Improvement of Fault Ride-Through Capability in Wind Farms Using VSC-HVDC

European Journal of Scientific Research
ISSN 1450-216X Vol.28 No.3 (2009), pp.328-337
© EuroJournals Publishing, Inc. 2009
http://www.eurojournals.com/ejsr.htm


Improvement of Fault Ride-Through Capability in Wind Farms
Using VSC-HVDC


Hanif. Livani
Electrical & Computer Engineering Department, Babol University of Technology
Shariati Street, Babol, Iran, P. O. Box 47135-484
E-mail: hanif.livani@gmail.com
Tel: +98-936-3688436; Fax: +98-111-3239214

Mohsen. Bandarabadi
Electrical Engineering Department, Islamic Aazad University of Minoodasht
Valiiasr Street, Minoodasht, Iran

Yosef. Alinejad
Electrical & Computer Engineering Department, University of semnan

Saeed. Lesan
Electrical & Computer Engineering Department, Babol University of Technology
Shariati Street, Babol, Iran
Tel: +98-936-3688436; Fax: +98-111-3239214

Hossein. Karimi-Davijani
Electrical & Computer Engineering Department, Babol University of Technology
Shariati Street, Babol, Iran
Tel: +98-936-3688436; Fax: +98-111-3239214


Abstract

This paper studies possible improvement of fault-ride through capability in
connection of 160 MW wind farm to transmission network using VSC-HVDC link. The
160 MW wind farm includes 80 individual 2 MW permanent magnet synchronous
generators which are divided into 4 groups with 40 MW nominal powers. The voltage at
the transmission network terminal should be re-established with minimized power losses
during wind speed fluctuations and after the clearance of grid side faults. It is also
important for the voltage of transmission network side to be supported by the VSC-HVDC
during short circuit faults in main grid which is named fault ride-through capability
improvement. This paper emphasizes on variable speed operation and fault ride-through
capability improvement in wind farm network and transmission network respectively.
Simulation is performed in PSCAD/EMTDC software to study behavior of wind farm,
transmission voltage and dc voltage for different changes in wind speed and three-phase
short circuit fault. The simulation results validate the connection method performance and
the fault ride- through capability improvement.

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Improvement of Fault Ride-Through Capability in Wind Farms Using VSC-HVDC
329
Keywords: Fault Ride-Through, PMSG, VSC-HVDC, Wind Farm

1. Introduction
Because of some challenges such as conventional energy sources consumption, pollution, global
climate change and security of energy supply, significant efforts have been made to develop renewable
energy sources such as wind energy. Wind power growth with a 20% annual rate has experienced the
fastest growth among all renewable energy sources science five years ago [1]. It is predicted that by
2020 up to 12% of the world's electricity will have been supplied by wind power [2]
In terms of wind power generation technology, as a result of numerous technical benefits
(higher energy yield, reducing power fluctuations and improving var supply) the modern MW-size
wind turbines always use variable speed operation which is achieved by electrical converters [3] .These
converters are typically associated with individual generators and they contribute significantly to the
costs of wind turbines. Between variable speed wind turbine generators doubly fed induction
generators (DFIGs) and permanent magnet synchronous generators (PMSGs) with primary converters
are emerging as the preferred technologies [4].
As a result of large-scale wind power generation, interconnecting large wind farms to power
grids and the relevant influences on the host grids need to be carefully investigated. Wind farms are
now required to comply with stringent connection requirements including [5]: reactive power support,
transient recovery, system stability and voltage/frequency regulation.
In recent years, with the development of power electronics, Voltage source converters (VSC)
based on HVDC (VSC-HVDC) transmission link, using self-commutated valves (IGBTs, IGCTs and
GTOs), have had an active role in electricity transmission and distribution improvement. Furthermore,
their significant advantages make this converters suitable for the connection of wind farms to
transmission networks [6,7]: (i) IGBT valve can switch off and on immediately, (ii) there is no
commutation failure problem, (iii) no telecommunication required between two stations of HVDC
system, (iv) Active and reactive power can be controlled independently, (v) reactive power
compensation is not required, (vi) only small filter is required to filter high frequency signal from
PWM.
In [4] decoupled control of active and reactive power is applied to VSC-HVDC to achieve
variable speed operation of wind generators, but basic requirements such as transient recovery and
voltage regulation of the grid aren’t investigated. In [6] coordinated control of voltage source converter
based on HVDC and wind turbines equipped with doubly-fed induction generators are introduced, but
focus is more on the variable frequency operation of the DFIG wind farm. In [5] an approximate linear
control strategy is applied to the VSC-HVDC link.
The objective of this paper is to study fault ride-through capability improvement of wind farm
using VSC-HVDC. Moreover, improvement of voltage quality and transient stability in transmission
network during short circuit faults and wind speed fluctuations and after the clearance of transmission
network side faults are investigated. Furthermore there is shown that the proposed control method for
VSC-HVDC converters is capable to achieve variable speed operation of PMSG wind turbines in wind
farm. Section 1 describes the wind farm and VSC-HVDC systems, and control strategies are described
in sections 3. Finally section 4 describes simulation results in PSCAD/EMTDC software.


2. Topology of Connection
In this paper, considered 160 MW wind farm consists of 80 individual wind generators and each of
them is a commercially available 2 MW, 4 KV unit based on permanent magnet synchronous
generators (PMSGs) [4]. Parameters of generators are given in Appendix A. The generators are divided
into 4 groups according to their geographic locations [4]. Each 40 MW group includes 20 generators
and there is a single 4 kV/90 kV transformer per group which increases the generator voltage to the
transmission level.

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330 Hanif. Livani, Mohsen. Bandarabadi, Yosef. Alinejad, Saeed. Lesan and Hossein. Karimi-Davijani
The construction of VSC based HVDC link used in connection of wind farm to the
transmission network is shown in Figure 1. The sample transmission network is chosen from [8].
Parameters of AC grids, AC transmission lines and DC link are given in Appendix B.
In order to maximize energy capture and reduce stress and noise, the wind turbines operate at
variable speed mode. Since wind farm control is transformed into each wind farm side VSC converters
control, converter systems are not used with individual PM generators.

2.1. Wind turbine model
In general, the relation between the wind speed and aerodynamic torque in a turbine can be described
by the following equation [9]:
1
2 3
P = ρπ R V C (θ ,λ)
w
2
w p
(1)
1
3 2
T = ρπ R V C (θ ,λ) /
w
λ
2
w p
Where Pw and Tw are the power and aerodynamic torque extracted from the wind [w], [N/m], ρ
is the air density [kg/m3], R is the wind turbine rotor radius [m], Vw is the equivalent wind speed [m/s],
θ is the pitch angle wind turbine blade [deg], λ= ωrot R /Vw is the tip speed ratio, where ωrot is the wind
turbine rotor speed [rad/s] and CP is the aerodynamic efficiency of the rotor.

Figure 1: Single-line Diagram of VSC-HVDC Based Connection of 160 MW Wind Farm



CP can be expressed as a function of the tip speed ratio (λ) and pitch angle (θ) by the following
equation [9]:
12.5/
C = 0.22(116/ - 0.4 - 5)

p
e
β
β
θ
−
(2)
Where β is defined as [9]:
1
β =
(3)
1
0.035
−
λ + 0 0
. 8θ
3
θ +1

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Improvement of Fault Ride-Through Capability in Wind Farms Using VSC-HVDC
331
3. Control Strategies
According to different wind speeds, two different control goals are considered in variable speed wind
turbines. Under the nominal wind speed a constant optimum tip speed ratio is maintained to get
maximum aerodynamic efficiency. In high wind speeds, the control goal is to regulate the turbine
torque and to keep the output power of wind generators at their rated values. On the hand, for
improving fault ride-through capability of the wind farm, it is important to implement a good control
strategy for VSC station in the transmission network side shown in Figure 1.

3.1. Pitch angle control
The aerodynamic model of the wind turbine has shown that the aerodynamic efficiency is strongly
influenced by the variations of the blade pitch angle with respect to the direction of the wind or to the
plane of rotation.
In details, below the rated wind speed, the maximum power coefficient is maintained by
keeping a constant maximum tip speed ratio and the turbine should simply try to produce as much
power as possible, therefore there is generally no need to vary the pitch angle. Above the rated wind
speed, to keep the rotor speed at a certain level and to maintain the machine at the rated output power
pitch control is applied. The relationship between the pitch angle and the wind speed is shown in
Figure 2.

Figure 2: The Relationship between Pitch Angle and Wind Speed in a Typical PMSG Wind Turbine



As can be seen from Figure 3, the rated wind speed is 12 m/s and below this speed, the pitch
angle is kept constant to achieve optimal aerodynamic efficiency CP by varying the turbine rotor speed
accordingly.

3.2. Speed control in wind generators
When the average wind speed varies, the average rotational speed of each group of generators has to be
adjusted accordingly. In order to acquire the maximum wind turbine power, VSC should control the
turbine speed to optimal value at any wind velocity.
The reference generator speed ωgref is calculated according to wind speed changes as Equation
(4) to enable maximum coefficient of performance [5]:
V K
w ts
ω
=
g
gref
r (4)
R
Where Vw is the wind speed, Kts is the optimal tip speed ratio, gr=77 is the gearbox ratio and
R=41 m is the turbine radius.
Figure 3 shows the aerodynamic efficiency (CP) tracking characteristic of a typical PMSG wind
turbine. As can be seen from Figure 3, in pitch angles which are lower than 5 degree, the maximum

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332 Hanif. Livani, Mohsen. Bandarabadi, Yosef. Alinejad, Saeed. Lesan and Hossein. Karimi-Davijani
aerodynamic efficiency can be achieved by applying the tip speed ratio around 7. So in Equation (4)
the optimal tip speed ratio (Kts) sets to 7.
According to the Equation (4) and corresponding to wind speed range 5(m/s)<Vw<12(m/s), the
generator speed operating range is 65(rad/s)<ωg<158(rad/s). Above the rated wind speed (Vwrated =
12(m/s)) the generator speed should be kept at 158 rad/s. In order to prevent generator flux saturation
and to enable suitable gain in the torque control loop, terminal voltage in each group is regulated to be
proportional to the generator speed (V/f control), therefore the ratio between the voltage and speed
reference is obtained as [5]:
Vn/ωg= 4 kV/314.15 rad/s = 0.0127 kVs/rad (5)
So the reference ac voltage in each generator’s group can be obtained by:
VacRef = 0.0127Wgref (6)
Where Wgref is the reference value of wind generator in each cluster.

Figure 3: The aerodynamic efficiency tracking characteristic of a typical PMSG wind turbine



The control diagram shown in Figure 4(a) is implemented to control each VSC in the wind farm
side. As can be seen in Figure 4(a), the AC voltage of each wind cluster is controlled by regulating
modulation index (Mi) through a PI controller. By controlling shift angle in the SPWM method, wind
generators speed is regulated according to Equation (4). The frequency of triangle carrier signal used
for SPWM generator is 33 times of fundamental frequency (fc = 1650 Hz).

Figure 4: (a) Controller diagram for wind farm side VSC, (b) Controller diagram for VSC on the transmission
network side



3.3. Control strategy for VSC on the transmission network side
A block diagram of the proposed control system is shown in Figure 4(b). It consists of two main
subsystems: DC voltage controller which maintains the DC voltage of HVDC link at constant value

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Improvement of Fault Ride-Through Capability in Wind Farms Using VSC-HVDC
333
(150 kV), and the AC voltage controller which is used to support the grid voltage during faults and to
improve fault ride-through capability in steady state. The AC and DC voltage control loops generate
modulation index (Mi) and shift angle (shift) respectively.


4. Simulation Results
In order to verify the performance of VSC-HVDC based wind farm interconnection method and its
effects on fault ride-through capability, an appropriate simulation for the system depicted in Figure 1
has been performed in PSCAD/EMTDC software [11].
First, simulations are performed to validate the variable speed concept in wind generators for
each cluster of PMSGs. Figure 5(a) shows the different wind speeds in each cluster of wind generators.
As a result of these wind speed variations, the generated power in each individual generators group and
the total power delivered to the transmission network through grid side converter (VSC5) vary
according to Figure 5(b). It can be realized by Figure 5(b) that based on wind speed reduction, the
output power of each wind generators group decreases. On the other hand, when wind speed exceeds
the rated wind speed (Vwrated = 12 m/s), the output power of permanent magnet synchronous generator
is kept at the nominal value of it (Po-nominal = 2 MW) by actuating pitch angle control in the wind
turbine.

Figure 5: System responses to different wind speeds. (a) Wind speeds in wind farms. (b) Generated power by
WFs and power received by ac network. (c) Generators speed. (d) AC voltage at WF buses. (e) DC
voltage at the WF side of dc link (Vdc1) and at the grid side of dc link (Vdc2). (f) AC voltage at the
PCC bus





According to Equation (4), the generator speed in each cluster should vary to achieve maximum
power coefficient (Cpmax). Figure 5(c) demonstrates the generators speed in different clusters.
Considering variable speed operation, in wind speeds lower than 12 m/s, the generators speed vary and
in wind speeds above 12 m/s, the control system of VSC on the wind farm side is capable to keep the
generator speed at its maximum value (ωgmax = 158 rad/s).

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334 Hanif. Livani, Mohsen. Bandarabadi, Yosef. Alinejad, Saeed. Lesan and Hossein. Karimi-Davijani
In the case of sudden changes in wind speeds, the proposed system is capable to transmit wind
power to the transmission network satisfactorily. AC voltages at each generators group are depicted in
Figure 5(d). These voltages track their reference value based on Equation (6) and technical limit of 4
kV in PMSG is maintained. The DC and AC voltage in the inverter terminal are also controlled at their
reference values. The DC voltage of dc link and AC voltage of PCC (Point of common Coupling)
during wind speed changes are illustrated in Figure 5(e) and Figure 5(f) respectively.
Among different short circuit faults, three-phase short circuit fault is the most severe fault, so in
this paper the three-phase short circuit fault is exerted to the end of line 4 near the PCC at t=5 second.
The protection relay in line 4 isolates it after 120 ms and reconnects it to the ac system after 300 ms,
considering a temporary fault is occurred. In this case, prior to fault, wind speeds in 4 groups are
around 12 m/s.

Figure 6: System responses to three-phase short circuit fault. (a) AC voltage at the PCC bus. (b) AC voltage
of wind farm. (c) Generators speed. (d) Generated power by WFs (Pw) and power received by ac
network. (e) DC voltage at the two sides of the cable.







The responses of the power system under the three-phase fault are shown in Figure 6. In Figure
6(a), we can observe that the ac voltage at the PCC bus manages to recover completely at 0.5 s after the

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Improvement of Fault Ride-Through Capability in Wind Farms Using VSC-HVDC
335
fault, so the requirement of grid code concerning the voltage mentioned in [12] is met and the WTs do
not have to be tripped. Figure 6(b)-(d) shows that the wind farms are not stressed seriously. So they can
remain connected during and after the fault, contributing to the transient stability of the system. The ac
voltages of wind farms remain actually unchanged during fault and after fault clearance [Figure 6(b)].
As it is shown in Figure 6(c), the generators speed in the wind farm is also slightly impacted. The real
power produced by wind farm (Pw) has a small dip and recovers to its pre-fault generation soon. So the
grid code requirement in which generated power must be recovered to 90% of the available power
within one second is met. Figure 6(e) shows the dc voltage at the two sides of the cable.


5. Conclusion
In this paper, to enhance the wind farm fault ride-through capability, a system of a HVDC link based
on VSCs for wind farm connection to the network is proposed. The proposed technology is simulated
in PSCAD/EMTDC software. The effects of wind speed changes and three-phase fault are investigated
in a case study. Simulation results prove that, the proposed control strategy is able to regulate the
generator speed and ac voltage of wind farm network according to wind speed changes. It is also
possible to regulate ac voltage of transmission and dc voltage during wind speed changes. In the case
of three phase short circuit fault in grid network, the ac voltage of PCC can meet the basic
requirements of grid network concerning voltage. On the other hand, according to simulation results,
wind farm is not significantly impacted in abnormal conditions. So it can remain connecting to the grid
during and after grid short circuit fault and contribute to improve transient stability of the power
system.

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336 Hanif. Livani, Mohsen. Bandarabadi, Yosef. Alinejad, Saeed. Lesan and Hossein. Karimi-Davijani
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Appendix A

Table 1:
Parameters of Permanent Magnet Synchronous Generators (PMSG)

Parameter value
Description
S=2 MVA
Rated power
U=4 kV
Rated voltage
f=50 Hz
Rated frequency
Rs=0.017 pu
Stator winding resistance
XLs=0.064 pu
Stator leakage reactance
Xd=0.55 pu
d axis unsaturated reactance
Xq=1.11 pu
q axis unsaturated reactance
Rkd=0.055 pu
d axis damper resistance
Xkd=0.62 pu
d axis damper reactance
Rkq=0.183 pu
q axis damper resistance
Xkq=1.175 pu
q axis damper reactance
P= 4
Number of poles
J= 2.7 kg/m2 Inertia
constant

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Improvement of Fault Ride-Through Capability in Wind Farms Using VSC-HVDC
337
Appendix B
The voltage level in the transmission network is 110 kV. The short circuit capacity (SCC) in AC grid 1
and AC grid 2 are 350 MVA and 300 MVA respectively. The parameters of AC lines, Load and DC
line are shown as follows:

Table 2:
Parameters of AC lines


Line 1
Line 2
Line 3
Line 4
Line 5
Length (km)
50
70
40
60
80
R (Ω/phase) 9.155
12.817 7.324
10.986
14.648
X (Ω/phase) 23.305
31.196 17.812
26.735 35.657
Y(mho/phase) 1.292.10-4 1.809.10-4 1.034.10-4 1.55.10-4 2.067.10-4

Table 3: Load
characteristics


P (MW)
Q (MVar)
Load 1
39
6
Load 2
24
6
Load 3
18
6

Table 4:
HVDC line Parameters

Parameter value
Description
Rdc = 5 Ω
Resistance of 100 km DC cable
C1=C2= 80 μF
DC-circuit capacitors in HVDC station 1
C3=C4= 120 μF
DC-circuit capacitors in HVDC station 2

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