Suspended Power Quality Hybrid Compensation Technologies In High Voltage Distribution Networks And Its Application

With rapid increasing of nonlinear loads, the problem of power quality is more and more serious including low power factor, serious harmonic pollution and large negative current. It results in high energy losses and frequent equipment failures. Because of the voltage level is too high to power electronic for power quality direct compensation in the high voltage distribution network (6/10/35kV), different topologies with power electronic device in series become a hot focus for solving the contradiction. At present, suspended technology is still restricted to reactive power compensation. LC and low voltage active power filter (APF) are the main ways for high voltage harmonic direct suppression, and the active suspended compensator is destitute. So a new high voltage suspended hybrid compensator (HVSHC) with its control and application is researched. The improved method of harmonic and reactive power fast extraction is advanced based on existing techniques, and the topological structure, operation principle and equivalent model of HVSHC are analysed. The control and optimization technologies of HVSHC are presented. The equipment of HVSHC is designed and applied to the distribution network of electric arc furnace, and the results demonstrate the validity of the proposed system.The major innovation research achievements include:(1) A new topological structure combined with H bridge cascade SVG and TCR of SVC is proposed. With the merits of rapid controllability of SVG and large capacity of SVC, it comprehensively compensates the reactive power, harmonic and negative sequence current in high voltage distribution network. SVC is used to compensate negative sequence current and slow change reactive power, and SVG is used to suppress of harmonic and rapid change reactive power. The effect of HVSHC is equivalent to the same capacity pure SVG, but avoids the difficulty of large capacity active compensation and reduces the cost.(2) A new method for fast extraction selective harmonic is proposed. It detects the control variables of selective harmonic by digital band-pass filter and multiple synchronous rotating reference coordinate in synchronism with frequency. Different AC variables are transformed DC components, which is easy to adjust by proportion-integral. With universal rotating transform, the proposed method decreases the delay time and calculation by two low power filters placed by a band pass filter.(3) The equivalent model of HVSHC is built, the active power and reactive power decoupling control strategy of H bridge cascade multilevel is advanced on multiple synchronous coordinates is proposed. The reactive power and harmonics are control at the same time. A new control technique of phase angle superposition is introduce to slove DC voltage unbalance. A new calculation and trigger strategy is put forward for fast change loads, and reactive computer and the time of control are improved. The response time of TCR is improved from40～60ms to20～30ms. These methods are validated by the results of the simulation and the experiment.(4) The cooperating control method based on rules and neural network prediction is proposed. It solved the problem of the function losing of reactive reserve and harmonic suppression by different response time of SVG and SVC. The output of SVG is limited by the change ratio of loads’ reactive power, and flexible track of reactive power is realized based on the voltage fluctuation prediction by Elman neural network.(5) An optimization method of adjustable modulation waveform amplitude and s variable switch frequency are invented. Output voltage distortion of H bridge cascade multilevel converter with DC voltage fluctuation is restrained by adjustable modulation waveform amplitude, and the ratio of active capacity is advanced by variable switch frequency. Switch frequency is divided to multi-sections by anticipant reactive power, and switch frequency is higher when output reactive power is lesser. The effect of harmonic suppression is improved.(6) The HVSHC device of power quality compensation system for35kV distribution network of electric arc furnace in cast steel factory, and its parameters are optimization. The application results indicated not only that power quality is promoted roundly, but also that smelting time is decreased form8hours to7hours. Moreover it reduces the power energy consumption is obviously.