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Pre Insertaion Impedance-XFMR Inrush

Pre Insertaion Impedance-XFMR Inrush

Pre Insertaion Impedance-XFMR Inrush

(OP)
One of the inexpensive methods offered to limit XFMR inrush is to insert an impedance for limited time and shorting it after transformer energization.
This will introduce a Z in series with xfmr impedance in circuit model.Objectiv is to limit inrush ,so voltage drop on upstream bus is limited. All calculation I have seen is based on 12X FLA for inrush for .1 sec. XMFR is 8MV at 25KV.
I can base my calculation on upstream voltage drop ,but I have my doubts if this will work!!  

RE: Pre Insertaion Impedance-XFMR Inrush

The generic calculation for voltage drop due to transformer inrush is:

Per Unit Voltage Drop = X / (X + 2.5 Xt)

where:
X = Thevenin reactance of system at transformer location (i.e. reactive part of Thevenin fault impedance in Ohms)
Xt = Transformer reactance in Ohms

This formula makes it easy to determine the effect of adding a closing impedance (would effectively increase Xt in the denominator).

Ian Dromey - www.dromeydesign.com

RE: Pre Insertaion Impedance-XFMR Inrush

(OP)
Thanks redfurry.
I need to do more research on the subject. Can you direct me to the source of the formula please?

What would be the difference between adding a inductor or resistance?
  

RE: Pre Insertaion Impedance-XFMR Inrush

The formula is described as a "back-of-the-envelope" approximation in a paper called "Assessing and Limiting Impact of Transformer Inrush Current on Power Quality" by Nagpal, Martinich, et al in IEEE Trans on Power Delivery, Apr 2006

From the paper (and the equation), I would assume that adding reactance and not resistance is the key.

Ian Dromey - www.dromeydesign.com

RE: Pre Insertaion Impedance-XFMR Inrush

Hi guys,

Please, note that the pre-insertion resistor will be in the (series) circuit for aprox. 8-10 ms (during the circuit-breaker closing).

The voltage drop in this pre-inserted resistor caused by the inrush current will reduce the magnetic flux in the core hence the inrush current.

Another way to reduce the transformer inrush current is by controlling the circuit-breaker closing (point-on-voltage-wave). This aproach is normally used in high voltage (230kV and above) transformer. However, it may be worth it to apply in MV transformer.

Regards,

H. Bronzeado
 

 

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