A "transformer short-circuit test," also known as a "short-circuit impedance test" or "impedance voltage test," is a method used to determine the impedance values of a power transformer. Impedance values are crucial for assessing the performance, efficiency, and fault behavior of a transformer within a power system.
The test is conducted by short-circuiting the secondary winding (low-voltage side) of the transformer while the primary winding (high-voltage side) is supplied with a reduced voltage. The objective of this test is to measure the voltage across the short-circuited secondary winding and the current flowing through the primary winding. Based on these measurements, the impedance values of the transformer can be calculated.
Here's how the test works:
Setup: The primary winding is connected to a reduced-voltage power source, typically around 5-10% of the transformer's rated primary voltage. This is to ensure that the test does not cause excessive current and damage to the transformer. The secondary winding is short-circuited using suitable equipment.
Measurement: The current flowing through the primary winding is measured using a current transformer, and the voltage across the short-circuited secondary winding is measured using a suitable voltmeter. These measurements are taken while the transformer is in the short-circuit condition.
Calculation: The short-circuit impedance of the transformer can be calculated using the following formula:
Short-Circuit Impedance (Zsc) = Short-Circuit Voltage (Usc) / Short-Circuit Current (Isc)
The Short-Circuit Voltage (Usc) is the voltage across the short-circuited secondary winding, and the Short-Circuit Current (Isc) is the current flowing through the primary winding. It's important to note that the impedance value calculated in this test is in terms of its per-unit value.
Conversion: The per-unit impedance value obtained from the test is then converted to the transformer's actual impedance value by multiplying it with the transformer's base impedance. The base impedance is calculated using the transformer's rated power and voltage values. This provides the actual impedance value in ohms.
The short-circuit impedance test is essential for determining the internal resistance and reactance of the transformer. These values are crucial for assessing the transformer's performance under various operating conditions and for evaluating its behavior during faults or short-circuit conditions in the power system.
It's important to perform this test accurately and safely, as improper testing procedures can lead to transformer damage or inaccurate impedance measurements. Therefore, experienced personnel and proper testing equipment are required to carry out this test effectively.