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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, [email protected]
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
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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
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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, [email protected]
1
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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)
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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
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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)
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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
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6 Frequency dependent resistance modelled.
Inter and intra-winding capacitance transformers need not be
modelled.
Transformers
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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
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8 Harmonic injections WTG transformer normally
at 0.69/33kV Single tuned filters
Wind turbine generator systems
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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 -