NCT

How to calculate NGR

Great website. Thanks for posting..

Prashanth

For LV transformers most national codes forbid the use of NGR's.

Why would you want to restrict the earth path of a system that is supplying the end user?

NGR is provided to restrict the earth fault current. Normally in India 415 V side is effectively earthed (solidly grounded). Hence no NGR is provided in neutral circuit.

However, the secondary side of transformer 33/ 6.9 kV is non-effectively earthed (grounded through resistance) so as to limit the earth fault current. The neutral side CT is connected to restricted earth fault relay for tripping the circuit breaker in case the current exceeds the set value. Motor protection relay is also provided with earth fault protection which can be set for lower earth fault current for protecting the winding of high voltage motor against any possible damage.

Primary current of NCT is selected by the system designer based on the maximum allowable earth fault current. In case of single phase to ground fault the fault current will return to the star point of secondary winding through NGR and will be sensed by the NCT.

33kV system is generally solidly grounded. However, in specific cases it may be resistance grounded depending on the type of load connected to the 33kV bus or on certain application specific issues which can't be commented upon without going into details.

Thanks for sharing this.

It's really enlightening to find out about how things are done in different places especially when explained by somebody who really seems to know their craft.

Cheers.