AckermannTh. (ed) Wind Power in Power Systems

Подождите немного. Документ загружается.

//INTEGRAS/KCG/PAGINATION/WILEY/WPS/FINALS_14-12-04/0470855088_05_CHA04.3D – 71 – [53–78/26] 20.12.2004

7:29PM

thereby reduce the losses and the amount of heat that has to be dissipated. This can lead

to a reduction in the size of the generator and to a higher efficiency of the wind turbine,

especially at higher loads. If the voltage of the machine matches the grid voltage, a grid

connection is possible without a transformer.

HVGs are manufactured both as synchronous generators and as asynchronous gen-

erators. HVGs are a potentially interesting alternative for large wind turbines exceeding

3 MW. The major disadvantages are the high cost of the total system, the uncertainty

regarding its long-term performance and the safety requirements, which are more

complex than those for low-voltage machines. The price of the HVG, the power

electronics and the auxiliary equipment, such as switchgears, increases substantially

with the size of the generator. The price could decrease in the future if the number of

wind turbines with HVGs increases significantly.

So far, only very few prototype wind turbines that have been designed by utilities or

large manufacturers of electric equipment have applied HVGs: Tjæreborg, wi th 2 MW,

applies an induction generator with an output voltage of 10 kV, and Growian, with

3 MW, uses a DFIG with an output voltage of 6.3 kV. Different companies have

initiated, with varying success, research projects on HVG wind turbines, during the

past few years. Lagerwey, for example, has started the series production of its LW72

2 MW turbine that has a synchronous generator with an output voltage of 4 kV.

Contrary to what was expected, the Windformer/ABB 3 MW concept has not been

successful. However, at present there are not many commercially available wind tur-

bines with HVGs. Instead of using HVGs, the trend has so far been towards moving the

step-up transformer up into the nacelle.

4.3.3.2 The switched reluctance generator

The SRG machine has shown over the years that it has a robust and simple mechanical

structure, a high efficiency, reduced costs and that it provides the opportunity to

eliminate the gearb ox (Kazmierkowski, Krishnan and Blaabjerg, 2002). It is interesting

for aerospace applications because of its ability to continue operating at reduced output

in the presence of faults in the generator itself. A survey of the positive and negative

properties of SRGs is included in L. H. Hansen et al. (2001). The literature on SRGs

related to wind turbines is not substantial, and much research remains to be done before

the SRG will be adapted to wind turbine applications.

The SRG is a synchronous generator with a doubly salient construction, with salient

poles on both the stator and the rotor. Excitation of the magnetic field is provided by the

stator current in the same way as it is provided for the induction generator. The SRG is

considered inferior to the PMSG machine because of its lower power density. The SRG

requires a full-scale power converter in order to operate as a grid-connected generator.

Moreover, the SRG has a lower efficiency than a PMSG and a lower power factor than

asynchronous generators (Kazmierkowski, Krishnan and Blaabjerg, 2002).

4.3.3.3 Transverse flux generator

The TFG machine topology is fairly new, but seems to be interesting. However, more

research is required before the TFG machine will be adapted so that it can be used as a

wind power generator.

Wind Power in Power Systems 71

//INTEGRAS/KCG/PAGINATION/WILEY/WPS/FINALS_14-12-04/0470855088_05_CHA04.3D – 72 – [53–78/26] 20.12.2004

7:29PM

The transverse flux (TF) principle may be applied to a range of machine types. It could

be used both in permanent magnet and in reluctance machines, for instance. The machine

will inherently behave as the generic type that is applied, but will have characteristics that

are influenced by the TF design. The high ratio of torque per kilogram of active material

seems to be very attractive (Dubois, Polinder and Fereira, 2000).

The nature of its operation is equal to that of a synchronous machine, and it will

function in principle in a way that is similar to any other PM machines. It can comprise

a very large number of poles, which may make it suitable for direct gearless applications.

However, the TFG has a relatively large leakage inductance. In the reluctance version,

this may cause the power factor to become very low during normal operation, and the

short-circuit current is insufficient to trip normal protection. There are similar problems

regarding the PM version, but because of the PM, they will not be as severe.

A disadvantage of the TFG is the large number of individual parts that it requires and

that a lamination technology has to be used. With the advance of powder technology,

this situation may improve.

4.4 Power Electronic Concepts

Power electronics is a rapidly developing technology. Components can handle higher

current and voltage ratings, the power losses decrease and the devices become more

reliable. The devices are also very easy to control with a megascale power amplification.

The price/power ratio is still decreasing, and power converters are becoming more and

more attractive as a means of improving the performance of wind turbi nes. In this

section we will present the power converter topologies that are most commonly used in

wind turbine applications, including their advantages and disadvantages.

4.4.1 Soft-starter

The soft-starter is a simple and cheap power electrical component used in fixed-speed

wind turbines during their connection to the grid (see Figure 4.1, Types A and B). The

soft-starter’s function is to reduce the in-rush current, thereby limiting the disturbances

to the grid. Without a soft-starter, the in-rush current can be up to 7–8 times the rated

current, which can cause severe voltage disturbances on the grid.

The soft-starter contains two thyristor s as commutation devices in each phase. They

are connected antiparallel for each phase. The smooth connection of the generator to

the grid, during a predefined number of grid periods, is achieved by adjusting the firing

angle () of the thyristors. The relat ionship between the firing angle () and the resulting

amplification of the soft-starter is highly nonlinear and is additionally a function of the

power factor of the connected element. After the in-rush, the thyristors are bypassed in

order to reduce the losses of the overall system.

4.4.2 Capacitor bank

The capacitor bank is used in fixed-speed or limited varia ble-speed wind turbines (see

Figure 4.1, Types A and B). It is an electrical component that supplies reactive power to

72 Generators and Power Electronics

//INTEGRAS/KCG/PAGINATION/WILEY/WPS/FINALS_14-12-04/0470855088_05_CHA04.3D – 73 – [53–78/26] 20.12.2004

7:29PM

the induction generator. Thus the reactive power absorbed by the generator from the

grid is minimised.

The generators of wind turbines can have a full load dynamic compensation, where

a certain number of capacitors are connected or disconnected continuously, depend-

ing on the average reactive power demand of the generator over a predefined period of

time.

The capacitor banks are usually mounted at the bottom of the tower or to the nacelle

(i.e. at the top of the wind turbine). They may be heavy loaded and damaged in the case

of overvoltages on the grid and thereby may increase the maintenance cost of the

system.

4.4.3 Rectifiers and inverters

A traditional frequency converter, also called an adjustable speed drive, consists of:

a rectifier (as AC-to-DC conversion unit) to converts alternating current into direct

current, while the energy flows into the DC system;

energy storage (capacitors);

an inverter (DC-to-AC with controllable frequency and voltage) to convert direct

current into alternating current, while the energy flows to the AC side.

Diodes can be used only in rectification mode, whereas electronic switches can be used

in the rectifying as well as in the inverting mode.

The most common rectifier solution is the diode rectifier, because of its simplicity, its

low cost and low losses. It is nonlinear in nature and, consequently, it generates

harmonic currents (Kazmierkowski, Krishnan and Blaabjerg, 2002). Another drawback

is that it allows only a unidirectional power flow; it cannot control the generator voltage

or current. Therefore, it can be used only with a generator that can control the voltage

and with an inverter (e.g. an IGBT) that can control the current.

The thyristor (grid-commutated) based inverter solution is a cheap inverter, with low

losses and, as its name indicates, it needs to be connected to the grid to be able to

operate. Unfortunately, it consumes reactive power and produces large harmonics

(Heier, 1998). The increasing demands on power quality make thyristor inverters less

attractive than self-commutated inverters, such as GTO inverters and IGBTs. The

advantage of a GTO inverter is that it can hand le more power than the IGBT, but this

feature will be less important in the future, because of the fast development of IGBTs.

The disadvantage of GTOs is that the control circuit of the GTO valve is more

complicated.

The generator and the rectifier must be selected as a combination (i.e. a complete

solution), while the inverter can be selected almost independ ently of the generator and

the rectifier. A diode rectifier or a thyristor rectifier can be used together only with a

synchronous generator, as it does not require a reactive magnetising current. As

opposed to this, GTO and IGBT rectifiers have to be used together with variable-speed

induction generators, because they are able to control the reactive power. However,

even though IGBTs are a very attractive choice, they have the disadvantages of a high

Wind Power in Power Systems 73

//INTEGRAS/KCG/PAGINATION/WILEY/WPS/FINALS_14-12-04/0470855088_05_CHA04.3D – 74 – [53–78/26] 20.12.2004

7:29PM

price and high losses. The synchronous generator with a diode rectifier, for example, has

a much lower total cost than the equivalent induction generator with an IGBT inverter

or rectifier (Carlson, Hylander and Thorborg, 1996).

There are different ways to combine a rectifier and an inverter into a frequency

converter. There are five applicable technologies for adjustable speed: back-to-back,

multilevel, tandem, matrix and resonant. An evaluation of each power converter topol-

ogy is included in L. H. Hansen et al. (2001).

4.4.4 Frequency converters

During recent years different converter topologies have been investigated as to whether

they can be applied in wind turbines:

back-to-back converters;

multilevel converters;

tandem converters;

matrix converters;

resonant converters.

A presentation of each of these converter topologies and their working principles,

including their advantages and disadvantages, is beyond the scope of this chapter. For

a detailed description, see L. H. Hansen et al. (2001). It is evident that the back-to-back

converter is highly relevant to wind turbines today. It constitutes the state of the art and

may therefore be used for benchmarkin g the other converter topologies. The analysis in

L. H. Hansen et al. shows that the matrix and multilevel converter are the most serious

competitors to the back-to-back converter and thus are recommended for further

studies.

This section focuses mainl y on the back-to-back converter, as it is today the most

widely used three-phase frequency converter. However, we will also include some br ief

comments regarding multilevel and matrix converters.

The back-to-back con verter is a bidirectional power converter consisting of two

conventional pulse-width modulated (PWM) VSC converters. The topology is shown

in Figure 4.3. The DC link voltage is boosted to a level higher than the amplitude of the

grid line-to-line voltage in order to achieve full control of the grid current. The presence

Inverter Rectifier

Grid

Generator

DC link

Figure 4.3 Structure of the back-to-back frequency converter

74 Generators and Power Electronics

//INTEGRAS/KCG/PAGINATION/WILEY/WPS/FINALS_14-12-04/0470855088_05_CHA04.3D – 75 – [53–78/26] 20.12.2004

7:29PM

of the boost inductance reduces the demands on the input harmonic filter and offers

some protection for the converter against abnormal conditions on the grid.

The capacitor between the inverter and rectifier makes it possible to decouple

the control of the two inverters, allowing the compensation of asymmetry on both the

generator side and the grid side, without affecting the other side of the converter.

The power flow at the grid-side converter is controlled to keep the DC link voltage

constant, and the control of the generator-side converter is set to suit the magnetisation

demand and the desired rotor speed. The control of the back-to-back converter in wind

turbine applications is described in several papers (e.g. Bogalecka, 1993; Pena, Clare and

Asher, 1996).

Kim and Sul (1993) mention that the presence of the DC link capacitor in a back-to-

back converter reduces the overall lifetime and efficiency of the system compared with a

converter without a DC link capacitor, such as the matrix converter (Schuster, 1998).

However, the protection of the matrix converter in a fault situation is not as good as

that of the back-to-back converter. Another aspect of the back-to-back converter is the

high switching losses compared with the switching losses of the matrix converter

(Wheeler and grant, 1993). The disadvantages of the matrix converter compared with

the back-to-back c onverter are higher cond uction losses and the limitation of the output

voltage converter (Wheeler and Grant, 1993).

In comparison with converters with constant DC link voltage and only two output

levels, the output harmonic content of the matrix converter is lower because of the fact

that the output voltage of the matrix converter is composed of three voltage levels.

However, considering the harmonic performance, the multilevel converter excels by

being the converter with the lowest demands on the input filters and therefore with

the best spectra on both the grid side and the generator side (Rodriguez et al., 1999).

To summarise the findings on the presented converters, it is concluded that the back-

to-back converter, the matrix converter and the multilevel converter are the ones

recommended for further studies in different generator topologies.

4.5 Power Electronic Solutions in Wind Farms

Today, and in the future, wind turbines will be sited in large concentrations with

hundreds of megawatts of power capacity. Wind farms of this size will often be

connected direct ly to the transmission grid and will, sooner or later, replace conven-

tional power plants. This means that the wind turbines will be required to have power

plant characteristics (A. D. Hansen, 2001; Sørensen et al., 2000), namely, to be able to

behave as active controllable components in the power system. Such large wi nd farms

will be expected to meet very high technical demands, such as to perform frequency and

voltage control, to regulate active and reactive power and to provide quick responses

during transient and dynamic situations in the power system. The traditional wind

turbines, where the active power is controlled by a simple pitching of the blades or by

using a dumping device or by disconnecting the wind turbines, do not have such control

capabilities and cannot contribute to power system stability as wi ll be required. Storage

technologies may be an option, too, but at the present such technologies are rather

expensive. Also, they are not a satisfa ctory solution in the case of large wind farms,

Wind Power in Power Systems 75

//INTEGRAS/KCG/PAGINATION/WILEY/WPS/FINALS_14-12-04/0470855088_05_CHA04.3D – 76 – [53–78/26] 20.12.2004

7:29PM

because of voltage stability issues. Power electronic technology will therefore becomes

more and more attractive for large wind farms that will have to fulfil future high

demands.

Presently, there are significant research activities to develop the electrical control

layout of such wind farms with different types of power electronic converters in order

for them to be able to comply with the high requirements and to be as cheap as possible

to install. Many control topologies are being investigated and some are already being

implemented in practice.

Depending on how the power electronic devices are used inside a wind farm, there are

several different topology options, each with its particular advantages and disadvan-

tages. The topology can include the following.

A completely decentralised control structure with an internal AC network con-

nected to the main grid, with each turbine in the wind farm having its own frequency

converter and its own control system, has the advantage that each wind turbine can

operate at its optimum level with respect to its local wind conditions. A practical

implementation of this structure is the 160 MW Horns Rev offshore wind

farm, based on wind turbines with doubly-fed induction generators of Type C (see

Figure 4.1).

A partly centralised, partly decentralised control structure where the power converter

is ‘split up’ and the output of each turbine is locally rectified and fed into a DC

network, with the whole farm still connected through a central inverter, has been

proposed by Dahlgren et al. (2000), who suggest use of a multipole high voltage

synchronous generator with permanent magnets. However, this solution has not yet

been implemented in practice. This configuration provides all the features of the

variable-speed concept, since each wind turbine can be controlled independently.

The generators could be SCIGs as well, if a VSC were used as rectifier.

A completely centralised control structure has a central power electronic converter

connected at the wind farm’s connection point. The turbines either could have SCIGs

or could have WRSGs. The advantage of such structure is that the internal behaviour

of the wind turbines is separated from the grid behaviour, and thus the wind farm is

robust to possible failures of the grid. The disadvantage of this concept is that all wind

turbines are rotating with the same average angular speed and not at an individual

optimal speed, thereby giving up some of the features of the variable-speed concept,

for each individual wind turbine.

An option of this concept is the centralised reactive power compensation topology

with an advanced static VAR compensation (ASVC) unit. Reactive power compensa-

tion units are widely used in power systems in order to provide the reactive power

balance and to impr ove voltage stability. ASVCs are inverters based on self-commu-

tated switches (i.e. with full, continuous control of the reactive power). They have the

advantage that, in the case of a voltage decrease (e.g. during a grid fault) their

available maximum reactive power decreases more slowly compared with the static

VAR compensation (SVC) units. In Rejsby Hede, Denmark, test equipment with an

8 Mvar ASVC unit has been incorporated into a 24 MW wind farm with 40 stall-

regulated wind turbines. Equipment and test results are described in Sto

ber et al.

(1998).

76 Generators and Power Electronics

//INTEGRAS/KCG/PAGINATION/WILEY/WPS/FINALS_14-12-04/0470855088_05_CHA04.3D – 77 – [53–78/26] 20.12.2004

7:29PM

Another option is the use of a high-voltage DC (HVDC) link as power transmission.

Such an installation is running on Gotland, Sweden (for more details, see Chapter 13).

All wind turbines are connected to the same power converter, and the entire wind farm

is connected to the public supply grid through another power converter. These two

converters are connected to each other through a long, HVDC link cable.

The application of power electronic technology in large wind farms seems thus to be

very promising. It plays a key role in complying with the high requirements that utility

companies impose on wind farms. This technology therefore needs substantial add-

itional research and development.

4.6 Conclusions

This chap ter has provided a brief and comprehensive survey of the generator and power

electronic concepts used by the modern wind turbine industry. A short introduction,

presenting the basic wind turbine topologies and control strategies, was followed by the

state of the art of wind turbines, from an electrical point of view. Old and new

potentially promising concepts of generators and power elect ronics based on technical

aspects and market trends were presented.

It is obvious that the introduction of variable-speed options in wind turbines increases

the number of applicable generator types and further introduces several degrees of

freedom in the combination of generator type and power converter type.

A very significant trend for wind turbines is that large wind farms will have to behave

as integral parts of the electrical power system and develop power plant characteristics.

Power electronic devices are promising technical solutions to provide wind power

installations with power system control capabilities and to improve their effect on power

system stability.

References

[1] Alatalo, M. (1996) Permanent Magnet Machines with Air Gap Windings and Integrated Teeth Windings,

technical report 288, Chalmers University of Technology, Sweden.

[2] Bogalecka, E. (1993) ‘Power Control of a Doubly Fed Induction Generator without Speed or Position

Sensor’, in EPE 5th European Conference on Power Electronics and Application, Volume 8, pp. 224–228.

[3] BTM Consults Aps. (2003) International Wind Energy Department – Word Market Update 2002, forecast

2003–2007, www.btm.du.

[4] Carlson, O., Hylander, J., Thorborg, K. (1996) ‘Survey of Variable Speed Operation of Wind Turbines’, in

1996 European Union Wind Energy Conference, Sweden, pp. 406–409.

[5] Dahlgren, M., Frank, H., Leijon, M., Owman, F., Walfridsson, L. (2000) Wind power goes large scale,

ABB Review 3 31–37.

[6] Dubois, M. R., Polinder, H., Fereira, J. A. (2000) ‘Comparison of Generator Topologies for Direct-drive

Wind Turbines’, in IEEE Nordic Workshop on Power and Industrial Electronics, IEEE, New York, pp. 22–26.

[7] Hansen, A. D., Sørensen, P., Janosi, L., Bech, J. (2001) ‘Wind Farm Modelling for Power Quality’, in

Proceedings of IECON’01, Denver.

[8] Hansen, L. H., Helle L., Blaabjerg F., Ritchie E., Munk-Nielsen S., Bindner, H., Sørensen, P., Bak-Jensen,

B. (2001) Conceptual Survey of Generators and Power Electronics for Wind Turbines, Risø-R-1205(EN),

Risø National Laboratory, Denmark.

[9] Heier, S. (1998) Grid Integration of Wind Energy Conversion Systems, John Wiley & Sons Ltd, Chichester,

UK, and Kassel University, Germany.

Wind Power in Power Systems 77

//INTEGRAS/KCG/PAGINATION/WILEY/WPS/FINALS_14-12-04/0470855088_05_CHA04.3D – 78 – [53–78/26] 20.12.2004

7:29PM

[10] Kazmierkowski, M. P., Krishnan, R., Blaabjerg, F. (2002) Control in Power Electronics – Selected

problems, Academic Press, London and New York.

[11] Kim, J. S., Sul, S. K. (1993) ‘New Control Scheme for AC–DC–AC converter without DC link Electro-

lytic Capacitor’, in IEEE, PESC’93, IEEE, New York, pp. 300–306.

[12] Krause, P. C., Wasynczuk, O., Sudhoff, S. D. (2002) Analysis of Electric Machinery and Drive Systems,

John Wiley & Sons Inc., New York.

[13] Larsson, A

. (2000) The Power Quality of Wind Turbines, PhD dissertation, Chalmers University of

Technology, Go

teborg, Sweden.

[14] Leonhard, W. (1980) Control of Electrical Drives, Springer, Stuttgart.

[15] Mitcham, A. J., Grum, N. (1998) ‘An Integrated LP Shaft Generator for the more Electric Aircraft’, in

IEE Coloquium on All Electric Aircraft, Institute of Electrical Engineers, London, pp. 8/1–8/9.

[16] Mohan N., Undeland, T. M., Robbins, W. P. (1989) Power Electronics: Converters, Applications and

Design, Clarendon Press, Oxford, UK.

[17] Novotny, D. V., Lipo, T. A. (1996) Vector Control and Dynamics of AC Drives, Clarendon Press, Oxford.

[18] Pena, R., Clare, J. C., Asher, G. M. (1996) ‘Doubly Fed Induction Generator using Back-to-back PWM

Converters and its Application to Variable Speed Wind-energy Generation’, IEE Proceedings on Elec-

tronic Power Application 143(3) 231–241.

[19] Rodriguez, J. Moran, L., Gonzalez, A., Silva, C. (1999) ‘High Voltage Multilevel Converter with

Regeneration Capability’, in IEEE, PESC’s 99, Volume 2, IEEE, New York, pp. 1077–1082.

[20] Schuster, A. (1998) ‘A Matrix Converter without Reactive Clamp Elements for an Induction Motor Drive

System’, in IEEE PESC’s, Volume 1, IEEE, New York, pp. 714–720.

[21] Sørensen, P., Bak-Jensen, B., Kristian, J., Hansen, A. D., Janosi, L., Bech, J. (2000) ‘Power Plant

Characteristics of Wind Farms’, in Wind Power for the 21st Century; Proceedings of the International

Conference Kassel, Germany.

[22] Sto

ber, R., Jenkins, N., Pedersen, J. K., Søbrink, K. H., Helgesen Pedersen, K. O. (1998) Power Quality

Improvements of Wind Farms, Eltra, Fredericia, Denmark.

[23] Van Custem, T., Vournas, C. (1998) Voltage Stability of Electric Power Systems, Kluwer Academic

Publishers, Boston, MA.

[24] Wallace, A. K., Oliver, J. A. (1998) ‘Variable-speed Generation Controlled by Passive Elements’, paper

presented at the International Conference in Electric Machines, Turkey.

[25] Wheeler, P. W., Grant, D. A. (1993) ‘A Low Loss Matrix Converter for AC Variable-speed Drives’, in

EPE 5th European Conference on Power Electronics and Application, Volume 5, EPE, pp. 27–32.

78 Generators and Power Electronics

//INTEGRAS/KCG/PAGINATION/WILEY/WPS/FINALS_14-12-04/0470855088_06_CHA05.3D – 79 – [79–96/18] 20.12.2004

7:30PM

Power Quality Standards for

Wind Turbines

John Olav Tande

5.1 Introduction

Injection of wind power into an electric grid affects the voltage quality. As the voltage

quality must be within certain limits to comply with utility requirements, the effect

should be assessed prior to installation. To assess the effect, knowledge about the

electrical characteristics of the wind turbines is needed or else the result could easily

be an inappropriate design of the grid connection. The electrical characteristics of wind

turbines are manufacturer specific but not site specific. This means that by having the

actual parameter values for a specific wind turbine the exp ected impact of the wind

turbine type on voltage quality when deployed at a specific site, possibly as a group of

wind turbines, can be calculated.

Seeing the need for consistent and replicable documentation of the power quality

characteristics of wind turbines, the International Electrotechnical Commission (IEC)

started work to facilitate this in 1996. As a result, IEC 61400-21 (IEC, 2001) was

developed and, today, most large wind turbine manufacturers provide power quality

characteristic data accordingly.

IEC 61400-21 describes procedures for determining the power quality character-

istics of wind turbines. T hese procedures are not presented in this chapter, but

rather the application of the characteristics i s explained. First, a brief description of

the power quality characteristics defined in IEC 61400-21 is given, and thereafter

Wind Power in Power Systems Edited by T. Ackermann

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

//INTEGRAS/KCG/PAGINATION/WILEY/WPS/FINALS_14-12-04/0470855088_06_CHA05.3D – 80 – [79–96/18] 20.12.2004

7:30PM

their application to determine the impact of wind turbines on voltage quality is

explained through a case study considering a 5  750 kW wind farm on a 22 kV

distribution feeder. The case study further demonstrates that possible voltage qual-

ity problems due to installation of a wind turbine may in many cases be overcome

simply by selecting an appropriate wind turbine type and/or by adjusting wind

turbine control parameters.

Until the development of IEC 61400-21 there were no standard procedures for

determining the power quality characteristics of a wind turbine, and simplified rules

were often applied for dimensioning the grid connection of wind turbines (e.g. requiring

a minimum short-circ uit ratio of 25 or that the wind farm should not cause a voltage

increment of more than 1 %). This approach has proved generally to ensure acceptable

voltage quality; however, it has been costly by imposing grid reinforcements not needed

and has greatly limited the development of wind farms in distribution grids. Actually, an

application of the ‘1 % rule’ would allow only one 550 kW wind turbine on the case

study grid. However, a detailed assessment that applies the power quality characteristics

of the wind turbines agrees with measurements that the 5  750 kW wind farm can be

accepted. Hence, simplified rules should be used with care, and it is preferab le to apply a

systematic approach as outlined in this chapter. Certainly, if the wind farm is large or

the grid is very weak, additional analyses may be required to assess the impact on power

system stability and operation.

5.2 Power Quality Characteristics of Wind Turbines

This section gives a brief description of the power quality characteristics of wind

turbines.

According to IEC 61400-21, the following parameters are relevant for characterising

the power quality of a wind turbine:

rated data (P

, Q

, S

, U

and I

);

maximum permitted power, P

(10-minute average);

maximum measured power, P

(60-second average) and P

0:2

(0.2-second average);

reactive power, Q, as 10-minute average values as a function of active power;

flicker coefficient c(

, v

) for continuous operation as a function of the network

impedance phase angle

and annual average wind speed v

;

maximum number of specified switching operations of the wind turbine within a

10-minute period, N

, and a 2-hour period, N

120

;

flicker step factor, k

(

), and voltage change factor, k

(

), for specified switching

operations of the wind turbine as a function of the network impedance phase angle,

;

maximum harmonic currents, I

, during continuous operation given as 10-minute

average data for each harmonic up to the 50th.

In the following sections, the above parameters are briefly described, providing infor-

mation to understand their relevance and indicating variations depending on the wind

turbine type.

80 Power Quality Standards