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Causes of three-phase imbalance in resistance

time:2020/11/9   source:华天电力  reading:542 time

The resistance in the DC resistance tester is unbalanced or the measured value is too different from the designed value (factory test value). Generally, there are several reasons:

1. Poor contact between the conductive rod and the inner lead in the transformer bushing. Many on-site transformers were overhauled and the bolts at the connection between the conductive rod and the inner lead in the bushing were not tightened tightly, causing the joint to heat up.

2. Poor contact of the decomposition switch. Due to the unclean interior of the tap changer, electroplating off, insufficient spring pressure, etc., the resistance of individual taps is too large and the three-phase resistance is unbalanced.

Transformer DC Winding Resistance Tester.png

3. The low-voltage winding of the large-capacity transformer adopts double-spiral or quad-spiral type. Because the spiral sees the wires move each other, the resistance between each phase winding is unbalanced.

4. Poor welding. Due to the poor welding quality of the lead wire and the winding, the resistance at the contact point is too large, or one or several strands of the multi-strand winding are not welded, resulting in a large resistance.

5. The difference between the resistance phases has exceeded the regulations when leaving the factory.

6. Wrong measurement wiring and experimental methods.

The wrong measurement wiring and test methods that cause resistance imbalance generally include:

1. The charging time is not enough and the measured value is read when the current is not stable.

2. The connection position of the measuring wiring and the transformer connector is wrong, that is, the voltage lead is outside the current lead or the same position of the current lead during measurement, so that the resistance of the contact is also included in the measured value.

3. When measuring a winding, the other windings are not disconnected from the grounding body, resulting in unstable charging.

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