Ground Fault Protection Methods

[timm333]

I am trying to figure out what is difference between ground fault relays: 50N, 51N, 59N, 87REF. Under what conditions are these used, can these be used interchangeably? Thanks.

 

[xptpcrewx]

I am trying to figure out what is difference between ground fault relays: 50N, 51N, 59N, 87REF. Under what conditions are these used, can these be used interchangeably? Thanks.

These protection elements are totally different from one another and much can be said about them. Plenty of references out there. They cannot be used interchangeably as they do different things and operate on different principles. The conditions where you would use these are generally application and industry specific. I suggest looking up the ANSI device numbers to first understand what they are. If you have a specific question about a particular element I would be happy to help but right now I don’t think your question is specific enough to answer without going into tangents.


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[timm333]

There are two methods to measure ground fault. The first is the current method in which CT's are used to feed the relays like ground overcurrent relays (50N, 51N) and restricted earth fault relay (87REF). The second method is voltage method in which broken delta VT's are used to feed 59N relay.

My questions is that under which conditions do we use the current method, and under which conditions is the voltage method preferred?

 

[xptpcrewx]

There are two methods to measure ground fault. The first is the current method in which CT's are used to feed the relays like ground overcurrent relays (50N, 51N) and restricted earth fault relay (87REF). The second method is voltage method in which broken delta VT's are used to feed 59N relay.

My questions is that under which conditions do we use the current method, and under which conditions is the voltage method preferred?

This comes down to the system grounding type and the magnitude of faults involved. 59N is generally measured directly when done with HRG systems. 64 detection on ungrounded systems is accomplished with a broken delta configured set of PT’s. In both of these applications the SLG fault current is very small and difficult to detect.

Imagine trying to use an overcurrent element to detect a fault on an ungrounded system. What magnitude of fault current do you expect to see? It would be much smaller than the load current. How would you tell the two apart (load vs fault)?


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[timm333]

The ground fault current in ungrounded system would be low as compared to load current. In solidly grounded and LRG, the ground fault current would be high and phase overcurrent relay can clear ground fault. So do we need the ground fault overcurrent relay only for Ungrounded/HRG?

Regarding the restricted earth fault protection, is it provided for windings, or is it also provided for the body of transformer/stator/rotor? Sometimes they call it 87REF and sometimes 64REF. I think it would make more sense to use 87REF for windings of transformer/stator/rotor; and 64 for the metal body of transformer/stator/rotor.

 

[xptpcrewx]

The ground fault current in ungrounded system would be low as compared to load current. In solidly grounded and LRG, the ground fault current would be high and phase overcurrent relay can clear ground fault. So do we need the ground fault overcurrent relay only for Ungrounded/HRG?

As mentioned, 59N is generally measured directly when done with HRG systems. This has the benefit of detecting a fault even with the HRG failed open.

You just stated, "The ground fault current in ungrounded system would be low as compared to load current", so using a ground fault overcurrent relay wouldn't make any sense. Where would you put the CT's for an ungrounded system?

Regarding the restricted earth fault protection, is it provided for windings, or is it also provided for the body of transformer/stator/rotor? Sometimes they call it 87REF and sometimes 64REF. I think it would make more sense to use 87REF for windings of transformer/stator/rotor; and 64 for the metal body of transformer/stator/rotor.

REF protection is achieved for anything located within its zone of protection (between Line CT's and the NCT). For example, applying this to a transformer, protection would be confined to the wye-connected windings of the transformer to detect phase-to-ground faults - anything outside of this zone is ignored (hence the term "restricted" earth fault protection). I believe 64 vs 87 has to do with how the fault currents are measured/calculated/compared. For example, 64 if residual quantities are used with a stabilizing resistor to pickup on voltage (high impedance - voltage operated), and 87 if residual quantities pickup on current (current operated).

 

[paulengr]

There are two methods to measure ground fault. The first is the current method in which CT's are used to feed the relays like ground overcurrent relays (50N, 51N) and restricted earth fault relay (87REF). The second method is voltage method in which broken delta VT's are used to feed 59N relay.

My questions is that under which conditions do we use the current method, and under which conditions is the voltage method preferred?

“Voltage method”? With currents the big difference is in terms of accuracy. You can use 3 phase CTs and mathematically add them together as vectors or tie them together in wye and measure the current with a 4th CT in the analog equivalent. It is subject to the errors of all 3 (or 4) but in a microprocessor relay it is “free” and close enough for many applications. A more accurate method uses one CT with all 3 phase leads running through it, called BYZ or core balanced CT. A big problem though is the low ratios. So a better approach is to use a higher ratio CT with a low output such as 25:0.925.

So far one advantage is all 3 methods are universal....they can be used anywhere. For instance the 3 CT numerical method is commonly implemented in advanced motor protection relays.

Another option is 87 relaying. This can be applied over an entire area (zone) with the simple idea of in equals out. It can be applied over transformers, busses, and even motors and generators if the wye is accessible. The downside is with the number of CTs the cost can get very high. It is typically only used for critical assets. This is aside from the common single phase uses (GFCI, GFPE).

In a resistance grounded system there is yet another option. If there is access to the resistor then a simple PT can be used with ohms law...V=IR. The current can also be measured directly by measuring it on the resistor. Frequently in these systems both are used. The primary mechanism is the CT. The PT provides backup protection but also if for some reason an open fault develops in the resistor it can protect against this as well.

In most situations it’s a trade off between accuracy, and cost. 87 relaying plus basic 50/51 protection can solve nearly any issue but at a prohibitively high cost,