Ring Bus - Short Circuit Values - Please Help 2

[Los123]

I need Help.

Using ASPEN One Liner I found the 3-phase, 2-phase and 1-phase short circuit values for each bus in my system.

However, my real system is build on ring buses.

The question is how can I calculate each breaker (in the ring bus) fault duty. I think this is called IBA (individual breaker analysis)

If I have 41kA 3 Phase fault (from Aspen), at a bus, made up from 9 breakers. What is the duty at each breaker ?

Thank you !

 

[rcw retired EE]

If the ring is closed, won't all of the breakers be subject to the same duty? When a breaker next to a faulted line opens, the adjacent breaker could feed the complete fault current available from the other feeders to the fault.

So for a ring bus, the breaker duty is equal to the sum of the fault currents from all feeders connected to the ring minus the smallest fault current contribution of the two feeders on either side of the subject breaker.

Most of the time it is easier to just assign the ring bus total fault current.

 

[cuky2000]

Assuming the calc is OK, the next standard interrupting rating is 63kA.

 

[Los123]

to rcwilson.

A little bit more info about my problem:

Using Aspen One liner I fault the bus itself, and not the adjacent feeders. I get 41 kA which is 1 kA more then my lowest breaker rating (40 kA).

I thought that all breakers would be subject to the same duty. However I was told that they are not. I was told that if I use "individual breaker analysis" i would find out that for each breaker the duty is below 40 kA. (?!)

So,do you think this still applies: "the breaker duty is equal to the sum of the fault currents from all feeders connected to the ring minus the smallest fault current contribution of the two feeders on either side of the subject breaker. "

I am new to power engineering.

Thank you,

 

[jghrist]

Seems to me that a breaker will have to interrupt the sum of all of the fault contributions from lines other than the one on the faulted bus section. The rest of the current may split until one breaker opens, but then the second breaker to open has to interrupt all of the current. The contribution from the line on the faulted bus section does not flow through any breakers.

If any line has no fault contribution, then the full bus fault will have to be interrupted by the breakers protecting this line.

 

[RalphChristie]

Not sure if I read your question correctly, but...

You simulate a fault at a bus.
The bus does have 9 breakers.
I assume with the term "ring-bus" you have two incoming feeders and 7 outgoing feeders.
If we assume each incomer contribute an equal amount of fault-current, then the contribution would be 20.5kA though each incomer during a fault. (Total fault current is equal to the sum of all the incoming feeds)
But each outgoing feeder would be subjected to 41kA, should the fault be on the outgoing side of that breaker.(supposing there is no fault-contribution from any downstream motors)
You have to simulate the fault with just one incomer closed, to see what will happen should one incomer trip.

I am not familiar with Aspen, but Digsilent shows the fault-current through each breaker that contributes to the fault.

 

[dpc]

This may be a limitation in ASPEN. Some programs (e.g. EasyPower) have a special breaker duty analysis function that determines that actual current through each breaker (not just the bus fault current)and then performs a complete ANSI C37 breaker short circuit analysis to determine the breaker duty.

I suspect other software has similar capabilities, but maybe not ASPEN. You might need to set up different switching scenarios to make sure you have found the worst case.

 

[rcw retired EE]

My comments about the maximum fault current seen by each breaker are based on simple math. A 9 breaker ring bus has 9 connections (feeders, incomers, transformers, whatever) between the breakers. Each breaker has a connection and a breaker on each side of it. Each of the connections could have some fault contribution to the bus. The maximum fault current through any breaker occurs when one of the adjacent breakers is open and there is a fault on that connection. The current from the other 8 connections flows around the bus and through the last breaker.

So the maximum current the breaker can see is the total bus current minus the smallest current contribution from the adjacent connections, because the faulted line does not deliver any current through the breaker.

I am not familiar with Aspen, but it must have some method to calculate the individual breaker duty. You may have to open each breaker, one at a time and determine fault levels.

I recommend doing a hand sketch, showing the contributions from each feeder and doing a quick hand calculation for each breaker's maximum current. The exercise will give you a feel for what is happening.

All of these comments assume that the ring bus impedance and breaker impedances can be ignored. I think that is a good assumption.

 

[dpc]

I believe with ASPEN that you need their "Breaker Rating Module" in order to do accurate breaker duty analysis.

If you are getting close to the breaker SC rating, you really need to refer to ANSI C37 (assuming breaker is tested to ANSI) for details on their required procedure for calculating the short circuit duty. It involves more than a standard short circuit calculation.

 

[jghrist]

I agree with rcwilson (I think it's the same thing that I said). In subtracting the current contribution from the adjacent connections, realize that it has to be a vector subtraction because all of the current contributions will not be at the same angle.

 

[Los123]

Thank you all !

Last question:

What would you recommend as a good reference book for a new transmission engineer ?

Thanks

 

[davidbeach]

Assuming North American Practice:

While it's been way too long since it's been updated, and it costs too much, the ABB (formerly Westinghouse) T&D book is considered by many to be the standard reference. Blackburn's Protective Relaying and Symmetrical Components books are both good references, as is Elmore's Protective Relaying book. The Alstom (now Areva, but the book still says Alstom) Network Protection and Automation Guide is much more up to date than the T&D book, but is also orientated much more to the IEC world. I presume you have a good Power System Analysis text from school. Anderson's Analysis of Faulted Power Systems covers most any off-nominal operation you might encounter.

After you get all of those finished, come back for some more suggestions. ;-) ;-)