Protection for 11/0.433 1 MVA ONAN tranformers operating in parallel 3

[durgeav]

Hi,

I have come across a very peculiar case during study of a power distribution system at one of manufacturing sites:

The power distribution system consists of 3 Panel 11kV VCB Switchboard (1 Incomer+ 2 Outgoings)equipped with Siemens 7SJ6015 relays. Each of outgoings feed 2- 11/0.433kV 1 MVA transformers which are operated in parallel.

The low voltage (415V) switchboard incomers (2) and bus-coupler(1) are air circuit breakers with LSIG solid state releases.

The transformers are with off circuit tap changers.

The concern here is do I have to provide a directional over current and earth fault element?

If yes, then where do I provide it- on LV or HV side?

Regards

durgeav

 

[ScottyUK]

Do you have to? No. Would it give you better ability to detect and clear a fault? Yes. Does it improve your security of supply? Yes.

Where do you put the relay? Do you have access to a VT on the HV side?

 

[davidbeach]

Transformer differential protection would serve you far better than directional overcurrent. You're only looking to keep from tripping both transformers for a fault in one of them; with only one source you aren't concerned about faults back on the source. The differential wouldn't require a VT and it would be fully selective, while a directional overcurrent scheme would gain its selectivity at the price of much slower tripping.

 

[melspuds]

I take it that the tie breaker is closed on the low voltage side. Can the loads on each low voltage buss setup so that the tie breaker stay open.
If the breaker is left open in normal operations then the transformers will not be in parallel.
Just a thought,
Good luck,
Dave

 

[durgeav]

Guys thanks for the advise.

This is a protection coordination study I am working on with review of protection schemes. The electrical infrastructure is too old (1959). The coordination of relays and releases in out of sorts.

I was planning to have the coordination in such a manner that if there is a fault on any of bus-section, then the bus-coupler would operate first to isolate healthy bus section from faulty one, followed by the incomer for respective bus section.

Scotty- VT is available on incomer of 3 Panel 11kV VCB Switchboard. Can you please comment on what if I don't go for directional OC release?

David- Transformer differential sounds a good solution but I won't be able to justify the cost.

Dave- I was exploring the option after running load flow studies and would be suggesting it. The only concern is that whether the site can afford a shutdown to shift feeders from one bus section to other.

Regards

durgeav

 

[krisys]

First of all this is not a correct design. As long as the primary source has a common breaker for both the transformers, we should not consider that the redundancy is available. Practically it works as one 2 MVA transformer.

Now for any problems in the transformer you have to trip the 11 kV breaker, thus switching off both the transformers.

Instead of breaking you head on this, you may advise on swapping of the primary connections for one each of the transformers. Probably this would give some degree of redundancy to the system.

 

[ScottyUK]

This is a fairly common design, with pro's and con's as with other designs. You're mistaken to call it 'incorrect'.

 

[durgeav]

Does the approach mentioned in my previous post on this thread good to provide adequate discrimination without use of directional overcurrent relay?

The incomers for LV panels have same LSI settings. When bus-coupler operates in event of fault on bus-bar then the sequence of operation shows that both incomers trip at same time after bus-coupler tripping.

Would it be safe to assume that if there is a fault on any of bus-section then bus-coupler would isolate the faulty bus-section and then incomer of that particular bus-section will only operate?

SocttyUK- Can I have a directional overcurrent relay on LV side,i.e. the CT and line VT just before incoming LV circuit breaker?

 

[ScottyUK]

Yes. You could use that position as a DOC element.

Does the bus section switch even have overcurrent? Often these are a dumb switch without protection.

Without DOC you risk losing both circuits to a single fault, with the one closest to the fault tripping first.

 

[durgeav]

The bus-coupler also has LSIG release.

Need your advise on ground fault protection on LV incomer and bus-coupler.

Currently these have been disabled. The outgoing feeders of LV board do not have earth fault protection device.

The largest outgoing feeder is 630A Air circuit Breaker.

 

[krisys]

If the LV side of the transformer is wye connected, you can use Restricted Earth Fault (REE-64R or Ground Differential protection-87N) protection to isolate the transformer. But in your case, if the transformer is tripped, you loose complete supply to the board.
However to isolate the faulty bus, you may use the partial differential protection.

 

[ScottyUK]

You need some sort of earth fault protection for the transformer. If you don't have E/F on the outgoing circuits then you may have difficulties where an earth fault on a load circuit may trip the incomer if that relay responds more quickly than the overcurrent protection on the outgoing circuit, but that doesn't remove the need to protect the transformer.

Sounds like you need a proper protection study by an engineer who has access to all the information and who understands how the protection relays interact.