The influence and solution of grid-connected photovoltaic power generation on power quality

1 Basic Principles

Photovoltaic power generation uses the photovoltaic effect present on the surface of the semiconductor to send a direct current through the light at both ends of the semiconductor material. When the sun shines on the semiconductor P-N node, a new electron-hole pair is formed, and after the photon excits the electron from the covalent bond, the electron flows to the N region and the hole flows to the P region, resulting in a potential difference between the two ends of the semiconductor. Once the circuit at both ends of the PN junction is connected, a current will be formed, flowing from the P zone to the N zone through the external circuit, and the electrical power will be output to the load.

2. Structure and classification of grid-connected photovoltaic power generation

Grid-connected photovoltaic power generation system is mainly composed of solar panel (module), high power tracking (MPPT) controller, DC-AC inverter several parts, using insulated gate bipolar transistor (IG-BT) as the switching element of photovoltaic inverter. The DC output of the solar cell goes through the DC-DC converter to increase the voltage level, and then the DC is converted into alternating current with the voltage amplitude, frequency and phase of the power grid through the DC-AC inverter, so as to realize the integration into the power grid or power supply to the AC load. The structure of photovoltaic power generation system is shown in Figure 1.

According to the grid-connected operation mode, the photovoltaic power generation system can be divided into three forms: countercurrent grid-connected, no countercurrent grid-connected and switching grid-connected. Grid-connected photovoltaic power generation system is directly connected to the power grid, does not require energy storage batteries, saves the floor area, greatly reduces the configuration cost, and the load power deficit is supplemented by the power grid. Therefore, grid-connected photovoltaic power generation system is the main development direction of solar power generation, and it is also a potential new energy power generation method at this stage.

3. The influence of grid-connected photovoltaic power generation on power quality of the grid

Photovoltaic power generation as a new energy generation, lighting, temperature and other external conditions of randomness, volatility, intermittent changes are the main factors of photovoltaic power generation impact on the grid. Wang Jinjing 13-77+443+09-92, among which DC-AC inverter is one of the main devices of grid-connected photovoltaic power generation system, the quality of photovoltaic inverter determines whether the power quality of photovoltaic power generation can meet the grid-connected requirements to a certain extent. When photovoltaic power generation is connected to the grid, problems such as harmonics, voltage fluctuations and flickers, DC injection, and island effect will occur, which will reduce the power quality of the grid and cause adverse effects on the grid. In serious cases, it will disrupt the safe and stable operation of the power supply system and photovoltaic power generation equipment itself.

3.1 DC Injection problem

Another key problem to be solved in grid-connected photovoltaic power generation system is DC injection. Dc injection affects the power quality of the grid, and also brings adverse effects to other equipment in the grid. IEEEStd929-2000 and IEEEStd547-2000 clearly stipulate that the DC current component injected into the grid by the grid-connected power generation device cannot exceed 0.5% of the rated current of the device. The main causes of DC injection are as follows: 1) the dispersion of power electronic devices and the inconsistency and asymmetry of drive circuits; 2) Zero drift and nonlinearity of measuring devices in high-power controller; 3) Asymmetry of line impedance of each switching device, influence of parasitic parameters and parasitic electromagnetic fields, etc.

At present, the main methods to suppress DC injection include: 1) detection compensation method; 2) Optimize and design the grid-connected structure of the inverter; 3) Capacitor straight separation; 4) Virtual capacitance method; 5) Device isolation transformer.

3.4 Impact of island effect

The islanding effect refers to the phenomenon that the grid power supply is interrupted due to human factors or natural factors, but each grid-connected photovoltaic power generation system fails to detect the power grid blackout state in time, so that the photovoltaic power generation system and the connected load still operate independently. With the continuous expansion of the access penetration rate of grid-connected photovoltaic power generation, the probability of island effect is gradually increasing.

The formation of island effect has adverse effects on the power quality of the entire distribution network, mainly including:

1) When the distribution network is switched to the island mode, only relying on the photovoltaic power generation system to supply electricity, if the power supply system capacity is too small or no energy storage device is installed, it is possible to cause voltage instability and flicker problems in the user load.