Protection for 11/0.433 1 MVA ONAN tranformers operating in parallel
Protection for 11/0.433 1 MVA ONAN tranformers operating in parallel
(OP)
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
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






RE: Protection for 11/0.433 1 MVA ONAN tranformers operating in parallel
Where do you put the relay? Do you have access to a VT on the HV side?
RE: Protection for 11/0.433 1 MVA ONAN tranformers operating in parallel
RE: Protection for 11/0.433 1 MVA ONAN tranformers operating in parallel
If the breaker is left open in normal operations then the transformers will not be in parallel.
Just a thought,
Good luck,
Dave
RE: Protection for 11/0.433 1 MVA ONAN tranformers operating in parallel
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
RE: Protection for 11/0.433 1 MVA ONAN tranformers operating in parallel
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.
RE: Protection for 11/0.433 1 MVA ONAN tranformers operating in parallel
RE: Protection for 11/0.433 1 MVA ONAN tranformers operating in parallel
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?
RE: Protection for 11/0.433 1 MVA ONAN tranformers operating in parallel
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.
RE: Protection for 11/0.433 1 MVA ONAN tranformers operating in parallel
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.
RE: Protection for 11/0.433 1 MVA ONAN tranformers operating in parallel
However to isolate the faulty bus, you may use the partial differential protection.
RE: Protection for 11/0.433 1 MVA ONAN tranformers operating in parallel
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.