A "transformer short-circuit test," also known as the "impedance test," is one of the standard tests performed on transformers to determine their equivalent circuit parameters, specifically the impedance values. The purpose of this test is to find the series impedance of the transformer, which includes the winding resistance and reactance. This information is crucial for assessing the transformer's performance, efficiency, and voltage regulation.
Here's how the transformer short-circuit test works and how it helps determine impedance values:
Test Setup: In the short-circuit test, one winding of the transformer (usually the low-voltage winding) is short-circuited while the other winding (usually the high-voltage winding) is connected to a low-voltage source. The test is conducted at rated frequency and voltage on the non-short-circuited winding.
Short-Circuiting: The low-voltage winding is short-circuited to ensure that the entire voltage across the winding is used to overcome its impedance. This allows for a current to flow through the winding, which will help measure the winding resistance and reactance.
Measurement: The current flowing through the non-short-circuited winding is measured using an ammeter, and the voltage across the winding is measured using a voltmeter. The current and voltage values are used to calculate the total impedance of the winding.
Calculation of Impedance: The impedance of the winding consists of two main components: resistance (R) and reactance (X). The formula to calculate impedance is:
Impedance (Z) = Voltage (V) / Current (I)
The impedance can also be expressed in terms of resistance and reactance:
Z = R + jX
Where j is the imaginary unit.
Per-Unit Impedance: The measured impedance values are typically converted to per-unit values for standardization. Per-unit values are useful because they allow for easier comparison between transformers of different sizes and ratings. Per-unit impedance values are calculated by dividing the measured impedance values by the transformer's rated values.
By conducting the short-circuit test and obtaining the impedance values, engineers can accurately determine the resistance and reactance of the transformer's winding. This information is then used to create an equivalent circuit model of the transformer, which helps in various aspects of transformer analysis, design, and performance assessment.
It's important to note that other tests, such as the open-circuit test, are also conducted to determine additional parameters of the transformer's equivalent circuit, such as the core loss resistance and magnetizing reactance. These parameters, along with the impedance values obtained from the short-circuit test, contribute to a comprehensive understanding of the transformer's behavior under different operating conditions.