In recent years, the number and capacity of photovoltaic power plants have been increasing, which has gradually increased the threat to the safe and stable operation of the power grid system. When the photovoltaic power station is suddenly disconnected from the grid, the voltage drop of the grid will further deteriorate the operating state of the grid. The power shortage caused in this process may cause the adjacent power station to trip, resulting in a larger area of power outages, and the low voltage ride through capability. It is considered to be one of the biggest challenges in the design and manufacturing control technology of photovoltaic grid-connected equipment, which is directly related to the large-scale application of photovoltaic power generation. Therefore, in order to ensure the safe and stable operation of the power grid after the photovoltaic power station is connected, it is necessary to study the low-voltage ride-through technology.
Large and medium-sized photovoltaic power stations should have a certain low-voltage ride-through capability. For large and medium-sized photovoltaic power stations, they should have low-voltage ride-through capability when the power grid falls. For the diversity of power system faults, large and medium-sized photovoltaic power stations should meet the following requirements: when the voltage profile shown in Figure 1 is above, it must ensure uninterrupted grid-connected operation and provide a certain amount of active and reactive power to the grid. When the voltage drops below the contour line, the photovoltaic power station can be cut out from the grid. When the voltage at the grid connection point is below the voltage contour line in the figure, the photovoltaic power station is allowed to be cut out from the grid. When the voltage of the grid-connected point drops to 20%, it needs to ensure uninterrupted operation for 1s. That is, T1=1: when the voltage of the grid-connected point drops to 90% of the nominal voltage within 3s (T2=3), the power station guarantees that it can not Intermittent grid-connected operation.
The photovoltaic grid-connected inverter is the core equipment of the photovoltaic grid-connected power generation system and the grid interface. The main purpose of the inverter control is to input the direct current generated by the solar cell to the inverter in the form of maximum power tracking point (MPPT), and convert it into In order to combine the same-phase alternating current with the grid voltage and then merge it into the grid, the output characteristics of photovoltaic power generation will have a great impact on the stability of the distribution grid during the voltage drop of the grid, especially the photovoltaic power station connected to the end of the distribution network. When the low voltage of the distribution network is disconnected from the grid, the operating state of the power grid will be further deteriorated. The low voltage ride-through capability of the photovoltaic power station directly affects the grid connection ability of the photovoltaic power station. If the photovoltaic inverter adopts an appropriate strategy during the low voltage period, it can be It emits reactive power to the system, and provides corresponding reactive power according to the magnitude of the voltage drop to support the operation of the grid for low voltage ride-through control of the photovoltaic power station.
Low voltage ride-through technology means that when the voltage of the grid-connected point of the photovoltaic array falls within a certain range, the photovoltaic grid-connected inverter can maintain uninterrupted grid-connected operation, and can even support the recovery of the grid by providing reactive power to the grid. When the power grid is stable, when the grid voltage drops, the stator flux linkage cannot follow the sudden change of the stator terminal voltage, and the rotor continues to rotate will generate a large slip, which will cause the rotor winding to generate overcurrent and overvoltage, which directly affects the safe operation of the equipment. However, after the photovoltaic power station voltage reaches the open-circuit voltage, the output current of the inverter is basically zero, and the DC side voltage will not continue to increase. Therefore, the main factor restricting the low voltage ride-through capability of photovoltaic power plants is the AC current output by the photovoltaic inverter. It is necessary to keep the inverter from being disconnected from the grid, and the inverter cannot be damaged or tripped due to overcurrent.
When photovoltaic power generation occupies a large proportion in the grid, an effective control strategy should be adopted to support the recovery of the grid during the transition period of the grid. The amount of reactive power provided to the grid can be allocated according to the depth of the grid drop. The control strategy is shown in the figure 2 shown.
When using the traditional double closed-loop vector control strategy, the grid voltage drop will cause the impact of the grid-connected current, which may cause the hardware protection to trip, or even burn the power devices; As a result, the power fed into the grid oscillates, which affects the safe operation of the photovoltaic inverter and the grid. In order to protect the safety of the photovoltaic power station and maintain the stability of the power system, the grid-connected control strategy of the photovoltaic inverter during the low voltage ride-through period needs to be carried out. Research.
When the grid voltage is normal, the photovoltaic grid-connected inverter works in a unit power factor state of 0, and only transmits active power to the electricity, and the reactive power is 0, but when the voltage drops, the grid-connected point voltage passes through and normal. Voltage comparison, through the PI regulator, based on the grid-connected control strategy, the external distribution of power and reactive current commands are re-distributed through the grid voltage outer loop, and the power and reactive current commands are dynamically externalized, so as to achieve the distribution of the output power of the inverter phase grid. The deeper the drop, the more reactive power the inverter supplies to the grid. Through the accurate detection of the grid voltage, it is judged whether the voltage fluctuation is within the normal range. If it is not exceeded, the inverter works in the grid-connected state of unity power factor, otherwise it needs to provide reactive power to the grid.
In addition, the photovoltaic inverter adopts the SVPWM double hysteresis current control strategy, which can enable it to achieve low voltage ride-through with better current tracking performance when the grid is disturbed or the voltage of the fault grid connection point drops, and sends reactive power to the grid and supports the grid connection point voltage. , which is beneficial to system security and stability.