| High voltage direct current (HVDC) transmission shows more and more significant advantages in long distance and large capacity electric power system,in the era of large power grid, as powerful supplement of the ac power, dc power transmission in the power system has important economic and technical value.When using thyristor as converter device of running HVDC system, due to thyristor commutation characteristic, the inverter commutation will absorb large amounts of reactive power, at this point, if we do not hang on the compensation on the spot of reactive power compensation device, system power factor will decline seriously,and also the efficiency of the generator.Therefore, this article in view of the HVDC transmission problem of reactive power compensation, carries on the design research with a variety of control strategy to the static var generator (SVG), when the system occurring failure on the exit, designs the resistance and capacity buffer to protect the device,and electronic protection circuit which based on fuzzy control as the protection of the converter system.Through analysis comprehensively the simulation results showed the performance of the SVG device, choosing the appropriate SVG; through the choice of model and parameters of the fuzzy PI control, makes the transient stability of the HVDC system show better.Paper analyzes the characteristics of reactive power in HVDC converter station firstly, in combination with the engineering calculation formula of capacitive and inductive reactive power, then introduces the reactive power compensation device, and the strategy which commonly used in engineering, all of which will lay a theoretical foundation for follow-up study of SVGTo improve power factor, the paper analyzes and studies the working principle of the SVG secondly.By introducing the three parts:the reactive power detection circuit, main circuit and control strategy in detail, select the main circuit model of system and reactive current and reactive power detection circuit.And in the measurement of reactive current and reactive power are for different control strategies, the control strategy uses tracking PWM technology and space voltage vector technology (SVPWM).Through the design of control method, the paper builds a simulation model of HVDC on MATLAB platform, in the simulation waveforms on all kinds of operation mode of the control strategy, and records the waveform of typical data:power factor, harmonic distortion rate, etc.The simulation results show that the triangle wave of direct current control method on both ends of the communication system, makes the system indicators of power factor and harmonic distortion in a optimal amount. In the equivalent inverter side system, based on the SVPWM technology of direct current control is the optimum system performance. In HVDC system, and discusses the inverter over voltage and over current fault reasons, analysis the pressure and flow simply.When the converter rectifier side occurring earthing fault at the exit, this paper designs over voltage and over current fault protection.The designs contain the resistance and capacity buffer to protect the device and the fuzzy control of electronic protection circuit to protect system. Through the simulation curves of typical data in dc side:the maximum overshoot、fault recovery time and so on, comparing the control effect,the simulation results show that the resistance and capacity buffer circuit enables the system follow better, improves system overshoot amount; Electronic protection circuit based on fuzzy PI control system has good transient and steady characteristics, and better tracking performance and enhance anti-jamming capability, to ensure the normal, reliable and stable operation of HVDC system. |
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