Power Transformer Protection

[timm33333]

I have a question about the power transformer protection. When we change the impedance of the transformer, the short circuit current and voltage drop across the transformer would change, and it will affect the settings of the relays on the secondary side of the transformer.

Will it also affect the settings of the relays on the primary side of the transformer? Thanks

 

[waross]

It's pretty hard to change the impedance of a transformer.
However if you replace a transformer with a transformer with a different impedance, then the protection should be reset appropriately for the replacement transformer.
The primary current may change slightly to reflect different losses.
The primary fault current will change as the secondary fault current changes.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[timm33333]

If we change the transformer with a different impedance, the voltage drop across transformer will change in addition to the change in short circuit current. Will this different voltage drop affect the settings of only the secondary relays, or both primary and secondary relays?

 

[HamburgerHelper]

If you can change the taps, you can change the impedance. Take a look at the documentation and if this is the case, they will list out the impedance at each tap.

You can change the zero sequence impedance by changing the grounding, too.


------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.

 

[bacon4life]

The relay settings on both the primary and secondary side of a transformer should be recalucated if a transformer is changed out with a transformer of a different impedance.

Note that changing the transformer has no direct impact the relay settings. The relay settings will stay wrong until the relay is reprogrammed.

 

[jghrist]

One of the factors in setting instantaneous overcurrent protection on the high side of a power transformer is the maximum through fault level. Normally the high side pickup is set below the maximum through fault level to avoid coordination problems with low side overcurrent relays. Higher impedance will lower the maximum through fault level. Coordination between high and low side inverse time overcurrent relays will be easier with higher impedance. Coordination will only have to be achieved for lower fault currents.

 

[davidbeach]

I think you meant that the high-side instantaneous would be set above the maximum through fault. That way if the instantaneous trips it had to have been a fault in the transformer.

 

[jghrist]

David,

That is correct.