According to the voltage level, photovoltaic power stations can be divided into three categories: firstly, photovoltaic power stations connected to the power grid with a voltage level of 66KV or above are called large-scale photovoltaic power stations; The second is that photovoltaic power stations connected to the power grid with a voltage level of 10-35KV are called medium-sized photovoltaic power stations; The third is that photovoltaic power stations connected to a low-voltage power grid with a voltage level of 0.4kV are called small-scale photovoltaic power stations.

A photovoltaic power station consists of four parts: a photovoltaic cell array, an inverter, a step-up transformer, and a control and protection device. The process of generating electricity and connecting to the power grid is to convert light energy into electrical energy through the photovoltaic cell array. The electrical energy is converted into AC power output in the form of DC power through the inverter. At this time, the AC power is low-voltage AC power, and then the voltage of the AC power is boosted and connected to the power grid through the step-up transformer. The power generated by a photovoltaic power station is measured by the amount of light it receives.

Photovoltaic power generation is affected by the environment, resulting in high harmonic content and unstable power generation, thereby affecting the power quality of photovoltaic power generation. The harmonics and high-order harmonic content generated by photovoltaic power stations, as well as the instability of their power generation, have brought a lot of pollution to the grid connection. So the photovoltaic power station must be connected to the power grid at the grid connection pointPower quality monitoring deviceLong term monitoring of photovoltaic power plant connection points and preservation of historical data for analysis.

With the development of power electronics technology, direct current transmission and high-power single-phase rectification technology are widely used in industrial sectors and electrical equipment, such as high-power thyristor devices, switching power supplies, variable frequency speed regulation, etc. These typical nonlinear loads will absorb or inject harmonic currents from the power grid, causing voltage distortion in the power grid, polluting the waveform, deteriorating power supply quality, increasing additional losses, and decreasing transmission capacity.

In the power grid, three-phase load imbalance, power system resonance grounding, and other factors can generate negative sequence, while high-power rectifiers and nonlinear equipment can generate harmonics.