Switchgear Protection Suggestion 1

[MeinKaun11]

Hi

I have attached a pdf file that shows breaker and bus arrangement for a 4.16 kV switchgear.

Background:

This project is replacement of a 1950 switchgear supplied by GE and has basic overcurrent protection on the feeders and 67 directional relays on the two main incomer feeders.
As part of the replacement, I have installed 751A relays on outgoing feeders and they would do most of the stuff typically part of feeder protection. for the bus protection, I am implementing bus blocking scheme using SEL 351-7 relays.

Question:

My dilemma is the existing 67 directional protection that was there before, why would I need a directional protection on incomers.
I would appreciate your input or any suggestions in regards to any other protection element that could be part of incoming protection scheme.

Regards,

 

[davidbeach]

If it can be operated tie through, you wouldn't want to trip both of your sources for a fault on one of them.

 

[collies99]

This is a radial scheme, therefore directional content is not required.

Query on why there are 2 mains, is it because of total ampacity of the bus and both breakers operate in tandem and treated as one.

751A for the feeder is Ok, and 351-7 for bud protection is also Ok as you are probably only using TOC elements. The feeder instantaneous is trickier to coordinate with a feeder fault at the breaker terminals, feeder side.

i would suggest looking st SEL-745 and incorporate the bus ZONE into the transformer zone. It has both TOC snd differential element so thst you csn coordinate with feeder OC and use the trsnsformer TOC as your bus TOC since it is feed from there anyways

The bus impedance may be to small to make fault levrls discernable gtom a nesrby fault on the feeder.

 

[collies99]

Oops, I meant SEL 787 for xformer diff relay.

 

[MeinKaun11]

Gentleman,

My apologies for submitting incorrect arrangement of the switchgear. I have attached the correct arrangement.

There is no tie breaker since the project is a like for like replacement. Based on the new attachment, do you see a requirement for a directional element. The only reason I see a need of directional element if the primary fuse of one the transformer blows away, there could be a backfeed from the adjoining transformer and I am looking for a way to handle that, either by reverse power or directional protection.
Any input please ?

Thanks

 

[collies99]

If your drawing is correct the ystem is still radial and you do not need a directional protection.

If the two breaker normal operating mode is both closed, then power is shared between the two transformer.

If they have fixed tap you have no issue, else you need block operation control scheme if the are in disagreement. The resulting current is not normally detected / prote4cted via directional elements.

Directional elements on transformer are for reverse power flow in the case 2 transformer feed a major but with tie close breker close or no breaker at all but feed from 2 lines or 2 sources. Directional protection would then operate if a source is lost and power would reverse through the transformer to feed the source end from the other source, this you do not have.

As we are guessing, what is the operating mode of the 2 breakers, what is the total load on the bus, what is the rating of the transformer and is the drawing totally complete ?

 

[waross]

If there is a fault in one feeder or one transformer, the other transformer will backfeed into the fault. Reverse power protection will provide a backup to transformer protection as well as protection in the event of loss of supply to one transformer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[slavag]

You will use 67 functionality in case of paralleling work of transformers.
Direction from LV side to transformer will be with other setting, than non directional.

 

[collies99]

Waross, Slavag:

In my shortcoming, I still failed to see the effective use of 67 device with a radial source feeding a parallel path into a common bus.

 

[slavag]

OK.
Both transformers are operated in parallel.
Please pay attention, without coupler ( bus tie ).
Fault in transformer 1, fuse will blown, but fault will feeded from transformer 2.
in this case we would like fast as possible LV breaker trip.
Best solution is 67 with time setting 0 or selectivity with fuse.
Non directional stage is selectivity with outgoing feeders protection.

 

[collies99]

Thank you !

 

[MeinKaun11]

I appreciate your time gentleman. So the recommendation is that I should have directional element enabled on both incomers breakers.

Any suggestions on reverse power ? will that do the job.

Note: There are no taps on these transformers and both transformers are operated in parallel.
Each transformers are rated at 3/4MVA, 4.16 kV and incoming breakers are rated at 1200 A and bus is rated at 2000 A.

 

[collies99]

You still have to work out the fault current levels on the high side as,fed from the other transformer. What ever the minimal current for phase and ground fault. If the high side is delta then you still need fault numbers for a low side terminal fault or as sensitive as possible for neutral end fault in the winding.

dwg does not have enough detail.

element 32 can be set up to operate below the required fault level, it is a directional device.

good luck

 

[slavag]

I'm not sure, if fuses is good idea for 3-4MVA transformers, breakers with protection on the HV side.
I would recommended you for such application use 87T, differential protection, isn't so expensive today .

Reverse power possible use as indication of fault on the HV side, in case of motor load on the LV bus.

 

[davidbeach]

Reverse Power is good for machine protection and enforcing tariff provisions that call for Reverse Power relaying but are very poor at fault detection and clearing. First off, there's no real power, to speak of, in a fault. Reverse MVAr would be more likely to see the fault, but probably way too slow. The SEL-751 (not A) has directional overcurrent and can do what you need.

 

[collies99]

If cost is not an issue, then high side breaker is the preferably scheme.

That configuration then becomes close to what a distribution substation looks like with the addition of a low side, operated in the close position. 87T would then become standard protection scheme along with bus O/C and feeder O/C potection.

There would still be no need for firectionsl protection since it is still radial feed into the sub.

Fault into thr transfirmer zone are back-up vis breaker failure which all will discover are much faster than any coordinated back-up protection from the adjacent / upstresm zone.