Transformer overexcitation protection (24) on HV side? 1

[Power0020]

a 230/115 kV power transformer, the owner sted that the 24 relay shall be provided on the HV side of the transformer tho there is +/- 10% OLTC. ABB RET670 recommends not to use the 24 on OLTC fitted side and the relay itself doesn't have an AVR flag.

I wonder how is that to operate...any clues?

 

[bacon4life]

This Taps tutorial power point discussed the impact tap changer location has on the flux in the transformer for various tap configurations. We typically only use volts/hertz on portions of the system that can be islanded with generation. Be sure the settings coordinate with the control set points and protection settings of any nearby capacitors or reactors.

 

[prc]

The over fluxing relay is set for V/f value of 1.1-1.2 values. On HV side, as the voltage will be varying with tap changing without changing flux density in core, it will not be possible to arrive at a proper V/f value setting. It will vary for each tap position. So the ABB recommendation seems correct.

 

[davidbeach]

My experience aligns with bacon's; V/Hz protection on GSUs but not on T&D transformers. If that one transformer gets into V/Hz trouble you could have dozens/hundreds (thousands, even) of transformers with V/Hz issues. What do you accomplish by tripping a bulk transformer under such circumstances?

 

[prc]

As I understood, over fluxing protection will be required in GSU during start up of generator prior to synchronization. Once it is locked to grid, are there chances of V/f going different than grid? In India, V/f relay is put on inter connecting auto units also. Does it mean it is of no use and can be saved?

 

[loadangle]

If the transformer is supplied with tap changer the protection must be connected to a side without tap changer. The side with the tap changer can withstand different voltages depending on the tap changer position and is therefore not suitable for overexcitation protection.