Research On Photovoltaic Generation System And Its Control Technologies

There exist many key problems in the development of photovoltaic power generation (PVPG) system and power-control of a grid-connected PVPG system, such as the parallel inverter technology of a grid-connected PVPG system, the maximum power track and control of a grid-connected PVPG system, the stability and reliability of the system, the influence of the unbalanced output-voltage of a three-phase inverter with unbalanced payload, etc. In this dissertation, we research deeply into these key problems, and propose corresponding solutions, i.e., a parallel control technology for a photovoltaic inverter, the maximum power track and control technology of a grid-connected PVPG system, harmonic suppression in a PVPG system, as well as a control algorithm for a three-phase unbalanced payload. The main research work and contributions of this dissertation are listed as follows:(1) A control scheme for high-performance paralleled-inverter in a PVPG system is proposed. This schemde is described below. Firstly, the frequency and phase of a sinusoidal-pulse width-modulation (SPWM) waveform is generated by a field programmable gate array (FPGA), which ensures the sysnchronization of the output voltages. After that, the three-phase alternating voltage and current are reconstructed from their single-phase ones. And then, they are transformed into two two direct components based on instantaneous reactive-power theory, which are transformed into their conresponding active and reactive currents using synchronous d-q transformation. Finally, the instantaneous reactive and active power of the two paralled-inverters are calculated and kept identical by a digital signal processor (DSP), thus the output current of the two inverters is current-sharing. Experiment results show that the control scheme is capable of power-sharing and small circumfluence between the inverters.(2) A maximum power-transmission method for PVPG grid-connected system is proposed. Its control strategy is described as follows:based on incremental conductance method, a control strategy of one cycle power-disturbance is applied in the DC/DC converter to track the PV battery maximum power; the maximum DC current of the PV battery maximum power output is used as the peak value of the instantaneous reference current for the AC feedback control of the output current of the grid-connected inverter in the DC/AC grid-connected inverter. The phrase and frequency of the grid voltage is used as those of the instantaneous AC reference current, the difference between which and the grid AC current is utilized for PI control. For assuring the stability and reliability of the grid-connected inverter, a grid-voltage feed-forward and a feedback of the filter-capacitor current are introduced. The principle and control strategy of the grid-connected control system are detailed analyzed. Simulation for the PV system is done using Matlab/Simulink tools, which validates the feasibility of the above-mentioned control-strategy.(3) This dissertation analyzed the causes that SPWM inverter power supply produces the harmonic and then designed a new three-phaseâ–³-band-pass filter. The theoretic analysis and the simulation showed that the filter can restrain the 3rd-19th components and the harmonic component are reduced from 31.5% to 3.1%. Consequently, the power factor and exploitation rate of equipment are improved and the loss-free line and loss of voltage are cut down considerable. Therefore, the PV system can work steadily and the quality of power is improved.(4) A split-phase control method of the output voltage of three-phase inverted power-supply is proposed. Applying the control of output line-voltage of a three-phase inverter, the balance of the three-phase voltage can be utilized, the traditional compensation method of the positive-, negative-and zero-sequence components of output voltage of the three-phase inverter under the case of unbalanced load is implemented by symmetrical component decomposing and superimpose theory in order to keep the the symmetric three-phase output voltage. However it is time-consuming, poor real-time and not easy to control. Thus, this dissertation proposed a novel individual phase control for output voltage of the three-phase inverter. The theoretic analysis and the simulation showed that this method is capable of reducing the maximal unbalance degree (rated load of A-phase,200% rated load of B-phase and no-load of C-phase) from traditional 5.49% to 1%, and the output voltage can reach the balance steadily and reliably when the unbalanced loads goes to 200%. Consequently, the voltage distorting caused by unbalanced load can be compensated effectively, so that the balanced output voltage is guaranteed under any unbalanced load.