Transformer Primary fuses selection

[cuky2000]

We are in process installing a temporary distribution substation with two small power transformers 9/12/15 MVA, Delta/Wye Gnd, 26kV/4.16 kV that will operated temporarily for one year while a new up-rated substation is under construction.

To protect the available temporary old transformer(>30 years in service), primary 34.5 kV fuses had been proposed with the following ratings:
1) Single 300E fuses with single disconnect switch per phase (larger single fuse available)
2) Twin 2-200E fuses and 2-switches per phase.

Some of the issues discuses are:
a) Choose single fuse even if limit the maxi. transformer near the top nameplate rating with limit emergency overloading.
b) Oversize primary fuses to allow the operator to overload the equipment beyond the nameplate capacity under emergency conditions.

Selecting option (a)the equipment could be loaded limited up to 13.5 MVA cont., 15 MVA daily for 1.3 hrs. and 18.4 MVA emergency for 1/2 hrs. Single fuse is also the most cost effective option.

Although, overloading equipment is not an unusual practice in the utility environment with relative new equipment, there is concern overloading >30 years in service equipment and be exposed to unknown risk of catastrophic failure in an active construction site at extra cost using two fuses and switches per phase.
Enclosed is a simplified one line diagram to illustrate this case.
We will appreciate your feedback and comment for this temporary application

Thanks

 

[jyj982001]

Is it right that you wonder the parameters of the 34.5kV fuses for the temporary old transformer which is 9/12/15 MVA, Delta/Wye Gnd, 26kV/4.16 kV? It's recommended not to oversize the fuse which could promote the power loss and reduce the power transformer's life ,even make the output power quality worse.

 

[jghrist]

A 300E fuse can handle 600A indefinitely without melting. The only concern might be if it can handle inrush when the transformer is energized. Check the minimum melting curve at 12 times full load and see if it melts in less than 0.1 seconds. If not, you should be OK with a 300E fuse. Check the clearing curve against the transformer damage curve. You might find that the twin 200A fuses will be too large to protect the transformer.

 

[prc]

Just a query -Why CB(vacuum or SF-6) not considered for 15 MVA at 24 kV? Only economy reasons? Cant one breaker handle two transformers in parallel? What type of fuses are planned instead of CB? It is extremely risky to overload a 30 year old transformer?

 

[cuky2000]

Hello all

Here are some of the issues to consider:

1. Reduction of fuse capability do to preloading and ambient temperature factors
“This preloading reduces the heat-dissipating ability of the fusible element, and hence reduces its melting time. “

http://www.sandc.com/edocs_pdfs/EDOC_023806.pdf

2. Overload vs. protection: We are trying to figure out how to compromise between accommodating the max. overload allows for a 30 year unit vs. the best protection that the fuse

To facilitate any future comments see the enclosed info.
===================================================================================

_{Here are some responses or clarification to your post.
jYj982001: Power Fuses are not “voltage critical”. We believe that the next std rate voltage of 34.5 kV fuses should work OK for lower operating voltage at 26 kV.
Jghrist: Initially I though like you that the fuse 300E could handle larger amp (>600A) but is only during fault conditions. For overloading conditions the continuous peak load capability is reduced to 300 A for the fuse 300E and ~524A (2-262) for the 2-200E for (see manufacturer table enclosed).
Prc: The nameplate rate of the secondary 4.16 kV breaker is 2000A continuous. However, the utility allow the breaker operate at maximum continuous load of 2100 Amps (~15 MVA, 323A @ 26 kV). During emergency conditions the breaker could be loaded up to 2600 Amps (~18.7 MVA, 416A @ 26 kV)}

 

[cuky2000]

Here is the info mentioned in the previous post

 

[Mbrooke]

Have you considered a micro processor recloser? I have applied these to transformer protection, and while more expensive it is possible to design almost any damage curve and current response. I find this to be the ultimate protection, coordination and reliability solution. At 15 MVA fuse choices become more restricted as well. Reclosers also allow for auto throw over on the primary side. They can latter be re-used as feeder protection if the substation is ever bypassed and the 4.16kv system is converted to 26kv or similar.

Also, do your outgoing feeders have current limiting reactors, or are the coils in the single line voltage regulators? Sorry for the elementary question.

In truth almost any fuse will work, even a 300E, but IMO the biggest factor outside of inrush is coordination with secondary breakers and OPCDs out on the line. This may be your biggest hurdle in that the fuses could clear before the secondary OCPD has a chance to open.

This has helped me a lot in the past, and it may help in your case (hoping lol):

http://www.sandc.com/edocs_pdfs/EDOC_025854.pdf

http://www.sandc.com/support/publications/sm-power-fuses-outdoor-distribution-pubs.asp

 

[cuky2000]

Mbrooke.
I like your suggestions. Unfortunately the temporary station is already in service and the only option at this time is for change the fuse size. The operator insists to add the 2-200E fuse for allow emergency overloading the transformer up to 18.5 MVA.
Many of us do not see the benefit of replacing the 1-300E fuses by 2-200E.
I hope this time the system allow to post the file posted and illustrate better the issue of the two fuses performance.

 

[Mbrooke]

How necessary is overloading the transformer? Does the unit have an overload history? Why is overlaod capacity needed? IMO it sounds more like the needs are overload/ reserve capacity driven.

Depending on the loading prior to the overload, and if the transformer has never been overloaded before you could overload the transformer without risk of short term failure down the road. If the unit has had a history of operating above its maximum hot spot temperature, overloading it can cause premature failure in service. Further in a case like that I personally would not feel comfortable subjecting a substation to such an extensive overload. A pole mounted unit I may not hesitate, but a large (compared to a pole pig) transformer I would.


I don't know the details of your system, but assuming this is an over head system load relief on the units could be done by partly off loading a few sections of the 4.16kv system through 26kv-4.16kv step down banks. 3 x 167kva or 3 x 250kva is typical. This method is generally done when the lower voltage system is being phased out. By adding step down banks several advantages are gained, one of which allows the existing 4.16kv system during conversion to be run right up to the banks while the lateral hold over can stay longer.

If the issue comes from an n+1 (failure of one 15 MVA transformer)are there any other 4.16kv supplies part or all of any one feeder could transfer? This might be out of the question, but if automatic switches are ever being added at any point (or already exist) they could be configured to create a forced load transfer scheme. During a failure of one 15MVA transformer, some load could be shifted to other 4.16kv sources off loading the single transformer taking on all the load after the failure. This isn't something all EEs do, but it is very common around here and something I frequently design into systems to increase normal loading of a substation while still being able to meet N+1 reliability requirements.