measurements and simulation for power quality of a wind farm

International Journal of Advanced Engineering Technology

Research Article

MEASUREMENTS AND SIMULATION FOR POWER QUALITY OF A WIND FARM 1

1

S.P. Shukla, 2Sushil Kumar Address for correspondence

Department of Electrical Engineering, Bhilai Institute of Technology, Durg, Chattisgarh (India) [email protected] 2 Department of Electrical & Electronics Engineering, Bhilai Institute of Technology, Durg, Chattisgarh [email protected] ABSTRACT This work investigates the impact of windfarm on the distribution network power quality. The quality of wind power is decided by IEEE Standard 519-1992 and IEEE Standard 1547. The experiments were carried out at the Vankuswade wind power project, Satara, (M.S.), India. The results are then compared by MATLAB simulation software (Version R2008a). The system is also simulated for system stability considering various faults on the simulated model. The field measurement results give important indications about the real effects of the integration of large interruptible renewable energy sources within the power system. The simulation results show an agreement with the measured response. KEY WORDS Wind power, Power Quality, Voltage Sag, Voltage Swell, Stability.

INTRODUCTION Wind power generation has experienced a

vicinity of a wind turbine at low voltage

very fast development worldwide mainly due

levels, and (ii) the interaction of wind farm

to environmental reasons. As the wind power

with the power system at the point of common

penetration into the grid is increasing quickly,

coupling with the grid at medium voltage

the influence of wind turbines on the power

level (Fig.1)

1,2, 11

quality is becoming an important issue

8

. Slow voltage variations,

.

flicker, voltage sags, transients and harmonics

The generation of wind power occurs with the

are measured by means of a modern digital

operation of multiple wind turbines in

measurement system. Important measuremed

windfarms. The integration of these wind

results are discussed.

parks into the power system may cause power

consisting

quality concerns. The important issue is how

Generators connected to the distribution

much the power quality will be affected by

network is simulated for various normal and

power production and connection of WTs to

abnormal conditions.

the grid. The purpose of this work is (i) to

The two results are then analyzed.

analyze the power quality in the electrical

IJAET/Vol. I/ Issue I/April-June, 2010/27-36

of

Then a wind farm

Doubly

Fed

Induction


Fig. 1. Wind turbine with DFIG.

Wind Farm Site Description

reactive

Measurements are carried out at Asia’s largest

Capacitors are not provided on H.V. side [2].

wind

at

The site has good power availability with an

Satara

average wind speed above 5m/s. Each

district and Gudhepanchgani in Sangli district

machine is active yaw and pitch regulated

of Maharashtra State (India). The wind

with

electric-

is

Experiments are performed at a wind farm

BHEL/NORDEX make, 3-Phase, 415 Volt,

sites to study the interaction of wind turbine

82/250 kW, 82/424 Amp, 8/6 Pole, 30/40

(induction type) generator with the utility grid.

Turbine RPM, 757/1008 Generator RPM, ∆-

A DFIG has a wound rotor that is connected

connected asynchronous machine. The wind

to

turbine is horizontal axis and installed at 30m

modulated IGBT frequency converter which

hub height. There are 8 nos. of machines

controls the excitation system in order to

installed at the 2 MW demonstration wind

decouple the mechanical and electrical rotor

power

frequency and to match the network and rotor

farm

installed

Vankusavade/Chalkewadi

generator

project

Development

of

sites

under

in

study

Maharashtra

Agency

Energy

(MEDA)

at

power

drawn

power/torque

the

network

from

control

through

a

the

grid.

capability.

pulse-width

frequency (Fig. 1). The wind turbine rotor is

Chalkewadi.

coupled to the generator through a gearbox

The generator is connected directly to the grid

which adapts the two different speeds of rotor

through 0.415/11 and 11/33 kV step-up

and generator. The control system usually

transformers as shown in Fig. 2. Three

keeps the power factor to unity, but the

switching capacitor banks (two of 37.5 kVAR

windfarm can also exchange reactive power

and one of 25 kVAR) are connected across the

with the rest of the network.

generator (i.e. on L.V. side) to compensate the IJAET/Vol. I/ Issue I/April-June, 2010/27-36


Fig. 2. Single line diagram of the test wind farm at Chalkewadi.

measure from 1 to 600V.

Power Quality Measurement Set-Up and Methodology When

•

power

probes

LEM

FLEX

RR3035, current ranges of 30/300/3000A,

performed on a system with wind production,

bandwidth: 10 Hz to 50 kHz, Accuracy:

due to the presence of electronic devices and

1%.

and

The measurement system was placed at the

currents are usually nonsinusoidal quantities.

wind turbine terminal, between the generator

In this application the measurement system

and the LV side of the step-up transformer

has to be carefully chosen and, in particular, it

according to a 3-Φ, 3 wire star connection.

has to be composed by transducers with high

Three channels have been connected to voltage

bandwidth,

blocks,

and current probes. The neutral has been

analog to digital converters a digital signal

connected to common and has been the

processing and, a storage unit.

reference for the three channels in order to

Low Voltage measurement system

measure

The following hardware components have

simplified diagram with only one phase is

been used:

provided in Fig. 3. The measurement system

•

•

converters,

measurements

current

are

frequency

quality

Three

analog

the

voltages

conditioning

the

phase-to-neutral

voltages.

A

Power monitoring instrument Dranetz

permits measuring the currents and voltages

PX5, 8 channels, 4 voltage and 4 current,

instantaneously. Parameters configured for the

256 samples/cycle, RMS Accuracy: ±0.1%

measurement

of Reading, ±0.05% Full Scale, over

minimum, RMS voltage and current values each

7KHz

flicker

minute; mean, minimum and maximum active

according to IEC 61000-4-15, complies

and reactive power and power factor each

with IEEE 1159, IEC 61000-4-30 Class A

minute; total harmonic distortion and individual

and EN50160 [1,5].

harmonics calculated each minute; flicker

bandwidth,

Instrument

voltage

measures

probes

that


can

are:

mean,

maximum

and

measurements, calculated as per IEC 61000-4-


15, with Pst (short term) interval equal to 10 min., Plt (long term) interval equal to 2 hours.

Fig. 3. LV measurement set-up. 3, 9

Medium Voltage measurement system

is equal to 2.77%

The following hardware components have been

are quite high, as the measurements have been

used:

done between the generator and the l.v. side of

. The harmonic emissions

•

Power monitoring instrument Dranetz PX5.

the transformer.

•

One PEARSON ELECTRONICS VD-305A

On the high voltage side of the transformer a

capacitive voltage divider, nominal division

reduced harmonic distortion level it is expected

ratio 2000:1, Maximum Pulse Voltage: 300

due to the damping features of the transformer.

kV, bandwidth: 30 Hz to 4 MHz.

Distortion levels are usually high in DFIG

Current probes LEM FLEX.

turbines due to the frequency converter,

•

In order to analyze the collective behavior of the

depending on the commutation frequency. The

wind power plant the measurement system has

distortion caused by the converter can be clearly

been placed at the point of common coupling

observed in Fig. 5.

(PCC) with the grid according to a single phase

Although harmonic distortion is an important

connection. A simplified connection diagram is

issue but due to the high switching frequencies,

provided in Fig. 4.

the advanced control algorithms and the filtering

Measurement Results and Discussion

techniques used in the wind farm allows

Voltage waveforms analysis

reducing the distortion well down the maximum

The

voltage

waveform

measured

at

the

generator terminals is shown in Fig. 5. The THD IJAET/Vol. I/ Issue I/April-June, 2010/27-36

value tolerated by standards11.


390.7V whereas at no load has been about 401V.

Long-term Voltage Variation Analysis The measured RMS voltage, under normal operating

conditions,

excluding

situations

Fig.5. Phase-to-neutral volt. at gen.

arising from faults or voltage interruptions,

Terminals.

during all observation intervals has been within the range of ± 5% of the nominal voltage. Voltage sags and swells PQ analyzer was set to register transients when thresholds values exceeded. Voltages below 95% and above 105% of the RMS nominal

Fig.6 Phase-to-neutral volt. at PCC.

value have been recorded. Fig. 8 shows voltage

Fig. 6 presents the voltage measured at the PCC between one phase and neutral, the THD is equal to 0.93%. The low distortion is due to the combined

smoothing

effect

due

to

the

aggregation of the generators. Compared with maximum harmonic levels for the power system

sag caused by the WT disconnection from the grid due to excessive

power

production

(overload condition). During the measurement, 18 voltage sags were registered at the generator terminal but no voltage sags were recorded at the PCC.

specified by EN 50160, even though this standard is not applicable to this context, these values of THD are largely within the limits. Voltage Variations with Power Produced It has been observed a voltage variation at the terminals of the generator depending on the power generated. Fig. 7 shows the trend of the

Fig.7 Voltage variation compared with power

active power and voltage RMS for a time period

generated.

with high power production At full load the phase-to-neutral voltage RMS has been about



The measurements at the LV terminals of the wind turbine showed that the flicker level (Pst) is correlated with the active power produced by the generator. In particular the flicker level Fig. 8 Voltage sag at the generator terminal. Flicker

increases with the production and remain about constant even though the power changes as

The torque from a horizontal axis wind turbine

depicted in Figs. 9-10.

has a periodic component at the frequency at which the blades pass the tower (1-2 Hz) caused by a variation of the wind speed seen by the blade.

Such

variation

depends

on

the

combination of the tower shadow, wind shear and turbulence. The torque fluctuations are directly translated into output power flicker as there

is

only

a

partial

buffer

Fig.10. Flicker measured at the generator terminal.

between

mechanical input and electrical output. Fixed speed wind turbine can cause high flicker whereas variable speed one can limit the flicker within reasonable values7. In the proposed measurement campaign the Flicker has been

Fig.11. RMS voltage at the PCC.

analyzed at the LV connection of a single WT and at the PCC of the wind farm.

Fig.12. Power generated one phase at the PCC. The flicker at the PCC depends on the power fluctuations caused by all the turbines of the Fig.9. Power generated by a single turbine.

wind farm are illustrated in Figs. 11-12. Measures


have

revealed

that

the

flicker


increases as the power produced decrease. In

CASE 1-

Fig. 11, part of this flicker effect could be

TURBINE RESPONSE TO A CHANGE IN

imputable to the regulation action of the on-load

WIND SPEED

tap changer (OLTC) installed in the substation

Effect of wind speed variation is shown in fig

transformer as well as caused by switching

14a & b. It increases from 8m/s -20m/s in steps.

operations of start and stop of wind turbines.

The generated power also increases, at t = 32

The results are in good agreement with other

sec. when V = 15.5m/s PGenerated becomes 9MW

contributions related to measurements in similar

i.e. rated capacity of the wind farm. A further

conditions3,4.

increment in wind velocity causes voltage swell

Simulation of a Wind Farm Using DFIG

of 2.5% at the generator terminals a swell of

Wind Turbines

0.25% at the converter terminals (Grid). Voltage

Model Description

swells are eliminated by the pitching of the

A 9-MW wind farm consisting of six 1.5 MW

blades. The pitch angle increases from 0 degree

wind turbines connected to a 25-kV distribution

as soon as the wind speed crosses 15m/s i.e. the

system exports power to a 120-kV grid through

speed for 1p.u. generation.

a 30-km, 25-kV feeder. A 2300V, 2-MVA plant consisting of a motor load (1.68 MW induction motor at 0.93 PF) and of a 200-kW resistive load is connected on the same feeder at bus B25. Both the wind turbine and the motor load have a protection system monitoring voltage, current

(a)

and machine speed. The DC link voltage of the DFIG is also monitored. 4, 6, 11 MATLAB Simulation and discussion of the results Following conditions were applied on the MATLAB model: 1. Turbine response to a change in wind speed. 2. Simulation for stability due to LG fault on the distribution system (25kV). 3. Simulation of a voltage sag on the system IJAET/Vol. I/ Issue I/April-June, 2010/27-36

(b) Fig. 14. (a) V, I , P& Q at Gen. Terminals, (b) VDC , Turbine and Wind speed & Pitch angle.


Fig. 4. MV measurement set-up.

Fig. 13 MATLAB model of DFIG connected to grid. CASE 2-

depicts

SIMULATION FOR STABILITY DUE TO

corresponding current. But the system regains

VARIOUS

steady state without losing stability when the

FAULTS

ON

THE

25-KV

a

large

drop

in

voltage

and

SYSTEM

fault is over. When the same fault was applied

The LG fault is applied on phase A at t=5sec.

for 9 cycles, it was observed that the windfarm

for a duration of 5 cycles. Its effect on load

gets

voltage and current is shown in Fig. 15, which

contribution (P, V & I) becomes zero as shown


tripped

from

the

network

and

its


in Fig. 16 a. This happens due to drop in

CONCLUSIONS

generator terminal voltage below 0.9 p.u. Thus

In this paper, the power quality of a large

the stability of the system is lost. When LLG

windfarm at the low voltage level and at the

fault is applied at t = 37sec. for 5 cycles, the

PCC with the HV grid is investigated. Flicker,

generated power suddenly drops to 37% from

harmonics, and voltage sags have been analyzed

the rated value but comes back to steady state

and correlated with wind characteristics of the

when the fault is over as shown in Fig. 17a and

site. Then same were observed on a simulation

b. When LLLG fault is applied at t = 37sec. for

model. The simulated and the measured results

5 cycles , the windfarm trips immediately from

exhibit a good agreement. The investigation also

the network and does not regain the stability.

shows that the windfarm works without a

Case-3 Simulation of a voltage sag on the

negative impact on the transmission network

system

when DFIG turbines are installed.

Fig. 16 b depicts that effect of LG fault on voltage sag is much severe at the generator terminals (approx.30%), then less severe on the medium voltage distribution network (approx. 10%) and least on the high voltage grid i.e. at PCC (approx. 1%). During LLG and LLLG fault similar behavior was obtained but with

(a)

deeper sag.

(b) Fig. 16. ( a) V, I & P at Gen. Terminals (b) Voltage at Generator, Distribution, Grid Fig 15 .Load (500KW) Voltage and Current after a 5 cycle LG fault. IJAET/Vol. I/ Issue I/April-June, 2010/27-36

terminals.


4. P. Sorensen, A.D. Hansen and P.A.C. Rosas, “Wind models for simulation of power fluctuations from wind farms”, Journal of wind

Engineering

Aerodynamics,

and

Vol.

Industrial

90,pp.

1381-

1402,December 2002. 5. IEEE

Standard

1547:

Interconnecting

Distributed Resources with Electric Power

(a)

System, IEEE Standard 1547-2003, p.p. 1-5, June 2003. 6. Yuriy Kazachkov and Steve Stapleton, “Modelling Wind Farms for power system stability

studies”

Power

Technology,

Newsletter Issue 95, April 2004. 7. S.P.Shukla, B.S.Narang “Harmonics in wind power systems” Applied science periodical, Vol.IX, No. 2, pp129-133, May, 2007. 8. Trinh Trong Chuong, Dragan “Voltage stability of grid investigation of grid

(b)

connected wind farm” PWASET, Vol. 32,

Fig. 17. (a) V, I & P at Gen. Terminals, (b)

August, 2008.

Voltage at Generator, Distribution, Grid

9. S.P.Shukla

terminals.

Recommended

al

“Transient

Control

Techniques in Synchronous Wind Turbine

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