scope of the deliverable is to give a high level review of the safety aspects of these topics and where possible to highlight links to regulatory material which may be useful when implementing such systems.

This deliverable is not a detailed review of the requirements and processes in implemen-tation of the demonstrators or similar installations, but is to be viewed as promoting dis-cussion and consideration of the topics contained. Some areas of potential absence in clarity or consideration by the regulations are pointed out but are not addressed in depth.

This deliverable was not intended to feed directly in to the safety methods used within the project, however during the project a requirement for a detailed study of the Evora demonstrator became necessary. The initial work on this study has been included here detailing the requirements. The simulation and results will be presented in deliverable D4.2.

2.2 References

Chapter 3

[1] Federal Energy Management Program, 2003. Flywheel Energy Storage. Federal Technology Alert 1–16. doi:10.1063/1.2998913

[2] Pjrensburg, o. J. Main Components of a Typical Flywheel [WWW Document]. URL https://commons.wikimedia.org/wiki/File%3AExample_of_cylindrical_flywheel_ro-tor_assembly.png (last accessed Oct. 31st .2016

[3] Nasa, o. J. Flywheel Module [WWW Document]. URL http://space-power.grc.nasa.gov/ppo/projects/flywheel/gallery.html (last accessed Oct. 31st .2016).

[4] Ashley, B.S., Editor, A., 1996. Designing Safer Flywheels.

[5] Thoolen, Franciscus, Johan, M., 2012. Containment, in particular for a Flywheel De-vice, and Containment Assembly. /EP2013514B1/

[6] Kress, G.R., 2000. Shape optimization of a flywheel. Structural and Multidisciplinary Optimization 19, 74–81. doi:10.1007/s001580050087

INTRODUCTION


Chapter 5

[1] SENSIBLE, “Deliverable D1.3 - Use cases and requirements.”

[2] Ministério da Economia, da Inovação e do Desenvolvimento, “Portaria n.º 596/2010“ “Anexo II, Regulamento da Rede de Distribuição”, 2010.

Chapter 6

[1] The Institution of Engineering and Technology, BS 7671:2008 Requirements for Electrical Installations, IET Wiring Regulations 17th Edition, 2008.

[2] Energy Networks Association, Engineering Recommendation G83/1-1, 2008.

[3] Energy Networks Association, Engineering Recommendation G59/2, 2010.

[4] British Standards Institute, BS EN 60898: Electrical accessories - Circuit breakers for overcurrent protection for household and similar applications, 2003.

[5] British Standards Institute, BS EN 60947-2: Low-voltage switchgear and control-gear. Circuit-breakers., 2006.

[6] British Standards Institute, BS EN 61009: Residual current operated circuit-break-ers with integral overcurrent protection for household and similar uses, 2016.

[7] British Standards Institute, BS1362: Specification for general purpose fuse links for domestic and similar purposes (primarily for use in plugs), 1973.

[8] British Standards Institute, BS 3036: Semi-enclosed electric fuses - Ratings up to 100 amperes and 240 volts to earth, 1958.

[9] British Standards Institute, BS 646: Specification for cartridge fuse links (rated up to 5 amperes) for a.c. and d.c. service, 1958.

[10] British Standards Institute, BS 88-2: Low-voltage fuses. Supplementary require-ments for fuses for use by authorized persons (fuses mainly for industrial applica-tion). Examples of standardized systems of fuses A to K, 2013.

[11] T. S. a. B. Eua-arporn, “Determination of Allowable Capacity of Distributed Gener-ation with Protection Coordination Consideration,” Engineering Journal, vol. 13, no. 3, pp. 29-44, 2009.

[12] J. K. a. B. Kroposki, “Understanding Fault Characteristics of Inverter-Based Distrib-uted Energy Resources,” National Renewable Energy Laboratory, 2010.

[13] Energy Networks Association, “Engineering Recommendation P26: The Estimation of Prospective Short Circuit Current for 3 Phase LV Supplies,” 1985.

INTRODUCTION


[14] I. X. a. M. Popov, “Smart Protection in Dutch Medium Voltage Distributed Genera-tion Systems,” in IEEE PES Innovative Smart Grid Technologies Conference Eu-

rope, 2010.

[15] Crown, The Electricity Safety, Quality and Continuity Regulations, 2002.

[16] Crown, The Electricity at Work Regulations, 1989.

[17] Crown, The Management of Health and Safety at Work Regulations, 1999.

[18] Energy Networks Association, The Distribution Code of Licensed Distribution Net-work Operators of Great Britain, 2015.

Chapter 7

[1] http://files.sma.de/dl/2485/FSSMAPPE-DEN1510-V10web.pdf

Chapter 8

[1] F. Blaabjerg, Y. Yang, K. Ma, X. Wang, “Power Electronics – The key Technology for Renewable Energy System Integration” 4th International Conference on Renew-able Energy Research and Applications, Palermo, Italy, Nov 2015

[2] H. Wang, M. Liserre, F. Blaabjerg, “Toward Reliable Power Electronics”, IEEE In-dustrial Electronics Magazine, June 2013

[3] BS EN 62477-1:2012 Safety requirements for power electronic converter systems and equipment, Part 1: General

[4] BS EN 62909-1 Ed 1.0, Bi-directional grid connected power converters, Part 1

[5] BS EN 62109-1:2010,Safety of power converters for use in photovoltaic power sys-tems, Part 1

[6] J.D. van Wyk, F.C. Lee, “ On a Future for Power Electronics”, IEEE Journal of Emerging and Selected Topics in Power Electronics, Vol. 1, No.2, June 2013

INTRODUCTION


2.3 Acronyms

AC – Alternating Current

BMS – Battery Management System

DC – Direct Current

DER – Distributed Energy Resource

DNO – Distribution Network Operator

ES – Energy Storage

ESS – Energy Storage System

GCPC - Grid-Connected Power Converters

IC - Integrated Circuit

MCB – Miniature Circuit Breaker

MCCB – Moulded Case Circuit Breaker

PCC – Point of Common Coupling

PCE - Power Conversion Equipment

PECS - Power Electronics Converter Systems

PV - Photovoltaic

RES - Renewable Energy Systems

RCBO – Residual Current Breaker with Overload Capacity

RCD – Residual Current Device

SoC – State of Charge

TSO – Transmission System Operator

ELECTROMECHANICAL STORAGE


3 Electromechanical Storage

An electromechanical storage in form of a flywheel is essentially a mechanical battery that stores electricity in the form of kinetic energy. Electricity is used to operate a motor that accelerates a rotating steel mass to a high speed. Because the flywheel is spinning in a vacuum there is no air drag and the rotational energy can be “stored” for relatively long periods of time with minimal parasitic losses. The flywheel’s momentum can then be harnessed to generate electricity on demand, [1].

GPTECH/UON

Mechanical energy storage devices protection and safety

Figure 3.1: The main components of a typical fly-wheel, here with a magnetic bearing – source [2]

3.1 Electrical Safety

--> see electrical safety for batteries, Chapter 4.

The amount of electrical energy in the system is significantly lower than in electrochem-ical storages. The mechanism for storing energy is mechanical, and without the transfor-mation of mechanical energy to electrical energy the quantity and power of electrical energy in the system is minor.

3.2 Physical Safety ( mechanical Safety)

High energy density storage has its drawbacks. A high-performance flywheel rotor spin-ning anywhere from 10,000 to more than 25,000 revolutions per minute has lots of inertia. Another formidable technical challenge is designing a lightweight, cost-effective safety containment system which can resist the impact of burst fragments and transmission of

Figure 3.2: Flywheel Module, NASA Image - source: [3].



high torque loads just milliseconds after flywheel failure [4]. Since the flywheel is a com-ponent subjected to extremely high mechanical stresses during operation, the contain-ment functions as a safety barrier in case of failure of the flywheel, the containment pre-vents parts of the flywheel from causing damage to persons and objects in the vicinity of the energy storage system [5].

Physical safety therefore is a very important issue for flywheels and it is referred to as mechanical safety. As far as this evaluation goes, only steel flywheels have been con-sidered, flywheels using composite materials are not included.

There are a number of reasons for these limitations: The physical transition from com-posite materials to metals and the other way around in an assembly can still be consid-ered as difficult and expensive to realise. Therefore, it was decided to avoid these if possible. Compared to metal based structures or so-called conventional material, the behaviour of composite and fibre enforced materials and components made of such ma-terials is extremely difficult to predict and calculate in events of damage, nowadays. As the consideration of events of damage is the core element when analysing safety for flywheels, this basically eliminated the possibility of utilising this class of materials for a comprehensive analysis. Risk management shows that there are exactly two ways to mitigate the risks of an event of damage:

Avoidance of occurrence and containment of avoidance are the two principles in risk management. Both approaches were investigated in depth to ensure a safe system and to enable an informed design to be performed.

3.2.1 Avoidance of occurrence

Avoidance addresses the fields of material quality, fatigue strength redundancy and the limitation of operation parameters. Through the use of certified materials the occurrence of flaws within materials can be reduced and therefore the risk of failures initiated through such flaws. Selection of dimensions towards a high level of safety considerations ad-dresses the same area and results in an improvement in the avoidance of the risk through fatigue,. Limiting the speed of the system, in this case of the rotational speed, is a meas-ure to ensure the dimensions are valid for the operation of the system in a safe and evaluated as well as simulated environment.

The measures for assuring the avoidance of occurrence are

• Avoidance of material flaws through material certification,

• Operation under subcritical conditions only,

• Bearing, monitoring

• Design and dimensioning for high fatigue strength.



3.2.1.1 Avoidance of material flaws through material certification

Utilisation of certified materials reduces the probability of material flaw occurrences. The goal of these certifications is to independently verify material conformity across the sup-ply chain, all the way back to the steelmaker and assure quality. Therefore, the probabil-ity of material flaws is reduced.

3.2.1.2 Operation on subcritical conditions only (resonance frequency)

It is important to limit the speed of the system to conditions that are subcritical regarding the system’s resonance frequency. This requires a proper analysis of the system’s res-onance frequency as well as the sensing system that ensure the operational parameters are kept at a safe margin away from the resonance frequency. Regarding a rotational system this means sensing the rotational speed on different levels (through the inverter as well as the system control) to ensure operation of the system in safe conditions.

3.2.1.3 Bearing failure

Bearing failure can occur to bearings and there is no way of fully eliminating this threat, but its occurrence can be reduced by eliminating the causes for these failures. One of those causes is imbalance in the rotating elements and the corresponding forces that the bearings are subjected too. Balancing the rotational mass as well as sensing for imbalance in the system during operation reduces the occurrence of bearing failures drastically and should be mandatory. For the prototypical admittance of flywheels a bal-ancing examination in the balanced condition with a speed factor of at least 1.1 is to be carried out. Moreover, the system needs to get switched off in case an imbalance or corresponding vibrations are detected. Additional to the already described sensors, tem-perature sensors can provide valuable input for detecting the status of the system. The motor temperatures as well as bearing temperatures are important for monitoring if the system is in a stable and safe state.

3.2.1.4 Design and dimensioning - high fatigue strength.

Safety factors make sure that the system is dimensioned in a safe way. This incorporates the dimensioning considering the system’s behaviour in certain situations. The worst-case scenario is a bursting event while the system is charged with a high level of me-chanical energy. The housing of the system needs to withstand a burst event.

3.2.2 Containment of avoidance

Containment addresses the safety of the system and the area around it in case of a failure. Regarding the flywheel system the danger lies mostly in the mechanical energy stored within the system where parts of the system might disintegrate. In these cases the critical question is where this energy goes or can go respectively, if the system can’t be discharged the regular way. Regularly there will be mechanical energy stored in rota-tional movement within the system and therefore the risk is in a failure of the rotating system itself. This can mean a failure in the rotational system which could evolve out of a bearing failure or a disintegration of the mass that stores the energy. Both would result in a disintegration of the energy-loaded rotating elements of the system. This case needs



to be considered during the design and dimensioning of the housing. The following in-vestigations need to be performed to ensure the safety regarding containment of failures:

• Shape optimisation on the rotational mass

• Design, simulation and realisation of concept housing, representing vacuum chamber and burst protection

• Design and simulation of concept for installing flywheel in container and fulfilling safety requirements for outdoor installation

3.2.2.1 Shape optimisation on the rotational mass

The design of the rotational mass has important influence on safety considerations [6]. Regarding the containment of a burst, it would be beneficial, if the rotational mass would scatter rather than start moving as a whole before colliding with the first or second level of housing or burst protection respectively. The flywheel mass can be designed and shaped, so that the integration of predetermined breaking points induce a disintegration in the event of a system failure. An alternative to the widespread Laval disk design is the use of a stack of disks with equal thickness and with a circular hole pattern, Fig. 3.3-3.5.

Figure 3.3: Disk stack, final design consists out of 10 stacks, e.q. around 100 disks



Figure 3.4: Simulation of stresses in top layer and next layer around the holes.

Figure 3.5: Disk of equal strength (Laval) replaced by disks of equal thickness with circular

holes (left), and a comparison of density: blue (laval disk) and red (disk with holes) do

match closely (right).

Depending on the operational parameters of the system, the rotational mass can now get designed so that disintegration occurs as soon as the internal forces exceed the considered operational forces by a certain factor. This would help in containing the me-chanical energy as smaller bits and pieces can only hold a smaller amount of energy and are therefore physically unable to burst through the housing.

3.2.2.2 Design, simulation and realisation of concept with bi-functional housing, rep-resenting vacuum chamber and burst protection

In the event of a failure of the flywheel its energy is released. The released parts of the system will leave in a rotatory as well as lateral movement with high speeds on an un-foreseeable motion path directed away from the axis of rotation. The kinetic energy bound within these machine parts can only be absorbed by one or multiple collisions or by deformation distortions which are on the part’s flight path.

To keep high energy loaded particles within the containment, both burst protection bar-riers are cylindrical. The cylindrical shapes aids containment of the initially high circum-ferential speeds particles will have, in the case that they are released from the main body of the flywheel.



Safety factors make sure that the system is dimensioned in a safe way. This incorporates the dimensioning considering the system’s behaviour in certain situations. The worst-case scenario is a bursting event while the system is charged with a high level of me-chanical energy. The housing of the system needs to withstand a burst. Therefore, the energy needs to be contained within the housing. In order to realise that, the housing needs to act as a burst protection. It is split into two different shells; the inner and the outer burst protection, see Figure 3.6. Different fragmentation scenarios are considered in the FE calculations, see Table 3.1.

Figure 3.6: Cross-sectional view of the geometry used



Table 3.1: Selected fragmentation scenarios for the FE calculations.

Sc. Description Number of fragments Mass ratio

Fragment/remaining

shaft

1 The flywheel mass simultaneously breaks

into two roughly equal fragments 2 B1: 50%, B2: 50%

2

A 50% fragment splits off, no shaft break-

age and the remaining shaft continues to

rotate

1

Examination as to

whether the remaining

shaft would actually not

break

F: 50%, RS: 50%

3



rotate

1

+ an examination in

which shaft breakage is

also assumed

F: 20%, RS: 80%

4



rotate

1

+ an examination in

which shaft breakage is

also assumed

F: 40%, RS: 60%

5

Asymmetrical break of the mass, in which

only 1/6th of the flywheel mass breaks

away in the upper section

1 F: 16%, RS: 84%

6

The same as scenario 5, except that the

break occurs at an angle (not at a right-

angle)

1

F: 16%, RS: 84%

The destroyed elements in all these scenarios need to be considered and analysed. It is crucial that the burst protection is not compromised to a point, where there would be particles leaving the housing. Additional to the strength of the enclosure, the forces in the fasteners need to be evaluated and they must not exceed the maximum permissible axial and shear forces. Otherwise there would be potential for the whole flywheel in becoming loose.



3.2.2.3 Design and simulation of concept for installing flywheel in container and fulfilling safety requirements for outdoor installation

The container and especially the floor assembly (Fig. 3.7) needs to be strong enough in regard to the stresses that occur when a flywheel operates and also for the case that the flywheel bursts. On the one hand, large deformations in the model with tipping of the flywheel energy store need to be avoided. On the other hand, there cannot be too much warping of the flanges of the floor assembly since those would lead to an element failure. Finally, the bolt forces cannot be allowed to be too high to maintain the connection be-tween the flywheel energy stores and container If these requirements cannot be fulfilled, there is a high risk of putting people in the vicinity in danger. This must be avoided by all means. Therefore, the dimensioning of the floor assembly needs to be designed to fulfil all requirements regarding its strength.

Fig. 3.7: Structure of a container (source: gfoellner.at)

Fig. 3.8: Model overview: Flywheel mounted on a containers floor assembly.

Floor assembly

Container hood



Fig. 1.9: Overview of adaptations in the container floor assembly

The results of the simulation of the model of the adapted concept will likely show that high stresses continue to occur even with the inclusion of the previously described rec-ommendations.. However, these stresses need to stay within an acceptable range so that it is possible for the system to be certified. Otherwise the concept cannot be used. A reinforced floor assembly that is strong enough to withstand the operational forces and even a burst, is shown in Fig. 3.9.

3.3 Summary

The key in ensuring the safety of a system storing kinetic energy lies in the dimensioning of the mechanical elements regarding avoidance of occurrence of major mechanical fail-ures. Additionally, it is crucial to ensure that the energy stored can be contained in case of a mechanical failure of the system which makes it impossible to discharge it by the usual operational method.

3.4 Regulations

Regulations are not referring to flywheels and burst protection directly. They are rather valid for systems and applications that are similar in their type of endangerment for the environment like dimensioning of rotors, safety of machinery and machine tools as well as protective measures in case of rotating parts and components:

EN ISO 23125:2015: Machine tools - Safety - Turning machines (ISO 23125:2015); Ger-

man version.

EN ISO 15641:2001: Milling cutters for high speed machining - Safety requirements (ISO

15641:2001); German version.

ISO 19499:2007: Mechanical vibration - Balancing - Guidance on the use and application

of balancing standards.



ISO 1940-1:2003: Mechanical vibration - Balance quality requirements for rotors in a con-

stant (rigid) state - Part 1: Specification and verification of balance tolerances.

ISO 1940-2:1997: Mechanical vibration - Balance quality requirements of rigid rotors -

Part 2: Balance errors.

ISO/FDIS 11342: Mechanical vibration - Methods and criteria for the mechanical balanc-

ing of flexible rotors.

EN ISO 13849-1:2015: Safety of machinery - Safety-related parts of control systems -

Part 1: General principles for design (ISO 13849-1:2015); German version.

EN ISO 12100:2010: Safety of machinery - General principles for design - Risk assess-

ment and risk reduction (ISO 12100:2010); German version.

ISO 14121-1:2005: Safety of machinery - Risk assessment - Part 1: Principles (ISO/DIS

14121-1:2005); German version prEN.

ELECTROCHEMICAL STORAGE


4 Electrochemical Storage

Currently, energy storage systems provide new methods to generate, distribute, accu-mulate and make use of available electrical energy. Different classifications of ESS exist according to the method by which energy is stored (e.g. mechanical, electrochemical, chemical, electrical or thermal). Each storage method requires consideration of various safety criteria and implementation of different protection systems depending on its con-stituent elements, chemicals or power rating.

In this section, the most relevant protection systems for electrochemical storage will be described as well as international regulations for energy storage system.

4.1 Protection Criteria for Energy Storage System (ESS)

When considering electrical protection criteria, multiple factors must be taken into ac-count, for example:

• Software response time. To detect an overload condition or internal / external short-circuit and to open principal contactors. This is not a fast protection method.

• Permitted current ratings (in overcurrent condition). Due to the “slow” response of software, software is not a reliable protection system for protection from short-circuit events. Besides, contactors do not support long time operation during overcurrent conditions (damage may by caused by overcurrent and it may not continue to operate correctly).

• Fuse response. This is the most effective device in overcurrent situations and it is the most economic. Fast response (< 10 milliseconds normally) is possible and can protect cells and ESS against overcurrent conditions.

• Maximum short-circuit time that a cell supports. This is the most important pa-rameter. It is the time that cells can support a short-circuit condition without suf-fering degradation or internal damage. This time must be higher than any fuse response time to assure the protection of the cells.

With these factors in mind, the most important considerations in an electrochemical ESS system are:

• Direct protection to users and ESS: In some occasions, users need to be able to handle ESS directly (in maintenance operations for example) or during failure / errors conditions; in these cases, protection systems are required to force the ESS in to a safe state (non-operative solution) and to disable the system if there is failure of the ESS.

Hardware and software solutions can be part of the protection mechanism.

• Protection to ESS´s devices, especially Li-ion cells: these protection devices must be fast (few milliseconds). The objective of this device is to protect ESS from overload or short-circuit. In this case, protections are exclusively imple-mented via hardware based solutions as it has to have an immediate and reliable response.



4.2 Electrical Safety (Software protection).

There are several devices capable of detecting overvoltage and overcurrent conditions. As it has been mentioned previously, these devices (software protection) do not have an ultrafast response because they consist of programmed devices and mechanical con-tractors. In fact, the main purpose of these devices is to put the system to a safe state.

Typically, ESS protection systems utilise: two contactors, one per terminal (the positive and the negative terminal), a current and voltage sensor, which are monitored and man-aged by a central battery management system (BMS).

In most cases, ESS have a modular configuration. Internally, the system is subdivided in to smaller modules consisting of cells. These modules have local BMSs where there maximum and minimum voltage are managed to prevent cell damage.

Some ESS have a differential current sensor. This device measures the input and output current of the ESS. If the sum of the input and output current is none zero the control system will respond and will open the contactors to disconnect the power line. This pro-tection system is used to detect ground leakage currents.

4.3 Physical Safety (Hardware protection).

The main objective of hardware protection is to protect the system against overload or overcurrent conditions. For that purpose, ESS s have an ultrafast fuse whereby the sys-tem is disconnect from its principal power connection.

Fuses must be selected according to the ESS specification with respect to its voltage and current ratings.

It must be taken into account that fuses are a series device and thus, must be correctly rated to allow operation of the system at full working current without inadvertent discon-nection of the ESS. Transient conditions also need to be considered as short lived cur-rents higher than rated current may occur under normal operation.

4.4 Recommendations for electrical protection.

1. High speed response fuses or thermomagnetic devices, connected between the converter and the grid connection to the point of common coupling (PCC).

With these protection devices, the grid is protected from overload, short-circuit or overcurrent that may be caused by an internal failure of the ESS´s cells.

In this case, the hazard is not produced by short-circuits in cells; instead it comes from a grid fault, so it is also necessary to protect ESS on the DC side of the system.

2. Installation of a manually operated device to allow isolation from the PCC. The system is installed to operate with the ESS in case of maintenance requirements or grid failure situations.



3. It is recommended to design the system to have isolation between the control and power systems. In this way, the effect of electromagnetic noise created by the power converter on the control electronics and measurements is reduced

4.5 Regulations.

There are many regulations regarding energy storage system and energy storage cells. The regulation cover many aspects including the electrical characteristics to mechanical and impact resistance. As we can see in Table 4.5.2.- Each country has their own regu-lations and requirements and their own classifications.

Table 4.5.1 shows a comparison of different countries according to its purpose and its application (cell, rack or module), only for energy storage system.

Target

Countries Standard Purpose

Cell/Mod-ule/Rack

International

IEC 62619 (on revi-sion)

Safety Cell/Rack

IEC 62620 Performance Cell/Rack

UN 38.3 Transportation Cell/Module/Rack

North America UL 1642 Safety Cell

UL 1973 Safety Module/Rack

Japan

SBA S1101 Safety

Cell/Module/Rack JIS C 8715-1 Performance

JIS C 8715-2 Safety

Table 4.5.1- Purpose of International regulation for ESS [LGChem]

Different normative must be used depending on the electrical characteristic.

• Overload situation is studied in IEC 62133 and IEC 62619.

• Furthermore, endurance cycles is limited to IEC 62620.

• One of the most important characteristics of a cell is its discharge performance and this parameter is in every standard: IEC 62133, IEC 62620, IEC 62619 or UL 1642.

• On the other hand, one safety parameter is to know the cell response under short-circuit condition, and IEC 62133 proposes a procedure to determine this parameter. IEC 62619 also cover this aspect and UL 1642 provide two tests in this field: external and internal short-circuit.



Other characteristics to be considered are mentioned in the regulations, for instance: temperature of use, internal resistance (D.C. and A.C.), high rate permissible current, performance at low temperature or abnormal charging tests.

On the other hand, there are mechanical tests such us crush, impact, shock or vibration test that affects to cells manufacturing.



Target Countries

Cell for IT applica-tion

Pack for IT application Automotive Battery ESS Battery

IEC/ISO Members

IEC 61959 IEC 61960 IEC 62133 IEC 62281

IEC 61959 ISO 12405-1 ISO 12405-2 ISO 12405-3 IEC 62660-1 IEC 62660-2

IEC 62619 IEC 62620

IEC 61960 IEC 62133 IEC 62281

60950-1

CB

EU - CE UN-ECE

-R100 with battery safety -EVS -GTR

-

US/Canadá - UL - -

Germany - TUV S

TUV GS (Bauart) - -

Russia - GOST - -

IEC Members - IEC 61000-4-2~6 - -

EU - EN 55022, 55024 - -

Australia/Newzea-land

- AS/NZC CISPR22 (C-Tick) - -

All UN 38.3 UN 38.3 UN 38.3 UN 38.3

Korea KC KC Security regulations for au-

tomotive vehicles. KBIA

US/Canada UL 1642 UL 2054 UL 2580 UL 1973

US IEEE 1725 IEEE 1625

IEEE 1725 IEEE 1625

UL 2580 UL 1973

China GB/T 18287-2013 GB/T 18287-2013 QC/T 743 -

Japan JIS 8712/8714 (PSE) JIS 8712/8714 (PSE) - S-mark

Thailand - TIS 2217-2548 (TISI) - -

Taiwan CNS 15364, CNS 14857-2

(BSMI) - - -

India IS 16046:2012 (BIS) - - -

Table 4.5.2.- Standards according to target and country

EVORA DEMONSTRATOR STUDY


5 Evora Demonstrator Study

5.1 Microgrid Protection

The majority of worldwide Power Electrical Systems existing nowadays were planned and built only accounting for unidirectional power flow, from big centralised generation units to the end customers (HV, MV and LV). With the development and growth of Dis-tributed Energy Sources (DES), namely micro-generation and Energy Storage Systems (ESS), bidirectional power flows became a reality which led to a change of the power distribution paradigm.

Although the use of locally produced energy and storage systems are of great benefit to the deferral of new investments in power electrical systems (namely the distribution net-work), it is also associated with some drawbacks. The integration of Distributed Gener-ation (DG) and storage systems, increases the complexity of the operation, control and protection of MV and LV power systems.

Conventional protection schemes that were designed for power systems without consid-erations of the DG impacts are insufficient and thus, a new generation of protection schemes that explicitly considers these requirements and adapts to the system operation are strongly required. Furthermore, the development of new protection schemes must take into account the types of operating modes of the grid to which they are intended, namely, grid-connected and islanded modes, these two being different which necessities must be met for each one of them.

In fact, the operation in islanded mode presents the greatest challenge when it comes to engineering a network protection scheme. A microgrid is basically a network comprising distributed generation sources (including storage systems) and controllable loads, which can operate in grid connected mode or, in case of fault, (or other motives) in isolated mode. Protection systems must respond to both main grid and micro grid faults. In case of the fault occurring upstream of the portion of the grid able to work disconnected from the main grid (microgrid) the protection systems must also be able to isolate the micro grid from the main grid as fast as possible The fast operation of protection, circuit break-ers and switches, has a beneficial effect on the quality of service delivered to client, by diminishing the exposure time to disturbances. Various protection issues can arise when DG and ESS are integrated at the distribution level network, namely:

• Changes in grid’s short-circuit current level of network in case of DG or ESS with high short-circuit current or in case of network with capability to operate on is-landed mode

• Possibility of sympathetic tripping (only in case of DG or ESS with high short-circuit current)

• Lack of operating conditions for distance relays

• Loss of relay coordination and unintentional islanding on networks not prepared for islanded operation.

• Possible non-detection and feeding of phase-to-ground faults in the MV network



When a micro grid is used to improve service continuity, distributed network protection requires modification. Automatic and fast operative devices are used to detect the fault in the grid and are responsible for sending a trip signal to the circuit breaker which dis-connects the micro grid almost instantly and automatically. To overcome problems aris-ing from bidirectional power flow and low fault current levels in micro grids with inverter based sources, new protection schemes are required and should include communication capabilities, where relays are checked and updated periodically assuring the continued safe and reliable operation of the grid.

5.2 Évora network protection

With 4 use cases developed under the distribution grid scope, Évora’s demonstrator is strongly focused on the distribution networks operation management, being two of those use cases related to the operation of a LV microgrid, UC10 and UC11 that can be found in [1].

This demonstrator will be installed in a live network which includes real-life clients of a network for which EDP Distribuição, as the DSO, is responsible. For this reason, EDP Distribuição has, and will continue to have, the principal role in the protection require-ments of this demonstrator. These are aimed at ensuring the safety of people, grid and equipment (including those of the LV clients).

Within the scope of SENSIBLE, EDP aims to ensure a flawless transition and operation in islanded mode through the optimal management, control and coordination of the RES, ESS and SS automation to be installed in one of the two networks that constitute the demonstrator. Évora demo will not only face the challenge of operating in islanding mode, but also the complexity of assuring a soft transition between connected and island operation and the synchronisation and reconnection to the main grid. Below is presented the hardware which will be deployed on-site and will enable the validation of the referred to use cases:

• RES

o 15 x 1,5 kWp PV Systems (Client owned)

• ESS

o 1 x 125/0,9 kW/kWh Flywheel (DSO owned)

o 1 x 50/44 kW/kWh Battery Energy Storage System (DSO owned)

o 1 x 30/23 kW/kWh Battery Energy Storage System (DSO owned)

o 10 x 3/3,3 kW/kWh Battery Energy Storage Systems (Client owned)

• SS equipment

o Distribution Transformer Controller

o Islanding CB and respective Protection Relay

o Islanding Manager (Programmable Logic Controller)

o Feeders CBs



Figure presents the single line diagram of the LV Switchgear specified by EDP Dis-tribuição (Portuguese DSO) to be installed inside every secondary substation (cabin type) as the one that will replace the existing one in Évora (on the network to operate in islanded mode). Highlighted in red, it can be noted that the present day protection of the LV network is assured through the use of fuses in each feeder, and a switch/disconnector for the network (highlighted in green), which reflects the main paradigm of protection systems used on nowadays LV grids. Due to the fuses characteristics, which are sized accordingly to the short-circuit current of the feeders, and all the constraints that a mi-crogrid present to a protection system, it is easy to acknowledge that the protection re-quirements of a microgrid may not be satisfied in present Évora grid, and with that in mind, a new protection paradigm must be developed.

Figure 5.1 - Single line diagram of Standard LV Switchgear

A thorough analysis of the Portuguese grid codes becomes indispensable to survey the requirements for the protection of the Portuguese electrical grids as well as the subse-quent comparison with Évora’s demonstrator protection requirements. In this analysis, two different perspectives must be addressed:

• The protection of the grid ESS to be installed in Évora grid.



• The protection of the network to operate in islanded mode.

Annex II of „Regulamento da Rede de Distribuição“ approved by „Portaria n.º 596/2010“ of 2010 July 30th [2] is the Portuguese regulation that establishes the technical condi-tions for the exploitation of the national electrical distribution grid as well as the technical conditions to the connection of consuming and generating facilities to the distribution grid. Chapter 4 dictates the technical conditions to establish a connection with the distri-bution grid. On the subject of protection of electricity generating facilities, this document stipulates that the owner must equip its facilities with interconnection circuit breakers (and protection relays) which can assure a fast and automatic disconnection of the dis-tribution grid according a specified set of parameters in „Guia técnico das instalações

eléctricas de produção independente de energia eléctrica“ that, translated to English, means the technical regulation for independent electrical energy generating facilities. The Portuguese Distribution Grid Code further states that the DSO is the responsible for conceiving, specifying, coordinating, regulating and inspection of all the protection in its grids. The DSO must establish or agree with the conditions to be monitored by the pro-tection equipment associated to the connections with the facilities of other entities.

The analysis of “Technical regulations for independent electrical energy generating facil-ities“ becomes essential, since due to its discharging ability, an grid ESS must be con-sidered an independent electrical energy generating facility illustrating the scheme to be followed on the connection of such facilities with generating capacity connected to the LV grid, and that can be found in the aforementioned document.

Regarding the Protection relay (highlighted in green) associated with the Interconnection circuit breaker (highlighted in red), this is intended to prevent the generation facility of disturbing the operation of the distribution grid to which is connected, as well as the other way around, decreasing the risk of accidents on the grid or the equipment to a minimum. For a better understanding of Figure 5.1, below is described the equipment that is fea-tured:

1) Switching device which must be accessible to the DSO for grid maintenance oper-ations.

2) Possible locations of the interconnection circuit breaker.

3) Interlocking with the Interconnection Circuit Breaker

4) Circuit Breaker or Contactor



ISC – Individual Switching Component

Figure 5.1 - Scheme for the connection of generation facilities to the LV network

The Portuguese technical regulations for connection to the grid also include a zero se-quence voltage protection because in case of a phase-to-ground fault in the MV network this is the only protection function that insures its detection by the DER with adequate levels of robustness.

The ESSs that SENSIBLE will deploy in Évora demonstrator are all connected to the LV grid and so, the automatic and instantaneous disconnection of the ESS must be assured by their Interconnection Circuit Breaker, which, for that purposes, must have relays that can detect 6 cases:

• Maximum and minimum frequency

• Maximum and minimum voltage

• Maximum current

• Maximum zero-sequence voltage trip for the ESS not connected to the wanted-island demonstrator



As the main topic of this section, the analysis of the protection system of the Évora net-work does not only takes into account the installation of the ESS but also the operation in islanded mode.

A LV network, when provided with systems (RES and ESS) that enable it to be self-sufficient can also, in scenarios of overproduction, feed the MV grid which can present several risks, namely feeding grid faults and forming undetected larger islands in which may not be granted the regulatory voltage. To guarantee the proper protection of the microgrid, the SS must be equipped with an interconnection circuit breaker (and protec-tion relay) which can assure a fast and automatic disconnection of the LV network in case of fault in the MV grid. According to the “Guia técnico das instalações eléctricas de

produção independente de energia eléctrica“, on a generating facility connected to the MV grid (as is the case of a LV network able to supply power to the MV grid) the Inter-connection Circuit Breaker protection must be able to detect 6 cases:

• Maximum and minimum frequency

• Maximum and minimum voltage

• Maximum zero-sequence voltage (protection against line-to-ground fault in MV grid, in case of Y/D power transformer)

• Maximum current

Figure 5.2 is a single-line diagram of all the equipment that will be installed inside the secondary substation of the network to operate disconnected from the MV grid. In addi-tion it also outlines the two ESS that will be directly connected to the SS busbar and exemplifies one of the ESS that will be connected along one LV feeder. The end result reflects the concern that EDP Distribuição has regarding the safety and security of peo-ple, grid and equipment. Due to a careful preliminary analysis, EDP required that all ESS acquired under the public tender should be supplied with their own current breaker equip-ment and independent protection relays. As the responsible partner for supplying the islanding circuit breaker and adapting its protection relay for Évora’s demonstrator, Sie-mens SA took also the responsibility of building a LV Switchgear that would satisfy all the protection requirements settled by the Portuguese distribution grid code and remain-ing regulatory documentation. Summaried below are all the protection devices and aux-iliary equipment predicted for the Évora demonstrator:

Measuring Equipment

o 1 VT (per phase) to connect on MV side of the Power Transformer

o 1 VT (per phase) to connect on LV side of the Power Transformer (below the is-landing CB)

o 1 VT (per phase) to connect on LV side of the Power Transformer (above the is-landing CB)

o 1 CT (per phase) to connect on LV side of the Power Transformer (below the is-landing CB)



• Switching equipment

o 1 MV switch

o 1 LV islanding CB and respective protection relay

o 1 LV switch

o 4 LV feeders CBs

o 4 LV feeders switches

• ESS

o 1 LV Interconnection CB and respective protection relay for each ESS

o 1 LV switch for each ESS

Figure 5.2 - SENSIBLE grid protection scheme



5.3 Summary

Although all requirements concerning measurement (VT, CT) and protection devices (re-lays, circuit breakers and switches) have been identified and addressed, the work re-garding the protection of the grid is not finished. All the acquired equipment must be adapted and parameterised so that they can meet the objective to which they are in-tended.

EDP Distribuição, as the Portuguese DSO, demands protection, coordination and selec-tivity simulation studies, and live tests, so that it can analyse the behaviour of the Évora demonstrator grid under fault conditions and validate that the solution found for the pro-ject meets all the requirements and standards. This simulation must include the behav-iour of the ESSs and the network under fault condition in transient and steady state mode. To proceed with this task, EDP Labelec carried out with the modelling of the LV network while the project partners responsible for supplying the inverters for the grid ESSs (GPTech, INESC and Siemens A.G.) performed the modelling of their own sys-tems. The complete model of Évora grid with all its new assets and the grid fault analysis is being developed by SENSIBLE consortium in order to take advantage of the experi-ence and valences of each partner. Since the simulations will have to undergo validation under laboratory testing scenario and real demonstration scenario, the results of the sim-ulations will be part of Deliverable 4.2.

EDP Labelec will also be conducting live tests on all ESS equipment to prove these fulfil all DSO and legal requirements.

EDP Distribuição will use the simulation and test results to approve all equipment, to be installed in its network, and to determine adequate settings for the ESS and SS protec-tion functions.

REVIEW OF UK REGULATIONS FOR GRID CONNECTION OF

ENERGY STORAGE FOCUSSED ON CLIENT PROTECTION


6 Review of UK regulations for grid connection of energy storage focussed on client protection

6.1 Electrical Regulations (UK)

6.1.1 Introduction

A review of relevant literature has not found any current standards which explicitly ad-dress the electrical protection of grid-connected energy storage. However, there are a number of relevant standards and recommendations concerning the connection of DERs and the protection of AC and DC supplies which are applicable. This review considers the existing standards and recommendations within the context of grid-connected energy storage and power system protection, with the aim of determining whether the current standards and recommendations are sufficient to address the protection requirements for grid-connected energy storage. This review has considered current standards relating to electrical installations; standards, codes of practice and engineering recommenda-tions relating to DERs and electrical networks; and some relevant research literature and legislation.

The main sources consulted while compiling this review are:

i) BS 7671:2008 “Requirements for Electrical Installations” – the IET Wiring Regulations, 17th Edition. Most installations must comply with these regula-tions and this includes energy storage installations [1].

ii) Engineering Recommendation G83/1-1 “Recommendations for the Connec-tion of Small-scale Embedded Generation (Up to 16A per Phase) in Parallel with Public Low-Voltage Distribution Networks.” Most energy storage instal-lations in domestic and small commercial properties are required to meet these recommendations, those that do not should meet the recommendations in ER G59/2 [2].

iii) Engineering Recommendation G59/2 “Recommendations for the Connection of Generating Plant to the Distribution Systems of Licensed Distribution Net-work Operators.” All energy storage installations not covered by ER G83/1-1 are required to meet these recommendations [3].

It should be noted that these documents are constantly being revised and that the current documents at any given time may contain revisions or amendments not covered in the editions used when compiling this review.

This review begins with an overview of generic protection requirements and the factors that affect circuit protection. An attempt has been made to determine whether the instal-lation of grid connected energy storage is likely to impact on existing network protection arrangements. The known protection issues caused by DERs are described and the spe-




cific impact of energy storage is discussed. The existing obligations upon DNOs, gener-ators and consumers are listed. A summary of the review, identifying weaknesses in the existing standards and gaps in the available literature is provided at the end of this report.

6.2 Low-voltage network protection

6.2.1 Supply-side protection

Within the context of this report supply-side protection is considered to be any electrical protection that is installed prior to the point-of-connection on consumer premises and is the responsibility of the DNO to operate and maintain. A wide variety of protective de-vices can be found on distribution and transmission systems, although on low-voltage networks, such as those in residential areas, fuses are common. Specific information regarding the types and ratings of protection devices installed on the distribution network should be available from the DNO.

6.2.2 Consumer protection

Consumer protection is any electrical protection which is installed after the point-of-con-nection on consumer premises and is the responsibility of the consumer to operate and maintain. Typical examples of consumer protection devices are fuses, MCBs, RCDs and RCBOs. In larger installations MCCBs may also be found. The general requirements for consumer protection can be found in the “Requirements for Electrical Installations” (BS 7671) [1], with specific protection devices being required to meet other relevant standards depending on the type and rating of the device. ER G83/1-1 [2] states that overcurrent protection must be provided to protect both installed DERs and the installa-tion to which it is connected and that this should comply with BS 7671 where appropriate.

6.3 Protective device requirements

The acceptable types of protective devices for consumers are listed in BS 7671. Ta-ble 53.4 of BS 7671 lists various types of protective devices and the standards they must adhere to. For circuit breakers the relevant standards to consider are BS EN 60898 [4], BS EN 60947-2 [5] and BS EN 61009-1 [6]. For fuses the relevant standards are BS 1362 [7], BS 3036 [8], BS 646 [9] and the BS 88 [10] series. Other standards apply to other types of protection, e.g. RCDs, but since these provide a specialist function ra-ther than generic protection from faults they are not discussed here.

Since any installation of grid-connected energy storage is very likely to be a recent in-stallation the most likely protective device to be found is an MCB which complies with BS EN 60898. Three different time-current tripping curves are defined in BS EN 60898 and these are referred to as “B”, “C” and “D” types. For a trip time of less than 0.1 sec-onds, a Type B MCB requires at least five times overcurrent and a Type C MCB requires at least ten times overcurrent. A Type D MCB requires at least twenty times overcurrent for a trip time of less than 0.4 seconds. MCBs should be selected such that they do not




operate at all when the current is below the rated circuit current and that expected over-load currents are allowed to flow for short periods of time.

The interrupt capacity of the MCB is of importance when selecting suitable protective devices. The interrupt capacity of the MCB should be greater than the prospective short-circuit current of the installation. Careful selection of protective devices based on inter-rupt capacity may become increasingly important if grid-connected energy storage is to be widely deployed because the additional energy sources can result in a rise in fault current levels and this could lead to the need for an increased safety margin when cal-culating the required interrupt capacity in order to future-proof installations [11].

No literature has been found suggesting that the electrical protection provided by MCBs is insufficient for use with grid-connected energy storage systems or other DERs.

6.4 Additional protection required for the connection of DERs

Additional protective devices may be needed in order to comply with the connection re-quirements for DERs. In addition to any overcurrent and earth leakage protection that is required, over- and under-voltage protection, over- and under-frequency protection and loss of mains protection are all required. For most DERs installed on low-voltage net-works the protection requirements are likely to be those found in ER G83/1-1 [2]: in most cases an over-voltage (> 264 V) or under-voltage (< 207 V) condition must result in dis-connection within 1.5 seconds, an over-frequency (> 50.5 Hz) or under-frequency con-dition (< 47 Hz) must result in disconnection within 0.5 seconds and a loss of mains condition must result in disconnection within 0.5 seconds. Furthermore, following discon-nection, the DER may not be reconnected until the distribution network has remained within the normal operating limits for a minimum of three minutes.

G83/1-1 states that when the generation is required to disconnect it is preferable to do so using a mechanical switching device, although a solid-state relay may also be used. If a solid-state relay is used but fails to operate then the output voltage must be controlled such that it falls to a value below 50 volts within 0.5 seconds.

6.5 Short-circuit current contributions from grid-connected energy storage

DERs provide an additional source of fault current resulting in an increase in the pro-spective short-circuit current of the installation. Neither ER G83/1-1 [2] nor ER G59/2 [3] comments specifically on grid-connected energy storage. However ER G83/1-1 includes a number of appendices giving specific recommendations for different DER types. In all cases where a DER is connected using a grid-tie inverter, as is expected to be the case with grid-connected energy storage, ER G83/1-1 states that “_ they are deemed to au-tomatically comply_ and no further tests are required.” An additional note suggests that grid-tie inverters may provide a small fault current up to 150% of their rated current for a few milliseconds. Test results have been found suggesting that the actual contribution could be closer to five times the rated current in some cases [12] and so further work on




the transient fault current contribution from grid-connected energy storage may be re-quired in future. Further investigation may also be required in order to determine if there is any change in the fault behaviour of the grid-connected energy storage when multiple installations are operating in parallel. Of particular concern is the effect on the system voltage during the fault and whether or not the expected fault current contribution rises linearly with installation size.

Where deployment of grid-connected energy storage is widespread the aggregated in-crease in fault currents may become a problem. For most consumers the fault level should not exceed the worst-case limit of 16 kA [13], but even causing fault levels to rise above much lower limits may be problematic if it causes the current to exceed the fault handling capabilities of lower-rated components.

6.6 Other factors that may have an impact on protection

6.6.1 Earthing requirements

The type of earthing system in use at an installation can have an impact on the protection requirements for that installation. Five different earthing systems are listed in BS 7671 [1]. These are TN-S, TN-C-S, TT, TN-C and IT. TN-S type earthing has separate neutral and earth conductors from the network to the consumer. In TN-C-S type earthing a single combined neutral and earth conductor is provided from the network and split into sepa-rate conductors at the boundary between the DNO’s system and the consumer. TT type earthing provides only a neutral conductor from the network to the consumer, although this is connected to earth at the source, and a local, direct earth connection is provided at the consumer’s property. TN-C earthing is strictly regulated and IT earthing is not found on low voltage networks that are accessible to the public. Of the three commonly found earthing systems, TT has the highest earth fault impedance (lowest earth fault current), TN-S is generally considered the safest system although it also has the highest installation and maintenance costs and TN-C-S provides a low cost alternative to TN-S.

No literature specifically considering the impact of different earthing systems on the be-haviour of grid-tie inverters or grid-connected energy storage during fault conditions has been found. The requirement to determine suitable earthing arrangements is mentioned in ER G59/2 but this only applies to larger installations which include their own trans-former and direct earth connection at the DER. In smaller installations the earthing sys-tem used generally remains fixed as a considerable amount of work is required in order to change the system in use.

6.6.2 Distribution circuit X/R ratio

When fault occurs on system which has a high source reactance relative to source re-sistance the fault current is asymmetric and contains an exponentially decaying DC com-ponent, an exponentially decaying AC component and a steady-state AC component [12]. The ratio of source reactance to source resistance is referred to as the X/R ratio. The choice of type and rating of protective devices depends on the system X/R ratio




although the X/R ratio is often ignored when considering low-voltage systems because it is assumed that the circuit resistance dominates over the reactance and therefore the fault current is expected to be symmetrical. G59/2 [3] states that the impact of an instal-lation on the distribution system X/R ratio should be taken into account.

No literature specifically considering the impact of X/R ratios on the behaviour of grid-tie inverters or grid connected energy storage during fault conditions has been found. Fur-thermore the change in system X/R ratio as a result of the presence of grid-tie inverters does not appear to have been addressed in the literature surveyed, although any contri-bution is expected to be small.

6.7 Known DER protection challenges

There are currently a number of well-documented issues with the protection of DERs of all types [11] [14]. The three main challenges are: a change in fault current levels due to the contribution from DERs; protection blinding – the failure of existing protection to func-tion as a result of the change in the fault current flowing through the protection device; and false tripping as a result of fault current being fed from a healthy circuit to a fault via circuit protection devices that would normally not be triggered. The change in fault cur-rent levels can have a significant effect on all parts of the network and on consumers’ own premises. Protection blinding and false tripping are expected to be primarily experi-enced by DNOs and TSOs, although they may also have an impact on individual con-sumers in some circumstances. Each of these DER protection issues shall be briefly described below and subsequently considered with specific reference to grid-connected energy storage. Islanding, where a part of the network is disconnected from the main supply but remains energised as a result of DERs present, is also a concern.

DERs have the potential to increase fault currents [11] [12]. In the case of inverter con-nected DERs, which includes battery energy storage, the inverter limits the maximum steady-state fault current contribution that can be made to the rated current of the inverter (or less if configured to do so) and so DER penetration must be high for the fault current to be significantly increased. However, this does not mean that the additional fault current can be assumed to have no impact. In addition, the transient contribution to the fault current, even when sourced from inverters, could be large. There is a possibility that the increase in fault currents sourced from DERs could result in the breaking capacity of existing protection being exceeded. This is most likely to affect individual consumers rather than DNOs.

A simple illustration of protection blinding is shown in Figure 6.1. The operation of the protective device is unpredictable, since some of the fault current is now sourced from DERs, resulting in a reduction in the fault current sourced from the main grid supply. A large current must be sourced from the DER supply in order to affect the protection in this manner. In practice, the breaker is unlikely not to trip. However, breaker operation




may be delayed compared to the response of the breaker when the DERs are not pre-sent. The delay in breaker operation may cause damage to infrastructure and increases the risk of fires or additional faults.

Figure 6.1 – Potential for protection blinding caused by a reduction in the fault current

sourced from the grid supply.

Figure 6.2 shows how false tripping may occur. In this scenario, the operation of the device protecting the circuit which includes the DER supply becomes unpredictable be-cause the fault current sourced from the DERs may cause the device to trip, disconnect-ing the healthy circuit. As with protection blinding, it is necessary to draw a large current from the DER supply in order to trip the breaker. With battery energy storage, connected using power electronic inverters which limit the maximum supplied current to the rated value, it is unlikely that either protection blinding or false tripping would occur at any realistic DER penetration level. False tripping leads to unnecessary outages of healthy circuits and can result in increased repair times as the faulted circuit may now take longer to find.

Figure 6.2 – Potential for false tripping caused by current flowing from a healthy circuit to

a fault.

In the specific case of grid-connected battery energy storage neither protection blinding nor false tripping are considered likely to be an issue. This is because the fault current contribution from the energy storage should be limited by the grid-tie inverter and should therefore not exceed the rated current of the inverter in steady-state as noted above. The grid connection may be expected to be capable of supplying a fault current consid-erably higher than the rated supply current and therefore can be assumed to contribute the majority of the fault current even at very high levels of energy storage penetration.




Figure 6.3 illustrates islanding; the main supply from the grid is lost, but the DER supply continues to feed the load. The formation of islands is generally not allowed, although in some situations using DERs, especially energy storage, to provide backup power during outages is acceptable. Formation of a stable island requires balance between the gen-erated and consumed powers, and this is typically difficult to achieve when the generator output cannot be controlled, as is the case with many DERs. As a result, the supply voltage and frequency are likely to vary considerably under island conditions and this may lead to equipment damage. In addition, islanded systems present a significant safety risk, either as a result of the full or partial energisation of a system that is expected to be isolated or as a result of voltage and phase mismatch between the network and island during reconnection.

Figure 6.3 - Islanding: the circuit remains powered, even after the grid supply has been

disconnected.

In the case of grid-connected battery energy storage islanding may become an issue. This is because the energy storage system is capable of supporting the island if the island load is less than the total power rating of the energy storage system and the bat-tery contains sufficient charge. It is expected that under these condition loss-of-mains detection should disconnect the energy storage system from the distribution network.

One further concern with islanding is that it results in a fall in short-circuit currents, par-ticularly when the island is powered by inverter-connected resources. In the case of en-ergy storage connected through grid-tie inverters the steady-state fault current is likely to be similar to the rated current of the faulted circuit. Although this may not immediately appear to be a problem it is likely to result in delayed breaker operation during fault conditions since the fault is perceived as an overload condition rather than a fault. In some cases breakers may not operate at all. The prolonged presence of the fault could lead to increased damage to wiring and an increased risk of electric shock of fire. Such a situation should hopefully be avoided if appropriate under-voltage protection is em-ployed since the system will not be able to remain at the normal voltage with the sub-stantially reduced fault current available.

6.8 Responsibilities of consumers and DNOs

Both generating consumers and the DNOs have a responsibility to ensure that any in-stallation that feeds power onto the distribution network does not adversely affect the




safety or normal operation of either the network or other another consumer’s equipment. This section identifies the obligations placed upon DNOs, generators and consumers which have already been identified and listed in standards.

6.9 Obligations as listed in ER G59/2

This sections contains table 6.1 of “Main Statutory and other Obligations,” taken from Appendix A13.8 of ER G59/2 [3]. The table outlines the obligations of DNOs, Generators and Users. The obligations outlined are a summarised list of the obligations found in a number of other sources: the “Electricity Supply Quality and Continuity Regulations 2002” (ESQCR) [15], the “Electricity at Work Regulations 1989” (EAWR) [16], the “Man-agement of Health and Safety at Work Regulations 1999” (MHSWR) [17], the “Require-ments for Electrical Installations” (BS 7671) [1] and the “Distribution Code” [18].

The obligations listed in ER G59/2 and reproduced below provide a good summary of the key responsibilities of various parties when connecting DERs to the distribution net-work. However a similar list is not found in ER G83/1-1, which is arguably a more appli-cable reference when considering the impact of small installations. ER G83/1-1 does not release the DNO, generator or user from any of their obligations although the installation of appropriately type-tested generation equipment – most commonly inverters in the con-text of grid-connected energy storage – does simplify the process of demonstrating com-pliance.

Users are very broadly defined in ER G59/2 as: “_ persons using the DNO’s Distribution System.” Such a definition could arguably include all domestic consumers with a con-nection to the distribution system, although this is unlikely to be the intended interpreta-tion as most consumers cannot be expected to be aware of the impact distributed energy resources have on their supply. Furthermore there is no general requirement for individ-uals to regularly inspect electrical installations in domestic properties and it is therefore likely to be impractical to enforce compliance of any obligations placed upon individual consumers.




Reference Obligation Responsibil-ity of:

ESQCR Reg 3 Ensure equipment is sufficient for purpose and electrically protected to prevent danger, so far as is reasonably practicable

DNO, Genera-tor

ESQCR Reg 4 Disclose information and co-operate with each other to ensure compliance with the ESQC Reg-ulations 2002

DNO, Genera-tor

ESQCR Reg 6 Apply protective devices to their network, so far as is reasonable practicable, to prevent overcur-rents from exceeding equipment ratings

DNO, Genera-tor

ESQCR Reg 7 Ensure continuity of the neutral conductor and not introduce any protective device in the neutral conductor or earthing connection of LV networks

DNO, Genera-tor

ESQCR Reg 8 Connect the network earth at or as near as rea-sonably practicable to the source of voltage; the earth connection need only be made at one point

DNO, Genera-tor

ESQCR Reg 11 Take all reasonable precautions to minimise the risk of fire from substation equipment

DNO, Genera-tor

ESQCR Reg 21 Ensure that switched alternative sources of en-ergy to distribution networks cannot operate in parallel with those networks and that such equipment which is part of an LV consumer’s in-stallation complies with BS 7671

Generator, User

ESQCR Reg 22 Not install or operate sources of energy in paral-lel with distribution networks unless there are: appropriate equipment, personnel and proce-dures to prevent danger, so far is as reasonably practicable; LV consumer’s equipment complies

Generator, User




with BS 7671; and specific requirements are agreed with the DNO

ESQCR Reg 24 DNO equipment which is on a consumer’s prem-ises but not under the consumer’s control is pro-tected by a suitable fused cut-out or circuit breaker which is situated as close as reasonably practicable to the supply terminals, which is en-closed in a locked or sealed container

DNO

ESQCR Reg 25 Not give consent to making or altering of con-nections where there are reasonable grounds to believe that the consumer’s installation does not comply with ESQCR/BS 7671 or, so far as is reasonably practicable, is not protected to pre-vent danger or interruption of supply

DNO

ESQCR Reg 27 Declare the number of phases, frequency and voltage of the supply and, save in exceptional circumstances, keep this within permitted varia-tions

DNO

ESQCR Reg 28 Provide a written statement of the type and rat-ing of protective devices

DNO

EAWR Reg 4 Construct systems including suitable protective devices that can handle the likely load and fault conditions

DNO, Genera-tor, User

EAWR Reg 5 Not put into service electrical equipment where its strength and capability may be exceeded in such a way as to pose a danger


EAWR Reg 11 Provide a suitably located means to protect against excess current that would otherwise re-sult in danger





MHSWR Reg 3 Carry out an assessment of risks to which em-ployees are exposed to at work and risks to other persons not employed arising from the ac-tivities undertaken


BS 7671 Provide protective devices to break over-load/fault current in LV consumer installations before danger arises

User

BS 7671 Take suitable precautions where a reduction in voltage, or a loss and subsequent restoration of voltage could cause danger

User

Distribution Code DPC4.4.4

Incorporate protective devices in Distribution Systems in accordance with the requirements of the ESQCR



Agree protection systems, operating times, dis-crimination and sensitivity at the ownership boundary



Normally provide back-up protection in case of circuit breaker failure on HV systems


Distribution Code DPC6.3

User’s equipment must be compatible with DNO standards and practices

Generator, User


Design protection systems that take into account auto-reclosing or sequential switching features on the DNOs network

Generator, User


Be aware that DNO protection arrangements may cause disconnection of one or two phases only of a three phase supply

Generator, User


Co-ordinate protection of embedded generator with DNO network and meet target clearance times

Generator





Agree protection settings at network ownership boundary in writing during the connection con-sultation process

DNO, Genera-tor


Generating units or power stations must with-stand NPS loading incurred during clearance of a close up phase to phase fault by system back-up protection

Generator


Agree transformer winding configuration and method of earthing with DNO

Generator


Assess the transient overvoltage effects at the network ownership boundary where necessary

DNO, Genera-tor

Table 6.1 - "Main Statutory and other Obligations" as found in ER G59/2 [3]




6.10 Summary

This review has considered the expected impact of grid-connected battery energy stor-age on electrical network protection and particularly consumer protection. Currently there are no standards dealing with grid-connected energy storage either exclusively or as a specifically addressed technology. It has been assumed that in almost all cases battery energy storage shall be grid-connected via an inverter and this has simplified the analysis somewhat as the impact of inverter connected resources has been considered in the existing standards and considered in much of the research literature available.

The factors likely to affect the behaviour of grid-connected energy storage during fault conditions have been considered. Although the current standards suggest that the fault current contribution for grid-connected energy storage is likely to be small there is some evidence to suggest that transient fault currents could be higher than expected. Further investigation of fault current contributions is required to determine how multiple parallel installations might be expected to behave and whether or not this has any adverse effect on the functioning of protective devices. It is also worth noting the potential for even relatively small increases in fault currents to cause a safety limit to be exceeded, espe-cially with older or poorly maintained installations. Further work is required to assess the influence of X/R ratio on the behaviour of grid-connected energy storage during fault conditions and on the potential for the grid-tie inverters themselves to change the X/R ratio. Further work is also required to determine what, if any, impact different earthing systems have on the behaviour of grid-connected energy storage during fault conditions.

Known problems that can occur specifically because of the presence of DERs on the distribution system have been considered and the expected contribution of grid-con-nected energy storage to these problems has been considered. In the case of protection blinding and false tripping it is not expected that grid-connected energy storage shall have much impact owing to their relatively low fault current contribution. In the case of islanding the impact is less clear. The stability of islands is very dependent on the current state of the system. Energy storage is less prone to sudden, unpredictable fluctuations in output than many other DERs, such as PV, and this could improve island stability. However, under current standards suitable loss-of-mains detection should cause the en-ergy storage to disconnect from the distribution network preventing the formation of an island for more than a very brief period.

The main electrical obligations for various parties with regards to DER installations and in accordance with current regulations has been included for reference. This is the same list as is found in ER G59/2 and gives a guide to the basic requirements that must be met for energy storage installations.

IMPACT OF INTEGRATION OF ENERGY STORAGE IN TO THE

LOCAL GRID ON EXISTING ELECTRICAL PROTECTION SYSTEM


7 Impact of integration of energy storage in to the local grid on existing electrical protection system

7.1 Introduction

A representative household (client) electrical installation was created in the flexelec lab to allow a series of overload tests to be conducted. These tests are to investigate how an energy storage system installed in a home may affect the operation of Miniature Cir-cuit Breakers (MCBs) which are commonly used in household distribution boards as cir-cuit protection devices against current overload.

Information from the tests can be further extrapolated to determine how a number of energy storage units in a community may affect a client’s protection system.

7.2 Test Setup

As part of the flexelec lab based demonstrator a representative fuse / distribution board as ay be used in a domestic home was installed according to the relevant UK regulations. Figure 7.1 shows the configuration of the test setup, Table 7.1 gives details of the specific equipment used during the tests. This setup includes a commercial PV and battery in-verter system which is one of the options for use in the Meadows demonstrator to be implemented in the UK.

To allow for a range of overload tests to be performed a switchable load bank has been used. This provides the by which high power loads can be switched on to the test circuit and simulated overload conditions. The overload tests are selected to stress the system and to approach short circuit faults, hence have low impedances and high overload fac-tors compared to the MCB protection device.




Figure 7.1 – Block diagram and single line diagram of test equipment setup and meas-

urement locations

ES SMA Sunny Island 4.4

Supply 230V 160A rated supply, tests on U phase ~235Vrms

Phase to neutral fault current 1.6kA

Voltage measurement Vsupply Pico TA041

Current measurement Ifault PEMUK CWT

Current measurement IES , GMC-I PROSyS - CP305 [300A setting = 1mV/A]

Current measurement Isupply , GMC-I PROSyS - CP305 [300A setting = 1mV/A]

F1.1 British General B6A MCB CUMB6-XD

F1 Schneider C 63A MCB iC60H

F2 Main breaker in building supply FEL1 cabinet 160A



Table 7.1 – Overload test equipment in Figure 5.2.1




7.2.1 Tests and Test Procedures

Each test performed is detailed in this section, an overview is given in table 7.2. The generic procedure is to setup the configuration of each test as in each test diagram, apply any base load and allow to reach steady state conditions. Once reached, the overload fault is switched in using a contactor and the data capture is triggered and the data stored. Tests were repeated to check for consistently in results.

For each test scenario a number of sub test were performed, these are with increasing levels of over load the details are given in table 7.3.

There are some limitations to the tests performed. Precise synchronisation with the grid voltage wave is difficult to repeatedly, this can affect the peak overload current seen. To attempt to mitigate against this at least 3 repeated test were performed for each overload condition and the most consistent results were in terms of fault timing were selected.

A direct comparison between over load values is difficult to perform, due to the differ-ences in contactors used for the different over load ratings. Each contactor having a variable response time and closing time, attempts were made to compensate for the variability, by adjustment of the timing of the over load trigger signal, but this achieved only limited success.

Test Sub tests (load rating) Description

1 a)20kW, b)40kW, c)100kW Grid only operation.

2 a)20kW, b)40kW, c)100kW Grid only with ES connected but disa-bled.32121

3 a)20kW, b)40kW, c)100kW Grid connected, ES is in auto discharge mode.

4 a)20kW, b)40kW, c)100kW Grid connected, ES is in manual discharge mode.

5 a) Base load ON b) Base load OFF

ES in self consumption mode, ES response time test.

Table 7.2 – Subtest loading conditions.

Steady state load at 240V

Steady State cur-rent (A)

Load Re-sistance (Ω)

Current rating overload factor of F1.1

20 kW 83 2.88 x14

40 kW 167 1.44 x28

100 kW 416 0.58 x69

Table 7.3 – Subtest loading conditions.




7.2.1.1 Test 1

This test is a base line set of tests the setup is shown in figure 7.2. There is a base load of 1.8kW attached the consumer on a different circuit to the circuit under test. The grid is the only source of energy and the ES is disconnected from the grid altogether.

Once steady state conditions have been reached the overload is switched on for each of the subtest conditions as given in Table 7.3.


urement locations




7.2.1.2 Test 2

This test is similar to test 1 the setup is shown in figure 7.3. There is a base load of 1.8kW attached the consumer on a different circuit to the circuit under test. The grid is the only source of energy though the ES is connected but disabled during this test.


urement locations




7.2.1.3 Test 3

In this test the ES is active and is in self-consumption mode. Its internal control is at-tempting to limit any export and use the battery to supply any load. ES operation is de-tailed in section 7.3. The setup is shown in figure 7.4. There is a base load of 1.8kW attached the consumer on a different circuit to the circuit under test. The grid is con-nected, though the source of energy for the load is the ES.


urement locations




7.2.1.4 Test 4

In this test the ES is active and is in manual control mode. Demands for power flow are set via MODBUS. ES operation is detailed in section 7.3. The setup is shown in figure 7.5. There is a base load of 1.8kW attached the consumer on a different circuit to the circuit under test. The grid is connected, though the source of energy for the load is the ES.


urement locations




7.2.1.5 Test 5

These test are different to the previous tests, they do not include overload testing. The two tests 5a and 5b are to investigate the ES system response to load changes. Test5a is an increase in load, test 5b is a decrease in load. The setup is shown in figure 7.6.


urement locations




7.3 Results

Results from the overload tests described in section 7.2.

Comments on the results are given in section 7.4.

7.3.1 Test 1

Figure 7.7 – Test 1, Grid only, subtest a 20kW overload




Figure 7.8 – Test 1, Grid only, subtest b 40kW overload

Figure 7.9 – Test 1, Grid only, subtest c 100kW overload




7.3.2 Test 2

Figure 7.10 – Test 2, Grid only ES connected, subtest a, 20kW overload

Figure 7.11 – Test 2, Grid only ES connected, subtest b 40kW overload




Figure 7.12 – Test 2, Grid only, ES connected, subtest c 100kW overload

7.3.3 Test 3




Figure 7.13 – Test 3, Grid with ES in self consumption mode, subtest a 20kW overload

Figure 7.14 – Test 2, Grid with ES in self consumption mode, subtest b 40kW overload




Figure 7.15 – Test 2, Grid with ES in self-consumption mode, subtest c 100kW overload

7.3.4 Test 4




Figure 7.16 – Test 2, Grid ES fixed demand output, subtest a 20kW overload

Figure 7.17 – Test 2, Grid ES fixed demand output, subtest b 40kW overload

Figure 7.18 – Test 2, Grid ES fixed demand output, subtest c 100kW overload




7.3.5 Test 5

Figure 7.19 – Test 5a, ES System response to step load increase.




Figure 7.20 – Test 5b, ES System response to step load decrease.




7.3.6 SMA Sunny Island and battery

The energy storage system used comprises an SMA Sunny Island 4.4 inverter with an attached LG Chem RESU 6.4 EX lithium-ion battery. The SMA inverter is rated for 3300W operation although can operate with powers up to 5500W for short dura-tions. The battery on the other hand has a peak power of 5000W with 6400Wh nominal energy.

The SMA Sunny Island 4.4 is a versatile energy storage inverter that is able to operate in various modes including back-up power, islanded and a self-consumption mode (the purpose of which is to increase a system‘s own use of generated energy such as that from PV modules). In self-consumption configuration, the SMA Sunny Island intends to operate in a manner that charges the attached ES rather than leaving the system to export energy from its domain or alternatively to discharge the ES rather than importing energy from the utility to supply the loads.

The SMA system typically uses an SMA Energy Meter in order to measure electrical conditions and energy flow at the point of connection of the property (for typical domestic systems). The energy meter communicates the values via the Ethernet based SMA Speedwire fieldbus including values of power either importing or exporting. In order to operate the SMA inverter requires these electrical values and, for the purposes of in-creasing self-consumption, operation can be based on the net flow of power particularly import or export and the magnitude.

That there is a separate device performing measurement that is then used to influence the inverter power set point, including having to communicate the value and potentially through Ethernet network devices, has the potential to instil delays into the response of the system to changes in power.

Also, the SMA inverter prioritises the protection of the battery condition. In order to not over charge and operate influenced by battery conditions the power is curtailed at ranges near the limits of the usable state of charge (SoC). In these ranges the SMA inverter will charge or discharge the battery as near to the required set point as is viable while it does not conflict with best operation for the battery. At the limit of usable SoC the SMA inverter will not allow any operation with power flow that would otherwise take the SoC out of the usable range while during charging as the SoC approaches full, the charge rate is inhib-ited.




Figure 7.21 Diagrams of a SMA Sunny Island installation. Images taken from SMA Flexible Storage

System [1]

Energy storage control systems and their inverters can have a wide range of reaction times to varying load or generation levels although they are inherently influenced by the response of the energy storage device attached. In an ideal world, the energy storage, inverter and controller would react instantaneously in order to best compensate or cap-




ture changing loads or excess generation however that cannot occur. Similarly, the re-sponse of protection elements to a change of load, such as that which would produce an overload in its section on the network, does not react instantaneously.

7.3.7 Controlled operation

During controlled operation the SMA inverter is given a power set point that it is to pro-duce at its AC connection. Operation of the battery, such as required current and voltage for the power is controlled internally in order to fulfil the required AC power demand. Provided the SMA inverter and battery are not within the curtailing region the power de-mand requested of the inverter is exchanged with the AC network. In this manner of operation, the SMA inverter can take up to five seconds to respond to a changed power demand that is issued.

7.3.8 Self-consumption operation

When configured for self-consumption the SMA inverter operates based on internally issued set points that depend on the values issued by the SMA Energy Meter. During times where the generation exceeds the consumption of the loads, where typically en-ergy would be exported to the utility, the inverter demands a charging power such that the excess generation is channelled to the battery for storage. Alternatively, where gen-eration is less than the power demanded by the loads, the inverter can control operation to discharge the battery thereby contributing all or some of the power required to supply the loads. Under ideal conditions, provided the power demands are within the rated power values of the inverter and battery and whilst the SoC and operation of the battery remains outside the curtailing region then the power exchange with the utility is reduced to zero.

7.3.9 Tests and SMA modes of operation

The SMA inverter was operated in different modes according to separate test condi-tions. Figures 7.22 to 7.25 indicate potential power flow in the different inverter modes of operation. In test 2 the inverter was forced to perform no power and therefore was con-nected but not contributing to any current flow, Fig. 7.22. In test 3 the system was con-figured to produce a fixed demanded power discharge, whereby changes to load or gen-eration would not influence the operation. The specific flow of power depends on the load value, Fig. 7.23. Finally in test 4 the inverter was reconfigured in to automated self-consumption operation whereby the system itself dictated its power set points based on whether its network was exporting or importing energy as well as the SoC of the attached energy storage, Fig. 7.24 & 7.25.




Figure 7.22 – Inverter is disabled

Figure 7.23– Inverter is enabled without battery storage and PV is generating

Figure 7.24 – Inverter is in self-consumption mode and supplying the load.




Figure 7.25 – Inverter is in self-consumption mode, PV generation is supplying load

and excess is stored in the battery.

7.4 Summary and Conclusion

An experimental setup was created to represent a house hold in the UK which has an ES system installed to investigate its effect on the overload protection devices installed in the consumer unit. A range of test scenarios where devised and a number of tests for increasing overload fault level where performed. Unfortunately the tests are somewhat inconclusive, mainly due to overload fault timing, variable delay in the overload contactor mean that the instantaneous supply voltage at the moment of the fault can vary. Though there are some conclusions that can be drawn from the results.

In the results for tests 1- 4 there is little difference in the measured fault currents when using energy storage or not. For the scenario tested of a home based system with a single ES unit the fault response is dominated by the grid connection. The response of the protection device is unaffected due to the presence of ES and the fault is cleared at the next zero crossing of the supply.

There are a few points to note, in test 3 when the ES is in self-consumption mode and responds automatically to changes in load, when the overload is connected there is no increase in the ES current. Also that in either mode of operation of the ES in test 3 & 4 the ES current remains constant before, during and after the overload is caused. This would mean that, although not discernible in the tests shown here, the ES will contribute to the fault current, though as stated before the response is dominated by the grid.

This contribution could become significant if a large amount of ES was active on the faulted phase and should be considered as adding to the short circuit current level sup-plies by the grid substation. As protection devices have fault current clearance ratings some protection devices may need to be changed or one with an increased clearance current rating fitted.




The additional fault current from ES is dependent on the power converter rating, so a large amount of energy storage is not necessarily implied. The combined total current output of connected ES converters should be considered. Though as shown in tests 5a & b the response time of the converter used in these tests to load changes is at least 2 seconds. The converter control response is unlikely to respond to the fault but maintain its previous current output prior to the fault, unless the fault is in place for a significant length of time. Even then to meet the UK grid regulations connected ES or PV should stop supplying power in to the grid during periods of grid voltage instability.

POWER ELECTRONIC SYSTEMS


8 Power Electronic Systems

Power electronics is the engineering discipline which enables, by implementing an effec-tive control strategy, the conversion of electrical power and is necessary wherever inter-connections are necessary between different power sources and loads. Such is the case, for example, for the power needed to supply home appliances, automotive traction, and industrial processes or to interface renewable resources (photovoltaic panels, wind tur-bines etc.) to the power distribution grid. Equally, Renewable Energy Systems (RES) are contributing to more and more environmentally-friendly electricity generation which is made possible only by using power electronic systems. The role of power electronic sys-tems in enabling generation of “clean” energy and transferring renewable energy to the power grid is fundamental and it is required to be done in a highly efficient and reliable manner, reducing energy losses and extending service time. Research into reliability of power electronics has highlighted a higher failure rate than other subsystems of renew-able energy systems which will need to be addressed by choosing better device pack-aging technologies, effective thermal management and robust design and validation with realistic mission profiles.

The following sections will be focused on the safety requirements aspects of the power electronic systems when implemented for RES.

Figure 8.1 – Power Electronics System in a RES

8.1 Safety requirements for power electronics converters systems (PECS) used in renewable energy systems

Any electrical system if improperly installed can lead to safety hazards such as electro-cution or fire and mechanical hazards. From a component point of view, the power con-verter itself not only undergoes a near continuous analysis and development cycle to create a more reliable design implementation, whose main reasons for failure and miti-gation will be described briefly, but should also comply with stringent safety requirements provided by International standards which will be mentioned in the following sections.



Power converter fundamental building blocks are power devices which switch, on and off, at a certain frequency and according to a certain control strategy to deliver the de-sired power conversion features. In order to be able to control these switching devices, a control interface between the latter and the controller will be needed. Therefore any of these elements, switching devices, its driving circuitry interface or the controller could at some point fail to function, sometimes causing a catastrophic failure which could lead to any of the safety hazards mentioned above.

The main causes for failures and suggested protection strategies for power converters, which would avoid safety hazards conditions, are listed below:

• Power device (IGBTs, Diodes etc.) failures are generally due to either over-volt-age, over-current or over-temperature. From an electrical point of view protective circuits consisting of snubber circuits or clamp over-voltage circuits will provide a temporary path for the current if for example the switching device fails open cir-cuit. If the switching device fails short circuit then a controlled shut down of the power converter is necessary, to avoid catastrophic failure of the converter itself. Also an effective cooling system for the power devices needs to be provided to avoid over-heating of the power converter

• Power devices driving circuitry interface failures are generally due to failure of the electronic components which this interface is made of. Some of these failures can be mitigated in a similar way as before, with snubber circuits or there are in-built protection mechanism such as under-voltage lock out feature in the Gate Driver IC which detects for example the sudden lack of supply to these ICs allow-ing a controlled and safe shut-down of the converter operation.

• Smart Effective Control which makes good use of all the information feedback by current, voltage, temperature, speed/position sensors to implement the appropri-ate control strategy for an efficient operation of a power converter also provides safety protection in a form of software shutdown of the functionalities of the power converter in case any of the readings from the sensors over take a threshold reference considered as safe operation mode.

The IEC 62477-1 2012 is an international standard issued by the International Electro-technical Commission which applies to Power Electronics Converter Systems (PECS) and equipment, the components necessary for power electronics conversion as men-tioned above and their control including protection, monitoring and measuring. It defines the minimum requirements for the design and construction of a PECS, to ensure its safety during installation, normal operating conditions and maintenance for its expected lifetime. Some of the areas covered by this standard are highlighted below:

• Protection against hazards: this includes design requirements which would lead to a PECS which operates free from faults and components failures and consequent required circuit analysis and testing to prevent unsafe operating con-ditions or to evaluate the entity of a hazard that an eventual component failure could cause. Guidance is provided on short circuit and overload protection, on



protection from electrical hazards, thermal hazards, mechanical hazards, envi-ronmental stress and sonic pressure hazard.

• Test requirements: As well as providing guidance on how to implement the cor-rect wiring and connections between the different parts of the system in order to perform a safe installation and avoid mechanical damage of the equipment, this standard also covers a whole series of procedures to perform tests on the PECS with the final objective of delivering a system which is not going to cause safety hazards. The tests and their types, specifications and procedures are highlighted and described in this standard in order to provide a tool to verify that the PECS and the equipment that comes with it conform to the requirements listed in the standard itself. Therefore the tests described are electrical, mechanical, environ-mental, material, hydrostatic pressure tests including guidance on visual inspec-tions.

• Information and marking requirements: this standard finally defines all the in-formation necessary to carry out a safe selection, installation (especially if the power electronics system will be installed in an enclosure, this document will specify the temperature requirements for the enclosure, its dimensions and mounting points), maintenance and operation of PECS, from defining the label-ling required to perform a correct installation to safety signs and signalling, visible and audible used while operating or performing a maintenance activity.

The following sections have been included with reference to the only two examples of power converters employed in a RES whose design requirements with a focus on safety have been described by the International Electrotechnical Commission in the related In-ternational standards IEC 62909-1 and IEC 62109-1 2010 (followed by the IEC 62109-2 2011)

8.2 Safety requirements for bidirectional power converters in grid connected applications

The IEC 62909 is the international standard that defines the bi-directional Grid-Con-nected Power Converters (GCPC), their components, system architecture, specifica-tions, performance and safety requirements. Figure 8.2 shows the structure of a bidirec-tional GCPC in a typical grid connected implementation.



Figure 8.2: Structure of a bidirectional GCPC

Figure 8.2 also shows the components used in a GCPC: two (or more) DC-DC convert-ers, one bi-directional converter and all the necessary control to operate them including the appropriate interface between converters and the grid or the distributed energy sources. The distributed energy resources could include, Photo-Voltaic, micro-turbine, wind generators or CHP plants together with storage resources such as electrochemical batteries or micro-flywheel systems.

The safety requirements covered in this standard recall all aspects of the safety require-ments covered by the IEC 62477-1 2012 standard which has been discussed in section 8.1.

IEC 62909 also provide a list of other IEC standards referenced or connected to all the individual aspects discussed in this document.

8.3 Safety of power converters for use in photovoltaic power systems

The IEC 62109-1 2010 (followed by the IEC 62109-2 2011 which includes some amended sections of the original document) is the International Standard which defines the requirements for the design and manufacture of Power Conversion Equipment (PCE) for use in photovoltaic (PV) applications with a specific focus on safety, providing infor-mation on how to avoid and how to protect the surrounding areas from electric, mechan-ical, thermal, chemical, sonic pressure and explosion hazards.

A great deal of emphasis is given in this standard to the testing requirements which the PCE used in PV applications need to be able to comply with so that by passing the testing procedures outlined they will conform to the design requirements defined in the standard itself. The testing requirements cover information such as testing in fault conditions, tests specifications of the equipment electrical ratings and finally guidance on tests to be con-ducted on the inverters used in grounded and ungrounded PV arrays.

The testing procedures described together with the subsequent sections in this standard outlining how to provide protections against various hazards (as mentioned above) have

Grid

Energy

Meter

CP Distribution

board GCPC

Distributed Energy

Resources

Distributed Energy

Resources

Distributed Energy

Resources



the common purpose of delivering an official and internationally valid guidance on de-signing, installing and maintaining a safe PEC for PV applications. IEC 62109 also pro-vide a list of other IEC standards referenced or connected to all the individual aspects discussed in this document.

8.4 Summary

Power electronic systems are an enabling technology necessary in achieving the goals of a ‘low carbon economy’. Anywhere electrical energy needs to be converted from one form to another or moved from one place to another, power electronic systems are nec-essary. Nowhere could this be more important than in the world of renewable energy generation and energy storage. This is typically due to the fact that many RES generation and energy storage media use DC to transfer or store the electrical energy whereas the typical domestic and distribution grids use AC. The continued development in terms of efficiency, reliability and cost are driving forces for the increased penetration of RES within the local and national energy portfolio. As with all things, safety of these systems is of paramount importance, especially where these systems are to be used in a domestic setting. This section describes in general the requirements placed on power electronic systems together with the international standards which are presently used to provide both guidelines on their development and construction and testing regimes to ensure that their safety is maintained.

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