by-passing transformer differential relay

Never! What are the problems you faced?IF during charging the transformer trips ,due to inrush ,increase the setting of 2nd Harmonic Restraint, used for restraining the relay operation during a switching inrush...........

Dear Anandeqq,

I suggest you to solve the problem first, then you can energize the transformer. And also do not add the timing of the differential relay for the purpose of initial stat-up. Based on my experience, at the initial start-up, transformer energization always failed due to diff. relay (microprocessor relay) tripped. I asked the contactor/vendor rep. about the harmonic restrain setting, he said already set (I did not know how to set it), they proposed to add the delay time for 500 milli seconds, I agreed on it, but I recommended to return it to normal setting after energization. Energizing is done safely.

Two month later, when the incoming breaker blown-up due to some reasons, we found that diff. relay did not operate. It is happened for two incoming breakers which are located far away from each other but both have typical feeder protection schemes.

We investigated the problem by using the relay differential record, revealed that breaker trip due to over current relay instead of diff. relay. So, I suggest the contractor to invite the relay manufacturer expert to investigate the problem. It is long and hard discussion have done.

The investigation revealed that the delay is still 500 ms (contractor forgot to return the delay time setting to normal), and the harmonic setting have not done yet. The vendor expert eliminated with proper setting.

At last, this system is OK right now.

Without going into the nitty gritty, or the fine details of what you wish to try, let me rephrase this into a generic question.

You have something that has a protection device that repeatedly goes into protection mode. You do not know why it goes into protection mode. So, you ask us if you can run something without protection?

Well, it depends on several things. First, I see two options for why your protection device maybe regularly going into protection mode, either the device is defective or it is doing its job. Now the safe method for identifying which possibility causes any protection device to trip does not involve bypassing the protection device. So troubleshooting this apparent fault is not your priority. This leads to my question of what is your intent? If your sole intent is to give the impression of completing the job then bypassing anything that might make the job appear incomplete can get you out of the job quickly. It can also quickly get you permanently out of this career. Now I'm sure there must be other intentions that will lead to bypassing a protection device. But they all run the risk of ruining a career, if not more.

To paraphrase Dirty Harry: Do you feel lucky? Well do ya?

You should get with this guy: intrinsic safe sov

Maybe the two of you can blow yourselves, and probably others, to hell while trying to do things ON THE CHEAP!!!!!!!!!!!!!!!!!!!

You should take two weeks off and then quit forever.

I shudder to think of by-passing the transformer differential relay. I suppose you have an older version of differential relay which senses the transformer in-rush current as differential current and causes a trip. You can go for a modern numerical differential relay which has a 2nd Harmonic Restrating Feature to avoid such a happening.

Or if your relay is already a numerical one, check the restraint settings or else check the TRF. thoroughly. There could be a problem too.

guys thanx for your views...BUt the real problem is transformer LV side switchboard is not presently available, which means there is no CT in the LV side presently for connecting differential relay in LV side (there are two differential relays communicating for trafo diff. protection). So can I just bypass diff. protection for running transformer on no-load. Is there a great risk in charging a transformer on no-load in this condition???

Just in case, there is a genuine internal fault in the transformer, irrespective of no-load or full-load, transformer might damage totally. So, no excuses for by-passing the diffl. relay.

I see you don't have CTs in the low voltage side of the transformer.. Do you have the other transformer protection - Buchholz..Pressure relief.. and others?.. I think you will have by pass the differential relay. Again I think the differential protection you have is useless since there is no CTs in the low voltage side of the transformer. If you any other CTs aside from the LV side of the TRAFO - like the main secondary breaker, you could use this CT. They are right that differential relays should not be by passed. But again it is of no use since you have only CT for your relay.

Dear Anand,

Oh ... this situation what you have? Sorry I never realize. My question what is the transformer capacity, is it less than 7.5 MVA or higher?

Our plant only recommend differential relay for transformer 7.5 MVA and higher. I do not know about other company philosophy and insurance company? especially your company.

If you have two differential relays, I assumed your sending end and receiving relay is far away? You can conduct internal fault test for differential relay even the secondary side not available. For commissioning purpose may be you can implement on one side only (sending end). Differential relay will not operate against magnetizing inrush current if you set it correctly. The differential will operate when you give loading on transformer, but you cannot conduct through fault test.

Many information you already have it know, you should not waiting us for making decision, because one company to the others are different philosophy, but all have the same concern on safe equipment and personal for energizing transformer.

I don't understand why you were asking what is the transformer size.. You are not reading, i think.

Sorry I was overlooking, yeah 50 MVA. By this capacity differential relay shall be installed completely, except if they are having a good justification.

Hi,

Your Diff relay without CT's on the secondary side of the TRFR will just act as an instantaneous overcurrent relay. The TRFR with no LV board in fact cannot have load so the LV CT's in this case would have no function anyway, and therefore the Diff relay does not need to be balanced. Should a genuine fault occur you would have the same effect whether the LV CT's are connected or not. If you just apply some decent settings, you should have no problem with the Diff relay. This may be negated if you have a digital Diff relay which may require some form of communication with the LV end, irregardless of load on the LV side to be stable.

Should this be a problem then just by setting your high set overcurrent element to be above inrush current and to an instantaneous value, you will protect your TRFR for any faults whilst the LV board is not connected. This setting will normally be well within the fault level of the system.

If you RE/F on both sides you will have high speed protection for earth faults on both sides of the TRFR, most likely fault is an earth fault anyway.

If you only RE/F on the LV side. ( I am assuming a Delta Star TRFR with earthed neutral on the LV side) Assuming the LV RE/F CT's are in the TRFR and not in the LV board in your case. (I prefer them in the LV board as you also get high speed E/F protection on the cable) You can also set your HT E/F (Residual connection in your overcurrent connection) to an instantaneous value and install a stabalising resistor in series with the E/F connection. 56 ohms for 1 amp CT's or 4.7 ohms for 5 amp CT's.

Unfortunately I do not have your actual connection diagrams so am unable to give the best advice as I am doing a bit of guessing. there are too many assumptions. the above is a good general guide.