Relays in Coordination Study

[timm33333]

I am trying to figure out whether it is required to include the differential relay in the coordination study. I heard that differential relay cannot be coordinated because it is instantaneous. Is it correct? Thanks.

 

[rcw retired EE]

I include the differential relay on the coordination curve, graphing the total tripping time including relay actuation time, lockout relay contact time and breaker arc clearing time. It helps to coordinate any backup overcurrent functions.

 

[RRaghunath]

Differential relays come under the category of unit protections with defined protection zone. Hence differential relay doesn't coordinate with any thing downstream of the protected equipment.
Thus, in principle differential relay doesn't have to be part of protection coordination study.
But with modern relay coordination software, it is possible to model differential and other unit protections and I have seen clients asking these to be included in the protection coordination charts.

 

[timm33333]

Thanks. How about sync check relay (25) and reverse power relay (32); should these be coordinated?

 

[jwatty]

I wouldn't necessarily plot differential relaying but it's good to be cognizant of the zone so you check appropriate contingencies. A differential around a transformer may change N-1 coordination with a highside overcurrent from one line out to system intact, for example. Same with pilot schemes - there are some lines that require dual pilot protection for stability, and if pilot is out they must be removed from service. Time delayed coordination on such lines may be unnecessary(depending on the utilities philosophy).

 

[timm33333]

Do we need to coordinate sync check relay (25) and reverse power relay (32)?

 

[jwatty]

Not usually. Sync check doesn't involve fault clearing, and reverse power tends to be either a control function or a last to operate type thing.

 

[bacon4life]

Some of the newer NERC standards require consideration of how differential relays interact with other kinds of relays, particularly when a protection system component misoperates. For examples transmission modeling software now includes relay models so that it automatically calculates the clearing time for faults as part of transient-stability studies. In order to verify correct operation of the system, we run the model both with normal conditions and with having the differential relay out of service.

Every relay function should be coordinated in the sense of acting harmoniously. Elements such as 87, 25, and 32 are often just a yes/no answer rather than being plotted on time current curves.