A Simple and Reliable Rectifier for Variable Speed SCIG Wind Turbines by Using Series Reactive Compensator Named MERS Takanori Isobe, Toshisato Ohno, Takayuki Kawaguchi and Ryuichi Shimada Tokyo Institute of Technology 2-12-1 O-okayama, Meguro-ku Tokyo 152-8550, Japan Phone: +81 (3) 5734-3328 Fax: +81 (3) 5734-3838 Email:
[email protected] URL: http://www.nr.titech.ac.jp/˜rshimada/
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Abstract This paper presents a simple rectifier configuration for SCIG (squirrel cage induction generator) applied to variable speed wind turbines. The proposed rectifier consists of a diode rectifier and a series reactive power compensator named MERS. Advantages of this configuration are reduced active semi-conductor device rating needed and line frequency switching operation of the series compensator. Experimental results with a 1.5 kW SCIG are shown.
Introduction Recently induction generator is started to be used for variable speed wind turbines because of simple structure and low maintenance needed [1]. In comparison to synchronous machine, the induction generator does not have the capability to generate voltage by itself, and reactive power supply is needed to generate the voltage. For that purpose, active rectifiers are essentially needed and usually VSC (voltage source converter) with PWM (pulse width modulation) is used. On the other hand, simple diode rectifier can be used for dc-excited synchronous generator. This paper proposes the use of variable series compensation with diode rectifier. Required rating for the series compensator corresponds to variation range of the required reactive power for the induction generator; therefore, is much lower than one for the full converter. Additionally, this paper proposes line frequency switching for the series compensator and that results in almost zero switching loss, low electromagnetic noise and no generator side filters needed.
Magnetic Energy Recovery Switch (MERS) Configuration and Characteristics Series compensator named magnetic energy recovery switch (MERS) [2, 3] and its application for wind turbine which uses a permanent magnet synchronous generator has been proposed [4, 5]. The MERS works as a series variable capacitor by line frequency switching and phase angle control. From a certain
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Fig. 1: Circuit configuration of MERS. (a)Full-bridge. (b)2-switch configuration.
aspect, the MERS is one of FACTS (flexible ac transmission system) device works as a series compensator [6] like GCSC (gate commutated series capacitor) [7, 8] and SSSC (static synchronous series compensator) [9], but has different operation range and implementation characteristics. GCSC and SSSC have been studied for power transmission applications well; however, not introduced to other applications. Fig. 1(a) shows the basic configuration of the MERS. The MERS consists of four semi-conductor switches and a capacitor. Semi-conductor switches are required to have reverse conductivity (or freewheeling diode is needed) and turn-off capability. At this moment, IGBTs and MOSFETs are candidates. In the full-bridge configuration, the capacitor is charged with unipolar voltage and electrolytic capacitors can be used. However, almost all current flows into the capacitor; therefore, electrolytic capacitors are usually not adequate due to their current ripple ratings. The MERS works as an active series compensator, in other words, adjustable series capacitor. In comparison to conventional converters, using the MERS has following advantages: 1. Line frequency switching, comparatively low switching frequency in usual system frequency of 50 / 50Hz. Additionally low conduction loss semi-conductor device can be used, if a special design is applied [10]. 2. Simple gate angle control like thyristor. 3. Soft-switching operation within a certain operating range. 4. The full-bridge configuration has a wide operating range compared to other series compensation technology.
Operation Principles Fig. 2 shows possible current paths of the full-bridge MERS. Two switches are turned on and off in pair, U-Y are always opposite to V-X. By controlling two pairs of switches, the capacitor can be connected to ac circuit in series alternately with different polarity, or shorted. Remarkable paths are (b) and (e), which can be achieved when the voltage of the capacitor is zero, and in these paths, the capacitor is shorted and does not inject any voltage to the ac circuit. By line frequency switching and controlling current paths in a cycle, this circuit can work as an adjustable series capacitor. Schematic waveforms are shown in Fig. 3. The phase difference between the flowing current and the switching, δ, can be controlled. When δ = 0 as shown in Fig. 3(a), the flowing current does not charge the capacitor; therefore, the MERS does not inject any voltage to the circuit. By increasing δ, generated voltage in the capacitor is increasing and this waveform mode is referred as discontinuous mode. Finally when δ = 90◦ , the injected voltage waveform becomes pure sinusoidal as shown in Fig. 3(c), and has the same amplitude as the case of using a fixed capacitor whose capacitance is equal to one of the MERS capacitor. This waveform mode is referred as Balance mode. Additionally, the full-bridge configuration can inject higher voltage by dc-offset mode as shown in Fig. 3(d). In all waveform modes, fundamental
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Fig. 2: Possible current paths of MERS. The flowing current is positive direction (left to right in the diagram) in (a), (b) and (c), and negative direction (right to left) in (d), (e) and (f). The capacitor is discharged in (a) and (d), and charged in (c) and (f). When the capacitor voltage is zero, the current flows in two parallel paths as shown in (b) and (e), which are referred as ’parallel bypass mode’.
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Fig. 3: Waveform modes of MERS. (a)No MERS. (b)Discontinuous mode. (c)Balance mode. (d)Dc-offset mode.
component of the injected voltage has 90 degree phase difference to one of the flowing current, and amplitude can be controlled. This clearly means that the MERS works as an adjustable capacitor about their fundamental components. The equivalent reactance of the MERS, Xmers , can vary from 0 to ∞. Normalized equivalent reactance of the MERS is shown in Fig. 4. Xmers can be controlled by δ in discontinuous mode. In dc-offset mode, δ is always 90◦ and Xmers cannot be determined by only δ. Practically, control is implemented by PLL (phase lock loop) based on generator voltage or by using rotary encoder,
Fig. 4: Normalized equivalent reactance of MERS, Xmers /Xc , as function of gate control angle, δ, where Xmers is the fundamental equivalent reactance of the MERS and Xc is the reactance of the equipped capacitor inside of the MERS. (a) to (d) are corresponding to waveform modes in Fig. 3.
and Xmers can be easily controlled by simple phase angle control with reference of generator voltage or rotor position. In discontinuous mode, soft-switching can be achieved. When switches are turned off, the capacitor voltage is applied to the switches but the voltage is almost zero within a short period of switching transient. Turn-on is performed naturally after the current is flowing in free-wheeling diodes. In line frequency switching operation, soft-switching does not contribute to loss reduction; however, still has advantages from points of EMI and voltage stress to semi-conductor switches.
2-switch Configuration Fig. 1(b) shows 2-switch configuration which also can work as a series compensator, but with reduced number of semi-conductor switches. In this configuration, the capacitor is charged and discharged without semi-conductor conduction, and only when the capacitor voltage is zero, current flows into semiconductor switches. This can be an attractive advantage from points of semi-conductor rating and conduction loss. If the operation is in discontinuous mode and very near to balance mode, semi-conductor conduction duration becomes very small. This will result in low conduction losses and small current ratings required as well. Additionally, two semi-conductor switches share their emitter (or source) terminals; therefore, gate drive circuit can be simple. On the other hands, as a drawback, the 2-switch configuration can not achieve dc-offset mode, since a remaining voltage in the capacitor will cause shorting charged capacitor and very high current flowing into semi-conductor switches. Therefore, the operating range of this configuration is limited as 0 < Xmers < Xc , where Xc is the reactance of the equipped capacitor.
Proposed Converter for SCIG Wind Turbine Fig. 5 shows the proposed configurations. The rectifier consists of a diode-bridge, a series connected MERS and a fixed capacitor. The fixed capacitor provides a partial reactive power required by the induction generator, and generates some voltage. This initial voltage and resulting generator current are needed to establish the phase control of the MERS. The MERS controls the reactive power provided to the induction generator and its terminal voltage. Fig. 6 shows an example of the required reactive power to maintain its terminal voltage of a SCIG as function of the load power. The SCIG requires a constant reactive power and a several percent of variable reactive power. Fig. 7 shows the equivalent circuit of the proposed configuration. The diode rectifier can
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Fig. 5: Proposed configurations using MERS for SCIG wind turbine. (a)With a voltage source type grid connected converter. (b)With a current source type grid connected converter.
Fig. 6: Required reactive power to maintain the terminal voltage of a 11 kW SCIG.
Fig. 7: Equivalent circuit of SCIG with a fixed capacitor, series reactive power compensatir and diode rectifier of the proposed configuration.
be modeled as a variable resistor, Rd , which can be controlled by the next stage converter. When the terminal voltage is maintained at a rated voltage, the fixed capacitor whose reactance is Xc generates a constant reactive power; therefore, the equivalent reactance of the MERS, Xmers , should be controlled to add an required reactive power to achieve the constant terminal voltage. By using two variables of Rd and Xmers , the active and reactive power can be controlled independently; therefore, the generator terminal voltage can be maintained with varying active power. Xc should be selected to achieve almost full reactive power supply when the active power is zero at the
Table I: Parameters of tested generator.
Rated power Rated voltage Rated current Rated frequency Pole number
1.5 kW 200 V 6.8 A 50 Hz 4
Table II: Circuit parameters.
Fixed shunt capacitor (per phase) MERS capacitor (per phase)
100 µF 120 µF
maximum mechanical speed. When the mechanical speed (therefore frequency) is reduced, the reactive power generation by the fixed capacitor becomes not enough; therefore, the MERS must generate more reactive power. Two configurations of the grid side converter can be possible. Fig. 5(a) shows the configuration with a voltage source converter as a grid connected converter. In this configuration, the diode rectifier does not have voltage step-up capability even with the MERS; therefore, an additional dc/dc converter for low rotation speed operation is needed. The additional step-up chopper introduces some complexity; however, the chopper is also needed in the converter for the dc-excited synchronous generator and the proposed configuration still can be said to be advantageous compared to the use of the active rectifier. Fig. 5(b) shows the configuration with a current source converter. The rectifier part and a current source converter for grid-connecting are connected via an inductor. By using the current link topology, the power from low speed operated generator can be transferred to the grid, whose voltage is constant and higher than the generator voltage, without additional step-up converter. Both line commutated converter and self-commutated converter with a high frequency switching can be used for the grid-side converter. The selection depends on rated power and requirements of harmonics improvement and reactive power controllability. The current dc-link topology brings possibility of series connection of several generators. By connecting the dc-side of diode rectifiers in series, comparatively high voltage, which is suitable for transmission, can be available. Additionally, one concentrated grid-connected converter can be used. These features are attractive for off-shore wind farms. However, generator windings can have high potential voltage; therefore, high level insulation of generators themselves or isolation transformers are needed.
Experimental Verification Setup To confirm the proposed concept, small scale experiments with a 1.5 kW induction generator were conducted. A motor-generator set shown in Fig. 8(a) was used for the experiments and its mechanical speed was controlled by the prime mover driven by an inverter. Parameters of the tested induction generator are listed in Table I. The circuit configuration for the experiments was almost same as the proposed configuration with the current source converter shown in Fig. 5(b); however, an electronic load and a series connected inductor were used instead of the current source type grid-connected inverter. The electronic load was operated in current control mode and the dc current set-point was given manually. The MERS operating set-point (gate phase angle) was also given manually in the experiments. Circuit parameters are listed in Table II.
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Fig. 8: Overview of the experimental setup. (a)Pilot device of the MERS series compensator. (b)Motor-generator set used for the experiments. The right side machine is the tested induction machine.
Fig. 9: Voltage, active power and currents profiles at 1500 min−1 with the rated generator terminal voltage as function of dc-link current.
Fig. 10: Voltage, active power and currents profiles with the rated generator current as function of rotation speed.
Power control with fixed rotation speed Firstly, the mechanical speed was fixed at rated speed and dc-link current was varied. The generator terminal voltage was maintained at the rated voltage by controlling the series compensation. The measured voltage, currents and active power are shown in Fig. 9. The ac-side current of the diode rectifier, which is equal to the current flowing into the MERS, is uniquely determined by the dc-link current, and the terminal voltage can be controlled by the reactive power control. Consequently, the constant voltage control results in the linear active power profile as function of the dc-link current. Measured waveforms with the rated terminal voltage and various dc-link current are shown in Fig. 11. The amplitude of the MERS injecting voltage was relatively low, which indicates semi-conductor devices
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Fig. 11: Voltages and current waveforms at 1500 min−1 and various dc-link current, Idc . (a) Idc = 5.5 A. (b) Idc = 3.0 A. (c) Idc = 1.0 A.
with low rated voltage can be used. The discontinuous mode of the MERS was observed in high current operations as shown in (a) and (b); however, the dc-offset mode was needed to maintain the rated voltage in low current operation as shown in (c), that means the full-bridge configuration of the MERS is needed to support low current (therefore low power and torque) operation.
Generator capability at each rotation speed Secondary, the mechanical speed was varied with rated generator current. To achieve required terminal voltage and current of the induction generator, both of the compensation degree of the MERS and the dc-link current were controlled. The terminal voltage was controlled to be linear to the mechanical speed, that means the generator capability was fully used at each rotation speed. The experimental results are shown in Fig. 10. High rotation speeds were firstly tested and then the rotation speed was decreased. The results confirmed that the proposed configuration achieves full use of the ratings of the induction machine in high rotation speed area. Fig. 12 shows the waveforms with various rotation speed. Only the discontinuous mode of the MERS is required for the reducing mechanical speed. In lower rotation speed area than 650 min−1 , the terminal voltage disappeared. The demagnetizing has irreversible characteristics. Once the terminal voltage disappears, the phase control of the MERS series compensator, which is based on the phase of the terminal voltage (or current), becomes not available. In low rotation speed operation, the initial voltage generation by the fixed shunt capacitor is not available since sufficient reactive power does not generated by the capacitor in such low frequency. To achieve much lower rotation speed operation, the MERS should be controlled with a feedback control to avoid unwanted terminal voltage reduction caused by varying rotation speed and load.
Conclusions This paper proposed a new configuration of rectifier for variable speed SCIG wind turbines. The main advantage of this configuration is reduced semi-conductor ratings. For generator applications, reactive power control is needed to achieve the full use of the generator capability but can not be achieved by
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Fig. 12: Voltages and current waveforms with the rated generator current and various rotation speed. (a) 1500 min−1 . (b) 1000 min−1 . (c) 650 min−1 .
a diode rectifier. Introducing reactive power compensator can achieve it with partial ratings of active semi-conductor switches. Using the MERS as the reactive power compensator has two advantages. Series compensation can be achieved without an inductor; therefore, the rectifier stage can consists of only semi-conductor devices and relatively small capacitors. Additionally the MERS is operated with line frequency switching, which is also an advantage of the series compensation. However, the series compensation has difficulties of device selection. The rating reduction means reduced voltage rating in the series compensation; therefore, appropriate design with well consideration of over voltage possibilities and protections is important. Experiments confirmed the advantage of the series compensation. Relatively small voltage injection performs good controllability of the generator. High impedance of the series compensator is needed for low active power operation; therefore, the full-bridge configuration of the MERS is desirable. However, the 2-switch configuration can be possible by giving up supporting a certain range of low power operation.
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