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

 

[davidbeach]

Voltage levels? Governing code?

 

[dfdt]

David,
I am talking about 110 & 220kV systems and as such there are no rules or codes mandating in this regards. Its upto individual TNSPs to decide their practice.

 

[davidbeach]

At those voltages I'll guess you're not ANSI/NESC. My personal experience is that it won't make a heck of a lot of difference. That's based on air insulated buswork well above your head and a reasonable set of remote zone 2 times.

 

[dfdt]

I agree with you as chances of surviving a bus fault is practically non existent at 110/220kV regardless if its cleared in ~100ms via busbar prot scheme or via remote end in zone 2 times of ~400ms.

 

[dfdt]

Is there a graph or a chart showing fatality survival rate Vs fault duration or current?

 

[davidbeach]

If the person is part of the fault they're toast. Period, full stop. But if they're merely in the vicinity, that's a different matter and I doubt it makes a lot of difference.

 

[krisys]

This is a good safety practice. So why you want to take risk?
First rectify the problem and then enter the substation.

Else,

Perform the Task Risk Assessment (TRA) and then if the TRA findings are safe then take a call to enter.
Like, if the weather conditions are good, no heavy wind, day time and similar factors shall be considered to be favorable.

 

[dfdt]

That iss true Krisys but some times one has to enter the switchyard to trouble shoot the very cause of busbar protection being out of service i.e. loose links in the junction box causes CT supervision to pickup which inturn shorts the scheme as designed. How do you handle such situations

 

[Mbrooke]

I'm confused. It sounds like its being said a bus fault will kill anyone in the vicinity, however my understanding is that the incident energy at ground level is actually very low as compared to say high current LV switchgear enclosed in a building.

 

[krisys]

That is the reason, I said perform TRA. Identify what is the task, check the weather condition, availability of competent people etc. Then take the call, whether to go ahead with entering the yard.

Don't forget to mitigate the perceived risks and document it and take approval from the line manager before proceeding with the job. So the risk will be spread with the concerned people, instead of one person assuming all the risk.

This is how we work.

 

[crshears]

i guess there must still be places in the world where 110 kV / 220 kV busbars are equipped with only single protection schemes...

All of ours are dual-protection-redundant; even the really old sh*t is equipped with both a primary and back-up protection, as a minimum...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Mbrooke]

x2, what protection system is used? How many CT circuits? Every single substation around here and those I have drawn up always have at least two buss bar protection relays per zone of protection. IE, a typical 4 bay single breaker single buss station would have a GE B90 low impedance relay for primary protection and an SEL587Z high impedance relay as a backup. When one is disabled the other takes over.

 

[dfdt]

Well in our part of the world it's getting increasingly difficult to justify redundant busbar protection schemes and there is some sort of backup via zone 2 from remote ends.

 

[Mbrooke]

Is this because you are using dynamic bus zones? In a single bus zone is very economical to implement a second relay.

 

[dfdt]

Not sure what you mean by dynamic bus zone. If we need to duplicate low imp BBP scheme then it means we need to lay new control and CT cabling from individual bays back to the control room which in my view would cost arm and a leg. We can justify redundant busbar schemes only if we can demonstrate that clearing bus faults via remote end zone two time of 400ms will jeopardise the system stability.

 

[davidbeach]

Relay maintenance compliance standards and other considerations means that it is becoming increasingly difficult to contemplate any transmission protection that isn't fully redundant.

 

[Mbrooke]

Not sure what you mean by dynamic bus zone.

How do you use these bus bars or more exactly does does any single breaker have access to only one bus bar or does any single breaker have access to two buss bars? If the latter, is one bus bar a spare or are half the breakers placed on one and the other half on the other?

If we need to duplicate low imp BBP scheme then it means we need to lay new control and CT cabling from individual bays back to the control room which in my view would cost arm and a leg. We can justify redundant busbar schemes only if we can demonstrate that clearing bus faults via remote end zone two time of 400ms will jeopardise the system stability.

At minimum you can add a relay in series with the old one.


Question. How many CT windings does each current transformer have? And how many CT circuits do you bring back to the relay house from each current transformer?

 

[dfdt]

True but in the eyes of general public TNSPs are perceived to be old plating their assets

 

[Mbrooke]

TNSPs?