Research On Single-phase Photovoltaic Grid-connected Power Conditioning System

As the energy and environmental problems have become increasingly prominent,the clean solar energy has been paid more and more attention. With the emergence of large nonlinear and impact load, harmonic and reactive current produces more and more pollution to the grid. To alleviate this problem, active power filter has become the focus of domestic and foreign research. The full bridge topology in photovoltaic grid connected inverter is the same with that in active power filter. When the photovoltaic grid connected inverter and active power filter are combined together,photovoltaic grid connected power system(PVPCS) comes into being. It injects active power to the grid, at the same time, compensates for reactive and harmonic current of nonlinear load, which not only improves the utilization rate of the equipment, saves investment, and improve the power quality of distribution network.PVPCS uses traditional double loop control strategy, which is PI voltage outer loop and quasi proportional resonant(Quasi-PR) current inner loop. There are two major problems: the first is when the environmental parameters such as light and temperature changes, the PV array output power and the power injected into grid are imbalance. Under the traditional PI voltage loop control, DC link voltage overshoot is large, dynamic response time is long, the grid current total harmonic distortion(THD)increases and may even endanger the stability of PVPCS. The second is that digital control is often adopted in the Quasi-PR current inner loop, so sampling, zero order holder and one-beat delay control are usually included, which degrades PVPCS current loop steady state and dynamic performance, increases grid current THD and the dynamic response time.To solve the two problems, the two-stage single-phase grid-connected PVPCS was discussed as an example. The mechanism that traditional control strategy causes DC link voltage fluctuation and the effects that digital control has on current inner loop were analyzed in theory. The new double loop control strategy, which is outer voltage loop with power feedforward control and the inner current loop with predictive control, as well as its control parameters, were designed. The simulation conclusions are as followed: digital control changes characteristics of the controller and the plant; the order of system characteristic equation and the phase lag increases;the amplitude resonance and phase jump arise at the switching frequency and its multiple frequency; the stable region of control parameters reduces; steady-state error becomes larger and dynamic performance declines because of system stability. The above effect is more serious when sampling frequency is lower. The outer voltage loop with power feedforward control can reduce the DC link voltage fluctuations and the dynamic response time. The predictive control inner current loop can output the control signal a beat in advance, which reduces the adverse effects of digital control,the grid current THD and dynamic response time. Under the new control strategy,PVPCS compensates for reactive and harmonic current of nonlinear load and injects active power to the grid to the greatest degree.The analysis on PVPCS was validated by both simulation and experiment. The PVPCS simulation model was established in MATLAB and the traditional control strategy and the new control strategy were compared. At the same time, the 1kW DSP-controlled PVPCS experimental platform were designed from the hardware and the software two aspects. The analysis correctness was verified through the experiment.