low voltage ride-through capability improvement of wind power

Low Voltage Ride-Through Capability Improvement of Wind Power

Generation Using Dynamic Voltage Restorer

NAOHIRO HASEGAWA, TERUHISA KUMANO

Department of Electronics and Bioinformatics, School of Science & Technology

Meiji University

1-1-1 Higashi-mita, Tama-ku, Kawasaki-shi, Kanagawa 214-8571

JAPAN

[email protected], [email protected]

Abstract: - Recently, the total amount of generation from wind power plants has been increased all over the world.

In this situation, a large amount of disconnection of wind generation may give a serious influence in the power

system. Consequently, Low Voltage Ride-Through (LVRT) is now required for wind power plants in many

countries. This paper studies LVRT capability enhancement using Dynamic Voltage Restorer (DVR), especially it

purposes to reduce DVR capacity. It shows that limit of DVR output and only reactive power output achieves to

reduce device MVA rating capacity and energy storage capacity.

Key-Words: - Wind power generation, Fixed-speed induction generator, Fault Ride-Through, Low Voltage Ride-

Through, Voltage sag, Dynamic Voltage Restorer, Energy storage

1 Introduction

In recent years, the total capacity of wind generation

connected to the power system has been increased

significantly due to its low environmental cost and low

installation cost compared with other renewable

energy. In this situation, the sudden disconnection of

wind power generation due to the power system

disturbance may collapse power balance between the

power supply and the power demand. In response to

this problem, transmission system operators have

revised grid codes in many countries, and they require

Fault Ride-Through (FRT) capability [1]. FRT is to

keep connection of the wind power generator to the

power system when power system disturbance (e.g.

voltage sag and swell, over and under frequency etc.)

occurs. In FRT, the case of voltage sag is called Low

Voltage Ride-Through (LVRT). However, sudden

voltage sag may cause unstable generator speeding

because of an unbalance between input power

(mechanical) and output power (electrical). In order to

meet the LVRT, the stabilization of the generator and

voltage recovery are needed. But it is very challenging

for wind power generation, especially Fixed-Speed

Induction Generator (FSIG) type wind power plants

because it can not control its active and reactive power

outputs.

There are two methods to enhance LVRT capability

of FSIG. One is to reduce mechanical input of wind

turbine and the other is to increase electrical output of

wind generator during fault and after fault clearance.

The method to reduce mechanical input is represented

by pitch angle controlling [2]. The methods to increase

electrical output are represented by using

mechanically switched capacitor [3], Static Var

Compensator (SVC) [4], STATic synchronous

COMpensator (STATCOM) [5]-[7], Unified Power

Quality Conditioner (UPQC) [6],[7], Dynamic

Voltage Restorer (DVR) [8], and Series braking

resistor [9] etc.

Pitch angle controlling can enhance LVRT

capability and has advantage in the cost. But, the

response of change in the pitch angle is slow in general,

so that this technique has a possibility not enough to

enhance LVRT capability. Using mechanically

switched capacitor has also cost-effective. However,

the ability to supply reactive power declines in

proportion to the square of the voltage, thus it may

degrade the contribution of the capacitor to enhance

LVRT capability. The same thing can be said to the

capacitor-based SVC. Reactive power output from

STATCOM during low voltage is larger than SVC or

capacitor, but STATCOM can not output during

voltage sag in order to avoid injection of additional

fault current into the power system. Therefore,

STATCOM has to be operated after fault clearance.

UPQC and DVR have good performance of LVRT

enhancement. However, both techniques often require

RECENT ADVANCES in ENERGY & ENVIRONMENT

ISSN: 1790-5095 166 ISBN: 978-960-474-159-5

high capital cost because UPQC and DVR need twin

inverter and energy storage device respectively. Series

braking resister is cost-effective but it does not work

effectively after the generator accelerates greatly.

This paper studies LVRT capability enhancement of

FSIG using DVR device, and proposes the method for

decreasing the energy storage and inverter capacity of

DVR. Section 2 describes how to use DVR to FSIG in

order to enhance LVRT capability. Section 3 explains

the simulation model. Section 4 shows numerical

simulation results using EMTP-ATP, where two

simulation cases are described. The first simulation

examines the stabilization effects of the wind

generator when Automatic Voltage Regulator (AVR)

of DVR is operated with its output limitation. The

second simulation shows the case that DVR is

operated after fault clearance. Section 5 is conclusions

in this work.

2 Application of DVR to Wind Power

System

There are two influences that the short circuit in the

power system exerts on FSIG type wind generator.

Firstly, the generator accelerates during voltage sag

caused by short circuit, so that it will be disconnected

by the over-speed relay if it exceeds the maximum

tolerable speed. This phenomenon results from the

fact that the active power output from IG declines by

the square of the terminal voltage, while the

mechanical input from the wind turbine is almost

constant. The maximum speed of generator depends

on the residual voltage, the inertia of generator and

turbine, input wind (mechanical) power and the

duration of the fault. Secondly, huge amount of

absorption of reactive power by IG after fault

clearance may disturb terminal voltage recovery. As a

result, the generator is further accelerated, and it will

be tripped by over-speed or under-voltage relays. For

these two reasons, voltage compensation is a good

solution in order to avoid disconnection of wind

generator (i.e. improvement of LVRT capability).

Therefore, the authors use DVR as voltage sag

compensator.

DVR is a series solid state device that connects

power system in order to regulate the load side voltage.

It has been introduced for the purpose of protecting

sensitive load such as semiconductor fabrication plant

from power system disturbance (e.g. voltage sag,

swell, harmonics, fault current etc.). It can compensate

for voltage sag by low device capacity compared with

UPS used for outage compensation or STATCOM.

This conventional role is modified to a wind generator

protection in this work. General configuration of DVR

consists of the series transformer, the harmonic filter,

the voltage source converter and the energy storage

device.

One-line diagram of the FSIG type wind farm and

the power system with DVR is shown in Fig.1. In this

figure, DVR boosts up the generator side voltage Vr

regulated by the DVR output voltage VDVR in the event

of supply side voltage Vs sags. By this voltage

insertion, DVR can absorb the excess power that

cannot be exported into the power system from the

generator, and inject necessary reactive power.

Block diagram of DVR controller in this work is

presented in Fig. 2. DVR has the function of AVR

because it aims to keep constant voltage usually.

Although there are some methods of voltage insertion,

In-Phase Compensation (IPC) is adopted in AVR

considering that wind power system is robust against

phase jump. IPC is the method that the injected DVR

voltage is in phase with the supply side voltage

regardless of current and the pre-fault voltage.

DVR

FSIG

FSIG

TR1

TR2

TR2

TR_DVR

Vs Vr

VDVR

VPCC

Infinite bus

Fig.1 Fixed-speed wind farm with DVR

Eq. 1: Vd=amp*cos, Vq=amp*sin [IPC]

Eq. 2: Vd=amp*cos(++), Vq=amp*sin(++)

Fig.2 DVR control model

Vs

abc

dq

+

Vr_ref

Vr_d

Vr

Vr_q

+

0

VDVR_d

VDVR_q

dq

abc

VDVR

abc

dq tan-1(Iq/Id)

(AVR)

Vr_d Vr_q

I amp

PLL

PI

PI

Eq.1

Eq.2


ISSN: 1790-5095 167 ISBN: 978-960-474-159-5

Vr_ref in this figure is the reference value of the

generator side voltage Vr, Vr is adjusted by using PI

controller in AVR for Vr_ref. In case of using AVR,

VDVR changes depending on Vs and Vr. In addition to

AVR, this control model can set amp (amplitude of

VDVR) and (phase angle between current and

VDVR). In case of setting amp and , it can control

active and reactive power independently (Eq.2 in this

figure). Eq.1 is used in case of using IPC as constant

voltage. These schemes are showed in Fig.3 by phasor

diagram.

3 Model Configuration

This section describes simulation model. The

studied system is shown in Fig.1. It is 11.4 MVA (10

MW) wind farm composed of 10 squirrel cage

induction generators with a rating of 1.14 MVA (1

MW). Each generator is connected to DVR by 1.2

MVA transformer (TR2:690V/6600V). Shunt

capacitors are adjusted so that the generator terminal

voltage becomes 1.0 p.u. at nominal output power

operation. Ratings of DVR and the series transformer

are 11.4 MVA. These are connected to the power

system by 11.4 MW transformer (TR1:

6600V/66000V). Wind farm is finally connected to

infinite bus through double circuit transmission line.

These parameters are presented in Appendix (Table 1,

2 and 3.). E.ON LVRT requirement [1] and 1.1 p.u.

generator speed limit are assumed.

4 Simulation Results

In this section the simulation results are shown

concerning the influence of DVR given to the power

system and the wind generator when voltage sag

occurred. Subsection 1 presents the results in case of

using AVR for its original purpose (to keep terminal

voltage to be constant during fault). In particular,

relation between DVR capacity and generator

stabilizing effect for various compensation voltage (by

changing Vr_ref in Fig. 2) is studied. Subsection 2

presents the case that DVR output inserts after fault

clearance without compensation during fault. This

subsection analyzes the influence when the DVR

output voltage phase ( in Fig. 2) changes under the

arbitrary voltage insertion.

The generator delivers nominal power (0.877 p.u.

11.4MVA base) to the power system under constant

nominal wind in both simulations. A voltage sag

occurs at infinite bus during t=1.0 to 1.1 s, which

simulates three-phase balanced short circuit. All

simulation cases are carried out numerically using

EMTP-ATP.

4.1 Stabilizing effects using AVR Fig. 4 shows the simulation results in case of using

AVR. It compares the three cases; (1) no control, (2)

voltage control with the reference value Vr_ref 1.0,

and (3) 0.7 p.u. is used as the reference value. In case

of 0.7 p.u. it is active only during fault (t= 1.0 to 1.1 s).

In No control case, voltage oscillation and

unsuccessful voltage recovery can be observed (see (a),

(b)), and the generator reaches over-speed limit after

t=3 s (see (c)). This oscillatory behavior of generator

speed can be explained by the mechanical elasticity

between the turbine and the generator. Active power

output from generator is reduced greatly during the

fault, which causes generator over-speeding in

consequence (see (d)). In contrast, both case of using

AVR can compensate terminal voltage, so that active

power output of the generator increases during fault

and generator speed is stabilized within one second

though some speed increase is noted. As a result of this,

the generator does not reach over-speed limit. The

effects of reference value setting on the resultant

acceleration cannot be observed too much. DVR

output (apparent power) is momentarily exceeded 2.0

p.u. (in case of Vr_ref=1.0) immediately after the fault

clearance because of over voltage due to PI controller

delay (see (e)). This problem is expected to be

mitigated by adjustment of PI parameter. Energy

storage capacity of DVR is shown in (f). Though it is

true that the energy storage capacity in the case of

Vr_ref=0.7 p.u. is lower than the case of 1.0 p.u., it

does not increase simply by a factor of 0.7 because of

taking time to stabilize generator.

[IPC (AVR:variable VDVR, Eq.1: constant VDVR)]

[ = 0 degree]

(only active power)

[ = -90 degree]

(only reactive power)

I Vr

Vs VDVR

I Vr

Vs

VDVR

I

Vr

= -90

Vs

VDVR

Fig.3 Phasor diagram of DVR control method


ISSN: 1790-5095 168 ISBN: 978-960-474-159-5

Fig. 5 shows the relation between the values of

Vr_ref, maximum generator speed and energy storage

capacity of DVR. Vr_ref is chosen from 0.4 p.u. to 1.0

p.u. , energy capacity is measured at t=1.1 s (just after

fault clearance) and t=4.0 s. All these cases can meet

LVRT. Though the maximum speed is decreased and

the absorbed energy at 1.1 s is increased as Vr_ref

increases, the absorbed energy at 4.0 s in case of low

Vr_ref (0.4 and 0.5 p.u.) is more than the case of

Vr_ref=0.6. This result is caused by the fact that it

takes time to stabilize due to low compensation

voltage.

By these results, it can be concluded that slightly

lowering compensation voltage leads to the energy

storage capacity reduction.

4.2 Stabilizing effects in case of post-fault

initiation of DVR This subsection studies the case that DVR is

activated after fault clearance and does not use AVR.

Compensation is started at t=1.12 s (1cycle delay after

fault clearance), and DVR voltage amplifier amp

(see Fig.2) is decreased from 0.1 p.u. to 0 p.u. in

proportion to the elapsed time from control beginning

to t=4.0 s in order to prevent over voltage after

stabilization. DVR voltage phase (see Fig. 2) is set

0 degree and -90 degree, and uses IPC (it follows

supply side phase). The case of 0 and -90 degree

correspond to active power absorption and reactive

power injection respectively, which are defined active

power compensation (APC) and reactive power

compensation (RPC) respectively in this paper.

Simulation results are showed in Fig. 6. It can be

observed that the voltage is decreased greatly and

generator speed increases temporarily in all cases

Fig.5 Maximum generator speed and energy storage

capacity of DVR in case that voltage reference of

AVR is changed

0

0.3

0.6

0.9

1.2

1.5

1

1.02

1.04

1.06

1.08

1.1

0 0.2 0.4 0.6 0.8 1

Ma

xim

um

gen

era

tor s

pee

d [p

u]

Vr_ref [pu]

Maximum

speed

Energy

(t=1.1s)

Energy

(t=4s)

Ener

gy [

MJ]

(f) DVR energy strage capacity (positive: absorb energy)

Fig.4 Response of the wind turbine and DVR using AVR

(a) DVR generator side voltage Vr

(b) voltage at PCC

(c) Generator speed

(d) Generator active power output

(e) DVR apparent power

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 0.5 1 1.5 2 2.5 3 3.5 4

vo

lta

ge [p

u]

Time [s]

No control Vref=1.0

Vref=0.7

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 0.5 1 1.5 2 2.5 3 3.5 4

vo

lta

ge

[pu

]

Time [s]

No control Vref=1.0

Vref=0.7 FRT(E.ON)

0.90

0.95

1.00

1.05

1.10

1.15

1.20

0 0.5 1 1.5 2 2.5 3 3.5 4

Gen

era

tor S

peed

[p

u]

Time [s]

No control Vref=1.0 Vref=0.7 speed limit

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

0.9 1 1.1 1.2 1.3 1.4 1.5

gen

era

tor a

cti

ve p

ow

er

[p

u]

Time [s]

No control Vref=1.0

Vref=0.7

0.0

0.5

1.0

1.5

2.0

2.5

0 0.5 1 1.5 2 2.5 3 3.5 4

Ap

pa

ren

t P

ow

er [

pu

]

Time [s]

Vref=1.0

Vref=0.7

-0.20

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

0 0.5 1 1.5 2 2.5 3 3.5 4

En

ergy

[M

J]

Time [s]

Vref=1.0

Vref=0.7


ISSN: 1790-5095 169 ISBN: 978-960-474-159-5

because DVR is activated after fault clearance (see (a),

(b) and (c)). However, in all cases of activating DVR

increases voltage after fault clearance, so that it finally

successfully stabilizes generator speed. The effects of

voltage compensation and rotation stabilization are the

biggest in IPC, then RPC. They are smallest in APC. It

is thought that this result, in which RPC has a better

performance than APC, arises from the characteristic

of induction generator. In high generator speed

situation compared with nominal operation point,

induction generator absorbs a large amount of reactive

power, while it cannot generate active power too much.

This is showed in Fig. 7. Therefore, the lack of

reactive power supply from DVR causes voltage drop

because APC cannot supply reactive power at all.

Apparent power outputs from DVR (see (d)) of the

three cases are almost the same, and they become

obviously low capacity compared with the case of

using AVR though simple comparison might be

misleading because of difference in voltage output.

Although they have almost the same apparent power

output, active power outputs are not the same as

shown in (e). APC absorbs the largest active power,

while IPC is the second. RPC does not absorb or inject

active power except for the small oscillation.

Consequently, energy storage capacity of DVR in case

of using RPC is zero, which result may be very helpful

because energy storage device such as battery or

electric double-layer capacitor are expensive now. In

addition, the cases of IPC and APC need bigger energy

storage at t=4 s compared with the case of using AVR

because long compensation time is necessary.

By these results, we conclude that it is possible to

stabilize only by handling reactive power in case of

activating DVR after fault clearance. But, this method

cannot be used in the case that generator reaches speed

limit during fault.

Fig.7 An example of generator output speed curve

0

0.5

1

1.5

2

2.5

3

1 1.05 1.1 1.15 1.2 1.25 1.3

Gen

era

tor O

utp

ut [p

u]

Generator speed [pu]

Reactive Power

Active Power

Fig.6 Response of the wind turbine and DVR in case of

starting operation after fault clearance

(a) DVR generator side voltage Vr

(b) voltage at PCC

(c) Generator speed

(d) DVR apparent power

(e) DVR active power (positive: absorb power)

(f) DVR energy strage capacity (positive: absorb energy)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 0.5 1 1.5 2 2.5 3 3.5 4

vo

lta

ge [p

u]

Time [s]

No control APC

RPC IPC

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 0.5 1 1.5 2 2.5 3 3.5 4

vo

lta

ge [

pu

]

Time [s]

No control APC

RPC IPC

FRT(E.ON)

0.90

0.95

1.00

1.05

1.10

1.15

1.20

0 0.5 1 1.5 2 2.5 3 3.5 4

Gen

era

tor S

pee

d [

pu

]

Time [s]

No control APC RPC IPC speed limit

0.00

0.10

0.20

0.30

0.40

0.50

0 0.5 1 1.5 2 2.5 3 3.5 4

Ap

pa

ren

t P

ow

er

[pu

]

Time [s]

APC

RPC

IPC

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

0 0.5 1 1.5 2 2.5 3 3.5 4Acti

ve P

ow

er

[pu

]

Time [s]

APC

RPC

IPC

-0.50

0.00

0.50

1.00

1.50

2.00

2.50

0 0.5 1 1.5 2 2.5 3 3.5 4

En

erg

y [

MJ]

Time [s]

APC

RPC

IPC


ISSN: 1790-5095 170 ISBN: 978-960-474-159-5

5 Conclusions

This paper analyzes LVRT enhancement of FSIG

based wind farm using DVR by numerical simulation.

It simulates two DVR control methods, one is to use

AVR by limiting output, while the other is to control

voltage phase of DVR which is activated after fault

clearance. This study concludes the following points.

(1) The stabilizing effect using AVR has good

performance, but DVR capacity and energy

storage capacity tend to become large.

(2) Limiting output using AVR can reduce DVR

capacity and energy storage capacity, but the

required energy capacity might increase in case

of low compensation voltage on the contrary.

(3) The method in which DVR is deactivated during

fault can also stabilize. In particular the method

with only reactive power injection has advantage

because of small storage capacity.

6 Appendix

Table 1 Wind generator parameters (1.14MVA base) Quantity Value

Nominal apparent power 1.14[MVA]

Nominal power 1.0[MW]

Nominal Voltage 690[V]

Nominal slip -0.0091[pu]

Stator resistance/reactance 0.0063/0.089[pu]

Rotor resistance/reactance 0.0095/0.092[pu]

Magnetizing reactance 2.85[pu]

Generator/Turbine inertia 0.5/3.0[s]

Spring constant 0.55[pu]

Table 2 Transformer parameters (self base) Quantity Value

[TR1] Primary/secondary voltage 66/6.6[kV]

[TR1] apparent power 11.4[MVA]

[TR1] resistance/reactance 0.008/0.08[pu]

[TR2] Primary/secondary voltage 6.6/0.69[kV]

[TR2] apparent power 1.2[MVA]

[TR2] resistance/reactance 0.008/0.08[pu] [TR_DVR] Primary/secondary voltage 6.6/0.44[kV]

[TR_DVR] apparent power 11.4[MVA]

[TR_DVR] resistance/reactance 0.008/0.08[pu]

Table 3 Grid parameter (1000MVA base) Quantity Value

Line resistance/reactance 0.286/3.217[pu]

References:

[1] J. Schlabbach, Low Voltage Fault Ride Through

Criteria for Grid Connection of Wind Turbine

Generators, 5th International conference on

European Electricity Market 2008, pp. 1-4 (2008)

[2] L. Holdsworth, I. Charalambous, J.B. Ekanayake

and N. Jenkins, Power System Fault Ride

Through capabilities of induction generator based

wind turbines, Wind Engineering, Vol. 28, No. 4,

pp. 399-412 (2004)

[3] A. Kehrli, M, Ross, Understanding frid

integration issues at wind farms and solution using

voltage source converter FACTS technology,

IEEE Power Engineering Society General Meeting,

vol. 3, pp. 1822-1828 (2003)

[4] T. Ahmed, O Noro, E. Hiraki, and M. Nakaoka,

Terminal Voltage Regulation Characteristics by

Static Var Compensator for a Three-Phase Self

Excited Induction Generator, IEEE Trans.

Industry Applications, Vol. 40, No. 4, pp. 978-988

(2004)

[5] L. Qi, J, Langston, and M. Steurer, Applying a

STATCOM for Stability Improvement to an

Existing Wind Farm with Fixed-Speed Induction

Generators, IEEE Power and Engergy Society

General Meeting, pp. 1-6 (2008)

[6] M. F. Farias, M. G. Cendoya and P. E. Battaiotto,

Wind Farms in Weak Grids Enhancement of

Ride-Through Capability Using Custom Power

Systems, IEEE/PES Transmission and

Distribution Conference and Exposition Latin

America 2008, pp. 1-5 (2008)

[7] N.G.Jauamto, M. Basu, M.F. Conlon and K.

Gaughan, Rating requirements of the unified

power quality conditioner to integrate the

fixed-speed induction generator-type wind

generation to the grid, IET Renewable Power

Generation, Vol. 3, pp. 133-143 (2009)

[8] H. Gaztanaga, I. Etxeberria Otadui, S. Bacha and

D. Roye, Fixed-Speed Wind Farm Operation

Improvement by Using DVR Devices, IEEE

International Symposium on Industrial Electronics

2007 (ISIE 2007), pp. 2679-2684 (2007)

[9] Andrew Causebrook, David J. Atkinson and Alan

G. Jack, Fault Ride-Through of Large Wind

Farms Using Series Dynamic Braking Resistors

(March 2007), IEEE Trans. on Power Systems,

Vol. 22, No. 3, pp. 966-975 (2007)

[10] S. S. Choi, B. H. Li, and D. M. Vilathgamuwa,

Dynamic Voltage Restoration with Minimum

Energy Injection, IEEE Trans. on Power Systems,

Vol.15, No.1, pp. 51-57 (2000)

[11] Bharat Singh, and S. N. Singh, Wind Power

Interconnection into the Power System: A Review

of Grid Code Requirements, The Electricity

Journal, vol. 22, Issue 5, pp. 54-63 (2009)


ISSN: 1790-5095 171 ISBN: 978-960-474-159-5

low voltage ride-through capability improvement of wind power

Documents