Medium voltage I.C. ratings from multiple sources

[salm10]

I have a situation where my client has a main-tie-main distribution setup at 4160 volts. The tie is normally open. When doing maintenance on one main breaker or the other, they typically close the tie and then open whichever main that is scheduled for maintenance, thus for a few seconds, the power sources are paralleled.

I did some fault calculations. Each source has a fault contribution of 138 mVA. The breakers are older magna-blast units rated at 250 mVA. The combined fault exceeds the rating of the breakers, but if you draw the circuit on a piece of paper, the fault current through either main or either bussing will never exceed 138 mVA, except for the exact point where the fault exists. If the fault occurred due to a failure within the mechanism of the tie breaker, then I could see where this breaker may have a violent failure during an operation.

This client is considering replacing this and other similar substations over the next few years. Upgrading all breakers, including his feeders, to 350 mVA represents an upcharge that easily is in excess of $100,000. Is this necessary? My feeling at this time is to recommend that only the ties have a 350 mVA rating, and the busswork is braced for 350 mVA.

Thanks for your help!

 

[kingpb]

It is common practice, and acceptable from an engineering standpoint to use only the through fault current. It has also been understood that the parallel of the the sources is very short and in the past, it has been accepted that the risk of fault, on the bus, during this momentary parallel operation is minimal, and therefore accepted that the bus is rated for only the fault current of one source.

The closing of the tie and tripping of the main signals should be done automatically, and not left up to an operator. This is to help guarantee that the operator doesn't go to lunch prior to opening the main, thereby leaving the sources paralleled indefinitely.

 

[salm10]

Kingpb, thanks for your opinion. The replacement gear will be designed with a remote operator that requires a permissive synch check signal, so the transfer time will always be one second or less. This is much better than their present mode of operation which is a manual sequence of steps.

 

[ron]

First my opinion.... Logic indicates that during closed transition for any length of time, there is a possibility of a fault during that period, 6 cycles, 30 cycles, 5 seconds et al. The SWGR should be designed to withstand and interrupt that amount of current coming from both sources.

Then there is the other opinion ..... There is no code or standard that specifically addresses this condition. I argue with my colleagues constantly regarding this issue, as it is almost always an issue, whether it is two utility sources, or the utility and the 'house' generators. Many of my colleagues feel that if it is less than a few seconds, not to consider that case. Some of my other colleagues agree with me.

The applicable code is NEC 110.9. It states that the gear should have a rating sufficient for the nominal circuit voltage and current on the line terminals of the equipment. So the question becomes, what is the available fault current at the line terminals of the equipment? If the fault occurs upstream of the SWGR, then the fault current at the line terminals will be equal to one source's contribution plus any building motor load. If the fault occurs at the tie breaker (or further downstream), the fault current at either line terminal will be equal to one source's contribution.
The catch is what if there is a fault on the line terminal of the equipment. Then you will have both sources and building's motors contributing. What is the likelihood that a fault will occur at the line terminal/bus, or during a few second closed transition?
It is for a designer to ponder the possibilities.

I understand that if you search in previous code cycles of the NEC ROP and ROC's, there was an attempt to have an exception listed for 110.9 that would eliminate the requirement during short overlap transitions, but the code committee voted it down indicating that the current from both sources needed to be calculated together.

 

[kingpb]

First my opinion.... Logic indicates that during closed transition for any length of time, there is a possibility of a fault during that period, 6 cycles, 30 cycles, 5 seconds et al. The SWGR should be designed to withstand and interrupt that amount of current coming from both sources.

I have heard this argument before, and from a purely hypothetical situation it has sound principle. However, what should not be overlooked is the fact that if a fault occurs on the bus, at the time the tie is closing and main opening, this means a catastrophic failure of the bus or the tie breaker has occurred. Consequently, it will not matter what the rating is, the equipment will be destroyed. Therefore, there is no value in trying to have the bus rated as high as both sources paralleled.

Further, in many cases, the combined contribution of the fault current will be so high, that equipment will not be available. In this situation you would need to add current limiting reactors in front of the mains, or have higher then normal impedances on transformers to keep the fault current down, which is not very practical from a financial, or operational perspective.