IEC60909 Impedance Correction Factor for Transformers 1

[EddyWirbelstrom]

Clause 3.3.3 of IEC60909-0 :2001 requires an "Impedance Correction Factor for two and three winding network transformers".
Depending on the 'c' Factor, the 'Transformer impedance correction factor' can either increase or decrease the transformer percentage impedance used for short-circuit calculations.
Can anyone explain the reason the the 'Transformer impedance correction factor' ?

 

[Alex68]

The meaning of the Kt factor is not explained but I suppose that the last sentence of the note at the end of the paragraph coulb be a suggestion:"if in special cases... it may be unnecessary to introduce the impedance correction factor"

I believe that the Kt is introduced to consider the impedance variations with the tap position.
Analising the Kt eq. 12b, the Kt directly depends on the effective maximum voltage and current of the system.

 

[RalphChristie]

Hi

The "c" factor is the voltage factor. It is the ratio between the equivalent voltage source and the nominal system voltage (Un).

The explanation of the voltage factor implies that the prefault voltages should be based on a power flow solution. In the abstance of a prefault power solution, the prefault voltage (Un nominal voltage) would be approximately 5% lower than the no-load voltage (Ur rated voltage) The allowance for high system voltages is accounted for by the voltage factor "c" shown below. The voltage factor cmax is used for maximum currents and the factor cmin is used for minimum currents.

Un = 100-1000V, cmax = 1.05, cmin = 1.0
Un = 230/400V (4 wire system), cmax = 1.00, cmin = 0.95
Un > 1kV to 230kV, cmax = 1.10, cmin = 1.0


A very good book regarding this issues is:
A Practical Guide to Short-Circuit Calculations by Conrad St. Pierre, ISBN:0-9717506-0-2

Regards
Ralph