In electrical engineering, Transformer Impedance (usually expressed as a percentage, $Z\%$) is one of the most critical parameters specified on a transformer’s nameplate. It plays a vital role in determining short-circuit currents and sizing protective devices like circuit breakers.
What is Percentage Impedance ($Z\%$)?
Percentage impedance is the percentage of the rated primary voltage required to circulate full-load rated current through the transformer secondary windings when the secondary terminals are short-circuited.
Mathematically, it represents the internal voltage drop of the transformer at full load due to its winding resistance and leakage reactance:
$$Z\% = \frac{V_{sc}}{V_{rated}} \times 100$$
Where:
- $V_{sc}$ = Impedance voltage (voltage required to produce rated current during short-circuit test)
- $V_{rated}$ = Rated primary voltage
Why is Transformer Impedance Important?
- Short-Circuit Fault Current: It directly determines how much current will flow during a fault. A lower impedance means higher short-circuit currents, requiring higher-rated circuit breakers.
- Parallel Operation: Two transformers operating in parallel must have identical percentage impedances. If they do not match, the transformer with lower impedance will take a higher share of the load and may become overloaded.
- Voltage Regulation: Higher impedance causes a greater voltage drop under full load conditions.
Typical Value Example
For industrial distribution transformers or dual-secondary models, impedance values typically range from 5% to 14% depending on the kVA rating and design application. For instance, if a client requests a higher fault-current limitation, a standard impedance value might be raised (e.g., from 7% to 14%) based on system feedback.
Tags: Transformer, Electrical Engineering, Power Systems
