AckermannTh. (ed) Wind Power in Power Systems

Подождите немного. Документ загружается.

//INTEGRAS/KCG/P AGIN ATION/ WILEY /WPS /FINALS_1 4-12- 04/0470855088_ 16_CHA15 .3D – 341 – [331–348/18]

20.12.2004 7:47PM

The wind turbines with induction generators (Type A in Aralvai and Karunkulam)

will increase reactive power consumption be cause of the leakage reactance in the

induction generators. Wind turbines with thyristor-based converters (Type D in

Karunkulam), will also have an increased reactive power consumption at rated power.

–0.40

–0.30

–0.20

–0.10

0.00

0.10

0.00 0.10 0.20 0.30 0.40 0.50 0.60

Fundamental power (p.u.)

Fundamental reactive power (p.u.)

Aralvai

Radhapuram

Karunkulam

PF = 0.95

PF = 0.85

(a)

–0.40

–0.30

–0.20

–0.10

0.00

0.10

0.00 0.10 0.20 0.30 0.40 0.50 0.60

Fundamental power (p.u.)

Fundamental reactive power (p.u.)

Dhank

Lamba

PF = 0.95

PF = 0.85

(b)

Figure 15.6 Reactive power consumption measured on the primary side of wind farm substations

in (a) Tamil Nadu and (b) Gujarat. Note:PF¼ power factor

Wind Power in Power Systems 341

//INTEGRAS/KCG/P AGIN ATION/ WILEY /WPS /FINALS_1 4-12- 04/0470855088_ 16_CHA15 .3D – 342 – [331–348/18]

20.12.2004 7:47PM

The wind turbines with IGB T-based converters (Type D, in Radhapuram) will not

consume reactive power, not even at rated power. However, as mentioned above, in

all wind farms, transformers and cables will consume reactive power, and this consump-

tion will increase accordingly if power increases to rated power.

15.4.3 Harmonic and interharmonic emission

According to the International Electrotechnical Commission (IEC) standard on the

measurement and assessment of power quality characteristics for grid connected wind

turbines, IEC 61400-21 (IEC, 2001), harmonic emissions have only to be measured and

assessed for wind turbines with power converters. There are reports regarding harmonic

emissions from a few wind turbines with directly grid connected induction generators, but

there is no agreed procedure for measuring harmonic emissions from induction machines.

Further, there is no reported instance of customer dissatisfaction or damage to equipment

as a result of harmonic emissions from such wind turbines. Therefore, IEC 61400-21 does

not require measurements of harmonic emissions from such wind turbines.

Measurements at the wind farm feeders of Dhank substation showed 5th and 7th

harmonic currents from wind turbines with directly grid connected induction generators

(Sørensen 2000). It was concluded that these harmonics are likely to originate from

induction generators with irregular windings. Another explanation is that the capacitors

together with the induction generator and grid reactance cause a resonance, typically

with an eigenfrequency around the 5th and 7th harmonic frequencies. These harmonics

have also been observed in other grids outside India.

Analyses of the harmonic power flow indicate that only one type of wind turbine

emits harmonics. IS 12360 does not include limits for harmonics, but the limits in the

European voltage quality standard EN 50160 are not exceeded. The standar ds do not

include absolute limits for current harmonics, because the grid conditions influence the

acceptable level. However, IEC 61000-3-6 (IEC, 1996a) gives some example values, and

it was concluded that the harmonic emission from the induction generators does not

exceed these limits, with the 5th harmonic current from one feeder reaching the limit.

Measurements at the 110 kV level of Radhapuram substation have indicated a

significant emission of harmonics and interharmonics to that high voltage level.

Figure 15.7 shows the spectrum of the amplitudes of each of the harmonics up to

the 200th harmonic (i.e. approximately 10 kHz). The majority of the wind turbines in

Radhapuram use a power converter with a switching frequency of 5 kHz. The

distribution of the distortions over frequencies in Figure 15.7 indicates that the

source of the distortion is the power converters in the wind farm. Distortions occur

at around multiples of the shifting frequency, but apparently the shifting frequency is

slightly below the stated 5 kHz.

Both voltage and current distortions measured at Radhapuram substation amount to

approximately 6 %. However, measurements with such a high bandwidth and on such a

high voltage level are subject to inaccuracies. The figure also shows that the distortions

are above the 50th harmonic and, consequently, it does not affect the total harmonic

distortion as defined in EN 50160. TNEB in Muppandal has not experienced any

problems with the distortions at Radhapuram substation.

342 Wind Farms in Weak Power Networks in India

//INTEGRAS/KCG/P AGIN ATION/ WILEY /WPS /FINALS_1 4-12- 04/0470855088_ 16_CHA15 .3D – 343 – [331–348/18]

20.12.2004 7:47PM

The measurements at neighb ouring substations at the 110 kV level show that the

harmonic distortions at Radhapuram substation have only a very weak effect on them.

This indicates that the distortion is filtered out by the capacitances of the 110 kV lines.

15.5 Grid Influence on Wind Turbines

Connecting wind turbines to weak grids affects the most essential aspects of wind

turbine operation, such as performance and safety. According to Rajsekhar, Hulle

and Gupta (1998), the inadequate transmission capacity from the wind farms in the

Lamba region has strongly affected the wind turbines’ performance and maintenance,

for instance. This section summarises the effect the connection to weak grids has on

wind turbines in India, regarding these and other aspects.

15.5.1 Power performance

The power performance of the wind turbines connected to the weak grids in India is

influenced by the following grid parameters:

outages;

frequency;

voltage imbalances;

steady-state voltage.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 50 100 150 200

Harmonic number

Amplitude (%)

Voltage

Current

Figure 15.7 Harmonic amplitudes measured at the 110 kV level of Radhapuram substation,

where 79 410 kW wind turbines with integrated gate bipolar transistor based power converters

are connected

Wind Power in Power Systems 343

//INTEGRAS/KCG/P AGIN ATION/ WILEY /WPS /FINALS_1 4-12- 04/0470855088_ 16_CHA15 .3D – 344 – [331–348/18]

20.12.2004 7:47PM

Grid outages reduce the performance simply because wind turbines shut down during

outages. The frequency varia tions in the grid mainly affect the performance of stall-

regulated wind turbines. The performance is modified because changes in the grid

frequency cause changes in the rotor speed of the wind turbine. Thi s changes the angle

of attack of the relative wind speed as seen from the rotating blades, which has a de cisive

influence on the performance of the wind turbine.

Figure 15.8(a) shows calculations of how the conventional 50 Hz power curve of a

stall-regulated wind turbine changes in the case of 48 Hz and 51 Hz grid frequency,

respectively. The figure shows that up to 10 m/s wind speed the performance is very

similar for the three frequencies. If the wind speed exceeds 10 m/s, the wind turbine on

the 48 Hz grid starts to stall. If the frequency, and consequently the rotor speed, is

higher, the stall and the maximum power point will shift to a higher wind speed, and the

maximum power will reach a higher value.

Figure 15.8(a) shows that for frequency variations from 48 Hz to 51 Hz the difference

between the maximum values of the power curves is approximately 20 %. This is a normal

frequency range in Indian grids, according to the measurement shown in Figure 15.5.

A stall-regulated wind turbine designed for a 50 Hz grid connection can pitch its

blades to avoid production above the rated 51 Hz. Alternatively, the wind turbine may

trip, as the generator gets too hot if power production is too high. In both cases, the

performance of the wind turbine will be affected.

Figure 15.8(b ) shows similar performance calculations for a pitch-regulated wind

turbine. It shows how the blade angle control ensures a maximum steady-state power

of 1 p.u. The 51 Hz power curve for the pitch-regulated wind turbine is almost identical

to the 50 Hz power curve, whereas the 48 Hz power curve lies slightly below the 50 Hz

power curve. Consequently, the performance of pitch-regulated wind turbines is

expected to be slightly inferior as a result of the frequency variations. However, the

influence of the frequency on pitch-regu lated wind turbines is much smaller than the

effect it has on stall-regulated wind turbines.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 5 10 15 20 25

Wind speed (m/s)

Power (p.u.)

50 Hz

48 Hz

51 Hz

(a)

Figure 15.8 Power curves for (a) a stall-regulated and (b) a pitch-regulated wind turbine, at

different frequencies

344 Wind Farms in Weak Power Networks in India

//INTEGRAS/KCG/P AGIN ATION/ WILEY /WPS /FINALS_1 4-12- 04/0470855088_ 16_CHA15 .3D – 345 – [331–348/18]

20.12.2004 7:47PM

The performance of wind turbines with power converters is in principle not affected

by frequency variations on the grid, because the frequency on the generator side of the

converter is controlled to optimise rotor speed, independent of grid frequency.

There are also other grid aspects that influence the performance of wind turbines. A

large voltage imbalance will increase losses in induction generators (Lakervi and

Holmes, 1995). The losses themselves cause a minor reduction in the produced power,

but the losses also cause a heating of the generator, which in some cases forces the wind

turbines to shut down.

Low voltage on the wind turbine terminals can cause the voltage relays to trip the

wind turbine. Low voltages also cause increased losses, which has only a small influence

on the performance. However, the losses also cause heating of the generator, which may,

in turn, trip the wind turbine.

15.5.2 Safety

Connection to weak grids also influences the safety of wind turbines:

The large number of grid outages imply an increased frequency of faults, which

reduces the safety.

Grid abnormalities, such as voltage and frequency variations or voltage imbalances,

can trip relays or result in the heating of the generator, for instance, which also

increases the risk of faults.

Low voltages are also critical because they reduce the maximum torque of the induction

generator, which is essential to the safety of wind turbines with directly grid connected

induction generators. Example calculations have shown that a 10 % voltage drop

reduces the maximum torque from approximately 2 times the rated torque to approxi-

mately 1.5 times the rated torque (Vikkelsø, Jensen and Sørensen, 1999).

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 5 10 15 20 25

Wind speed (m/s)

Power (p.u.)

50 Hz

48 Hz

51 Hz

(b)

Figure 15.8 (continued)

Wind Power in Power Systems 345

//INTEGRAS/KCG/P AGIN ATION/ WILEY /WPS /FINALS_1 4-12- 04/0470855088_ 16_CHA15 .3D – 346 – [331–348/18]

20.12.2004 7:47PM

15.5.3 Structural lifetime

The lifetime of the mechanical components is also affected by connection to a weak grid.

In particular, the drive train is exposed. If a wind turbine stops, the generator normally

remains connected to the grid until the brakes have reduced the power. That way the

generator serves as an electrical brake. In the event of a grid outage, the wind turbine

will first accelerate, and then the mechanical brakes will have to provide full braking.

Wind turbi nes with pitch control or active stall control will be able to reduce the stress

by using the pitch to decelerate and stop the wind turbine.

Also, startups in very high winds will put stress on the mechanical system. In

particular, for stall-controlled wind turbines with directly grid connected induction

generators (Type A0), the load on the drive train can be high during connection at high

wind speeds, and this can affect the lifetime of the gearbox.

15.5.4 Stress on electric components

Switching operations of the capacitor banks in wind turbines with directly g rid con-

nected induction generators cause transient overvoltages, which stress the capacitors.

Consequently, the number of switching operations can affect the lifetime of the capaci-

tors.

IEC 831-1 (IEC, 1996b) and IE C 931-1 (IEC, 1996c) include the capacitor design

limits for transient switching voltages. They are limited to a maximum of 5000 switching

operations per year, each with a maximum peak voltage of twice the rated voltage

amplitude. Simulations of switching 100 kvar capacitors to a 225 kW wind turbin e

indicate that the peak value of the voltage does not exceed the specified value of twice

the rated amplitude. The number of 5000 switching operations per year is a very high

limit for a wind turbine, which only switches in capacitors during startup. Capac itors

designed according to standards should therefore not suffer from ageing due to grid

abnormalities, if only no-load compensation is used. Also, the temperature variations in

India are within the limits specified in IEC 831-1 and IEC 931-1.

Poor voltage quality puts stress on directly grid connected induction generators, too.

Voltage imbalances and voltage drops cause increased losses in the generator, which will

heat the generator. In addition, experience with induction motors shows that unbalanced

currents in the generator cause reverse torque, which tends to retard the generators.

15.5.5 Reactive power compensation

As stated above, the react ive power consumption of wind farms has an important

influence on the power system. However, the power system also affects the reactive

power consumption of the wind farms. This is particularly the case for wind turbines

with directly grid connected induction generators and capacitor compensation (mainly

Type A). Mainly the voltage, but also the frequency, of the grid will affect the power

factor. Example calculations have shown that a 10 % voltage drop reduces the power

factor from 0.96 to 0.94, wher eas the influence of the frequency on the power factor is

insignificant.

346 Wind Farms in Weak Power Networks in India

//INTEGRAS/KCG/P AGIN ATION/ WILEY /WPS /FINALS_1 4-12- 04/0470855088_ 16_CHA15 .3D – 347 – [331–348/18]

20.12.2004 7:47PM

15.6 Conclusions

The findings in India show that wind turbines connected to weak grids have an

important influence on the power system, but also that weak grids have an important

influence on wind turbines. These findings apply in general to wind farms connected to

weak grids, but the Indian case is pronounced regarding the weakness of the grids as

well as the wind energy penetration level.

The main concern related to the effect that wind turbines have on the grid is the

variation in the steady-state voltage and the reactive power consumption, as well as the

influence on voltage fluctuation, and harmonic and interharmonic distortions on the grid.

The weak grid in India is characterised by large voltage and frequency variations,

which affects wind turbines regarding their power performance, safety and the life-

time of mechanical and electrical components. Moreover, grid variations modify the

reactive power consumption of the wind farms. Hence, it is not only grid companies

and customers that should have an interest in strong networks, but also wind farm

operators.

References

[1] DEFU (Danish Utilities Research Association) (1998) ‘Connection of Wind Turbines to Low and

Medium Voltage Networks. Elteknikkommiteen’, DEFU KR 111-E, DEFU, Lyngby, Denmark.

[2] European Standard (1999) ‘Voltage Characteristics of Electricity Supplied by Public Distribution Sys-

tems’, EN 50160, European Committee for Electrotechnical Standardisation (CENELEC).

[3] Government of India (1998) ‘Wind Power Development in India: Towards Global Leadership’, Ministry

of Non-conventional Energy Sources, Delhi, India.

[4] Hammons, T. J., Woodford, D., Loughtan, J., Chamia, M., Donahoe, J., Povh, D., Bisewski, B., Long,

W. (2000) ‘Role of HVDC Transmission in Future Energy Development’, IEEE Power Engineering

Review Volume 20, p. 10–25.

[5] IEC (International Electrotechnical Commission) (1996a) ‘Electromagnetic compatibility (EMC). Part 3:

Limits. Section 6: Assessment of emission limits for distorting loads in MV and HV power systems – Basic

EMC publication’, IEC 61000-3-6, IEC, Geneva, Switzerland.

[6] IEC (International Electrotechnical Commission) (1996b) ‘Shunt Power Capacitors of the Self-healing

Type for A.C. Systems Having a Rated Voltage up to and Including 1000 V, General Performance,

Testing and Rating – Safety Requirements – Guide for Installation and Operation’, IEC 831-1, IEC,

Geneva, Switzerland.

[7] IEC (International Electrotechnical Commission) (1996c) ‘Shunt Power Capacitors of the Non-self-

healing Type for A.C. Systems Having a Rated Voltage up to and Including 1000 V, General Perfor-

mance, Testing and Rating – Safety Requirements – Guide for Installation and Operation’, IEC 931-1,

IEC, Geneva, Switzerland.

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

of Grid Connected Wind Turbines’, IEC 61400-21, IEC, Geneva, Switzerland.

[9] Indian Standard (1988) ‘Voltage Bands for Electrical Installations Including Preferred Voltages and

Frequency’, IS 12360, Delhi, India.

[10] Lakervi, E., Holmes, E. J. (1995) ‘Electricity Distribution Network Design’, IEE Power Engineering

Series 21, 2nd Edition. Peter Peregrinus Ltd, UK 1995.

[11] Ponnappapillai, S., Venugopal, M. R. (1999) ‘Policy of TNEB in Developing Wind Farms in Tamil

Nadu’, presented at the Workshop on Wind Power Generation and Power Quality Issues, Trivandrum,

India.

[12] Rajsekhar, B., Hulle, F. V., Gupta, D. (1998) ‘Influence of Weak Grids on Wind Turbines and Economics

of Wind Power Plants in India’, Wind Engineering 22(3) p. 171–181.

Wind Power in Power Systems 347

//INTEGRAS/KCG/P AGIN ATION/ WILEY /WPS /FINALS_1 4-12- 04/0470855088_ 16_CHA15 .3D – 348 – [331–348/18]

20.12.2004 7:47PM

[13] Sørensen, P. (2000) ‘Harmonic and Interharmonic Emission from Wind Turbines’, presented at the

Nordic Wind Power Conference (NWPC), NWPC 2000, Trondheim, Norway.

[14] Sørensen, P., Unnikrishnan, A. K., Lakaparampil, Z. V. (1999) ‘Measurements of Power Quality of Wind

Farms in Tamil Nadu and Gujarat’, presented at the Workshop on Wind Power Generation and Power

Quality Issues, Trivandrum, India.

[15] Sørensen, P. E., Unnikrishnan, A. K., Mathew, S. A. (2001) ‘Wind Farms Connected to Weak Grids in

India’, Wind Energy 4 137–149.

[16] Sørensen, P., Madsen, P. H., Vikkelsø, A. , Jensen, K. K., Fathima, K. A., Unnikrishnan,

A. K., Lakaparampil, Z. V. (2000) ‘Power Quality and Integration of Wind Farms in Weak Grids

in India’, publication R-1172(EN), Risø National Laboratory, Roskilde, Denmark.

[17] Tande, J. O., Nørga

rd, P., Sørensen, P., Sønderga

rd, L., Jørgensen, P., Vikkelsø, A., Kledal,

J. D., Christensen, J. S. (1996) ‘Power Quality and Grid Connection of Wind Turbines’, Sum mary

Report, publication Risø-R-853(Summ.) (EN), DEFU-TR-362 E, Risø National Laboratory, Roskilde,

Denmark.

[18] Taylor, C. W. (1994) ‘Power System Voltage Stability’, California, USA McGraw-Hill, 1994.

[19] Vikkelsø, A., Jensen, K. K., Sørensen, P. (1999) ‘Power Factor Correction in Weak Grids using Power

Capacitors’, presented at the Workshop on Wind Power Generation and Power Quality Issues,

Trivandrum, India.

348 Wind Farms in Weak Power Networks in India

//INTEGRAS/KCG/P AGIN ATION/ WILEY /WPS /FINALS_1 4-12- 04/0470855088_ 17_CHA16 .3D – 349 – [349–364/16]

17.12.2004 10:38PM

Practical Experience with Power

Quality and Wind Power

ke Larsson

16.1 Introduction

Perfect power quality means that the voltage is continuous and sinusoidal, with a

constant amplitude and frequency. Power quality can be expressed in terms of physical

characteristics and electrical properties. It is most often described in terms of voltage,

frequency and interruptions. The quality of the voltage must comply with the require-

ments stipulated in national and international standards. In Europe, voltage quality is

specified in EN 50160 (CENELEC, 1994). In these standards, voltage disturbances are

subdivided into voltage variations, flicker, transients and harmonic distortion. Figure 16.1

shows a classification of different power quality phenomena.

Grid-connected wind turbines may affect power quality. The power quality depends

on the interaction between the grid and the wind turbine. This chapter is subdivided into

five sections. Four secti ons focus on different aspects of voltage disturbances, such as

voltage variations (Section 16.2), flicker (Section 16.3), harmonics (Section 16.4) and

transients (Section 16.5). Section 16.6 focuses on frequency variations. A wind turbine

normally will not cause any interruptions on a high-voltage grid. Interruptions therefore

will not be considered.

16.2 Voltage Variations

Voltage variations can be defined as changes in the RMS (root mean square) value of

the voltage during a short period of time, mostly a few minutes. Voltage variations on

the grid are caused mainly by variations in load and power production units . If wind

Wind Power in Power Systems Edited by T. Ackermann

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

//INTEGRAS/KCG/P AGIN ATION/ WILEY /WPS /FINALS_1 4-12- 04/0470855088_ 17_CHA16 .3D – 350 – [349–364/16]

17.12.2004 10:38PM

power is introduced, voltage variations also emanate from the power produced by the

turbine. All types of wind turbine cause voltage variations. The power production from

wind turbines may vary widely and not only as a result of variations in the wind. It may

also momentarily drop from full to zero power production in the event of an emergency

stop or startup in high winds.

Several methods are used to calculate voltage variations. There are several computer

programs for load-flow calculations available on the market. Utility companies use such

software for predicting voltage variations caused by load variations. Load-flow calcula-

tions can advantag eously be used to calculate variations in the voltage caused by wind

turbines. Another analyt ical method is simply to calculate the voltage variation caused

by the grid impeda nce Z, the active power P and the reactive power Q (Ballard and

Swansborough, 1984). The analyt ical method uses a simple impedance model, shown in

Figure 16.2. U

is the fixed voltage at the end of the power system and U

is the voltage

at the point of common connection (PCC).

The vo ltage at the PCC can be expressed as

¼ a þða

 bÞ

1=2

; ð16 :1Þ

where

a ¼

ðRP  XQÞ;

b ¼ðP

þ Q

ÞZ

and R is the resistance and X is the reactance.

Figure 16.3 shows the calculated voltage of the grid at the PCC for different X/R

ratios and for a constant short-circuit ratio. The sho rt-circuit ratio is defined as the ratio

between the short-circuit power of the grid at the PCC and the rated power of the wind

Power quality

Voltage Frequency Interruptions

Voltage variations Flicker

Harmonics

Transients

Figure 16.1 Classification of different power quality phenomena

Z = R +jXU

Figure 16.2 Simple impedance model. Note: U

¼ fixed voltage at the end of the power system;

¼ fixed voltage at the point of common connection; Z ¼ grid impedance; R ¼ resistance;

X ¼ reactance; P ¼ active power; Q ¼ reactive power

350 Practical Experience with Power Quality