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Proceedings of the IEEE Energy Conversion Congress and Exposition (ECCE 2014), Pittsburgh, PA, USA, 14-18 September, 2014.
Sizing of the Series Dynamic Breaking Resistor in a Doubly Fed Induction Generator Wind Turbine Hammam Soliman Huai Wang Dao Zhou Frede Blaabjerg Mostafa I. Marie
Suggested Citation H. Soliman, H. Wang, D. Zhou, F. Blaabjerg, and M. I. Marie, “Sizing of the series dynamic breaking resistor in a doubly fed induction generator wind turbine," in Proc. IEEE Energy Convers. Congr. and Expo., 2014, pp. 1842-1846.
Sizing of the Series Dynamic Breaking Resistor in a Doubly Fed Induction Generator Wind Turbine Hammam Soliman, Huai Wang, Dao Zhou, Frede Blaabjerg
Mostafa I. Marie Electrical Power Engineering and Machines Department Ain Shams University Cairo, Egypt
[email protected]
Energy and Technology Department Aalborg University (AAU) Aalborg, Denmark
[email protected],
[email protected],
[email protected],
[email protected] Abstract – This paper investigates the effect of Series Dynamic Breaking Resistor (SDBR) sizing on a Doubly Fed Induction Generator (DFIG) based wind power conversion system. The boundary of the SDBR value is firstly derived by taking into account the controllability of the rotor side converter and the maximum allowable voltage of the stator. Then the impact of the SDBR value on the rotor current, stator voltage, DC-link voltage, reactive power capability and introduced power loss during voltage sag operation is evaluated by simulation. The presented study enables a trade-off sizing of the SDBR among the above performance factors.
I.
INTRODUCTION
There are several types of the wind turbine generators (WTGs), one of which is the Doubly Fed Induction Generator (DFIG). In the last decades, the DFIG has been widely applied in the variable speed – constant frequency wind power technology. This is due to its simple control strategy, low losses, and it being capable of regulating the active and reactive power separately. However, the DFIG suffers from its sensitivity to grid faults, as a result of its direct connection to the grid. The stator of the DFIG is connected to the grid directly, whereas the rotor is connected to the grid through the back to back converter. Voltage sag in one of the dominant type of grid faults. It causes a transient stator flux in the generator, along with an increase in the rotor current due to the mutual coupling between the stator and rotor [1]. The increased rotor current flowing through the Rotor Side Converter (RSC) may exceed the maximum allowable one, inducing damage to the converter and DC-link. The DC-link voltage rises as its capacitors are charged above their nominal voltage. Some means of protection is required to prevent the converter from high inrush current faults. Therefore, prior art research has been devoted to analyzing the behavior of wind turbines during faults and presenting Fault Ride-Through (FRT) solutions for wind turbines. Generally the solutions can be divided into two categories. The first one is by introducing additional hardware components or circuits, either in the stator side,
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rotor side, or on the DC-link of the RSC. The second one is by additional control schemes, e.g., demagnetization current or flux compensation to the DFIG system. The existing solutions to improve the performance and the FRT capability of DFIG systems are reviewed as follows. 1) Crowbar circuit In order to protect the RSC from over-current damage, the generator rotor circuit is shorted and the RSC is blocked through a crowbar system. There are two types of crowbar systems, passive and active. Active crowbar is more popular because of the fully controllable characteristics [2]. This type of crowbar consists of semiconductor switches such as GTOs or IGBTs. If either the rotor current or DC link voltage levels exceed their limits due to faults, the IGBTs of the RSC are blocked and the active crowbar is turned on. After the fault clearance, the RSC is restarted and the reactive power is ramped up in order to support the grid. 2) Static Synchronous Compensator (STATCOM) In addition to crowbar approach, a hardware modification can be used to improve the FRT capability of DFIG wind turbines. The strategy is to modify the turbine behavior by a set of resistors, series antiparallel Thyristors, Static Series Compensator (SSC), or STATCOM [3-5]. However, this approach is usually more expensive than the crowbar approach. 3) DC-chopper A chopper circuit with a resistance can be added to the DC-link with a similar control function to that of the rotor side crowbar in order to reduce the DC-link voltage [6]. The chopper facilitates a voltage-raising action from the converter terminals, enabling the limitation of the DC-link discharges, and a fast recovery of the DC-link voltage. The protective device in this scheme is a simple chopper circuit and a resistance [7]. A control signal will activate the IGBT when the DC-link voltage exceeds its nominal value, and thus, the chopper is turned on and the energy is dissipated by the internal resistance.
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4) Flux compansation/demagnetaization current In order to avoid rotor over voltage, and RSC saturation, it is important to cancel the transient flux, this can be done through generating a rotor current from RSC, which opposes DC and negative sequence components of the stator flux linkage during a fault [8]. If the RSC saturation can be avoided the currents in the rotor circuit can be controlled and limited during the fault which stands for successful FRT without using any external devices, such as a crowbar. Currents are also used in [10] to oppose DC and negative sequence components of the stator flux linkage. They define these currents as demagnetizing currents. However, they also point out that the amount of demagnetizing current needed to cancel the transient flux is very high. Thus, the RSC should be over dimensioned in order to have a large current capacity. It should be noted, however, that the main advantage of a DFIG concept is small dimensioned converters. Instead of over dimensioning the FRT process in [10] relies on the combination of crowbar protection and demagnetizing current injection. After a voltage dip, the crowbar is connected to the rotor circuit in order to reduce the rotor current and the RSC is disconnected. After the current is reduced lower than the current capability of the RSC the crowbar is disconnected and the RSC is activated again and it starts to feed demagnetizing current to the rotor circuit. Also, the RSC starts to feed reactive power to the network. In both [9] and [10], the operation of a Grid Side Converter (GSC) is not taken into account. 5) Stator dynamic braking resistor (SDBR) SDBR approach introduces a resistor bank connected in series with the stator side or rotor side. It is used to increase the stator voltage or reduce the rotor current under grid voltage sags, respectively, therefore, mitigating the destabilizing depression of electrical torque and power during fault [11]. This strategy has been discussed in [12] – [17]. However, the sizing of the SDBR and its associated impact on the system performance is not well discussed in prior-art research. This paper investigates the sizing of SDBR connected in stator side and its impact on the performance of DFIG wind turbine systems. Section II derives the boundary of the SDBR value by considering the controllability of the RSC and the maximum allowable stator voltage during grid fault operation. Section III presents a study case on a 1.5 MW DFIG system and the impact of SDBR values within the boundary on the rotor current, stator voltage, DC-link voltage, followed by the conclusions. II.
DERIVATION OF THE BOUNDARY OF THE SDBR VALUE
Fig. 1 shows a DFIG system with SDBR. The stator side of the DFIG is connected in series with the SDBR, then directly to the grid side transformer. The rotor side is connected to the grid side transformer with an interface of a back-to-back converter, composed of line filters, RSC, and GSC. Either a bypass switch or a circuit breaker can be used
Fig. 1. A DFIG system with SDBR [18]. for the SDBR control strategy. The bypass switch solution is less expensive than that of circuit breaker, which is shown in Fig. 1. The bypass switch for the SDBR is in on-state under normal operation. At the presence of faults resulting in an increase of rotor current or DC-link voltage to a specified limit, it is turned off. Therefore, the stator currents are diverted to the series connected resistors from the bypass switch. When the faults are cleared and the system is recovered, the bypass switch is turned on and the stator circuit restores to its normal state. In the SDBR control strategy, the stator voltage is equal to the summation of the grid side voltage and the voltage across the SDBR VSDBR. Therefore, the function of the SDBR is to maintain the stator voltage at a minimum required level under grid voltage sags. It benefits to limit the rotor voltage induced by the transient stator flux, therefore, to limit the rotor current peaks. In addition, the rotor current limitation can also reduce the charging current to the DC-link capacitor, hence avoiding DC-link overvoltage which could damage the DFIG power converter. As discussed later in Section III, the SDBR can also improve the reactive power capability of the DFIG under grid faults operation, and thus, can also improve the FRT capability. This paper investigates the sizing of the SDBR and its impact on the rotor current, stator voltage, DC-link voltage and reactive power capability under grid voltage sags. The boundary of the SDBR value is determined by two criteria. The first one is to avoid the loss control of the RSC, implying that the rotor voltage due to the transient stator flux at the presence of grid faults should be limited to the maximum allowable input voltage of the RSC. The second one is that the summation of the grid side voltage under faults and the voltage across the SDBR should not exceed the specified maximum stator voltage during the abnormal operation. Under grid faults operation (e.g., voltage sags), the transient evolution of the stator flux induces a voltage Vr0 in the rotor voltage Vr. According to the discussions in [19], it can be derived that
Vr 0,max =
Lm ⎡ s V2 + (1 − s )(V1 − V2 ) ⎤⎦ Ls ⎣
(1)
where Vr0, max is the maximum Vr0 induced by the transition evolution of the stator flux. Lm is the magnetizing inductance
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conversion system. Therefore, the following section investigates the impact by a case study on a DFIG wind turbine system.
Is
V1 (1-p)
III.
V2
VSDBR
Fig. 2. Phasor diagram with the effect on stator voltage [11]. of the generator, Ls is the stator winding inductance, s is the slip, V1 is the stator voltage under normal grid conditions, V2 is the stator voltage under grid faults. With the added SDBR as shown in Fig. 2, V2 can be presented as
V2 = V1 (1 − p ) + VSDBR
(2)
where VSDBR is the voltage across the SDBR as shown in Fig. 1. The voltage Vr0 due to the stator flux can be considered as a voltage supply in series connected to the rotor transient inductance and rotor resistance, as the input of the RSC. To avoid loss control of the RSC current, it should fulfill that Vr 0,max ≤ VRSC ,max
(3)
where VRSC,max is the maximum allowable input voltage of the RSC, which is limited by the current controllability of the RSC as illustrated above.
VRSC ,max =
(1 − α 2)(1 − β )VDC 3
The rotor current, stator voltage, DC-link voltage, and reactive power capability of a 1.5 MW DFIG system are studied under different values of SDBR within the boundary defined by (7). A worse case with full voltage dip is chosen in the study (i.e., p = 1). Table I shows the specifications of the DFIG system and the operation conditions in the study case. The simulated voltage dip is set to 100% voltage dip and initiated at t = 1s and cleared at t = 1.1s. For the case study with parameters listed in table I, and according to (7), it can be derived that the boundary of RSDBR is 0.11625 pu/0.0255 Ω RSDBR 1.25 pu/0.275 Ω. A. System without SDBR Fig. 3 shows the system behavior during voltage dip without SDBR. It can be noted that the stator voltage drops to nearly zero and the rotor current reaches to almost four times of its nominal value. Due to the transients and high rotor current peaks, a large voltage overshoot appears in the DC-link, which may cause the failure of DC-link capacitors. TABLE I. SPECIFICATIONS OF THE DFIG SYSTEM AND THE
(4)
where α is the peak-to-peak voltage ripple of the DC-link capacitor bank with respect to the DC-link voltage, β is the maximum voltage drop across the input side inductors of the RSC with respect to the DC-link voltage. According to (1)(4), it can be obtained that VSDBR ≥
⎡ Ls (1 − α 2 )(1 − β )VDC ⎤ 1 ⎢(1 − p ) s V1 + (1 − s ) pV1 − ⎥ 1 − s − s ⎣⎢ 3Lm ⎦⎥
(5)
Therefore, the controllability of the RSC determines the lower limit of the VSDBR. From the limitation of the stator voltage level, the upper limit of VSDBR is limited to
VSDBR ≤ Vs ,max − V1 (1 − p )
THE IMPACT OF SDBR SIZING ON A 1.5 MW DFIG WIND TURBINE SYSTEM
OPERATION CONDITIONS OF THE CASE STUDY
Rated active power (P) DFIG output rated voltage Nominal DC-link voltage (VDC)* Nominal rms stator voltage (V1) Maximum rms stator voltage (Vs,max) Nominal stator current (is) Magnetization inductunce (Lm) Stator inductance (Ls) Rotor inductance (Lr) Slip (s) Voltage dip ratio (p) Ripple effect on DC-link capacitor (α) Losses due to inductances on RSC (β)
1.5 MW 690 V 2 pu 575 V 1 pu 0.8 pu 2.9 pu 3.08 pu 3.06 pu -0.2 1 10% 5%
*VDC = 2 pu (base value is the line-to-line stator voltage 575 V).
(6)
where Vs,max is the maximum allowable stator voltage during grid faults operation. By substituting VSDBR = Is RSDBR into (5) and (6), it results 1
(1 − s − s ) I ≤ RSDBR ≤
s
⎡ Ls (1 − α 2)(1 − β )VDC ⎤ ⎢(1 − p ) s V1 + (1 − s ) pV1 − ⎥ 3Lm ⎣⎢ ⎦⎥
Vs ,max − V1 (1 − p )
(7)
Is
The previous equation is a generic derivation of the boundary of the SDBR value for DFIG based wind power conversion system. Within the lower and upper limit of (7), the value of SDBR has impact on the rotor current, stator voltage, DC-link voltage and reactive power capability of the
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Fig. 3. Simulated waveforms of stator voltage, rotor current, and DC-link voltage of the DFIG without SDBR.
Fig. 6. Active and reactive power during voltage dip without SDBR.
Peak Rotor Current (pu)
Fig. 4. Simulated waveforms of stator voltage, rotor current, and DC-link voltage of the DFIG with SDBR. 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0
Maximum rotor current limit
Fig. 7. Improved active and reactive power with SDBR. 0
0.05
0.1
0.15
0.2
0.25
0.3
RSDBR (Ω)
Fig. 5. Relationship between the peak rotor current and the value of SDBR. B. System with SDBR The DFIG system with different SDBR values within 0.0255 Ω and 0.275 Ω are evaluated by simulations as shown in Fig. 4. It can be noted that the stator voltage is increased with the SDBR value, implying an improved capability to avoid the loss of control of RSC and to limit the rotor current. Accordingly, the rotor current is reduced with the increase of the SDBR value. It should be noted that the RSC is usually designed with a rating of 2-3 times of the rated rotor current in DFIG systems. Therefore, the maximum allowable rotor current is set as 2 pu in this study. To achieve this target, the SDBR value should be no less than 0.0965 Ω according to the waveforms shown in Fig. 4. To further correlate the relationship between rotor current and SDBR, Fig. 5 plots the peak value of rotor current under different SDBR values. It is shown that a larger value of SDBR is beneficial to a lower rotor current. However, a larger value of SDBR implies an increased power rating and power loss of the SDBR, which is not desirable. The sizing of the SDBR is based on the trade-off of these two factors to fulfill the rotor current limitation with minimum power loss induced by the resistors.
Fig. 8. The DC-link voltage enhanced with DC-chopper. The results shown in Fig. 4 also reveal that the DC-link voltage overshoot under voltage sag operation cannot be effectively limited by SDBR. To overcome this issue, a DCchopper is needed with the SDBR operation, which is discussed later in this section. C. SDBR effect on FRT capability The SDBR effect on the FRT capability is studied on a simulated wind farm consisted of six DFIG wind turbines studied above. Unlike the analyzed stator voltage, rotor current and DC-link voltage on a single wind turbine level, it is more important to investigate the impact of SDBR on the FRT capability from a wind farm perspective. The simulation results from the 9 MW wind farm without SDBR is shown in Fig. 6. It can be noted from Fig. 7 that the reactive power capability of the wind farm is improved with the increase of SDBR value. With the selected value of
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0.0965 Ω, the six wind turbines can provide about 4 MVAR reactive power to the grid. D. Enhanced DC-Link performance with DC-Chopper As discussed above, a DC-chopper is added in the DClink, which is usually applied in wind turbines as a practice. It protects the IGBTs from overvoltage dissipating the excessive energy, which has negligible effect on the rotor current. The results shown in Fig. 8 reveals that the DC-link voltage peak is reduced from 2400 V and 1800 V to 1200 V with SDBR values of 0.275 Ω and 0.0965 Ω, respectively. IV.
[6]
[7]
[8]
[9]
CONCLUSION
This paper discusses the sizing of stator side SDBR to improve the FRT capability of DFIG wind power conversion systems. A generic derivation of the boundary of the SDBR value is obtained, applicable for different voltage sag conditions. A study case on a 1.5 MW DFIG system is demonstrated and the sizing of the SDBR within the boundary is determined by considering the rotor current, stator voltage, and power loss of the SDBR during the abnormal operation. It reveals that a resistance value of 0.0965 Ω is optimal in terms of limiting the rotor current and power loss of the added resistors. It is also observed that high DC-link overshoots still appear with SDBR only, which is mitigated by introducing an additional DC-chopper in the study.
[10]
[11]
[12]
[13]
[14]
[15]
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