What is transformer impedance?

Impedance is the current limiting characteristic of a transformer and is expressed in percentage. It is used for determining the interrupting capacity of a circuit breaker or fuse employed to protect the primary of a transformer. The impedance (or resistance to current flow) is important and used to calculate the maximum short circuit current which is needed for sizing, circuit breakers and fuses. This percentage represents the amount of normal rated primary voltage which must be applied to the transformer to produce full rated load current when the secondary winding is short circuited. The maximum short circuit current that can be obtained from the output of the transformer is limited by the impedance of the transformer and is determined by the multiplying the reciprocal of the impedance timed the full load current.

Electrical impedance of the load is expressed in ohms, and the relationship between the current and the voltage in the circuit is controlled by the impedance in the circuit. In general, impedance has a complex value, which means that loads generally have a resistance to the source that is in phase with a sinusoidal source signal and reactance that is out of phase with a sinusoidal source signal. The total impedance is the vector sum of the resistance and the reactance. When a signal source, such as our composite video output, sees a very low-impedance circuit, it produces a larger than intended current; when it sees a very high-impedance circuit, it produces a smaller than intended current. These mismatched impedance redistribute the power in the circuit so that less of it is delivered to the load than the circuit was designed for because the nature of the circuit is that it can't simply readjust the voltage to deliver the same power regardless of the rate of current flow.

The impedance is measured by means of a short circuit test. With one winding shorted, a voltage at the rated frequency is applied to the other winding sufficient to circulate full load current. The most economical arrangement of core and windings leads to a 'natural' value of impedance determined by the leakage flux. The leakage flux is a function of winding ampere turns and the area and length of the leakage flux path. These can be varied at the design stage by changing the volts per turn and the geometric relationship of the windings.
A transformer with a lower impedance will lead to a higher fault level (and vice versa). However, the zero sequence impedance is dependent upon the path available for the flow of zero sequence current and the balancing ampere turns available within the transformer. Generally, zero sequence current requires a delta winding, or a star connection with the star point earthed. Any impedance in the connection between the star point and earth increases the overall zero sequence impedance. This has the effect of reducing the zero sequence current and is a feature that is frequently put to practical use in a distribution network to control the magnitude of current that will flow under earth fault conditions.