Generator Differential and Earth Faults

I suppose u re referring to differential protection protection of a generator.It is employed on large generators and transformers and in some transmission lines.It will protect windings within a zone. It compares quantities derived from input and out put currents of the protected circuit in such a manner that for all the healthy and through fault conditions qtys balance each other and protection remains inoperative while for internal fault conditions ie for faults within the zone the balance is disturbed and the protection operates. Earth Fault relay will be for any kind of earth fault it will operate.In practice a relay protection scheme is an art and one needs lots of experience

Dear nesurba,

thank you for your reply you have cleared my basic doubt. the differential relay is for localised fault within the circuit of relay and earth fault is external fault. Thank you once again.

No not so.

Differential protection of a 3 φ gen set will protect the set in case of un-balanced conditions, even if the fault is external meaning un-balanced loads beyond the gen set capabilities. An earth fault detector will detect any current leakages going to earth. This may in some connecting arrangements cause un-balance or shorted output. and just like the differential current detector, will activate the main CB, or in extreme cases such as internal shorts, kill the engine.

Wangito.

Good day, I am Ekolo. I was searching for possible solutions to a problem at my job side ( powerplant). I am a mechanical engineer. We keep having athe differential fault alarm when we try to increase the energy up to1 MW....we have tried several possibilities to no avail. Please I highly solicitate proposals for posible causes....

We have changed all the CTs from alternator to bus and transfo. But we still have the same fault .....pls help.

thanks Ekolo

I must respectfully disagree with Wangito. Generator differential protection involves at least 2 current transformers per phase, one at each end of the zone of protection. One CT is located on the phase winding connection just prior to the neutral star connection. The other CT is often located in the stator bushing which connects to the output bus, but may be at the step-up transformer or the main CB. The differential relay compares the polarity and magnitude of both CT's. A vector sum greater than zero (within calibration tolerance) indicates that current is leaving the phase at some point other than the neutral or output bushing. If this occurs, the generator and prime mover are tripped. For a fault which occurs outside the zone of protection, the current passes through both CT's on that phase, the sum remains zero, and the relay does not act. Many stations have multiple zones of protection; generator, step-up transformer, and bus. The zones overlap, which provides a ready means of localizing the fault. The only relay to trip will be the one in whose zone the fault occurred.

There are 2 different types of earth fault relay. One type (commonly called a neutral overcurrent relay) has a separate CT in the neutral line after the star connection. If the neutral current rises above the setpoint, either because of a single phase fault or because of load imbalance between the phases, the relay will trip the generator main CB. The second type (sometimes called a phase balance relay) uses all 3 phase CT's and calculates a vector sum of the phase currents. In an ideal generator, all phases are equally loaded, and the vector sum is zero. In the real world, there is never a perfect balance, so the vector sum will always be somewhat greater than zero. The phase balance relay is set to trip at a point where the imbalance is considered to be excessive.

Regards,

Tony

Hi Tony,

I respectfully must agree, your answer is a lot better and more in depth than mine. but basically, we both say, considering the above, the same.

I liked your slogan...

Wangito.

Hi Tony,

It is really fantastic! you have further cleared my doubt. The explaination of purpose of providing differential relay you made it crystal clear. Thank you for your valuable information

rangacharya

Most or all the answers are pretty good.

But most of all, the problem must be faced as a system problem.

Therefore, it is of utmost importance to know what kind of grounding is beeing used.

In regular generating power plants of power larger than 20MW, the grounding system is through a distribution transformer with a resistor across its secondary.

For this reason, the system much behaves like an ungrounded system, say, no phase to ground faults are easily detected.

For these systems, generator differential protection is mostly used for phase to phase or three phase faults.

Another type of task will cope for the phase to ground task: the stator to ground fault.

There are many different methods of stator grounding protections and you should refer to electrical engineer specialists on this matter.

A good idea would be to contact protection relays manufacturers, such as ABB, GE, SCHWEITZER and SIEMENS. You can find them by using the "Godfather" Google.

They will have a representative to your region and will certainly be pleased to ask your questions about the matter.

Regards