Busbars Insulators

What does IEC 865 say?

Thanks for your reply.

IEC 865 just refer the short-circuit current as a variable to calculate the electrodynamic force, and not clarify the assumptions to calculte it.

What do you think you should do?

In my opinion we should consider just the max of two values, as the sum of two current just happen in one point.

As you can see in the image, the lenght l1 are "affected" by 30kA current and the length l2 by 20kA. The point where the short-circuit happen the length is about zero.

As the electrodynamic force is the result of total length between the two insulators, the worst case is considering the 30kA and the l1+l2, and despite the point where the short circuit happen.

You agree with this explanation?

P.S.: Sorry for some mistake in my english.

I don't intend to check how the short-circuit was determined, but I am convinced you verified all case and this will be the maximum. I guess the maximum could be close to generator terminals. I think you don't have to worry about the insulator involved in grounding fault-it is already exploded.

But for the rest of the insulators you have to use the maximum.

When a ground fault occurs on an overhead transmission line in a power

network with grounded neutral, the fault current returns to the grounded neutral through

the tower structures, ground return paths and ground wires.

If each tower is locally grounded then ,when a fault appears, a part of the ground fault current will get to the ground through the faulted tower, and the rest of the fault current will get diverted to the ground wire and other towers.

Therefore the current will be different in different point.

I suppose your system is three phase.

Also you are designing or sizing insulators for mechanical strength against effect of fault current flowing through the bus or buses.

First of all, stress on insulator due to electro-magnetic forces, you have to consider the maximum of two values and Peak Value not RMS value of fault current (I peak = 1.414 * I rms).

Secondly, You have to decide which type of fault have damaging effect on the Insulators and supports.

We know all insulators are strong in compression and weak in tension.

Ground fault will have less damaging effect on the insulators. Reason being the earth fault returning to source (Grid or generator) will flow in direction opposite to direction of fault in the bus (through the ground or Bus Duct in your case). The two currents, flowing in opposite direction will create two opposing magnetic fields - attracting each other. (use right hand thumb rule, thumb along direction of current and fingers of closed hand direction of magnetic field around conductor). This will result in COMPRESSIVE FORCES on the Support Insulators.

A 2 phase or 3 phase fault will have more damaging effect on the Insulators. Because the currents Will result in either pulling inward or repelling the two bus apart (Depending upon instant of magnetic field or currents in different phases). So effect of phase to phase or 3 phase fault is like a cyclic bending of insulators in either direction at a rate 100HZ resulting in failure due to fatigue stress in shafts).

Your case when designing is more of a case of mechanical design that electrical design. It will not be possible to accurately design the system, unless sufficient data of insulators being selected is available for tensile as well as fatigue conditions.

Hence best is to go for Type Tested Bus Bar System tested for Maximum fault level expected in the system and giving some margin for future expansion.

Also use full design information is also available in ABB Electrical Handbook, I had seen it but did not ever use (earlier it was known as BBC Electrical Design Handbook and was available free of charge to EPC and packagers from BBC. You may find this on net ).

Best of luck.

You answer was enlightening.

Thanks

The max value not the sum is appropriate. The other consideration not mentioned by others is that the dynamic strength of a insulator is quite different from the static strength. The problem is that the static strength of an insulator is most commonly given since it is easiest to test. Complicating this is that sometimes only a tensile and compressive strength is given when a cantaliver or sheer strength would be more appropriate. With a very brittle insulator, like poor quality porcelain, the dynamic strength can be quite a bit less than the static strength. A high quality porcelain or glass or composite however might have dynamic strength much more than the static strength even as much as 7 times. When we (GE) tested transformers for actual short circuit test survivability so we could guarantee that, we had to throw out a lot of old formulas that had been used for years. The results were a mixed blessing of cost reductions, product improvements, and marketing strength until others caught up. No one really got caught up, but a few claimed they did. The thing we found that helped was that the dynamic strength of materials was much higher than predicted.

I agree with others that a multiple phase to phase short may be worse, but my experience in bus duct failures is that the line to ground and differential are the fastest relays. Also if the duct does not have good isolation, which few do, the fault will involve all phases on phase to phase and phase to ground within less than a second. Which relay takes it out is pretty much a relay race. I have seen the cases where the relays are fast enough that the damage is minor. In fact, at times it is hard to find. What I have not seen are insulator mechanical failures. Most bus ducts are designed with plenty of extra strength in the supports. We frequently needed to replace supports from flash damage, but I have never seen a broken support from a fault. Forklifts are another story.