Differential & Restricted Earth Fault Protection Of Transformer

Usually CTs rated to be used for differential protection will not be as sensitive if applied for earth fault protection, due to the high primary current rating. Loading up the secondaries of these CTs with other burdens of questionable value also risks the best protection you have for your transformer.

Earth fault down to 10% of the windings can be achieved by monitoring current or voltage directly in the neutral, leaving the poor protection (?<50% of the windings) available by measuring the line current out of the protection scheme altogether.

Sometimes belt & suspenders can create more risk than K.I.S.S. If you supply the model of the relay you have in mind, and the details of your high impedance grounding, then more help may be available.

Thanks for the answer. But still I have doubts on below points

1. Differential & High impedance protection can be achieved from same set of PS class CT or not! In this case the e/f element of the relay will be series connected with the main CT neutral along with the NCT at the trafo end. The differential coils will be connected as usual with the star & delta side of the transformer. Balance REF & Differential part will be derived by the relay itself. To confirm.

2. Differential & low impedance protection can be achieved from same set of PS class CT or not! In this case the e/f element will be connected with the NCT only. Balance REF & Differential part will be derived by the relay itself. To confirm.

Is it possible ? My main intention is to save one set of separate PS class CT for REF. I am considering one set of PS class CT with sufficient VK value ( Considering the % impedance of Trafo as well).

Hope I am clear with the points. Kindly share your views & confirm the possibilities.

Regards

What does the instruction manual of the relay you are intending to use for these functions say? Usually the manual provides typical schematics for how the relay is to be connected to operate properly. Since you have not identified the relay for us, we can't begin to tell you if it will work. The manufacturer is the best one to say! A call to them should help immensely.

Thanks for the reply. The relay to be used is Micom P642 where both differential 87T & 64R features are there.Manual doesn't elaborate on the detail of REF with a single CT.

Now with a single PS class CT at the star side both Diff. & REF can be achieved or not!

If yes then 1. High impedance or low impedance REF will be possible?

2. How diff & REF can be differentiate in case any fault occurs inside the zone ?

Separate PS class CT for both diff & REF is at all justified or not?

Any suggestion please.

On seeing relay connection diagram from manual of P642 I found that there is provision for connection of two phase CTs (HV and LV of transformer) and two neutral CTs (in case of star-star transformer) so it become obvious that you can use same CT for differential and for REF protection.

Now coming to choosing high impedance or low impedance REF, it depends upon how your transformer (which is to be protected) neutral is grounded, if it is high impedance earthed you should opt for high impedance REF and if transformer neutral is solidly grounded you should opt for low impedance REF.

Hope it helps.

The protection you get from a residual connection in the differential CTs is pretty poor, is what I am saying. Not worth the effort to compromise the differential protection by adding a useless pair of connections and failure point. We stopped using that protection method in the 1970s.

You can connect whatever you want in the CT circuit, and it will work providing you don't exceed the burden rating of the CTs for the accuracy you are looking for. You must do the calculations regardless, wire diameter, length, CT ratings, burdens of the devices connected to the CTs, all this is required for the basic installation.

I would disable the ground fault protection that is backup to the backup on your protective relay, it is so insensitive it is not worth using.

You also need to explain your power system grounding better, you are mixing high impedance and low impedance grounding terms, due to translation problems. The important part of this is how is the neutral of the Y winding earthed? Can you consider the delta side effectively grounded? Probably not...

Also, you need to consider how to coordinate the ground fault protection with the rest of the power system, so that the operator can figure out just where the fault is?

BTW, 64 function is derived from voltage inputs to the relay, but you must have all 3 phases brought in, not an open delta set of PTs. I have used this function when effectively ungrounded system, such as on your delta side. Better than nothing.

Dear Sirs,

Thanks for guiding me . Now gradually I am clear with the requirements. Actually both diff & ref protection from a single set of PS class CT can cover the zone of protection what I understood from the discussion. Now for a defined fault inside the zone either for a ph to earth fault or an internal trafo fault how differential & REF setting could be done to visualize in the event record? Throw some light.

Also for high impedance REF what should be the value of resistance connected in series with the E/F element of the relay!

Also for 50-51/N o/c & e/f ,3 nos current coil of the relay could be sufficient for a derived earth fault protection as 4-th coil will be used for REF as described above. Hope it is ok .

Hope I am clear with my views.

Your opinion please.

Regards,

I'm not clear what REF refers to.

The reason for 2 sets of CTs is usually you can pick a better ratio for your differential protection (better protection) than for your through fault protection (temperature limit rise for large through faults), also to prevent burning up your protective relay coils from too much continuous current. This is something you need to check when you do your coordination. make sure your downstream overcurrent relays clear the source of the high current before you trip off the transformer, and then with that elapsed time, you don't burn up the protective relays.

Having a dedicated set of CTs for your differential protection makes it much less likely that someone in maintenance will inadvertently bypass them while working on metering or backup protection. The differential protection is the best way to ensure your transformer lives to see another day after a severe problem. First line of defense is your sudden pressure relay, 63, that will clear the transformer off line and minimize damage & fire possibility.

Missing still is the grounding primary & secondary of the transformer, without this information you cannot design ground fault protection.

You need to know the inductance of your potential transformers and estimate the line capacitance at your connection point to avoid ferroresonance, a source of much discussion you can look up for yourself. One conclusion suggests that the resistance should be as low as possible within the thermal capacity of your PTs. An example cited for a 35kV install noted that a 16 ohm resistor was not low enough, the system required an 11 ohm resistor. The resistor wattage rating is for the 208V applied during fault conditions. There could be some residual current flow during normal conditions, but if you size the resistor for operating with an uncleared ground fault, then you should be covered.

I don't understand any of the last sentence, maybe assumes knowledge of REF. You need to know external ground fault & overcurrent coordination to make decisions on 50-51N, also through current rating (short term) of transformer.