Fault current on tap changing power transformer
Fault current on tap changing power transformer
(OP)
How does fault current vary on the secondary of a power transformer (ie 115kv to 13.8kv 40MVA) as the tap changer adjusts up and down in relation to primary side voltage? Does the X/R ratio exhibit any change? Do I need a tester for exact values or can they be adjusted easily by hand in relation to those of the normal neutral position?






RE: Fault current on tap changing power transformer
This is only slight and generally the tolerance considered on the rated %Z while calculating the fault current will cover this.
Rompicherla Raghunath
RE: Fault current on tap changing power transformer
RE: Fault current on tap changing power transformer
Depending on where on the winding the taps in question are connected an increase in ratio between high and low could result in either an increase or a decrease in impedance.
Generally the differences should be small enough to fall into the model's general uncertainty bucket; if the third significant digit is important you probably want to get the impedance tested at that location.
Haven't tried it, but it would seem that if there's a known load that can be switched on and off at different tap settings and you've got really good metering on both sides of the transformer it would seem that you could at least determine whether the impedance went up or down for that change is load. Since the transformer impedance is mostly reactance, a reactive load, cap bank or shunt reactor, would give a better indication than a strictly real power load. It is also possible that the R and X components of the impedance change in opposite directions with the overall change dominated by the change in X.
RE: Fault current on tap changing power transformer
Can I just test that position with an omicron or do I need to test all taps?
RE: Fault current on tap changing power transformer
RE: Fault current on tap changing power transformer
The nameplate is based in the neutral tap, right?
BTW- do they even make ATC step tap power transformers? I don't think I have ever seen one outside of off load selection.
RE: Fault current on tap changing power transformer
In some cases, the increase/decrease will on pro-rata basis. In other cases it is not.
Where the impedance is not on pro-rata basis, one should get the impedances at the extreme tap positions. Then for the intermediate taps, it is the linear variation. Hence increased or decreased based on the tap position.
RE: Fault current on tap changing power transformer
In the above 4 alternatives(there are still further options like with series injection winding etc which are nowadays not common) there can be further 4 variations in each category - (1) tappings on winding (2)separate tap winding outer most (3) tap winding innermost (4) tap winding between HV and LV. In all these 16 alternatives impedance variation with tap changing will be different.
Of course under each, still there are variations-linear tap winding, reversing tap winding, coarse-fine tap windings etc
To avoid confusion, generally in the type mentioned by M brooke, it will be a1 or a2 option for which change in % impedance is negligible. But with (b), constant % impedance change will be negligible, with b1 or b3 option.Remember with constant %z at all taps, fault current will be maximum at lowest voltage tap.(highest tap number)
Sometimes customer may ask constant fault current at all taps(ie constant ohmic impedance -not %impedance-at all taps) - a requirement in olden days- for which also transformer designers have solutions!
RE: Fault current on tap changing power transformer
RE: Fault current on tap changing power transformer
RE: Fault current on tap changing power transformer