HV bus coupler

[countingcrow]

Hi,

We have an 11kV Ring Mains system taken from 2 incomers from the grid. The ring feeds some 12 transformers (which then transform down to LV for the production buildings).
Located on the ring is a generator site currently delivering around 95% of site requirements.
The 2 incomers are coupled via the bus coupler but the loop is closed as well.
As the previous electrical engineer has moved on and I am new here - and there are no records about how the system was designed - i have no idea why the system has been set up this way.
To my mind either the coupler should be closed or the loop should be closed but not both. For one thing i would assume it increases the fault level.

Could it be that the reason is the on-site generators?
(they are located halfway around the loop at a single substation)

Other than that I really have no idea.

Regards

 

[Marmite]

If the system was designed to run with everything closed then I would expect to see unit protection installed, such as Solkor or Translay. Each of the substations on the ring would have circuit breakers equipped with unit protection. This would provide a high standard of security and fast tripping for faults. If your ring substations are equipped with basic ring main units, ie two ring switches and a tee off circuit breaker/switchfuse, and your main incomers have basic IDMT protection then the system should be run with an open point at one of the ring main units (probably half way around the ring). If not, you will likely lose everything if you get a fault, and will be off supply until you can identify and isolate the faulty section.
Regards
Marmite

 

[RRaghunath]

Countingcrow,

The arrangement you described is rather a common one in industrial substations. The two grid incomers paralleled at the bus through bus coupler is preferred, as trip of any of the incomers due to problems upstream is not likely to affect the loads.

Having said that, the protection scheme needs to take the above in to account and if not done, an uncleared downstream fault or bus fault in one section of the busbar is likely to result in trip of both the incomers.

This is done by adopting Partial differential principle based OCEF protection for the incomers.

 

[countingcrow]

Thanks Gents,

We do have solkor protection on the incomers, but the RMUs are just the basic ones you describe.
I think the incomer protection was changed relatively recently so i'll try and dig out data about why they were changed.

Much thanks!

 

[slavag]

Hi.
From my excpirence.
We use only open ring with simple O/C and E/F relays. Trip sended by infeed protection relay only and to infeed CB only. Other relays send trip signal to PLC or SCADA and system disconnect fualty segment and back infeed to work. Simple and cheaper solution.
Regards.
Slava