Technical indicators for selection of off-grid photovoltaic inverters

The off-grid photovoltaic inverter is the main component and important part of the off-grid photovoltaic power generation system. In order to ensure the long-term normal operation of the solar photovoltaic system, in addition to determining the technical indicators of the photovoltaic power generation system and referring to the manufacturer’s product manual data, the selection of the off-grid inverter should also focus on the following technical indicators.

1. Rated output power

The rated output power represents the ability of the inverter to supply power to the load. Inverters with higher rated output power can carry more electrical loads. When selecting an inverter, the first consideration should be to have enough rated output power to meet the electrical power requirements of the equipment under the maximum load, as well as system expansion and some temporary load access. When the electrical equipment is dominated by pure resistive loads or the power factor is greater than 0.9, an inverter with a rated output power that is 10% to 15% larger than the total power of the electrical equipment is generally selected. At the same time, the inverter should also have the ability to resist capacitive and inductive load shocks. For general inductive loads, such as motors, refrigerators, air conditioners, washing machines, water pumps, etc., their instantaneous power may be 5-6 times their rated power when starting. At this time, the inverter will bear a large instantaneous surge current. For such systems, the rated output power of the inverter should have sufficient margin to ensure that the load can be started reliably.

2. Adjustment performance of output voltage

The regulation performance of the output voltage represents the voltage regulation capability of the inverter output. Generally, inverters give the percentage of fluctuation deviation of the output voltage of the inverter when the DC input voltage varies within the allowable range, which is usually called the voltage regulation rate. A high-performance inverter should also give the deviation percentage of the output voltage of the inverter when the load changes from 0 to 100%, which is usually called the load regulation rate. The voltage regulation rate of the inverter with excellent performance should be less than or equal to ±3%, and the load adjustment rate should be less than or equal to ±6%.

3. Machine efficiency

The overall efficiency indicates the size of the power loss of the inverter itself. Inverters with larger capacity also give efficiency values under full load and low load operation. Generally, the efficiency of inverters below the kilowatt level should be 85%~95%, the efficiency of the 10KW level should be 95%~97%, and the efficiency of higher power must be 98%~99%. The efficiency of the inverter has an important impact on improving the effective power generation of the photovoltaic power generation system and reducing the cost of power generation. Therefore, when choosing an inverter, try to choose a product with higher overall efficiency.

4. Startup performance

The inverter should be guaranteed to start reliably under rated load. High-performance inverters can be started at full load multiple times in a row without damaging power switching devices and other circuits. Small inverters sometimes use soft-start or current-limiting start-up measures or circuits for their own safety.

The above items are the main basis for the design and purchase of off-grid inverters, and are also important indicators for evaluating the technical performance of off-grid inverters.

The off-grid system generally selects the DC input voltage of the inverter according to the DC voltage determined by the design of the photovoltaic power generation system, determine the power and phase number of the inverter according to the type of load, and determine the power margin of the inverter according to the impact of the load. The continuous power of the inverter should be greater than the power of the load, and the starting power of the load should be less than the maximum impact power of the inverter. When selecting the model, a certain margin should be reserved for the future expansion of the photovoltaic power generation system, which can be determined by referring to the following formula:

Inverter power = resistive load power × (1.2~1.5) + inductive load power × (5~7)

In the off-grid photovoltaic power generation system, the selection of the system voltage should be determined according to the requirements of the load. The higher the load voltage requirements, the higher the system voltage should be. When there is no 12V, 24V DC load in the system, the system voltage is best to choose 48V, 96V, 144V, 192V, etc., so that the current in the DC circuit part of the system can be reduced. The higher the system voltage, the smaller the system current, so that the system and line losses can be reduced.

The performance of inverters used in photovoltaic power generation systems involves many aspects. The inverter needs to have high efficiency when converting photovoltaic power from DC to AC, and it needs to be able to accurately track the maximum power point of the photovoltaic power generation system in different environments and working conditions, and at the same time, it should be able to meet the requirements of the power grid in different regions during operation. All functions must guarantee stable operation for many years and require as little maintenance as possible. In many cases, inverters need to operate in extremely harsh environments, such as high temperature and sand and dust environments in desert areas, high humidity and salt spray environments at seaside, etc. Inverters are required to maximize energy output and minimize costs throughout the product life cycle for maximum economic return.