Photovoltaic power generation, as a new and pollution-free power generation method today, has greatly alleviated the demand for traditional electric energy. However, for the grid-connected photovoltaic power generation system, due to its inherent randomness, volatility, and intermittent characteristics, and the grid-connected photovoltaic power generation system contains a large number of nonlinear power electronic components, compared with traditional power generation methods, photovoltaic power generation has The power quality of the grid has a great impact.
1. Basic principles of photovoltaic power generation
Photovoltaic power generation utilizes the photovoltaic effect existing on the surface of a semiconductor to emit direct current through light at both ends of the semiconductor material. When sunlight shines on the P-N node of the semiconductor, new electron-hole pairs are formed. After the photons excite the electrons from the covalent bond, the electrons flow to the N region and the holes flow to the P region, thereby generating a potential difference across the semiconductor. Once the circuit at both ends of the PN junction is connected, a current will form, flowing from the P area to the N area through the external circuit, and output electric power to the load.
2. Structure and classification of grid-connected photovoltaic power generation
The grid-connected photovoltaic power generation system mainly consists of solar panels (components), high power tracking (MPPT) controller, and DC-AC inverter. Insulated gate bipolar transistors (IGBT) are used as the switching elements of the photovoltaic inverter. . The DC power output by the solar cell increases the voltage level through the DC-DC converter, and then converts the DC power into AC power with the same amplitude, frequency, and phase as the grid voltage through the DC-AC inverter, so as to be integrated into the grid or supplied to the AC. Load power supply, photovoltaic power generation system structure is shown in below picture.
According to the grid-connected operation mode, photovoltaic power generation systems can be divided into three forms: grid-connected with countercurrent, grid-connected without countercurrent, and switching grid-connected. The grid-connected photovoltaic power generation system is directly connected to the grid and does not require energy storage batteries, which saves floor space and greatly reduces configuration costs. The load power deficit is supplemented by the grid. Therefore, grid-connected photovoltaic power generation systems are the main development direction of solar power generation and are also the most promising new energy power generation methods at this stage.
3. The impact of grid-connected photovoltaic power generation on grid power quality
Photovoltaic power generation is a type of emerging energy power generation. Random, volatile, and intermittent changes in external conditions such as light and temperature are the main factors that affect the power grid. Among them, the DC-AC inverter is one of the main components of the grid-connected photovoltaic power generation system. The quality of the photovoltaic inverter determines to a certain extent whether the power quality of the photovoltaic power generation can meet the grid connection requirements. When photovoltaic power generation is connected to the grid, problems such as harmonics, voltage fluctuations and flickers, DC injection, and islanding effects will occur, which will degrade the power quality of the power grid and adversely affect the power grid. In severe cases, it will disrupt the power supply system and photovoltaic power generation equipment. own safe and stable operation.
3.1 Harmonic influence
Photovoltaic power generation converts solar energy into direct current through photovoltaic modules, and then converts the direct current into alternating current through a grid-connected inverter to achieve grid connection. In photovoltaic power generation systems, inverters are the main equipment that generates harmonics. The extensive application of power electronic components inside the grid-connected inverter has improved the informatization and intelligent processing of the system, but it has also added a large number of nonlinear loads, causing waveform distortion and bringing a large number of harmonics to the system. The delay in the switching speed of the inverter will also affect the output of the overall dynamic performance within the power grid system and produce a small range of harmonics. If the weather (irradiance, temperature) changes greatly, the fluctuation range of harmonics will also become larger. Although the output current harmonics of a single grid-connected inverter are small, when multiple grid-connected inverters are connected in parallel, the harmonics of the output current will be superimposed, causing the output current harmonics to exceed the standard. In addition, inverters connected in parallel are prone to produce parallel resonance, which in turn leads to coupled resonance phenomena, causing the expansion of specific grid-connected harmonic currents and causing the problem of excessive grid-connected current harmonic content.
In view of the power quality problems after photovoltaic access, effective methods to suppress harmonics are proposed:
1) Starting from the source of harmonic generation, transforming the harmonic source to reduce harmonic injection.
2) Install active or passive filters to absorb harmonic currents of certain specific orders.
3) Install additional harmonic compensation devices.
3.2 Voltage fluctuation and flicker
In traditional distribution networks, changes in active power and reactive power over time will cause system voltage fluctuations. For photovoltaic power generation, changes in the active power of the photovoltaic power generation system are the main factors causing voltage fluctuations and flickers at the access point. The high power point of the photovoltaic panel, the core component of the photovoltaic power generation system, is closely related to factors such as irradiation intensity, weather, season, temperature, etc. Random changes in these natural factors cause large changes in output power, causing the load power to change frequently within a certain range. This causes voltage fluctuations and flickers at the load terminals of grid-connected users.
The current solutions to photovoltaic voltage fluctuation and flicker problems mainly include:
1) Optimizing the photovoltaic grid-connected inverter control strategy to improve voltage stability;
2) Increasing the bus short-circuit capacity of the substation;
3) When the capacity of the photovoltaic power station is determined under certain circumstances, its power factor is improved to increase the total amount of active power, thereby reducing the change in reactive power and meeting the limit requirements for voltage fluctuations.
3.3 DC injection problem
Another key issue that needs to be solved urgently in grid-connected photovoltaic power generation systems: DC injection. DC injection affects the power quality of the power grid and also brings adverse effects to other equipment in the power grid. IEEEStd929-2000 and IEEEStd547-2000 clearly stipulate that the DC current component injected into the grid by a grid-connected power generation device cannot exceed 0.5% of the rated current of the device. The main reasons for DC injection are:
1) The dispersion of power electronic devices and the inconsistency and asymmetry of the drive circuit;
2) The zero-point drift and nonlinearity of the internal measurement devices of the high-power controller;
3) The inconsistency of the line impedance of each switching device Symmetry, the influence of parasitic parameters and parasitic electromagnetic fields, etc.
At present, the main methods to suppress DC injection include:
1) detection and compensation method;
2) optimized design of inverter grid-connected structure;
3) capacitor DC isolation;
4) virtual capacitor method;
5) installation of isolation transformer, etc.
3.4 Impact of island effect
The islanding effect refers to the phenomenon that the power supply of the grid is interrupted due to human factors or natural factors, but each grid-connected photovoltaic power generation system fails to detect the power outage status of the grid in time, so the photovoltaic power generation system and the load connected to it still operate independently. As the penetration rate of grid-connected photovoltaic power generation continues to expand, the probability of islanding effect also gradually increases. The formation of the islanding effect has adverse effects on the power quality of the entire distribution network, mainly including:
1) Where the islanding effect occurs, the voltage and frequency fluctuate greatly, which reduces the power quality, and the voltage and frequency in the island are not affected by the power grid. Control may cause damage to system electrical equipment and reclosing failure, and may also cause personal safety hazards to power grid maintenance personnel.
2) During the power supply restoration process, surge current will be generated due to asynchronous voltage phases, which may cause the power grid waveform to drop instantaneously.
3) After the islanding effect occurs in the photovoltaic power generation system, if the original power supply mode is a single-phase power supply mode, it may cause a three-phase load asymmetry problem in the distribution network, thereby reducing the overall quality of power consumption for other users.
4) When the distribution network is switched to the island mode and relies solely on the photovoltaic power generation system to supply electric energy, if the capacity of the power supply system is too small or no energy storage device is installed, it may cause voltage instability and flicker problems in user loads.
For the impact of the island effect, there are mainly the following solutions:
1) Optimize the islanding detection method of grid-connected photovoltaic power generation systems, analyze the impact of photovoltaic power generation on the magnitude, direction and distribution of fault current in the distribution network, and improve the load shedding speed and islanding selection technology under fault conditions.
2) Improve the reliability of islanding detection technology, configure fast and effective anti-islanding protection functions, accurately determine the islanding status under abnormal circumstances and quickly and effectively interrupt the grid connection.
