Selection of Primary Over-current Protection for Three-Phase Transformers 4

[Waleed93]

In regards to transformer damage curves.

How much of those curves must a protection device cover ( "be to the left and under" when plotting protection device TCC curve on top of TR damage curves) ?
What is the standard allowable margin?

Thank you.

 

[davidbeach]

Ideally all. Realistically, somewhat less. What's the application, what size is the transformer? For substation transformers with overhead distribution and automatic reclosing I'd really want to protect the whole damage curve. For indoor stuff where the design avoids sustained overloads I'd concentrate on the shorter-term mechanical damage portion and allow the overall design to take care of the longer term thermal portion.

 

[Waleed93]

Thank you for your reply and for that valuable information.

I am not trying to find a margin for a specific transformer.

I am trying to find a standard that specifies how much of the transformer through-fault curves shall be covered by primary side main protection device for all:
[ul]
[li]IEEE Std C57.12.1 Dry type Transformers category I ,II, III[/li]

[li]IEEE Std C57.12.00 Oil filled transformers Cat. I , II, III, IV[/li]
[/ul]

 

[dpc]

I think ANSI C57 is going to say the all of the curve should be covered. But these C57 standards are not legal requirements and as David said, full protection is difficult, and with primary fuses, often impossible. The NEC has no requirement for protecting transformer against these damage curves.

 

[Waleed93]

Thank you for your post dpc.

So what is a good guideline to follow when protecting these curves?

 

[dpc]

Again, as David said, the most important portion is the knee of the curve at 2 seconds, assuming you are using the frequent fault curve with mechanical damage. The thermal portion between 100 and 1000 seconds is the area that will be hard stay under. For forced-cooled transformers, the rated amps comes pretty close to the damage curve up at 1000 sec. In this region, the oil temp and winding temp (if monitored) give a better indication of the transformer overloading.

 

[Waleed93]

Thank you for your reply dcp .