Switchgear Protection Suggestion
Switchgear Protection Suggestion
(OP)
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,
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,






RE: Switchgear Protection Suggestion
RE: Switchgear Protection Suggestion
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.
RE: Switchgear Protection Suggestion
RE: Switchgear Protection Suggestion
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
RE: Switchgear Protection Suggestion
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 ?
RE: Switchgear Protection Suggestion
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Switchgear Protection Suggestion
Direction from LV side to transformer will be with other setting, than non directional.
RE: Switchgear Protection Suggestion
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.
RE: Switchgear Protection Suggestion
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.
RE: Switchgear Protection Suggestion
RE: Switchgear Protection Suggestion
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.
RE: Switchgear Protection Suggestion
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
RE: Switchgear Protection Suggestion
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.
RE: Switchgear Protection Suggestion
RE: Switchgear Protection Suggestion
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.