Measuring the DC resistance of a transformer winding or motor coil is an essential step in assessing the condition of electrical equipment. The primary purpose of this measurement is to check the quality of the welding joints within the winding, detect any turn-to-turn short circuits, and ensure that the contacts of the voltage tap changer are functioning properly. It also helps identify issues such as broken leads, loose strands, or damaged windings. This test should be conducted after maintenance, when the tap position has been changed, or following a short-circuit fault at the outlet.
The most common methods for measuring DC resistance include the current-voltage meter method and the balanced bridge method. The current-voltage method, also known as the voltage drop method, involves passing a DC current through the winding and measuring the resulting voltage drop. Using Ohm’s Law, the resistance can then be calculated. However, the internal resistance of the ammeter and voltmeter can affect the accuracy of the readings, so careful attention must be paid to how these instruments are connected in the circuit.
The balanced bridge method uses a bridge circuit to measure resistance by balancing the arms of the bridge. Commonly used devices for this method include the QJ23A single-arm bridge and the QJ44 or QJ57 double-arm bridges. When performing this test on a transformer, it is crucial to disconnect the high-voltage leads and power off the transformer first. For large power transformers, the RL charging time constant can be very long, which means the test may take significant time to complete. To speed up the process, modern digital micro-ohm meters like the PC9A from Shanghai Diyi Instrument Factory, or the SB2234/3 and SB2234B/10 DC resistance testers, are often used.
The standard for acceptable DC resistance values varies depending on the size of the transformer. For transformers above 1600kVA, the difference between the phase resistances should not exceed 2% of the average value of the three phases. For windings without a neutral point lead-out, the difference should not be more than 1%. For transformers rated at 1600kVA and below, the phase-to-phase difference should generally not exceed 4%, while the line-to-line difference should not exceed 2%. Additionally, any changes compared to previous measurements should not exceed 2% to ensure consistency and reliability.
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