Three Common Misconceptions About Grid-tied Inverters

As the "brain" of photovoltaic (PV) systems, solar inverters play a crucial role in the operation and output of the entire system. When technical issues arise, such as unexpected standby mode, shutdowns, alarms, faults, underperformance, or data monitoring interruptions, maintenance personnel typically start by examining the inverter to identify causes and solutions. Despite the rapid development of distributed PV systems globally, several common misconceptions about inverters persist.

Today, we'll address these misconceptions.

Do inverters produce voltage?

The parameter "AC output voltage" is commonly found in inverter specifications and is a key characteristic defining an inverter's performance. While it might seem to refer to the voltage output from the inverter's AC side, this is a misunderstanding.

An inverter doesn't produce voltage independently; rather, it synchronises with the grid voltage. It's a current-source device that must connect to the grid to safely transmit the generated electricity. During operation, it continuously monitors the grid's voltage (V) and frequency (F). The inverter's ability to export power depends on its synchronisation with these grid parameters.

For example, to deliver 10kW at 400V (line-to-line), the required output current per phase would be approximately 14.5A (10000W ÷ (400V × √3)). If the grid voltage increases to 430V, the output current would decrease to about 13.4A to maintain the same power output.

Two important points:

1)    Grid voltage fluctuates continuously.

2)    The inverter must operate within a specified voltage range. If the grid voltage deviates from this range, the inverter must detect this and shut down to protect equipment and personnel safety.

The term "AC output voltage" persists due to industry convention and to maintain consistency with output current terminology, despite not being entirely accurate.

Do Inverters Always Have Anti-Islanding Protection?

Yes, anti-islanding protection is a fundamental feature of grid-tied inverters. This safety mechanism prevents the inverter from circulating electricity within the system, which could pose serious safety risks to utility workers and equipment.

When the grid power fails, the inverter must quickly detect this condition and cease power export. This is achieved through various detection methods, both passive and active. Regulations typically require inverters to disconnect from the grid within 2 seconds of detecting an islanding condition.

Does Higher DC String Voltage Always Mean More Power Generation?

Not necessarily. Inverters have an optimal operating voltage range, often referred to as the Maximum Power Point Tracking (MPPT) range. The inverter operates most efficiently when the DC input voltage is within this range, typically closer to the lower end of the range.

If the string voltage is too low, the inverter may struggle to reach its rated AC output voltage, reducing efficiency. Conversely, if the string voltage is too high, it may exceed the inverter's maximum input voltage rating, potentially causing damage or triggering protective shutdowns.

For example, an inverter rated for 1000V DC maximum input is estimated to have an MPPT range of 550-850V. The optimal operating point could be around 650-750V, depending on the specific model.

When designing PV systems, it's crucial to consider both Standard Test Conditions (STC) and real-world conditions. Temperature changes affect panel voltage output, so designers must account for both cold-weather and  hot-weather conditions.

For current mainstream modules:

1)    182mm cell modules: typically 13-17 modules per string

2)    210mm cell modules: typically 16-22 modules per string

These recommendations should be adjusted based on specific module and inverter parameters. Always consult manufacturer guidelines and use string sizing tools for accurate calculations.

Conclusion

Understanding these concepts is fundamental for anyone working with PV systems. Correct knowledge aids in system design, component selection, grid integration, and efficient troubleshooting, ultimately leading to better-performing and more reliable solar installations.