| Due to the dual-pressure of environment and energy issues,distributed generations have become an important part in modern power system and will become a key component in future power system.Under the guidance of governments,distributed generations have become global highlights all over the world.As an important part of distributed energy generations,small-scale wind power generations also gain a bright future.Owing to merits such as wide speed range,high system efficiency,good fault tolerant ability,high reliability and low manufactory cost etc.,switched reluctance generators(SRGs)are proved to be suitable for small-scale wind power generations in mountainous villages,skyscrapers,broder posts and isolated islands etc,al.However,comparing with permanent magnet generators,which are widely used for small-scale wind power generations,SRG suffers from relatively low power density and system efficiency.In this paper,we focus on improving the maximum output power range and system efficiency of SRG system by optimizing control parameters,phase current modulation and power converter design,which are listed in details as following.By analyzing the model of low-voltage SRG system,it's obtained that both the output power range and system efficiency can be improved by optimizing the excitation and demagnetization current trajectory.Moreover,the system efficiency is mainly killed by copper loss in the machine and device loss in the power converter.Since both current trajectories and power losses can be improved by optimizing excitation and demagnetization voltages,this paper suggests to increase the excitation and demagnetization voltages to improve the system efficiency and output power range.To simplify the current control method,a phase current control scheme is proposed for phase current modulation in the entire operating range.For low speed operations,phase current is kept “flat-top” by regulating the demagnetization voltage.An analytical method is proposed to build a given SRG whose geometric parameters are unknown.With the analytic model,the best turn-off angle can be calculated and only the turn-on angle and DC-link voltage are to be optimized.Then the system output power range is studied by differential evolution algorithm and is increased by optimizing the turn-on angle and DC-link voltage.To find the best efficient point on-line,a double-loop control strategy is proposed.In the proposed double-loop control strategy,the required output power is achieved by the power control loop and the system efficiency is optimized by the efficiency optimization loop.When zero-steady state is achieved in the power loop,the controller evaluates the system efficiency and optimizes the turn-on angle in the efficiency loop.In the end,with the proposed double-loop control strategy,the system finds the best efficiency point under desired output power.To accelerate the on-line tuning process,an efficiency optimization method is proposed for low-voltage SRG system.Since the major loss in low voltage SRG system is propotional to the root-mean-square value of phase current,the proposed efficiency optimization method improves the system efficiency by maximizing the average-torque ampere ratio.First,the torque profile is analyzed and the rule to maximize average-torque ampere ratio is proposed.In order to calculate the current command on-line,the torque profile is modeled by cubic polynomial.The current command equation is solved according to rotor position and torque coefficient.Since it's hard to build SRG phase current in the excitation interval and modulate SRG phase current in the demagnetization interval,a dynamic firing angle regulation strategy is proposed.The turn-on angle are regulated according to the actual current trajectory in the excitation interval and the ideal current trajectory in the excitation interval;the turn-off angle is regulated according to actual current trajectory in the demagnetization interval and ideal current trajectory in the demagnetization interval.Experimental results show that both output power and system efficiency can be kept in relatively high levels.To implement the current regulating scheme in low-cost system,a position detecting scheme is proposed to improve the position resolution with conventional photoelectric position sensor.The resolution of proposed detecting scheme is 0.105° at the rated rotor speed.Since the system performance,including the output power range and system efficiency,can be improved by boosting the excitation and demagnetization voltage.An integrated power converter is proposed for the low-voltage SRG system.The integrated power converter is achieved by adding a front-end circuit,which consists of one MOSFET and two diodes,in front of conventional asymmetric half-bridge power converter.By controlling the MOSFET in front-end circuit,the voltage of boost capacitor can be controlled directly,while the output voltage can be steady at the rated value simultaneously.Then two voltage control strategies are compared for the boost voltage control in the front-end circuit: the voltage hysteresis control strategy and the power flow based voltage control strategy.The hysteresis control is simple,but high output voltage ripples might be obtained at the output capacitor.By comparing the delivered current and load current,charging current of the boost capacitor is distributed to minimize the output voltage ripple.Simulation and experimental results are given to verfy the effectiveness of proposed integrated power converter and power flow control strategy.Then the proposed converter is compared with conventional asymmetric half-bridge power converter.The proposed converter shows advantages in minizing output voltage ripple,improving system output power range and system efficiency.Comparing with other existing boost converters for SRG control,the proposed converter shows advantages in voltage control and can be evoluted from conventional asymmetric half-bridge power converter convinently.Although the performance of asymmetric power converter can be improved by augmenting the front-end circuit,power loss in the asymmetric power converter for SRG control is still high.To overcome the drawback of conventional asymmetric half-bridge power converter,a simplified power converter is proposed for SRG control in stand-alone wind power systems.The power converter is simplified from asymmetric power converter and only needs m+2 MOSFETs and m diodes to drive an m phase SRG.By reducing MOSFETs in the excitation path and diodes in the freewheeling path,the system efficiency is improved by the reduced RMS current that flows through power devices.Then a DC-DC converter is employed for the power flow control in the simplified converter.With the DC-DC converter,the battery coordinates with SRG convinently.Since the DC-DC converter shows fast dynamic response and good robust,the output voltage can be steadied quickly in the proposed converter.In the end,the effectiveness of proposed converter and control strategy is also verified by experiments.The simplified converter shows high efficiency,good robust and quick dynamic response. |
PDF Full Text Request