Oct.26-28, 2011, Thailand OP-11 CIGRE-AORC 2011

1

Voltage sag ride through of distribution grid-connected Wind energy system

with D-STATCOM

T. Menaneanatra1, N. Chattranont, S. Wanchana and J.Kaewmanee

2

1Better Care and Power Quality Department

Metropolitan Electricity Authority

132/18 Charansanitwong Rd. Bangkoknoi Bangkok 10700, Thailand

2 Metropolitan Electricity Authority Wat Liab District

121 Chackaphet Rd. Pranakorn Bangkok 10200, Thailand

SUMMARY

In this paper, a wind turbine generator and dynamic reactive power equipment (e.g. D-STATCOM) for

the improvement voltage sag ride through of Doubly Fed Induction Generator (DFIG). Low voltage ride

through (LVRT) performance of the wind turbine and the associated power quality are determined on the

basis of measurement, assessment and the norms followed according to the guideline specified in

International Electro-technical Commission standard, IEC-61400-21. The D-STATCOM connected at a

point of common coupling (PCC) with battery energy storage system (BESS) to improve voltage sag ride

through is simulated using PSCAD/EMTDC software tool. The battery energy storage system is integrated

in this case study in order to support the real power source under voltage fluctuating of the wind power.

The D-STATCOM control scheme for the distribution grid connected wind turbine generation system to

improve voltage sag ride through is implemented. A DFIG 2.5 MW distribution grid connected is

simulated to verify the effectiveness of this method. In the case study, various voltage sags due to

symmetrical and unsymmetrical faults are taken into consideration as the network disturbances. The test

result is fully documented and discussed, moreover, the application of the D-STATCOM in wind turbine

generator to cope with the LVRT is further corroborated.

KEYWORDS

Wind Energy System, DFIG, Low Voltage Ride Through, D-STATCOM, PSCAD/EMTDC

Oct.26-28, 2011, Thailand OP-11 CIGRE-AORC 2011

www.cigre-aorc.com

Email : [email protected]

2

INTRODUCTION

Due to the increased concern on environmental pollution and energy shortage, the interest in

renewable energies in general and wind power in particular has increased tremendously in last few

decades. It is predicted that 12 percent of global electricity demand will be supported by wind energy

by 2020 in terms of its annual growth rate in excess of 30 percent [1]. With the advantages of partial

active and reactive power control capabilities, lower converter costs and less power losses, the doubly

fed induction generator (DFIG) becomes a common used wind turbine (WT) in large wind farms.

However, a major drawback of a DFIG is its high sensitivity to grid faults because of the limited partial

rating of the power converter, even the faults occur far away from the location of the WT [2]. The faults in the

power system can cause voltage dip at the terminal of a WT. This voltage dip leads to surge current in the

stator windings, coupling surge current in the rotor windings.

MODELING OF THE WIND TURBINE AND D-STATCOM

The basic configuration of a DFIG wind turbine is shown in Fig. 1. The wind turbine is connected to the

induction generator through a mechanical shaft system., the power flow between the rotor circuit and the grid must

be controlled both in magnitude and in direction. Therefore, the VFC consists of two IGBT PWM converters

(a rotor-side converter RSC and a grid-side converter GSC) connected back-to-back by a dc-link capacitor.

Control of the DFIG is achieved by controlling the VFC, which includes control of the RSC and control of the

GSC, as shown in Fig. 1. [1]

Wind

wV

mP

l l

GearBox

WindTurbine

DFIG

fL

rabci

Wind TurbineControl

RSC Control GSC Control

rabcv

RSC dcV

C

GSC gabci

gabcv

gr gL

Filter gC

gQgP

eP eQ

Grid

labcisabcvs sP Q

r rP Q

sabci

Fig. 1. Configuration of a DFIG wind turbine connected to a MEA Distribution grid

A. Reactive Power Control

Both the RSC and the GSC can be applied to control the updating, and improving engineering models

and turbine reactive power of the DFIG , as shown in Fig. 3. In the d-q power controllers generate the

reference signals for the inner-loop current controllers of the RSC and GSC, respectively. The

commands of the reactive power can be generated by a supervisory controller of the wind farm, which

in turn can be designed to control for example the power factor or the voltage at the grid connection

point of the wind farm at a desired value. [2]

3

Wind Farm

Supervisory

Controller

∑

∑

*sQ

*gQ

sQ

gQ

+

+

_

_

PI

PI

*dri

*qgi

Current

Controller

Current

Controller

RSC

GSC

∑

*r

r

+_

PI

*qri Current

ControllerGSC

Fig. 2 Reactive Power Control of DFIG wind

turbine

Fig 3 Speed Control of DFIG wind turbine

B. Active Power Control

The RSC usually controls the active power generated by the wind turbine, depending on the available wind

energy at a specific moment. At a certain below-rated wind speed, there exists a unique turbine shaft speed

where the wind turbine extracts the maximum power from the wind. This optimal operating point (i.e.,

optimal shaft speed or maximum power point) is usually determined from the wind turbine power

characteristics. The RSC control system then regulates the stator active power or shaft speed of the DFIG at

this optimal point, as shown in fig. 3. [2]

C. Modeling of the D-STATCOM

The D-STATCOM can be used to increase the line power transmission capacity, to enhance the

voltage/angle stability, or to damp the system oscillatory modes. The objective of D-STATCOM in this

paper is to regulate voltage at the point of common coupling (pcc) in the desired level, by injecting or

absorbing reactive power. Therefore, D-STATCOM can enhance the low-voltage ride-thorough capability

of a wind energy system.[3] Fig. 4 shows the schematic representation of the D-STATCOM. The VSC

converts the dc voltage across the storage device into a set of three-phase ac output voltages. These

voltages are in phase and coupled with the ac system through the reactance of the coupling transformer.

Suitable adjustment of the phase and magnitude of the D-STATCOM output voltages allows effective

control of active and reactive power exchanges between the D-STATCOM and the ac system.[4]

VSC

DC Energy

Storage

Coupling

Transformer

Sensitive

Load

D STATCOM

DistributionBus

T TV

c cV TjX

Fig.4. Schematic representation of the D-STATCOM as a custom power. [4]

In general, transmission of power P jQ over a feeder line with impedance R jX results in a

voltage drop

R P X QV

V

(1)

It can be seen that change in the voltage (V) is directly proportional to the reactive power (Q) as X>> R in

a feeder line. Therefore, supplying reactive power during voltage sags (e.g. grid fault) can improve voltage

stabilization and better dynamic performance of power system, so that only reactive power flows between

4

D-STATCOM and grid. Reactive current IC flowing between D-STATCOM and grid depends on the

voltage difference between VC and VT.

LOW VOLTAGE RIDE THROUH

This contrasts with the utility requirement, just a few years back, when all wind turbines were

required to disconnect during grid faults. The major technologies and solution to achieve LVRT of DFIG

wind turbines using external reactive compensation. A LVRT characteristic requirement curve in IEC

61400-21 [5] The characteristic shows voltage sag following grid disturbance down to 0.2 pu. Voltage at

point of common coupling, wind turbine should keep on-line at least 500 milliseconds. It can be seen the

remained voltage value at the point of common coupling and the minimal sustained duration the DFIG has

to be able to endure.

SIMULATION RESULTS AND CONCLUSSIONS

A. Case study wind turbine connected distribution grid

It is assumed that the DFIG is part of a wind turbine and that the voltage dip occurs somewhere in

the 24 kV distribution grid. The DFIG is connected to this grid through transformers and a feeder line

and operated at rated as shown in Fig. 1. The 690 V stator voltage of the DFIG is transformed to 24

kV by transformer at PCC. Data of a 2.5 MW wind turbine with a DFIG have been used during the

simulations. The machine parameters, that can be found [6].

Voltage dip of 80%, implying, that only 20% of the grid voltage remains and a duration of 0.5 second

due to three phase fault at PCC as shown in Fig. 6. The d-axis and q-axis component of the rotor current are

shown in the figure. It can be seen that the rotor currents oscillates to about 2.5 times the rated current (see

Figure 7). If nothing is done to protect the converter, it will be destroyed completely.

B. Voltage sag ride through by D-STATCOM

When the grip faults (three phase fault) and dip hold on for a longer time, it can be required that the

generator supplied reactive power, the GSC may not be able to provide sufficient reactive power and

voltage support due to it small power capacity. As a result, there can be a risk of voltage instability and the

subsequent tripping of the wind turbine generator. To prevent such a contingency, external dynamic

reactive compensation. The proposed protection schemed also offers D-STATCOM 3 MVAR is

installed at PCC to supply reactive power during the grid fault and immediately after the fault is cleared.

The resulting in an improved voltage profile (see Figure 8) and keeping the rotor current to stay within the

rated values, apart from some peaks during the transient are shown in Figure 9, respectively.

Fig. 6 Stator Voltage for a Voltage dip of 80%,

0.5 s Fig. 7 Rotor current di (bottom) and qi (top) for a

Voltage dip of 80%, 0.5 s without

D-STATCOM

5

Fig. 9 Stator Voltage with D- STATCOM

connected at PCC

Fig. 7 Rotor current di (bottom) and qi (top) for a

Voltage dip of 80%, 0.5 s with D-TATCOM

BIBLIOGRAPHY [1] Fernando D. Bianchi. (Wind Turbine Control Systems: Principles, Modelling and Gain

Scheduling Design, Springer, 2006).

[2] Gesche Krause. (From Turbine to Wind Farms - Technical Requirements and Spin-Off Products,

InTech, 2011).

[3] Lingfeng Wang, Chanan Singh, and Andrew Kusiak. (Wind Power Systems: Applications of

Computational Intelligence, Springer, 2010).

[4] Thomas Ackermann. (“Wind Power in Power Systems”, John Wiley & Sons, 2005)

[5] S. M. Muyeen. (Stability Augmentation of a Grid-connected Wind Farm, Springer, 2008).

[6] Gaston Orlando Suvire. (Wind Farm - Impact in Power System and Alternatives to Improve the

Integration, InTech, 2011).

[7] Wei Qiao, "Dynamic modeling and control of doubly fed induction generators driven by wind

turbines,". IEEE/PES Power Systems Conference and Exposition, pp.1-8, 2009.

[8] J.Morren and S.W.H. de Haan, "Ridethrough of wind turbines with doulby-fed induction generator

during a voltage dip," IEEE Transactions on Energy Conversion, vol.20, pp.435-441, 2005.

[9] Mohod S.W., Aware, M.V., "A STATCOM-Control Scheme for Grid Connected Wind Energy

System for Power Quality Improvement" IEEE, vol. 4, Issue: 3, pp. 346 – 352, 2010.

[10] Olimpo Anaya-Lara and E. Acha, “Modeling and Analysis of Custom Power Systems by

PSCAD/EMTDC” IEEE Transactions on Power Quality, Vol.17,No.1,January 2002.

[11] Wind turbine generator systems-Part21, INTERNATIONAL STANDARD-IEC 61400-21, 2001.

[12] Shan-Ying Li, Yu Sun, Tao Wu, Yu-Zhi Liang, Xiao Yu, Jian-Ming Zhang, "Analysis of Low

Voltage Ride through Capability in Wind Turbine Based on DFIG," Electrical and Control

Engineering (ICECE) International Conference, pp.3331-3334, 2010 .

[13] Chi Jin and Peng Wang, "Enhancement of Low Voltage Ride-Through Capability for Wind

Trubine Driven DFIG with active Crowbar and Battery Energy Storage System," IEEE Power

and Energy Society General Meeting, pp.1-8, 2010

Tanit Menaneanatra received the M.Eng. degree in electrical engineering in 2008 from the King

Mongkut's University of Technology North Bangkok, Thailand

Currently, he is electrical engineer at Better Care and Power Quality Department, Metropolitan Electricity

Authority (MEA) Bangkok, Thailand. His research covers electric systems for voltage sag mitigation by

improving protection in distribution system, stochastic voltage sag prediction in distribution system by

Monte Carlo simulation and PSCAD/EMTDC.

Tanit MeananeatraNaris Chattranont Voltage sag ride through of distribution

id d Wi d i hgrid-connected Wind energy system with D-STATCOM

Suraphol Wanchana

Jutanon Kaewmanee*

AORCAORC--CIGRECIGREASIAASIA OCEANIA REFIONAL COUNCIL OF CIGREOCEANIA REFIONAL COUNCIL OF CIGRE

Better Care and Power Quality Department andWat Lieab District*ASIAASIA--OCEANIA REFIONAL COUNCIL OF CIGREOCEANIA REFIONAL COUNCIL OF CIGRE

October October 2626--28 28 ,,20112011. Chiang Mai.. Chiang Mai. Metropolitan Electricity AuthorityThailand

OutlinesOutlinesOutlinesOutlines

Introduction

Modeling of The Wind turbine and D-STATCOMModeling of The Wind turbine and D-STATCOM

Simulation Results

Conclusion

AORCAORC--CIGRE ASIACIGRE ASIA--OCEANIA REFIONAL COUNCIL OF CIGRE October OCEANIA REFIONAL COUNCIL OF CIGRE October 2626--28 28 ,,20112011. Chiang Mai.. Chiang Mai.

IntroductionIntroductionIntroductionIntroduction

A major drawback of a DFIG is its high sensitivity to gridA major drawback of a DFIG is its high sensitivity to gridfaults because of the limited partial rating of the power converter, even the faults occur far away from the location of the WT [2]. The faults in the power system can cause voltage dip at the terminal of a WT. This voltage dip leads to surge current in the stator windings coupling surge current in the rotor windingsstator windings, coupling surge current in the rotor windings.


IntroductionIntroductionIntroductionIntroduction

This paper presents a wind turbine generator and dynamic reactiveThis paper presents a wind turbine generator and dynamic reactivepower equipment (e.g. D-STATCOM) for the improvement voltage sagride through of Doubly Fed Induction Generator (DFIG) in which timed i l i i i d t i th PSCAD/EMTDC ftdomain analysis is carried out using the PSCAD/EMTDC softwarepackage.

l id h h ( ) f f h i d biLow voltage ride through (LVRT) performance of the wind turbine is referred to IEC-61400-21.

The developed tool is tested with a distribution grid of Metropolitan Electricity Authority (MEA).


Modeling of The wind turbineModeling of The wind turbineModeling of The wind turbineModeling of The wind turbine

Wind mP

Gear d

labcisabcvs sP Qsabci

wVlω lω

GearBox

Wind

DFIG

L gQ gP eP eQ

Grid

r rP QWindTurbine fL

rabciβ

Wind Turbine

RSC dcV

C

GSC gabcigr gL

gQ g

Wind TurbineControl

RSC Control GSC Control

rabcv gabcv Filter gC

Fig.1. Configuration of a DFIG wind turbine connected to a MEA Distribution grid


Modeling of Control strategy of DFIGModeling of Control strategy of DFIGModeling of Control strategy of DFIGModeling of Control strategy of DFIG

The commands of the reactive power can be generated by a supervisoryThe commands of the reactive power can be generated by a supervisory controller of the wind farm, which in turn can be designed to control for example the power factor or the voltage at the grid connection point of the wind farm at a desired value

*QsQ

*i

the wind farm at a desired value.

sQ

gQ

dri

*gQ

*qgi

Fig. 2 Reactive Power Control of DFIG wind turbine


Modeling of Control strategy of DFIGModeling of Control strategy of DFIGModeling of Control strategy of DFIGModeling of Control strategy of DFIG

Th RSC ll t l th ti t d b th i d t biThe RSC usually controls the active power generated by the wind turbine, depending on the available wind energy at a specific moment.

The RSC control system then regulates the stator active power or shaft speed of the DFIG at this optimal point, as shown in fig. 3

*rω

rω*qrir qr

Fig 3 Speed Control of DFIG wind turbine


LOW VOLTAGE RIDE THROUGH (LVRT)LOW VOLTAGE RIDE THROUGH (LVRT)LOW VOLTAGE RIDE THROUGH (LVRT)LOW VOLTAGE RIDE THROUGH (LVRT)

A LVRT h t i ti i t i IEC 61400 21 [5] ThA LVRT characteristic requirement curve in IEC 61400-21 [5] The characteristic shows voltage sag following grid disturbance down to 0.2 pu. Voltage at point of common coupling, wind turbine should keep on-line at least 500 milliseconds.

Fig 4 LVRT profile referred IEC 61400 21


Fig 4 LVRT profile referred IEC 61400-21

Modeling of DModeling of D STATCOMSTATCOM

The objective of D STATCOM in

Modeling of DModeling of D--STATCOMSTATCOM

The objective of D-STATCOM in this paper is to regulate voltage at the point of common coupling (pcc) in the d i d l l b i j ti b bi

SensitiveLoad

T TV θ∠

desired level, by injecting or absorbing reactive power. Coupling

TransformerDistribution

Bus

V θ∠TjX

The change in the voltage (ΔV) is directly proportional to the reactive power (Q) as X>> R in a feeder line as

VSC

DC Energy

D STATCOM−

c cV θ∠

power (Q) as X>> R in a feeder line as equation below.

Fig 5. Schematic representation of

DC EnergyStorage

( )R P X QV

⋅ + ⋅ΔFig 5. Schematic representation of

the D-STATCOM as a custom power

( )V

VΔ =


Simulation Results : Case studySimulation Results : Case study

A. Case study wind turbine connected distribution grid

B. Voltage sag ride through by D-STATCOM


Simulation Results : ConditionsSimulation Results : Conditions

It is assumed that three phase fault at distribution gridIt is assumed that three phase fault at distribution grid occured at 0.6 s, duration 0.5 s.

Th i d i d d d lThe wind energy system is operated at rated power. and normal poeration mode of DFIG. ( ,speed control mode and blade angle is fixed)

* 0gQ =

The D-STATCOM ± 3 MVAR is installed at PCC for improving

angle is fixed)

voltage sag ride through.


Simulation Results : A. Case study wind turbine connected distribution grid

Fig. 6 Stator Voltage for a Voltage dip of 80%, duration 0.5 s

Fig. 7 Rotor current id (bottom) and iq (top) for a Voltage dip of 80%, 0.5 s without D-STATCOM


Simulation Results : B. Voltage sag ride through by D-STATCOM

Fig. 8 Stator Voltage with D- STATCOM connected at PCC

Fig. 9 Rotor current id (bottom) and iq (top) for improving by D-STATCOM


ConclusionConclusion

The Rotor current i (proportional reactive power) and i (proportional activeThe Rotor current id (proportional reactive power) and iq (proportional active power) is increasing about 2.6 times the rated current, when voltage dip 80%,duration 0.5 s. If nothing is done to protect the converter, it will be destroyed.

When the dip holds on for a longer time,it can be required that the generator supplied reactive power.

The Rotor current id (proportional reactive power) and iq (proportional active power) is decreasing about 1 5 times the rated current when voltage dip as same

pp p

The installation of D-STATCOM can be improving LVRT of DFIG cause to

power) is decreasing about 1.5 times the rated current, when voltage dip as same.

voltage dip 80%,duration 0.5 s (referred IEC 61400-21)


Documents

Oct.26-28, 2011, Thailand OP-11 CIGRE-AORC 2011