1 DC Resistance Measurement
1.1 Measurement Method
Measuring DC resistance is an important item in transformer testing. Through measurement, defects such as poor contact, poor welding, coil faults, and wiring errors in the conductive circuit of the equipment can be detected. In the actual measurement of medium and small transformers, the DC bridge method is mostly used. When the resistance value of the tested coil is above 1 ohm, a single-arm bridge is generally used for measurement, while for below 1 ohm, a double-arm bridge is used. When wiring with a double-arm bridge, the potential terminal of the bridge should be close to the measured resistance, and the current terminal should be connected above the potential terminal. Before measurement, estimate the resistance value of the tested coil, set the bridge multiplier switch to an appropriate position, short-circuit and ground the non-tested coils, then turn on the power switch to charge. After sufficient charging, press the galvanometer switch and quickly adjust the measuring arm to move the galvanometer pointer toward the zero line in the middle of the galvanometer scale, making fine adjustments. When the pointer stops steadily at zero, record the resistance value. At this time, the tested coil resistance = multiplier × measuring arm resistance. After measurement, first release the galvanometer button, then turn off the power switch.
1.2 Precautions
During the measurement process, in addition to strictly complying with electrical safety regulations and equipment testing regulations, special attention should be paid to:
1) Perform measurement when the coil temperature is stable, requiring that the temperature difference between the upper and lower parts of the transformer oil tank does not exceed 3°C;
2) Due to the inductance of transformer coils, the charging current during measurement is not very stable. Be sure to count only after the current is stable, and if necessary, take measures to shorten the charging time;
3) Minimize the contact resistance of wires in the test circuit. The tap changers of operating transformers are often affected by contaminants such as oil films, causing poor contact. Generally, switch several times before measurement to avoid misjudgment.
2 Analysis of Measurement Results
2.1 Specification Requirements
According to specification requirements, for three-phase transformers, the line-to-line resistance should be measured; for transformers with a neutral point lead, the phase resistance should be measured; for coils with tap changers, during major overhauls and acceptance tests, the coil resistance at all tap positions should be measured, while during minor overhauls and preventive tests, only the coil resistance at the used position needs to be measured. Due to differences in transformer manufacturing quality, maintenance level of the operating unit, accuracy of the instruments used by test personnel, and measurement wiring methods, the measured three-phase resistance values also differ. Usually, the following error formula is introduced for judgment:
△R% = [(Rmax - Rmin)/RP] × 100%
RP = (Rab + Rbc + Rac) / 3
Where △R% –––– error percentage
Rmax –––– maximum value in actual measurement (Ω)
Rmin –––– minimum value in actual measurement (Ω)
RP –––– average value of actual measurement in three phases (Ω)
The specification requires that for transformers above 1600KVA, the difference among the DC resistance values of each phase coil should not be greater than 2% of the three-phase average value; for transformers below 1600KVA, the difference among the DC resistance values of each phase coil should not be greater than 4% of the three-phase average value, and the difference between lines should not be greater than 2% of the three-phase average value; compared with the previous measurement value, the change should not be greater than 2% of the previous measurement value.
2.2 Related Conversion
When performing comparative analysis, it must be done at the same temperature. If the temperature is different, convert to the resistance value at 20°C using the following formula:
R20℃ = RtK, K = (T + 20) / (T + t)
Where R20℃ –––– DC resistance value at 20°C (Ω)
Rt –––– DC resistance value at t°C (Ω)
T –––– constant (234.5 for copper conductors, 225 for aluminum conductors)
t –––– temperature during measurement
To determine the phase where the defect is located, for three-phase transformers without a neutral point lead, it is also necessary to convert the measured line-to-line resistance into per-phase resistance. Suppose the measurable line-to-line resistances of the three-phase transformer are Rab, Rbc, Rac, and the per-phase resistances are Ra, Rb, Rc. When the transformer coil is connected in Y-type, the phase resistance is:
Ra = (Rab + Rac - Rbc) / 2
Rb = (Rab + Rbc - Rac) / 2
Rc = (Rac + Rbc - Rab) / 2, if the three phases are balanced, the phase resistance is equal
to 0.5 times the line resistance; when the transformer coil is connected in △-type, and a connects y, b connects z, c connects x,
Ra = (Rac - RP) - RabRbc / (Rac - RP)
Rb = (Rab - RP) - RacRbc / (Rab - RP)
Rc = (Rbc - RP) - RabRac / (Rbc - RP)
When the transformer coil is connected in △-type, and a connects z, b connects x, c connects y,
Ra = (Rab - RP) - RacRbc / (Rab - RP)
Rb = (Rbc - RP) - RabRac / (Rbc - RP)
Rc = (Rac - RP) - RabRbc / (Rac - RP)
Where RP = (Rab + Rbc + Rca) / 2, if the three phases are balanced, the phase resistance is equal to 1.5 times the line resistance.
3 Case Analysis
From the actual measurement results, it can be seen that there are many factors causing the transformer coil resistance value to exceed the specification requirements. In terms of measurement technology, the main ones are insufficient bridge accuracy, incorrect measurement wiring, excessive lead resistance and wiring resistance, short charging time of the transformer, insufficient bridge voltage, etc.; in the transformer itself, the main ones are poor contact of tap changers, poor welding or breakage of coils or leads, poor contact between bushing conductor rod and lead, short circuits between turns, layers, or phases of coils, etc. For transformers with delta connection, if the resistance value already reflects that the defect is only in one phase, the following simplified formula can be used to obtain the phase resistance for analysis and determination, that is, when Rab = Rbc ≠ Rac, then Rb = Rc ≠ Ra. The measurement results of several common fault phenomena are analyzed as follows:
