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Regular testing of these transformers is essential to ensure their safe, efficient, and reliable operation. Here's an overview of some common tests performed on distribution transformers:
Visual Inspection: Before any electrical testing, a thorough visual inspection is conducted to check for any physical damage, leaks, corrosion, or other abnormalities.
Insulation Resistance (Megger) Test: This test measures the resistance between the windings and the transformer’s casing (or between windings) using a high voltage DC source.
Polarity Test: Determines the correct phase relationships between the primary and secondary windings to ensure proper electrical connections.
Winding Resistance Test: Measures the resistance of each winding using a low voltage DC source. High resistance could indicate poor connections or damaged conductors.
Ratio Test ( Turns Ratio Test): Verifies that the transformer is stepping up or stepping down voltage as per its design specifications. This is done by comparing the voltage ratios of the primary to the secondary windings.
Excitation Current Test: Measures the no-load current drawn by the transformer when it’s energized. Abnormal values can indicate core issues or incorrect tap changer settings.
Open-Circuit Test (OCT): Performed on the primary side with the secondary open-circuited. It helps determine the transformer’s magnetizing characteristics, including core loss and no-load current.
Short-Circuit Test (SCT): Conducted on the secondary side with the primary side short-circuited. It measures the transformer’s copper losses and short-circuit impedance, indicating the transformer’s ability to handle load currents.
Dielectric Strength (Hi-Pot) Test: Applies a high voltage across the windings and between windings and ground to check for any breakdown in the insulation. This ensures the transformer can withstand overvoltage conditions without failure.
Oil Sampling and Analysis (for oil-filled transformers): Samples of the transformer oil are taken and analyzed for moisture content, acidity, dielectric strength, and the presence of dissolved gases. This helps detect early signs of insulation degradation or overheating.
Temperature Rise Test: Determines the temperature rise of the transformer during operation, ensuring it stays within safe limits.
Tap Changer Testing: For transformers equipped with tap changers, tests are performed to verify the correct operation of the tap changer mechanism, including resistance measurements and transition checks.
Regular testing not only helps identify potential problems before they cause failures but also contributes to extending the transformer's lifespan and maintaining overall system reliability. It's important to follow established standards and safety protocols during these tests, such as IEEE and IEC standards, and to have trained personnel perform the tests.
