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Poster Code: P1

Harmonic modelling for simulation of HVDC Converter Stations and Wind Farms connected to a transmission system

Kah Leong KOO, PhD*

*Principal Power Systems Engineer, Power Networks, Parsons Brinckerhoff, klkoo@theiet.org

Keywords: Harmonic assessment, Harmonic compliance, HVDCs, Wind farms, Engineering Recommendations-G5/4.

Abstract Large capacity offshore wind farms, onshore wind farms and HVDC connections will be common place in the next decade. In the UK, offshore transmission networks are built to accommodate the growth of offshore wind farm connections, leading to the integration of DC (CSC and VSC type HVDC) and AC connections for offshore wind farms and European network integration. These connections are harmonic sources into the transmission system. Therefore, detailed power quality studies for the connections need to be performed by utilities, wind farm and HVDC developers to derive the harmonic limits. The limits become a contractual agreement between the utility and the customer; this is to comply with the relevant engineering standards. For example, Engineering Recommendation G5/4 is applied to the connections in the UK as referred to by the UK Grid-code. This paper presents the different technical modelling and assessment criteria for wind farms and HVDC converter stations connected to the UK transmission system.

1 Introduction It is expected that the capacity of wind farms in the UK will reach 18 GW offshore and 10 GW onshore by 2020, while offshore could potentially reach over 40GW by 2030[1]. The ability to transfer this power through identified corridors to high demand locations (far from the generation sources) requires the application of new technologies, including HVDC. The expected investment of 19 billion for the UK alone is of pan-European significance nationally and regionally. HVDC connections are being used for large scale offshore and onshore wind farms to speed up connection and resolve tight corridors to help deliver this programme[2].

2 Harmonic Assessment and Specifications Assessing Offshore Wind Farms Offshore wind farms can connect to a substation at EHV (normally at 400kV in the UK). Transmission utilities will need to ensure that harmonic limits are not exceeded once the wind farms are connected. Figure 1 illustrates a general radial

connection outwards from an onshore TO (Transmission Owner) substation to the off shore wind farm.

Figure 1 Typical arrangement of an offshore wind farm

In the process of performing any harmonic assessment, a TO has to consider the impact of the prospective connection on existing harmonic levels in its own system and downstream to any DNO (Distribution Network Operator) systems, including any power stations and industrial systems. The process of setting the limits must include: 1. Determining the maximum amplification of the

harmonic voltages up to 5 KHz (for a 50Hz system) for all anticipated network conditions at different system demand conditions - Equation (1).

2. Determination of the maximum distortion allowed at the point of connection to the transmission system (designated interface point in Figure 1) due to harmonic emissions from the wind farm (Equation (2)) for all anticipated network conditions and demand conditions as (1).

For Equation (1), the variation in the background harmonic levels are determined from the network impedance changes before (Area 1 in Figure 1) and after (with Area 2 in Figure 1) the wind farm connection. The detailed modelling of the wind farm for the passive and active elements for harmonic studies can be referenced [3].

bgbg VMV u ' (1)

Reactive power compensation and

harmonic mitigation

N/O N/O N/O

Area 2

Interface Point Boundary

N/O: normally open MV Offshore collector bus usually 33 kV

More turbines More strings

EHV Grid connection usually 400 or 275

kV, TO asset

Area 1

2

Vbg is the background harmonic level after wind farm is connected for harmonic.

Vbg is the background harmonic level before wind farm is connected for harmonic.

M is the magnification or attenuation of the harmonic level for harmonic.

Equation 2 gives a generic definition of the harmonic voltage distortion calculated at the interface point which includes all phase relationships from each wind turbine generator and all other active harmonic sources.

)(,(

)(,(

,1

,1

rstransformecircuits

Nsvcn

nsvc

rstransformecircuits

Nwtgn

nwtg

ZZFnZVFnV

ZZFnZVFnVVdist

(2)

Implementation of a detailed model to calculate the voltage distortion (Equation 2) as opposed to the aggregation method in IEC 61000-3-6 has been previously discussed [4] to allow accurate estimation of calculated voltage distortion. An equivalent harmonic voltage source of the wind turbine of complex quantity is usually available from the wind farm developer (note that the data also provides the effect of interaction of wind turbine generators in clusters). It is noted that data provided according to IEC 61400-21 would not be suitable using this approach. Assessing Onshore Wind Farms For onshore wind farms, overhead AC lines as opposed to long MV (132-150kV) cables are normally used, unless routes transgress areas which are protected by law. The method for performing the harmonic assessment will be the same as for offshore wind farms but the effects as defined by Equation (1) and Equation (2) will be different due to less cable dominance and emissions electrically closer to the interface point. Assessing HVDC Converter Stations HVDC converter stations located onshore are close to the transmission system and have pronounced effects on harmonic levels. In the UK, the Western HVDC link (CSC) is expected to go into operation late 2015 and the Eirgrid link (VSC) currently being commissioned are used to achieve the increase transfers of renewable wind energy. The harmonic assessment for both these HVDC links will produce harmonic limits specifications requirements similar to that for wind farms. This has been considered an acceptable approach leading to optimized filter design where planning levels and compatibility levels referred to in ER G5/4 are currently used. The filters are designed for (1) mitigation of the characteristic emissions (2) attenuation of pre-existing system harmonic levels and (3) provision of reactive VARs consumption for CSC type HVDC and maintenance of required system reactive exchange.

The recommended approach for assessment of an HVDC converter station differs to that of wind farms and details of this work can be referenced [5, 6]. The assessment criteria are:

1. Determination of the maximum distortion allowed at the point of connection to the transmission system (designated as interface point in Figure 1 where the HVDC is connected to the transmission system) is assessed for the interface point and for all other nodes referred back to the interface point in the transmission system, affected nodes in the DNO systems including any power stations and industrial systems (Equation (3)) for all anticipated network and demand conditions.

2. The maximum amplification of the harmonic voltages up to 5 KHz (for a 50Hz system) for all anticipated network conditions at different system demand conditions is dependent on (1) and planning levels stipulated in Engineering Recommendations G5/4.

R-PCC

BKG@RPL@RHVDC@IP G

VVV

(3) VHVDC@IP is the permissible incremental distortion at the

interface point due to the converter, VPL@R is the planning level as per ER G5-4 at a remote

node, VBKG@R is the background distortion at the associated

remote node and, GPCC-R is the harmonic voltage gain from the interface point

to the associated remote node.

Assessment under UK Offshore transmission system regime Figure 2 shows a generic diagram for the interconnection of offshore wind farms to the UK transmission system and the European Grid via an offshore transmission system. The methods for performing the harmonic assessment and setting the limits depends on whether these are wind farms or HVDC converter stations. Using Figure 2, the assessments are performed by: 1. Offshore transmission owner OFTO1, who will perform

the harmonic assessments for offshore wind farms (WF1, WF2, WF3), HVDC1 and HVDC2.

2. Onshore transmission owner OSTO1, who will perform the harmonic assessments for HVDC3.

3. Offshore wind farm developer WF4, will perform the harmonic assessments up to the interface point owned by OSTO1 as WF4 owns the offshore transmission assets and takes the role of offshore transmission owner.

4. Onshore transmission owner OSTO1, who will perform the harmonic assessments for directly connected onshore wind farm WF5.

3 Conclusion The paper has set out the modelling differences in wind farms and HVDC converter stations required in studies to determine harmonic limits at interface points for these connections. In the future world where many developers, transmission owners and system operators will interact closely, these studies will be required by different parties and will present many technical challenges. The foreseeable challenges are maintaining a consistent approach to these activities and coordinating these activities to allow an optimised design which avoids inadequate system performance and wastage.

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Figure 2 A simplistic illustration of possible layout with onshore and offshore transmission systems

Acknowledgements The author would like to express his gratitude to Parsons Brinckerhoff for its technical and financial support for the preparation and presentation of this paper.

References [1] Department of Energy and Climate Change. UK

Renewable Energy Roadmap, July 2011. [2] ENTSO, ENTSO-E 10-Year Network Development

Plan 2012 Project For Consultation, Draft Version for Public Consultation, 1 March 26April 2012.

[3] Kah Leong Koo, Harmonic assessments for filter design specifications at U.K. National Grid Electricity Transmission System for large wind farms, 45th International Universities Power Conference, Cardiff, Wales, 31st Aug 3rd Sept 2010.

[4] Foroozan Ghassemi, Kah-Leong Koo. Equivalent Network for Wind Farm Harmonic Assessments, IEEE Transactions on Power Delivery, VOL. 25, NO.3 July 2010.

[5] Z.Emin, F.Ghassemi, J.J.Price, Harmonic Performance Requirements of an HVDC Connection; Network Owner Perspective, ACDC 2010, 9th International Conference on AC and DC Power Transmission, London, United Kingdom, Paper 0061, 20-21 October 2010.

[6] K.L.Koo, Z.Emin, F.Ghassemi and J.J.Price, Harmonic performance specification of a VSC type HVDC, Cigre International Symposium, Recife, Pernambuco, Brazil, Paper 73, April 3-6, 2011.

OSTO1 (UK)

OSTO2 (Europe)

OSTO3 (Europe)

Offshore AC substation

WF5

WF4

WF1 WF2 WF3

UK shoreline

HVDC1

HVDC3

HVDC2 OFTO1 AC

Harmonic modelling for simulation of HVDC Converter Stations and Wind Farms connected to a transmission system

Kah Leong KOO, PhD* *Principal Power Systems Engineer, Power Networks,

Parsons Brinckerhoff, Leong.Koo@pbworld.com

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2Rationale for this IET Paper

Sharing experiences and challenges in harmonic assessments of offshore wind farms and HVDC projects in the UK :

RWE Gwynt-Y-Mor 635MW offshore wind farm at Bodelwyddan new 400kV substation (currently being commissioned)

EON London Array Stage 1 635MW offshore wind farm at Cleve Hill new 400kV substation (currently being commissioned)

Centrica Lincs 270MW offshore windfarm at Walpole 400kV substation (currently being comissioned)

Eirgrid 500MW VSC type HVDC (EWIP)connecting at 400kV Deeside substation (UK) and Woodland (Ireland) (currently being commissioned)

Western HVDC CSC 2.2 GW link between England (new Connahs Quay 400kV substation) and Hunterston in Scotland (contract being awarded to Siemens expected commissioning in 2015)

3R e act ive pow e r com pe nsat ion and

har mon ic mit iga t ion

N /O N /ON/ O

Ar e a 2

In terfa ce Po in tB oundar y

N /O : no r m a lly openM V O f fs hore c o lle c to r b us usuall y 33 kV

M ore tu r b ines M ore str ings

EH V G ri d conne ct ionusual ly 400 o r 2 75

kV , TO a sse t

A re a 1

Typical arrangement of an offshore wind farm

4Wind Farm harmonic model required for harmonic analysis

Sub-transmission network (cables, transformers)

Dynamic and passive compensation equipments (SVCs, MSCs, SHRs ..)

Wind turbine generator systems (WTG harmonic source, WTG transformer, Filters)

5 Not adequate to lump capacitance for wind farms MV network and aggregate harmonic injections at Interface Point (S8.3 IEC 61400-21) .

Reference - Foroozan Ghassemi and Kah-Leong Koo, Equivalent Network for Wind Farm Harmonic Assessments, IEEE Trans. on Power Delivery, Vol. 25, No.3, July 2010.

Cable capacitance of wind farms

6 Frequency dependent resistance modelled.

Inter and intra-winding capacitance transformers need not be modelled.

Transformers

7 Dynamic type equipments like SVCs modelled.

Passive type equipments modelled (e.g. filters).

Self Impedance from HV tertiary transformer with and without filters

Dynamic and passive compensation equipments

8 Harmonic injections WTG transformer normally

at 0.69/33kV Single tuned filters

Wind turbine generator systems

9Background harmonic levels changes latest measurements

Harmonic levels demonstrating 3 consecutive years

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Modification of background harmonic levels

bgharmbgharm VMV u 'Where: Vharmbg is the background harmonic level after wind farm is

connected. Vharmbg is the background harmonic level before wind farm

is connected. M is the magnification or attenuation of the harmonic level.

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Voltage distortion calculated at Interface Point from all harmonic injections

Where: R.H.S. 1 represents all wind turbine emissions accounting for all MV circuit elements R.H.S. 2 represents all active compensation equipments accounting for all MV circuit

elements

)(,( ,1

rstransformecircuits

Nwtgn

nwtg ZZFnZVFnVVdist

)(,( ,1

rstransformecircuits

Nsvcn

nsvc ZZFnZVFnV

R.H.S. 1

R.H.S. 2

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HVDC approach for Voltage distortion calculated at Interface Point

Where: VHVDC@IP is the permissible incremental distortion at interface point, VPL@R is the planning level as per ER G5-4 at remote node, VBKG@R is the background distortion at remote node, GIP-R is the harmonic voltage gain from the interface point to remote node

R-IP

BKG@RPL@RHVDC@IP G

VVV

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A simple network illustrating Interface Point and remote nodes where gains are calculated

New Load

Load

Load

Load

LoadInterface Point

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O ST O 1 (U K )

O S TO 2 (E u r op e)

O ST O 3 ( E u r op e)

O f f shor e A C s ubs ta t io n

W F5

W F4

W F 1 W F 2 W F 3

UK sho r el ine

H V D C1

H VD C 3

H V D C 2O F TO 1 A C

A simplistic illustration of possible layout with combinations of multiple onshore and offshore transmission systems near future??

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Conclusions

Modelling differences for limit assessments when dealing with wind farms and HVDCs.

Looking forward HVDC and Wind farm owners/developers, transmission owners and system operators have different roles in meeting many power quality challenges.

Foreseeable challenges and certainly not by any measure exhausted include:

All parties maintaining consistent approach to assessment activities.

Coordinating these activities to optimise design of these connections.

Timely delivery of these assessments to avoid unnecessary delays in these connections.

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References

Kah Leong Koo, Harmonic assessments for filter design specifications at U.K. National Grid Electricity Transmission System for large wind farms, 45th InternationalUniversities Power Conference, Cardiff, Wales, 31st Aug 3rd Sept 2010.

Foroozan Ghassemi, Kah-Leong Koo. Equivalent Network for Wind Farm Harmonic Assessments, IEEE Transactions on Power Delivery, VOL. 25, NO.3 July 2010.

Z.Emin, F.Ghassemi, J.J.Price, Harmonic Performance Requirements of an HVDC Connection; Network Owner Perspective, ACDC 2010, 9th International Conference on AC and DC Power Transmission, London, United Kingdom, Paper 0061, 20-21 October 2010.

K.L.Koo, Z.Emin, F.Ghassemi and J.J.Price, Harmonic performance specification of a VSC type HVDC, Cigre International Symposium, Recife, Pernambuco, Brazil,Paper 73, April 3-6, 2011.

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Questions??

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Poster P1 to look for further discussions and details -