Analysis of Three-phase Unbalance in DC Resistance Test

The three-phase resistance is unbalanced or the measured value is too different from the design value (factory test value). Generally, there are several reasons:

1. Poor contact between the conductive rod and the inner lead in the transformer bushing. For example, after the on-site transformer is overhauled, the bolts at the connection between the conductive rod in the bushing and the inner lead are not tightened tightly, causing the joint to heat up.

2. The tap changer is in poor contact. The resistance of individual taps is too large and the three-phase resistance is unbalanced due to the uncleanness of the inside of the tap changer, the peeling of the electroplating layer, and the insufficient spring pressure.

3. The low-voltage windings of large-capacity transformers are of double-helix or four-helix type. Due to the mutual displacement of the wires between the helices, the resistance of each phase winding is unbalanced.

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

5. The phase-to-phase difference of the resistance has exceeded the specification when it leaves the factory.

6. Wrong measurement wiring and experimental methods.

Incorrect measurement wiring and test methods that cause resistance unbalance are generally:

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 measurement 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 at the contact is also included in the measured value.

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

JYR-20S DC winding resistance tester

1. Testing lines are connected to the transformer in one trial, without resetting testing line during testing process.
2. Adopting simultaneous testing of three phases when Yn type is tested, high-voltage YN three-phase simultaneous measurement compensation method (including O-phase resistance),and calculate balance rate of resistance of three phases automatically.

3. Under Yn/D(Y)type can be tested phase by phase, and calculate balance rate of resistance of three phases automatically, it solves the problem that the leading wire resistance of neutral point can not be measured.

4. Apply automatic magnetic assist for testing low voltage when testing large capacity YnD11 transformer (with magnetic assist box inside).

5. Auxiliary to judge the data stability, automatically calculate the percentage of 5", 15" or 30" resistance deviation, and accurately judge the authenticity of the test data

6. Large screen display, humanized operation, printing with USB and 485 communication interface.

7. Protection function from misconnection 380V power, power off and back EMF, discharging indication is clear.

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