Nov 10, 2020
The working principle and basic common sense of photovoltaic off-grid inverter
The small off-grid photovoltaic power generation system is composed of photovoltaic arrays, battery packs, photovoltaic controllers, photovoltaic inverters, and AC and DC loads. During the day, the photovoltaic array converts solar energy into electrical energy, and under the control of the photovoltaic controller, it provides electrical energy for the AC and DC loads and charges the battery; at night, the electricity stored in the battery is supplied by the photovoltaic controller to the AC and DC loads.
Among them, the photovoltaic inverter converts the direct current from the photovoltaic array or the battery into alternating current, which is the core equipment of the entire photovoltaic power generation system. The inverter structure is composed of input circuit, inverter circuit, output filter circuit, control circuit, protection circuit and auxiliary power supply.
The input circuit shapes and filters the DC power from the photovoltaic array or battery. The inverters of small off-grid photovoltaic systems often use single-phase inverters. The inverter circuit converts DC power into single-phase AC power. If open loop control is used, the output There is no need to feed back to the control circuit, and if closed-loop control is used, the output will also be fed back to the control circuit. The function of the control circuit is to generate and adjust a series of control pulses as required to control the on and off of the inverter switch, so as to cooperate with the inverter main circuit to complete the inverter function. The function of the auxiliary power supply is to provide DC working voltage for the control circuit. The protection circuit mainly realizes overvoltage protection, undervoltage protection, overload protection, short circuit protection, etc.
Taking into account the energy consumption of the transformer in the inverter circuit using the transformer, the topology adopts a transformerless inverter circuit. After the DC power from the battery or photovoltaic array is boosted by the DC, it is filtered by the inverter and converted into a load. Directly used alternating current.
Single-phase inverters generally have three structural forms: push-pull, half-bridge, and full-bridge. The switch tube in the push-pull circuit needs to withstand twice the DC voltage. It is relatively difficult to select the switch tube, and the use of the transformer The phase rate is low, which reduces the efficiency of the entire inverter system to a certain extent; if the half-bridge circuit wants to get the same power as the push-pull and full-bridge, the switch tube needs to withstand twice the current. The full-bridge inverter circuit overcomes the shortcomings of the above two structures. The steady-state operating voltage of the switch tube is equal to the DC input voltage. Under the same power, the current of the switch tube is half smaller than that of the half-bridge type, so the full bridge is used here. The structure of the inverter. The main inverter circuit consists of two parts, the DC-DC boost part and the single-phase full-bridge inverter part.