Fault Ride-Through Study of Wind Turbines

doi:10.4236/jpee.2013.15004

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Journal of Power and Energy Engineering, 2013, 1, 25-29

http://dx.doi.org/10.4236/jpee.2013.15004 Published Online October 2013 (http://www.scirp.org/journal/jpee)

Fault Ride-Through Study of Wind Turbines

Xinyan Zhang1, Xuan Cao2, Weiqing Wang1, Chao Yun1

1School of Electrical Engineering, Xinjiang University, Urumqi, China; 2School of Electronics Information Engineering, China Civil

Aviation University, Tianjin, China.

Email: [email protected]

Received September 2013

ABSTRACT

The installation of wind energy has increased rapidly around the world. The grid codes about the wind energy require

wind turbine (WT) has the ability of fault (or low voltage) ride-through (FRT). To study the FRT operation of the wind

farms, three methods were discusse d. First, the rotor short c ur r e nt of doubly-fed induction generator (DFIG) was limited

by introducing a rotor side protection circuit. Second, the voltage of DC bus was limited by a DC energy absorb circuit.

Third, STATCOM was used to increase the low level voltages of the wind farm. Simulation under MATLAB was stu-

died and the corresponding results were given and discussed. The methods proposed in this paper can limit the rotor

short current and the DC voltage of the DFIG WT to some degree, but the vo ltage support to the power system dur ing

the fault largely depend on the installation place of STAT COM.

Keywords: Wind Energy; Fault Ride-Through; Doubly-Fed inducti on Generat or; Wi nd Farm

1. Introduction

Large scale of wind power has been installed in every

where around the world. Both the wind farm installation

capability and the WT capacity have increased rapidly.

Although the time start to use of wind power in China is

very late, the development is very fas t. There wer e many

constant speed wind turbines (CSWT) using stall control

with squirrel cage induction generators (SCIG) installed

in the wind farms in China. Newly installed WTs are

dominated by variable-speed wind turbines (VSWT) us-

ing pitch control with DFIG. VSWT with direct-driven

permanent-magnet synchronous generator (DDPSG) has

been successfully studied and installed. Now the 6MW

DDPSG WT used for the offshore wind farm has being

developed. The wind power capacities installation in China

has increased very rapidly. Figure 1 is the wind power

capacities installation in one prov ince of China.

Power system operation with increasing wind power

penetration will become more and more difficult. More

conventional power plants will be replaced by wind

farms and accordingly the stability of the power system

will be affected. So the new grid codes have proposed

even strict requirement to the wind power. Not only the

wind turbines should be kept connection in grid during

the fault and fast recover power generation after fault

clearance, but also the wind turbines or wind farm can

provide the voltage support and generate capacitive reac-

tive power [1]. To meet this requirement, there are many

researchers put their study on this subject. The studies

can be mainly divided into three categories. The first one

is mainly concern about the protection of the rotor side

converter of DFIG WT during the fault. Paper [2] has

given a through discussion about the operation process of

the rotor side crowbar of DFIG WT. Paper [3] has pro-

vided a method to improve the vector control and limit

the rotor current. Paper [4] has proposed a new magnet

excitation method to counteract the transient DC flux.

The second one is mainly concer n about how to make the

wind turbine fast recover power generation after the fault

clearance. Paper [5] has discussed some methods to pro-

tect the VSWT with DFIG and DDPSG and their fast

recover power generation capabilities. Paper [6] has stu-

died the double vector control method. Paper [7] has

made a detail study of the post-fault behavior of the

power system with wind power connected. The third one

is mainly concern the voltage and reactive power support

to the power system during the fault. Paper [8] has pro-

vided a rotor side converter reactive control. Paper [9]

has discussed the DDPSG WT FRT methods under unity,

leading and lagging power factor respectively. There are

new publications, as discussed in paper [10-12] also give

some suggestions and simulation results about the fault

ride-through of wi n d turbine generator systems.

The rotor current and DC voltage limitation is mainly

discussed in this paper. The method to give a voltage

support to power system by using STATCOM in the

wind farm is also proposed here.

Fault Ride-Through Study of Wind Turbines

Figure 1. The WTs installation capabilities in Xinjiang of China.

2. Fault Ride-Through Requirement of WTS

The increasing and expansion of wind power has set

some new problems to power system. The power system

with large scale wind power will involve problems not

only in steady state operation but also in contingency

condition. FRT requires keep the WTs on the grid during

faults so that they can contribute to the stability to the

power transmission system. Experts have done many re-

searches about the behaviors of WTs. Figure 2(a) gives

the simulation results of the behavior of induction gene-

rator based on WT following grid faults. We can find

after 250 ms, if the fault still can not be cleared, it will

lead to voltage collapse. F igure 2(b) is the basic re-

quirements of fault ride-through of E.ON of Germany. In

the second (blue) area, the WTs should be kept on grid,

but if the WTs face overloads and stability problems,

they can short time interrupt (STI), but the STI time must

be far less than 2s.

3. Protection Measure Study of WT for FRT

Voltage drop is the most common fault occurred in the

power system. The depths of the voltage sags are differ-

ent according to the place where the fault occurrence.

The lowest depth of voltage sag can be zero. To meet the

requirement of grid for FRT of WTs, WT must keep in

grid while there is a fault in the power system, such as

single-phase ground, three-phase ground, etc. Nowadays,

the majority WTs use DFIG as electrical power generator.

The stator of DFIG is directly connected to the grid,

while its three-phase rotor windings are coupled to the

grid through a back-to-back partial size (about 20% - 30%

of the rated power of DFIG) converter. A short circuit

within the grid may lead to a 5 - 8 times of the rated sta-

tor current of DFIG. So the mechanical drive and shaft

will subject to a considerable stress. And the very high

rotor current may lead to the rotor side converter damage

(a)

(b)

Figure 2. The voltages under fault and FRT requirement, (a)

the voltages under fault with different fault clearance times,

(b) boundary conditions for FRT requireme nt .

and the DC bus over-voltage. Therefore, most measures

are proposed to protect the converter and DC bus. Ac-

tually, the stator will also end ure a high transien t current.

At present, the protection circuit of VSWTs with DFIG

mainly includes three parts. They are rotor side crowbar,

DC bus chopper and stator side crowbar used to protect

rotor side converter, DC bus and the DFIG. In this sec-

tion, we major discuss the rotor side and DC bus protec-

Fault Ride-Through Study of Wind Turbines

tion. The block diagram is shown in Figure 3.

3.1. The Rotor Side Protection Measure

During the fault of the power system, the stator of DFIG

will endure a large fault current because it is directly

connected to the grid. The disturbance will be further

transmitted to the rotor of DFIG for the reason that the

rotor and stator is magnetically coupled and the flux must

be conservative. So the rotor current will be very high

and this may lead to over current to the rotor side con-

verter. We use IGBT and resister to make an over current

limiter (shown in Figure 3) to protect th e rotor side con-

verter. Figure 4(a) gives the simulation results of rotor

current. We can find that u se the limiter, th e rotor curr ent

can be limited within the rated range.

The DC voltage with and without rotor side over cur-

rent limiter are shown in Figures 4 (b) and (c) respec-

tively. The simulation result shows that if there is not

rotor current limiter, the DC voltage will be 1275 V

when the fault occur and it will be 1273 V when the fault

is just cleared; if there is the current limiter, the DC vol-

tage can be dropped in some degree, the value is 1260 V

when the fault occur and is 1242 V when the fault is just

cleared. So there is still a 5% over voltage.

3.2. The DC Bus Protection Measure

Use When the fault (s hor t circuit) occurs, the voltage will

drop considerably and the depth is zero in the place

where the fault is located. In the fault moment, the grid

side converter (GS C) lost its ability to transfer the pow er

from the rotor side converter (RSC) to the grid, but the

WT is still driven by the wind energy, so there will be a

lot of energy gathered in the DC part of the back-to-back

power electronic converter. This will make the DC capa-

citor be over charged and consequently the voltage of the

DC bus will be very high. Use the rotor current limiter,

the transient over current can b e reduced, this will lead to

the drop of the DC over voltage. But just as discussed in

subsection A of this section, the DC bus still has a con-

siderable over voltage. So we introduce a DC voltage

limiter by using the chopper shown in Figure 2 to absorb

the additional energy. Figure 5 shows the simulation

result of the DC voltage while th e DC over voltage limi-

ter is used. It can be found the over voltage has been re-

duced from 1300 kV to 1235 kV, but there is still 3% at

the moment when the fault is cleared. And the voltage

drop can not be removed.

4. Voltage Support

Conventional synchronous generator is able to supply

high short-circuit current to the fault location and holds

the voltage in the power system so the low voltage area

be reduced. As the increasing of the wind power penetra-

Figure 3. The block diagram of protection measure for FRT

of DFIG WT.

(a)

(b)

(c)

Figure 4. The simulation results of rotor current and DC

voltage of DFIG WT, (a) the rotor current during fault; (b)

the DC voltage without rotor current limitation; (c) the DC

voltage with rotor current limitation.

Figure 5. Simulation result of the DC voltage with DC over

voltage limiter.

Fault Ride-Through Study of Wind Turbines

tion in power system more and mor e conventional power

will be replaced by the wind power. So the wind farm

must have the ability to give the power system voltage

support during the fault condition. There are some re-

searches in this aspect. One is major in study how to

control the reactive current and then let the WT has the

ability to generate reactive power. The other is major in

how to improve the voltage of the wind farm and give

reactive power to the system at the same time. Because

the reactive power generated by the WT is limited, more

attention now is drawn to improve the voltage by use

FACTS, especially in the condition that th e wind farm is

built by SCIG WTs for the reason that this kind of WTs

can not generate reactive power by the control within the

generator.

We use a simplified system of a power system with

wind farm shown in Figure 6 to study the voltage sup-

port of the wind farm by using STATCOM, because

STATCOM not only can improve the voltage level but

also can generate both lagging and leading reactive pow-

er. In Figure 6, LV means low level voltage, MV means

middle le ve l v oltage, H V means high l evel volt a ge.

Figure 7 shows the simulation result when the wind

farm is built by DFIG WTs.

From Figure 7(a), we can find using STATCOM the

absorbed reactive power can be reduced, especially at the

moment that the fault is occurred and cleared. Figure 7(b)

shows the LV voltage condition while there is fault. From

this result, we find the voltage can be increased. But at

the moment the fault is cleared, the voltage has more

than 10% over voltage. Figure 7(c) shows the LV vol-

tage condition while there is serious voltage dip. From

this result, we find the voltage can be increased to 90%

of the rated voltage level. But at the moment the fault is

cleared, the voltage has also nearly 10% over voltage.

About this problem we need further study in future.

Figure 8 shows the simulation result when the wind

farm is built by SCIG WTs.

Figure 8(a) shows the LV voltage and MV voltage

when there is fault occurred in the power system; Figure

8(b) shows the voltages of LV bus and MV bus while th e

STATCOM is connected in the MV bus during the fault;

Figure 8(c) shows the voltages of LV bus and MV bus

while the STATCOM is connected in the LV bus during

the fault. From the simulation results, we can find using

STATCOM the voltage s of both LV bus and MV bus can

be increased. But the voltage amplitudes increased are

different according to the place where STATCOM lo-

cated. STATCOM connected to the LV bus is better for

the wind farm.

Figure 6. The power system with wind farm.

(a)

(b)

(c)

Figure 7. Simulation result under fault condition when the

wind farm is built by DFIG WTs, (a) the reactive power of

the wind farm during the voltage dip, (b)the LV voltage of

the wind farm during the serious voltage dip.

5. Conclusion

The power system fault will lead to voltage dip on WTs.

To maintain the grid stability, wind farm is required to

keep connected in the power system for a defined time

period under grid fault, this is called FRT. Actually, the

voltage is not always dip to zero, it can be just a voltage

sag. So many researchers put their efforts to deal with the

so called low voltage ride-through problem. The main

differences in FRTs requirement of different countries

are the depth of voltage drop, the time period and the

boundary where WTs can be tripped. New FRT needs

not only the WTs keep on grid but also can provide vol-

tage support or generator reactive power to the power

system. So nowad ays, the FRT researchers care even more

subjects. How to protect the converter and DC bus of

WTs, how to control the generator to generate reactive

power and how to increase the voltage under grid fault

are the three main study directions.

6. Acknowledgements

The work is supported by NSFC of Xinjiang of China

(No. 2011211A016) and Doctor Project of Xinjiang Uni-

Fault Ride-Through Study of Wind Turbines

(a)

(b)

(c)

Figure 8. The LV and MV voltage of system shown in Fig-

ure 6 under fault when wind farm is built by SCIG WTs, (a)

without STATCOM, (b) STATCOM connected in the MV

bus, (c) STATCOM connected in LV bus.

versity (No. BS100122) .

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