RECENT ADVANCES in ENERGY & ENVIRONMENT
Low Voltage Ride-Through Capability Improvement of Wind Power
Generation Using Dynamic Voltage Restorer
NAOHIRO HASEGAWA, TERUHISA KUMANO
Department of Electronics and Bioinformatics, School of Science & Technology
Meiji University
1-1-1 Higashi-mita, Tama-ku, Kawasaki-shi, Kanagawa 214-8571
JAPAN
gmdfg278@gmail.com, kumano@isc.meiji.ac.jp
Abstract: - Recently, the total amount of generation from wind power plants has been increased all over the world.
In this situation, a large amount of disconnection of wind generation may give a serious influence in the power
system. Consequently, Low Voltage Ride-Through (LVRT) is now required for wind power plants in many
countries. This paper studies LVRT capability enhancement using Dynamic Voltage Restorer (DVR), especially it
purposes to reduce DVR capacity. It shows that limit of DVR output and only reactive power output achieves to
reduce device MVA rating capacity and energy storage capacity.
Key-Words: - Wind power generation, Fixed-speed induction generator, Fault Ride-Through, Low Voltage RideThrough, Voltage sag, Dynamic Voltage Restorer, Energy storage
turbine and the other is to increase electrical output of
wind generator during fault and after fault clearance.
The method to reduce mechanical input is represented
by pitch angle controlling [2]. The methods to increase
electrical output are represented by using
mechanically switched capacitor [3], Static Var
Compensator (SVC) [4], STATic synchronous
COMpensator (STATCOM) [5]-[7], Unified Power
Quality Conditioner (UPQC) [6],[7], Dynamic
Voltage Restorer (DVR) [8], and Series braking
resistor [9] etc.
Pitch angle controlling can enhance LVRT
capability and has advantage in the cost. But, the
response of change in the pitch angle is slow in general,
so that this technique has a possibility not enough to
enhance LVRT capability. Using mechanically
switched capacitor has also cost-effective. However,
the ability to supply reactive power declines in
proportion to the square of the voltage, thus it may
degrade the contribution of the capacitor to enhance
LVRT capability. The same thing can be said to the
capacitor-based SVC. Reactive power output from
STATCOM during low voltage is larger than SVC or
capacitor, but STATCOM can not output during
voltage sag in order to avoid injection of additional
fault current into the power system. Therefore,
STATCOM has to be operated after fault clearance.
UPQC and DVR have good performance of LVRT
enhancement. However, both techniques often require
1 Introduction
In recent years, the total capacity of wind generation
connected to the power system has been increased
significantly due to its low environmental cost and low
installation cost compared with other renewable
energy. In this situation, the sudden disconnection of
wind power generation due to the power system
disturbance may collapse power balance between the
power supply and the power demand. In response to
this problem, transmission system operators have
revised grid codes in many countries, and they require
Fault Ride-Through (FRT) capability [1]. FRT is to
keep connection of the wind power generator to the
power system when power system disturbance (e.g.
voltage sag and swell, over and under frequency etc.)
occurs. In FRT, the case of voltage sag is called Low
Voltage Ride-Through (LVRT). However, sudden
voltage sag may cause unstable generator speeding
because of an unbalance between input power
(mechanical) and output power (electrical). In order to
meet the LVRT, the stabilization of the generator and
voltage recovery are needed. But it is very challenging
for wind power generation, especially Fixed-Speed
Induction Generator (FSIG) type wind power plants
because it can not control its active and reactive power
outputs.
There are two methods to enhance LVRT capability
of FSIG. One is to reduce mechanical input of wind
ISSN: 1790-5095
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RECENT ADVANCES in ENERGY & ENVIRONMENT
high capital cost because UPQC and DVR need twin
inverter and energy storage device respectively. Series
braking resister is cost-effective but it does not work
effectively after the generator accelerates greatly.
This paper studies LVRT capability enhancement of
FSIG using DVR device, and proposes the method for
decreasing the energy storage and inverter capacity of
DVR. Section 2 describes how to use DVR to FSIG in
order to enhance LVRT capability. Section 3 explains
the simulation model. Section 4 shows numerical
simulation results using EMTP-ATP, where two
simulation cases are described. The first simulation
examines the stabilization effects of the wind
generator when Automatic Voltage Regulator (AVR)
of DVR is operated with its output limitation. The
second simulation shows the case that DVR is
operated after fault clearance. Section 5 is conclusions
in this work.
UPS used for outage compensation or STATCOM.
This conventional role is modified to a wind generator
protection in this work. General configuration of DVR
consists of the series transformer, the harmonic filter,
the voltage source converter and the energy storage
device.
2 Application of DVR to Wind Power
System
One-line diagram of the FSIG type wind farm and
the power system with DVR is shown in Fig.1. In this
figure, DVR boosts up the generator side voltage Vr
regulated by the DVR output voltage VDVR in the event
of supply side voltage Vs sags. By this voltage
insertion, DVR can absorb the excess power that
cannot be exported into the power system from the
generator, and inject necessary reactive power.
Block diagram of DVR controller in this work is
presented in Fig. 2. DVR has the function of AVR
because it aims to keep constant voltage usually.
Although there are some methods of voltage insertion,
In-Phase Compensation (IPC) is adopted in AVR
considering that wind power system is robust against
phase jump. IPC is the method that the injected DVR
voltage is in phase with the supply side voltage
regardless of current and the pre-fault voltage.
TR2
FSIG
TR2
Infinite bus
TR1
VDVR
VPCC
Vs
Vr
TR_DVR
DVR
Fig.1 Fixed-speed wind farm with DVR
There are two influences that the short circuit in the
power system exerts on FSIG type wind generator.
Firstly, the generator accelerates during voltage sag
caused by short circuit, so that it will be disconnected
by the over-speed relay if it exceeds the maximum
tolerable speed. This phenomenon results from the
fact that the active power output from IG declines by
the square of the terminal voltage, while the
mechanical input from the wind turbine is almost
constant. The maximum speed of generator depends
on the residual voltage, the inertia of generator and
turbine, input wind (mechanical) power and the
duration of the fault. Secondly, huge amount of
absorption of reactive power by IG after fault
clearance may disturb terminal voltage recovery. As a
result, the generator is further accelerated, and it will
be tripped by over-speed or under-voltage relays. For
these two reasons, voltage compensation is a good
solution in order to avoid disconnection of wind
generator (i.e. improvement of LVRT capability).
Therefore, the authors use DVR as voltage sag
compensator.
DVR is a series solid state device that connects
power system in order to regulate the load side voltage.
It has been introduced for the purpose of protecting
sensitive load such as semiconductor fabrication plant
from power system disturbance (e.g. voltage sag,
swell, harmonics, fault current etc.). It can compensate
for voltage sag by low device capacity compared with
ISSN: 1790-5095
FSIG
(AVR)
Vr_ref
Vr_d
abc
Vr
－
+
－
+
VDVR_d
PI
dq
VDVR_q
Eq.1
Eq.2
θ
PLL
φ
abc
I
Vr_q
Vr_d
0
Vs
abc
PI
Vr_q
dq
amp
tan-1(Iq/Id)
ψ
Eq. 1: Vd=amp*cosθ, Vq=amp*sinθ [IPC]
Eq. 2: Vd=amp*cos(ψ+φ+θ), Vq=amp*sin(ψ+φ+θ)
Fig.2 DVR control model
167
VDVR
dq
ISBN: 978-960-474-159-5
RECENT ADVANCES in ENERGY & ENVIRONMENT
occurred. Subsection 1 presents the results in case of
using AVR for its original purpose (to keep terminal
voltage to be constant during fault). In particular,
relation between DVR capacity and generator
stabilizing effect for various compensation voltage (by
changing Vr_ref in Fig. 2) is studied. Subsection 2
presents the case that DVR output inserts after fault
clearance without compensation during fault. This
subsection analyzes the influence when the DVR
output voltage phase (ψ in Fig. 2) changes under the
arbitrary voltage insertion.
The generator delivers nominal power (0.877 p.u.
11.4MVA base) to the power system under constant
nominal wind in both simulations. A voltage sag
occurs at infinite bus during t=1.0 to 1.1 s, which
simulates three-phase balanced short circuit. All
simulation cases are carried out numerically using
EMTP-ATP.
“Vr_ref” in this figure is the reference value of the
generator side voltage Vr, Vr is adjusted by using PI
controller in AVR for Vr_ref. In case of using AVR,
VDVR changes depending on Vs and Vr. In addition to
AVR, this control model can set “amp” (amplitude of
VDVR) and “ψ” (phase angle between current and
VDVR). In case of setting “amp” and “ψ”, it can control
active and reactive power independently (Eq.2 in this
figure). Eq.1 is used in case of using IPC as constant
voltage. These schemes are showed in Fig.3 by phasor
diagram.
[IPC (AVR:variable VDVR, Eq.1: constant VDVR)]
I
Vr
φ
Vs
VDVR
[ψ = -90 degree]
(only reactive power)
[ψ = 0 degree]
(only active power)
4.1 Stabilizing effects using AVR
I
φ
φ
VDVR
φ
Fig. 4 shows the simulation results in case of using
AVR. It compares the three cases; (1) no control, (2)
voltage control with the reference value “Vr_ref” 1.0,
and (3) 0.7 p.u. is used as the reference value. In case
of 0.7 p.u. it is active only during fault (t= 1.0 to 1.1 s).
In “No control” case, voltage oscillation and
unsuccessful voltage recovery can be observed (see (a),
(b)), and the generator reaches over-speed limit after
t=3 s (see (c)). This oscillatory behavior of generator
speed can be explained by the mechanical elasticity
between the turbine and the generator. Active power
output from generator is reduced greatly during the
fault, which causes generator over-speeding in
consequence (see (d)). In contrast, both case of using
AVR can compensate terminal voltage, so that active
power output of the generator increases during fault
and generator speed is stabilized within one second
though some speed increase is noted. As a result of this,
the generator does not reach over-speed limit. The
effects of reference value setting on the resultant
acceleration cannot be observed too much. DVR
output (apparent power) is momentarily exceeded 2.0
p.u. (in case of Vr_ref=1.0) immediately after the fault
clearance because of over voltage due to PI controller
delay (see (e)). This problem is expected to be
mitigated by adjustment of PI parameter. Energy
storage capacity of DVR is shown in (f). Though it is
true that the energy storage capacity in the case of
Vr_ref=0.7 p.u. is lower than the case of 1.0 p.u., it
does not increase simply by a factor of 0.7 because of
taking time to stabilize generator.
Ψ= -90
I
Vr
Vs
VDVR
Vs
Vr
Fig.3 Phasor diagram of DVR control method
3 Model Configuration
This section describes simulation model. The
studied system is shown in Fig.1. It is 11.4 MVA (10
MW) wind farm composed of 10 squirrel cage
induction generators with a rating of 1.14 MVA (1
MW). Each generator is connected to DVR by 1.2
MVA transformer (TR2:690V/6600V). Shunt
capacitors are adjusted so that the generator terminal
voltage becomes 1.0 p.u. at nominal output power
operation. Ratings of DVR and the series transformer
are 11.4 MVA. These are connected to the power
system by 11.4 MW transformer (TR1:
6600V/66000V). Wind farm is finally connected to
infinite bus through double circuit transmission line.
These parameters are presented in Appendix (Table 1,
2 and 3.). E.ON LVRT requirement [1] and 1.1 p.u.
generator speed limit are assumed.
4 Simulation Results
In this section the simulation results are shown
concerning the influence of DVR given to the power
system and the wind generator when voltage sag
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RECENT ADVANCES in ENERGY & ENVIRONMENT
1.1
1.4
1.5
1.2
1.06
0.9
Maximum
speed
Energy
(t=1.1s)
Energy
(t=4s)
1.04
1.02
0.6
volta ge [pu]
1.2
Energy [MJ]
Maximum generator speed [pu]
1.0
1.08
0.8
0.6
0.4
No control
0.2
Vref=1.0
Vref=0.7
0.0
0
0.5
1
1.5
2
Time [s]
2.5
3
3.5
4
(a) DVR generator side voltage Vr
0.3
1.2
1.0
1
0
0.2
0.4
0.6
0.8
0.8
1
voltage [pu]
0
Vr_ref [pu]
Fig.5 Maximum generator speed and energy storage
capacity of DVR in case that voltage reference of
AVR is changed
0.4
0.2
No control
Vref=1.0
Vref=0.7
FRT(E.ON)
0.0
0
Fig. 5 shows the relation between the values of
Vr_ref, maximum generator speed and energy storage
capacity of DVR. Vr_ref is chosen from 0.4 p.u. to 1.0
p.u. , energy capacity is measured at t=1.1 s (just after
fault clearance) and t=4.0 s. All these cases can meet
LVRT. Though the maximum speed is decreased and
the absorbed energy at 1.1 s is increased as Vr_ref
increases, the absorbed energy at 4.0 s in case of low
Vr_ref (0.4 and 0.5 p.u.) is more than the case of
Vr_ref=0.6. This result is caused by the fact that it
takes time to stabilize due to low compensation
voltage.
By these results, it can be concluded that slightly
lowering compensation voltage leads to the energy
storage capacity reduction.
0.5
1
1.5
2
Time [s]
2.5
3
3.5
4
(b) voltage at PCC
1.20
Generator Speed [pu]
1.15
1.10
1.05
1.00
0.95
No control
Vref=1.0
Vref=0.7
speed limit
0.90
0
0.5
1
1.5
2
Time [s]
2.5
3
3.5
4
(c) Generator speed
genera tor a ctive power [pu]
4.0
4.2 Stabilizing effects in case of post-fault
initiation of DVR
No control
3.0
Vref=1.0
Vref=0.7
2.0
1.0
0.0
-1.0
-2.0
0.9
1
1.1
1.2
Time [s]
1.3
1.4
1.5
(d) Generator active power output
This subsection studies the case that DVR is
activated after fault clearance and does not use AVR.
Compensation is started at t=1.12 s (1cycle delay after
fault clearance), and DVR voltage amplifier “amp”
(see Fig.2) is decreased from 0.1 p.u. to 0 p.u. in
proportion to the elapsed time from control beginning
to t=4.0 s in order to prevent over voltage after
stabilization. DVR voltage phase “ψ” (see Fig. 2) is set
0 degree and -90 degree, and uses IPC (it follows
supply side phase). The case of 0 and -90 degree
correspond to active power absorption and reactive
power injection respectively, which are defined active
power compensation (APC) and reactive power
compensation (RPC) respectively in this paper.
Simulation results are showed in Fig. 6. It can be
observed that the voltage is decreased greatly and
generator speed increases temporarily in all cases
ISSN: 1790-5095
0.6
2.5
Vref=1.0
Appa rent Power [pu]
2.0
Vref=0.7
1.5
1.0
0.5
0.0
0
0.5
1
1.5
2
Time [s]
2.5
3
3.5
4
3
3.5
4
(e) DVR apparent power
Energy [MJ]
1.40
1.20
Vref=1.0
1.00
Vref=0.7
0.80
0.60
0.40
0.20
0.00
-0.20 0
0.5
1
1.5
2
Time [s]
2.5
(f) DVR energy strage capacity (positive: absorb energy)
Fig.4 Response of the wind turbine and DVR using AVR
169
ISBN: 978-960-474-159-5
RECENT ADVANCES in ENERGY & ENVIRONMENT
because DVR is activated after fault clearance (see (a),
(b) and (c)). However, in all cases of activating DVR
increases voltage after fault clearance, so that it finally
successfully stabilizes generator speed. The effects of
voltage compensation and rotation stabilization are the
biggest in IPC, then RPC. They are smallest in APC. It
is thought that this result, in which RPC has a better
performance than APC, arises from the characteristic
of induction generator. In high generator speed
situation compared with nominal operation point,
induction generator absorbs a large amount of reactive
power, while it cannot generate active power too much.
This is showed in Fig. 7. Therefore, the lack of
reactive power supply from DVR causes voltage drop
because APC cannot supply reactive power at all.
Apparent power outputs from DVR (see (d)) of the
three cases are almost the same, and they become
obviously low capacity compared with the case of
using AVR though simple comparison might be
misleading because of difference in voltage output.
Although they have almost the same apparent power
output, active power outputs are not the same as
shown in (e). APC absorbs the largest active power,
while IPC is the second. RPC does not absorb or inject
active power except for the small oscillation.
Consequently, energy storage capacity of DVR in case
of using RPC is zero, which result may be very helpful
because energy storage device such as battery or
electric double-layer capacitor are expensive now. In
addition, the cases of IPC and APC need bigger energy
storage at t=4 s compared with the case of using AVR
because long compensation time is necessary.
By these results, we conclude that it is possible to
stabilize only by handling reactive power in case of
activating DVR after fault clearance. But, this method
cannot be used in the case that generator reaches speed
limit during fault.
1.2
1.0
volta ge [pu]
0.8
0.6
0.4
0.2
No control
APC
RPC
IPC
0.0
0
0.5
1
1.5
2
Time [s]
2.5
3
3.5
4
(a) DVR generator side voltage Vr
1.2
1.0
volta ge [pu]
0.8
0.6
0.4
No control
RPC
FRT(E.ON)
0.2
APC
IPC
0.0
0
0.5
1
1.5
2
Time [s]
2.5
3
3.5
4
(b) voltage at PCC
1.20
Genera tor Speed [pu]
1.15
1.10
1.05
1.00
0.95
No control
APC
RPC
IPC
speed limit
0.90
0
0.5
1
1.5
2
Time [s]
2.5
3
3.5
4
3
3.5
4
3
3.5
4
(c) Generator speed
0.50
APC
Apparent Power [pu]
0.40
RPC
IPC
0.30
0.20
0.10
0.00
0
0.5
1
1.5
2
Time [s]
2.5
(d) DVR apparent power
0.40
APC
Active Power [pu]
0.30
3
RPC
0.20
IPC
0.10
0.00
-0.10 0
0.5
1
1.5
2
2.5
-0.20
Reactive Power
2
Time [s]
Active Power
(e) DVR active power (positive: absorb power)
1.5
2.50
APC
2.00
1
Energy [MJ]
Generator Output [pu]
2.5
0.5
0
1
1.05
1.1
1.15
1.2
Generator speed [pu]
1.25
1.50
RPC
IPC
1.00
0.50
0.00
1.3
-0.50 0
0.5
1
1.5
2
2.5
3
3.5
4
Time [s]
Fig.7 An example of generator output – speed curve
(f) DVR energy strage capacity (positive: absorb energy)
Fig.6 Response of the wind turbine and DVR in case of
starting operation after fault clearance
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RECENT ADVANCES in ENERGY & ENVIRONMENT
[2] L. Holdsworth, I. Charalambous, J.B. Ekanayake
and N. Jenkins, “Power System Fault Ride
Through capabilities of induction generator based
wind turbines”, Wind Engineering, Vol. 28, No. 4,
pp. 399-412 (2004)
[3] A. Kehrli, M, Ross, “Understanding frid
integration issues at wind farms and solution using
voltage source converter FACTS technology”,
IEEE Power Engineering Society General Meeting,
vol. 3, pp. 1822-1828 (2003)
[4] T. Ahmed, O Noro, E. Hiraki, and M. Nakaoka,
“Terminal Voltage Regulation Characteristics by
Static Var Compensator for a Three-Phase Self
Excited Induction Generator”, IEEE Trans.
Industry Applications, Vol. 40, No. 4, pp. 978-988
(2004)
[5] L. Qi, J, Langston, and M. Steurer, “Applying a
STATCOM for Stability Improvement to an
Existing Wind Farm with Fixed-Speed Induction
Generators”, IEEE Power and Engergy Society
General Meeting, pp. 1-6 (2008)
[6] M. F. Farias, M. G. Cendoya and P. E. Battaiotto,
“Wind Farms in Weak Grids Enhancement of
Ride-Through Capability Using Custom Power
Systems”,
IEEE/PES
Transmission
and
Distribution Conference and Exposition Latin
America 2008, pp. 1-5 (2008)
[7] N.G.Jauamto, M. Basu, M.F. Conlon and K.
Gaughan, “Rating requirements of the unified
power quality conditioner to integrate the
fixed-speed induction generator-type wind
generation to the grid”, IET Renewable Power
Generation, Vol. 3, pp. 133-143 (2009)
[8] H. Gaztanaga, I. Etxeberria Otadui, S. Bacha and
D. Roye, “Fixed-Speed Wind Farm Operation
Improvement by Using DVR Devices”, IEEE
International Symposium on Industrial Electronics
2007 (ISIE 2007), pp. 2679-2684 (2007)
[9] Andrew Causebrook, David J. Atkinson and Alan
G. Jack, “Fault Ride-Through of Large Wind
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[10] S. S. Choi, B. H. Li, and D. M. Vilathgamuwa,
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5 Conclusions
This paper analyzes LVRT enhancement of FSIG
based wind farm using DVR by numerical simulation.
It simulates two DVR control methods, one is to use
AVR by limiting output, while the other is to control
voltage phase of DVR which is activated after fault
clearance. This study concludes the following points.
(1) The stabilizing effect using AVR has good
performance, but DVR capacity and energy
storage capacity tend to become large.
(2) Limiting output using AVR can reduce DVR
capacity and energy storage capacity, but the
required energy capacity might increase in case
of low compensation voltage on the contrary.
(3) The method in which DVR is deactivated during
fault can also stabilize. In particular the method
with only reactive power injection has advantage
because of small storage capacity.
6 Appendix
Table 1 Wind generator parameters (1.14MVA base)
Quantity
Nominal apparent power
Nominal power
Nominal Voltage
Nominal slip
Stator resistance/reactance
Rotor resistance/reactance
Magnetizing reactance
Generator/Turbine inertia
Spring constant
Value
1.14[MVA]
1.0[MW]
690[V]
-0.0091[pu]
0.0063/0.089[pu]
0.0095/0.092[pu]
2.85[pu]
0.5/3.0[s]
0.55[pu]
Table 2 Transformer parameters (self base)
Quantity
[TR1] Primary/secondary voltage
[TR1] apparent power
[TR1] resistance/reactance
[TR2] Primary/secondary voltage
[TR2] apparent power
[TR2] resistance/reactance
[TR_DVR] Primary/secondary voltage
[TR_DVR] apparent power
[TR_DVR] resistance/reactance
Value
66/6.6[kV]
11.4[MVA]
0.008/0.08[pu]
6.6/0.69[kV]
1.2[MVA]
0.008/0.08[pu]
6.6/0.44[kV]
11.4[MVA]
0.008/0.08[pu]
Table 3 Grid parameter (1000MVA base)
Quantity
Line resistance/reactance
Value
0.286/3.217[pu]
References:
[1] J. Schlabbach, “Low Voltage Fault Ride Through
Criteria for Grid Connection of Wind Turbine
Generators”, 5th International conference on
European Electricity Market 2008, pp. 1-4 (2008)
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