Leakage protection devices (also known as ground fault circuit interrupters) are commonly used in electrical circuits. However, when installed upstream of a frequency converter (variable frequency drive, VFD), we often receive feedback from users that the leakage protector frequently trips as soon as the VFD starts operating. Why does this happen?

Principle of Leakage Protection Devices

A leakage protection device operates on the principle that under normal conditions, the vector sum of currents in the live and neutral wires (zero-sequence current) should be zero. However, when a frequency converter (VFD) is used, the zero-sequence current is unlikely to remain zero. Why?

The output voltage of a VFD is a high-frequency PWM wave, not a standard sine wave. The long motor cables exhibit capacitive coupling with the ground. During VFD operation, this capacitance charges and discharges, allowing current to flow through the capacitor to the ground and then back to the VFD through the grounding wire on the input side, forming a current loop. If a leakage protection device is installed on the input side and the leakage current exceeds its threshold, the device will trip and cut off the power. Therefore, it is generally not recommended to use a standard leakage protection device with a VFD, as it may lead to frequent tripping.

What to Do When Leakage Protection is Mandatory?
If regulatory requirements or circuit constraints necessitate the use of a leakage protection device, here are two solutions:

1. Adjust the Sensitivity of the Leakage Protector:
   Most leakage protection devices feature an adjustable sensitivity setting. Increase the threshold slightly above the leakage current generated by the VFD. Gradually adjust the setting until the VFD operates normally without causing tripping.

2. Replace with a VFD-Specific Leakage Protector:
   Consider switching to a leakage protection device specifically designed for use with VFDs. These specialized devices are engineered to tolerate higher-frequency leakage currents without unnecessary tripping.

With over a decade of experience in VFD applications, the engineers at Nanjing Oulu Electric Co., Ltd. offer the above solutions to address leakage protection tripping issues. We hope these insights prove helpful to users! For further questions regarding VFDs, feel free to follow us, contact our experts, or leave a message for consultation.