The Transition Process of Wind Turbine Based on Squirrel-Cage Induction Generator When Voltage Sag and Its Low Voltage Ride-Through Method Xiangwu Yan, Member, IEEE, Liming Yang, Bo Zhang, Student Member, IEEE, Lei Sun, Yuzhao Liang State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206, China
E-mail:
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Abstract -- This paper reveals two typical phenomena of Squirrel-Cage Induction Generator (SCIG) based wind turbine during the voltage sag, such as asynchronous induction generator out of step and the shocks of the electromagnetic torque with polarity reversal, the result shows low voltage ride-through(LVRT) capability of SCIG based wind turbine is very poor. A method of using a stator series resistance device is proposed for SCIG based wind turbine LVRT. Both detailed analysis and simulation results demonstrate it is a safety and effective method to ensure the SCIG based wind turbine against the impact of the PCC voltage sag, and provide maximum active and reactive power to support the grid stability and security. Index Terms-- squirrel-cage induction generator (SCIG) based Wind turbine, Voltage sag, Low voltage ride through (LVRT)
I.
INTRODUCTION
Constant speed wind turbine with squirrel-cage induction generator has simple structure, good robustness, and low cost advantages; it is one of the grid-connected wind power mainstream models and has occupied a certain market share[1]. With the increasing number of wind turbines directly connected to the network, when grid faults, wide range of voltage drop occurs in varying degrees. A large number of wind turbines due to low voltage protection exit may result in grid voltage collapse[2],[3]. Therefore, the Germany in 2004 developed the first requirements about wind turbine low voltage ride through, later, other European countries and the United States have developed their own standards[4-6]. China’s low voltage ride-through regulations began on trial in 2009, it requires that wind turbine must be able to keep connection to grid at least 625ms when PCC voltage sag down to 20% of the nominal voltage at a symmetrical short-circuit fault (three-phase to ground), and must have the ability to ride through the asymmetric short-circuit faults (phase to phase to ground, single phase to ground, phase to phase) occurred in PCC. Because early grid-connected wind turbines and its wind farms do not have the low voltage ride-through capability, the
issues that wind turbines effect the security and stability operation of the grid are increasingly apparent with the rapid expansion of wind power. The structure of squirrel-cage asynchronous induction generator is simple, and its rotor side without connecting power electronic device, the overvoltage and over-current withstand capability is stronger than other types of wind turbines. Therefore, most people think that the capability of low voltage ride through of SCIG based wind turbine is also stronger. However, is it really be as the same as people expected? This paper shows electromagnetic and electromechanical transient variation process of 5th order SCIG based wind turbine during the voltage sag, and reveals two typical phenomena of SCIG based wind turbine during the voltage sag, such as asynchronous induction generator out of step and the electromagnetic torque shocks with polarity reversal, which will result in the shaft and the gear box to withstand large mechanical stress and cause the shaft and gear box fatigue damage[7]. The result shows low voltage ride-through capability of SCIG based wind turbine is very poor. Hence, a improved method using a stator side series resistance device directly to limit the shocks of stator current and electromagnetic torque to improve the LVRT capability of SCIG based wind turbine[8] during PCC voltage sags is proposed in this paper. Further, the theoretical analysis and simulation results show that SCIG based wind turbine with a stator side series resistance device is able to ride through the symmetrical short-circuit fault (three-phase to ground) and the asymmetric short-circuit faults (phase to phase to ground, single phase to ground, phase to phase) occurred in PCC, voltage dip to 15% of the nominal voltage, 150ms and 625ms fault durations. II. INTRODUCTION ELECTROMAGNETIC AND ELECTROMECHANICAL TRANSIENT CHARACTERISTICS OF SCIG WIND TURBINE DURING GRID VOLTAGE FAULT A.
SCIG wind turbine system model
There are many types of mathematical models to represent induction generators in power system studies. The induction generator may be modeled as a transient reactance
with a voltage source. However, in order to reveal the electromagnetic transients, the time domain dynamic models must be used. This paper builds a 5th order time domain model for it can give more satisfactory transient response, and uses a two-mass model because of its need to deal with flexible modes induced by the low-speed shaft stiffness more effectively. Fig.1 is an electrical diagram that shows a typical 2MW SCIG wind turbine with the grid connection for the study.
Fig.1 Block diagram of a grid connecting wind turbine with a SCIG
Without own excitation circuit, SCIG based wind turbine excitation depends on the absorption of reactive power from grid. The electromagnetic torque Te of SCIG is proportional to the square of the terminal voltage Us under any given speed, that is: (1) Te=KsUs2 Here, K is Generator parameters; s is motor slip; SCIG rotor motion equation is[9]:
dω J = Tm − Te (2) dt Here, J is inertia time constant; Tm is mechanical torque of generator rotor shaft; ω is generator rotor rotation angular rate. Stator voltage equation is[10]: U s = −(r + jx ') I s + E '
(3)
Here, Us、Is、E′ and x′ respectively are generator stator voltage, stator current, transient potential and transient reactance. B. The transient electromagnetic response of SCIG wind turbine in the grid fault In order to reveal the impact on SCIG based wind turbine of the voltage sags, this paper respectively demonstrate the electromagnetic transient process of a 2WM SCIG wind turbine without any low voltage ride-through measure when symmetry short-circuit fault and asymmetrical short-circuit fault occur in the grid. The concerns include the stator voltage, stator current, generator electromagnetic torque, rotational speed, active power and reactive power. Fig.2 is the responses of 5th SCIG based wind turbine without any LVRT measures when the PCC voltage sags down to 70% of the nominal voltage. At the moment of faults happening, the speed and the electrical torque change greatly, the maximum instantaneous electromagnetic torque Te is within 200%-250%TN, which shows that voltage sag not
being serious, it has small impact on SCIG based wind turbine. However, when the PCC voltage sags down to 20% of the nominal voltage, as is shown in Fig.3, the maximum instantaneous electromagnetic torque Te is over 200%-250%TN, accompanied by the electromagnetic torque acuteness oscillation and torque polarity repeated reversal. The transient current and torque of SCIG based wind turbine will suffer a great influence by the voltage sags, causing wind turbine rotating shaft and e gear box to withstand the mechanical stress greatly. The low voltage ride-through capability of SCIG based wind turbine is very poor. Because of the SCIG based wind turbine’s structure and its operating principle, the SCIG based wind turbine will absorb reactive power from the grid on normal condition, or on fault cases, what can be seen in Fig.2 and Fig.3. Transition waves indicate that voltage sag will cause a great impact on SCIG wind turbines, and the impact will seriously harm the safety of the SCIG wind turbine gearbox and shaft. So the measure of low voltage ride through for SCIG wind turbine is necessary. Four types of typical fault at different voltage dip depths and different durations have been simulated. The transition behaviors of SCIG wind turbine show: (1) The simulation results are consistent with actual situations of SCIG wind turbine on normal condition, or on fault cases at PCC voltage dip to 70% of the nominal voltage, 625ms fault duration. It is shows that the simulation model of 5th SCIG based wind turbine system is correct and accurate. (2) In addition to commonly showing strong changes of SCIG electrical quantities during voltage sag, two new behavior characteristics appeared on SCIG wind turbines: the first one is the polarity reversal phenomenon of the electromagnetic torque Te, even repeated polarity reversal of the torque Te, that is, The SCIG operation state alternates between the generator state and the motor state, or repeatedly alternating during voltage sag; the second is the out-of-step phenomenon of the SCIG wind turbine, it generally occurs in a long voltage sag duration. III.
ANALYSIS OF THE SCIG ELECTROMAGNETIC AND ELECTROMECHANICAL TRANSIENT PROCESS
A.
Te polarity reversal causes While a SCIG based wind turbine operating in a steady state, the electromagnetic torque Te is equal to its input mechanical torque Tm. At the grid voltage sag moment, SCIG stator winding appears large current, active power and reactive power also change dramatically, a larger transient electromagnetic torque Te occurs at the same time, Te > Tm, SCIG rotor rotation speed decreases rapidly under the action of the brake torque Te (Here Te is the brake torque in
(a)
(b)
(c)
(d)
Fig.2 The transition responses of a SCIG wind turbine without any LVRT measure at voltage sag to Upcc=0.7p.u., 625ms fault duration, (a) 3φ fault, (b) 2φn fault, (c) 1φn fault, (d) 2φ fault.
(a)
(b)
(c)
(d)
Fig.3 The transition responses of a SCIG wind turbine without any LVRT measure at voltage sag to Upcc=0.2p.u., 625ms fault duration, (a) 3φ fault, (b) 2φn fault, (c) 1φn fault, (d) 2φ fault.
generator state of SCIG), and the SCIG operation state will change from super-synchronous generator state to sub-synchronous motor state after SCIG rotor rotation speed goes down to the point of synchronous speed. When the grid voltage recovers after the fault cleaned, the machine side voltage increases correspondingly, and the electromagnetic torque Te increases rapidly as the same time. As the SCIG operation in sub-synchronous motor state the electromagnetic torque Te is drive torque in this moment, the speed of SCIG rotor will be accelerated rapidly in the double role of Te and Tm. At this stage, the polarity reversal of the electromagnetic torque Te occurs, it can be seen in Fig. 3 (a)-(d). This process continues until the rotor accelerated to super synchronous speed. It is noteworthy that wind turbine and SCIG double drive the rotor shaft and the gear system in the same time, which will give the shaft and gear box an extremely negative impact. And the SCIG operating as a squirrel-cage induction motor absorbs a large number of active and reactive power from the grid during the period of voltage recovery, which is though the disadvantage of security and stability on the grid. When the rotor recover to the super-synchronous speed, the SCIG just return to generator state, and the electromagnetic torque Te becomes the brake torque in super-synchronous area. What’s more, the rotor speed decreases until the brake torque Te equals to the input mechanical torque Tm. Experiencing a period of oscillation, SCIG gradually return to pre-fault operating point. If the voltage sag is extreme and shorter time, the repeat polarity reversal phenomenon of the electromagnetic torque Te will occur in the later period of voltage recovery, it can be seen in Fig. 3 (a)-(d). B.
Principle of SCIG going to stall
Fig.4 is the curve of SCIG torque – angular velocity (Te-ω), The first quadrant indicates power generator characteristics of SCIG, the third quadrant means motor characteristics of SCIG, ωas>ω0>ωs>ω1>ωcr>ω2>0, among them, ω0 is angular velocity of pre-fault operating point of the SCIG, ωs is synchronous angular velocity of the SCIG, ωcr is critical angular velocity of the fault cleaned, ωas is angular velocity of stall. In steady state operation, Te = Tm, the SCIG is stable operating in the ω0. At the PCC voltage sag moment, SCIG stator winding appears large current, a larger transient electromagnetic torque Te occurs at the same time, Te > Tm, SCIG rotor rotation speed decreases rapidly under the action of the brake torque Te, then the SCIG operation enters into sub-synchronous region. While the grid voltage recovered after the fault cleaned, the machine side voltage increases correspondingly, the electromagnetic torque Te increases rapidly as the same time. As the SCIG operation in sub-synchronous motor state the electromagnetic torque Te is drive torque in this moment,
SCIG rotor will be accelerated the speed rapidly in the double role of Te and Tm. for example, SCIG rotor is accelerated from ω1 on the Fig. 4, if the acceleration area SABF< the deceleration area SAEI, the SCIG will back to pre-fault operating point. If the acceleration area SADH> the deceleration area SAEI, the SCIG will lose the stability. The acceleration area SACG= the deceleration area SAEI, this is a critical state, so the ωcr is called the critical angular velocity of the fault cleaned.
Te
Tm 0
ω2 ωcr ω1 ωs ω0
ωas
ω
Fig.4 The curve of SCIG torque – angular velocity
Therefore, the angular velocity ω of SCIG rotor is less than the critical angular velocity ωcr when the grid faults cleaned, the SCIG will lose the stability. This cases can be seen in Fig. 3 (b) and (d). IV.THE CONTROL OF ASYNCHRONOUS WIND TURBINE SYSTEM USING RESISTANCE BUFFER DURING GRID FAULTS
A.
The analysis of the theory of resistance device
SCIG wind turbine and the grid network can be presented in Thevenin's circuit as shown in Fig.5 (a).The voltage of power generator is:
U S = ES − Z S I S
(4)
The stator current of the power generator is:
Is =
U s − Vg ZT + Z l
(5)
If symmetrical short-circuit fault happens at the point PCC, the circuit is divided into two part, while SCIG wind turbine is short circuit through transfer ZT and partial circuit resistance Z l .As shown in Fig.5(b),a equivalent impedance can be insert between SCIG and network grid to limit the big
change of stator flux linkage λs ,stator current is ， rotor
resistance device is shown in fig. 7.
current ir and electromagnetic torque Te .The value of the
The algebraic model of the stator series resistance device can be expressed by the following equation: v 1 1 (7) i s = S P s + ( S s1 + S s2 )(v g − v s ) RP Rs1 Rs 2
transit is: .
Z eq =
Eg .
Is
+ Zg
(6)
(a) Toplogy of a SCIG connected to grid (b)The use of equivalent impedance
Fig.5 Single-phase equivalent circuit of SCIG wind turbine LVRT
If the equivalent impedance can be invert immediately when the grid fault happens, there would not have big change on generator stator voltage, stator flux linkage λs ,stator current is , rotor current ir and electromagnetic torque Te during the grid fault. This is exactly the main source that SCIG with stator side series resistance device is capable of low voltage ride through. B. The methodology of the SCIG wind turbine LRVT using resistance device From the analysis of simulation results of the four typical faults, we can recognize that the low voltage ride-through capability of SCIG wind turbine is very poor. It is necessary to take the corresponding LRVT measures for ensuring the stability and security of SCIG wind turbine and the grid. Based on reference [11] and [12] a improved method with the stator series resistance device for SCIG wind turbine LRVT is proposed as shown in fig. 6.
Fig.6
Schematic diagram of SCIG wind turbine LVRT
Fig.7 Single phase equivalent circuit model of the stator series resistance device
C.
The topology of the stator series resistance device Single phase equivalent circuit model of the stator series
Where SP=1 represents the switch SP is closed, SP =0 indicates that the switch SP is opened. Switch Ss1, Ss2 and SP are three-phase operation switch. The states of switches Ss1 and Ss2 are similar as switch SP. When the grid voltage sags down to 80% of nominal voltage, switch Ss1, Ss2 and SP act after detecting the voltage sag at the same time, insert resistance of Rs1, Rs2 and SP between the SCIG and grid. After voltage recovery, the switch Ss1, Ss2 and switch SP are closed by order. Resistance Rs1, Rs2 and SP don’t work under the normal voltage. V. THE SIMULATION RESULTS OF SCIG LOW VOLTAGE RIDE-THROUGH USING STATOR SERIES RESISTANCE DEVICE This paper demonstrates the low voltage ride-through performance of a 2MW SCIG wind turbine using the stator series resistance device. Fig.8 is the responses of SCIG wind turbine with the stator series resistance device when the PCC voltage sags down to 70% of the nominal voltage in four type of typical faults respectively. Fig.9 is the responses of SCIG wind turbine with the stator series resistance device when the PCC voltage sags down to 20% of the nominal voltage in four type of typical faults respectively. The simulation results show insertion of the stator series resistance device between SCIG wind turbine and grid during grid fault, which can limit the stator short-circuit current, prevent the stator voltage dropping, and restrains the electromagnetic torque Te shock during grid voltage sags and voltage recovery. The maximum stator current is less than 2 times of the nominal current. The maximum instantaneous electromagnetic torque Te is restricted within 200%-250%TN without polarity reversal. The active and reactive power of SCIG can also maintain a balance before and after the grid fault. This makes clear that SCIG wind turbine with the stator series resistance device is capable of LVRT. The proposed method can meet the grid code requirements of domestic and foreign. Seen from the simulation results, at the moment of the voltage sag, though the peak initial values will have great fluctuations, the values will decrease to the allowable value soon with the stator series resistance device. Thus, the instantaneous peak values will do help to the wind turbines’ protection relays. So the peak initial values are acceptable. VI.CONCLUSION This paper reveals two typical phenomena of 5th SCIG based wind turbine during the voltage sag, such as asynchronous induction generator going into stall and the shocks of the electromagnetic torque with polarity reversal. The simulation result shows low voltage ride-through
capability of SCIG based wind turbine is very poor. A method of utilizing a stator series resistance device is proposed for SCIG based wind turbine LVRT. Both detailed analysis and simulation results show that SCIG with stator side series resistance device is capable of low voltage ride through, that include varying fault duration and varying voltage sag depth, even voltage down to 20% of the nominal voltage remaining due to the symmetrical short-circuit fault (three-phase to ground) and the asymmetric short-circuit faults (phase to phase to ground, single phase to ground and phase to phase) occurred in the grid. It is apparently know that this method can ensure the SCIG wind turbine against the impact of the PCC voltage sag, and provide maximum active and reactive power to support the grid stability and security. Further, due to the topology and its control strategy of the device are simple, the proposed device is easy to control, capable of off-line operation for high efficiency and low cost in manufacturing and maintenance. REFERENCES [1]
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(a) Fig.8
(b)
(c)
(d)
The transition responses of a SCIG wind turbine with a stator series resistance device at voltage sag to Upcc=0.7p.u., 625ms fault duration, (a) 3φ fault, (b) 2φn fault, (c) 1φn fault, (d) 2φ fault
(a) Fig.9
(b)
(c)
(d)
The transition responses of a SCIG wind turbine with a stator series resistance device at at voltage sag to Upcc=0.2p.u., 625ms fault duration, (a) 3φ fault, (b) 2φn fault, (c) 1φn fault, (d) 2φ fault
