Impact On Power Quality Of The Distributed Photovoltaic Grid

Nowadays, the photovoltaic(PV) power grid, which is feeding on solar energy, can incorporate into distribution network. It can relieve the situation of power emergency effectively. The distributed PV generation has flexible power supply mode. When PV gird produces more electricity than users consume, the rest of electricity will be exported to distribution network. When PV gird produces less electricity than users consume, the lack of electricity will be imported by distribution network. Although the technique has lower costs and release of energy emergency, it brings some power quality problems at the same time, such as harmonic pollution?islanding effect?DC injection?residual current, etc. According to the operation and output power of distribution network, this paper analyzes the impact on power quality of distribution network under its voltage and current influence.First, the main component of PV grid is simply introduced, and its principles are analyzed. Based on distributed PV device model, there comes the conclusion: PV grid current always maintains the same frequency and phase with the voltage no matter how the changes of user load.Second, reactive power and harmonic is introduced. This is one of the main measures of power quality. Based on the simulation model, the current total harmonic distortion(THD) is 4.92% under the rated power of inverter. When the inverter operates at 10% rated power, the current THD will be increased to 7.63%. So the inverter must be operated under rated power.Third, islanding effort is illustrated according to the topological structure of PV grid and distribution network. Its common detection methods are simply introduced. Based on the simulation model, before the island state, the gird is under stable operation as well as its voltage and current are in standard permission scope. Under the island state, the bad change of voltage and current would bring a lot of pressure to operation of PV grid and distribution network, even breakdown.Then the definition of DC component is introduced by mathematical method. And its origin and control methods are explained. The existence of DC component causes the supersaturation of DC magnetic bias in the distribution transformer. Therefore the output voltage waveform will loss at its peak, meanwhile the output current waveform has a pulse. The entire grid would be impacted.Finally, the definition and influence of residual current are briefly described. Based on the simulation model of single-phase full-bridge inverter, the current and voltage of distribution network and residual current of the inverter are analyzed.