How to calculate the differential relay setting?


Re: How to calculate the differential relay setting?

I assume for a generator, not a generator and transformer?

The easiest way is to run all 6 (3 phases, 3 neutrals) or 4 (3 phases and neutral) leads through a large window CT and connect it to a simple overcurrent relay. This is not always very easy. You can also use individual CT's, wire all the secondaries in parallel and in parallel with the overcurrent relay. It is very important the CT's are matched exactly. Thus the percentage comes into play.


Percentage differential is the difference between the operating current and the restraint current. Think of it this way, operating current (phase current) tries to close the relay and the restraint current (neutral) tries to hold it open. If it closes, the breaker trips. So a 10% differential relay setting says that the operating current needs to be over 10% more than the restraint current.  


This gets much more complicated when you have a zone that includes the generator and the transformer.


What type of protective relay do you have?


I will gladly share a scan if you will send me an email.



New member

Re: How to calculate the differential relay setting?

Dear Sir,

I had an old relay DBP/G-3 OF MICROELETTRIC ITALIA  and setting was ka =2,kr=3,a0=2 r0=3 and In=5A. and all the dip switches are slected on (active)

what would be the trip setting out of this setting please. in order to load on new realy MD32-G same brand

as I calculated 1.6 and I am not sure !



Super Contributor

Re: How to calculate the differential relay setting?

The best answer is to hire an engineer with the proper experience to develop the settings for you based on you particular application and installed equipment.


Some relays require identical CT's on both ends, some systems require different CT's if a GSU (Generator Step Up transformer). If you have to pickup large transformers in your system you may need 2nd harmonic blocking, if you have bus differential you will have to assure settings are correctly coordinated for the overlapping zones.


In general you can develop general trip slope, thresholds and delays, but actual settings generation is dependent on what relay you're using.  Properly applied differential protection can prevent stator core damage in the event of a fault, preserving a valuable asset, improperly done settings will result in nuisence trips or improper protections. In short, not something left to someone not fully experienced and capable of producing the proper settings.


This Basler publication offers a general overview of generator protection with a discussion of differential protection. A Google search will also yield a number of resources.


Mike L.

Super Contributor

Re: How to calculate the differential relay setting?