System Earthing for Unit Transformer Fed High Voltage Motors 1

[brijgoyal]

I have a specification which insist high resistance earthing, limiting the earth fault current 10A to prevent core burning of the motor, for unit transformer fed high voltage motors.

On the other hand we have high voltage motors directly supplied from high voltage system with resistance earthed, limiting earth fault current to 100 A.

My question is why it is necessary to limit the earth fault current to 10 A for unit transformer fed motors when it is acceptable to limit it to 100 A for directly supplied motors.

 

[Kiribanda]

What is the nominal system voltage of your HV system? Having a HR earthing system for your HV system may not be practically possible
due to capacitance effect during an earth fault.
What is the nominal system voltage of the system where you are limiting the earth fault current to 10A?

 

[brijgoyal]

High resistance earthing with 10 A limit is being proposed only for the motors which are supplied via its own unit transformers on the motor side. HV motor voltage rating could be 3.3, 6.6, or 11 kV.
Thanks

 

[RRaghunath]

The High resistance earthing is always beneficial as the core burning is prevented in case of earth fault in the stator of the motor.
The motor can be reused in case of winding fault after rewinding but, if the core happens to be involved in the stator earth fault, the motor has to be replaced ((no repair is possible). Thus, it becomes expensive.
For high resistance earthing to be effective, the capacitance of the system needs to be limited. In other words, it is possible to have it when there is dedicated transformer supplying a motor but not a large transformer supplying a busbar with many motors and other feeders are connected.

 

[brijgoyal]

Raghunath,

Your response is a great help.
It is now clear to me why we should limit the earth current to 10 A for unit transformer fed motors and not for other motors directly on the system.

Thank you