The power quality problems caused by grid connected photovoltaic power generation system mainly include harmonics, voltage fluctuations, flicker, etc., which affect the characteristics of active and reactive power flow and frequency control. Online power quality monitoring of photovoltaic power generation system is needed.

Due to the influence of weather, environmental temperature, installation position of photovoltaic panels and other factors, the output power of photovoltaic power generation will also change, with a large change rate even exceeding 10% of the rated value. Therefore, the instability of power generation will occur, which will affect the harmonics fed into the grid.

The change of photovoltaic power with sunshine intensity has an impact on the load characteristics of power grid. Its access changes the power flow direction of power grid, and will affect the planning and dispatching operation mode of existing power grid. Moreover, photovoltaic power generation units do not have the function of dispatching automation, which increases the difficulty of grid control and dispatching operation. If a large number of photovoltaic power generation systems are connected to the power grid terminals, the voltage fluctuation will be intensified, and the voltage and reactive power regulation devices may act frequently; if a high proportion of photovoltaic power generation system is introduced, the distribution network will change from the traditional single source radial network into a double terminal or even multi terminal network, thus changing the size and duration of fault current, affecting the protection of the system.

Therefore, it is necessary to carry out on-line monitoring of power quality for photovoltaic grid connected power generation, and the following conditions must be met:

1. Real time monitoring of all parameters of power quality.

2. Be able to analyze harmonics, inter harmonics, higher harmonics and other functions.

3. When power quality event occurs, it can give real-time alarm.

Power quality problem data analysis function, at present, the domestic power quality online monitoring device which can meet the technical standards of photovoltaic power grid connected power quality, together with powerful background software, constitutes the power quality monitoring system of photovoltaic power grid connection.