Power Quality and Grid Connection of Wind Turbines
M. A. Nielsen
Product Manager
Vestas Wind Systems A/S
Smed Hansens Vej 27
6940 Lem Denmark
Abstract
This paper describes the effect of grid connected wind turbines on power quality. Special attention has been paid to
stationary voltage conditions, flicker, and harmonics.
In connection with power quality it is essential to distinguish between 1) conventional wind turbines with fixed
rotational speed, 2) inverter connected wind turbines with variable rotational speed and 3) Optislip wind turbines with
semi-variable rotational speed.
The paper presents a comparison between the three concepts in relation to power quality. Part of the comparison is
based on actual measurements.
1
Introduction
An increasing number of wind turbine installations and ever more wind power capacity connected to the grid has made
the impact of wind turbines on voltage quality a major issue. To avoid unnecessarily conservative limits on the amount
of wind power that can be connected to an electric grid, it is a necessity to be able to predict wind turbines impact on
voltage quality at the Point of Common Coupling (PCC). This can only be done if the power quality parameters and
characteristics of the wind turbine are known.
The costs of new transmission lines are considerable. The costs of grid connection are therefore to a large extent
dependent on whether the wind turbine installation can be connected to the local grid, or whether new transmission
capacity has to be installed. Whether or not the local grid can be used without reinforcement depends on serval factors,
such as the wind characteristics, the wind turbine design, the grid stiffness etc.
The high costs of grid reinforcement is a major barrier to further expansion of wind power. It is therefore important to
be able to point out methods of reducing those costs, while at the same time maintaining standards of power quality.
This paper describes grid connected wind turbines effect on power quality. Special attention has been paid to stationary
voltage conditions, flicker, harmonics and wind turbine design. The paper is made by using publications from
Risø/DEFU and TC88 (Technical Committee 88).
In connection with turbine design and power quality it is essential to distinguish between:
a)
Conventional wind turbines with fixed rotational speed, stall or pitch
b)
Inverter connected wind turbines with variable rotational speed.
c)
Optislip wind turbines with semi variable rotational speed.
controlled.
The paper also presents a comparison based on measurements between the different concepts in relation to power
quality.
References is made to the "strength" or "stiffness" of a point on a electricity network. A "week" point on a network is
one where changes in the real and reactive power flows into or out of the grid will cause significant changes in the
voltage at that point, and at neighbouring points on the grid. Therefore the stronger the grid, the less likely it is that
wind turbines will cause power quality problems. Grids in remote areas are generally weaker than in urban or industrial
areas. Weak grids can also be referred to as having a "low short-circuit level" or a "low fault level".
2
Stationary Voltage Conditions
In Denmark DEFU (Research association of Danish utilities) prepares operational recommendations based on, among
other things, Europeans standard. The DEFU recommendation KR-77 /1/ gives a simplified method of assessing
stationary voltage conditions for grid connection of wind turbines. It says that the wind turbine can be connected to
10kV radial with consumers without further check of the stationary voltage conditions, provided that the wind turbine
alone does not give rise to an increase in voltage ∆U
∆U =
U0 − U R ⋅ P + Q ⋅ X
=
≤ 0,01
U
U2
The simplified consideration above has been used, and is still being used in many countries in relation to grid
connection of wind turbines.
The increasing share of wind power and lager wind turbines has made it interesting to assess the voltage conditions of
grid connection of wind turbines in more detail by using load flow analyses. In a load flow calculation, the voltage in
each node is determined for a given load situation. In a radial with turbines and consumers, minimum voltage will
typically occur at zero production and maximum load, and maximum voltage at maximum production and minimum
load. If both the minimum and maximum voltage are within the permissible levels according to current practice, the
situation can be accepted; otherwise the conclusion is that the grid must be reinforced.
To be able to make the above mentioned calculations, it is necessary to have precise and verified information regarding
maximum output power and reactive power.
2.1
Maximum output power
The wind turbine maximum output power is an essential characteristic in determination of the required grid stiffness at
the PCC of the wind turbine installation. The following information are therefore necessary:
Reference power; defined as the maximum point of the power curve specified according to IEC 1400-12 /2/.
Maximum continuous power; defined as 10 minutes average power which shall not be exceeded irrespective of
weather (wind and air density) and grid conditions. In practical terms this means that the wind turbine must have a
mechanism that controls operation so that continuous power never exceeds a certain limit. The maximum continuous
power can be found by assessing the control system of the turbine or by measurements according to IEC 1400-12 /2/.
A stall controlled wind turbine with fixed speed has large variations in the continuous power due to the behaviour of the
stall control and variation in air density. The pitch controlled wind turbine with fixed or variable speed provides a ideal
output limitation of the continuous power due to the active pitch/power control.
Maximum instantaneous power; defined as the maximum instantaneous output power from the turbine during normal
operation conditions and standard air density. The maximum instantaneous power can be measured according to IEC
1400-12 /2/.
Fixed speed wind turbines with stall or pitch control can produce peaks of output power well above nominal output,
whereas Optislip and variable speed wind turbines also provide ideal output limitations in the instantaneous power
because of the variable slip or speed.
2.2
Reactive power
The wind turbines reactive power (consumption or production) is also an essential characteristic in determination of the
required grid stiffness at the PCC of a wind turbine installation. The reactive power (or power factor) can be determined
from measurements and specified as a function of the output power of the wind turbine.
The reactive power consumption will typically be zero for a wind turbine attached to a frequency converter (variable
speed wind turbine), whereas in case of conventional types of wind turbines with induction generators the reactive
power consumption will vary as a function of the active production. Wind turbines with induction generators are usually
compensated so that the power factor goes from 1.0 at zero production down to about 0.98 at nominal production,
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Proceedings of Solar ’97 – Australian and New Zealand Solar Energy Society
Paper 154: Nielsen
depending on size and design of the generator. By connecting extra capacitors to the wind turbine unity power factor
can be reached if required.
3
Flicker
Flicker describes the effects of rapid voltage variations on incandescent lights. Rapid voltage variations may be due to
consumer loads which cause fluctuations in real power and reactive power flows. Wind turbines are also a source of
power fluctuations, predominantly due to the effect of wind gusting (turbulence) and tower shadow which cause a
periodic power fluctuation at the frequency at which the rotor blades pass the tower. Flicker also depends on the X/R
ratio and the short-circuit level (fault level) at the PCC.
The flicker contribution, Pst from a wind turbine can be measured directly by using dedicated measuring instruments, or
indirectly from measured current or output power time series. In the following a distinction is made between wind
turbine in continuous operation and switching operation.
3.1
Continuous operation
Risø I-939 /3/ gives a method allowing calculations of flicker contribution Pst in continuous operation of a given wind
turbine at specified grid and wind conditions.
The flicker contribution from several wind turbines connected to the same node will be added up. Given that the wind
power production from an individual wind turbine, for instance in a wind farm, within a period of 10 minutes can be
regarded as stochastic noise, and as independent of the variation in the wind power production from other wind
turbines, the flicker contribution from N wind turbines according to IEC 10003-7 can be added up as following:
Pst =
N
∑P
2
st ,i
i =1
where Pst,i is the flicker contribution of individual wind turbines.
3.2
Switching operations
The flicker emission due to switching operations is characterised by the maximum number of switching operations
during a 10 minutes period and the inrush current due to the switching operations. The following cases shall be
documented:
1.
2.
Inrush current during generator connection at cut-in wind speed.
Inrush current during generator connection at reference power wind
speed.
In normal conditions a wind turbine will as a maximum start or stop once within a 10 minutes period. Risø I-939 /3/
gives a method allowing calculations of flicker contribution Pst in switching operations
Where serval wind turbines are connected to the same node, it is possible that there will occur several start/stops within
the same 10 minutes period. If the start/stops of wind turbines are regarded as random single events, it can be shown
that N start/stops within a 10 minutes period of wind turbines in, for instance, a wind farm, will produce a total flicker
contribution of:
Pst =
N
3, 2
∑P
3, 2
st , i
i =1
where Pst,i is the flicker contribution from each start/stop.
The measured inrush current can be presented as time series plots, and for each of these, the maximum half- period
RMS inrush current (Imax) and the current spike factor (k) can be calculated.
The current spike factor (k) is defined as:
Proceedings of Solar ’97 – Australian and New Zealand Solar Energy Society
Paper 154: Nielsen
3
k=
3.3
I MAX
In
Flicker coefficient
The flicker coefficient (c) can then be determined by applying:
c = Pst , ref ⋅
S k ,ref
Sn
where Sn is the reference apparent power of the wind turbine and Sk is the reference short-circuit apparent power of the
grid.
The flicker coefficient shall be limited to the degree needed to avoid unacceptable flicker emissions at the PCC by
applying the following relation:
c ≤ Plim ⋅
Sk
Sn
Recommended flicker emission limits for equipment connected to medium and high voltage levels are given in IEC
1000-3-7. There are limits both for short-term and long-term flicker emissions. The short-term relates to 10 minutes
period and the long-term to 2 hours period. The short-term limit is 0.35 and the long-term limit is 0.25.
Fixed speed turbines with stall or pitch control produce in generally higher flicker in continuous operation than OptiSlip
or variable speed wind turbines. The flicker coming from the switching operation is lower on the pitch controlled
turbines than on the stall controlled wind turbine. The pitch controlled wind turbines can control the generator speed
before grid connection by using the pitch system, this leads to a smaller inrush current (k-factor) corresponding to a
lower flicker contribution.
4
Harmonics
Voltage deviations from the perfect sinus shaped 50Hz curve give harmonics and noise. The content of harmonics and
noise causes increased losses, and in serious cases it can lead to overloading of capacitor batteries, transformers, and
electrical appliances as well as disturbances of communication systems and malfunctioning of control equipment. A
variable speed wind turbine with a frequency converter will cause harmonic voltage in the grid, because a frequency
converter generates an imperfectly sinus shaped current. And because of the grid impedance, harmonic current will
cause harmonic voltage. The amplitude of the harmonic voltage depends on the amplitude of the harmonic current and
the grid impedance at the current frequency. Modern PWM frequency converters for variable speed wind turbines will
typically operate with a switch frequency over 2kHz, and ideally noise will only be generated around integer multiples
of switch frequency.
CENELEC EN50160 gives limits on the content of harmonics up to the 40th order. Standards for harmonics in the
frequency range above 2 kHz, i.e. for harmonics over the 40'th order are not present indicated directly in any known
standard.
Wind turbines without frequency converters will in generally not cause harmonic voltage problems because the
generator is connected directly to the grid.
5
Power Quality Measurements
Measurements of a wind turbines electric characteristics is now a standard for grid connected wind turbines in
Germany. Results from the measurements are certified data sheets, which are required in order to obtain the permission
for grid connection from the local utilities. The following is an overview of the power quality measurements performed
in Germany during the last three years. The overview is based on 25 commercial available wind turbines ranging from
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Paper 154: Nielsen
80kW to 600kW, with respect to flicker, power peaks (instantaneous and averaged) and current spike factor (k-factor).
The overview is based on material from DEWI (Deutsches Windenergie-Institut).
Flicker Coefficient c for two Wind
Turbine Power Classes
2.50
80
Average
70
Max.
60
Min.
Average
Power / rated power
Flicker coefficient c
90
Instantaneous Power Peaks
2.00
Max.
Min
1.50
50
40
1.00
30
20
0.50
10
0
0.00
Pitch <
400kW
Stall <
400kW
Pitch
400600kW
Stall
400600kW
Pitch <
400kW
1 - Minute - Average Power
Peakes
Pitch
400600kW
Stall
400600kW
Comparison of current spike
factor k
5.00
1.3
Average
1.25
4.50
Max.
Min
1.2
1.15
K-factor
Power / rated power
Stall <
400kW
1.1
1.05
Average
4.00
Max.
3.50
Min.
3.00
2.50
2.00
1.50
1
1.00
0.95
0.50
0.9
0.00
Pitch <
400kW
Stall <
400kW
Pitch 400- Stall 400600kW
600kW
Pitch <
400kW
Stall <
400kW
Pitch 400- Stall 400600kW
600kW
Fig.1:
Flicker, power and current peaks generated by different turbine types and sizes. The average value in each
turbine class is shown as well as the difference between the best and worst machine.
6
Conclusion
Wind turbines with semi-variable speed (Optislip) or variable speed in combination with pitch control will provide ideal
output power limitation, so that maximum power does not exceed the nominal output, whereas fixed speed turbines with
stall control may produce peaks of output considerably above the nominal output. This means that, all other things
being equal, the stationary voltage conditions will be affected less by OptiSlip or variable speed wind turbines than by
fixed speed turbines.
Wind turbines with OptiSlip or variable speed will generate less output variations in continuous operation as by
switching operations than fixed speed wind turbines. It is therefore a fact that wind turbines with variable speed or
OptiSlip, all other things being equal, will produce less contribution to flicker than corresponding fixed speed wind
turbines.
Proceedings of Solar ’97 – Australian and New Zealand Solar Energy Society
Paper 154: Nielsen
5
Variable speed wind turbines with frequency converters will generate harmonics, the level of harmonics depends on
design, components, switch frequency, etc. Wind turbines without frequency converters will in generally not cause
harmonic voltage problems.
The aspects of wind turbine power quality are today a very important design parameter for the turbine manufactures and
it will become even more important as the size of the wind turbine installations will increase even more in the future.
High power quality should help prevent unnecessarily strict utility requirements, and allow more wind turbine capacity
to be connected to public grid.
7
References
/1/
DEFU KR 77 (1988) Grid connection of wind turbines. (In Danish)
/2/
/3/
6
IEC 1400-12 (1996) Wind Turbine Generator Systems. Part 12: Power Performance Measurements
Techniques. (Draft)
Risø I-939(EN) (1995) Methods for calculation of the flicker contributions from wind turbines.
Proceedings of Solar ’97 – Australian and New Zealand Solar Energy Society
Paper 154: Nielsen