A Novel Structure for Three Phase Four Wire Distribution System Utilizing UPQC

A Project Report On

A NOVEL STRUCTURE FOR THREE-PHASE FOUR-

WIRE DISTRIBUTION SYSTEM UTILIZING UNIFIED

POWER QUALITY CONDITIONER (UPQC)

Submitted to

Jawaharlal Nehru Technological University Ananthapur,

Ananthapuramu.

in partial fulfillment of the requirements

For the award of the degree of

Bachelor of Technology

In

Electrical & Electronics Engineering

By

Gufran Ali Ahamed Syed (108Z1A0220)

Anwesh Katakam (108Z1A0204)

Chaitanya Rangani (108Z1A0210)

Kranti Kiran (108Z1A0230)

Under the esteemed guidance of

Ch. Lenin Babu, M.Tech, MISTE, MIAENG

Assistant Professor

Dept. Of EEE

Department of Electrical & Electronics Engineering

PRIYADARSHINI INSTITUTE OF TECHNOLOGY (Affiliated to Jawaharlal Nehru Technological University Anantapur, ANANTAPURAMU)

Kanuparthipadu, Nellore-524004

(2010-2014)

PRIYADARSHINI INSTITUTE OF TECHNOLOGY (Affiliated to Jawaharlal Nehru Technological University Anantapur, ANANTAPURAMU)

Kanuparthipadu, Nellore-524004

DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING

Certificate

This is to certify that the Project entitled

A NOVEL STRUCTURE FOR THREE-PHASE FOUR-

WIRE DISTRIBUTION SYSTEM UTILIZING UNIFIED

POWER QUALITY CONDITIONER (UPQC)

Is the bonafide work done by

In the Department of Electrical and Electronics Engineering, Priyadarshini

Institute of Technology, SPSR Nellore and is submitted to Jawaharlal Nehru

Technological University Anantapur, Anantapuramu, in partial fulfillments of the

requirements for the award of B. Tech Degree in Electrical and Electronics

Engineering.

This work has been carried out under my guidance





Guide:

Ch. Lenin Babu, M. Tech, MISTE, MIAENG

Asst. Professor,

Department of E.E.E.

H.O.D.:

G. Venkata Narayana, M. Tech

Assoc. Professor,

Department of E.E.E.

Internal Examiner External Examiner

(i)

Acknowledgement

We are deeply indebted to our supervisor Mr. Ch. Lenin Babu, M.Tech, MISTE,

MIAENG, Associate Professor, Department of Electrical and Electronics

Engineering for valuable guidance, constant encouragement, constructive criticism

and keen interest evinced throughout the course of our work. We are really fortunate

to associate ourselves with such an advising and helping guide in every possible way,

at all stages, for the successful completion of this Project work.

We express our deep sense of gratitude and thanks to Mr. G. Venkata

Narayana, M. Tech, Associate Professor and Head of the Department, Dept. Of

Electrical and Electronics Engineering, for supporting in completing our Project

work.

We express our gratitude to our Principal Prof. Dr. Prasad H. K. P, Ph.D., and

the Management for providing all the facilities and supporting in completing our

Project work successfully. We express heartfelt thanks to all our Teachers in the

EEE department of Priyadarshini Institute of Technology for their moral support

and good wishes.

Finally we have a notation to express sincere thanks to our friends, one and all

those who guided, inspired and helped us in the completion of our Project work

successfully.

Project Associates





(ii)

Declaration

We hereby declare that our Project report entitled, “A Novel Structure for Three-Phase

Four-Wire Distribution System Utilizing Unified Power Quality Conditioner (UPQC),” being

submitted by us for award of Degree of Bachelor of Technology in ELECTRICAL &

ELECTRONICS ENGINEERING, to Jawaharlal Nehru Technological University Anantapur,

Anantapuramu and is a bonafied record of work done in PRIYADARSHINI INSTITUTE OF

TECHNOLOGY, SPSR NELLORE and has not been submitted to any other courses or

University for award of any degree.

Project Associates





(iii)

Abstract

This project deals with a novel structure for a three phase four-wire

distribution system utilizing Unified Power Quality Conditioner (UPQC).

Synchronization of the voltage frequency and phase allows electrical systems to

function in their intended manner without significant loss of performance or life. The

term is used to describe electric power that drives an electrical load and the load's

ability to function properly. Without the proper power, an electrical device (or load)

may malfunction, fail prematurely or not operate at all. There are many ways in which

electric power can be of poor quality and many more causes of such poor quality

power. Power quality determines the fitness of electrical power to consumer devices.

In this project, a Three-Phase Four-Wire system is realized from a Three-Phase

Three-Wire system where the neutral of series transformer used in series part of

UPQC is considered as the fourth wire for the Three-Phase Four-Wire system. A new

control strategy to balance the unbalanced load currents is also done and presented in

this document. The neutral current that may flow towards transformer’s neutral point

is compensated by using a four-leg voltage source inverter topology for shunt part.

Thus, the series transformer’s neutral will be at virtual zero potential during all

operating conditions. The simulation results based on MATLAB/Simulink are

presented to show the effectiveness of the proposed UPQC-based Three-Phase Four-

Wire distribution system.

http://en.wikipedia.org/wiki/Grid_(electricity)

http://en.wikipedia.org/wiki/Electrical_load

http://en.wikipedia.org/wiki/Electrical_power

http://en.wikipedia.org/wiki/Consumer

Contents

Page.

No. Acknowledgement i

Declaration ii

Abstract iii

List of Figures vi-viii

List of Tables ix

Chapter 1 Introduction…………………………………... 02-05

1.1 Distribution Systems…………………………………... 02 1.2 History of Power Quality Conditioning……………….. 04

1.3 Outline of Project……………………………………… 05

Chapter 2 Power Quality………………………………… 07-13

2.1 Introduction……………………………………………. 07

2.2 Power Quality Problems……………………………….. 08

2.3 Power quality can be improved through……………….. 09

2.4 Benefits of Power Quality……………………………... 10

Chapter 3 FACTS Devices in Distribution System…….. 15-21

3.1 Introduction……………………………………………. 15

3.2 FACTS Devices used in Distribution for Power

Conditioning……………………………………………. 18

Chapter 4 P-Q Theory…………………………………… 23-26

4.1 Introduction……………………………………………. 23

4.2 P-Q Theory Power Components……………………….. 23

Chapter 5 Pulse Width Modulation……………………... 28-32

5.1 Introduction…………………………………………….. 28

5.2 Sinusoidal Pulse Width Modulation…………………… 29

Chapter 6 Voltage Source Inverter……………………… 34-38

6.1 Introduction…………………………………………….. 34

6.2 General Structure of Voltage Source Inverters………… 34

Chapter 7 Active Power Filters………………………….. 40-43

7.1 Introduction…………………………………………….. 40

7.2 Types of Active Power Filters…………………………. 41

Chapter 8 Modelling of Test System…………………….. 45-52

8.1 Three Phase Four Wire Distribution System Utilizing

UPQC …………………………………………………. 45

8.2 UPQC Controller……………………………………….. 47

Chapter 9 MATLAB Design and Simulation of Test

Results…………………………………………. 54-77 9.1 UPQC to R Load……………………………………….. 54 9.2 UPQC to RL Load……………………………………… 60 9.3 UPQC to RLC Load……………………………………. 66

9.4 UPQC TO IM LOAD…………………………………... 72

Chapter 10 Conclusion and Future Scope………………... 79-80

10.1 Conclusion……………………………………………… 79

10.2 Future Scope……………………………………………. 79

References 82

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List of Figures

S.NO NAME OF THE FIGURES PAGE. NO

3.1 Operational limits of transmission lines for different voltage levels 16

3.2 Overview of major FACTS-devices 17

3.3 Classification of Distribution Power Conditioners 18

3.4 A Schematic diagram of DVR 19

3.5 Schematic diagram of DSTATCOM 20

3.6 Schematic diagram of UPQC 21

4.1 p-q theory power components 26

5.1 Simple Voltage Sourced Inverter 29

5.2 Principal of Pulse Width Modulation 30

5.3 SPWM with fc/fm = 48, L/R = T/3 31

5.4 Over Modulation: m = 1.3 32

6.1 Principal of Pulse Width Modulation 37

7.1 Generalized block diagram for active power filters 40

7.2 Active power filter topologies implemented with PWM VSI.

(a) Shunt active power filter.

(b) Series active power filter.

(c) Hybrid active power filter. 42

8.1 3P4W distribution system: neutral provided from generation station 45

8.2 3P4W distribution system: neutral provided from Δ–Y transformer 45

8.3. 3P3W UPQC structure 46

8.4. Proposed 3P4W system realized from a 3P3W system utilizing UPQC 47

8.5 Shunt active filter control block diagram.

(a) Proposed balanced per-phase fundamental active power estimation.

(b) DC-link voltage control loop.

(c) Reference source current generation.

(d) Neutral current compensation. 52

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9.1. MATLAB model of UPQC for R Load 54

9.2. Utility Voltage (VS_abc) in UPQC2R Load 54

9.3. Load voltage (VL_abc) in UPQC2R Load 55

9.4. Injected Voltage (Vinj_abc) in UPQC2R Load 55

9.5. DC-link Voltage (Vdc) in UPQC2R Load 56

9.6. Load Current (IL_abc) in UPQC2R Load 56

9.7. Source Current (IS_abc) in UPQC2R Load 57

9.8. Shunt Compensating Current (ISh_abc) in UPQC2R Load 57

9.9. Current flowing through load neutral wire (IL_n) in UPQC2R Load 58

9.10. Shunt neutral compensating current (ISh_n) in UPQC2R Load 58

9.11. Neutral current flowing towards series transformer (ISr_n)

in UPQC2R Load 59

9.12. MATLAB model of UPQC for RL Load 60

9.13. Utility Voltage (VS_abc) in UPQC2RL Load 60

9.14. Load voltage (VL_abc) in UPQC2RL Load 61

9.15. Injected Voltage (Vinj_abc) in UPQC2RL Load 61

9.16. DC-link Voltage (Vdc) in UPQC2RL Load 62

9.17. Load Current (IL_abc) in UPQC2RL Load 62

9.18. Source Current (IS_abc) in UPQC2RL Load 63

9.19. Shunt Compensating Current (ISh_abc) in UPQC2RL Load 63

9.20. Current flowing through load neutral wire (IL_n) in UPQC2RL Load 64

9.21. Shunt neutral compensating current (ISh_n) in UPQC2RL Load 64


in UPQC2RL Load 65

9.23. MATLAB model of UPQC for RLC Load 66

9.24. Utility Voltage (VS_abc) in UPQC2RLC Load 66

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9.25. Load voltage (VL_abc) in UPQC2RLC Load 67

9.26. Injected Voltage (Vinj_abc) in UPQC2RLC Load 67

9.27. DC-link Voltage (Vdc) in UPQC2RLC Load 68

9.28. Load Current (IL_abc) in UPQC2RLC Load 68

9.29. Source Current (IS_abc) in UPQC2RLC Load 69

9.30. Shunt Compensating Current (ISh_abc) in UPQC2RLC Load 69

9.31. Current flowing through load neutral wire (IL_n) in UPQC2RLC Load 70

9.32. Shunt neutral compensating current (ISh_n) in UPQC2RLC Load 70


in UPQC2RLC Load 71

9.34. MATLAB model of UPQC for IM Load 72

9.35. Utility Voltage (VS_abc) in UPQC2IM Load 72

9.36. Load voltage (VL_abc) in UPQC2IM Load 73

9.37. Injected Voltage (Vinj_abc) in UPQC2IM Load 73

9.38. DC-link Voltage (Vdc) in UPQC2IM Load 74

9.39. Load Current (IL_abc) in UPQC2IM Load 74

9.40. Source Current (IS_abc) in UPQC2IM Load 75

9.41. Shunt Compensating Current (ISh_abc) in UPQC2IM Load 75

9.42. Current flowing through load neutral wire (IL_n) in UPQC2IM Load 76

9.43. Shunt neutral compensating current (ISh_n) in UPQC2IM Load 76


in UPQC2IM Load 77

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List of Tables

Table No. Name of Table Page. No.

7.1 Active filter solutions to power quality problems 43

Chapter 1

Introduction

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1. Introduction

1.1 Distribution Systems In recent years, the total number of distributed generation such as photovoltaic

generation system and wind power generator system connected to the distribution system

has been increasing because of the reduction of effects on the environment in Japan. In

the distribution system, it is necessary to keep reliability of power and power quality.

However, the harmonic troubles in a distribution system are apprehended in the

background of the increase of connection of DGs through the inverters and the spread of

the power electronics equipment. So far, the authors had studied the influence which

harmonics generated from DGs has on harmonics of distribution network and the restraint

method of harmonics in a distribution system by active filters. In this paper, the authors

propose a restraint method of voltage total harmonic distortion in a whole distribution

system by active filters operation of distributed generation with inverter.

Electricity distribution is the final stage in the delivery of electricity to end users.

A distribution system's network carries electricity from the transmission system and

delivers it to consumers. Typically, the network would include medium-voltage (2kV to

34.5kV) power lines, substations and pole-mounted transformers, low-voltage (less than

1 kV) distribution wiring such as a Service Drop and sometimes meters.

The electric power industry comprises electricity generation (AC power), electric

power transmission and ultimately electricity distribution to an electricity meter located

at the premises of the end user of the electric power. The electricity then moves through

the wiring system of the end user until it reaches the load. The complexity of the system

to move electric energy from the point of production to the point of consumption

combined with variations in weather, generation, demand and other factors provide many

opportunities for the quality of supply to be compromised.

The modern distribution system begins as the primary circuit leaves the sub-

station and ends as the secondary service enters the customer's meter socket by way of a

service drop. Distribution circuits serve many customers. The voltage used is appropriate

for the shorter distance and varies from 2,300 to about 35,000 volts depending on utility

standard practice, distance, and load to be served. Distribution circuits are fed from a

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transformer located in an electrical substation, where the voltage is reduced from the high

values used for power transmission.

Conductors for distribution may be carried on overhead pole lines, or in densely

populated areas, buried underground. Urban and suburban distribution is done with three-

phase systems to serve residential, commercial, and industrial loads. Distribution in rural

areas may be only single-phase if it is not economical to install three-phase power for

relatively few and small customers.

Only large consumers are fed directly from distribution voltages; most utility

customers are connected to a transformer, which reduces the distribution voltage to the

relatively low voltage used by lighting and interior wiring systems. The transformer may

be pole-mounted or set on the ground in a protective enclosure. In rural areas a pole-

mount transformer may serve only one customer, but in more built-up areas multiple

customers may be connected. In very dense city areas, a secondary network may be

formed with many transformers feeding into a common bus at the utilization voltage.

Each customer has a service drop connection and a meter for billing. (Some very small

loads, such as yard lights, may be too small to meter and so are charged only a monthly

rate.)

A ground connection to local earth is normally provided for the customer's system

as well as for the equipment owned by the utility. The purpose of connecting the

customer's system to ground is to limit the voltage that may develop if high voltage

conductors fall down onto lower-voltage conductors which are usually mounted lower to

the ground, or if a failure occurs within a distribution transformer. If all conductive

objects are bonded to the same earth grounding system, the risk of electric shock is

minimized.

However, multiple connections between the utility ground and customer ground

can lead to stray voltage problems; customer piping, swimming pools or other equipment

may develop objectionable voltages. These problems may be difficult to resolve since

some of them often originate from places other than the customer's premises. But many

problems occur in the quality of power too. Voltage sags, voltage swells, interruptions of

power, Harmonics etc. So, to tackle these problems of Power Quality, a new method for

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Three Phase Four Wire Distribution System was developed Using Unified Power Quality

Conditioner (UPQC), which is discussed in this thesis.

1.2 History of Power Quality Conditioning Both electric utilities and end users of electric power are becoming increasingly

concerned about the quality of electric power. The term power quality has become one of

the most prolific buzzwords in the power industry since the late 1980s. It is an umbrella

concept for a multitude of individual types of power system disturbances. The issues that

fall under this umbrella are not necessarily new. What is new is that engineers are now

attempting to deal with these issues using a system approach rather than handling them as

individual problems.

There are four major reasons for the increased concern:

1. Newer-generation load equipment, with microprocessor-based controls and

power electronic devices, is more sensitive to power quality variations than was

equipment used in the past.

2. The increasing emphasis on overall power system efficiency has resulted in

continued growth in the application of devices such as high-efficiency, adjustable-speed

motor drives and shunt capacitors for power factor correction to reduce losses. This is

resulting in increasing harmonic levels on power systems and has many people concerned

about the future impact on system capabilities.

3. End users have an increased awareness of power quality issues. Utility

customers are becoming better informed about such issues as interruptions, sags, and

switching transients and are challenging the utilities to improve the quality of power

delivered.

4. Many things are now interconnected in a network. Integrated processes mean

that the failure of any component has much more important consequences.

To overcome these power quality problems, power conditioning methods were

developed using FACTS (Flexible AC Transmission System). In FACTS, there are

different types of devices that are used for power conditioning. Some of these devices are

Dynamic Voltage Restorer (DVR), STATCOM (STATic synchronous COMpensator),

SVC (Static VAR Compensator), Unified Power Quality Conditioner(UPQC), Unified

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Power Flow Controller (UPFC) etc. Here in this project we have made use of UPQC for a

Three Phase Four Wire Distribution System.

1.3 Outline of Project The use of sophisticated equipment/loads at transmission and distribution level

has increased considerably in recent years due to the development in the semiconductor

device technology. The equipment needs clean power in order to function properly. At

the same time, the switching operation of these devices generates current harmonics

resulting in a polluted distribution system. The power-electronics-based devices have

been used to overcome the major power quality problems. To provide a balance,

distortion-free, and constant magnitude power to sensitive load and, at the same time, to

restrict the harmonic, unbalance, and reactive power demanded by the load and hence to

make the overall power distribution system more healthy, the unified power quality

conditioner (UPQC) is one of the best solutions.

A three-phase four-wire (3P4W) distribution system can be realized by providing

the neutral conductor along with the three power lines from generation station or by

utilizing a delta-star (Δ–Y) transformer at distribution level. The UPQC installed for

3P4W application generally considers 3P4W supply. This paper proposes a new

topology/structure that can be realized in UPQC-based applications, in which the series

transformer neutral used for series inverter, can be used to realize a 3P4W system even if

the power supplied by utility is three phase three-wire (3P3W). This new functionality

using UPQC could be useful in future UPQC-based distribution systems. The unbalanced

load currents are very common and yet an important problem in 3P4W distribution

system. This paper deals with the unbalanced load current problem with a new control

approach, in which the fundamental active powers demanded by each phase are

computed first, and these active powers are then redistributed equally on each of the

phases.

Chapter 2

Power Quality

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2. Power Quality

2.1 Introduction Both electric utilities and end users of electric power are becoming increasingly

concerned about the quality of electric power. The term power quality has become one of

the most prolific buzzwords in the power industry since the late 1980s. It is an umbrella

concept for a multitude of individual types of power system disturbances. The issues that

fall under this umbrella are not necessarily new. What is new is that engineers are now

attempting to deal with these issues using a system approach rather than handling them as

individual problems.

There are four major reasons for the increased concern:

1. Newer-generation load equipment, with microprocessor-based controls and

power electronic devices, is more sensitive to power quality variations than was

equipment used in the past.

2. The increasing emphasis on overall power system efficiency has resulted in

continued growth in the application of devices such as high-efficiency, adjustable-speed

motor drives and shunt capacitors for power factor correction to reduce losses. This is

resulting in increasing harmonic levels on power systems and has many people concerned

about the future impact on system capabilities.

3. End users have an increased awareness of power quality issues. Utility

customers are becoming better informed about such issues as interruptions, sags, and

switching transients and are challenging the utilities to improve the quality of power

delivered.

4. Many things are now interconnected in a network. Integrated processes mean

that the failure of any component has much more important consequences.

In distribution system power quality is important. As power quality problem is

defined as a problem manifested in voltage, current, or frequency deviations that results

in failure or misoperation of customer equipment, it has more to do with the distribution

system as distribution system is the connecting segment of power system and consumers.

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2.2 Power Quality Problems For the purpose of this article, we shall define power quality problems as:

‘Any power problem that results in failure or improper operation of customer equipment

manifests itself as an economic burden to the user, or produces negative impacts on the

environment.’

When applied to the container crane industry, the power issues which degrade power

quality include:

• Power Factor

• Harmonic Distortion

• Voltage Transients

• Voltage Sags or Dips

• Voltage Swells

The AC and DC variable speed drives utilized on board container cranes are

significant contributors to total harmonic current and voltage distortion. Whereas SCR

phase control creates the desirable average power factor, DC SCR drives operate at less

than this. In addition, line notching occurs when SCR’s commutate, creating transient

peak recovery voltages that can be 3 to 4 times the nominal line voltage depending upon

the system impedance and the size of the drives. The frequency and severity of these

power system disturbances varies with the speed of the drive. Harmonic current injection

by AC and DC drives will be highest when the drives are operating at slow speeds. Power

factor will be lowest when DC drives are operating at slow speeds or during initial

acceleration and deceleration periods, increasing to its maximum value when the SCR’s

are phased on to produce rated or base speed. Above base speed, the power factor

essentially remains constant.

Unfortunately, container cranes can spend considerable time at low speeds as the

operator attempts to spot and land containers. Poor power factor places a greater kVA

demand burden on the utility or engine-alternator power source. Low power factor loads

can also affect the voltage stability which can ultimately result in detrimental effects on

the life of sensitive electronic equipment or even intermittent malfunction. Voltage

transients created by DC drive SCR line notching, AC drive voltage chopping, and high

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frequency harmonic voltages and currents are all significant sources of noise and

disturbance to sensitive electronic equipment.

It has been our experience that end users often do not associate power quality

problems with Container cranes, either because they are totally unaware of such issues or

there was no economic Consequence if power quality was not addressed. Before the

advent of solid-state power supplies, Power factor was reasonable, and harmonic current

injection was minimal. Not until the crane Population multiplied, power demands per

crane increased, and static power conversion became the way of life, did power quality

issues begin to emerge. Even as harmonic distortion and power Factor issues surfaced, no

one was really prepared.

Even today, crane builders and electrical drive System vendors avoid the issue

during competitive bidding for new cranes. Rather than focus on Awareness and

understanding of the potential issues, the power quality issue is intentionally or

unintentionally ignored. Power quality problem solutions are available. Although the

solutions are not free, in most cases, they do represent a good return on investment.

However, if power quality is not specified, it most likely will not be delivered.

2.3 Power quality can be improved through

• Power factor correction,

• Harmonic filtering,

• Special line notch filtering,

• Transient voltage surge suppression,

• Proper earthing systems.

In most cases, the person specifying and/or buying a container crane may not be fully

aware of the potential power quality issues. If this article accomplishes nothing else, we

would hope to provide that awareness.

In many cases, those involved with specification and procurement of container

cranes may not be cognizant of such issues, do not pay the utility billings, or consider it

someone else’s concern. As a result, container crane specifications may not include

definitive power quality criteria such as power factor correction and/or harmonic

filtering. Also, many of those specifications which do require power quality equipment

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do not properly define the criteria. Early in the process of preparing the crane

specification:

• Consult with the utility company to determine regulatory or contract

requirements that must be satisfied, if any.

• Consult with the electrical drive suppliers and determine the power quality

profiles that can be Expected based on the drive sizes and technologies proposed for the

specific project.

• Evaluate the economics of power quality correction not only on the present

situation, but consider the impact of future utility deregulation and the future

development plans for the terminal

2.4 Benefits of Power Quality

Power quality in the container terminal environment impacts the economics of the

terminal operation, affects reliability of the terminal equipment, and affects other

consumers served by the same utility service. Each of these concerns is explored in the

following paragraphs.

1. Economic Impact

The economic impact of power quality is the foremost incentive to container terminal

operators. Economic impact can be significant and manifest itself in several ways:

a. Power Factor Penalties

Many utility companies invoke penalties for low power factor on monthly

billings. There is no industry standard followed by utility companies. Methods of

metering and calculating power factor penalties vary from one utility company to the

next. Some utility companies actually meter kVAR usage and establish a fixed rate times

the number of kVAR-hours consumed. Other utility companies monitor kVAR demands

and calculate power factor. If the power factor falls below a fixed limit value over a

demand period, a penalty is billed in the form of an adjustment to the peak demand

charges.

A number of utility companies servicing container terminal equipment do not yet

invoke power factor penalties. However, their service contract with the Port may still

require that a minimum power factor over a defined demand period be met. The utility

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company may not continuously monitor power factor or kVAR usage and reflect them in

the monthly utility billings; however, they do reserve the right to monitor the Port service

at any time. If the power factor criteria set forth in the service contract are not met, the

user may be penalized, or required to take corrective actions at the user’s expense. One

utility company, which supplies power service to several east coast container terminals in

the USA, does not reflect power factor penalties in their monthly billings, however, their

service contract with the terminal reads as follows:

‘The average power factor under operating conditions of customer’s load at the

point where service is metered shall be not less than 85%. If below 85%, the customer

may be required to furnish, install and maintain at its expense corrective apparatus which

will increase the Power factor of the entire installation to not less than 85%. The

customer shall ensure that no excessive harmonics or transients are introduced on to the

[utility] system. This may require special power conditioning equipment or filters.

The Port or terminal operations personnel, who are responsible for maintaining

container cranes, or specifying new container crane equipment, should be aware of these

requirements. Utility deregulation will most likely force utilities to enforce requirements

such as the example above.

Terminal operators who do not deal with penalty issues today may be faced with

some rather severe penalties in the future. A sound, future terminal growth plan should

include contingencies for addressing the possible economic impact of utility deregulation.

b. System Losses

Harmonic currents and low power factor created by nonlinear loads, not only

result in possible power factor penalties, but also increase the power losses in the

distribution system. These losses are not visible as a separate item on your monthly utility

billing, but you pay for them each month. Container cranes are significant contributors to

harmonic currents and low power factor.

Based on the typical demands of today’s high speed container cranes, correction

of power factor alone on a typical state of the art quay crane can result in a reduction of

system losses that converts to a 6 to 10% reduction in the monthly utility billing. For

most of the larger terminals, this is a significant annual saving in the cost of operation.

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c. Power Service Initial Capital Investments

The power distribution system design and installation for new terminals, as well

as modification of systems for terminal capacity upgrades, involves high cost,

specialized, high and medium voltage equipment. Transformers, switchgear, feeder

cables, cable reel trailing cables, collector bars, etc. must be sized based on the kVA

demand. Thus cost of the equipment is directly related to the total kVA demand. As the

relationship above indicates, kVA demand is inversely proportional to the overall power

factor, i.e. a lower power factor demands higher kVA for the same kW load.

Container cranes are one of the most significant users of power in the terminal.

Since container cranes with DC, 6 pulse, SCR drives operate at relatively low power

factor, the total kVA demand is significantly larger than would be the case if power factor

correction equipment were supplied on board each crane or at some common bus location

in the terminal. In the absence of power quality corrective equipment, transformers are

larger, switchgear current ratings must be higher, feeder cable copper sizes are larger,

collector system and cable reel cables must be larger, etc.

Consequently, the cost of the initial power distribution system equipment for a

system which does not address power quality will most likely be higher than the same

system which includes power quality equipment.

2. Equipment Reliability

Poor power quality can affect machine or equipment reliability and reduce the life

of components. Harmonics, voltage transients, and voltage system sags and swells are all

power quality problems and are all interdependent.

Harmonics affect power factor, voltage transients can induce harmonics, the same

phenomena which create harmonic current injection in DC SCR variable speed drives are

responsible for poor power factor, and dynamically varying power factor of the same

drives can create voltage sags and swells. The effects of harmonic distortion, harmonic

currents, and line notch ringing can be mitigated using specially designed filters.

3. Power System Adequacy

When considering the installation of additional cranes to an existing power

distribution system, a power system analysis should be completed to determine the

adequacy of the system to support additional crane loads. Power quality corrective

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actions may be dictated due to inadequacy of existing power distribution systems to

which new or relocated cranes are to be connected.

In other words, addition of power quality equipment may render a workable

scenario on an existing power distribution system, which would otherwise be inadequate

to support additional cranes without high risk of problems.

4. Environment

No issue might be as important as the effect of power quality on our environment.

Reduction in system losses and lower demands equate to a reduction in the consumption

of our natural nm resources and reduction in power plant emissions. It is our

responsibility as occupants of this planet to encourage conservation of our natural

resources and support measures which improve our air quality.

Chapter 3

FACTS Devices in

Distribution System

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3. FACTS Devices in Distribution System

3.1. Introduction Flexible AC Transmission Systems, called FACTS, got in the recent years a well-

known term for higher controllability in power systems by means of power electronic

devices. Several FACTS-devices have been introduced for various applications

worldwide. A number of new types of devices are in the stage of being introduced in

practice.

In most of the applications the controllability is used to avoid cost intensive or

landscape requiring extensions of power systems, for instance like upgrades or additions

of substations and power lines. FACTS-devices provide a better adaptation to varying

operational conditions and improve the usage of existing installations. The basic

applications of FACTS-devices are:

• Power flow control,

• Increase of transmission capability,

• Voltage control,

• Reactive power compensation,

• Stability improvement,

• Power quality improvement,

• Power conditioning,

• Flicker mitigation,

• Interconnection of renewable and distributed generation and storages.

The usage of lines for active power transmission should be ideally up to the

thermal limits. Voltage and stability limits shall be shifted with the means of the several

different FACTS devices. It can be seen that with growing line length, the opportunity for

FACTS devices gets more and more important.

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The influence of FACTS-devices is achieved through switched or controlled shunt

compensation, series compensation or phase shift control. The devices work electrically

as fast current, voltage or impedance controllers. The power electronic allows very short

reaction times down to far below one second.

Fig. 3.1. Operational limits of transmission lines for different voltage levels

The development of FACTS-devices has started with the growing capabilities of

power electronic components. Devices for high power levels have been made available in

converters for high and even highest voltage levels. The overall starting points are

network elements influencing the reactive power or the impedance of a part of the power

system. Figure 3.2 shows a number of basic devices separated into the conventional ones

and the FACTS-devices.

For the FACTS side the taxonomy in terms of 'dynamic' and 'static' needs some

explanation. The term 'dynamic' is used to express the fast controllability of FACTS-

devices provided by the power electronics. This is one of the main differentiation factors

from the conventional devices. The term 'static' means that the devices have no moving

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parts like mechanical switches to perform the dynamic controllability. Therefore most of

the FACTS-devices can equally be static and dynamic.

Fig. 3.2. Overview of major FACTS-devices

The left column in Figure 3.2 contains the conventional devices build out of fixed

or mechanically switch able components like resistance, inductance or capacitance

together with transformers. The FACTS-devices contain these elements as well but use

additional power electronic valves or converters to switch the elements in smaller steps or

with switching patterns within a cycle of the alternating current. The left column of

FACTS-devices uses Thyristor valves or converters. These valves or converters are well

known since several years. They have low losses because of their low switching

frequency of once a cycle in the converters or the usage of the Thyristors to simply bridge

impedances in the valves.

The right column of FACTS-devices contains more advanced technology of

voltage source converters based today mainly on Insulated Gate Bipolar Transistors

(IGBT) or Insulated Gate Commutated Thyristors (IGCT). Voltage Source Converters

provide a free controllable voltage in magnitude and phase due to a pulse width

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modulation of the IGBTs or IGCTs. High modulation frequencies allow to get low

harmonics in the output signal and even to compensate disturbances coming from the

network. The disadvantage is that with an increasing switching frequency, the losses are

increasing as well. Therefore special designs of the converters are required to compensate

this.

3.2 FACTS Devices used in Distribution for Power Conditioning In distribution system, the FACTS devices used for power conditioning are

classified below in fig. 3.3.

Fig. 3.3. Classification of Distribution Power Conditioners

Power conditioning is essential for improving power quality. In fact, power

conditioning means improving of power quality. The devices that are used for power

conditioning at the distribution side of power system are mentioned in the above figure.

They are classified as:

A. DVR

B. DSTATCOM

C. UPQC

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A. DVR

DVR stands for Dynamic Voltage Restorer. It is used to correct Voltage Sags,

Voltage Swells, and Voltage Harmonics. A schematic diagram of Dynamic Voltage

Restorer is shown in the following fig. 3.4.

Fig. 3.4. A Schematic diagram of DVR

Dynamic Voltage Restoration (DVR) is a method and apparatus used to sustain,

or restore, an operational electric load during sags, or spikes, in voltage supply. Often

used in manufacturing areas requiring significant power to run tools/equipment, and

utility plants, this custom device mitigates potential damage to equipment and

undesirable slowdowns to the production line caused by an abrupt change in electric load.

This method uses critical devices such as an automatic Transfer switch and IGBT

Modules in order to operate. In normal conditions, the dynamic voltage restorer operates

in stand-by mode. However, during disturbances, nominal system voltage will be

compared to the voltage variation. This is to get the differential voltage that should be

injected by the DVR in order to maintain supply voltage to the load within limits.

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B. DSTATCOM

DSTATCOM stands for Distribution STATic synchronous COMpensator. It

compensates Reactive Current, Current Harmonics etc. A schematic diagram of

DTATCOM is shown in the following fig. 3.5.

Fig.3.5. Schematic diagram of DSTATCOM

Usually a STATCOM is installed to support electricity networks that have a poor power

factor and often poor voltage regulation. There are however, other uses, the most

common use is for voltage stability. A STATCOM is a voltage source converter (VSC)-

based device, with the voltage source behind a reactor. The voltage source is created from

a DC capacitor and therefore a STATCOM has very little active power capability.

However, its active power capability can be increased if a suitable energy storage device

is connected across the DC capacitor. The reactive power at the terminals of the

STATCOM depends on the amplitude of the voltage source. For example, if the terminal

voltage of the VSC is higher than the AC voltage at the point of connection, the

STATCOM generates reactive current; on the other hand, when the amplitude of the

voltage source is lower than the AC voltage, it absorbs reactive power. The response time

of a STATCOM is shorter than that of an SVC, mainly due to the fast switching times

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provided by the IGBTs of the voltage source converter. The STATCOM also provides

better reactive power support at low AC voltages than an SVC, since the reactive power

from a STATCOM decreases linearly with the AC voltage (as the current can be

maintained at the rated value even down to low AC voltage).

C. UPQC

UPQC stands for Unified Power Quality Conditioner. It compensates for Reactive

Current, Current Harmonics, Voltage Sags, Voltage Swells, Voltage Harmonics etc. A

schematic diagram of UPQC is shown in the following fig.3.6.

Fig. 3.6. Schematic diagram of UPQC

The provision of both DSTATCOM and DVR can control the power quality of the source

current and the load bus voltage. In addition, if the DVR and STATCOM are connected

on the DC side, the DC bus voltage can be regulated by the shunt connected

DSTATCOM while the DVR supplies the required energy to the load in case of the

transient disturbances in source voltage. The configuration of such a device (termed as

Unified Power Quality Conditioner (UPQC)) is shown in Fig. This is a versatile device

similar to a UPFC. However, the control objectives of a UPQC are quite different from

that of a UPFC.

Chapter 4

P-Q Theory

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4. P-Q Theory

4.1. Introduction

In 1983 Akagi et al. proposed a new theory for the control of active filters in

three-phase power systems called “Generalized Theory of the Instantaneous Reactive

Power in Three-Phase Circuits", also known as “Theory of Instantaneous Real Power and

Imaginary Power”, or “Theory of Instantaneous Active Power and Reactive Power”, or

“Theory of Instantaneous Power”, or simply as “p-q Theory”.

The theory was initially developed for three-phase three wire systems, with a brief

mention to systems with neutral wire. Later, Watanabe et al. and Aredes et al. extended it

to three-phase four-wire systems (systems with phases a, b, c and neutral wire). Since the

p-q theory is based on the time domain, it is valid both for steady-state and transient

operation, as well as for generic voltage and current waveforms, allowing the control of

the active filters in real-time. Another advantage of this theory is the simplicity of its

calculations, since only algebraic operations are required. The only exception is in the

separation of some power components in their mean and alternating values.

However, as it will be shown in this thesis, it is possible to exploit the

symmetries of the instantaneous power waveform for each specific power system,

achieving a calculation delay that can be as small as 1/6 and never greater than 1 cycle of

the power system frequency. It is also shown that calculations for reactive power and

zero-sequence compensation do not introduce any delay. Furthermore, it is possible to

associate physical meaning to the p-q theory power components, which eases the

understanding of the operation of any three-phase power system, balanced or unbalanced,

with or without harmonics.

4.2 P-Q Theory Power Components

The p-q theory implements a transformation from a stationary reference system in

a-b-c coordinates, to a system with coordinate’s α-β-0. It corresponds to an algebraic

transformation, known as Clarke transformation, which also produces a stationary

reference system, where coordinates α-β are orthogonal to each other, and coordinate 0

corresponds to the zero-sequence component.

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The zero sequence calculated here differs from the one obtained by the

symmetrical components transformation, or Fortes cue transformation, by a 3 factor. The

voltages and currents in α-β-0 coordinates are calculated as follows:

……………. (4.1)

Where,

………………………... (4.2)

The p-q theory power components are then calculated from voltages and currents in

the α-β-0 coordinates. Each component can be separated in its mean and alternating

values (see Fig. 1), which present physical meanings:

A. Instantaneous Zero-Sequence Power ( p0 )

…………… (4.3)

p0 − Mean value of the instantaneous zero-sequence power. It corresponds to the energy

per time unity that is transferred from the power source to the load through the zero-

sequence components of voltage and current.

-Alternating value of the instantaneous zero-sequence power. It means the energy per

time unity that is exchanged between the power source and the load through the zero-

sequence components of voltage and current.

The zero-sequence power exists only in three-phase systems with neutral wire.

Moreover, the systems must have both unbalanced voltages and currents, or the same

third morder harmonics, in both voltage and current, for at least one phase. It is important

to notice that cannot exist in a power system without the presence of . Since is

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clearly an undesired power component (it only exchanges energy with the load, and does

not transfer any energy to

the load), both and must be compensated.

B. Instantaneous Real Power ( p )

……………. (4.4)

-Mean value of the instantaneous real power. It corresponds to the energy per time

unity that is transferred from the power source to the load, in a balanced way, through the

a-b-c coordinates (it is, indeed, the only desired power component to be supplied by the

power source).

- Alternating value of the instantaneous real power. It is the energy per time unity that

is exchanged between the power source and the load, through the a-b-c coordinates.

Since does not involve any energy transference from the power source to load, it must

be compensated.

C. Instantaneous Imaginary Power ( q )

…………… (4.5)

- Mean value of instantaneous imaginary power.

- Alternating value of instantaneous imaginary power.

The instantaneous imaginary power, q, has to do with power (and corresponding

undesirable currents) that is exchanged between the system phases, and which does not

imply any transference or exchange of energy between the power source and the load.

Rewriting equation (4.5) in a-b-c coordinates the following expression is obtained:

………… (4.6)

This is a well-known expression used in conventional reactive power meters, in

power systems without harmonics and with balanced sinusoidal voltages. These

instruments, of the electro dynamic type, display the mean value of equation (4.6). The

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instantaneous imaginary power differs from the conventional reactive power, because in

the first case all the harmonics in voltage and current are considered. In the special case

of a balanced sinusoidal voltage supply and a balanced load, with or without harmonics,

is equal to the conventional reactive power

…………….(4.7)

Fig. 4.1. p-q theory power components

It is also important to note that the three-phase instantaneous power ( p3 ) can be

written in both coordinates systems, a-b-c and α-β-0, assuming the same value:

…………….. (4.8)

……………… (4.9)

Thus, to make the three-phase instantaneous power constant, it is necessary to

compensate the alternating power components and . Since, as seen before, it is not

possible to compensate only , all zero-sequence instantaneous power must be

compensated.

Moreover, to minimize the power system currents, the instantaneous imaginary

power, q, must also be compensated. The compensation of the p-q theory undesired

power components ( , p0 and q ) can be accomplished with the use of an active power

filter. The dynamic response of this active filter will depend on the time interval required

by its control system to calculate these values.

Chapter 5

Voltage Source Inverter

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5. Pulse Width Modulation

5.1 Introduction Pulse-width modulation (PWM), or pulse-duration modulation (PDM), is a

modulation technique that confirms the width of the pulse, formally the pulse duration,

based on modulator signal information. Although this modulation technique can be used

to encode information for transmission, its main use is to allow the control of the power

supplied to electrical devices, especially to inertial loads such as motors. In addition,

PWM is one of the two principal algorithms used in photovoltaic solar battery chargers,

the other being Maximum power point tracking (MPPT).

The average value of voltage (and current) fed to the load is controlled by turning

the switch between supply and load on and off at a fast pace. The longer the switch is on

compared to the off periods, the higher the power supplied to the load is.

The PWM switching frequency has to be much faster than what would affect the

load, which is to say the device that uses the power. Typically switching have to be done

several times a minute in an electric stove, 120 Hz in a lamp dimmer, from few kilohertz

(kHz) to tens of kHz for a motor drive and well into the tens or hundreds of kHz in audio

amplifiers and computer power supplies.

The term duty cycle describes the proportion of 'on' time to the regular interval or

'period' of time; a low duty cycle corresponds to low power, because the power is off for

most of the time. Duty cycle is expressed in percent, 100% being fully on.

The main advantage of PWM is that power loss in the switching devices is very

low. When a switch is off there is practically no current, and when it is on, there is almost

no voltage drop across the switch. Power loss, being the product of voltage and current, is

thus in both cases close to zero. PWM also works well with digital controls, which,

because of their on/off nature, can easily set the needed duty cycle.

A duty cycle is the percentage of one period in which a signal is active. A period

is the time it takes for a signal to complete an on-and-off cycle. As a formula, a duty

cycle may be expressed as:

D = (T/P) x100 %............(5.1)

Where D is the duty cycle, T is the time the signal is active, and P is the total

period of the signal. Thus, a 60% duty cycle means the signal is on 60% of the time but

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off 40% of the time. The "on time" for a 60% duty cycle could be a fraction of a second,

a day, or even a week, depending on the length of the period.

Duty cycles can be used to describe the percent time of an active signal in an

electrical device such as the power switch in a switching power supply

5.2 Sinusoidal Pulse Width Modulation The switches in the voltage source inverter (See Fig. 5.1) can be turned on and off

as required. In the simplest approach, the top switch is turned on If turned on and off only

once in each cycle, a square wave waveform results. However, if turned on several times

in a cycle an improved harmonic profile may be achieved.

Fig. 5.1. Simple Voltage Sourced Inverter

In the most straightforward implementation, generation of the desired output

voltage is achieved by comparing the desired reference waveform (modulating signal)

with a high-frequency triangular ‘carrier’ wave as depicted schematically in Fig.5.2.

Depending on whether the signal voltage is larger or smaller than the carrier waveform,

either the positive or negative dc bus voltage is applied at the output. Note that over the

period of one triangle wave, the average voltage applied to the load is proportional to the

amplitude of the signal (assumed constant) during this period.

The resulting chopped square waveform contains a replica of the desired

waveform in its low frequency components, with the higher frequency components being

at frequencies of a close to the carrier frequency. Notice that the root mean square value

of the ac voltage waveform is still equal to the dc bus voltage, and hence the total

harmonic distortion is not affected by the PWM process. The harmonic components are

merely shifted into the higher frequency range and are automatically filtered due to

inductances in the ac system. When the modulating signal is a sinusoid of amplitude Am,

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and the amplitude of the triangular carrier is Ac, the ratio m=Am/Ac is known as the

modulation index. Note that controlling the modulation index therefore controls the

amplitude of the applied output voltage.

With a sufficiently high carrier frequency (see Fig.5.3 drawn for fc/fm = 21 and t

= L/R = T/3; T = period of fundamental), the high frequency components do not

propagate significantly in the ac network (or load) due the presence of the inductive

elements. However, a higher carrier frequency does result in a larger number of

switching’s per cycle and hence in an increased power loss. Typically switching

frequencies in the 2-15 kHz range are considered adequate for power systems

applications. Also in three-phase systems it is advisable to use so

that all three waveforms are symmetric.

Fig. 5.2. Principal of Pulse Width Modulation

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Fig. 5.3. SPWM with fc/fm = 48, L/R = T/3

Note that the process works well for there are periods of the

triangle wave in which there is no intersection of the carrier and the signal as in Fig. 5.4.

However, a certain amount of this “over modulation” is often allowed in the interest of

obtaining a larger ac voltage magnitude even though the spectral content of the voltage is

rendered somewhat poorer.

Note that with an odd ratio for fc/fm, the waveform is anti-symmetric over a 360

degree cycle. With an even number, there are harmonics of even order, but in particular

also a small dc component. Hence an even number is not recommended for single phase

inverters, particularly for small ratios of fc/fm.

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Fig. 5.4. Over Modulation: m = 1.3

Chapter 6

Pulse Width Modulation

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6. Voltage Source Inverter

6.1 Introduction

A voltage source inverter (VSI) is one that takes in a fixed voltage from a device, such as

a dc power supply, and converts it to a variable-frequency AC supply. Voltage-source

inverters are divided into three general categories: Pulse-width Modulated (PWM)

Inverters, Square-wave Inverters, and Single-phase Inverters with Voltage Cancellation.

Pulse-width modulation inverters take in a constant dc voltage. Diode-rectifiers are used

to rectify the line voltage, and the inverter must control the magnitude and the frequency

of the ac output voltages. To do this the inverter uses pulse-width modulation using its

switches. There are different methods for doing the pulse-width modulation in an inverter

in order to shape the output ac voltages to be very close to a sine wave. These different

methods will be discussed further with a focus on sinusoidal-PWM. Squire-wave

inverters have their input connected to a controlled dc voltage in order to control the

magnitude of the output ac voltage.

The inverter only controls the frequency of the output where the input voltage is

controlled the magnitude. The output ac voltage has a waveform similar to a square wave

which is where the inverter got its name. Lastly, Single-phase inverters with voltage

cancellation take in a constant dc source and output a square-wave like ac voltage. They

can control both the frequency and the magnitude of the output but do not use PWM and

therefore have a square-wave like output. These inverters have combined characteristics

of the previous two inverters. The voltage cancellation only works with single phase

inverters, not three phases.

6.2 General Structure of Voltage Source Inverters

Figs. 6.1 (a) and 6.1(b) show the typical power-circuit topologies of a single-

phase and a three-phase voltage source inverter respectively. These topologies require

only a single dc source and for medium output power applications the preferred devices

are n-channel IGBTs. ‘Edc’ is the input dc supply and a large dc link capacitor (Cdc) is put

across the supply terminals. Capacitors and switches are connected to dc bus using short

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leads to minimize the stray inductance between the capacitor and the inverter switches.

Needless to say that physical layout of positive and negative bus lines is also important to

limit stray inductances. Q1, Q2, Q3 etc. are fast and controllable switches. D1, D2, D3 etc.

are fast recovery diodes connected in anti-parallel with the switches. ‘A’, ‘B’ and ‘C’ are

output terminals of the inverter that get connected to the ac load. A three-phase inverter

has three load-phase terminals whereas a single-phase inverter has only one pair of load

terminals.

The current supplied by the dc bus to the inverter switches is referred as dc link

current and has been shown as ‘idc’ in Figs 6.1(a) and 6.1(b). The magnitude of dc link

current often changes in step (and sometimes its direction also changes) as the inverter

switches are turned on and off. The step change in instantaneous dc link current occurs

even if the ac load at the inverter output is drawing steady power. However, average

magnitude of the dc link current remains positive if net power-flow is from dc bus to ac

load. The net power-flow direction reverses if the ac load connected to the inverter is

regenerating. Under regeneration, the mean magnitude of dc link current is negative.

The dc link current may conceptually be decomposed into its dc and ac

components. The individual roles of the ‘dc voltage source’ and the ‘dc link capacitor’

may be clearly seen with respect to the dc and ac components of the dc link current. For

the dc component of current the capacitor acts like open circuit. As expected, under

steady state, the capacitor does not supply any dc current. The dc part of bus current is

supplied solely by the dc source. A practical dc voltage source may have some resistance

as well as some inductance in series with its internal emf. For dc component of bus

current, the source voltage appears in series with its internal resistance (effect of source

inductance is not felt). But for ac component of current, the internal dc emf of source

appears as short and its series impedance (resistance in series with inductance) appears in

parallel with the dc-link capacitor. Thus the ac component of current gets divided into

these two parallel paths. However, the high frequency component of ac current mainly

flows through the capacitor, as the capacitive impedance is lower at high frequencies.

The step change in dc link current is associated with significant amount of high frequency

components of current that essentially finds its path through the capacitor.

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For an ideal input (dc) supply, with no series impedance, the dc link capacitor

does not have any role. However a practical voltage supply may have considerable

amount of output impedance. The supply line impedance, if not bypassed by a

sufficiently large dc link capacitor, may cause considerable voltage spike at the dc bus

during inverter operation. This may result in deterioration of output voltage quality, it

may also cause malfunction of the inverter switches as the bus voltage appears across the

non-conducting switches of the inverter. Also, in the absence of dc link capacitor, the

series inductance of the supply line will prevent quick build up or fall of current through

it and the circuit behaves differently from the ideal VSI where the dc voltage supply is

supposed to allow rise and fall in current as per the demand of the inverter circuit.

It may not be possible to reduce supply line inductance below certain limit. Most

dc supplies will inherently have rather significant series inductance, for example a

conventional dc generator will have considerable armature inductance in series with the

armature emf. Similarly, if the dc supply is derived after rectifying ac voltage, the ac

supply line inductance will prevent quick change in rectifier output current. The effect of

ac line inductance is reflected on the dc side as well, unless this inductance is effectively

bypassed by the dc side capacitor. Even the connecting leads from the dc source to the

inverter dc bus may contribute significantly to the supply line inductance in case the lead

lengths are large and circuit lay out is poor. It may be mentioned here that an inductance,

in series with the dc supply, may at times be welcome. The reason being that for some

types of dc sources, like batteries, it is detrimental to carry high frequency ripple current.

For such cases it is advantageous if the dc source has some series inductance. Due to

series inductance of the source, the high frequency ripple will prefer to flow through the

dc link capacitor and thus relieve the dc source.

The dc link capacitor should be put very close to the switches so that it provides a

low impedance path to the high frequency component of the switch currents. The

capacitor itself must be of good quality with very low equivalent series resistor (ESR)

and equivalent series inductor (ESL). The length of leads that interconnect switches and

diodes to the dc bus must also be minimum to avoid insertion of significant amount of

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stray inductances in the circuit. The overall layout of the power circuit has a significant

effect over the performance of the inverter circuit.

(a) (b)

(c)

Fig.6.1. (a) Topology of 1- Phase VSI; (b) Topology of a 3- Phase VSI

(c) Topology of a 1-Phase Half Bridge VSI

One of the thumb rules for good circuit layout is to put the conductor pairs

carrying same magnitude but opposite direction of currents close by, the minimum

distance between them being decided only by their voltage isolation requirement. Thus

the positive and negative terminals of the dc bus should run close by. A twisted wire pair

may be an example of two closely running wires.

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The details of the inverter circuits shown in Figs. 6.1(a) and 6.1(b) are discussed

in later lessons. However it may be mentioned here that these circuits are essentially

extension of the half bridge circuit shown in Fig. 6.1(c). For example, the single-phase

bridge circuit of Fig. 6.1(a) may be thought of as two half-bridge circuits sharing the

same dc bus. Thus the single phase ‘full-bridge’ (often, simply called as ‘bridge’) circuit

has two legs of switches, each leg consisting of an upper switch and a lower switch.

Junction point of the upper and lower switches is the output point of that particular leg.

Voltage between output point of legs and the mid-potential of the dc bus is called as ‘pole

voltage’ referred to the mid potential of the dc bus. One may think of pole voltage

referred to negative bus or referred to positive bus too but unless otherwise mentioned

pole voltages are assumed to be referred to the mid-potential of the dc bus. The two pole

voltages of the single-phase bridge inverter generally have same magnitude and

frequency but their phases are 1800 apart. Thus the load connected between these two

pole outputs (between points ‘A’ and ‘B’) will have a voltage equal to twice the

magnitude of the individual pole voltage. The pole voltages of the 3-phase inverter

bridge, shown in Fig. 6.1(b), are phase apart by 1200 each.

Chapter 7

Active Power Filters

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7. Active Power Filters

7.1 Introduction Active Filters are commonly used for providing harmonic compensation to a

system by controlling current harmonics in supply networks at the low to medium voltage

distribution level or for reactive power or voltage control at high voltage distribution

level. These functions may be combined in a single circuit to achieve the various

functions mentioned above or in separate active filters which can attack each aspect

individually. The block diagram presented in section shows the basic sequence of

operation for the active filter. This diagram shows various sections of the filter each

responding to its own classification.

The block diagram shown in figure represents the key components of a typical

active power filter along with their interconnections. The reference signal estimator

monitors the harmonic current from the nonlinear load along with information about

other system variables. The reference signal from the current estimator, as well as other

signals, drives the overall system controller. This in turn provides the control for the

PWM switching pattern generator. The output of the PWM pattern generator controls the

power circuit through a suitable interface. The power circuit in the generalized block

diagram can be connected in parallel, series or parallel/series configurations, depending

on the transformer used.

Fig. 7.1. Generalized block diagram for active power filters

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7.2 Types of Active Power Filters The technology of active power filter has been developed during the past two

decades reaching maturity for harmonics compensation, reactive power, and voltage

balance in ac power networks. All active power filters are developed with PWM

converters (current source or voltage source inverters). The current-fed PWM inverter

bridge structure behaves as a non-sinusoidal current source to meet the harmonic current

requirement of the nonlinear load. It has a self-supported dc reactor that ensures the

continuous circulation of the dc current. They present good reliability, but have important

losses and require higher values of parallel capacitor filters at the ac terminals to remove

unwanted current harmonics. Moreover, they cannot be used in multilevel or multistep

modes configurations to allow compensation in higher power ratings.

The other converter used in active power filter topologies is the voltage-source

PWM inverter. This converter is more convenient for active power filtering applications

since it is lighter, cheaper, and expandable to multilevel and multistep versions, to

improve its performance for high power rating compensation with lower switching

frequencies. The PWM voltage source inverter has to be connected to the ac mains

through coupling reactors. An electrolytic capacitor keeps a dc voltage constant and

ripple free.

Active power filters can be classified based on the type of converter, topology,

control scheme, and compensation characteristics. The most popular classification is

based on the topology such as shunt, series or hybrid. The hybrid configuration is a

combination of passive and active compensation. The different active power filter

topologies are shown in Fig.7.2

Shunt active power filters (Fig. 7.2 (a)) are widely used to compensate current

harmonics, reactive power and load current unbalanced. It can also be used as a static var

generator in power system networks for stabilizing and improving voltage profile. Series

active power filters (Fig.7.2 (b)) is connected before the load in series with the ac mains,

through a coupling transformer to eliminate voltage harmonics and to balance and

regulate the terminal voltage of the load or line. The hybrid configuration is a

combination of series active filter and passive shunt filter (Fig. 7.2 (c)). This topology is

very convenient for the compensation of high power systems, because the rated power of

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the active filter is significantly reduced (about 10% of the load size), since the major part

of the hybrid filter consists of the passive shunt LC filter used to compensate lower-order

current harmonics and reactive power.

(a) (b)

(c)

Fig. 7.2. Active power filter topologies implemented with PWMVSI.

(a) Shunt active power filter. (b) Series active power filter. (c) Hybrid active power filter.

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Due to the operation constraint, shunt or series active power filters can

compensate only specific power quality problems. Therefore, the selection of the type of

active power filter to improve power quality depends on the source of the problem as can

be seen in Table 7.1.

So, Active Power Filters are used to remove the disturbance that Load injects into

Supply and vice versa and hence improves the power quality in the system. To have this

property of improving power quality we have used APF (Active Power Filters) in this

project.

Chapter 8

Modelling of Test System

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8. Modelling of Test System

8.1 Three Phase Four Wire Distribution System Utilizing UPQC

Fig. 8.1. 3P4W distribution system: neutral provided from generation station.

Generally, a 3P4W distribution system is realized by providing a neutral

conductor along with three power conductors from generation station or by utilizing a

three-phase Δ–Y transformer at distribution level. Fig. 8.1 shows a 3P4W network in

which the neutral conductor is provided from the generating station itself, whereas Fig.

8.2 shows a 3P4W distribution network considering a Δ–Y transformer.

Assume a plant site where three-phase three-wire UPQC is already installed to

protect a sensitive load and to restrict any entry of distortion from load side toward

utility, as shown in Fig. 8.3. If we want to upgrade the system now from 3P3W to 3P4W

due to installation of some single-phase loads and if the distribution transformer is close

to the plant under consideration, utility would provide the neutral conductor from this

transformer without major cost involvement. In certain cases, this may be a costly

solution because the distribution transformer may not be situated in close vicinity.

Fig. 8.2. 3P4W distribution system: neutral provided from Δ–Y transformer.

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Recently, the utility service providers are putting more and more restrictions on

current total harmonic distortion (THD) limits, drawn by nonlinear loads, to control the

power distribution system harmonic pollution. At the same time, the use of sophisticated

equipment/load has increased significantly, and it needs clean power for its proper

operation. Therefore, in future distribution systems and the plant/load centers, application

of UPQC would be common. Fig. 8.4 shows the proposed novel 3P4W topology that can

be realized from a 3P3W system.

This proposed system has all the advantages of general UPQC, in addition to easy

expansion of 3P3W system to 3P4W system. Thus, the proposed topology may play an

important role in the future 3P4W distribution system for more advanced UPQC based

plant/load center installation, where utilities would be having an additional option to

realize a 3P4W system just by providing a 3P3W supply.

Fig. 8.3. 3P3W UPQC structure.

As shown in Fig. 8.3, the UPQC should necessarily consist of three-phase series

transformer in order to connect one of the inverters in the series with the line to function

as a controlled voltage source. If we could use the neutral of three-phase series

transformer to connect a neutral wire to realize the 3P4W system, then 3P4W system can

easily be achieved from a 3P3W system (Fig. 8.4). The neutral current, present if any,

would flow through this fourth wire toward transformer neutral point.

This neutral current can be compensated by using a split capacitor topology or a

four-leg voltage-source inverter (VSI) topology for a shunt inverter. The four-leg VSI

topology requires one additional leg as compared to the split capacitor topology. The

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neutral current compensation in the four-leg VSI structure is much easier than that of the

split capacitor because the split capacitor topology essentially needs two capacitors and

an extra control loop to maintain a zero voltage error difference between both the

capacitor voltages, resulting in a more complex control loop to maintain the dc bus

voltage at constant level.

Fig. 8.4. Proposed 3P4W system realized from a 3P3W system utilizing UPQC.

In this paper, the four-leg VSI topology is considered to compensate the neutral

current flowing toward the transformer neutral point. A fourth leg is added on the

existing 3P3W UPQC, such that the transformer neutral point will be at virtual zero

potential. Thus, the proposed structure would help to realize a 3P4W system from a

3P3W system at distribution load end. This would eventually result in easy expansion

from 3P3W to 3P4W systems. A new control strategy to generate balanced reference

source currents under unbalanced load condition is also proposed in this paper and is

explained in the next section.

8.2 UPQC Controller The control algorithm for series active power filter (APF) is based on unit vector

template generation scheme, whereas the control strategy for shunt APF is discussed in

this section. Based on the load on the 3P4W system, the current drawn from the utility

can be unbalanced. In this paper, a new control strategy is proposed to compensate the

current unbalance present in the load currents by expanding the concept of single phase

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p–q theory. According to this theory, a signal phase system can be defined as a pseudo

two-phase system by giving π/2 lead or π/2 lag, i.e., each phase voltage and current of the

original three-phase system can be considered as three independent two-phase systems.

These resultant two phase systems can be represented in α–β coordinates, and

thus, the p–q theory applied for balanced three-phase system can also be used for each

phase of unbalanced system independently. The actual load voltages and load currents are

considered as α-axis quantities, whereas the π/2 lead load or π/2 lag voltages and π/2 lead

or π/2 lag load currents are considered as β-axis quantities. In this document, π/2 lead is

considered to achieve a two-phase system for each phase.

The major disadvantage of p–q theory is that it gives poor results under distorted

and/or unbalanced input/utility voltages. In order to eliminate these limitations, the

reference load voltage signals extracted for series APF are used instead of actual load

voltages.

…………… (8.1)

……………………..….. (8.2)

Where represents the reference load voltage and VLm represents the desired load

voltage magnitude.

Similarly, for phase b, the load voltage and current in α–β coordinates can be represented

by π/2 lead as

……………… (8.3)

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……………………….…………. (8.4)

In addition, for phase c, the load voltage and current in α–β coordinates can be

represented by π/2 lead as

…………………… (8.5)

……………….. (8.6)

By using the definition of three-phase p–q theory for balanced three-phase system, the

instantaneous power components can be represented as

Instantaneous active power

pL,abc = vL,abc_α * iL,abc_β + vL,abc_β * iL,abc_α ………… (8.7)

Instantaneous reactive power

qL,abc = vL,abc_α * iL,abc_β - vL,abc_β * iL,abc_α ……….…… (8.8)

Considering phase a, the phase-a instantaneous load active and instantaneous load

reactive powers can be represented by

…………. (8.9)

Where

………… (8.10)

…………. (8.11)

In (10) and (11), and represent the dc components that are responsible for

fundamental load active and reactive powers, whereas and represent the ac

components that are responsible for harmonic powers. The phase-a fundamental

instantaneous load active and reactive power components can be extracted from PLa and

qLa, respectively, by using a low pass filter.

Therefore, the instantaneous fundamental load active power for phase a is given by

……………… (8.12)

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And the instantaneous fundamental load reactive power for phase a is given by

…………… (8.13)

Similarly, the fundamental instantaneous load active and the fundamental instantaneous

load reactive powers for phases b and c can be calculated as

Instantaneous fundamental load active power for phase b

……….. (8.14)

Instantaneous fundamental load reactive power for phase b

…….… (8.15)

Instantaneous fundamental load active power for phase c

……….. (8.16)

Instantaneous fundamental load reactive power for phase c

………… (8.17)

Since the load current drawn by each phase may be different due to different loads

that may be present inside plant, therefore, the instantaneous fundamental load active

power and instantaneous fundamental load reactive power demand for each phase may

not be the same. In order to make this load unbalanced power demand, seen from the

utility side, as a perfectly balanced fundamental three-phase active power, the unbalanced

load power should be properly redistributed between utility, UPQC, and load, such that

the total load seen by the utility would be linear and balanced load. The unbalanced or

balanced reactive power demanded by the load should be handled by a shunt APF.

The aforementioned task can be achieved by summing instantaneous fundamental

load active power demands of all the three phases and redistributing it again on each

utility phase, i.e., from (12), (14), and (16),

………… (8.18)

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…………. ……………..(8.19)

Equation (8.19) gives the redistributed per-phase fundamental active power

demand that each phase of utility should supply in order to achieve perfectly balanced

source currents. From (8.19), it is evident that under all the conditions, the total

fundamental active power demanded by the loads would be equal to the total power

drawn from the utility but with perfectly balanced way even though the load currents are

unbalanced.

Thus, the reference compensating currents representing a perfectly balanced

three-phase system can be extracted by taking the inverse of (8.9)

………… (8.20)

In (8.20), pdc/ph is the precise amount of per-phase active power that should be

taken from the source in order to maintain the dc-link voltage at a constant level and to

overcome the losses associated with UPQC. The oscillating instantaneous active power

should be exchanged between the load and shunt APF.

The reactive power term (qLa) in (20) is considered as zero, since the utility

should not supply load reactive power demand. In the above matrix, the α-axis reference

compensating current represents the instantaneous fundamental source current, since α-

axis quantities belong to the original system under consideration and the β-axis reference

compensating current represents the current that is at π/2 lead with respect to the original

system.

Therefore,

…………….. (8.21)

Similarly, the reference source current for phases b and c can be estimated as

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…………… (8.22)

……………… (8.23)

The reference neutral current signal can be extracted by simply adding all the sensed load

currents, without actual neutral current sensing, as

………….. (8.24)

……………………….. (8.25)

The proposed balanced per-phase fundamental active power estimation, dc-link

voltage control loop based on PI regulator, the reference source current generation as

given by (8.21)–(8.23), and the reference neutral current generation are shown in Fig.

8.5(a)–(d), respectively.

Fig. 8.5. Shunt active filter control block diagram. (a) Proposed balanced per-phase

fundamental active power estimation. (b) DC-link voltage control loop. (c) Reference

source current generation. (d) Neutral current compensation.

Chapter 9

MATLAB Design and

Simulation of Test Results

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9. MATLAB Design and Simulation of Test Results

9.1 UPQC to R Load

Fig. 9.1. MATLAB model of UPQC for R Load

V(v)

T(ms)

Fig. 9.2. Utility Voltage (VS_abc) in UPQC to R Load

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V(v)

T(ms)

Fig. 9.3. Load voltage (VL_abc) in UPQC to R Load

V(v)

T(ms)

Fig. 9.4. Injected Voltage (Vinj_abc) in UPQC to R Load

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V(v)

T(ms)

Fig. 9.5. DC-link Voltage (Vdc) in UPQC to R Load

I(A)

T(ms)

Fig. 9.6. Load Current (IL_abc) in UPQC to R Load

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I(A)

T(ms)

Fig.9.7. Source Current (IS_abc) in UPQC to R Load

I(A)

T(ms)

Fig. 9.8. Shunt Compensating Current (ISh_abc) in UPQC to R Load

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I(A)

T(ms)

Fig. 9.9. Current flowing through load neutral wire (IL_n) in UPQC to R Load

I(A)

T(ms)

Fig. 9.10. Shunt neutral compensating current (ISh_n) in UPQC to R Load

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I(A)

T(ms)

Fig. 9.11. Neutral current flowing towards series transformer (ISr_n) in UPQC to R

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9.2 UPQC to RL Load

Fig. 9.12. MATLAB model of UPQC for RL Load

V(v)

T(ms)

Fig. 9.13. Utility Voltage (VS_abc) in UPQC to RL Load

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V(v)

T(ms)

Fig. 9.14. Load voltage (VL_abc) in UPQC to RL Load

V(v)

T(ms)

Fig. 9.15. Injected Voltage (Vinj_abc) in UPQC to RL Load

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V(v)

T(ms)

Fig. 9.16. DC-link Voltage (Vdc) in UPQC to RL Load

I(A)

T(ms)

Fig. 9.17. Load Current (IL_abc) in UPQC to RL Load

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I(A)

T(ms)

Fig. 9.18. Source Current (IS_abc) in UPQC to RL Load

I(A)

T(ms)

Fig. 9.19. Shunt Compensating Current (ISh_abc) in UPQC to RL Load

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I(A)

T(ms)

Fig. 9.20. Current flowing through load neutral wire (IL_n) in UPQC to RL Load

I(A)

T(ms)

Fig. 9.21. Shunt neutral compensating current (ISh_n) in UPQC to RL Load

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I(A)

T(ms)

Fig. 9.22. Neutral current flowing towards series transformer (ISr_n) in UPQC to RL Load

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9.3 UPQC to RLC Load

Fig. 9.23. MATLAB model of UPQC for RLC Load

V(v)

T(ms)

Fig. 9.24. Utility Voltage (VS_abc) in UPQC to RLC Loa

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V(v)

T(ms)

Fig. 9.25. Load voltage (VL_abc) in UPQC to RLC Load

V(v)

T(ms)

Fig. 9.26. Injected Voltage (Vinj_abc) in UPQC to RLC Load

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V(v)

T(ms)

Fig. 9.27. DC-link Voltage (Vdc) in UPQC to RLC Load

I(A)

T(ms)

Fig. 9.28. Load Current (IL_abc) in UPQC to RLC Load

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I(A)

T(ms)

Fig. 9.29. Source Current (IS_abc) in UPQC to RLC Load

I(A)

T(ms)

Fig. 9.30. Shunt Compensating Current (ISh_abc) in UPQC to RLC Load

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I(A)

T(ms)

Fig. 9.31. Current flowing through load neutral wire (IL_n) in UPQC to RLC Load

I(A)

T(ms)

Fig. 9.32. Shunt neutral compensating current (ISh_n) in UPQC to RLC Load

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I(A)

T(ms)

Fig. 9.33. Neutral current flowing towards series transformer (ISr_n) in UPQC to RLC

Load

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9.4 UPQC TO IM LOAD

Fig. 9.34. MATLAB model of UPQC for IM Load

V(v)

T(ms)

Fig. 9.35. Utility Voltage (VS_abc) in UPQC TO IM Load

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V(v)

T(ms)

Fig. 9.36. Load voltage (VL_abc) in UPQC TO IM Load

V(v)

T(ms)

Fig. 9.37. Injected Voltage (Vinj_abc) in UPQC TO IM Load

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V(v)

T(ms)

Fig. 9.38. DC-link Voltage (Vdc) in UPQC TO IM Load

I(A)

T(ms)

Fig. 9.39. Load Current (IL_abc) in UPQC TO IM Load

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I(A)

T(ms)

Fig. 9.40. Source Current (IS_abc) in UPQC TO IM Load

I(A)

T(ms)

Fig. 9.41. Shunt Compensating Current (ISh_abc) in UPQC TO IM Load

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I(A)

T(ms)

Fig. 9.42. Current flowing through load neutral wire (IL_n) in UPQC TO IM Load

I(A)

T(ms)

Fig. 9.43. Shunt neutral compensating current (ISh_n) in UPQC TO IM Load

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I(A)

T(ms)

Fig. 9.44. Neutral current flowing towards series transformer (ISr_n) in UPQC TO IM

Load

Chapter 10

Conclusion

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10. Conclusion and Future Scope

10.1 Conclusion A new 3P4W topology for distribution system utilizing UPQC has been proposed

in this paper. This proposed topology would be very useful to expand the existing 3P3W

system to 3P4W system where UPQC is installed to compensate the different power

quality problems, which may play an important role in future UPQC-based distribution

system. A new control strategy to generate the balanced reference source current under

unbalanced load condition is also presented in this paper. The MATLAB/Simulink-based

simulation results show that the distorted and unbalanced load currents seen from the

utility side act as perfectly balanced source currents and are free from distortion. The

neutral current that may flow toward the transformer neutral point is effectively

compensated such that the transformer neutral point is always at virtual zero potential.

10.2 Future Scope

In high power applications, the filtering task cannot be performed for the whole

spectrum of harmonics by using a single converter due to the limitations on switching

frequency and power rating of the semiconductor devices. Therefore, compensating the

reactive harmonic components to improve the power quality of the DG integrated system

as well as to avoid the large capacity centralized APF, parallel operation of multiple low

power APF units are increasing. Like APF, UPQC can also be placed at the PCC or at a

high voltage distribution line as a part of DG integrated network or in micro-grid system

to work both in interconnected or islanded mode. At this place, capacity enhancement is

achieved by using Multi-level topologies to reach the higher power levels.

These options are as follows:

i. Multi-level converter based UPQC

ii. Multi-module converter based UPQC

iii. Multi-module (power cell) unit based UPQC

A multi-level converter is proposed to increase the converter operation voltage,

avoiding the series connection of switching elements. However, the multilevel converter

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is complex to form the output voltage and requires an excessive number of back-

connection diodes or flying capacitors or cascade converters.

A multi-module H-bridge UPQC can also be connected to the distribution system

without series injection transformers. It has the flexibility in expanding the operation

voltage by increasing the number of H-bridge modules isolated through a single-phase

multi winding transformer.

These Multi-module techniques allow the symmetrical distribution of the load

power among the components of the topology, but the classical design procedure must be

modified or refined to ensure the power cell components should be within its maximum

ratings. Therefore, a new design procedure of UPQC with a feature of extending capacity

based on a modular approach is presented.

It is found that research in recent years has placed more emphasis on CPDs,

especially on UPQC, and its application in DG or micro-grid system. Capacity

enhancement has been achieved using multi-level or multi-module and central control

mode, however, the flexibility of UPQC to increase its capacity in future and to cope up

with the increase load demand in low voltage distribution level has not been achieved.

References

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References

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[10] M. Aredes, K. Heumann, and E. H.Watanabe, “An universal active power line

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A Novel Structure for Three Phase Four Wire Distribution System Utilizing UPQC

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