AckermannTh. (ed) Wind Power in Power Systems

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the same type, the flicker emission would further increase above the acceptable limit.

Hence, for the example system, flicker emission due to continuous operation may be a

constraint for a further expansion of the wind farm. To overcome this constraint, the

straightforward approach is to select a different wind turbine type that has a smaller

value of c. First of all, c ¼ 10:9 is rather high for a fixed-speed, stall-regulated wind

turbine, so this value should not be assumed to be typical for this type of wind turbine.

Certainly, a (semi)variable-speed wind turbine would yield a much lower value of c,

whereas a fixed-speed, pitch-regulated type could actually yield a higher value of c.

5.3.5 Voltage dips

A voltage dip is defined in EN 50160 as a sudden reduction of the voltage to a value

between 1 % and 90 % of U

followed by a voltage recovery after a short period of time,

conventionally 1 ms to 1 min. The expected number of voltage dips during a year may

vary from a few dozen to a thousand. According to EN 50160, dips with depths of

between 10 % and 15 % of U

are commonly due to the switching of loads, whereas

larger dips may be caused by faults.

The startup of a wind turbine may cause a sudden reduction of the voltage followed

by a voltage recovery after a few seconds. Assuming that each wind turbine is char-

acterised by a voltage change factor, k

(

), the sudden voltage reduction may be

assessed (deducted directly for definition):

d ¼ 100 k

: ð5:8Þ

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 20 40 60 80 100 120

Power (% of rated power)

Flicker emission, P

Measured (wind farm)

Measured (individual turbine)

Estimated (wind farm)

Figure 5.4 Measured and estimated flicker emission from a 5  750 kW wind farm. The

estimated flicker emission is calculated as the measured emission from one wind turbine within

the farm multiplied by the square root of the number of wind turbines in the farm. The good

agreement between the estimated and measured flicker emission verifies Equation (5.7)

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As it is unlikel y that several wind turbines in a wind farm start up at the exact same time,

Equation (5.8) is not a function of the number of wind turbines as are the quantities in

Equations (5.5)–(5.7).

For the example wind turbines, d ¼ 3:0 %. This sudden voltage reduction is in most

cases acceptable, especially consider ing that this would imply a voltage of less than 0.90

p.u. (i.e. a voltage dip) only for the case when startups coincide with a high load on the

network. Further, as Equation (5.8) is not a function of the number of wind turbines in

the farm, it can be concluded that, for the example system, voltage dips are not a

constraint on further expansion of the wind farm.

5.3.6 Harmonic voltage

Nonlinear loads (i.e. loads that draw a current that is not a linear function of the

voltage) distort the voltag e waveform and may in se vere cases cause an overheating

of neutral conductors and electrical distribution transformers, the malfunctioning of

electronic equipment and the distortion of communication systems. Examples of non-

linear loads are power electronic converters, arc furnaces, arc welders, fluorescent lamps

and mercury lamps. The distorted waveform may be expressed as a sum of sinusoids

with various frequencies and amplitudes, by application of the Fourier transform.

The sinusoids with frequencies equal to an integer multiple of the fundamental frequency

are denoted harmonics, whereas the others are denoted interharmonics.

Harmonic voltages, U

, where h denotes the harmonic order (i.e. an integer multiple

of 50 Hz) can be evaluated individually by their relative amplitude:

: ð5:9Þ

According to EN 50160, the 10-minute mean RMS values of each u

have to be less

than the limits given in Figure 5.5 during 95 % of a week. Further, the total harmonic

distortion (THD) of the voltage, calculated according to Equation (5.10), has to be

8%:

THD ¼

h¼2

ðu

1=2

: ð5:10Þ

With regard to higher-order harmonics, EN 50160 does not specify any limits but states

that higher-order harmonics are usually small, though largely unpredictable. In the

same manner, EN 50160 does not indicate any levels for interharmonics. These are

under consideration, though.

To ensure that the harmonic voltages are kept within acceptable limits, each source of

harmonic current can be allowed only a limited contribution. The limits depend on

network specifications, such as harmonic impedance and voltage level, and can be found

using IEC 61000-3-6 (IEC, 1996a) as a guide.

A wind turbine with an induction generator directly connected to the grid without an

intervening power electronic converter (i.e. wind turbine Types is A and B; see Section

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4.2.3) is not expected to distort the voltage waveform . Power electronics applied for soft-

start may give a short-term burst of higher-order harmonic currents, though the dur-

ation and magnitude of these are, in general, so small that they can be accepted without

any further assessment. So, for the example system with fixed-speed wind turbines,

emission limits for harmonics are not a constraint. However, if we considered variable-

speed wind turbin es using power electronic converters, their emission of harmonic

currents should be assessed against given or calculated limits.

Thyristor-based converters are expected to emit harmonic currents that may influence

the harmonic voltages. EN 50160 includes limits for this. Such converters are, however,

rarely used in modern wind turbines. Their converters are, rather , transistor-based and

operate at switching frequencies above 3 kHz. As a consequence, the impact of such new

wind turbines on the voltage waveform is commonly negligible and probably not a

constraint for wind power development.

In general, the connection of electric equipment does change the harmonic impedance

of the network; for example, e.g. capacitor banks, which are often installed as part of

wind farms consisting of fixed-speed wind turbines, may shift the resonance frequency

of the harmonic impedance. Hence, possible harmonic sources already present in the

network may, then, given unfortunate conditions, cause unacceptable harmonic volt-

ages. Consequently, for networks with significant harmonic sources, the connection of

new appliances such as wind farms with capacitor banks should be carefully designed in

order to avoid an ill-conditioned modification of the harmonic impedance.

5.4 Discussion

Prior to the development of IEC 61400-21 there were no standard procedures

for determining the power quality characteristics of a wind turbine. Basically, the

2 4 6 8 10 12 14 16 18 20 22 24

Order

Relative voltage (%)

Figure 5.5 Values of individual harmonic voltages as a percentage of the nominal phase-to-phase

voltage, U

, according to EN 50160 (EN, 1995); the third harmonic is commonly significantly

lower at medium voltage

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manufacturer provided only rated data, giv ing little support for assessing the impact

of wind turbines on voltage quality. Therefore, utilities often applied simplified rules

(e.g. requiring the voltage increment due to a wind turbine installation to be less than

1 %). A large number of wind turbines have been connected and successfully operated

applying such simplified rules. Therefore, it is relevant to discuss if the more detailed

approach suggested in this chapter will lead to any real improvement in securing

voltage quality and still allow a fair amount of wind power to be connected at a

minimum cost.

A simplified assessment would typically consider only the rated data of the wind

turbine installation (e.g. for the case study wind farm the active power, P, is 3750 kW,

and the reactive power, Q, is 0 kvar). Then, the feeder impedance between the wind farm

and an assumed stiff point would be applied to calculate the voltage increment due to

the wind farm. For the example system, the resistance, R,is8:83  and the reactance, X,

is 9:17 , giving a voltage increment U  6:8 % by applying Equation (5.11):

U 

RP þ XQ

: ð5:11Þ

Assuming that onl y a 1 % voltage increment is permitted, which is common for such

simplified assessments, the wind farm cannot be connected according to this 1 %

criterion. Hence, the simplified assessment would conclude that the grid must be

reinforced or the wind farm be limited. In fact, leaving the grid as it is, the application

of the 1 % rule would allow only one 550 kW wind turbine.

This is indeed very much in contrast to the results obtained by the detailed assessment

made in Section 5.3, suggesting that the wind farm can be connected to the existing grid,

and it is also supported by measur ements on a similar real-life system (SINTEF TR

A5330; see Tande, 2001). Hence, it c an be concluded that the simplified assessment

unnecessarily limits the acceptable wind farm capacity. A simplified assessment is there-

fore not recommended for dimensioning the grid connection of a wind farm.

5.5 Conclusions

Procedures for determini ng the power quality characteristics of wind turbines are

specified in IEC 61400-21. The need for such consistent quantification of the power

quality characteristics of wind turbines was verified in Section 5.2. In Section 5.3, the

application of these characteristics for assessing the impact of wind turbines on voltage

quality was explained.

A5 750 kW wind farm on a 22 kV distribution feeder was used as an illustration.

The detailed analysis suggested that the wind farm capacity can be operated at the grid

without causing an unacceptable voltage quality. For comparison, a simplified design

criterion was considered, assuming that the wind farm is allowed to cause a voltage

increment of only 1 %. According to this criterion, only a very limited wind power

capacity would be allowed. Measurements, however, confirm the suggestion of the

detailed analysis, and it is concluded that a simplified design criterion such as the ‘1 %

rule’ should not be used for dimensioning the grid connection of a wind farm. Rather,

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the following analyses are suggested, which are made possible through the developm ent

of IEC 61400-21:

Load-flow analyses should be conducted in order to assess whether slow voltage

variations remain within acceptable limits (Section 5.3.3).

Maximum flicker emission from the wind turbi nes should be estimated and compared

with given or calculated limits (Section 5.3.4).

Possible voltage dips due to wind turbine startup should be assessed (Section 5.3.5).

Maximum harmonic currents from the wind turbines (if relevant) should be estimated

and compared wi th given or calculated limits (Section 5.3.6).

It should be emphasised that the above may have to be supported by additional

analyses, for instance, regarding the system stability in the case of large wind farms or

weak grids, and regarding the impact on grid frequen cy in systems where wind power

covers a high fraction of the load (e.g. in isolated systems).

References

[1] EN (1995), Voltage Characteristics of Electricity Supplied by Public Distribution Systems, EN 50160,

www.cenelec.org.

[2] Hatziargyriou, N. D., Karakatsanis, T. S., Papadopoulos, M. (1993) ‘Probabilistic Load Flow in Dis-

tribution Systems Containing Dispersed Wind Power Generation’, IEEE Transactions on Power Systems,

8 (1), 159–165.

[3] IEC (International Electrotechnical Commission) (1996a) EMC, Part 3: Limits. Section 6: Assessment of

Emission Limits for Distorting Loads in MV and HV Power Systems, IEC 61000-3-6, Basic EMC publica-

tion (technical report), www.iec.ch.

[4] IEC (International Electrotechnical Commission) (1996b) EMC, Part 3: Limits. Section 7: Assessment of

Emission Limits for Fluctuating Loads in MV and HV Power Systems, IEC 61000-3-7, Basic EMC

publication (technical report), www.iec.ch.

[5] IEC (International Electrotechnical Commission) (1997) EMC, Part 4: Testing and Measurement Tech-

niques. Section 15: Flickermeter – Functional and Design Specifications, IEC 61000-4-15, IEC. www.iec.ch.

[6] IEC (International Electrotechnical Commission) (2001) Measurement and Assessment of Power Quality

Characteristics of Grid Connected Wind Turbines, IEC 61400-21, IEC. www.iec.ch.

[7] Tande, J. O. G. (2000) ‘Exploitation of Wind-Energy Resources in Proximity to Weak Electric Grids’,

Applied Energy 65 395–401.

[8] Tande, J. O. G. (2001) ‘Wind Power in Distribution Grids – Impact on Voltage Conditions’, SINTEF

Energy Research TR A5330 (in Norwegian).

[9] Tande, J. O. G. (2002) ‘Applying Power Quality Characteristics of Wind Turbines for Assessing Impact

on Voltage Quality’, Wind Energy, 5, 37–52.

[10] Tande, J. O. G., Jørgensen, P. (1996) ‘Wind Turbines’ Impact on Voltage Quality’, in Ed. E. Sesto,

Proceedings of the EWEA Special Topic Conference: Integration of Wind Power Plants in the Environment

and Electric Systems, Rome, Italy, 7–9 October 1996, ISES, Rome, paper 3.4, pp. 3–8.

[11] Tande, J. O. G., Relakis, G., Alejandro, O. A. M. (2000) ‘Synchronisation of Wind Turbines’, in

Proceedings of the EWEA Special Topic Conference: Wind Power for the 21st Century, 25–27 September

2000, Kassel, Germany, WIP-Renewasle Energies, Germany, pp. 152–155.

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Power Quality Measurements

Fritz Santjer

6.1 Introduction

The term ‘power quality’ in relation to a wind turbine describes the electrical perform-

ance of the turbine’s electricity generating system. It reflects the generation of grid

interferences and thus the influence of a wind turbine on the power and voltage quality

of the grid. The main influences of wind turbines on the grid concerning power quality

are voltage changes and fluctuations, especially at the local level, and harmonics for

wind turbines with electronic inverter syst ems. Other parameters are reactive power,

flicker, power peak s and in-rush currents.

The grid interferences have different causes, which are mostly turbine-specific. Aver-

age power production, turbulence intensity and wind shear (i.e. the difference of wind

speed between the top and the bottom of the rotor) refer to causes that are determined

by meteorological and geographical conditions. However, the technical performance of

the wind turbine may also have an influence on grid interferences. This performance of

the wind turbine is determined not only by the characteristics of the electrical compon-

ents, such as generators, transformers and so on, but also by the aerodynamic and

mechanical behaviour of rotor and drive train.

Wind turbines and their power quality will be certified on the basis of measurements

according to national or international guidelines. These certifications are an important

basis for utilities to evaluate the grid connection of wind turbines and wind farms.

Wind Power in Power Systems Edited by T. Ackermann

Ó 2005 John Wiley & Sons, Ltd ISBN: 0-470-85508-8 (HB)

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6.2 Requirements for Power Quality Measurements

6.2.1 Guidelines

The following guidelines provide rules and requirements for the measurement of the

power quality of wind turbines:

International Electrotechnical Commission (IEC) guideline IEC 61400-21: ‘Wind

Turbine Generator Systems, Part 21: Measurement and Assessment of Power Quality

Characteristics of Grid Connected Wind Turbines’ (IEC, 2001);

MEASNET guideline: ‘Power Quality Measurement Procedure of Wind Turbines’

(MEASNET, 2000);

The German Fo

rdergesellschaft Windenergie (FGW) guideline: ‘Technische Richtlinie

r Windenergieanlagen, Teil 3: Bestimmung der Elektrischen Eigenschaften’

(Technical guidelines for wind turbines, part 3: determination of the electrical char-

acteristics; FGW, 2002.

IEC 61400-21 is one of the most important guidelines for power quality measure-

ments of wind turbines. The first edition of this guideline was published at the end of

2001. It specifies which characteristics that are relevant to the power quality of wind

turbines have to be measured (e.g. flicker, harmonics, power, switching operations) and

which measurement methods should be used. As a result of a power quality measure-

ment according to IEC 61400-21 the measuring institute will issue a data sheet, which

comprises all the relevant electrical characteristics of the measured wind turbine. All the

measurement results that are contained in the data sheet are given in a normalised form,

independent of the specific site and of the specific network of the measured wind

turbine. Thus the measured data can be used for nearly all sites and networks, and it

is necessary only to measure the electrical characteristics of one wind turbine of each

type. Often, the prototype will be measur ed. All the other wind turbines of this type

should have the same electrical characteristics, as long as they are identical to the

measured wind turbine. Measurements and investigations on several wind turbines of

the same type, but at different sites, have shown that the electrical characteristics,

measured according to IEC 61400-21 at the one site, are similar or identical to the

electrical characteristics measured at another site. Thus it is not necessary to measure

each individual wind turbine, but only one wind turbine of each type.

The data sheet with the electrical characteristics of the wind turbine provides the basis

for the utility’s assessment regarding a grid connection of a wind turbine or of a wind

farm. In order to decide whet her to connect a wind farm project at a specific site to its

grid, the utility will also require site-specific data (e.g. the number of wind turbines and

the annual average wind speed), and data of the specific network (e.g. the short-circuit

power and the impedance angle of the grid). In addition to the measurement req uire-

ments, IEC 61400-21 also gives recommendations on how a utility should perform such

an assessment. Unless there are specific guidelines – such as that of the Verband der

Elektrizita

tswirtschaft (VDEW, 1998) or the Danish utilities Research Association

(DEFU, 1998) – the utility has the option to follow the recommendations of IEC

61400-21.

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MEASNET is a measuring network of wi nd energy institutes, which has the aim to

harmonise measuring procedures and recommendations in order to achieve compar-

ability and mutual recognition of the measurement results by its member institutes.

Regular round robin tests (mutual checks) ensure the high quality of the measurement

results of the member institutes. The ‘Power Quality Measurement Procedure’

of MEASNET (2000) coincides with the requirements of IEC 61400-21. However,

the MEASNET guideline requires more extended measurements regarding harmonic

currents.

In Germany, the FGW technical guideline (FGW, 2002) is used for measuring the

power quality of wind turbines. The guideline was introduced at the beginning of

the 1990s and since then has proved to be a useful instrument for obtaining objective

criteria for the grid connection of wind turbines. It has been continuously improved

and updated and incorporates new research results and new turbine concepts. In

principle, the methods and the procedures given in this guidel ine are similar to those

in IEC 61400-21. There are, however, some differences between the methods of the

two guidelines, so that the data measured according to IEC 61400-21 are not

comparable to the data measured according to the german guideline. The specific

differences between the two guidelines are explained elsewhere (Santjer, 2000). A data

sheet, the so-called ‘Extract of the Test Report’, summarises the results of a power

quality measurement according to the technical guideline (FGW, 2002). This data

sheet contains all the relevant power quality characteristics of the measured turbine

and, together with the VDEW (1998) guideline, which includes the requirements and

limits to be observed, it is used by German utilities for the assessment of the grid

connection.

6.2.2 Specification

According to the above-mentioned guidelines, the power quality measurements are

performed for harmonics, flicker and transients, during normal and switching oper-

ations, separately, as well as for power factor, reactive power consumption and power

peaks.

Although measurement and evaluation methods of the IEC guideline and the MEAS-

NET guideline are similar to the regulations of the German technical guideline, results

may differ considerably. This is partly because of the different averaging periods used

but also because of the differences in the evaluation methods, which in some cases have

a strong influence on the results. An overview of the requirements of the guidelines is

given in Table 6.1.

Power peaks

A power peak is the maximum active power output of the wind turbine over a specified

averaging time during continuous operation (without star t or stop of the turbine).

Power peaks of the turbine are determined over three different time intervals: instant-

aneous, 1 minute and 10 minutes. The power pe aks ha ve to be measured. Only under

the IEC guideline is the 10-minute power peak determined based on manufacturers’

information.

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Table 6.1 Requirements of the guidelines

Requirement IEC 61400-21 MEASNET German technical guideline

Instantaneous power peaks Average value over 0.2 s Identical to IEC 61400-21 Average value over 8 to 16 line

periods

1-minute power peak Included Included Included

10-minute power peak Maximum permitted power,

based on manufacturer’s

information

Identical to IEC 61400-21 Measurement at 10-minute

intervals

Reactive power or power

factor

Reactive power as 10 minute

average value

Identical to IEC 61400-21 Power factor as 1-minute

average value

Flicker in normal operation 10-minute intervals Identical to IEC 61400-21 1-minute intervals

Integer harmonic currents Up to 50th order, 10-minute

average values

Up to 50th order, grouping,

10-minute average values

Up to 40th order, 8 line period

value

Interharmonic currents No Up to 2 kHz, grouping, 10-

minute average values

Up to 2 kHz, 8 line period

value

Current distortion in the higher

frequency range

No Frequency range 2–9 kHz,

grouping, 10-minute average

values, bandwidth 200 Hz

Frequency range 2–9 kHz, 8

line period value, bandwidth

200 Hz

Flicker and voltage change

during switching operations

of the wind turbine

For switching operations: Identical to IEC 61400-21 For switching operations:

cut-in at cut-in wind speed

cut-in at nominal wind speed

switching between generator

stages

cut-off at nominal wind

speed

switching between generator

stages

Separate evaluation of flicker

and voltage change

Same evaluation of flicker and

voltage change