National Renewable Energy Laboratory 

Innovation for Our Energy Future 

A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 

Power Quality Aspects in a 

Wind Power Plant 

Preprint 

E. Muljadi and C.P. Butterfield 

National Renewable Energy Laboratory 

J. Chacon 

Southern California Edison 

H. Romanowitz 

Oak Creek Energy Systems, Inc. 

To be presented at the IEEE Power Engineering Society 

General Meeting 

Montreal, Quebec, Canada 

June 18–22, 2006 

Conference Paper 

NREL/CP-500-39183 

January 2006 
NOTICE

The submitted manuscript has been offered by an employee of the Midwest Research Institute (MRI), a 

contractor of the US Government under Contract No. DE-AC36-99GO10337. Accordingly, the US 

Government and MRI retain a nonexclusive royalty-free license to publish or reproduce the published form of 

this contribution, or allow others to do so, for US Government purposes. 

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any 

warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or 

usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not 

infringe privately owned rights. Reference herein to any specific commercial product, process, or service by 

trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, 

recommendation, or favoring by the United States government or any agency thereof. The views and 

opinions of authors expressed herein do not necessarily state or reflect those of the United States 

government or any agency thereof. 

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy 

and its contractors, in paper, from: 

U.S. Department of Energy 

Office of Scientific and Technical Information 

P.O. Box 62 

Oak Ridge, TN 37831-0062 

phone: 865.576.8401 

fax: 865.576.5728 

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from: 

U.S. Department of Commerce 

National Technical Information Service 

5285 Port Royal Road 

Springfield, VA 22161 

phone: 800.553.6847 

fax: 703.605.6900 

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/ordering.htm

Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste
POWER QUALITY ASPECTS IN A WIND POWER PLANT 

 E. Muljadi1 C.P. Butterfield1 J. Chacon2 H. Romanowitz3 . 

 1

National Renewable Energy Laboratory 2) Southern California Edison 3) Oak Creek Energy Systems Inc.

 1617 Cole Blvd. P.O. Box 800 14633 Willow Springs Rd. 

 Golden, CO 80401 Rosemead, CA 91770 Mojave, CA 93501

Abstract— Like conventional power plants, wind 

power plants must provide the power quality 

required to ensure the stability and reliability of the 

power system it is connected to and to satisfy the 

customers connected to the same grid. When wind 

energy development began, wind power plants were 

very small, ranging in size from under one megawatt 

to tens megawatts with less than 100 turbines in each 

plant. Thus, the impact of wind power plant on the 

grid was very small, and any disturbance within or 

created by the plant was considered to be in the noise 

level. 

In the past 30 years, the size of wind turbines and 

the size of wind power plants have increased 

significantly. Notably, in Tehachapi, California, the 

amount of wind power generation has surpassed the 

infrastructure for which it was designed. At the same 

time, the lack of rules, standards, and regulations 

during early wind development has proven to be an 

increasing threat to the stability and power quality of 

the grid connected to a wind power plant. 

Fortunately, many new wind power plants are 

equipped with state of the art technology, which 

enables them to provide good service while producing 

clean power for the grid. The advances in power 

electronics have allowed many power system 

applications to become more flexible and to 

accomplish smoother regulation. Applications such 

as reactive power compensation, static transfer 

switches, energy storage, and variable-speed 

generations are commonly found in modern wind 

power plants. 

Although many operational aspects affect wind 

power plant operation, this paper, focuses on power 

quality. Because a wind power plant is connected to 

the grid, it is very important to understand the 

sources of disturbances that affect the power quality. 

In general, the voltage and frequency must be kept as 

stable as possible. The voltage and current 

distortions created by harmonics will also be 

discussed in this paper as will self-excitation, which 

may occur in a wind power plant due to loss of line. 

Index Terms—wind turbine, wind farm, wind power 

plant, power quality, wind energy, aggregation, 

power systems, renewable energy, reactive power 

compensation, self-excitation, harmonics. 

I. INTRODUCTION

In the past 30 years, the size of wind power plants has 

increased significantly. Notably, in Tehachapi, 

California, the amount of wind power generation has 

surpassed the capability of the infrastructure for which it 

was designed. The infrastructure was built to support 

small, scattered wind generation. Similarly, because 

wind plants were so small in the past, the rules 

governing wind generation were more relaxed to 

encourage development. For example, in the past, wind 

turbines were only required to have capacitor 

compensation at each turbine to satisfy the no-load 

reactive power generation. But as the amount of wind 

generation increases, the lack of rules, standards, and 

regulations during early wind development has proven to 

be an increasing threat to the stability and power quality 

of the interconnected grid. 

Although many operational aspects affect wind 

power plant operation, in this paper, we focus on power 

quality. Because a wind power plant is connected to the 

grid, it is very important to understand the sources of 

disturbances that affect its power quality. In general, the 

voltage and frequency must be kept as stable as possible; 

therefore, voltage and current distortions created by 

harmonics will also be discussed in this paper. Selfexcitation, which may occur in a wind power plant due 

to loss of line, will also be presented. 

Section II describes the voltage and frequency 

variations, and Section III discusses single and multiple 

turbines representation. Section IV presents harmonics 

and self-excitation, and Section V presents the summary. 

 We used Power Systems Simulation for Engineers 

(PSSETM) from Siemens Power Technologies Inc., and 

Visual Simulation (Vissim) from Visual Solution Inc. 

II. VOLTAGE AND FREQUENCY VARIATIONS

A. Overview

This section describes the interaction between the 

wind power plant, reactive power compensation, and the 

power system network. The Tehachapi power system 

network used in this study is based on a power system 

diagram from 1999 [1]. Thus, the simulation results 

1
presented in this paper do not necessarily reflect the 

current situation. The current power system network has 

been improved to accommodate more wind power 

generation. 

Because the loads and the wind power plants' output 

fluctuate during the day, the use of reactive power 

compensation is ideal for maintaining normal voltage 

levels in the power system network. Reactive power 

compensation can minimize reactive power imbalances 

that can affect the surrounding power system. 

In this section, we will show how the contribution of 

wind power plants affects the power distribution 

network and how the power distribution network and 

reactive power compensation interact when the wind 

changes. 

About 24 wind power plants are included in the 

simulation. The wind power plants represent 1- to 70-

MW wind power plants. The wind power plants are 

connected to the rest of the transmission network. Bus 

1, which is Antelope Substation, is used as the gateway 

to the much larger network outside the area and is 

treated as an infinite bus. Each wind power plant 

represents the following characteristics: 

1. The characteristics of the wind resource (turbulence 

level, average speed, air density, etc.). 

2. Diversity of the wind speeds with respect to the 

locations of each wind power plant in the Tehachapi 

area. For a more detailed discussion on simulation of 

wind power plant aggregation, see reference [2]. 

3. The characteristics of the wind turbine (Cp-TSR 

characteristics) and induction generator 

4.The P-Q (real and reactive power) electrical 

characteristic of the individual wind power plants. 

When the work presented in this paper was developed, 

no wind turbine or wind power plant models were 

commercially available, thus, the model presented here 

was developed at NREL. Today many wind turbine 

models are commercially available and can be 

downloaded from the GE website, the Siemens Power 

Technologies International website, and other software 

vendors. 

B. Voltage Variation 

This section presents the voltage variations caused by 

the wind speed changes and a comparison between an 

uncompensated and a compensated system. The term 

compensated refers to the use of static reactive power 

compensation to improve the voltage characteristics of 

the wind power plant. The CalCement substation (bus 

22) has 18 MVAR capacitor compensation installed. 

Other buses in the area are compensated to a total of 

77.3 MVAR. The total load in the area (including 

surrounding small towns) is about 259 MW and 46.4 

MVAR. The generations at Kern River and Bailey are 

24 MW and 20 MW, respectively. The simulation was 

fed by non-uniform wind speeds to simulate the entire 

area. A total simulation time of 6000 seconds (100 

minutes) is used to cover the spectrum of possible wind 

fluctuations. For a wind power plant power system, IEC 

Standard 61400-21 stated that the 10-minute average of 

voltage fluctuation should be within + 5% of its nominal 

value [3]. 

A typical reactive power compensator may be 

implemented by using a fixed capacitor, a switched 

capacitor, or static compensator [4-5]. The simplest 

form of reactive power compensation is the static VAR 

compensator (SVC). By choosing the correct size of 

inductor and capacitor, the SVC can be operated to 

generate reactive power, which varies from -Qlolim to 

+Qhilim. For example, with a 100-MVAR capacitor and a 

200-MVAR inductor, a range of +100 MVAR can be 

achieved and adjusted continuously. If negative reactive 

power is not required, a combination of a 100-MVAR 

Reactive power from CalCement to 

Antelope (no-compensation).

 1.0 p.u. - 26 MVAR 

Voltage at CalCement 0.905 p.u. 

4500 6000 

 Time (seconds) 

Figure 1. Reactive power from bus 22 to bus 1, and 

voltage at bus 22 without reactive power compensation. 

Reactive power from CalCement to 

Antelope (with-compensation).

- 18 MVAR 

 1.0 p.u. 

0.95 p.u. 

Voltage at CalCement 

4500 6

 

000

 Time (seconds) 

Figure 2. Reactive power from bus 22 to bus 1, and 

voltage at bus 22 with reactive power compensation.

2
capacitor and a 100-MVAR inductor provides a range of 

0 MVAR to +100 MVAR 

The reactive power compensation is located inside the 

wind power plant at VarWind, bus 40. Figures 1 and 2 

compare an uncompensated wind power plant and a 

compensated wind power plant. Without reactive power 

compensation, the voltage variation at CalCement (bus 

22) drops to 0.905 p.u., and with proper compensation, 

the voltage drops to 0.95 p.u., which is considered 

allowable for a wind power plant. Note, that the reactive 

power flows from the Antelope drops significantly (from 

26 MVAR to 18 MVAR) when the reactive power is 

compensated locally. Notice that the voltage traces 

follow the reactive power traces closely and the 

minimum voltage occurs at the minimum reactive power 

flowing from CalCement (or maximum reactive power 

flowing from Antelope to CalCement). 

C. Frequency Variation 

Frequency variation is affected by the rate of change 

in real power flow. Thus, there is a difference in the 

trend between the voltage-reactive power traces 

compared to frequency-real power traces relationship. 

The reactive power improves the voltage characteristic 

of the wind power plant and surrounding area. 

Comparing Figures 3 and 4, the real power flow is not 

affected by the reactive power compensation. Although 

the frequency variation is barely noticeable, it indicates 

that there are differences in the rate-changes in the real 

power flow between compensated and uncompensated 

systems. It shows that there is a reduction in the 

frequency swings; however, because the frequency 

swing is very small to begin with, it is probably not 

warranted to claim that the reactive power compensation 

helps reduce the frequency variations. Note, that the 

minimum frequency does not occur at the maximum 

power flow but rather the maximum rate of change of 

the power flow. 

Real power 

III. SINGLE TURBINE AND MULTIPLE TURBINES

A. Single Turbine Representation (STR) 

In this section, we look at a wind power plant 

represented by one group of wind turbines. This is the 

worst-case assumption because we assume that all the 

wind turbines in this group are synchronized. Thus, the 

same wind fluctuations and tower shadow effects will 

affect the output power of the wind power plant and the 

power quality at the PCC. 

24. 01μHz 

- 21. 6 μHz 

Frequency at CalCement 

B. Multiple Turbine Representation (MTR) 

In this section, we focus on the aggregation impact on 

the wind power plant output at the PCC. We use the 

same wind turbulence intensity and the same impedance 

of the transmission line. We measured the real and 

reactive power fluctuations, the voltage fluctuations, and 

at the PCC of a wind power plant. We quantified the 

difference in power and voltage fluctuations level if we 

treat a wind power plant as a single turbine or as 

multiple groups of turbines. The flicker level 

measurement can be implemented using design 

specification in IEC 61000-4-15 [1]. 

Ideally we would like to model every wind turbine on 

the wind power plant. Unfortunately, a large wind 

power plant can have more than 100 wind turbines on 

site. Therefore, it is not possible to represent all the 

turbines simultaneously, because the computing time 

would be excessive. To closely represent a real wind 

power plant without simulating each wind turbine, we 

made the following assumptions: 

4500 6000 

 Time (seconds) 

Figure 3. Real power from bus 22 to bus 1, and frequency at 

bus 22 without reactive power compensation. 

 A large wind power plant (200 turbines) is divided 

into several groups of wind turbines. 

- 21. 6 μHz 

Real power 

21. 4 μHz 

- 20. 2 μHz 

Frequency at CalCement 

4500 6000 

 Time (seconds) 

Figure 4. Real power from bus 22 to bus 1, and frequency at 

bus 22 with reactive power compensation. 

3
· The wind speed is uniform for each group of wind 

turbines. 

· The groups are arranged in sequence. 

· Our interest is in the long-term simulation, thus the 

start-up of each turbine is not a major concern. Besides, 

because the wind speeds are different at each turbine 

location, all the wind turbines do not start at the same 

time. 

· All the turbines in the wind power plant are exposed to 

of 18.7 m/s and turbulence level of 19.7%. The time 

series wind speed shifts by 1 minute for each group. 

· The contribution of each group is chosen randomly. 

For example, a wind power plant with 3 groups of 

turbines may be proportioned as 35% from the first 

group, 25% from the second group, and 40% from the 

third group. 

· This concept of grouping is repeated for different 

numbers of groups, but the total wind turbines in the 

entire wind power plant is kept the same—200 turbines. 

· Eventually we will compare the impact of wind 

turbine distribution by comparing the flicker and the 

voltage fluctuations based on groupings (one group only, 

two groups, four groups, and eight groups). 

C. Comparison between STR and MTR 

To start, consider the time series of wind speed shown in 

Figure 5. In an STR, the wind speed is applied to a 

single turbine and the output of the single wind turbine is 

multiplied by the number of the turbines within the wind 

power plant. In an MTR, the time series of the wind 

speed is subdivided into several sections and each 

subdivision is applied to a different group of turbines. 

For example, for the figure shown, the time series of 

wind speed is divided into four different files with the 

starting time (t=0) at w1, w2, w3 and w4. This 

assumption is an approximation of the time it takes for 

the wind speed to travel from one group of turbines to 

another group of turbines down wind. Although this 

assumption is not perfect, by assuming that the wind 

speed has a characteristic of frozen turbulence, and that 

the turbulence does not change as it passes a wind 

turbine, we can more closely simulate the real situation. 

Let us consider the output of STR and MTR and place 

the two graphs next to each other for a better 

comparison. Figure 6 shows variation of real power for 

both representations taken at the point of 

interconnection. The time scale is changed to make an 

easier observation of the nature of power fluctuations 

within a short time frame. In these particular traces, the 

trace of tower shadow is very visible. Tower shadow 

effect is the effect of power fluctuations due to power 

production deficit every time a blade passes the turbine 

tower. Usually the tower shadow has a frequency 3 per 

revolution. This effect is commonly known as 3 p 

effects. Besides the tower shadow, the power variation is 

also caused by the wind speed variations with time. For 

the STR, the power fluctuation reflects the power 

fluctuation of a single turbine. It is amplified by the 

number of turbines within the wind power plant. For the 

MTR, the power fluctuation is the collective behavior of 

several groups of wind turbines with each group fed by a 

different time series as illustrated in Figure 5. 

W ind Farm P & Q

T im e (sec)

1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9 2 0

P (MW) at PCC

0

1 0

2 0

3 0

4 0

5 0

6 0

The label WP1G is a single-group representation and 

WP16G is a 16-group representation. Comparing the two 

graphs, it is obvious that there is some smoothing effect 

in the power fluctuations if we consider that the wind 

power plant consists of sixteen different groups of wind 

turbines. Figure 7 shows the voltage fluctuations as the 

wind speed varies with two different representations. 

The STR obviously shows very large variations of the 

voltage at the point of interconnection as the wind speed 

Real P ower

Figure 5. Wind speed variation applied to the wind turbines. 

WP1G 

W ind Farm Real P ower (16 groups)

T ime (sec)

10 11 12 13 14 15 16 17 18 19 20

P at PCC

0

10

20

30

40

50

60 Real P ower (MW )

WP16G 

Figure 6. Real power output of a wind farm in a WP1G 

and in a WP16G. 

W ind Speed (group1)

T ime (sec)

0 100 200 300 400 500 600

Wind Speed (m/s)

0

10

20

30

40

w1

w2

w3

w4

4
varies while the output voltage for 16 groups 

representation shows a much smaller voltage swings. 

IV. SELF EXCITATION AND HARMONICS 

A. Overview 

Traditional wind turbines are equipped with 

induction generators. Induction generators are preferred, 

because they are inexpensive, rugged, and require very 

little maintenance. Unfortunately, induction generators 

require reactive power from the grid to operate and some 

capacitor compensations are often used. Because 

reactive power varies with the output power, the 

capacitor compensation is adjusted as the output power 

varies. The interactions among the wind turbine, the 

power network, and the capacitor compensation (shown 

in Figure 8) are important aspects of wind generation 

[6]. In this section, we will show the interactions among 

the induction generator, capacitor compensation, power 

system network, and magnetic saturations and examine 

the cause of self-excitation and harmonic currents. 

B. Self Excitation 

Self-excitation can occur in a fixed-speed wind 

turbine equipped with an induction generator. Fixed 

capacitors are the most commonly used method of 

reactive power compensation in a fixed-speed wind 

turbine. Induction generators alone cannot self excite. It 

requires reactive power from the grid to operate 

normally. The grid dictates the voltage and frequency of 

the induction generator. 

Although self-excitation does not occur during 

normal grid-connected operation, it can occur during offgrid operation. For example, if a wind turbine operating 

in normal mode becomes disconnected from the power 

line due to a sudden fault or disturbance in the line 

feeder, the capacitors connected to the induction 

generator will provide reactive power compensation. 

However, the voltage and the frequency are determined 

by the balancing of the systems. 

One disadvantage to self-excitation is the safety 

aspect. Because the generator is still generating voltage, 

it may compromise the safety of the personnel inspecting 

or repairing the line or generator. Another disadvantage 

is that the generator’s operating voltage and frequency 

are determined by the balance between the system’s real 

power and the reactive power. Thus, if sensitive 

equipment is connected to the generator during selfexcitation, the equipment may be subjected to 

over/under voltage and over/under frequency operation. 

In spite of the disadvantages of operating the induction 

generator in self-excitation, some people use selfexcitation for dynamic braking to help control the rotor 

speed during an emergency such as a grid loss condition. 

Thus, with the proper choice of capacitance and resistor 

load (to dump the energy from the wind turbine), the 

wind turbine can be brought to a safe operating speed 

during grid loss and mechanical brake malfunctions. 

In an isolated operation, the conservation of real and 

reactive power must be preserved. The equation 

governing the system can be simplified by looking at the 

impedance or admittance of the induction machine. To 

operate in an isolated fashion, the total admittance of the 

induction machine and the rest of the connected load 

must be zero. The voltage of the system is determined y 

the flux and frequency of the system. Thus the level of 

saturation plays an important role in sustaining or 

collapsing the self-excitation process. A more detailed 

description of the self-excitation process in a wind 

turbine operating off grid can be found in reference [6]. 

Figure 7. Voltage at the point of interconnection for WG1G and WP16G 

Figure 8. The physical diagram of the system under investigation. 

Infinite 

Bus

Line Feeder Transformer 

Wind 

Turbine

Vph at Infinite Bus and at PCC (in p.u.)

T ime (sec)

0 100 200 300 400 500 600

Per phase voltage (in p.u.)

.90

.95

1.00

1.05

1.10

Caps 

To local loads 

Vph at PCC (in p.u.) - 16 groups

T ime (sec)

0 100 200 300 400 500 600

Per phase voltage (in p.u.)

.90

.95

1.00

1.05

1.10 Per phase voltage at PCC

Per phase voltage at PCC

WP16G 

WP1G 

5
s r

h

s

h

S

h

ω ω

ω

− =

Figure 9 shows traces of rotor speed, output power, 

and voltage of an induction generator compensated with 

local capacitors when it was separated from the grid at 

about t=3.1 sec, feeding a local load, and reconnected to 

the grid at t=6.5 sec. During the transient, the generator 

output power and rotor speed varies with the induction 

generator slip, the level of saturation, and the load 

connected to the island network. The voltage increases 

by 11%, and the frequency increases by 4.2%. The 

voltage and frequency settle at the balance of real power 

and reactive power. 

C. Harmonics 

We replaced the power network, shown in Figure 8, 

with a per phase equivalent circuit, shown in Figure 10, 

to represent harmonic components of the circuit. In this 

circuit representation, the harmonics is denoted with h to 

indicate the higher harmonics multiples of 60 Hz. Thus 

h=5 indicates the fifth harmonics (300 Hz). In wind 

turbine applications, the induction generator, 

transformer, and capacitors are three phases and either 

Wye or Delta is connected. Thus, the flow of even 

harmonics, the third, and its multiples do not exist (only 

h = 5, 7, 11, 13, 17. . . etc.). 

 1) Infinite Bus and Line Feeder 

The infinite bus and the line feeder connecting the wind 

turbine to the substation are represented by a simple 

Thevenin representation of the larger power system 

network. Thus, we consider a simple RL line 

representation. 

 2) Transformer 

We considered a three-phase transformer with standard 

impedance of 6%. Because the magnetizing inductance 

of a large transformer is usually very large compared to 

the leakage inductance, only the leakage inductance will 

be considered. Assuming the efficiency of the 

transformer is about 98% at full load, and the copper 

loss is equal to the core loss (general assumption for an 

efficient, large transformer), we can approximate the 

winding resistance, which is generally very small for an 

efficient, large transformer. 

 3) Capacitor Compensation 

The capacitors representing the compensation of the 

wind turbine are switched capacitors. Although the 

manufacturer equipped the wind turbine with only 400 

kVAR of reactive power compensation, the wind turbine 

we considered is equipped with an additional 1.5 MVAR 

reactive power compensation. The wind turbine is 

compensated at different levels of compensation 

depending on the level of generation. The capacitor is 

represented by the capacitance C. In series with the 

capacitance, is the parasitic resistance (Rc), representing 

the losses in the capacitor. This resistor is usually very 

small for a good quality capacitor. 

 4) Induction Generator 

The induction generator (1.5 MW, 480V, 60 Hz) used 

for this wind turbine can be represented as the per phase 

equivalent circuit shown. Figure 8 shows the equivalent 

circuit of the induction generator. The operating slip of 

the generator at fundamental frequency (60Hz) is around 

1%. The slip of induction generator at harmonic 

frequency can be computed as: 

where 

Sh = slip for hth harmonics 

h = harmonics order 

ωs = synchronous speed of the generator 

ωr = rotor speed of the generator 

 

 j hXline

 Rline 

 Rxf/2 j hXxf/2 Rxf /2 

 j hXxf/2 

rotor speed 

output power 

 Vh Rc Rgen 

(j hωC )-1 j hXgen

Harmonic Representation 

Figure 10. The per phase equivalent circuit of the simplified model for 

harmonic analysis. 

Figure 9. Traces during transient versus the time 

a) the rotor speed and output power 

b) voltage variation during transient

6
Thus for higher harmonics (5th and higher) the slip is 

close to one (Sh = 1) and for practical purposes is 

assumed to be one. From the circuit diagram we can 

compute the impedance seen by the harmonic source as: 

 can 

Z C,h = Z + 0.5 Z // 0.5 Z + Z //Z ( )( ) line xfmr ( ) xfmr C gen

The admittance can be found from the impedance. 

 ( ) ( )

Z C,h = Z + 0.5 Z // 0.5 Z + Z //Z ( )( ) line xfmr ( ) xfmr C gen

( ) ( )

1 Y C,h = Z C,h

The admittance corresponds to the harmonic current for 

a given harmonic voltage excitation. In this section, we 

analyze the system admittance from the most dominant 

harmonic frequency (up to 23rd harmonics, excluding 

even and non-triplen harmonics) and vary the size of the 

capacitor compensation. Because the data in the field 

only consists of the total harmonic distortion, and does 

not provide information about individual harmonics, we 

can only compare the trends shown by the admittance. 

Figure 11a shows that the total admittance computed for 

all higher harmonics of interest up to 23rd harmonics 

(odd and non-triplen harmonics) is plotted as a function 

of the total reactive power (per unit) as the size of 

switched capacitors varied. For a comparison, the 

measured data of the total harmonic distortion as a 

function of the total reactive power (per unit) is 

presented in Figure 11b. Both Figures 11a and 11b 

show that when the size of the capacitor compensation is 

increased, the circuit admittance (in Figure 11a) or the 

total harmonic distortion (in Figure 11b) shows 

resonance at different higher harmonics. The two graphs 

show the same trend. There are two values of 

capacitances that amplify the total harmonic distortions 

revealed from the calculation and from the field 

measurement. Although the magnitudes do not match 

completely, the size of reactive power at which the two 

resonance frequencies occur shows a good correlation 

between the field measurement and the calculated 

values. 

Plot

Time (sec)

4.95 4.96 4.97 4.98 4.99 5

-800

-600

-400

-200

0

200

400

600

800

Voltage Current 

Fig.12. The voltage and current of a transformer under 

light load condition. 

Ytot vs Reactive Power

0

50

100

150

200

250

300

0.00 0.20 0.40 0.60 0.80 1.00

Reactive Power (p.u.)

Ytot (mhos)

Fig. 11a. The total admittance for higher harmonics (odd and nontriplen) as a function of reactive compensation. 

From Figure 11, we can say that the circuit will 

resonate at different higher harmonics when that 

particular harmonics presents or excites the circuit. The 

source of harmonics can be traced back from the 

saturation level of the magnetizing branch of the 

transformer, creating a nonlinear magnetizing inductance 

and a distorted line current. Figure 12 shows distorted 

current as a result of the transformer operating in a 

saturation condition. Note that the deviation of the line 

current from sinusoidal wave shape may excite the 

harmonics circuit created by the combination of line 

inductances and capacitance of the capacitor 

compensation in the collector system network. 

PU VAR Support on a MW Class WT Near 

Full Generation Magnitude of VAR 

Support, 0.95 PF Leading at Rated KW

-0.3

-0.1

0.1

0.3

0.5

0.7

0.9

1.1

0.0 0.2 0.4 0.6 0.8 1.0

PU Supplemental VARs

PU Volts

0

1

2

3

4

5

6

7

8

% TH

D

PU V 

% THD Cap A 

% THD U 

V. CONCLUSION

This paper describes various aspects of power quality 

within the wind power plant. There are many others that 

Figure 11b. Total harmonic distortion of the current as a 

function of the reactive compensation in per unit. 

7
could be included in this paper such as flicker, faults, 

start-up transients etc., but, because of the space limit, 

those topics are not included in this paper. 

In general, the voltage at each turbine within the wind 

power plant varies independently because of the 

difference in wind speeds, the line impedances, and 

compensations among the wind turbines. However, the 

most important point is the point of interconnection that 

provides a gateway for the wind power plant to the real 

world. In this paper, we have shown that the reactive 

power compensation, strategically placed, affects the 

voltage behavior of a very large area consisting of many 

wind power plants. We also show that when studying a 

large wind power plant, we cannot represent the plant as 

a single turbine. A very large wind power plant should 

be represented by groups of wind turbines representing 

its unique characteristics with respect to the location, the 

type of turbines, the control setting, and the line 

impedance. Finally, we show that capacitor 

compensation used by induction generators in a wind 

power plant can lead to self-excitation and harmonics. 

VI. ACKNOWLEDGMENT

We acknowledge the support of the U.S. Department 

of Energy, Oak Creek Energy, and Southern California 

Edison. 

VII. REFERENCES

[1] Muljadi, E.; Butterfield, C.P.; Yinger, R.; Romanowitz, H.; "Energy 

Storage and Reactive Power Compensator in a Large Wind Farm," A 

Collection of the 2004 ASME Wind Energy Symposium Technical 

Papers Presented at the 42nd AIAA Aerospace Sciences Meeting and 

Exhibit, Reno, NV, January 5-8, 2004. 

[2] Muljadi, E.; Butterfield, C.; Gevorgian, V.; "The Impact of the Output 

Power Fluctuation of a Wind Farm on a Power Grid," Conference 

Record, Third International Workshop on Transmission Networks for 

Offshore Wind Farms, Royal Institute of Technology, Stockholm, 

Sweden, April 11 - 12, 2002. 

[3] IEC Standard 61400-21 Measurement and Assessment of Power Quality 

of Grid Connected Wind Turbines. 

[4] Timothy J.E. Miller, Editor, “Reactive Power Control in Electric 

Systems” John Wiley & Sons, 1982. 

[5] Mathur, R.M., Editor, “Static Compensators for Reactive Power 

Control”, Cantext Publications, Winnipeg, Canada, 1984.

[6] Muljadi, E.; Butterfield, C.P.; Romanowitz, H.; Yinger, R.; “SelfExcitation and Harmonics in Wind Power Generation,” Transactions of 

ASME, Journal of Solar Energy Engineering, November 2005, Vol. 

127, pp. 581-587. 

8
F1147-E(12/2004) 

REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing the burden, to Department of Defense, Executive Services and Communications Directorate (0704-0188). Respondents 

should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a 

currently valid OMB control number. 

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

January 2006 

2. REPORT TYPE

Conference paper 

3. DATES COVERED (From - To)

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

Power Quality Aspects in a Wind Power Plant: Preprint 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/CP-500-39183 

5e. TASK NUMBER

WER6.2112 

6. AUTHOR(S)

E. Muljadi, C.P.Butterfield, J. Chacon, and H. Romanowitz 

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 

8. PERFORMING ORGANIZATION 

REPORT NUMBER

NREL/CP-500-39183 

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

11. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

Although many operational aspects affect wind power plant operation, this paper focuses on power quality. Because 

a wind power plant is connected to the grid, it is very important to understand the sources of disturbances that affect 

the power quality. 

15. SUBJECT TERMS

wind turbine; wind farm; wind power plant; power quality; wind energy; aggregation; power systems; renewable 

energy; reactive power compensation; self-excitation; harmonics 

16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON 

a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified

c. THIS PAGE 

Unclassified

17. LIMITATION 

OF ABSTRACT

UL 

18. NUMBER 

OF PAGES 

19b. TELEPHONE NUMBER (Include area code)

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18