In some areas without electricity, installing photovoltaic off grid energy storage systems is more economical and environmentally friendly than using oil powered power generation. Compared to grid connected systems, off grid systems are more complex and need to consider the user's load, electricity consumption, and local weather conditions. Especially, the load situation is diverse, including inductive loads like water pumps and resistive loads like electric furnaces, which can be single-phase or three-phase. For photovoltaic off grid systems larger than 10kW, single machine or multi machine parallel connection can be used, but each has its own advantages and disadvantages.

This article mainly introduces the design method of a medium to high-power photovoltaic off grid system built using multiple off grid inverters.

The integrated SPF5000TL HVM model with Guriwat off grid control inverter supports up to 6 parallel units and can build photovoltaic off grid systems up to 30kW. It can form both a single-phase system of 30kW and a three-phase system of 30kW. Considering that the three-phase load may not be balanced, there are multiple configuration methods available when six 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 electricity consumption of a user.

This system is relatively special, with two types of loads: single-phase load and three-phase load, and the three-phase is unbalanced. We will first select and design inverters based on the distribution of loads. The total load power of the system is 24kW. The user stated that not all loads will operate simultaneously, and the maximum power is around 20kW. Therefore, we will use six 5kW single-phase off grid inverters, with three for phase A totaling 15kW, two for phase B totaling 10kW, and one for phase C totaling 5kW, forming a 30kW three-phase unbalanced off grid system. The single-phase inverter output has two lines: phase line and zero line. The zero lines of 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 lines of 1 inverter are connected to phase C.

Multiple inverters are connected in parallel, and each machine needs to be connected to a communication line. The three machines in phase A are connected together, while the two machines in phase B are connected together. After connecting the lines, connect the battery, close the output circuit breaker, and set the phase of the inverter on the panel. SPF5000 enters the 23rd setting item. Three machines in phase A are set to 3P1, two machines in phase B are set to 3P2, and one machine in phase C is set to 3P3. Once the setting is completed, it can be run.

After selecting the inverter, we will calculate the usage of the components. The system requires an average of 80 kWh of electricity per day, and the local peak sunshine hour 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, which is about 0.7. This calculation of 80/(3.5 * 0.7) requires about 32kW of photovoltaic modules. The design uses 120 280W modules, 20 for each inverter, with a power of 5.6kW. The modules are connected to the inverter in 10 series and 2 parallel, with a total system power of 33.6kW.

Let's calculate the battery capacity again. After understanding, most of the time the user uses electricity during the day, which is about 50 degrees Celsius, and when there is no sun at night, which is about 30 degrees Celsius. The customer requests that the battery be equipped as little as possible to meet the basic requirements. Therefore, the design uses 16 12V250AH lead carbon batteries with a total capacity of 48 degrees Celsius, a discharge depth of 0.7, and a usable power of 33.6 degrees Celsius, which basically meets the customer's needs. The battery adopts a 4-series and 4-parallel method, The batteries of all 6 inverters are shared.

Compared to a medium power off grid inverter with a complete machine of 30kW, using multiple small power single-phase parallel inverters is a more complex wiring and debugging method, but it is cheaper and has high flexibility. Even if one or two inverters are damaged, the system can still continue to operate, which is a good choice.