Coordination Curves??

[Mike01]

How could a transformer secondary ocpd trip upon the energization of the associated transformer? Looking at a coordination curve I was told that if the secondary ocpd device curve is located to the left of the xfmr inrush current it could possible trip the breaker. And that the secondary would have to remain closed, the xfmr would be energized and then the secondary closed. I guess I do not understand how could the secondary ocpd trip when the xfmr has to saturate first before the secondary devices receives its voltage?? I could see this for the primary ocpd but why the secondary?

 

[ron]

Transformer inrush will effect the primary OCPD not the secondary.

 

[physis]

For one thing, with a typical breaker at least, the things don't operate fast enough to care about the transformer's field charging.

Second, The primary and secondary are literally two different circuits. Whatever math is associated with the two of them are essentially seperate.

From what you're saying I'd be looking for an issue on the secondary side.

Edit: Either that or your breakers don't match up for current on either side of the transformer.

 

[brian john]

For one thing, with a typical breaker at least, the things don't operate fast enough to care about the transformer's field charging.

THEY DON'T...I have been doing things wrong for years.

 

[kingpb]

Trying to start a load on the LV side of the transformer at the same time your energizing the transformer could be the problem with the LV OCPD.

 

[jghrist]

Maybe the problem is "cold load pickup" instead of transformer inrush. After an extended outage, diversified loads such as hvac or refrigeration will become undiversified and all come on as soon as the transformer is energized.

If the lv ocpd was sized for a diversified load, it may trip on cold load pickup.

 

[coulter]

Only one other thing I can think of: Some protective schemes have the secondary trip tied to a primary trip. For example, one system I deal with has an 86T tied to several primary trips. An 86T trip opens the secondary CB.

carl

 

[brian john]

Maybe the problem is "cold load pickup" instead of transformer inrush. After an extended outage, diversified loads such as hvac or refrigeration will become undiversified and all come on as soon as the transformer is energized.

That's one way of thinking about it.