We are planning to Source the standard OC + EF numercal relay for standard feeder protection , howver woudl ike to be adivsed as what shoudl be nos of stages that this OC element should have and some information of LOW -SET and HIGH set setting and thumb rules basis your experiences.

Thanks in advance...

I believe there are standard IEC/US C settings for the OC


· Very Inverse Time Overcurrent

The very inverse time overcurrent protection provides also the discriminations as in the standard inverse curves. This type of curve however is more appropriately used as operating curves when there is a substantial drop in fault current level on a radial feeder as the fault location is further from the source. This scenario could arise when there is significant increment in the impedances of the interconnecting cable/s or transformer/s moving from the relay point into the radial feeder.

· Extremely Inverse Time Overcurrent

The extremely inverse time overcurrent protection can has its operating time inversely proportional to the fault current squared. This overcurrent characteristic is very suitable for protection of feeder circuits subjected to frequent peak currents that could be resulted from switching in of circuit breakers. Using these extreme inverse curves, the overcurrent protection prevents any mal operation during transient on a “low” overcurrent condition without compromising quick isolation on high fault current.

· Definite Time Overcurrent

Definite time overcurrent protection has no inverse characteristic i.e. the operation of overcurrent protection has no reciprocal relation with the fault current measured.

This type of protection is particularly useful when the fault current varies very little on a feeder from the relay point due to the changes in the source impedance. Given this, there is little advantage to discriminate protection relays in series by fault current level. With definite time grade under this circumstance, it will be more appropriate to grade the protection relays in downstream. Definite time overcurrent also has an advantage of providing good coordination of the relays operating at lower fault current.

· High Set (HS) Instantaneous Overcurrent

The high set instantaneous overcurrent protection is always designed to assist the time overcurrent to improve tripping time at very high fault levels. With HS, the overall time grading of protections can become more discriminative at very high fault currents.

Overcurrent protection relays can be set to the standard IEC or US curves for the protection operating times. The standard curves have the operating and reset times in the following equations

The complete study of the overcurrent protection to coordinate tripping times of the circuit breakers in a connected system requires a detailed study of the connected network. This kind of fault study requires the system parameters such as the voltage levels, equipment MVA ratings, impedances of the interconnecting cables/ conductors, current transformer ratios etc as inputs. Only with the fault current levels computed from these parameters, the overcurrent protection settings can be graded correctly.