PERCENTAGE IMPEDANCE

In our college studies we may know about Impedance. Impedance is nothing but the sum of Resistance and Reactance in circuit.

Z = √R↑2 + X↑2 where, R= Resistance; X= Reactance,

Than in General form, In A.C component circuit Z = V / I,

In Our transformer Impedance generally used to measure the Reactance as

X = √Z↑2 - R↑2

Once we entered in to professional these studied formulas, methods are not handy for get easy results, we must apply them at appropriate place,

Impedance Voltage: This is nothing but (HV) Primary voltage which required to create a full load current at short circuited secondary (LV).

** Percentage Impedance Voltage (3 phase Transformer)

= √3 X 100 X (HV Applied voltage to get full load current / HV Rated voltage)

*** Without knowing the Impedance it is unable to measure the Short Circuit withstand current.

** Trans withstand Short Circuit Current = 100 X Full Load Current (Rated Current) / Percentage Impedance

Measurement of Impedance:

1. Short the LV side

2. Start to apply 5 or 10% of rated voltage at primary side

3. Gradually increase the voltage level up to current flow in shorted LV side is full(Rated Current) value.

4. Then measure the applied voltage at primary as Va.

* Take rated primary voltage as Vr

Percentage Impedance (% Z) = (Va / Vr) X √3 X 100

Example:

In 25 MVA, 220 / 11 KV Transformer YNy0, HV rated current 65.61 A, after short flow of rated current in primary, the applied voltage is 20094 V.

% Z = (20094 / 220000) X √3 X 100 = 0.09133 X √3 X 100 = 15.81 % (At Measured Temperature)

We must express the %Z and other losses etc.. at 75 C only then it will be apprx. %Z = 15.99%.