fault current reduction

[don_resqcapt19]

Thanks guys. This has just become only a learning experience as they have found the original documentation for the MCCs that show the bus is rated for 42,000. It is only the buckets that are rated at 22,000. The will be ordering new buckets and changing them out...still costly but nowhere near as costly as replacing the MCCs.

 

[Besoeker]

Thanks guys. This has just become only a learning experience as they have found the original documentation for the MCCs that show the bus is rated for 42,000. It is only the buckets that are rated at 22,000. The will be ordering new buckets and changing them out...still costly but nowhere near as costly as replacing the MCCs.

Sounds like a good solution.
But buckets?
I'm sure it is a colloquial term for you but it's not one I'm familiar with in the context of MCCs.
Please can you put me out of my ignorance?

 

[iwire]

Sounds like a good solution.
But buckets?
I'm sure it is a colloquial term for you but it's not one I'm familiar with in the context of MCCs.
Please can you put me out of my ignorance?

The removable sections that generally contain the over current devices and contactors.

 

[Pierre C Belarge]

In the early days of my career it was common practice to use transformers with non-standard impedances in situations such as you have described. You let the transformer manufacturer do the cable coiling for you. However 1500kVA was about the top that I remember and it would appear you would need something on the order of a 10%Z.

I remember reading somewhere that the utility companies have issues with transformers used in the service rated at such a high Z. Maybe it was the amount of change for the customer billing?

 

[jim dungar]

I remember reading somewhere that the utility companies have issues with transformers used in the service rated at such a high Z. Maybe it was the amount of change for the customer billing?

Because utilities are usually required to keep spare transformers on-hand, they are reluctant to provide a special %Z. However, customers are usually able to purchase 'primary' power and then use, and maintain whatever special transformer they want.

 

[rbalex]

I remember reading somewhere that the utility companies have issues with transformers used in the service rated at such a high Z. Maybe it was the amount of change for the customer billing?

As Jim noted, the client owned the transformers. Remember, my early career was predominately in refinery applications and we distributed power throughout the facility at anywhere from 5 to 69kV, depending on the overall size. We commonly had distribution voltage primary to 480V secondary transformers all over the site. Short circuit duities of 22kA was typical at 480V based on 1000kVA transformers with a standard 5.75% impedance. Motor contribution was usually ignored since steam was the common power source for larger rotating equipment at the time. It was rare to need anything larger than 1000kVA transformers.

When something larger was needed though, a common application was a 1500kVA transformer with a 8% impedance. That usually dropped the available faults nicely into the 22kA range, when all other relevant system impedances were considered.

Typically, the increase in impedance was achieved by a serpentine routing of winding conductors inside the transformer tank. In other words, the extra winding conductor was run back and forth parallel to the core, rather than encircling it. That reduced the magnetic flux linkage but increased the resistive component of the impedance - read that tremendous I^2R power losses. The no-load losses took an especially heavy hit comparitively.

When I started in the business, power was relatively cheap for refineries; but short-circuit duty ratings above 22kA weren't - if you could get them at all.

 

[ron]

How big of an issue is the magnetic field around an air core reactor? Is it an issue with the normal loads or only when there is a fault? It is my understanding that an air core reactor is prefered for this application because a solid core one could go into saturation at high currents and not provide the required current reduction.

Don,
My understanding is the magnetic field is 24/7, as the current passing through the reactor (reactor is installed in series with the conductor)makes the field, and since there is no core, it just "flies around".
Depending on the size, I understood that I couldn't have anything ferrous for several feet in any direction, or else inductive heating would melt it.

 

[weressl]

Laszlo,
What would be a large motor for this application? I don't think there would be any over 100 hp on any of the MCCs in question.

You have not supplied sufficient information to be able to give a definitive answer.

So my comment was generic as to what issue(s) need to be addressed, to be thought of, when such issues arise and such changes are contemplated.