Transformer impedance commonly represented as a percentage is an important factor in defining a transformer’s size, rating and performance.
Impedance is the opposition a transformer provides to the flow of current under short-circuit conditions and it is affected by both
- Winding resistance and
- Leakage reactance.
A greater impedance limits the short-circuit current protecting downstream equipment but it also results in some voltage drop under full load which must be taken into account when sizing the transformer.
When selecting a transformer the impedance value affects the current rating necessary to provide a specific load.
For the same load, a transformer with a higher impedance will have lower short-circuit currents which is potentially reducing mechanical stress on windings and fault damage.
A transformer with very low impedance, on the other end, allows for higher short-circuit currents requiring stronger construction and possibly a larger core & windings to effectively manage fault currents.
Furthermore, transformer impedance effects voltage regulation.
A transformer with a low impedance produces voltage closer to nominal under full load but it may require a bigger size to securely manage short-circuit conditions.
High-impedance transformers may be smaller & less expensive but they have a higher voltage drop when loaded.
As a result, while designing (or) selecting a transformer engineers have to consider
- Impedance,
- Transformer size,
- Voltage control and
- Fault current capabilities
to assure the transformer’s and the linked system’s reliability, efficiency, and protection.
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