Research On Critical Techology Of The Single-phase Power Electronics Load

With the development of society, various electrified products are widely applied in today’s manufactures and life. The quality of power sources would directly affect the production efficiency and living level. Therefore, power sources burn-in test is required. Traditional test equipments are energy-dissipating, which consists of resistance, inductance, capacitor and other passive devices. For the concept of energy saving and environment protection, the green testing system emerge as the times require.This paper designs an equipment of Single-phase AC Power Electronic Load based on back-to-back structure according to the testing standards of power sources (GB/T7260.3-2003) and the grid-connection requirements (GB12325-1990, GB/T14549-1993, GB/T15945-1995). The pre-stage Load Simulator Convertor (LSC) of this equipment simulates various load characteristics, including linear load like resistance, resistance-inductance and resistance-capacitance and nonlinear load. The last-stage Grid-connected Convertor (GCC) feeds back the energy from tested source to the grid to realize green energy-saving.As the PWM convertor is the basic composition unit of back-to-back structure, its mathematic model is established. Depending on the whole structure of Single-phase Power Electronics Load (SPEL), this paper displays the control topology of SPEL under classical PI control and designs the parameters of primary circuit hardware and PI control to make the system stable in different working conditions. By the simulations and experiments, it is found that the non-resistance load characteristic could not be simulated precisely and the THD of grid current can not accord with the grid-connection requirements. So an improvement scheme is proposed. The compound control with P control and repetitive control is adopted in the pre-stage LSC, which could flexibly simulate load characteristic with zero steady-state error. While a dual-loop control strategy, which voltage-loop using PI control and current-loop using quasi-PR (proportion&resonance) control, is adopted in the last-stage GCC to guarantee the stability of dc-bus voltage and high efficiency of grid-connected current feedback to the grid.To accurately simulate load characteristic, two key technologies have to be solved:the generation method of load simulation current reference and tracking of current reference with zero steady-state error. There are linear and nonlinear current references. Usually the acquisition of linear current reference comes from constant current mode or constant impedance mode. This paper put forward an improved method of constant-impedance current reference after a comparison of the two modes. Moreover, four feasible methods to obtain nonlinear current reference based on DSP are also given. For the limit band width of P control, there are amplitude decay and phase lag in medium and high frequencies in the stead output, which can not meet demand of the stability and zero tracking steady-state error at the same time. So the compound control based on P control and repetitive control is introduced to improve the accuracy of simulating input current and to achieve zero gain and zero phase-shifting in a certain bandwidth. Both the series compound control and parallel compound control are detailed analyzed about their characteristics, implementation methods and design gist. It is testified in the simulation and experiment that compound control has widen the bandwidth and achieved zero gaining and zero phase-shifting within this bandwidth. Furthermore, series compound control has similar control effect with parallel control from the bode diagrams of system’s closed-loop transfer function for a consistent characteristic equation. But series compound control has a control object with an extra proportional link, which would reduce the burden of repetitive controller. Therefore, series compound control is superior to be applied in experiments and a relative satisfying control effect is acquired.According to the state grid-connected standards of energy quality for public grid harmonic (GB/T14549-1993), the THD of grid-connected output current should be less than5%. On the view of energy saving, the unit power factor control for GCC is needed. According to active power balance principle, the twice-frequency pulsation in instantaneous power of single-phase system would cause second harmonic in dc-bus voltage, which would make a rich third harmonic in grid-connected current reference via voltage loop. To overcome this problem, filtering processing for voltage loop and current loop is carried out by low-pass filter, notch filter, band-pass filter and mean filter. From the perspective of zero-pole point distribution, the control effects of these four filters are compared to choose two proper filters for different load conditions. The quasi-PR control, which not to affect dynamic characteristic of the system, is adopted in current-loop to achieve zero gaining and zero phase-shifting of grid-connected current reference at base frequency.For series compound control and mean filter would cause long regulating time, feedforward control strategy is proposed to quicken the dynamic response speed and to inhibit the disturbance from input voltage, input current and grid-connected voltage. This paper analyzes the feedforward control of input voltage, grid-connected current reference and grid-connected voltage and designs the feedforward parameters to improve the dynamic characteristic, especially emphasizing on the effects of voltage fluctuation and distortion in constant impedance mode. All the control strategy above has been verified in digital signal simulation and experiment.By quoting the concept of low voltage ride through (LVRT) and applying appropriate control algorithm, SPEL could cope with the grid voltage drop well and reliable work without stopping, which would provide reference basis for intelligent control of SPEL.