Earth fault protection is designed to detect a short circuit between a phase and earth, on the output side of the VSD, and immediately shuts down the converter. This protection is generally not intended for protection of human life from electric shock, as the trip points are set much higher than acceptable human safety limits. This feature is mainly for the protection of the AC converter itself.
Earth fault protection is usually implemented by means of a core balance current transformer. This is constructed from a toroidal magnetic core through which either the DC bus cables or the output motor phase cables are passed. A low current secondary winding is wound around the toroid and connected to the protection circuit. If the vector sum of all the currents passing through the core add up to zero, the flux in the core will be zero. A net zero flux is the normal operating situation.
If an earth fault occurs and there is a path for current to earth, the sum of the currents through the core balance transformer will no longer be zero and there will be a flux in the core as shown in Figure below.
This flux will result in a current being generated in the secondary protection winding, which is converted to a voltage via a burden resistor. A comparator circuit detects the fault and shuts down all the power device drives. Typically, the protection trip level is around 5 amp.
Care must be taken in establishing the set point for the earth fault trip circuit. In all
PWM VSDs, some leakage current will always take place to earth due to the high frequency components of the motor current waveform and the capacitance of the motor cables to earth. High leakage currents can sometimes cause some nuisance tripping of the earth fault protection.
Reference: Practical Variable Speed Drives and Power Electronics
Malcolm Barnes CPEng, BSc(ElecEng), MSEE, Automated Control Systems,