Disravel what is a string inverter and its development

Reducing costs and improving quality and efficiency are the development trends of the modern photovoltaic industry, which puts forward higher requirements on the performance of photovoltaic products, including photovoltaic inverters.

Although string inverter accounts for a small proportion of the initial investment, their role as a bridge connecting the grid and photovoltaic power generation units cannot be underestimated.

The inverter is the core equipment of the photovoltaic power station, and through the power electronic inverter technology, the direct current generated by the photovoltaic module is converted into alternating current that can be integrated into the grid. In the field of PV inverter, this article mainly describes the mainstream technology of string inverter.

1. What is a string inverter

The main function of the photovoltaic grid-connected inverter is to convert the direct current generated by the photovoltaic module into alternating current that meets the requirements of the power grid, which is the core device in the photovoltaic system.

String inverter can track one or more strings of PV modules individually at maximum power points, and their power ranges from 1.5kW to 500kW. 

String photovoltaic inverter is the direct current generated by photovoltaic module directly converted into alternating current after boosting, grid connected, so the power of the inverter is relatively small, photovoltaic power plants generally use string photovoltaic inverters below 50kW.

The string inverter system is a series circuit, and the photovoltaic modules in the system are arranged in series, and the string inverter is connected in series with each "string". When the system is running, the cumulative voltage of the whole string can generally reach 600V~1000V.

2. Advantages of string inverter

Compact string inverter is most favored by small and medium-sized photovoltaic power plants. It has a wide range of application scenarios and plays a key role in industrial and commercial distributed rooftops and household photovoltaic systems.

The reason why string inverter can occupy a place in the market is due to the following advantages:

• Easy to install, wall-mounted does not occupy space, especially suitable for small and medium-sized distributed pv system application scenarios.
• Flexible technology, up to 4 MPPT channels, flexible component access scheme, suitable for various component installation orientation.
• Wide maximum efficiency tracking range for longer power generation times.
• Easy to maintain, and equipment maintenance adopts the replacement of the whole machine, without the maintenance of professional technicians.

For the needs of customers in the industrial, commercial and residential industries, new energy solutions and integrated energy service providers can provide customers with four values: efficient power generation, safety and reliability, intelligent management and good economy.

● Efficient power generation

Adapt to various complex terrain and effectively increase power generation. The maximum inverter conversion efficiency is 99%. Ultra-wide MPPT operating voltage range.

● Safe and secure

High protection level, outdoor dustproof and waterproof design. Long life cycle, providing auxiliary power for tracking brackets. Arc detection design to ensure safe DC transmission.

● Intelligent management

Real-time curve detection, fault recording function, fault diagnosis with data support. Intelligent online upgrade, convenient system maintenance.

● Good economy

The inverter AC output direct transformer saves the cost of bus equipment and cables. The system efficiency is increased by 1%-3%, and the power generation efficiency is significantly increased. Accurately identify and locate abnormal components to avoid loss of power generation.

3. Disadvantages of string inverter

String inverter is based on a modular concept, with each PV string (1-5kw) passing through an inverter and several panels forming a string with a power of several hundred watts to thousands of watts.

The access low power single phase inverter has maximum power peak tracking at the DC end and is connected to the grid in parallel at the AC end, which has become the most popular inverter on the market now. So what are the disadvantages of string inverters?

When multiple string inverter is connected in parallel, the total harmonics will be superimposed, and it is difficult to suppress and easy to generate resonance. There are many electronic components, power devices and signal circuits on the same board, the design and manufacture are difficult, and the reliability is slightly worse.

Power devices have small clearances and are not suitable for high altitudes. Outdoor installation, wind and sun can easily lead to aging of the shell and heat sink. In large-scale ground power stations, the number of string inverters is large, the total failure rate will increase, and system monitoring is difficult.

4. The difference between photovoltaic centralized inverter and string inverter

● Centralized inverter

The centralized inverter is to convert the aggregated direct current into alternating current, therefore, the power of the inverter is relatively large, and the centralized inverter above 500KW is generally used in photovoltaic power plants.

Centralized inverters are generally used in large-scale power generation systems such as large plants, desert power stations, and ground power stations with uniform sunlight, and the total power of the system is large, generally megawatt-level anomalies.

● Key advantages

• Easy maintenance and management.
• The inverter has high integration and high power density.
• The inverter has complete various protection functions, and the safety of the power station is high.
• Active power factor adjustment function, high and low voltage ride-through function.

● Main disadvantages

• The DC combiner box has a high failure rate, which affects the entire system.
• The MPPT voltage range of the centralized inverter is narrow, generally 450-820V, and the component configuration is not flexible.
• On rainy days, areas with a lot of fog have a short power generation time.
• The inverter itself consumes power and the ventilation and heat dissipation noise of the computer room is large, and the system maintenance is relatively complicated.

In the centralized grid-connected inverter system, the PV module array reaches the inverter through two confluences, and the inverter maximum power tracking function cannot monitor the operation of each module.

Therefore, it is impossible to make each component at the optimal working point, and when one component fails or is obscured by shadows, it will affect the power generation efficiency of the entire system.

There is no redundancy capability in the centralized grid-connected inverter system, and if there is a failure or shutdown, the entire system will stop generating electricity.

5. Future power trends of string inverter

PV grid connected inverters, the efficiency is getting higher and higher, and the price is getting lower and lower.

● Limitations of power devices

The power device is the core device of the string inverter, which is easy to fail under high current and voltage conditions when working. The increase in power devices increases the difficulty of management, so high-power inverters are generally designed with power modules that integrate multiple discrete devices.

● Limit on the number of strings

In terms of operational feasibility, one inverter can connect many components. However, in practical applications, the length of photovoltaic cable and the loss of DC cables should be considered. PV connector selection and crimping are important, the longer the operation time, the greater the internal resistance.

6. How to choose the right string inverter

The factors that determine the selection of string inverter include output voltage, load power factor, rated output current, rated output capacity, rated output efficiency, overload capacity and other technical parameters.

In addition, there are other requirements such as environmental conditions of use, electromagnetic interference and noise requirements, protection measures, safety performance requirements and so on. Specifically, you can refer to the following points:

● Rated output capacity

Indicates the ability of a string inverter to supply power to the load. It should first be considered to have sufficient rated capacity to meet the electrical power requirements of the equipment under the maximum load, as well as the expansion of the system and the access of some temporary loads.

● Output voltage regulation performance

Indicates the regulation capability of the inverter output voltage. Generally, the string inverter provides voltage regulation and load regulation.

• Voltage regulation: the deviation (%) of the output voltage of the inverter input DC voltage within the allowable fluctuation range should be ≤3%.
• Load regulation: The high-performance inverter should also give the deviation (%) of the output voltage of the inverter when the load changes from 0 to 100%, which should be ≤6%.

The off-grid photovoltaic power generation system uses the battery as the energy storage device, and the voltage change at the battery end can reach about 30% of the nominal voltage. Therefore, in order to ensure that the photovoltaic power generation system is supplied with a stable AC voltage, the string inverter must be required to have good voltage regulation performance.

● Overall machine efficiency

Indicates the magnitude of the power loss of the string inverter itself. Inverters with larger capacities should also provide efficiency values for full load and low load operations.

In the design of the special inverter for photovoltaic power generation system, special attention should be paid to reducing its own power loss and improving the efficiency of the whole machine.

This is because the actual efficiency of the 10kW class general purpose inverter is only 70% to 80%, and when used in the photovoltaic power generation system, it will bring 20% to 30% of the total power generation loss. Therefore, when the home energy storage system does not use electricity, the inverter should be turned off to reduce the loss when no electricity is used.

● Protection features

It should have overvoltage, overcurrent and short circuit automatic protection, over-temperature protection function, open circuit, phase loss protection and other functions.

● Startup performance

Should be maintained for reliable start-up at rated load. High-performance inverters can be started at full load many 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.

7. Conclusion

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