OIL CIRCUIT BREAKERS

[reactive]

Searching for opinions with this one.

We are doing a study on an existing plant and have found the symmetrical breaking capacities of some indoor metalclad Hawker Siddeley 11kV OCBs to be marginal (18,4kA). The prospective breaking fault current has been considered at 5 and 10 cycles (50Hz) (including motor contribution) giving respective maximum overloads of 13% at 5 cycles and 5% at 10 cycles.

Upgrading of the breakers would be an extremely costly excercise and the client does not have the money available. The other option is of course fault limiting reactors but this is also costly.

It would be interesting to hear some opinion on whether the potential 13%/5% breaking overload is acceptable, bearing in mind that the breakers, although well maintained are pretty old (20 years). Does anyone have any knowledge of the old manufacturing standards and what factors of safety were used when rating the OCBs?

Thanks and best regards

 

[dpc]

Speaking only for myself, this would not be an acceptable situation. As the responsible engineer, I would have to recommend that this switchgear be upgraded, or some other method found to reduce the prospective fault current.

Once the risk has been identified, I don't think you can ignore it.

Just ask yourself if you would be able to explain your decision in court, or worse yet, on "60 Minutes".

 

[electricpete]

Sorry I don't have any useful input. I agree with dpc, it sounds ugly. I'm sure it's not your decision on whether to upgrade, but you might want to walk thru a couple of worst-case scenario's of failure-to-interrupt with your client.

 

[peterb]

I'm with dpc and electricpete 100% on this - you have a definite professional responsibility here.
Besides installation of current limiting reactors (or current limiters), it may be possible to reduce the fault level by operating with the buses split in such a way as to minimise fault currents in each section - I suggest that you explore that possibilty fully, as it could be a minimum or no cost option.

 

[cuky2000]

CURRENT LIMITER could be a compromising cost-effective technical sound solution for those special cases when the available fault current grow beyond the capability of existing switchgear.

Those devices are available in the market for few years with several thousands applications. They operate under the principle of continuos monitoring both the momentary value of the current and the rate of rise in less than 3 ms according with the manufacturer performance data.

Could be installed at the main breaker or bus tie that limits fault current and is capable of handling high continuous and fault currents up to 36 kV. There is available rated voltage of 12 kV with 75 kV BIL and various continuos current rating from 1200 to 4500 A.

The down side is if there is any trip, the cartridges need to be returned to the manufacturer. Therefore, in the total cost of this installation the spare parts needs to be considered.

 

[peterb]

cuky2000, I agree (see above) that current limiters could work, but I should point out that in my experience it is an expensive alternative. Both first cost and element replacement costs are high.

 

Considering that these breaker will be bulk oil I would be even less inclined to allow any overload. You really have to consider the consequences of not implementing some form of replacement or CLRs or a limited switching regime. If you decide to replace them, Hawker Siddeley Swgr can still offer vacuum breakers which will fit within the existing cubicles.

 

[jbartos]

Suggestion: In some cases, the current limiting fuses do not allow the fault to reach its calculated or as-built value. Have you considered those? Else, the more appropriate approches have been already posted.

 

[reactive]

Thanks for all the input. Obviously our opinion and recommendation will be that either the OCBs are upgraded or we get the fault level down. However, at the end of the day it is the client's decision whether to implement the recommendation or not and the more info. we give the client, the better position he will be in to make his decision. No need to get the blinkers out.

My biggest concern is obviously the safety aspect.

Due to operational constraints and loadings it is unfortunately not possible to reconfigure the network to bring down the 11kV fault level.

I don't see how current limiting fuses can work. If they are installed on the main incomers, any major downstream fault will take them into pre-arcing and result in total loss of plant. No co-ordination can exist with the downstream relay protection.

Once again, many thanks.

 

[peterb]

The current limiter consists of a current limiting fuse in parallel with a shorting bridge. When the sensing circuit detects a rate of rise of current and an instantaneous value that indicates that the prospective fault current will be above the switchgear rating, an explosive charge is detonated to rupture the shorting bridge. This leaves the current flowing through the fuse, which interrupts the fault current well before the first peak. Yes, the circuit is then dead until the assembly is replaced, but there isn't a pile of smoking metal.
Another option (higher cost again) would be to install current limiters in parallel with reactors - normal operation is with the reactors shorted out & no voltage drop. After a fault the reactors remain in circuit to feed the buses, with the consequent reactor voltage drop.

 

[dpc]

At least in the U.S., the use of upstream current-limiting fuses would not be a legally acceptable solution, unless the current-limiting fuse had been tested in combination with the particular circuit breaker in question. This is certainly the case at low voltages and I am fairly certain it would also apply to medium-voltage.

If this solution did not work as planned, the design engineer would be holding all the cards, and all the cards would be bad.

I understand that the current-limiter makes sense from an engineering perspective, but I just wanted to point out that whoever recommends this solution is buying a huge amount of liability if something doesn't work right.

As an interim solution, you may want to recommend that the owner immediately restrict access around the under-rated breakers.

 

[reactive]

Many thanks for all the input.