crowbar effect dfig

8/11/2019 Crowbar Effect DFIG

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Analysis of the By-Pass Resistance of an Active

Crowbar for Doubly-Fed Induction Generator Based

Wind Turbines under Grid Faults

Wei Zhang, Peng Zhou, Yikang He,Senior Member, IEEECollege of Electrical Engineering, Zhejiang University, China

Email: [email protected], [email protected], [email protected]

Abstract-The doubly-fed induction generator (DFIG) based

wind turbines of today are often equipped with a crowbar, which

can be used to limit the inrush rotor current and protect the ac

excitation converter from damage via a set of resistors that are

connected to the rotor windings of the DFIG. When the rotor

current becomes too high due to the severe grid voltage dips on the

transmission line, the crowbar is activated and the rotor sideconverter is disconnected from the rotor windings at the same

time, which makes the DFIG behave as a squirrel cage induction

generator with increased rotor resistance. This paper analyzes the

influence of the crowbar resistors on the electrical behavior of the

DFIG during the grid voltage dips. Approximate expression of the

fault stator and rotor currents are deduced from mathematical

analysis and verified by simulation. Different crowbar by-pass

resistances and their impacts on the maximal fault currents and

electromagnetic torque are examined by simulations with different

parameter sets of the DFIG. Based on the simulation results, the

appropriate crowbar by-pass resistance can be effectively

predicted.

Index Terms-Crowbar, by-pass resistance, doubly fed induction

generator, wind turbine, grid voltage dip

I. INTRODUCTION

With the steadily increasing of wind turbines connected to the

grid, the interaction between large scale wind farms and the

regional power systems are becoming more and more

remarkable. As the penetration of wind power increases, wind

turbines are required to romaine connected during grid fault and

contribute to system stability, according to the recent grid codes

[1]. Since the doubly-fed induction generator (DFIG) has been

widely used in wind energy conversion, the low voltage ride

through (LVRT) technology of the DFIG has been investigated

extensively in recent times [2-12].

When the grid voltage dips occur, the power generated by the

DFIG can not be transmitted to the grid in time, which will

induce large fault currents in the stator and rotor windings.

Various improved control strategies have been proposed to deal

This work was supported by the National High Technology Research and

Development of China (the 863 Program Number: 2007AA05Z419) and the

National Natural Science Foundation of China (Project Number: 50577056).

with the inrush over-currents [2-3]. However, these methods

basically restrain the faulty rotor current at the expense of higher

rotor control voltage and, thus, the control capability is

restricted seriously by the limited capacity of the rotor side

converter (RSC). In case of severe grid voltage dips, the most

popular and reliable way to protect the RSC is to apply a

crowbar to short circuit the rotor windings and bypass the faultrotor current [4-9], as shown in Fig. 1.

Once the crowbar is activated, all of the IGBTs in the RSC are

turned off simultaneously, then the DFIG operates as an

asynchronous generator and the controllability over the DFIG

will get lost. At this condition, the resistance of the by-pass

resistors is of great importance on the dynamic behavior of the

DFIG. Several papers have discussed the crowbar resistance and

two main requirements are summarized [10-12]. Firstly, the

resistance should be high enough to limit the inrush rotor

current. Secondly, it should be sufficiently low to avoid high

voltage on the rotor terminals, which can result in the

breaking-down of isolation materials and the charging of dc bus

capacitor through the antiparallel diodes in the RSC. However,

some other factors should also be taken into consideration in

order to determine the appropriate crowbar resistance, such as

the maximum transient torque and the decaying speed of the

fault currents, which have not been investigated and published

in detail yet.

In order to determine the crowbar resistance, the transient

behavior of the DFIG during grid faults should be analyzed

firstly. Several papers have discussed the dynamic response of

the DFIG to grid voltage dips. A theoretical analysis of the

DFIG under voltage dips was proposed in [13]. But the focus of

the analysis was only on the transient rotor voltage, and the fault

currents were not detailedly investigated. The short circuitcurrent of the DFIG was discussed in [14], and the Laplace

transformation was applied to calculate the fault current.

However, the calculation was too complicate to be adopted.

Based on the transient equivalent circuit of the DFIG, reference

[15] proposed a simpler method to calculate the short circuit

current. But it is still hard to calculate the amplitude of the fault

current because of the many unknown parameters in the

expression of the current. In [11], the approximate equations for

the maximum short circuit currents were determined and

compared with the simulated results. Moreover, the crowbar

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B. Faulty Stator and Rotor Currents

In this section, the approximate equations of the faulty stator

and rotor currents are deduced. Some assumptions are made

primarily in order to simplify the analysis.

The assumptions are: firstly, the DFIG rotor is short circuited

by the crowbar as soon as the dip occurs. Secondly, the DFIG is

rotating at the synchronous speed s when the dip occurs.

Thirdly, the DFIG is producing the rated active power before thedip occurs. Finally, the three phase grid voltage drops to zero, i.e.

p=0.

Substituting (4) and (5) into (3), the faulty stator current can

be obtained as [11]

' '/ ( ) /0( ) [ ]s s rt T j t t T ss

t e e eL

=

I (6)

where L is the sum of stator and rotor leakage inductances, i.e.

s rL L L = + , and'

s sT L R= ,

'

r rT L R= . And s0 is the

initial value of the stator flux after the dip occurs, which can be

expressed as0 0

( )s s s

t j= U .

According to (3), since the magnetizing inductance of the

DFIG is much bigger than the leakage inductances, the stator

and rotor current vectors possess approximately the same

amplitude and opposite direction. Based on (6), the maximal

faulty stator and rotor currents can be calculated as

max max

2 ss r

s

UI I

L = (7)

The amplitude of the simulated fault rotor current is shown in

Fig. 4. DFIG 1 is used in this simulation and its parameters are

tabulated in the appendix. As the total leakage inductance

L=0.327, the maximal rotor current calculated from (7) is about6pu, which is highly close to the simulated result.

When the crowbar is applied, the equivalent circuit of the

DFIG can be drawn as in Fig. 5.

Assuming that the by-pass resistance Rcrow is much bigger

than the stator and rotor resistance, the stator and rotor side

equivalent impedance can be calculated as

2

( )

( )

m s crows

s crow

r crow

s L L sL RZ s

sL R

Z s sL R

+=

+

= +

(8)

According to (8), the stator and rotor transient time constants

are derived as

2 2' 1

2

1

'

( )

crow ss

s crow

r crow

R L LT

L L R

T L R

+=

=

(9)

Substituting (8) and (9) into (6) and (6) becomes

'

'/

/0 0( ) ( )s

s r

t Tj t t Ts s rdc

s r

s crow

j et t e e

j L R L

=

I I (10)

(

)

rI

pu

( )t s0 0.05 0.1 0.15 0.2

0

1

2

3

4

5

6

Rcrow=0

Rcrow=0.2 pu

Rcrow=0.5 pu

Rcrow=1 pu

Rcrow=2 pu

Fig. 4. The amplitude of the faulty rotor current with different crowbar

resistances applied

sR sL rLsI rR

mL

rI

s rsd

dt

crowR

sZ rZ

Fig. 5. Equivalent circuit of the DFIG after the crowbar applied

AsRcrowgrows bigger, Trwill become smaller than T

s, which

means that the second term on the right hand side of (10)

attenuates faster than the first one, as shown in Fig. 4 when

Rcrow=0.2pu. Therefore, the faulty stator and rotor currents are

primarily determined by the first term in (10), so that the

maximal faulty current can be deduced as

2 2

max max ( )s r s s crowI I U L R = + (11)

The maximal faulty rotor current obtained from simulationand (11) are shown in Fig. 6. As can be seen, the simulated

results are highly close to the calculated ones for crowbar

resistance above a certain value (about 0.5pu). When Rcrow is

relatively small, the errors between the simulated and calculated

results come from rdc0, which has been ignored in (11).

max

(

)

rI

p

u

( )crowR pu0 0.5 1 1.5 2 2.5 3

0

1

2

3

4

5

6

Calculated Results by (11)

Rotor speed 0.7 pu

Rotor speed 1 pu

Rotor speed 1.3 pu+

Simulated Results

Calculated Results by (12)

Fig. 6. The maximal fault rotor current versus the crowbar resistance

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C. Factors that Affect the Faulty Currents

In section B, some important assumptions are made so as to

simplify the transient state analysis of the DFIG. However, these

assumptions are not always valid for a wind turbine driven

DFIG. So that some factors should be taken into account in order

to make the analytical results more veracious.

Firstly, the rotor speed r at the instant of voltage dip is

examined. Generally, the rotor speed of a DFIG is within therange of 0.3pu around the synchronous speed. Based on the

simulation results, it is found that the higher the rotor speed is,

the bigger the maximal faulty currents are. The relationship

between rand the maximal faulty currents can be expressed as

max max 2 2( )

srs r

s r crow

UI I

L R

=

+

(12)

Fig. 6 shows the maximum fault rotor current obtained from

simulation and calculating by using (12). As can be seen, these

two results agree with each other very well when Rcrow is big

enough.

Secondly, the pre-fault operation condition of the DFIG isconsidered. During the above analysis, it is assumed that the

DFIG produces the rated active power and zero reactive power

before the dip occurs. When this operation condition changes,

the dc rotor flux rdc0, rather than the forced rotor flux rf0, will

change accordingly. However, the impact of rdc0 can be

neglected ifRcrowis big enough, so that the pre-fault operation

condition does not play an important role in the DFIG transient

behavior during the dip when the crowbar is applied.

Finally, if the voltage does not drop to zero, the case will

become more complicated. Besides the attenuated dc flux, there

will be non-attenuated ac component in the stator flux [16], as

expressed in (4). As a result, the faulty currents can likewise beseparated into attenuated and non-attenuated components, and

the amplitude of the faulty currents can be expressed as

'/

2 2[(1 ) cos( )]

( )

st Tsrs r s

s r crow

UI I p e p t

L R

= + +

+

(13)

where is the angle related to the pre-fault operation condition

of the DFIG.

Fig. 7 shows the amplitude of the simulated faulty rotor

current when the rotor speed is fixed at 1.3pu. It is clearly shown

that when the grid voltage does not drop to zero, there will be a

non-attenuated component with the frequency of 50Hz existingin the faulty current, which is induced by the remaining grid

voltage after the dip. As p grows bigger, the maximal faulty

current will become smaller, which means that the largest faulty

current will appear when a short circuit occurs right at the

terminals of the DFIG.

III. SELECTION OF THE CROWBAR BY-PASS RESISTANCE

A. Appropriate Value of the Crowbar Resistance

As mentioned in the introduction, the by-pass resistance of the

crowbar should be high enough to limit the fault currents and

low enough to avoid over-voltages on the rotor terminals. Based

on these two considerations, an appropriate range of the crowbar

resistance can be acquired.

Firstly, the fault currents should be limited in a safe range.

Based on the analysis of last section, the largest faulty current

will appear when the grid voltage drops to zero and the pre-fault

rotor speed is the highest, i.e. 1.3pu. According to (12), thefollowing expression can be described

2 2( )

srsafe

s r crow

UI

L R

crowbar effect dfig

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