Earth Fault in Parallel Operating Gensets

It is unusual to see solidly grounded MV gensets and further grounding all the parallel connected gensets. It is more common to see only one of the generators grounded that too through resistor to limit the current in case of earth fault inside the generator stator.

Regarding the question about the generator trip in case of fault in one of the generators, it depends on the location of fault, if the fault happens to be close to the line terminals of the generator, all the generators 51N relays will see the same magnitude of the fault current and hence trip together.

In case the fault happens to be deep in to the windings, the current seen by the 51N relay in the faulted generator circuit could be lower (again depending on the resistance at the point of fault) than the current seen in other unfaulted generators and hence the unfaulted generators are likely to trip first. It also depends on what kind of settings are adopted in the relay 51N (DT or IDMT specifically) and the magnitude of fault current.

More importantly, I think it is a good idea to review the earthing system to save the generators from stator core burning in case of earth fault in the stator windings.

Thankyou for your reply, you mean to say that NGR to be included in the earthing to reduce the fault. If NGR is using only one gensets neutral could be earthed. even thoug the magnitude of the fault current flowing through each neutral will be same, ie. all gensets can be tripped together during fault. my question is how to run the rest of the system by isolating the fault one only.

thanks.

How to run the rest of the system by isolating the fault(y) one only(?)

This once again depends on where the earth fault is in the network.

If the fault is on the common bus where the switchboard is, then each generator protection relay will trip its respective CB. No power source can connect to the switchboard until the damage has been repaired.

If the fault is at a load, then the CB protecting the load will trip if setup correctly.

If the fault is on the line side of the generator CB, then I suggest using REF or differential current protection. Either of these types of protection schemes will only trip its respective gen CB for a fault within the zone of protection.

sir,

already there is diffential and ref protection in addition the e/f protection. my doubt is if only one genset nuetral is earthed, how the current path for the earth fault current in other gensets will be completed? for ex. if gen-1 neutral is earthed and earth fault occured in gen-4 winding how gen-4 neutral ct will sense the fault current.

thanks.

if only one genset nuetral is earthed, how the current path for the earth fault current in other gensets will be completed?

It will be completed through the substation earth grid, it is easier if you create a single line drawing so you can see the current circuit for the earth fault condition.

if gen-1 neutral is earthed and earth fault occured in gen-4 winding how gen-4 neutral ct will sense the fault current.

Presuming that the switchboard only has the three phase connections (i.e. no neutral connections), then only gen-1 neutral CT will sense any current at all.

For the case you presented in the OP, all generators will feed into the fault and therefore all 51N protections will trip. Note that there may be some impedance differences for the current path on the earth grid to each generator, but if the codes are followed correctly for the earth grid design, this will usually have a negligable effect.

Note that 51N is inverse time overcurrent protection, and as such, the time taken to trip is inversely proportional to the current. Also, the protection device will only pick up at a minimum current level originally set by the protection engineer.

For REF / differential protection, if it is set up correctly, when there is a fault within the zone of protection, this protection will trip the associated CB. Note that the minimum current level for the REF / current differential protection is usually much lower than the 51N protection, so this protection will usually pick up and trip before 51N (once again if set up correctly).

Every feeder including the generator feeder is expected to have CBCT based sensitive earthfault protection. The setting for this protection would be sensitive and the SBEF protection (using generator neutral CT) will be set to coordinate with the feeder earth fault protection.

So, when gen-1 is earthed and the earthfault occurs in generator-4, generator-4 trips on the earthfault protection. Same thing will be true when the fault is in one of the outgoing load feeders.

The SBEF protection in all those generators whose neutrals are disconnected from earth, remain disabled.

dear peterjh and raghun,

Thanks for your replies and sorry for the dealy in responding i was travelling,

The function no. is not 50N its 50G, its only for earth fault not earth leakage. please refer the attached sketch as per your your explanations. pls. check it. i hope you mean its the way of completion of current path. when a fault happens at point "A". ie. gen-1 e/f protection realys contact to be used for the protection trip of all the rest? no need individual e/f relays? the SBEF or unbalance no need to trip its time delayed. but e/f is instantaneous trip. thanks

Okay, the original sketch shows 51N, not 50N or 50G. Regardless, ANSI 50 is a current measuring element, and will trip if the current level is above the set point for the preset time delay.

Presuming that the ANSI 50 relay is sensing the current between the earth and the alternator star point, it will sense some or all of the current (depending whether there are any mains sources with a neutral-earth connection also supplying to the load). Note, however, if there is a phase to phase earth fault, then the ANSI 50 relay as described at the start of this paragraph will not measure any of the fault current.

It is not for me to advise whether your protection scheme has been design correctly. Even if I will, I do not know the full details of the installation (i.e. what types of loads, any other sources not mentioned, cable lengths etc). You need to engage a qualified power systems engineer to properly assess your installation and advise what the best setup should be.