Introduction
The doubly fed induction machine using an AC-AC converter iin the rotor circuit (Scherbius drive) has long been a standard drive option for high-power applications involving a limited speed range. The power converter need only be rated to handle the rotor power.Vector-control techniques for the independent control of torque and rotor excitation current are well known, whilst Jones and Jones, for example,that a vector-control strategy can be used for decoupled control of active and reactive power drawn from the supply. Wind-energy generation is regarded as anatural application for the Scherbius DFIG system, since the speed range (from cut-in to rated wind velocity)may be considered restricted.
Most Scherbius FIG systems reported employ either a current-fed(naturally commutated) DC-Link converter or cycloconverter in the rotor circuit. Smith et al.describe the rated speed settings, gearbox ratios, and machine and converter ratings for variable-speed wind generation using the DFIG. Cardici and Ermis, and Uctug et al,have presented strategies aimed at maximising the total electrical power output from the DFIG. The use of a current-fed DC-link converter has a number of disadvantages: the DC-link choke is expensive, and an extra commutation circuit is required for operation at synchronous speed (which lies within the operational speed range), and this has resulted in poor performance at low slip speeds. In addition, such a converter draws rectangular current waveforms from the supply.
The problem at synchronous speed may be overcome by use of a cycloconverter, and vector- controlled Scherbius schemes with 6-pulse cycloconverters have been described by Leonhard and Walczyna. Yamamoto and Motoyoshi have presented a detailed analysis of the current harmonics drawn from the supply, which is still a problem in this type of drive. Machmoum et al,have presented an implementation with a simpler 3-pulse cycloconverter,whilst Holmes and Elsonbaty describe a similar converter to excite a divided-winding doubly-fed machine, which improves the speed range to 50% slip at the expense of increased machine complexity. Both of these schemes have the disadvantage of requiring a transformer to form the neutral; in addition, naturally commutated DC-link and cycloconverter schemes may, in many cases, require a transformer for voltage matching.The disadvantages of the naturally commutated DClink and cycloconverter schemes can be overcome by the use of two PWM voltage-fed current-regulated inverters connected back-to-back in the rotor circuit.The characteristics of such a Scherbius scheme,IN follows:
operation below, above and through synchronous speed with the speed range restricted only by the rotorvoltage ratings of the DFIG operation at synchronous speed, with DC currents injected into the rotor with the inverter working in chopping mode low distortion stator, rotor and supply currents independent control of the generator torque and rotor excitation Control of the displacement factor between the voltage and the current in the supply converter, and hence control over the system power factor.
The doubly fed induction machine using an AC-AC converter iin the rotor circuit (Scherbius drive) has long been a standard drive option for high-power applications involving a limited speed range. The power converter need only be rated to handle the rotor power.Vector-control techniques for the independent control of torque and rotor excitation current are well known, whilst Jones and Jones, for example,that a vector-control strategy can be used for decoupled control of active and reactive power drawn from the supply. Wind-energy generation is regarded as anatural application for the Scherbius DFIG system, since the speed range (from cut-in to rated wind velocity)may be considered restricted.
Most Scherbius FIG systems reported employ either a current-fed(naturally commutated) DC-Link converter or cycloconverter in the rotor circuit. Smith et al.describe the rated speed settings, gearbox ratios, and machine and converter ratings for variable-speed wind generation using the DFIG. Cardici and Ermis, and Uctug et al,have presented strategies aimed at maximising the total electrical power output from the DFIG. The use of a current-fed DC-link converter has a number of disadvantages: the DC-link choke is expensive, and an extra commutation circuit is required for operation at synchronous speed (which lies within the operational speed range), and this has resulted in poor performance at low slip speeds. In addition, such a converter draws rectangular current waveforms from the supply.
The problem at synchronous speed may be overcome by use of a cycloconverter, and vector- controlled Scherbius schemes with 6-pulse cycloconverters have been described by Leonhard and Walczyna. Yamamoto and Motoyoshi have presented a detailed analysis of the current harmonics drawn from the supply, which is still a problem in this type of drive. Machmoum et al,have presented an implementation with a simpler 3-pulse cycloconverter,whilst Holmes and Elsonbaty describe a similar converter to excite a divided-winding doubly-fed machine, which improves the speed range to 50% slip at the expense of increased machine complexity. Both of these schemes have the disadvantage of requiring a transformer to form the neutral; in addition, naturally commutated DC-link and cycloconverter schemes may, in many cases, require a transformer for voltage matching.The disadvantages of the naturally commutated DClink and cycloconverter schemes can be overcome by the use of two PWM voltage-fed current-regulated inverters connected back-to-back in the rotor circuit.The characteristics of such a Scherbius scheme,IN follows:
operation below, above and through synchronous speed with the speed range restricted only by the rotorvoltage ratings of the DFIG operation at synchronous speed, with DC currents injected into the rotor with the inverter working in chopping mode low distortion stator, rotor and supply currents independent control of the generator torque and rotor excitation Control of the displacement factor between the voltage and the current in the supply converter, and hence control over the system power factor.
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