Adding bus differential to 230kV overhead bus

[rockman7892]

We are currently looking into replacing/modifying some 230kV OH bus in a customer owned substation and in the process of doing so I was wondering if it made sense to add bus differential to the OH bus system.

The attached One-Line shows the existing 230kV OH customer owned bus being fed by two incoming breakers (52-1 & 52-2) which are supplied by a local utility transmission line. All breakers and bus shown on the one-line are owned by customer, with the utility having a line differential circuit wrapping on the load side of each breaker going back to their local switchyard in close proximity.

Currently the only bus protection resides with 50/51 settings in the 52-1 & 52-1 breaker relays with the Instantaneous setting set with no delay, but above the secondary let though current of any of the transformers in order to avoid nuisance tripping for secondary faults. I'm curious to hear weather or not that instantaneous setting is considered adequate protection for the bus or if adding a buss differential would add significant protection? There are a number of spare CT's already available for use that are currently wired back to the main substation control panel. It would likely just be a matter of adding a relay to existing panel, connecting CT's, and integrating necessary controls.

Interested in hearing others thoughts

Thanks

 

[davidbeach]

I'd certainly add bus protection. I'd even make it redundant. I'm surprised at the lack of redundancy all the way around, particularly at 230kV.

 

[Mbrooke]

If its that easy- then its certainly worth doing. I mean your 13.8kv already has bus differential protection.

 

[rockman7892]

davidbeach

Thanks for the response. I’m curious to hear more about the added redundancy you are suggesting.

What additional protection advantage does this buss diff provide over the INST setting on the main breaker relays? Won’t these INST settings and buss diff essentially have the same clearing times?

 

[davidbeach]

So, I work for a utility and live under the requirements of PRC-005, among others. Relays fail but that shouldn’t be a cause for dumping customer load. Testing is mandatory, but outages are hard to schedule. Redundancy makes everything easier and makes easier to sleep at night.

For a fault on one of the two FPL sources, it looks like around half the fault current will be seen by the 351, will it trip? If so, it’s a horrible approach.

A 230kV bus and yard is so expensive that doing the protection right vs. doing it cheap is just a rounding error in the overall budget.

 

[Mbrooke]

Maybe I'm just tired, but I only see one FPL source?

 

[davidbeach]

Must have been looking at it on too small a screen. You’re right, only one source from outside that drawing. So never mind about a fault on the “other” source.

 

[DTR2011]

If I’m reading the dwg correctly, I think you have go utilize LoZ protection due to the unequal CTRs.

 

[Mbrooke]

I'd imagine the disconnect between Bus B2A and B2C or between B2C and B2B is normally open which would require dynamic differential.


I also see "To FP&L Bus Differential Protection" CTs below breaker 52-1 and 52-2, so I'm making a guess that Bus B2 has some type of differential protection perhaps including the incoming line.

 

[davidbeach]

Add one more breaker on bus B2 and eliminate B2A/B2C/B2B along with a net reduction of 3 motor operated switches. The SEL-351s can be eliminated and an issue with a single transformer only trips that transformer.

 

[rockman7892]

Davidbeach

With adding a 3rd breaker would you not still want to have some sort of tie capability on 230kv at load side of breakers in the event of a breaker being out of service. Wouldn’t you still need to be able to feed any transformer if a respective transformers breaker is out of service?

I don’t see where the reduction in 3 switches comes from. You will eliminate tie switches but won’t you need to add two for the new breaker (line & load) and thus have a net of zero?

Do you think the cost of adding a new breaker is more cost effective than replacing all of the 230kv switches which the customer is
Currently looking at doing?
In the current arrangement do you gain anything by having bus diff over the INST setting on the 351’s?

 

[davidbeach]

With four breakers you could also do a 4 position ring so that any one breaker could be out of service at any given time without impact to the rest of the ring. With a ring, the entire bus is part of one position or another so there wouldn't separate bus protection needed. In the present configuration it appears that any transformer fault results in tripping of all of three transformers; the ring would allow each transformer to be tripped independently of the others. If the process can continue on a reduced basis that could be worth something, but if the process requires all three to do anything then the design as shown may not matter.

In a simple bus installation of transformers connected to the bus there's no need for a disconnect between the transformer and the breaker on either side. Anything you want out - high-side breaker, transformer, or low-side breaker - requires the other two out anyway so the number of disconnects can be reduced compared to having one on each side of each breaker.

With just one source, the 351s might be sufficient, but I haven't studied it in any depth.

 

[Mbrooke]

A transformer fault appears to take out at most two units. I only say that because I can't imagine all the disconnects running normally closed. I would run 89-M3 open. And I'm imagining the trip coil output on the SEL787-T3 and other two TX differential relays is switched based on the disconnects.


Around here something like this would get 3 circuit switchers, at least today. I personally would move breaker 52-2 over to T3 and place T2 on a circuit switcher.


But to be frank your current instillation is fine in that regard- I'd be more concerned about replicating your relays and making sure you have DTT over to FP&L.