Ever wondered what is the %Z or %impedance or short circuit impedance that everyone refers to when talking about power transformers? %Z is a key parameter in power transformers and its value determine the available short circuit current at the secondary of the transformer (to a large extent), voltage regulation, impedance offered to harmonics originating at the secondary side etc. Often times engineers do not have a clear idea on what this parameter is and how it influences the performance characteristics of the transformer. If you are in the same situation, keep reading!
There are two key tests for a transformer soon after production. One is open circuit test and the other is short circuit test. Open circuit test establishes the no-load core losses etc. For short circuit test, the secondary (usually the low voltage side) is short circuited and a variable AC source is connected to the other side (usually the high voltage side). The variable AC source voltage is increased until rated secondary current flows. The % of primary voltage that is required to produce the rated secondary current is what is known as %impedance or per-unit impedance.
There is another way of defining the %Z or % Impedance or leakage reactance. A transformer is a nothing but two (or more) coils coupled by magnetic field. Ideally all the magnetic field flux from the primary side couples 100% to the secondary and hence the following ideal equation can be established.
V1* I1= V2* I2
In a practical transformer, not all primary flux will be one hundred percent coupled to the secondary and there is some ‘leakage flux’. This leakage flux will cause voltage drop on a loaded transformer. In general leakage reactance increases with:
Higher primary voltage (Thicker insulation between windings)
KVA rating
Larger core
The leakage reactance value is under the control of the transformer designer and manufacturers can make custom impedances for customers if required. A situation where high leakage reactance (or high %Z) is required is when controlling the magnitude of prospective secondary fault current is desired.
Areas where % Z is important:
Lower %Z allows higher secondary fault current.
Important parameter when paralleling transformers.
Certain type of drives requires a minimum source impedance for functioning properly. In this situation an isolation transformer with required %Z is inserted in front of the drive.
Isolation transformers with appropriate %Z can be inserted in front of large non-linear loads to control harmonics, reduce voltage notching.
To explore the effects of changing %impedance on the secondary short circuit current, the following calculator can be used.
Transformer Short Circuit Current Calculator
Some times knowing the actual impedance of the transformer in ohms is useful for certain calculations. However, it should be noted that the actual value in ohms for a transformer is not a constant value. It depends on the voltage level at which the value is desired. This is because the %impedance (or %Z) is a per unit value. The calculated impedance in ohms will vary depending on the whether the value is desired at primary or secondary terminals. It will also vary for the different tap settings if available. In contrast per unit %impedance do not change with voltage and hence is the preferred method for power system engineers.
