Among some glaring issues in Solar Plant Operation and Maintenance, Ground fault a is very common phenomenon, specifically arising during monsoon and wet conditions. Occurrence of ground faults affect the generation as well as lead to safety issues.
In accordance with the IEC60364, earthing systems are configured as TT, TN or IT networks. The DC field is susceptible to fire and the solar inverters are earthed in IT type configuration. In IT network configuration the inverter DC side has resistance grounding and the connected components of inverter have direct earthing. A special Ground Fault Monitoring Device(GFMD) is installed to protect the system from high leakage current in case of ground faults.
For DC side ground fault monitoring, the negative or positive pole of DC input of the inverter is grounded with high resistance. The entire connection is called ground fault monitoring device(GFMD). For DC ground fault, the leakage current flows through the GFMD. If the leakage current crosses the pre-set value which usually less than 32mA, the GFMD breaks the path for leakage current and passes the ground fault trip signal to the inverter and eventually inverter trips. Once the ground fault on DC side is rectified, the inverter starts automatically.
Ground Fault Monitoring
The inverters in solar PV plants has a AC system which are typically ungrounded. For AC side ground faults, a separate mechanism trips the inverter with current imbalance. The internal current transducers measure the line currents of the inverter. If the Phasor sum of line currents crosses 8% of the actual measured sum value, the inverter receives current imbalance trip signal and eventually inverter trips.
Phasor sum of phase currents = Ir + Iy + Ib = 0A (balance network)
If the sum of phase currents is non-zero, the circuit is imbalance due to external AC fault. If this non-zero value crosses the 8% of the numerical sum, the inverter trips with current imbalance.
Hence during design phase it is recommended to choose inverters with ground fault monitoring device.— By Raju kaki- Electrical Engineer