Typical design of photovoltaic off-grid inverter parallel operation

In some areas without electricity, installing a photovoltaic off-grid energy storage system is more economical and environmentally friendly than using oil generators to generate electricity. Compared with the grid-connected system, the off-grid system is more complicated and needs to consider the user's load, power consumption, and local weather conditions. In particular, the load conditions are diverse, including inductive loads such as water pumps and resistive loads such as electric furnaces, and both single-phase and three-phase. For photovoltaic off-grid systems greater than 10kW, a single machine or multiple machines in parallel can be used, but each has its own advantages and disadvantages.

This article mainly introduces the design method of medium and large power photovoltaic off-grid systems built with multiple off-grid inverters.

Growatt's off-grid control inverter SPF5000TL HVM model supports up to 6 units in parallel, and can build a 30kW off-grid photovoltaic system. It can form a 30kW single-phase system or a 30kW three-phase system. Considering that the three-phase load is not necessarily balanced, there are many configuration methods when 6 inverters form a three-phase system, such as 222, 321, 411, etc., which can meet the needs of users in different scenarios. The following table shows the actual load and power consumption of a user.

This system is quite special, with single-phase load and three-phase load, and the three phases are unbalanced. According to the distribution of load, we first select and design the inverter. The total load power of the system is 24kW. Users said that not all loads will run at the same time, and the maximum power is about 20kW. Therefore, the design uses 6 5kW single-phase off-grid inverters, 3 for phase A with a total of 15kW, 2 for phase B with a total of 10kW, and 1 for phase C with a total of 5kW, forming a 30kW three-phase unbalanced off-grid system. The output of the single-phase inverter has two wires: the phase line and the neutral line. The neutral lines of the 6 inverters are all connected together, the phase lines of 3 inverters are connected to phase A, the phase lines of 2 inverters are connected to phase B, and the phase line of 1 inverter is connected to phase C.

When multiple inverters are connected in parallel, each inverter needs to be connected to a communication line. The three inverters in phase A are connected together with the current lines, and the two inverters in phase B are connected together with the current lines. After the lines are connected, connect the battery, turn off the output circuit breaker, set the inverter phase on the panel, SPF5000 enters the 23rd setting, the three inverters in phase A are set to 3P1, the two inverters in phase B are set to 3P2, and the one inverter in phase C is set to 3P3. After the setting is completed, it can be operated.

After selecting the inverter, we calculate the component usage. The system requires an average of 80 kWh of electricity per day. The local peak sunshine hours data is an average of 3.5 hours per day. The efficiency of the off-grid system is lower than that of the grid-connected system, about 0.7. So 80/(3.5*0.7) is calculated, and about 32kW of photovoltaic components are needed. The design uses 120 280W components, 20 for each inverter, with a power of 5.6kW. The components are connected to the inverter in a 10-series and 2-parallel manner, and the total system power is 33.6kW.

Next, we calculate the battery capacity. We learned that users consume electricity during the day most of the time, about 50 degrees, and about 30 degrees at night when there is no sun. The customer requires as few batteries as possible to meet basic requirements. Therefore, the design uses 16 12V250AH lead-carbon batteries with a total capacity of 48 degrees, a discharge depth of 0.7, and an available power of 33.6 degrees, which basically meets customer needs. The batteries are in 4 series and 4 parallel mode, and the batteries of all 6 inverters are shared.

Compared with a 30kW medium-power off-grid inverter, the method of connecting multiple small-power single-phase inverters in parallel is more complicated in wiring and debugging, but it is cheaper and more flexible. Even if one or two inverters are damaged, the system can continue to operate, which is a good choice.