Abstract
Experimental and theoretical studies have confirmed that, relative to a one-shot voltage fault, a doubly-fed induction generator (DFIG) will suffer a greater transient impact during continuous voltage faults. This paper presents the design and application of an effective scheme for DFIGs when a commutation failure (CF) occurs in a line-commutated converter based high-voltage direct current (LCC-HVDC) transmission system. First, transient demagnetization control without filters is proposed to offset the electromotive force (EMF) induced by the natural flux and other low-frequency flux components. Then, a rotor-side integrated impedance circuit is designed to limit the rotor overcurrent to ensure that the rotor-side converter (RSC) is controllable. Furthermore, coordinated control of the demagnetization and segmented reactive currents is implemented in the RSC. Comparative studies have shown that the proposed scheme can limit rotor fault currents and effectively improve the continuous fault ride-through capability of DFIGs.
TO achieve CO2 emissions peaking before 2030 and carbon neutrality by 2060 in China, large-scale power generation by renewable energy sources transported by line-commutated converter based high-voltage direct current (LCC-HVDC) transmission systems will be more prominent [
Various improved control strategies have been proposed to prevent and mitigate CFs such as optimal control based on the voltage-dependent current order limit (VDCOL) [
In order to maintain the stability of the sending terminal during a CF, a DFIG needs to maintain uninterrupted operation. Regarding low-voltage ride through (LVRT) and high-voltage ride through (HVRT), there has been plenty of research that can help a DFIG ride through faults. When a slight low-voltage fault occurs, a reduction in the fault current of the rotor and accelerated attenuation of the natural flux can be achieved by modifying the control strategies of the rotor-side converter (RSC) and grid-side converter (GSC), e.g., demagnetization control [
Owing to DC blocking, a high-voltage fault will occur at the PCC of a wind farm. Although research on HVRT is not as sufficient as that on LVRT, many scholars have also conducted research on it. The virtual damping control is proposed in [
However, LVRT and HVRT are basically studied separately. Few studies have considered continuous faults. The transient characteristics of a DFIG when a step-type continuous fault occurs are analyzed in [
This paper presents a new continuous FRT scheme for DFIGs that combines improved control with a current-limiting circuit. The DFIG model under continuous faults is presented in Section II. The proposed continuous FRT scheme is explained in Section III. Simulation studies are conducted to demonstrate the effectiveness of the scheme in Section IV. Finally, conclusions are drawn in Section V.
The configuration of an LCC-HVDC transmission system that includes a wind farm at the sending terminal is shown in

Fig. 1 Configuration of LCC-HVDC transmission system.
Different continuous faults are shown in

Fig. 2 RMS of stator voltage of DFIG under single continuous fault or multiple continuous faults. (a) Single continuous fault. (b) Multiple continuous faults.
The DFIG voltage and flux equations in the stationary stator reference frame can be expressed as:
(1) |
(2) |
where , , and are the voltage, current, and flux space vectors, respectively; the subscripts s and r denote the stator and the rotor, respectively; the superscript s denotes the stationary stator reference frame; Rs and Rr are the stator and rotor resistances, respectively; Ls, Lr, and Lm are the stator, rotor, and magnetizing inductances, respectively; and is the angular frequency of the rotor.
It can be observed that, unlike a step-type low-voltage fault, the voltage amplitude of a continuous fault does not suddenly change but first decreases and then transitions to a high value after some time. According to [
(3) |
where is the decay time constant of the natural flux of the stator; is the angular frequency of the stator; and C1 is the initial value of the natural flux.
According to the flux continuity theorem, the stator flux during the entire fault period can be obtained as:
(4) |
where C2, C3, and C4 are the initial values of the natural flux during different stages.
In (4), C1, C2, C3, and C4 can be expressed as:
(5) |
where is the voltage magnitude.
According to (1) and (2), the rotor voltage can be obtained as:
(6) |
The first item on the right-hand side of (6) is the open-circuit voltage of the rotor, i.e., the rotor-induced electromotive force (EMF). The rotor EMF during t3-t4 can be calculated as:
(7) |
If only one high-voltage fault with a step increase has occurred after the steady-state operation, the open-circuit voltage of the rotor is:
(8) |
It can be observed from (7) and (8) that the voltage swell of a continuous fault is different from a single voltage swell, which is affected by the low-voltage stage of a continuous fault. The EMF induced in the high-voltage stage is greater when the decrease in the voltage is higher and the time over which the voltage decrease occurs is shorter. Compared with a single fault, the transient flux component generated in the previous stage will be superimposed on the latter stage, resulting in a larger rotor EMF, which causes the RSC to face larger fault currents and threatens the safe operation of wind turbines. Therefore, the effect of the accumulated stator flux should be mitigated while limiting the fault current of the rotor to reduce the harm to the DFIG due to continuous faults.
According to [

Fig. 3 Block diagram of improved demagnetization control.
As shown in
The transient demagnetization current in the improved control can be expressed as:
(9) |
where and are the d- and q-axis demagnetization currents of the rotor, respectively; and Kd1 and Kd2 are the compensation coefficients, which are positive.
A larger compensation coefficient can better restrain the rotor overcurrent, but the RSC output voltage is higher. A smaller compensation coefficient will lower the output voltage of the RSC and weaken the ability to restrain the rotor overcurrent. The introduction of demagnetization current in the active current loop will cause an instantaneous active power and electromagnetic torque pulsation. In this study, two types of compensation coefficients are designed, which can reduce Kd1 in the case of shallow faults, thereby preventing a large active power and electromagnetic torque pulsation. In the case of deep faults, Kd1 and Kd2 can take the same limiting value to accelerate the attenuation of the transient stator flux.
Owing to the limited capacity of the RSC, the magnitude of the demagnetization current for deep faults will exceed the allowable current of the RSC. The peak value of the output voltage of the RSC cannot exceed the DC voltage; hence, the output voltage of the RSC should be within a certain range. From (6), it can be observed that it is the difference between the EMF and the rotor voltage that produces a larger current in the transient impedance of the rotor. Therefore, an increase in the impedance of the rotor can limit the fault current of the rotor. It can be observed from [
A control system for a DFIG integrated with the proposed scheme is shown in

Fig. 4 Control system for a DFIG integrated with proposed scheme.
After inserting the impedance, the rotor voltage can be expressed as:
(10) |
where and are the equivalent transient resistance and inductance of the rotor, respectively.
According to (1) and (2), the stator voltage can be re-expressed as:
(11) |
where is the transient resistance of the stator.
It can be observed from (11) that the attenuation of the transient flux of the stator can be accelerated by selecting a suitable impedance value according to (3).
The selection of impedance parameters should be considered on the basis of more serious conditions. According to (4)-(7), for typical low- and high-voltage continuous faults, the most serious situation is that the transient flux induced by a low-voltage fault and the transient flux induced by a high-voltage fault are in the same direction. In addition, both low- and high-voltage faults can be considered to be step-type faults since the amplitude of the transient flux generated by a step-type voltage fault is greater than that of a voltage fault with a certain change over time. In a relatively short time, two faults can be equivalent to a single voltage fault, ignoring the attenuation of the transient flux. At this time, the series impedance is inserted into the rotor. According to transient analyses of the rotor current in [
(12) |
where is the maximum output voltage of the RSC; is the coupling factor of the stator; and is the prefault current.
The impedance parameters can be selected according to (12). Although the overcurrent of the rotor can be limited more extensively when the selected impedance parameters are larger, this may weaken the controllability of the RSC and result in overvoltage of the rotor. The resistance and reactance of the series impedance in this paper are and , respectively.
Multiple grid codes require a DFIG to support the grid voltage during a fault; that is, the DFIG should absorb/output additional reactive power.
The E.ON grid code requires a wind turbine to have LVRT capability and the ability to provide dynamic reactive and voltage support. Taking the E.ON grid code as an example, the additional reactive current of the stator when a voltage dip and swell fault occurs needs to satisfy:
(13) |
According to the relationship between the stator and rotor currents, the q-axis reactive current reference of the rotor can be set as:
(14) |
According to (13) and (14), the segmented reactive current strategy proposed in this paper includes two modes, i.e., the reactive current output mode during the low-voltage stage and the reactive current absorption mode during the high-voltage stage. In addition, it can be observed from
After the current-limiting impedance is activated on the rotor side, the equivalent impedance of the rotor increases. Compared with the situation in which there is no series impedance, the amplitude and phase of the demagnetization current need to be modified. In order to offset the transient component of the EMF caused by the transient flux, the demagnetization current needs to satisfy:
(15) |
From (15), the demagnetization current is:
(16) |
According to (15), the demagnetization current is injected into the rotor current reference, which can offset the EMF of the rotor and accelerate the attenuation of the transient flux of the stator. According to (16), the amplitude of the transient demagnetization current greatly decreases owing to the increase in the transient impedance of the rotor.
A flowchart of the proposed scheme is shown in

Fig. 5 Flowchart of proposed scheme.
1) The series impedance is inserted into the rotor when a fault is detected to limit the fault current of the rotor and ensure that the RSC can be controlled.
2) The active current is set to be 0 to ensure that the RSC has sufficient current capacity for the demagnetization current, the reactive current, and part of the active current. On the basis of the series impedance parameters and the grid voltage level, coordinated control can be implemented according to (14) and (16).
3) When the voltage returns to the normal range, the active and additional reactive currents are reset to be 0 after a delay of 50 ms. After a delay of 100 ms, the series impedance is deactivated to ensure that the DFIG is not affected when the series impedance is cut off. Finally, the DFIG returns to steady-state operation after a delay of 50 ms.
The 1.5 MW DFIG shown in
Parameter | Value |
---|---|
Rated power | 1.5 MW |
Stator voltage | 690 V |
Rotor voltage | 1725 V |
DC-link voltage | 1200 V |
Stator resistance | 0.005 p.u. |
Stator inductance | 0.171 p.u. |
Rotor resistance | 0.007 p.u. |
Rotor inductance | 0.156 p.u. |
Mutual inductance | 2.9 p.u. |
Grid frequency | 50 Hz |
To verify the effectiveness of the proposed improved demagnetization control, the RSC uses different current control strategies. During a fault, the active current reference is set to be 0, and the reactive current reference still retains its prefault value. Moreover, the demagnetization current is added to the rotor current. The following four control strategies are considered.
1) Strategy A: the conventional demagnetization control strategy in [
2) Strategy B: the improved control strategy in [
3) Strategy C: the natural stator current tracking control strategy in [
4) Strategy D: the proposed improved demagnetization control strategy.

Fig. 6 Simulation results of DFIG with different control strategies under single continuous fault. (a) Stator voltage. (b) Rotor current amplitude. (c) DC-link voltage. (d) Transient flux amplitude. (e) Electromagnetic torque.
Although the improved demagnetization control has a certain limiting effect on the fault current, the demanded demagnetization current is very large, and the safe operation of a DFIG cannot be guaranteed when severe faults occur. Comparative studies have been conducted. The following three schemes and the proposed scheme are considered.
1) Scheme A: with conventional VC.
2) Scheme B: with the crowbar and DC-chopper combination.
3) Scheme C: with an energy storage inverter connected in parallel on the rotor side and coordinated control of the demagnetization and reactive currents in [
The simulation results under a single continuous fault with different schemes are shown in

Fig. 7 Simulations results of DFIG under a single continuous fault with different schemes. (a) Stator voltage. (b) Rotor current amplitude. (c) DC-link voltage. (d) Reactive power. (e) Electromagnetic torque.
Although the peak rotor current of Scheme C is 3.74 p.u., an energy storage converter with the same capacity as the RSC is connected in parallel with the rotor, and the peak output current of the RSC is 1.87 p.u., which is still greater than that of the proposed scheme. The proposed scheme can effectively limit the rotor overcurrent and protect the RSC. In conventional VC, the continuous fault causes excess energy to accumulate on the DC-link capacitor, resulting in an increase in the DC voltage to 1.99 p.u.. In Schemes B and C, the peak values of the DC-link voltage are 1.22 and 1.08 p.u., respectively. However, the lowest value of the DC-link voltage is 0.73 p.u. for Scheme B, which is below the safety threshold of the DC-link voltage. With the help of the DC chopper in proposed scheme, the peak value of the DC voltage is 1.08 p.u., and the DC-link voltage-limiting rate is 45.7%.
It can be observed from
The occurrence of multiple CFs may result in multiple continuous faults. Simulation results of DFIG under multiple continuous faults with different schemes are shown in

Fig. 8 Simulations results of DFIG under multiple continuous faults with different schemes. (a) Stator voltage. (b) Rotor current amplitude. (c) DC-link voltage. (d) Reactive power. (e) Electromagnetic torque.
The minimum and maximum values are p.u. and 0.84 p.u., respectively, and the peak-to-peak value is 6.78 p.u.. With the proposed scheme, the electromagnetic torque is restrained between -1.98 p.u. and 1.14 p.u.. Thus, the rate for limiting torque oscillation is 54.0%. It can be observed that the proposed scheme can still achieve better transient response under multiple continuous faults and help DFIGs successfully ride through continuous faults.
This paper proposes an effective scheme to enhance the continuous FRT capability for DFIGs when CFs occur in LCC-HVDC transmission systems. The core idea is to combine a series impedance circuit on the rotor side with coordinated control to protect the RSC and accelerate the attenuation of the transient flux of the stator. The effectiveness of the scheme is verified under different continuous faults. The main conclusions are as follows.
1) Improved demagnetization control without filters is proposed for continuous faults, which can weaken the EMF of the rotor caused by the accumulation effect of the flux.
2) The design and operation principles of the series impedance circuit integrated on the rotor side are provided. Considering a continuous fault, a design method is provided for this circuit.
3) Coordinated control of the demagnetization and reactive currents is proposed. Segmented reactive current control can meet the requirements of grid codes during the low- and high-voltage stages.
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