Grid Integration of Offshore Wind Farms via VSC-HVDC
- Dynamic Stability Study
Hongzhi Liu and Prof. Zhe Chen
Department of Energy Technology, Aalborg University, Denmark
Introduction
Voltage Stability Enhancement
VSC-HVDC Link
Offshore VSC
Onshore VSC
Converter control
scheme
Voltage support
capability
VSC-HVDC
Converter capacity
DC Cable
• Impact investigation on power
system stability
• Stability enhancement of main
AC system
Offshore Wind Farm
Trajectory sensitivity analysis
(TSA) based Method
Short-term voltage
stability
Start
Main AC System
Time domain simulation
1). 150MVA (Sbase) VSC
2). 155MVA VSC
No
V < 0.9 pu
at t = 1.5s
Yes
X
(1.5s )  X base (1.5s )
Sen(1.5s )  155 MVA
155  150
Impact on Power System Stability
Steady state voltage profile
With suitable control, the VSC-HVDC link can
benefit offshore wind farm integration by
improving the penetration level, enhancing
dynamic voltage stability and transient angle
stability.
Dynamic voltage stability
Transient angle stability
S 
Time domain simulation with ( Snew  1MVA)
*
Get X new
(1.5s )
X exp (1.5s )  X base (1.5s )
*
*
Sen* (1.5s)  X new
(1.5s)  X base
(1.5s)
Sen(1.5s )
S 
*
Snew  Sbase  S
*
Time domain simulation with Snew ( Sbase )
*
Get X base (1.5s )
*
X exp (1.5s )  X base
(1.5s )
Sen* (1.5s )
*
*
Snew
 Sbase
 S *  Sbase  S  S *
End
A TSA-based two-stage design method is developed to identify the
suitable onshore VSC capacity with which VSC-HVDC can provide enough
support for stabilizing the PCC voltage following a grid disturbance.
Coordinated Frequency Regulation
PMSG
Grid
Application of Battery Energy Storage System
Under-frequency Controller
(fmeas<49.8Hz)
PWM converter
1
1  sT
f meas
AC grid
Pinertia
-
f nom +
Kinertia
df dt
f
1
1  sT
Pdroop
1R
+
V
=
-
Battery model
DC
Pmd
gen
Vbatt
Punder
P _ ref
P
+
+
Pmq
Ibatt
Current
controller
Pg*

id* lim
P,V
Pover
iq* lim
V
f
f meas
1
1  sT
f
1
K grad
100
 ( f  f up )  Kover
y0
Frequency
controller
PWF
id*
PV
controller
Pref
Smoothing
controller
iq*
Power management unit
Over-frequency Controller
(fmeas>50.2Hz)
50.1
BESS with 29.45MVA converter
(Hz)
Frequency
Frequency (HZ)
Onshore
network
±150kV
Offshore
network
Primary frequency reserve of BESS in different charging/discharging states
49.9
Onshore VSC
Offshore VSC
49.7
Smoothing power
(MW)
Potential primary
control reserve (MW)
Case1
-20 (charging)
49.45
Case2
-10 (charging)
39.45
125
Case3
0
29.45
115
Case4
10 (discharging)
19.45
Case5
20 (discharging)
9.45
49.5
49.3
0
5
10
15
(a)
20
25
30
PI
i +
+
-
Vwf*
i
f wf
Pm
PI
-
The proposed ancillary
control strategy for VSCHVDC is coordinated with
the ancillary frequency
control of offshore wind
turbines
to
achieve
significant
frequency
response enhancement.
F0
PI
+
f grid
Frequency signaling
Offshore VSC control
Ancillary DC voltage controller
0.1 pu
+
Kf
f nom
1  sT f
-0.1 pu
*
vdcf
vdc +
imax
-
id*
PI
+
-
v grid
+
PI
Pmd
-
*
vdc
id
PI
Normal
v*grid
Fault
Current limiter
f grid
iq*
Pmq
+
PI
iq
Onshore VSC control
With the increasing of
VSC-HVDC communication
delay, the enhancement
on
nadir
or
peak
frequency of the proposed
control strategy will be
weakened
but
the
restoration
of
the
frequency will get better.
105
0
5
Case1
10
Case2
15
(b)
Time (s)
Case3
20
Case4
25
Case5
30
Case6
50.1
Frequency (HZ)
-
A new power management strategy as well as a new size evaluation
method of BESS power capacity is developed to enable the BESS to
enhance the system frequency response in cooperation with
smoothing wind power fluctuation .
49.9
49.7
49.5
49.3
Combined P (MW)
(onshore VSC and BESS)
Vwf
Combined PP(MW)
(MW)
Combined
(onshore VSC and BESS)
135
*
SOC
Charge
controller
2.
3.
4.
5.
6.
7.
5
10
15
(a)
20
25
30
0
5
10
15
(b)
Time (s)
20
25
30
135
125
115
105
1.
0
Hongzhi Liu, Zhe Chen, "Aggregated Modelling for Wind Farms for Power System Transient Stability Studies," in Asia-Pacific
Power and Energy Engineering Conference (APPEEC), Shanghai, China, March 2012.
Hongzhi Liu, Zhe Chen, "Fault ride-through and grid support of permanent magnet synchronous generator-based wind farms
with HVAC and VSC-HVDC transmission systems," in IEEE International Energy Conference and Exhibition (ENERGYCON),
Florence, Italy, September 2012.
Hongzhi Liu, Zhe Chen, "Impacts of large-scale offshore wind farm integration on power systems through VSC-HVDC," in IEEE
Grenoble PowerTech (POWERTECH), Grenoble, France, June 2013.
Hongzhi Liu, Zhe Chen, Chengxi Liu, “Enhanced Dynamic Voltage Stability Support by VSC-HVDC for Offshore Wind
Applications using Trajectory Sensitivity Analysis”, in 12th International Workshop on Large-scale Integration of Wind Power
into Power Systems as well as on Transmission Networks for Offshore Wind Power Plants, London, UK, October 2013.
Hongzhi Liu, Zhe Chen, “Coordinated frequency regulation by offshore wind farms and VSC-HVDC transmission,” International
journal of smart grid and clean energy, vol. 3, no. 1, pp 1-7, January 2014.
Hongzhi Liu, Zhe Chen, “Contribution of VSC-HVDC to Frequency Regulation of Power Systems with Offshore Wind
Generation,” IEEE Trans. On Energy Conversion (Under review).
Hongzhi Liu, Zhe Chen and Jiakun Fang, “Enhancement of Voltage Support Capability of VSC-HVDC for Offshore Wind
Applications Using Trajectory Sensitivity Analysis,” Renewable Energy (Under review).