Track 3: POWER AND SYSTEMS ENGINEERING
VOLTAGE QUALITY OF GRID CONNECTED WIND TURBINES
Trinh Trong Chuong
Ha Noi University of Industry
ABSTRACT
Grid connected wind turbines may cause voltage quality problems, such as voltage variation and
flicker. This paper discusses the voltage variation and flicker emission of grid connected wind turbines
with doubly-fed induction generators. A method to compensate flicker by using a voltage source
converter (VSC) based STATCOM (Static Synchronous Compensator) is presented, which shows it is
an efficient mean to improve voltage quality.
1. INTRODUCTION.
Recently wind power generation has been
experiencing a rapid development in a global
scale. The size of wind turbines and wind farms
are increasing quickly; a large amount of wind
power is integrated into the power system. As the
wind power penetration into the grid increases
quickly, the influence of wind turbines on the
power quality is becoming more and more
important. Voltage quality, such as voltage
variation and flicker, is an important issue of grid
connection. Voltage variation and flicker are
caused by power flow changes in the grid. The
output power of grid connected wind turbines
may vary considerably due to wind speed
variations, and it is also depend on the wind
power generation technology applied. Variable
speed operation of wind turbines has the
advantage that the faster power variations are not
transmitted to the grid but are smoothed by the
flywheel action of the rotor so as to reduce the
output power fluctuations.
Although variable speed wind turbines have
better performance in comparison with fixed
speed wind turbines, compensation and
mitigation may still become necessary as the
wind power penetration level increases. The
voltage source converter (VSC) based
STATCOM technique has many advantages to
perform compensation, such as relative
independent from the voltage at the connection
point, faster response, flexible voltage control
and smooth reactive power control. Using high
frequency PWM (Pulse Wide Modulation), the
converter will produce smooth current with low
harmonic content. In this paper, the voltage
variation and flicker caused by wind turbines are
analyzed. Then the flicker during continuous
operation is discussed, the factors that affect
flicker emission of wind turbines, such as wind
speed, turbulence intensity, and short circuit
capacity, are illustrated. A method of improving
voltage quality by using a STATCOM is
described. Simulation results show that
STATCOM is an effective means to improve
voltage quality.
2. RELATIONSHIP BETWEEN SYSTEM
VOLTAGE
AND
WIND
POWER
GENERATION
In normal operational condition, the voltage
quality of a wind turbine or a group of wind
turbines may be assessed in terms of the
following parameters:
- Steady state voltage under continuous
production of power
- Voltage fluctuations
+ Flicker during operation
+ Flicker due to switching
Fig. 1 illustrates an equivalent wind power
generation unit, connected to a network with
equivalent short circuit impedance, Zk. The
network voltage at the assumed infinitebusbar
and the voltage at the Point of Common
Coupling (PCC) are Us and Ug, respectively. The
output power and reactive power of the
generation unit are Pg and Qg, which corresponds
toa current Ig.
International Symposium on Electrical & Electronics Engineering 2007 - Oct 24, 25 2007 - HCM City, Vietnam
-176-
Track 3: POWER AND SYSTEMS ENGINEERING
 Sg
Ig = 
U
 g
*
 Pg − jQg
 =
Ug

(1)
The voltage difference, ∆U, between the system
and the connection point is given by:
 Pg − jQg 
U g − U s = ∆U = ZkU g = ( Rk + jX k ) 

 U
g


Rk Pg + X k Qg
Pg X k − Qg Rk
=
+j
= ∆U p + j∆U q
Ug
Ug
(2)
The voltage difference, ∆U, is related to the short
circuit impedance, the real and reactive power
output of the wind power generation unit. It is
clear that the variations of the generated power
will result in the variations of the voltage at PCC.
Us
Z k ∠ψ k
Grid
Ig
∼
PCC Ug
Pg, Qg
Wind
turbine
(a)
U
δ
∆U
Us
∆Uq
(b)
∆Up
Fig 1. A simple system with an equivalent wind
power generator connected to a network. a)
System circuit b) Phasor diagram
Equation (2) indicates the relationship between
the voltage and power transferred into the
system. The voltage difference, ∆U, can be
calculated with load flow methods as well as
other simulation techniques. The voltage at PCC
should be maintained within utility regulatory
limits. Operation of wind turbines may affect the
voltage in the connected network. If necessary,
the appropriate methods should be taken to
ensure that the wind turbine installation does not
bring the magnitude of the voltage outside the
required limits. Fluctuations in the system
voltage (more specifically in its rms value) may
cause perceptible light flicker depending on the
magnitude and frequency of the fluctuation. This
type of disturbance is called voltage flicker, often
it is shortened as flicker. The allowable flicker
limits are generally established by individual
utilities. Rapid variations in the power output
from a wind turbine, such as generator switching
and capacitor switching, can also result in
variations in the RMS value of the voltage. At
certain rate and magnitude, the variations cause
flickering of the electric light. In order to prevent
flicker emission from impairing the voltage
quality, the operation of the generation units
should not cause excessive voltage flicker. The
short term flicker emission, Pst is used to
describe the degree of flicker emissions, and may
be estimated with the coefficient and factors,
cf(Ψk, va) and kf(Ψk) obtained from the
measurements, which are usually provided by
wind turbine manufacturers.
3. VOLTAGE QUALITY ASSESSMENT OF
WIND TURBINES.
A. Steady-state voltage
The utility service voltage and the wind turbine
voltage should be maintained within the utility
limits. Operation of a wind turbine may affect
the steady-state voltage in the connected
network. It is recommended that load-flow
analyses be conducted to assess this effect to
ensure that the wind turbine instllation does not
bring the magnitude of the voltage outside the
required limits.
Depending on the scope of the load-flow
analysis, a wind turbine installation may be
assumed as a PQ node, which may use ten
minutes average data (Pmc and Qmc ) or 60 s
average data (P60 and Q60) or 0.2 s average data
(P0.2 and Q0.2). A wind farm with multiple wind
turbines may be represented with its output
power at the PCC. Ten minute average data (Pmc
and Qmc ) and 60 s average data (P60 and Q60) can
be calculated by simple summation of the output
from each wind turbine, whereas 0.2 s average
data (P0.2 and Q0.2) may be calculated according
to equations (3) and (4) below [1,2,3]:
N wt
P0.2 ∑ = ∑ Pn ,i +
i =1
N wt
Q0.2 ∑ = ∑ Qn ,i +
i =1
N wt
∑ ( P0.2,i − Pn,i )
2
(3)
i =1
N wt
∑ (Q
0.2,i
i =1
− Qn ,i )
2
(4)
where:
Pn,i , Qn,i are the rated real and reactive power of
the individual wind turbine;
International Symposium on Electrical & Electronics Engineering 2007 - Oct 24, 25 2007 - HCM City, Vietnam
-177-
Track 3: POWER AND SYSTEMS ENGINEERING
Nwt is the number of wind turbines in the group.
B. Voltage fluctuations
There are two types of flicker emissions, the
flicker emission during continuous operation and
the flicker emission due to generator and
capacitor switchings. Often, one or the other will
be predominant. The flicker emissions from a
wind turbine installation should be limited to
comply with the flicker emission limits. It is
recommended [2] that in 10 kV-20 kV networks
a flicker emission of Pst = 0.35 as a weighted tenminute average to be considered acceptable for
loads in an installation. This is also assumed to
be acceptable for wind turbine installations,
however, different utilities may have different
flicker emission limits. The assessments of the
flicker emissions are described below.
1) Continuous operation
The flicker emission from a single wind turbine
during continuous operation may be estimated
by [2]:
Pst = c f (ψ k , va )
Sn
Sk
(5)
where:
cf (Ψk, va) is the flicker coefficient of the wind
turbine for the given network impedance phase
angle, Ψk , at the PCC, and for the given annual
average wind speed, va, at hub-height of the wind
turbine.
A table of data produced from the measurements
at a number of specified impedance angles and
wind speeds can be provided by wind turbine
manufactures. From the table, the flicker
coefficient of the wind turbine for the actual Ψk
and va at the site may be found by applying linear
interpolation The flicker emission from a group
of wind turbines connected to the PCC is
estimated using equation (6):
Pst ∑ =
1
Sk
N wt
∑ ( c (ψ
f ,i
i =1
k
, va ) S n ,i )
2
(6)
where:
cf,i (Ψk, va) is the flicker coefficient of the
individual wind turbine;
Sn,i is the rated apparent power of the individual
wind turbine;
Nwt is the number of wind turbines connected to
the PCC.
If the limits of the flicker emission are known,
the maximum allowable number of wind turbines
for connection can be determined.
2) Switching operations
The flicker emission due to switching operations
of a single wind turbine can be calculated as:
Pst = 18 × N100.31 × k f (ψ k )
Sn
Sk
(7)
where
kf(Ψk) is the flicker step factor of the wind
turbine for the given Ψk at the PCC. The flicker
step factor of the wind turbine for the actual Ψk
at the site may be found by applying linear
interpolation to the table of data produced from
the
measurements
by
wind
turbine
manufacturers. The flicker emission from a
group of wind turbines connected to the PCC can
be estimated from [1]:
Pst ∑
3.2 
18  N wt
=  ∑ N10,i ( k f ,i (ψ k ) S n ,i ) 
S k  i =1

0.31
(8)
where
N10,i and N120,i are the number of switching
operations of the individual wind turbine within
10 minute and 2 hour period respectively.
kf,i(Ψk ) is the flicker step factor of the individual
wind turbine;
Sn,i is the rated apparent power of the individual
wind turbine;
Again, if the limits of the flicker emission are
given, the maximum allowable number of
switching operations in a specified period, or the
maximum permissible flicker emission factor, or
the required short circuit capacity at the PCC
may be determined. The flicker assessments can
also be conducted with simulation method if
appropriate simulation models are developed.
The flicker assessment by simulation will be
reported in this paper.
IV. MODELING AND CONTROL OF WIND
TURBINES
WITH
DOUBLY-FED
INDUCTION GENERATORS.
International Symposium on Electrical & Electronics Engineering 2007 - Oct 24, 25 2007 - HCM City, Vietnam
-178-
Track 3: POWER AND SYSTEMS ENGINEERING
The wind turbine with a doubly-fed induction
generator, using back-to-back PWM voltage
source converters in the rotor circuit is a popular
configuration. Fig. 2 illustrates the main
components of such a grid connected wind
turbine. Two of main control schemes used for
the wind turbine are speed control and pitch
control. The speed control can be realized by
adjusting the generator power or torque. The
pitch control is a common control method to
regulate the aerodynamic power from the
turbine. Vector-control techniques have been
well developed for doubly-fed induction
generators using back-to-back PWM converters
[4]. Two vector-control schemes are applied
respectively for the rotor-side and grid-side
PWM converters, as shown in Fig. 2. Normally,
the objective of the vector-control scheme for the
grid-side PWM converter is to keep the DC-link
voltage constant and to remain a unity power
factor for the power flow between the rotor
circuit and the grid. It may also be responsible
for controlling reactive power flow into the grid.
The vector-control scheme for the rotor-side
PWM converter ensures decoupling control of
stator-side active and reactive power. It provides
the generator with wide speedrange operation,
which enables the optimal speed tracking for
maximum energy capture from the wind.
Fig. 2. Block diagram for the vector-control
schemes of doubly fed induction generator.
V. VOLTAGE COMPENSATION
Voltage variation and flicker emission of grid
connected wind turbines is related to many
factors, including.
- Mean wind speed, v
- Turbulence intensity, In
- Short circuit capacity ratio, SCR = S k / S n
where Sk is the short circuit capacity of the grid
where the wind turbines are connected, Sn is the
rated power of the wind turbine.
The voltage difference in (2) may be
approximated as:
∆U ≈ ∆U p =
Pg Rk + Qg X k
Ug
(9)
With the grid impedance angle ψk and the wind
turbine power factor angle ψ being defined as:
tagψ k = X k / Rk
tagψ = Qg / Pg
(10)
Equation (9) can be written as:
P R (1 + tagψ k .tagψ ) Pg Rk cos(ψ −ψ k )
∆U p = g k
=
Ug
U g cosψ k cosψ
(11)
It can be seen from equation (11) that when the
difference between the grid impedance angle ψk
and the wind turbine power factor angle ψ
approaches 90 degrees, the voltage fluctuation is
minimized. Equation (11) also indicates if the
reactive power can be regulated with the real
power generation, voltage variation and flicker
will be minimized.
The variable speed wind turbine with doubly-fed
induction generator is capable of controlling the
output active and reactive power respectively.
Normally the output reactive power of the wind
turbine is controlled as zero to keep the unity
power factor. It is possible to control the output
reactive power appropriately with the variation
of the output real power so that that may cancel
the voltage changes from the real power flow
from the reactive power flow.
It is also possible to change the reactive power
flow by using reactive shunt compensators, such
as STATCOM, to mitigate the voltage variation
during normal operation of grid connected wind
turbines.
The STATCOM consists of a controllable PWM
voltage source converter, which generates a
voltage with the amplitude and phase being
continuously and rapidly controlled, so as to
effectively perform reactive power control. By
control of the voltage source converter output
voltage in relation to the grid voltage, the voltage
source converter will appear as a generator or
absorber of reactive power. A circuit
configuration of using STATCOM as a
compensation device is shown in Fig. 3.
International Symposium on Electrical & Electronics Engineering 2007 - Oct 24, 25 2007 - HCM City, Vietnam
-179-
Track 3: POWER AND SYSTEMS ENGINEERING
PWT
QStatcom
PWT
QWT = 0
Wind
turbine
G
2
Z
1
Zth
Eth
PStatcom = 0
QStatcom
Statcom
Fig. 3: STATCOM and wind turbine in the
system for voltage quality improvement.
In this study, the STATCOM is connected in
shunt to bus 2 (PCC) to mitigate the flicker level
during continuous operation of the grid
connected wind turbine. The reactive power
generated or absorbed by the STATCOM may be
varied with the output active power of the wind
turbine. Since the wind turbine output reactive
power QWTG is normally controlled as zero to
keep the unity power factor, regulating the
reactive power generated or absorbed by the
STATCOM, QSTAT , may change the reactive
power flow on the line 1-2. As mentioned before,
when the difference between the grid impedance
angle ψk and the line power factor angle ψ
approaches 90 degrees, the flicker emission is
minimized. Therefore the reactive power
absorbed by the STATCOM, QSTAT , can be
controlled in proportion to the wind turbine
output active power, PWTG , so that the power
factor angle ψ of the line 1-2 is adjusted at the
value of 90 + ψk degrees.
A vector-control approach can be used, with a d
q reference frame such positioned to enable
independent control of the real and reactive
power between the grid and STATCOM. The
PWM converter is current regulated, with the d
axis current regulating the DC-link voltage and
the q-axis current regulating the reactive power.
Fig. 4 shows the schematic of the STATCOM
control system.
Fig. 4. Vector-control scheme for STATCOM.
VI. SIMULATION STUDIES
A. Simulation model
The simulation model of the wind turbine is
developed in the dedicated power system
analysis tool, PSCAD/EMTDC. A complete
wind turbine model includes the wind speed
model, the aerodynamic model of the wind
turbine, the mechanical model of the
transmission system and models of the electrical
components, namely the induction generator,
PWM voltage source converters, transformer, the
control and the supervisory system. The system
for the simulation study is shown in Fig. 3,
where a wind farm with doubly-fed induction
generators represented by a single machine
which is integrated to the external power system
represented by a constant voltage source
connected in series with its Thevenin’s
equivalent impedance. Bus 2 is the point of
common coupling. The external power system is
connected to bus 2 through a line 1-2. The wind
turbine generates 2 MW real power during the
rated state operation, while the output reactive
power of the wind turbine is controlled to be zero
to keep the unity power factor. Fig. 5 shows the
wind speed and output power produced by the
simulation model and the corresponding power
spectra are shown in Fig. 6 where the 3p effect
can be clearly seen.
International Symposium on Electrical & Electronics Engineering 2007 - Oct 24, 25 2007 - HCM City, Vietnam
-180-
Track 3: POWER AND SYSTEMS ENGINEERING
Fig. 5. Wind speed and output power of a wind
turbine.
Fig. 6. Spectrum of wind speed and output power
of the wind turbine.
The level of flicker is quantified by the short
term flicker severity Pst, which is normally
measured over a ten-minute period. According to
IEC standard IEC 61000-4-15 [5], a flickermeter
model is built to calculate the short-term flicker
severity Pst as shown in Fig. 7.
Fig. 7. Flickermeter model according to IEC
61000-4-15.
B. Results of flicker simulation study
As shown in Fig. 8, when the angle difference
(ψ − ψ kl ) approaches to 90 degrees, the flicker
level is minimized. It indicates that the flicker
level is significantly reduced if the angle
difference ( ψ − ψ kl ) is regulated to be 90
degrees by controlling the reactive power flow of
the STATCOM. The variation of short-term
flicker severity Pst with mean wind speed is
illustrated in Fig. 9. As it is shown, at low wind
speeds (less than 7.5 m/s), the Pst value is very
low due to little output power. Then the Pst value
increases approximately linearly with the mean
wind speed until it reaches 11.5 m/s, where the
pitch angle begins to change obviously. For
higher wind speeds, where the wind turbine
reaches rated power, the pitch angle modulation
reduces the turbulence-induced fluctuations
reflected in the output power of the wind turbine,
which results in the reduced flicker levels. The
relationships between the Pst and turbulence
intensity varies with different mean wind speeds
are given in Fig. 10 and Fig. 11. As it is shown in
Fig. 10, in low wind speeds (for example, 9 m/s),
the Pst has an almost linear relation with the
turbulence intensity. The more turbulence in the
wind results in larger flicker emission. However,
in high wind speeds (for xample, 18 m/s) as
shown in Fig. 11, where the wind turbine is
controlled to keep the rated output power, the
relationship between the Pst and turbulence
intensity is quite different. When the turbulence
intensity of the wind is low, the wind profile
varies in a small range that corresponds to a rated
output power. The Pst value is low due to little
power fluctuation as a result of the aerodynamic
control. As the turbulence intensity increases, the
wind profile changes significantly which results
in a large variation of output power. As a
consequence, the flicker emission becomes
serious.
Fig. 12 illustrates an approximately inversely
proportional relationship between the short-term
flicker severity Pst and the short circuit capacity
ratio. The higher the short circuit capacity ratio,
the stronger the grid that the wind turbine is
connected. As expected, the wind turbine would
produce greater flicker in weak grids than in
stronger grids. From Figs. 9–12, it can be
concluded that the STATCOM is an effective
means for flicker mitigation by controlling the
reactive power flow regardless of mean wind
speed, turbulence intensity and short circuit
capacity ratio.
International Symposium on Electrical & Electronics Engineering 2007 - Oct 24, 25 2007 - HCM City, Vietnam
-181-
Track 3: POWER AND SYSTEMS ENGINEERING
Fig. 8. Short-term flicker severity, Pst, variation with
angle difference, ψ- ψk, (v=9m/s, ln=0.1, SCR=20, ψk
= 63.4 o )
Fig. 9. Short-term flicker severity, Pst, variation with
mean wind speed ( ln=0.1, SCR=20, ψk =63.4o ) _ _
_*_ _ _ _ : without STATCOM, --- --_----- -- : with
STATCOM.
Fig. 11. Short-term flicker severity, Pst, variation with
turbulence intensity (v=18m/s, SCR=20, ψk =63.4o ) _
_ _*_ _ _ _ : without STATCOM ------_-- ----- : with
STATCOM.
Fig. 12. Short-term flicker severity, Pst, variation with
short circuit capacity ratio (v=9m/s, ln=0.1, ψk =63.4o
) _ _ _*_ _ _ _ : without STATCOM, ------ _------- :
with STATCOM.
VII. CONCLUSIONS
Fig. 10. Short-term flicker severity, Pst, variation with
turbulence intensity (v=9m/s, SCR=20, ψk =63.4o ) _
_ _*_ _ _ _ : without STATCOM, --- --_--- ---- : with
STATCOM.
This paper discusses the voltage quality issue of
grid connected doubly fed induction generator
wind turbine systems. The voltage quality
requirements in normal operation, voltage
variation and flicker, are discussed. It is shown
that the voltage variation and flicker caused by
the variation of real power generation may be
compensated by controllable reactive power. A
STATCOM connected in shunt in the system can
provide fast and smooth reactive power control,
therefore, effectively control the system voltage
level and mitigate the flicker during continuous
operation of the grid connected wind turbines.
The flicker mitigation has been studied in detail
by simulation, the results show that the
STATCOM is an effective means to improve
system voltage quality.
International Symposium on Electrical & Electronics Engineering 2007 - Oct 24, 25 2007 - HCM City, Vietnam
-182-
Track 3: POWER AND SYSTEMS ENGINEERING
VIII. REFERENCES
[1] IEC Standard 61400-21; Measurement and
Assessment of Power Quality of Grid Connected
Wind Turbines.
[2] N.Dizdarevic, M.Majstrovic, D. Bajs…;
Connection Criteria at the Distribution network
for Wind Power Plants in Croatia; Energy
Institute HRVOJE POZAR, Zagreb, Croatia,
2004..
[3] IEC 61400-12; Wind turbine generator
systems. Power performance measurement
techniques (2001).
[4] R. Pena, J. C. Clare, G. M. Asher; Doubly fed
induction generator using back-to-back PWM
converters and its application to variablespeed
wind-energy generation; IEE Proc. Electr.
Power Appl., vol. 143, no. 3, pp. 231-241, May
1996.
[5]
IEC
61000-4-15;
Electromagnetic
Compatibility (EMC), Part 4: Testing and
measurement
techniques,
Section
15:
Flickermeter,
Functional
and
design
specifications; Bureau Central Commission
Electrotech. Int., Geneva, Switzerland, Nov.
1997.
[6] Ake Anderson; Power quality of wind
turbines; Thesis Degree of Doctor of Philosophi;
Chalmers Unversity of Technology; Goteborg
Swedent 2000.
[7] F. Blaabjerg, Z. Chen; Power electronics as
an enabling technology for renewable energy
integration; Journal of Power Electronics, vol. 3,
no. 2, April 2003.
[8] N. G. Hingorani, L. Gyugyi; Understanding
FACTS: Concepts and technology of flexible AC
transmission systems; New York: IEEE press,
2000.
[9] Z. Zhang, N. R. Fahmi, W. T. Norris; Flicker
analysis and methods for electric arc furnace
flicker (EAF) mitigation (a survey); in 2001
IEEE Porto Power Tech Proceedings, Porto,
Portugal, Sep. 2001.
[10] T. Larsson, C. Poumarede; STATCOM, an
efficient means for flicker mitigation; in
Proceedings of the IEEE Power Engineering
Society 1999 Winter Meeting, New York, 1999.
[11] M.P. Papadopoulos, S.A. Papathanassiou,
S.T. Tentzerakis, N.G. Boulaxis; Investigation of
the flicker emission by grid connected wind
turbines; in Proceedings of the 8th International
Conference on Harmonics And Quality of Power,
Athens, Greece, Oct. 1998.
International Symposium on Electrical & Electronics Engineering 2007 - Oct 24, 25 2007 - HCM City, Vietnam
-183-