Short Circuit rating @ 480V with M-T-M closed

[kvramesh]

Hello,

I am designing a low voltage distribution system for an industrial facility. I have a double ended substation with 2 x 2500KVA transformers with secondary 480V. The tie breaker is normally open. But during maintenance of one of the transformers or for any reason, when decided to supply load on one transformer, the facility wants to close the tie first and open one of the main breakers.

I am afraid that it may result in double the short circuit available current at the low voltage switchgear which is close coupled to the transformers. There are MCCs fed from this double ended unit substation. In case of SC, they contribute 4 x FLA according to ANSI. MCCs are also located close to the switchgear in the same e-room

From my calculations, the short ciruit is turning around 125KA which is way higher than 65kA and higher than 100kA which most of the equipment is designed for. I was able to find LVS available for 150KA but the MCCs are capable to handle upto 100kA only. The only way I see to limit the short circuit lower than 100kA is by increasing the impedance of the transformers which may lead to unreasonable voltage drop, I have not checked this option. Are there any other ways of resolving this issue other than increasing the impedance of the transformer?

The fact that all the three breakers are closed at the same time is certain even for a few seconds. What can we do to limit the short circuit or how the system can de distributed other than resizing the transformers and make them 1500KVA etc.

Thanks for any assistance/comments/suggestions

 

[zog]

You either need key interlocks to prevent closing all 3 breakers or have breakers rated for the available fault current. We have hundreds of 480V breakers with 200kA interuption ratings.

 

[kvramesh]

This is the facility requirement that they want to close the tie first. So, I cannot take exception to that.

How about MCCs rated for 200kA? Where can I go?

 

[jghrist]

First, you have to calculate the fault current at the MCCs. It will drop off fairly quickly with length of cable.

 

[kvramesh]

MCCs are about 15 feet away from LVS

 

[jim dungar]

Have you looked into buying transformers with higher than normal %IZ?
While this method may generate other problems (i.e. voltage regulation), it has helped me in similar situations.

 

[bob]

You can install Current Limiters or Current Limiting Reactors that will solve your problem. Google "Current Limiters" an "Current Limiting Reactors" for more information.

 

[zog]

Current limiters is the 2nd best solution here, the 1st best being convincing your client to install key interlocks so you wont ever have the high fault current problem.

Wait til they do thier arc flash analysis for the M-T-M all closed.

 

[kvramesh]

I am thinking of increasing the impedance of the transformer to limit the short circuit on the secondary. If I increase the impedance to 7.7%, short circuit is coming around 38kA for one transformer which would solve the SC problem.

Now comes the voltage drop issue? How do I calculate the voltage drop...is it 7.7% x 480 = 37volts. Isnt it way high? Please some one help me to calculate the voltage drop.

I understand that we could increase the voltage by lowering the primary taps, but want to know how much I should lower atleast to bring the voltage to 466 volts? Thanks

 

[jghrist]

I am thinking of increasing the impedance of the transformer to limit the short circuit on the secondary. If I increase the impedance to 7.7%, short circuit is coming around 38kA for one transformer which would solve the SC problem.

Now comes the voltage drop issue? How do I calculate the voltage drop...is it 7.7% x 480 = 37volts. Isnt it way high? Please some one help me to calculate the voltage drop.

I understand that we could increase the voltage by lowering the primary taps, but want to know how much I should lower atleast to bring the voltage to 466 volts? Thanks

The voltage drop depends on the load current.
Approximate %VD = (%I?%R?cos? + %I?%X?sin?)?100
where %I is load in percent of the transformer rated load
%R is transformer full load loss as a percent of transformer rating
%X = sqrt(%Z?-%R?)
cos? = load power factor

How much you lower the primary taps depends on what the primary voltage is and what the voltage would be at light load (to avoid having the voltage too high at light load).

 

[ron]

You cannot use current limiters to consider downstream MCC to be protected unless there is a series rating with the upstream limiter or fuse.

Use 200kAIC rated switchgear.

For the MCC's, I try to utilize more lower ampacity MCC's rather than less higher ampacity MCC's, which allows you to have smaller conductor size and higher impedance.

I have had to resort to having the EC's run the wire/conduit around the room a few times, until I had enough wire impedance to reduce the fault current at the MCC or panelboard or whatever.

 

[mdshunk]

I have had to resort to having the EC's run the wire/conduit around the room a few times, until I had enough wire impedance to reduce the fault current at the MCC or panelboard or whatever.

:grin: I often wondered if anyone ever considered or did that.

 

[LarryFine]

I have had to resort to having the EC's run the wire/conduit around the room a few times, until I had enough wire impedance to reduce the fault current at the MCC or panelboard or whatever.

Don't forget a pull box for each 'lap' of the conduit. :wink:

 

[noonan]

Hmmm, could you use the same pull box for each lap, sized properly of course?

 

[weressl]

Hello,

I am designing a low voltage distribution system for an industrial facility. I have a double ended substation with 2 x 2500KVA transformers with secondary 480V. The tie breaker is normally open. But during maintenance of one of the transformers or for any reason, when decided to supply load on one transformer, the facility wants to close the tie first and open one of the main breakers.

I am afraid that it may result in double the short circuit available current at the low voltage switchgear which is close coupled to the transformers. There are MCCs fed from this double ended unit substation. In case of SC, they contribute 4 x FLA according to ANSI. MCCs are also located close to the switchgear in the same e-room

From my calculations, the short ciruit is turning around 125KA which is way higher than 65kA and higher than 100kA which most of the equipment is designed for. I was able to find LVS available for 150KA but the MCCs are capable to handle upto 100kA only. The only way I see to limit the short circuit lower than 100kA is by increasing the impedance of the transformers which may lead to unreasonable voltage drop, I have not checked this option. Are there any other ways of resolving this issue other than increasing the impedance of the transformer?

The fact that all the three breakers are closed at the same time is certain even for a few seconds. What can we do to limit the short circuit or how the system can de distributed other than resizing the transformers and make them 1500KVA etc.

Thanks for any assistance/comments/suggestions

The general conscensus is to rate the gear for parallel operation.

However. These schemes use relaying that allows the paralleling of the transformers only for a couple of cycles, e.g. close then open the selected main breaker. In the case of loosing one of the transformer the tie closes automatically, unless the loss of voltage was due to a bus fault, so in that scenario, the parallel operation of the trasnformers is not the case. In the reverse transfer mode when the off duty transformer is bought back on line, the respective main breaker would be closed first, then the tie would be opened within a few cycles. Since the parallel operation is not a continuous operation and it is limited to a few cycles, the potential of a full short circuit developing simultaneously is virtually non-existent, therefore it is OK to size the gear to the open tie duty. This was confirmed by NFPA.

 

[don_resqcapt19]

Since the parallel operation is not a continuous operation and it is limited to a few cycles, the potential of a full short circuit developing simultaneously is virtually non-existent, therefore it is OK to size the gear to the open tie duty. This was confirmed by NFPA.

As a formal interpretation?

 

[zog]

I too have seen these systems, commony called "Electronic Kirk Keys", the swgr was not rated for MTM closed even though it could be for a short period of time.

 

[jim dungar]

I too have seen these systems, commony called "Electronic Kirk Keys", the swgr was not rated for MTM closed even though it could be for a short period of time.

I have also seen these. The rational is that if a bolted fault condition did not exist prior to the temporary paralleling it is all but impossible for one to occur during an "overlapping" transfer.

 

[weressl]

I have also seen these. The rational is that if a bolted fault condition did not exist prior to the temporary paralleling it is all but impossible for one to occur during an "overlapping" transfer.

That would be consistent with many stipulations how we approach the Code.

One example is the area classification in Class I, Division 2 areas. It is recognized that:

1./ There will be ignitable mixtures in the area as the result of process equipment failure.
2./ Equipment approved for the use in these areas are not a source of ignition, but could become if the equipment malfunctions.
3./ It is unlikely that 1./ and 2./ will occur simultaneously.

I stipulate that the potential of a fault occuring during the <10 cycle of transfer is far less likely than the 1./ and 2./ occuring simultaneously. In addtition the number of electrical equipment in Class I, Division 2 areas are degrees of magnitude greater than the number of transfer schemes.

 

[zog]

The one I have seen had longer times. You would close the Tie (With both mains closed) manually and then have like 30 seconds to open a Main before the system opened a Main for you (Or maybe it opened the Tie, cant recall) but I do know it was an adjustable time and the one I worked on was set at 30 seconds. There are several of these at the Toledo Jeep plant.