Simple Earth Fault Protection in Motors




To protect a Motor, Motor protection Relay shall need following minimum protection features are needed :-
1.Thermal protection against winding overheating (Stall protection/Locked Rotor/Overload/Protection through RTD)
2. Single Phasing or Phase Unbalance.
3. Short Circuit.
4. Earth Fault.
5. Overvoltage or Undervoltage

The most sensitive protection of all the above is Earth Fault protection as it ensures correct equipment as well as personal safety.
Normally it is detected through ground relay with 50N protection as below:


  
This system detects the fault through zero sequence currents. However, its sensitivity may be affected due to below reasons:
1)       Current Unbalance in the three CTs.
2)       CT saturation due to high starting currents.
3)       Minor variations in the CTs in Manufacturing
Hence an alternate method used is sensitive earth fault protection by means of CBCT and Earth Leakage Relay (ELR).
This method shall eliminate any errors due to current unbalance and CT defects. It shall cancel out nuisance tripping due to negative sequence currents or DC components. 


When using a grounded conductor, the toroidal CT should encompass only the phase conductors and not the grounded conductor. Figure shows two methods of grounding the cable sheath when using a toroidal CT. Note that the cable shield ground is established on the load side of the CT in Figure (a) and, therefore, does not contribute to the flux within the CT. In Figure (b), the shield lead is doubled back from the source side to the load side of the CT, resulting in magnetic flux due to current in the shield summing to zero within the CT. If the cables in Figure (b) have individual shielding tape or braiding, they can be jumpered together on the source side with a shield lead brought back through the CT. The purpose of both methods is to ensure the current measured by the ground relay reflects the ground-fault current and elimination of the outgoing phase current with the returning current in the grounded sheath.


By correct matching of CBCT and ELR, we can achieve efficient earth fault protection.  It can be used in of the common applications involving motors. In case of large HV motors, directional earth fault protection might be needed.
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