What is a Short-Circuit Impedance Test?
The short-circuit impedance test is a vital procedure in transformer testing that helps determine the equivalent impedance and copper losses under load conditions. This test is essential for assessing the transformer's performance, ensuring safety, and verifying its design specifications. A short-circuit impedance test measures the transformer's series impedance (comprising resistance and reactance) by applying a reduced voltage to one winding while the other winding is short-circuited. The test is typically performed on the low-voltage side with the high-voltage winding shorted, or vice versa, depending on equipment availability, to avoid excessive currents or damage.
What is the purpose of the Short-Circuit Impedance Test?
- Determine the equivalent series impedance (Zsc) of the transformer.
- Measure the copper losses or load losses of the transformer at rated current.
- Calculate voltage drop during full-load operation.
- Verify the transformer's nameplate impedance value.
- Assess the transformer's internal condition and quality.
How to calculate the short-circuit impedance (Zsc)
During testing, Z% can be calculated using:
Zpu% = (Vm * Ir) / (Vr * Im) * 100
Example
For a power transformer with the following nameplate data:
YNyn0
Rating: 90 MVA
Voltage: 132 / 33 kV
Rated current (HV): 393.65 A
Rated current (LV): 1574.6 A
Nameplate impedance: Z% = 22%
This means that under short-circuit conditions, rated current will flow when 22% of the rated voltage is applied:
Z% = Vsc / Vr * 100
So on the HV side:
Vsc = Z% * Vr
In-site, applying such a high voltage is not practical, so the short-circuit condition is simulated by applying a low-voltage three-phase supply (e.g. 400 V) on the HV side while short-circuiting the LV side.
Using the nameplate Z% and equation (1), the expected short-circuit current under this condition can be estimated.
In this case, the measured HV current during the test is approximately 5 A.
Substituting this measured current into equation (1), the calculated per-unit impedance will be very close to the nameplate 22%, which verifies the transformer impedance.
Note that this explanation is concise, and there are more details about the test that were not included.