Transformer Frequent Fault

[HamidEle]

we are dealing with Utility company recnetly. They provide 1 single transformer for a 4.16 kv SWGR. The sencondary of the transformer is direct buried. Based on IEEE, we designed our downstream protection according to infrequent fault of the transformer. But the Utility company insists on using frequent fault and asked us to speed up our protection. I don't see any point. Do they have different protection guideline ecxept IEEE?

 

[DanDel]

The transformer Frequent Fault damage curve is typically used when the transformer modeled will see more secondary faults (like when the transformer is feeding an overhead line) than what is normal for a substation transformer. There is a cutoff point for the amount of faults in the IEEE 242, but I don't have access to it right now.

 

[dpc]

10 through-faults in the lifetime of the transformer is the break-off point in the ANSI standards between frequent and infrequent.

If you can protect for the frequent fault damage curve, your transformer will be better protected and likely last longer regardless of how many faults it actually experiences, so why not do it if possible.

 

[bacon4life]

The curve shown in IEEE 242 for Category II and III transformers ends at 2 seconds. So this means the transformer can take 10-2 second though faults at it's secondary bushings? How about if they are less than 2 seconds, can it take an infinite amount, or is it more of a total I^2*t limitation? Thus if they are only 1 second, can it take 20?

Do faults close to a substation often take that long to clear?

 

[apowerengr]

They own the transformer and get to set their protection requirements - sounds like they are really asking for your system to coordinate instead of your faults taking out their overcurrent protection. Wasn't this question asked of the utility when the service application was made?

 

[dpc]

The damage from through faults is cumulative to a large extent. The transformer life will probably be shortened by any through fault. The ANSI requirements just attempt to define a reasonable level of protection that will provide the average transformer an average life expectancy. There is no "infinite" capacity to withstand through faults. If your transformer fails on a through fault and it is out of warranty (usually one or two years) you are probably out of luck regardless of what the ANSI curves say. That is why it is prudent to provide as good a level of protection as is practical and economical.

Fault clearing time depends on the type of fault and fault magnitude. A bolted fault close to the substation should clear within a few cycles. A downed conductor could take much longer, but current will be less.

 

[HamidEle]

in Delta/Y connection Transformer, do we need to shift the curve to the left by 0.58 factor for the Phase to phase fault on the secondary? In one phase on the Pri, the fault is 1.15 x Secondary fault, but on the other phases, the fault current is only 0.58x secondary, I realy see other engineers to shift the curve. Why?

 

[dpc]

You are confusing two different things - phase to ground faults and phase-to-phase faults.

The IEEE Red Book or Buff Book would be really good resources for you and will explain why the transformer damage curve must be shifted when considering secondary line-to-ground faults and primary protection on a delta-wye transformer.