Abstract —Inter-turn short circuit fault is the main fault type of doubly-fed induction generator (DFIG), so online monitor and fault diagnose are particularly necessary for DFIG. However, due to the presence of the generator external imbalances, winding inter-turn fault diagnosis is difficult. Based on the finite element model, we analyzed the negative sequence current and harmonic components generated by the rotor winding inter-turn fault of DFIG. We get the features of rotor inter-turn fault in DFIG, and the fault characteristics in the case of grid imbalance. Index Terms—Doubly fed induction generators (DFIG), inter-

turn fault of rotor winding, finite element method (FEM), negative sequence component

I. INTRODUCTION

DFIG is electromechanical integration equipment which integrates high power frequency conversion device, rotating generators and modern control systems. DFIG has many advantages, such as, it has a wide range of variable-speed operation, the capacity of the inverter device requires only one-third of the whole volume, it is cost-effective, etc. Currently, the wind turbine generators which inputted commercial operation mainly included doubly fed induction generator, squirrel cage induction generator and permanent magnet synchronous generator. And, DFIG obtained the majority market share by virtue of its dominants [1]. DFIG has become the main generator of wind turbines [2-3]. Because of the work environment and structural reasons, DFIG is also the model which often fails. The fault of DFIG mainly includes three aspects, such as abnormal vibration caused by turbine rotating system [4-5], fault of converter [6-7], and generator winding faults. And generator winding fault is the multiple faults.

All faults of generator are produced and developed with certain failure mechanisms. As long as we analyze the fault mechanism carefully and sum the law of the fault, we can accurately and timely realize generator winding fault diagnosis. The generator’s electrical and non-electrical quantities, for example, voltage, current, impedance, inductance, temperature, vibration, noise, etc, will show its standard value early designed when generator is on normal operation state. If generator winding fault, it is bound to change these electrical or non-electrical quantities. Therefore, once we know the This work was supported by Natural Science Foundation of Hebei Province in China (E2010001705)

development trends of these quantities with the fault, the fault can be diagnosed effectively. Current and voltage are easy to collect and contain rich feature information, so current and voltage signals are often as carriers in the online monitor and fault diagnose for the DFIG. It can be gotten from the method of symmetrical components that the asymmetric circuit system can generate negative sequence components in the winding current. Both generator winding fault and grid imbalance can lead to unbalanced three-phase circuit. In this paper, we analyze the negative sequence currents and their harmonic components in these two cases.

II. WORKING PRINCIPLE AND HARMONIC ANALYSIS OF DFIG

A. Working principle of DFIG DFIG is asynchronous generators which excited by AC. In

order to equalize the angular frequency of rotating magnetic field which generated by the stator and the grid angular frequency 1ω , the angular frequency of rotor current is

2 1ω ω ω= − when the rotating angular frequency ω of rotor varied with the different wind speed. The slip of induction generator should be

1

1

sω ω

ω−

= (1)

So, the rotor current frequency is 2 1f sf= . 1f is stator current frequency.

The relative speed to rotor of circular rotating field, which generated by the rotor AC excitation, 2n is

22 60

fn

p= (2)

Where, p is the number of pole pairs. As stator is stationary, the relative speed to stator of the

magnetic field generated by the rotor 1n is 1 2n n n= ± (3)

Where, n is the speed of rotor, the sign “+” is taken when the direction of rotor and the magnetic field direction generated by the rotor is the same; the sign “-” is taken when the direction of rotor and the magnetic field direction generated by the rotor is the opposite.

At this time, the rotating magnetic field generated by the rotor cut the stator by the speed of 1n . The frequency of

Rotor Winding Inter-turn Fault Analysis of Doubly-fed Induction Generator Based on

Negative Sequence Component Li Junqing, He Long, Wang Dong

School of Electrical and Electronic Engineering, North China Electric Power University, Baoding, China E-mail: helong69@126.com

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2013 International Conference on Electrical Machines and Systems, Oct. 26-29, 2013, Busan, Korea

978-1-4799-1447-0/13/$31.00 ©2013 IEEE

induced electromotive force on the stator is

1 21

( )=60 60n n nf p p ±= (4)

B. Harmonic analysis of DFIG on imbalanced grid voltage When the grid voltage is imbalance, the stator windings can

induce in oval rotating magnetic field. It can be obtained from the symmetrical component method that the rotating magnetic field generated by the stator current can be decomposed into two circular rotating magnetic fields with the same speed

1n and the opposite direction. Setting the counterclockwise direction as the positive one, rotor rotating speed is n with positive direction, the relative speed to rotor of clockwise rotating magnetic field is ( 1n n+ ). This reverse rotating magnetic field can induce the harmonic e.m.f in the rotor winding whose frequency is

' 1( )60

n nf p += (5)

Based on the conclusion of (1), (2), (3) and (5), (6) can be obtained. '

1(2 )f s f= ± (6) Where, the sign “-” is taken when the direction of rotor and the magnetic field direction generated by the rotor is the same; the sign “+” is taken when the direction of rotor and the magnetic field direction generated by the rotor is the opposite.

It can be drawn from the above analysis that the harmonic component which frequency is '

1(2 )f s f= ± will be obtained from the rotor current when the grid voltage is imbalanced.

III. MODEL OF THE GENERATORS In this paper, take YR132M-4 winding induction generator

for model, the parameters are as below: rated power is 5.5NP kW= , rated frequency is 50Nf Hz= , the number of

stator slots is 1 36Z = ，the number of rotor slots is 2 24Z = ，

the number of pole pairs is p=2 and rotor speed is 1560r/min. Stator winding is connected in a triangle, there are two branches in parallel in per phase and six coils in every branch. Rotor winding structure applies star connection, and there is one branch in a phase and eight coils in every branch. The simulation model of the machine is established by ANSOFT MAXWELL, shown as Fig.1.

Fig.1 Finite element model of the generator

This model is based on the ANSOFT analysis software and winding failure is set by the method of field-circuit coupled. The short-circuit fault is set in NO.1 coil of rotor A-phase winding. The external circuit is shown in Fig.2.

Fig.2 The external circuit of rotor

IV. SIMULATION AND ANALYSIS

A. Simulation of DFIG on balanced grid voltage Set the generator speed to 1200r/min. First, we analyze the

line current of the rotor under the normal winding condition, the result is shown in Fig.3. Then, the rotor line current and its negative sequence component are respectively analyzed under different fault degrees, which specifically is set short-circuit 1 turn, 5 turns and 10 turns, shown in Fig.4 to Fig.6. From the figures we can get the following information. The three-phase line current of the rotor is symmetrical on normal operation. When inter-turn short fault occurs, the magnitude and phase of three-phase line current are no longer symmetrical. And with the deepening of the fault, asymmetry is further deepened.

I(A)

Fig.3 The rotor current under normal rotor

Fig.4 The rotor current under one turn short-circuit

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0.3 0.35 0.4 0.45 0.5-20-15-10-505101520

Time(s)

I(A)

IAIBIC

Fig.5 The rotor current under five turns short-circuit

I(A)

Fig.6 The rotor current under ten turns short-circuit

The rotor effective value currents in different failure degrees

are shown in TABLE I. AS TABLE I showing, the three-phase effective value line currents of the rotor are symmetrical on normal operation. When A-phase of the rotor occur inter-turn short current fault, A-phase and B-phase currents increase significantly, C-phase current is essentially constant.

TABLE I

The current effective value under different degrees of the rotor fault Shorted turns Ia(A) Ib(A) Ic(A)

0 10.5501 10.4012 10.7323

1 10.7140 10.8357 10.6377

5 11.2971 11.8757 10.6992

10 13.0223 13.7529 10.9986

The rotor current phase difference in different failure degrees are shown in TABLE Ⅱ. AS TABLE Ⅱ showing, when inter-turn short fault occurs, the phase difference of AB-phase and CA-phase angle increased. Conversely, the phase difference of BC-phase reduced.

With the deepening of the fault, the angle of the fault phase is further increased, the phase difference between non-fault phases significantly reduced.

TABLEⅡ The current phase difference under various degrees of the rotor fault

Shorted turns AB-phase (°) BC-phase (°) CA-phase (°)

0 120.07 120.29 119.64

1 120.16 119.36 120.48

5 120.87 116.65 122.48

10 121.82 113.84 124.34

The harmonic analysis of the three-phase line currents under

different degrees of the rotor faults are shown as Fig.7 to Fig.10. As shown in the figures, when the generator is normal, the amplitudes of the three-phase fundamental current are substantially the same and harmonic components are very

small. With the deepening of the fault, the amplitudes of the three-phase fundamental current are significantly imbalance. There has been significant third harmonic on fault operation which increases with the deepening of the fault. Moreover, there will be the fifth harmonic when the fault is serious(10-turns short).

Fig.7 Rotor current spectrum when rotor is normal

Fig.8 Rotor current spectrum when rotor is one turn fault

Fig.9 Rotor current spectrum when rotor is five turns fault

Fig.10 Rotor current spectrum when rotor is ten turns fault

TABLE Ⅲ is the negative and positive sequence currents under different degree of fault. These negative and positive sequence currents are derived from the fundamental component of the rotor current. As the table shown, the negative sequence current is very small on normal operation. As inter-turn short fault occurs, the negative sequence current increased significantly. The ratio of negative sequence current and positive sequence current (I2/ I1) increased with the deepening of the fault. In the TABLE Ⅲ, I1 and I2 represent

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positive sequence current and negative sequence current respectively.

TABLE Ⅲ The negative and positive sequence current under different degree of fault Shorted turns I2 (A) I1 (A) I2/ I1

0 0.0600 7.3328 0.8200%

1 0.1779 7.3392 2.4200%

5 0.5200 7.3898 7.0400%

10 1.1058 7.6904 10.9986%

B. Simulation of DFIG on unbalanced grid voltage As we all know, the grid voltage fluctuation is allowed. And,

it also can produce negative sequence component and negative current in the rotor. So, it is necessary to find the different features of external imbalances and the rotor inter-turn fault to make the diagnosis more reliable.

Set A, B, C three-phase voltage amplitude is 311, 295, 311 volts to simulate the unbalanced grid voltage. Fig.11 to Fig.13 is the harmonic analysis of rotor A-phase line current in different cases.

Fig.11 is the rotor current spectrum on balanced grid voltage when rotor is normal. Fig.12 is the rotor current spectrum on unbalanced grid voltage when rotor is normal. Fig.13 is the rotor current spectrum on unbalanced grid voltage when the rotor winding includes 10-interturn fault. Comparing Fig.11 and Fig.12, it can be seen that there is apparent 1(2 )s f− harmonics when grid voltage is imbalance. Comparing Fig.12 and Fig.13, it can be seen that rotor current contains not only the 1(2 )s f− harmonics but also the third harmonic and fifth harmonic on unbalanced grid voltage when the rotor winding is interturn fault. So, the 1(2 )s f− harmonic is the features of unbalanced grid voltage. And the third harmonic and fifth harmonic appear when the rotor winding is interturn fault.

I(A)

Fig.11 The rotor current spectrum on balanced grid voltage

I(A)

Fig.12 The rotor current spectrum on unbalanced grid voltage

Fig.13 The rotor current spectrum on unbalanced grid voltage and 10-turn

fault of rotor

V. CONCLUSIONS

In this paper, we analyzed the negative sequence current and harmonic components generated by the rotor winding inter-turn fault of the DFIG. We get the fault features of the DFIG rotor inter-turn fault, and the fault characteristics in the case of grid imbalance. The following conclusions are gotten.

(1) The inter-turn fault of rotor winding in DFIG can generate negative sequence component in the rotor line current. And the proportion of negative sequence current has rising trend with the deepening of the fault degree.

(2) The effective value and the phase difference of line current can be impacted by the fault. The effective value of the three-phase rotor currents are significantly imbalance and the phase angle between the non-fault phases significantly reduce when the fault occur in rotor winding. Both of them also increase with the deeper level of fault.

(3) The third and fifth harmonics occur when rotor failure. Negative sequence third harmonic is obviously when grid is imbalance.

(4) There is apparent 1(2 )s f− harmonics in the rotor line current when grid voltage is imbalance.

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