Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission:

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May, 8th, 2007
S. Bozhko , G. M. Asher, J. C. Clare, L. Yao, and M. Bazargan
Grid Integration of Large Offshore Wind Farms
Using STATCOM-Controlled HVDC Power Transmission
Control and Engineering Issues

Reporter Dr S. Bozhko
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Introduction

• World electricity demand to be covered for up
  to 12 by 2020
• Offshore wind conditions are better, planning
  restrictions are reduced
• HVDC vs HVAC
• VSC HVDC vs LCC HVDC
• SG vs DFIG
• DFIG STATCOM LCC HVDC well studied as
  separate components
• Existing studies consider the overall system
  concept and possible control paradigms no
  detailed study or rigorous design procedure

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The power system studied
Total wind farm power 1GW (set of DFIG-based WTG
3.3MVA each) Collection Bus Voltage 33kV
Offshore Bus Voltage 132kV Onshore Grid 400kV
_at_ SCR2,5 HVDC Link 1GW (2kA_at_500kV)
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Control system should provide

• optimal tracking of collected wind power and its
  transfer into the HVDC link
• control of voltage and frequency of the offshore
  grid

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(No Transcript)
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Control Approach
Simplified diagram of the studied system
IG
L0
R0

TS
TC
E0
IC
IS
CS
VS
VG
VC
ES
V0
_
Cf
VS ABC
AOR (a)
STATCOM
HVDC
AC F
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Control Approach
Reduced plant of control
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Detailed block-diagram of the proposed control
structure
controllers
controlled plant
CS
ES
Isd
VSd
VSa
VS ABC
VGd
VSß
2/3
Isq
VSq
?e
VGq ( 0)
RS
?
2p?50
?
LS
ISd
ISa
IS
?Cf
ISß
ISq
?Cf
IG
VGa
VGd
VG
VGß
VGq
Cf
ICa
ICd
IC
ICß
ICq
I0
AOR (a)
_
I0

V0
L0
I0
R0
IGa
IGd
E0
IGq
IGß
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Control Approach
PSCAD/EMTDC simulations of the proposed control
system

• Detailed PSCAD/EMTDC simulation model is used

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Control Approach
Simulation results

• Confirm high performance in both normal
  conditions and during a severe fault

• Raise engineering concerns regarding STATCOM
  rating (1.3pu in order to handle the fault)
• Also raise concerns regarding STATCOM capacitor
  overvoltage (1.92pu)
• Some measures must be undertaken to improve the
  system practicality

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STATCOM DC-link capacitor sizing

• Energy stored in this capacitor

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STATCOM DC-link capacitor sizing

• Can be used to derive a criterion for the STATCOM
  capacitor sizing in order to guarantee that the
  capacitor overvoltage during a fault will not
  exceed the acceptable level

CS MIN F(tf, td, tG, tC, kV, PG0, PC0, PL0)
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Power system operation during a fault
0.25 pu
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Influence of communication delay td on STATCOM
rating
The dynamics of HVDC rectifier AC currents is
twice as faster than the dynamics of HVDC DC-link
current loop!
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STATCOM rating issue (continued)

• STATCOM rating can be reduced substantially only
  if no communication delay or if it is very small
  compare to HVDC DC-link current control time
  constant

• If communication delay exceeds some value, the
  STATCOM apparent power demand during faults can
  reach the value of wind farm delivered apparent
  power

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Power system operation during a fault
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Reduction of the STATCOM rating can be achieved
by

• Suppression of STATCOM DC-link voltage control
  fault detection scheme can set the HVDC current
  demand I0 to some value I0fin in order to absorb
  the AC filters reactive power by HVDC link, not
  by STATCOM

• Reduction of wind farm output power via fast DFIG
  current control loops

• Communication delay td due to distant location if
  WTGs should be lowered

• Reactive power capabilities of DFIGs front-end
  converters the reactive current reference as a
  function of reactive current component at HVDC
  input

• Active power support through rotor q-current
  controls the q-current reference as a function
  of active current component at HVDC/filters input

• Improvement of HVDC DC-link current control need
  adaptation to fault conditions

• Lowering the bandwidth of offshore grid voltage
  and frequency controls

• Hard Limits on STATCOM currents.

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Simulation of fault in the enhanced system

• STATCOM active and reactive power demand is
  significantly lowered

• STATCOM DC-link overvoltage is reduced from 94
  to 25

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Conclusions

• A large offshore wind farm with a LCC HVDC
  connection to the main onshore grid is considered
  

• The proposed control system is proven to provide
  high performance control of the offshore grid and
  wind power transfer to onshore

• Engineering issues related to the STATCOM sizing
  is considered

• Recommendations for control system enhancement
  are given

• The proposed system can be a satisfactory
  solution for integrating large offshore
  DFIG-based wind farms into existing AC networks

Acknowledgement
Authors would like to express their appreciation
for the partial funding support from the New and
Renewable Energy Programme of the DTI, UK under
the contract K/KL/00340/00/00.
THANK YOU!