Generator content

In practical applications, nonlinear loads exist in both the client and the network. Half wave load and rectifier bridge circuit are common nonlinear load forms. Rectifier bridge circuit is the most common input circuit form in power electronic devices, and also the most common nonlinear load form at present. The biggest harm caused by nonlinear load is the distortion of the system voltage and current waveform, which leads to the degradation of the output waveform quality. Therefore, the current relevant standards in China clearly specify the technical indicators of the output waveform quality of grid-connected inverters. Harmonics are two key factors that affect the output waveform quality of grid-connected inverters. The output of PV grid-connected inverter has the problem of DC injection, that is, there is DC component in the grid-connected current. In recent years, with the wide application of uninsulated grid-connected inverters, the harm caused by this problem has become more and more serious, and has attracted wide attention.

In photovoltaic systems using uninsulated grid-connected inverters, DC components will be directly injected into the grid, causing serious harm to the grid, such as DC magnetic bias of transformers at all levels of substations. In grid-connected photovoltaic systems using grid-connected inverters, although the isolation transformer can inhibit the output DC component of the grid-connected inverter to a certain extent, the magnetic core saturation, output waveform distortion, increased loss, and shortened service life will also lead to a significant increase in cost, and the DC component will also damage the transformer itself. The research results of Spanish scholars on the output DC component of inverters show that there is still DC component in the grid-connected current of photovoltaic inverters. Therefore, the research on DC injection of PV grid-connected inverters has important practical significance.

2. High and low voltage crossing technology

In recent years, with the rapid development and wide application of photovoltaic grid-connected power generation, the proportion of photovoltaic power generation in the world's energy consumption and power supply is increasing. Once the grid fails, the security of the grid becomes increasingly high. The traditional high penetration rate (proportion in the grid) of photovoltaic power supply is disconnected, which further worsens the operation of the grid, causing serious accidents, and cannot meet the requirements of the normal operation of the power system. The State Grid Corporation of China has also issued the "Technical Regulations for Photovoltaic Power Station grid-connection", which clearly stipulates that when the power grid is abnormal, small photovoltaic power stations are used as the load and the power grid is cut off as soon as possible; For large and medium-sized photovoltaic power stations, it should be considered as a power supply, with a certain low-voltage overcurrent capacity, and provide reactive power support for the system to maintain grid stability. The research of grid-connected inverter overcurrent technology mainly focuses on the low-voltage overcurrent of wind power generation. To -D. When an accident or power system interruption causes the power supply and grid voltage to drop, the grid-connected power supply can ensure continuous operation within a certain voltage drop range and time interval (without connecting the grid). This paper mainly studies the low-voltage overcurrent problem of large-scale photovoltaic power plants. This is because when the grid voltage drops, the grid-connected inverter as the grid-connected power interface reduces the injected power of the grid due to its current capacity limitation. Therefore, the imbalance of input and output power will lead to DC side overvoltage. If the DC voltage remains stable, the output current of the grid-connected inverter is too large, endangering the safety of power electronic equipment.

In addition, according to the characteristics of the photovoltaic array, the output power of the photovoltaic array decreases with the increase of the output voltage until the open circuit voltage is reached and the output power is zero. Therefore, the current research on low-voltage overcurrent of photovoltaic grid-connected power supply is mainly to suppress the overcurrent of photovoltaic grid-connected inverters. The technology of high voltage switch corresponding to low voltage switch has not been paid enough attention. At present, the relevant regulations of our country are mainly off-grid operation. Since wind power is generally located at the end of the grid, the voltage fluctuation of the grid is mainly caused by the drop, and the intersection must be mainly low-voltage. Therefore, once the grid voltage suddenly rises, it is also necessary to study the high-voltage switch of the photovoltaic grid-connected power supply.

3. No interconnection line parallel technology

With the development of the world economy and the increasing number of power equipment, people have put forward higher and higher requirements for the power level and reliability of the power system. Due to the limitation of the power level of the inverter power supply, people have higher and higher requirements for the inverter power supply. The power supply mode of a single inverter can not meet the requirements of high power and ultra-high power. Therefore, the focus is on increasing the power level of the inverter by running multiple inverter modules in parallel. Parallel inverter technology is not only an important means for power system to develop to high power. At the same time, it is also a key technology for the development from centralized to distributed. It appeared in the early 21st century and has been rapidly developed and applied in the following years. The United States, Germany, Japan and other developed countries have conducted in-depth research on this.