backup

[facetoface]

could any one give me more informatio for 51V relay which is used in generatot back-up protection , as principle and function please , and is there equation specify the inverse relation between Var/V

Best regard

 

[dpc]

There are two flavors of 51V - voltage restrained and voltage controlled. It sounds like you are talking about the voltage restrained variety.

The basic principle is that for backup protection of a generator you need to be able to sense fault currents that are less than the generator full-load current because the synchronous reactance of a generator is generally over 100%.

The 51V relay lets you set a time-current curve below load current but restrained by normal operating voltage. When the fault occurs, the voltage goes down and the relay curve is shifted way down (voltage-restrained), or the guard relay drops out to arm the TOC element (voltage-controlled)

Do a Google search for "art and science of protective relaying" and you'll find more info at the GE website.

Cheers.

 

[facetoface]

Thanks for yoyr repaly
if assume the fault occurs , the relay will be immediatlty activated if the fault cuurent less than overload current and the voltage will be become bellow the nominal voltage if the overcurrent has mistake, is this right please

 

[dpc]

Yes, that's the basic idea. It's really backup protection, except for very small generators.

 

[rbulsara]

Just to clarify, its not that the primary 51 relay (if installed) malfuncitons, but it may not pick up if the fault is close the generator terminals, driving down the generator voltage and that in turn ofen limits the fault current below the rated full load amps. Because the normal 51 relay pick is set at like 125% of the FLA.

So the 51V is set up to pick up only upon low voltage (generally below 80%) condiitons and still sensing current which may be below FLA.