The key technologies in the photovoltaic power generation system mainly include the maximum power point tracking control technology, the detection and prevention of the islanding effect, and the low voltage ride through function.
1. Maximum power point tracking control technology
Because the volt-ampere characteristics of solar cells have strong nonlinearity, their output voltage and current are closely related to factors such as light, temperature, and load in the working environment. But in a specific working environment, the battery has a unique maximum power output point (Maximum Power Point, MPP). Therefore, in order to maximize the output power of the solar cell, the operating point of the solar cell should be adjusted in real time, so that it always works near the maximum power point, so as to maximize the conversion of light energy into electrical energy, namely Maximum Power Point Tracking (MPPT) technology. At present, the commonly used MPPT methods include constant voltage method, conductance increment method, disturbance observation method and so on. These methods have their own advantages and disadvantages, and the specific application can be selected according to the different requirements of the photovoltaic grid-connected inverter.
2. Detection and prevention of islanding effect
Islanding effect means that when the power grid is powered off due to electrical failure, natural factors or power outage maintenance, the grid connected inverter does not detect the power outage and continues to supply power to the power grid, so that the photovoltaic grid connected inverter and load form a power supply “island” beyond the control of the power company. The islanding effect can cause serious harm to maintenance personnel, electrical equipment and even the power grid. Therefore, considering factors such as power safety and power quality, the islanding effect is not allowed. Once the islanding effect occurs, the grid-connected system must quickly and accurately disconnect the grid-connected inverter from the grid.
The islanding effect detection method can generally be divided into passive and active. Passive detection methods mainly use grid monitoring status such as voltage, frequency, phase, etc. as the basis for judging whether the grid is faulty. However, when the output power of the photovoltaic power generation system is balanced with the local load power, the passive detection method will lose the islanding effect detection capability. The active islanding detection method refers to controlling the inverter so that there is a certain disturbance in its output voltage, frequency or phase. When the grid is working normally, these disturbances cannot be detected due to the balance of the grid. Once the grid fails, the disturbance of the inverter output will quickly accumulate and exceed the allowable range, thereby triggering the islanding effect detection circuit. This method has high detection accuracy and small non-detection area, but the control is more complicated, and the power quality output by the inverter is reduced. The anti-islanding protection of photovoltaic power plants should have both active and passive types, and at least one active and passive anti-islanding protection should be provided. The current anti-islanding strategy of grid-connected inverters generally uses a combination of passive detection schemes and an active detection scheme.
3. Low voltage ride through function
The early new energy system was designed to protect the power station itself. In the event of a grounding or phase-to-phase short-circuit fault, the relay protection adopts a full off-grid removal mode, which greatly reduces the stability of the power system. When the proportion of new energy is relatively large, it will cause the consequences of power system oscillations and even grid disassembly. Therefore, all countries in the world have stipulated the terms of low voltage ride through in the grid connection technical conditions of large new energy power stations. The so-called low voltage ride-through refers to the instantaneous grounding short-circuit or inter-phase short-circuit, because the distance between the short-circuit point and the grid connection point is different, the phase voltage of a certain phase grid connection point will decrease to a certain threshold (generally equal to or lower than 20%). At this time, large-scale wind or photovoltaic power plants cannot be disconnected or disconnected from the grid, and need to be charged and provide reactive current to the system; at the same time, the voltage, frequency, and phase of the power system are automatically tracked; and no harmful inrush current is generated during automatic reclosing, so as to quickly connect to the grid to restore power.
Figure 1 shows the low voltage ride through characteristic curve of a photovoltaic grid-connected inverter. In Figure 1, UL0 is the lowest voltage limit for normal operation, generally 0.9 times the rated voltage. UL1 is the lower limit of voltage to withstand, T1 is the time required to maintain the grid connection when the voltage drops to UL1, and T2 is the time for the photovoltaic power station to recover to UL0 after the voltage drops. During this time period, the photovoltaic power station should ensure continuous operation without disconnecting from the grid. The value determination of UL1, T1, T2 needs to consider the actual conditions such as protection and reclosing time. It is recommended that UL1 be set to 0.2 times the rated voltage, T1 is 1s, and T2 is 3s.
The key technologies in the photovoltaic power generation system mainly include the maximum power point tracking control technology, the detection and prevention of the islanding effect, and the low voltage ride through function.
1. Maximum power point tracking control technology
Because the volt-ampere characteristics of solar cells have strong nonlinearity, their output voltage and current are closely related to factors such as light, temperature, and load in the working environment. But in a specific working environment, the battery has a unique maximum power output point (Maximum Power Point, MPP). Therefore, in order to maximize the output power of the solar cell, the operating point of the solar cell should be adjusted in real time, so that it always works near the maximum power point, so as to maximize the conversion of light energy into electrical energy, namely Maximum Power Point Tracking (MPPT) technology. At present, the commonly used MPPT methods include constant voltage method, conductance increment method, disturbance observation method and so on. These methods have their own advantages and disadvantages, and the specific application can be selected according to the different requirements of the photovoltaic grid-connected inverter.
2. Detection and prevention of islanding effect
Islanding effect means that when the power grid is powered off due to electrical failure, natural factors or power outage maintenance, the grid connected inverter does not detect the power outage and continues to supply power to the power grid, so that the photovoltaic grid connected inverter and load form a power supply “island” beyond the control of the power company. The islanding effect can cause serious harm to maintenance personnel, electrical equipment and even the power grid. Therefore, considering factors such as power safety and power quality, the islanding effect is not allowed. Once the islanding effect occurs, the grid-connected system must quickly and accurately disconnect the grid-connected inverter from the grid.
The islanding effect detection method can generally be divided into passive and active. Passive detection methods mainly use grid monitoring status such as voltage, frequency, phase, etc. as the basis for judging whether the grid is faulty. However, when the output power of the photovoltaic power generation system is balanced with the local load power, the passive detection method will lose the islanding effect detection capability. The active islanding detection method refers to controlling the inverter so that there is a certain disturbance in its output voltage, frequency or phase. When the grid is working normally, these disturbances cannot be detected due to the balance of the grid. Once the grid fails, the disturbance of the inverter output will quickly accumulate and exceed the allowable range, thereby triggering the islanding effect detection circuit. This method has high detection accuracy and small non-detection area, but the control is more complicated, and the power quality output by the inverter is reduced. The anti-islanding protection of photovoltaic power plants should have both active and passive types, and at least one active and passive anti-islanding protection should be provided. The current anti-islanding strategy of grid-connected inverters generally uses a combination of passive detection schemes and an active detection scheme.
3. Low voltage ride through function
The early new energy system was designed to protect the power station itself. In the event of a grounding or phase-to-phase short-circuit fault, the relay protection adopts a full off-grid removal mode, which greatly reduces the stability of the power system. When the proportion of new energy is relatively large, it will cause the consequences of power system oscillations and even grid disassembly. Therefore, all countries in the world have stipulated the terms of low voltage ride through in the grid connection technical conditions of large new energy power stations. The so-called low voltage ride-through refers to the instantaneous grounding short-circuit or inter-phase short-circuit, because the distance between the short-circuit point and the grid connection point is different, the phase voltage of a certain phase grid connection point will decrease to a certain threshold (generally equal to or lower than 20%). At this time, large-scale wind or photovoltaic power plants cannot be disconnected or disconnected from the grid, and need to be charged and provide reactive current to the system; at the same time, the voltage, frequency, and phase of the power system are automatically tracked; and no harmful inrush current is generated during automatic reclosing, so as to quickly connect to the grid to restore power.
Figure 1 shows the low voltage ride through characteristic curve of a photovoltaic grid-connected inverter. In Figure 1, UL0 is the lowest voltage limit for normal operation, generally 0.9 times the rated voltage. UL1 is the lower limit of voltage to withstand, T1 is the time required to maintain the grid connection when the voltage drops to UL1, and T2 is the time for the photovoltaic power station to recover to UL0 after the voltage drops. During this time period, the photovoltaic power station should ensure continuous operation without disconnecting from the grid. The value determination of UL1, T1, T2 needs to consider the actual conditions such as protection and reclosing time. It is recommended that UL1 be set to 0.2 times the rated voltage, T1 is 1s, and T2 is 3s.
