Busbar faults

[dfdt]

We have a legacy practice of not allowing peeople to enter live switch yard if busbar protection is out of service and wonder what is your experience or practice in this regards?
thanks for your input in advance

 

[dfdt]

TNSP= Transmission Network Service Providers i.e. Utilities dealing with Electricity transmission

 

[dfdt]

our switchyards are generally disconnector and a half type arrangement i.e. any bay/line can be switched from one bus to another via switching one of the tree motorised disconnectors provided bus coupler CB tieing the busses A & B is closed. Each line has its own CB and line side discoonector.
Disc 1 connecting to Bus A
Disc 2 connecting to Bus B
Disc 3 connecting above disconnectors

 

[Mbrooke]

If bus A and B are normally used (half circuits on bus A half on B), then it is safe to say you have a dynamic buss zone protection system. Remember that when a bus fault occurs the bus relay needs to know what breakers are on the faulted bus so it can trip only those.



In such a case it will be much harder to implement secondary protection, but in theory I guess you could feed all the CT circuits in series with another dynamic relay and wire all else in parallel. That way if one buss relay is taken out the other will still function without issue. Just remember to have CT isolation switches:

http://www.statesproducts.com/

Another possibility is if you are ok with clearing both A and B for a fault would be to convert the existing relay to a single zone which will allow a low cost substitute instead of anther (set) of pricey dynamic relays.

 

[marks1080]

To the author: "GOLD" plating assets has nothing to do with having redundant protections. Especially when thinking about protecting a Bus at a HV station. It's about protecting the system. You can tell your 'TNSPs' they are idiots and dumb. Please post their replys for us all the read

 

[ScottyUK]

marks1080,

It's not the TSO / TNSP folks who are making the assumption of gold-plating, it's 'experts' from outside the industry who lack any depth of understanding but make up for it with their wealth of ill-informed opinion.

I'm facing similar difficulties in the oil sector where people from the chemical industry (cheap = good) are meeting us folks from the utilities (high availability = good) in a culture clash similar to two freight trains colliding. I fear that today's focus on cost-cutting to the detriment of quality may turn around and bite us very hard in the decades to come.

 

[marks1080]

ScottyUK,

The utility I work for just got a new CEO from the Industrial Farming industry. His first statement to the company is that he wants to take us into the commodity world. Day 1 he was able to let everyone know that he has no idea about electricity. It'll never be a commodity based industry until someone figures out how to keep all the electrons inside the grain silo. Never the less we have a 'market-based' system to 'buy and sell' electricity. On a good day we give our excess power away for free to neighboring jurisdictions. On a normal day we pay them money to take it.

From what I've seen gold plating never referred to an electrical system or sub system being over-designed. It refereed to the fact that all the hand rails in the plant were made of solid brass. Some old plants actually had marble floors. That's gold plating. The only time I've seen something way-over engineered in this business were on the generator protections of one of the first nuke plants ever built in my region. I believe the reason for this was because it was the first time using these super expensive generators so they were maybe a little nervous, and the extra cost to over-design the protection was nothing compared to a failure of protection.


There's a book written by an ex-CIA guy, I believe it's called "Confessions of an Economic Hitman" or something like that. He talks about going to third world countries back in the 60's through the 80's on behalf of a North American utility companies to help build the nations power system. His alternative agenda (the CIA agenda) was to over build the power systems so much that the country would be paying interest to a N.A. company for eons to come. That's gold plating from a design perspective. Where we would normally see a Duel bus station here in N.A. some of these countries were sold Quad bus stations, with all the extra transmission circuits that would go along. Basically the systems are quad-redundant from the primary side of things. I would consider that gold-platted design, until i realized that i've just introduced that much more of a security risk to the system by adding more elements.

The Art of P&C is understanding that there's always a trade-off between security and selectivity. It would be difficult to over build a system and call it "gold platted" due to the extra security risk. Anyone who figures out how to increase both at the same time should win a Nobel prize imo.

 

[dfdt]

Mark- Perhaps i used the word "gold plating" in wrong context. The point I was making is that duplicating busbar prot schemes is good to have and not must have unless you can demonstrate that delayed clearance via zone 2 from the remote ends will impinge on system stability i.e. pole slipping of generators or system collapse due to voltage instability etc.

 

[davidbeach]

Or simply that you can't get a outage of a non-redundant protection system. Different places, different rules and considerations.

 

[bacon4life]

Historically our 115kV double bus scheme had a single bus differential relay. In addition to being out of service for maintenance, the bus relay was also disabled anytime any of the circuits were on the bypass position breaker.

The physical risks from either arc flash or step & touch potential are relatively straightforward to calculate once you figure out the zone 2 remote clearing times. The arc flash exposure is low compared to many 480V installations because 115 kV is open air (instead of a box) and because of much larger working space. Ground grids are usually designed for lengthy clearing times, so step and touch potential probably wouldn't demand bus relay redundancy.

Complying with NERC regulations rather than personnel protection is a major factor for installing redundant bus relays. Although NERC regulations might in the long run turn out to provide more dependability, in the shorter term the regulations have had detrimental effects on security and complexity.

 

[ScottyUK]

marks1080,

We haven't seen that kind of thing in UK plants since the days of the CEGB, and even then I'm not sure we ever had a marble turbine hall. Certainly not within my lifetime.

I think what I'm seeing here is that necessity in one industry is regarded as 'gold plating' by another. I agree with you that duplicate schemes are an operational necessity rather than a luxury. Your last few words are right on the mark.

 

[davidbeach]

We have the same "discussion" in house. IT thinks that 80% is good; while T&D thinks that 99.999% begins to approach good enough. Remember, an electric utility that "only" achieves 99% leaves the lights off for 14 minute day. 99% is a great grade; it's a major fail.

 

[marks1080]

Whenever I'm stuck in a situation trying to explain these concepts to people with business degrees I usually fall back on "what's the worst thing that will happen with a single failure - ie: a failed bus protection on a non-redundant system?" The typical answer is a look of confusion, followed by a look of absolute fear when i give them the answer. Actually, I'd be lying if I said some part of me didn't enjoy those moments.

 

[marks1080]

bholas: In my world non-redundant bus protection at a high-voltage station would be completely unacceptable, and our operators would seriously consider taking the entire bus out of service. There's always a case to be made for one off scenarios, but its not usual and typically would have a lot of other factors to consider.

If you end up with a fault on a HV bus without instantaneous protection BEST case scenario is a massive over trip taking out many more zones than necessary and potentially causing system stability issues. WORST case scenario is much, much, MUCH more expensive, not to mention potential health and safety risks. The industry really looks down on killing workers these days. At least in my jurisdiction.

 

[Mbrooke]

If you end up with a fault on a HV bus without instantaneous protection BEST case scenario is a massive over trip taking out many more zones than necessary and potentially causing system stability issues. WORST case scenario is much, much, MUCH more expensive, not to mention potential health and safety risks. The industry really looks down on killing workers these days. At least in my jurisdiction.

I take there is no way to configure these relays so they will trip the breakers immediately connected to the buss instead of relying on remote zone 2 breakers?

 

[ScottyUK]

Yes, and that's what they are designed to do. But when a relay fails, or a relay is out of service for maintenance, then you either need a duplicate relay scheme to keep the high speed fault clearance capability, or you rely on slower backup protection.

 

[Mbrooke]

I mean the feeder (line) relaying, not the bus differential.

 

[davidbeach]

One could, in theory, put a contact on the relay for each position that closes when the relay sees a reverse fault and wire all of those in series to trip the bus lockout. Directional capability on transformer relays is relatively recent, so most transformer positions won't have the capability and the scheme won't work. Redundant bus protection is the preferred solution.

 

[dfdt]

I think we have digressed a bit but never the less some great contributions and comments.The question is will you be comfortable to let workers move around in live switchyard when busbar protection is out of service?

 

[MatthewDB]

Why is the bus protection specifically mentioned vs. any other protection?

How about a transformer, reactor or cap bank on zone 2? Are they not a bomb?

Or how about a line position outside the bus zone? Isn't that a risk too?

 

[dfdt]

The reason we were discussing busbar protection scheme is that it covers bulk of switchyard in terms of area or real estate whereas line scheme or any other schme would cover limited area of the switchyard. yes agree the risk is similar