AckermannTh. (ed) Wind Power in Power Systems
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turbine. As can be seen in Figure 16.3, a low X/R ratio will increase the voltage at the
PCC whereas a high X/R ratio will lower the voltage. Low X/R ratios are found mainly
in weak grids in rural areas where long overhead lines with a voltage level below 40 kV
are used.
Figure 16.4 presents the measured voltage variation of a wind turbine at Risholmen,
Sweden. The calculated voltage variation obtained by Equation (16.1) is a lso shown. At
this specific site, the short-circuit ratio is 26 and the X/R ratio at the PCC is approxi-
mately 5.5. As can be seen in Figure 16.4, the voltage variations are within tight limits, at
this specific site, because of a high X/R ratio at the PCC.
0.98
0.99
1.00
1.01
1.02
1.03
1.04
0 0.2 0.4 0.8 1.0
P (p.u.)
U
2
(p.u.)
X/R = 1
X/R = 0
X/R = infinity
0.6
Figure 16.3 Voltage variations at different ratios of reactance, X, to resistance, R. The short-
circuit ratio is constant. Note : U
2
¼ fixed voltage at point of common connection; P ¼ active
power
0.994
0.996
0.998
1.000
1.002
1.004
1.006
0 0.2 0.4 0.6 0.8 1.0
Active Power, P (p.u.)
Voltage, U (p.u.)
Figure 16.4 Measured (solid line) and calculated (dashed line) voltage variations of the wind
turbine site at Risholmen, Sweden
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16.3 Flicker
Flicker is the traditional way of quantifying voltage fluctuations. The method is based
on measurements of variations in the voltage amplitude (i.e. the duration and
magnitude of the variations). Flicker is included in standard IEC 60868 and Amend-
ment 1 (IEC, 1990a; IEC, 1990b). Figure 16.5 shows the magnitude of maximum
permissible voltage changes with respect to the number of voltage changes per second,
according to IEC 60868.
The fluctuations are weighted by two different filters. One filter corresponds to the
response of a 60 W light bulb and the other filter corresponds to the response of the
human eye and brain to variations in the luminance of the light bulb (Walker, 1989).
Flicker from grid-connected wind turbines has been the subject of several investiga-
tions (Gardner, 1993; Saad-Saoud and Jenkins, 1999; Sørensen et al., 1996). Flicker
from wind turbines occurs in two different modes of operation: continuous operation
and switching operations.
16.3.1 Continuous operation
Flicker produced during continuous operation is caused by power fluctuations. Power
fluctuations mainly emanate from variations in the wind speed, the tower shadow effect
and mechanical properties of the wind turbine.
Pitch-controlled turbines will also have power fluctuations caused by the limited
bandwidth of the pitch mechanism, in addition to fluctuations caused by the tower
shadow. This is valid for all pitch-controlled turbines. The method to overcome this
drawback and to prevent such power fluctuations from being transferred to the grid is to
use a variable-speed generator in the wind turbine. The power of pitch-controlled
turbines is controlled by the angle of the blades. This means that at high wind speeds,
normally between 12–14 m/s, and the cut-off wind speed, at 20–25 m/s, the steady-state
0.1
1
10
0.001 0.01 0.1 1 10 100
Frequency,
f (Hz)
Voltage variation, ∆U/U (%)
Figure 16.5 Flicker curve according to IEC 60868 (IEC, 1990a)
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value of the power output should be kept close to the rated power of the generator. This is
achieved by pitching the blades. Figure 16.6(a) shows the steady-state value of the power
(solid line) as a function of wind speed. At wind speeds exceeding 12 m/s the steady-state
value of the power is kept equal to rated power. However, pitching the blades implies that
the power curve shifts. This is illustrated in Figure 16.6(a) where the dotted line shows the
instantaneous power curve, when at a wind speed of 15 m/s the blades are pitched for
operation at rated power.
Unfortunately, the wind speed is not constant but varies all the time. Hence, instan-
taneous power will fluctuate around the rated mean value of the power as a result of
gusts and the speed of the pitch mechanism (i.e. limited bandwidth). As can be seen in
Figure 16.6(a), variations in wind speed of 1 m/s give power fluctuations with a
magnitude of 20 %. Figure 16.6(b) shows the power from a stall-regulated turbine
under the same conditions as the pitch-controlled turbine in Figure 16.6(a). Variations
4 6 8 101214161820
Wind speed (m/s)
Power (p.u.)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Steady-state power
Instantaneous power
(a)
Wind speed (m/s)
Power (p.u.)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
4 8 10 12 14 16 18 206
(b)
Figure 16.6 Power as function of wind speed from (a) a pitch-controlled turbine and (b) a stall-
regulated turbine. Reprinted, with permission, from A. Larsson, ‘Flicker Emission of Wind Turbines
During Continuous Operation’, IEEE Transactions on Energy Conversion 17(1) 114–118 Ó 2002 IEEE
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in wind speed cause also power fluctuations in the stall-regulated turbine, but they are
small in comparison with those in a pitch-controlled turbine.
Figure 16.7 shows the measured power of a Type A1 wind turbine with a rated power
of 225 kW in high winds. The figure shows variations in the power produced by the
turbine. As previously mentioned, Type A wind turbines produce power pulsations as a
result of wind speed gradients and the tower shadow.
(1)
The frequency of the power
pulsations is equal to the number of blades multiplied by the rotational speed of the
turbine (i.e. the 3-p frequency). The figure also indicates the power fluctuations caused
by gusts and the speed of the pitch mechanism.
Measurements are required to determine flicker emission produced during the con-
tinuous operation of a wind turbine. IEC 61400-21 warns that flicker emission should
not be determined from voltage measurements, as this method will be influenced by the
background flicker of the grid (IEC, 1998). The method proposed to overcome this
problem is based on measurements of current and voltage. The short-term flicker
emission from the wind turbine should be calculated based on a reference grid using
the measured active and reactive power as the only load on the grid.
16.3.2 Switching operations
Switching operations will also produce flicker. Typical switching operations are the
startup and shutdown of a wind turbine. Startup, stop and switching between generators
or generator windings will cause a change in power production. The change in power
production will lead to voltage changes at the PCC. These voltage changes will, in turn,
cause flicker.
180
200
220
240
260
024681012
Power,P (kW)
Time (s)
Figure 16.7 Measured power during normal operation of a Type A1 wind turbine (solid line). In
the figure, the steady-state power is also plotted (dotted line). Reprinted, with permission, from
A. Larsson, Flicker ‘Emission of Wind Turbines During Continuous Operation’, IEEE Transactions
on Energy Conversion 17(1) 114–118 Ó 2002 IEEE
(1)
For definitions of wind turbine Types A to D, see Section 4.2.3.
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16.3.2.1 Startup
The startup sequences of variable-speed wind turbines as well as fixed-speed wind
turbines are all different. Variable speed wind turbines are normal ly equipped with
pitch control. Generally, owing to the controllable speed of the turbine and the pitch
control, the starting sequence of variable-speed wind turbines is smoother than that of
fixed-speed wind turbines.
Fixed-speed wind turbines
Figure 16.8(a) shows the measured power during the startup of a Type A1 wind turbine.
The startup of the turbine occurs at time t ¼ 30 s. As can be seen, the wind turbine
consumes reactive power in order to magnetise the generator. The soft-starter operates
for two or three seconds in order to limit the current to the rated value. The reactive
power is then compensated by shunt capacitor banks. It can be seen that the capacitors
are switched in four steps, at time intervals of approximately 1 s. Once all capacitor
banks have been switched in at approximately t ¼ 35 s, the blades of the turbine are
pitched, which increases power production. The power production also affects the
reactive power consumption.
Figure 16.8(b) shows the corresponding terminal voltage of the wind turbine. The
voltage change caused by the startup of the turbine can be divided into two parts. The
first part is caused by the reactive power consumption of the generator. As can be seen,
the reactive power consumption causes a voltage drop. Once the capacitors are con-
nected and the reactive power consumption returns to zero, the voltage level is restored.
The second part is caused by the power production. As the power production increases,
the voltage level begins to rise.
Variable-speed wind turbine
Figure 16.9 shows the active and reactive power during the startup of a Type D1 wind
turbine in high winds. At time t ¼ 30 s the generator is connected to the grid via the
converter. The active power increases smoot hly from zero to half the rated power in 30 s.
During this period the active power increases; the reactive power is controlled in order
to keep the power factor constant. At this particular site, a power factor of 0.98 was
chosen. Again, the starting sequence of variable-speed wind turbines is smoother than
that of fixed-speed wind turbines.
16.3.2.2 Shutting down
If the wind speed becomes either too low or too high, the wind turbine will stop
automatically. In the first case the turbine will be stopped in order to avoid a negative
power flow. In the second case it will be stopped to avoid high mechanical loads. At low
wind speeds (3–4 m/s) the active power is almost zero. The stop will be rather soft and
the impact on the voltage at the PCC will be small at such low wind speeds. The impact
may be larger at high wind speeds (greater than 25 m/s) since the turbine produces at
rated power on these occasions. If the turbine is stopped and the power decreases from
rated power to zero production, the voltage at the PCC will be affected.
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Fixed-speed wind turbines
Figure 16.10 shows the stop of a Type A0 wind turbine with a rated power of 600 kW.
The wind turbine operates at approximately half the rated power when the turbine is
stopped. Once the turbine is stopped, the capacitor bank for reactive power compensation
is switched off. After a couple of seconds, brakes reduce the turbine speed. Figure 16.10
illustrates that the turbine’s speed is reduced by braking at time t ¼ 15 s. In order
–300
–200
–100
0
100
200
300
400
0 6 12 18 24 30 36 42 48 54 60
Active power, P (kW)
Reactive power, Q (kVAR)
Time (s)
Power
(a)
388
392
396
400
404
408
12 18 24 30 36 42 48 54 60
Voltage, U (V)
Reactive power
compensation
Active
power
production
06
Time (s)
(b)
Figure 16.8 (a) Measured power and (b) Measured voltage during startup of a Type A1 wind
turbine with a rated power of 600 kW Reprinted, with permission, from A. Larsson, ‘Flicker
Emission of Wind Turbines caused by Switching Operations’, IEEE Transactions on Energy
Conversion 17(1) 119–123 Ó 2002 IEEE
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to make sure that the turbine is actual ly stopped, the generator is disconnected when the
power is reversed.
Variable-speed wind turbines
Figure 16.11 shows the stop of a Type D1 wind turbine in high winds. The turbine
operates at rated power. The power from the wind turbine begins to decrease at time
t ¼ 6 s. Four seconds later, the power has dropped from rated power to zero. Similar to
the startup, the stop is very gentle and smooth.
–100
0
100
200
300
0
10 20 30 40
50
60
Time (s)
Active power, P (kW)
Reactive power, Q (kVAR)
Power
Figure 16.9 Measured power during startup of a Type D1 wind turbine with a rated power of
500 kW Reprinted, with permission, from A. Larsson, ‘Flicker Emission of Wind Turbines caused
by Switching Operations’, IEEE Transactions on Energy Conversion 17(1) 119–123 Ó 2002 IEEE
–500
–400
–300
–200
–100
0
100
200
300
400
0 5 10 15 20 25 30
Time (s)
Active power, P (kW)
Reactive power, Q (kVAR)
Power
Figure 16.10 Measured power during stop of a Type A0 wind turbine with a rated power of
600 kW Reprinted, with permission, from A. Larsson, ‘Flicker Emission of Wind Turbines caused
by Switching Operations’, IEEE Transactions on Energy Conversion 17(1) 119–123 Ó 2002 IEEE
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According to IEC 61400-21, switching operations have to be measured during the cut-in
of a wind turbine and for switching operations between generators. Switching operations
between generators are only relevant for wind turbines with more than one generator or a
generator with multiple windings.
16.4 Harmonics
Voltage harmonics are virtually always present on the utility grid. Nonlinear loads,
power electronic loads, and rectifiers an d inverters in motor drives are some sources that
produce harmonics. The effects of the harmonics include overheating and equipment
failure, faulty operation of protective devices, nuisance tripping of a sensitive load and
interference with communication circuits (Reid, 1996).
Harmonics and interharmonics are defined in IEC 61000-4-7 and Amendment 1 (IEC
1991, 1997). Harmonics are components with frequencies that are multiples of the
supply frequency (i.e. 100 Hz, 150 Hz, 200 Hz, etc.). Interharmonics are defined as
components having frequencies located between the harmoni cs of the supply frequency.
The signal that is to be analysed is sampled, A/D-converted and stored. These samples
form a window of time (‘window width’) on which a discrete Fourier transformation is
performed. The window width, according to the standard, has to be 10 line-periods in a
50 Hz system. This window width will provide (the) distance be tween two consecutive
interharmonic components of 5 Hz. Figure 16.12 shows the interharmonic components
between 1250 Hz and 1300 Hz of the measured current from a variable-speed wind
turbine. The current has been analysed in accordance with IEC 61000-4-7.
Fixed-speed wind turbines are not expected to cause significant harmonics and inter-
harmonics. IEC 61400-21 does not include any specifications of harmonics and inter-
harmonics for this type of turbine. For variable-speed wind turbines equipped with a
–200
–100
0
100
200
300
400
500
600
0 5 10 15
Time (s)
Active power, P (kW)
Reactive power, Q (kVAR)
Power
Figure 16.11 Measured power during stop of a Type D1 wind turbine with a rated power of
500 kW Reprinted, with permission, from A. Larsson, ‘Flicker Emission of Wind Turbines caused
by Switching Operations’, IEEE Transactions on Energy Conversion 17(1) 119–123 Ó 2002 IEEE
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converter, the emission of harmonic currents during continuous operation has to be
specified. The emission of the individual harmonics for frequencies of up to 50 times the
fundamental grid frequency are to be specified, as well as the total harmonic distortion.
According to IEC 61000-4-7, the following equation has to be applied to determine the
harmonic currents that origi nate from more than one source connected to a common
point:
I
n
¼
X
k
I
n; k
!
1=
; ð 16 :2Þ
where I
n
is the harmonic current of order n; I
n, k
is the harmonic current of order n from
source k; and is an exponent, chosen from Table 16.1. This recommendation is valid
for wind farm applications.
Figure 16.13 shows the harmonic content of two Type D1 wind turbine equipped with
forced-commutated inverters. The harmonic content is obtained in two different ways. One
way is to use Equation (16.2) and the measured time series from each wind turbine. The
other way is to use the measured time series of the total current from both wind turbines.
0.0
0.2
0.4
0.6
0.8
1.0
1250 1255 1260 1265 1270 1275 1280 1285 1290 1295 1300
Frequency (Hz)
I
n
/I
1
(%)
Figure 16.12 Current interharmonic content between 1250 Hz and 1300 Hz. Note: I
n
¼ harmonic
current of order n
Table 16.1 Exponent, , for harmonics
Harmonic number, n
1 n < 5
1.4 5 n 10
2 n > 10
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16.5 Transients
Transients seem to occur mainly during the startup and shutdown of fixed-speed wind
turbines (Demoulias and Dokopoulos 1996). The startup sequence of a Type A wind
turbine is performed in two steps. First, the generator is switched on. A soft-starter is
used to avoid a high in-rush current. Once the soft-starter begins to operate and the
generator is connected to the grid, the shunt capacitor banks are switched in. The shunt
capacitor banks are switched directly to the grid without any soft switching devices. Once
the shunt capacitor banks are connected, a high current peak occurs (see Figure 16.14).
This transient may reach a value of twice the rated wind-turbine current and may
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
2 6 10 14 18 22 26 30 34 38
Harmonics number, n
I
n
/I
1
(%)
E2 + E3
IEC
Figure 16.13 Calculated harmonic content from two Type D1 wind turbines equipped with
forced-commutated inverters (i) by using the measured time series of current from each wind
turbine (E2 þ E3) and by using Equation (16.2) in text and (ii) by using the measured time series of
the total current from both wind turbines (IEC). Note: I
n
¼ harmonic current of order n; IEC ¼
standard IEC 61400-21 (IEC, 1991, 1997)
–1000
–500
0
500
1000
3.3 3.31 3.32 3.33 3.34 3.35
Time (s)
Current, I (A)
Figure 16.14 Measured oscillating current when connecting shunt capacitors during the startup
sequence of a 225 kW wind turbine of Type A1
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