A "transformer short-circuit test," also known as the "impedance test" or "short-circuit impedance test," is a method used to determine the impedance of a power transformer. Impedance is a measure of the opposition that a circuit presents to the flow of alternating current (AC), and it includes both resistance and reactance components.
In a power transformer, impedance is an important parameter because it affects the voltage regulation and the ability of the transformer to handle fault currents without excessive heating or damage. The impedance test helps to quantify this impedance by providing a way to measure the voltage drop that occurs when a rated current flows through the transformer's short-circuited secondary winding.
Here's how the transformer short-circuit test works:
Setup: The primary winding of the transformer is connected to a low-voltage AC supply, typically at rated frequency (50 Hz or 60 Hz), while the secondary winding is short-circuited. This means that the secondary winding is connected together using a short-circuiting link or a low-impedance load. The primary voltage is adjusted to ensure that the rated current flows through the short-circuited secondary winding.
Measurement: Instruments are used to measure the primary current, primary voltage, and the secondary voltage (which is essentially zero due to the short-circuit). These measurements are used to calculate the impedance of the transformer.
Calculation: The impedance (Z) of the transformer can be calculated using the following formula:
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Z = V_primary / I_short_circuit
Where:
Z is the impedance of the transformer.
V_primary is the primary voltage.
I_short_circuit is the current flowing through the short-circuited secondary winding.
By measuring the primary voltage and the short-circuit current, the test effectively simulates the condition where the transformer is delivering its rated current under short-circuit conditions. The resulting impedance value gives insights into the transformer's ability to regulate voltage and handle fault conditions. A higher impedance indicates greater voltage drop and potentially reduced performance during high load or fault situations.
It's important to note that the transformer impedance obtained through this test is usually expressed in percentage values of the transformer's rated voltage on the primary side and rated current on the secondary side. This allows for easy comparison across different transformers of varying sizes and ratings.
The transformer short-circuit test is one of the routine tests performed during the manufacturing process and periodic maintenance of power transformers to ensure their reliability and proper functioning in power distribution networks.