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Measurement of DC Resistance of Transformer Coils and Analysis of Results

Time: 2019-01-18 Browse: 1326 times
Introduction

Measurement of DC Resistance of Transformer Coils and Analysis of Results
1 DC resistance measurement 1.1 Measurement method Measuring DC resistance is an important item in the transformer test. Through measurement, you can check whether the conductive circuit of the device is connected.

Defects such as poor contact, poor welding, coil failure, and wiring errors. In the actual measurement of medium and small transformers, most of them use DC

In the bridge method, when the resistance of the coil under test is above 1 ohm, it is generally measured by a single-arm bridge, and below 1 ohm by a double-arm bridge.

When using a two-arm electrical bridge wire, the potential stake of the bridge should be close to the measured resistance, and the current stake should be connected above the potential pile. measuring

Before, you should first estimate the resistance of the coil under test, set the bridge magnification selector to an appropriate position, short-circuit and ground the non-test coil, and then

Turn on the power switch to charge, press the galvanometer switch after fully charged, and quickly adjust the measuring arm so that the pointer of the galvanometer is in the scale of the galvanometer.

Move in the direction of the zero line between the two, perform fine adjustment, and record the resistance value when the pointer stops at the zero position smoothly. At this time, the measured coil resistance value =

Magnification number × Measurement arm resistance value. After the measurement is completed, first open the galvanometer button and then release the power switch.
1.2 Precautions In the measurement process, in addition to strictly observing electrical safety regulations and equipment test regulations, special attention should be paid to:
1) The measurement is performed under the condition that the coil temperature is stable, and the difference between the upper and lower temperature of the transformer tank is not more than 3 ° C;
2) Because the transformer coil has inductance, the charging current during measurement is not stable. Be sure to count after the current is stable.

When necessary, measures to shorten the charging time;
3) Minimize the contact resistance of the wires in the test circuit. The transformer taps during operation are often affected by dirt such as oil films.

Poor contact, usually need to switch several times before measuring, so as not to cause discrimination errors.
2 Analysis of measurement results 2.1 Specification requirements According to the specification requirements, three-phase transformers should measure the line resistance, and transformers with a neutral point should measure the phase resistance;

The coils of the joints must be tested for the coil resistance at all tap positions during overhaul and handover tests. During minor repairs and pretests, only the

Find out the coil resistance at the use position. Due to the quality of transformer manufacturing, the maintenance level of the operating unit, the accuracy of the instruments used by testers, and

The measured three-phase resistance values are different depending on the measurement wiring method. Usually, the following error formula is introduced to determine △ R% = [(Rmax-Rmin) / RP] × 100%
RP = (Rab Rbc Rac) / 3
In the formula, △ R ％ ———— Percentage error Max ———— The larger value in actual measurement (Ω)
Rmin --- smaller value in actual measurement (Ω)
RP ———— Measured average in three phases (Ω)
The specification requires that the DC resistance of the coils of each phase of the transformer above 1600KVA should not be greater than 2 of the three-phase average.

%, For transformers below 1600KVA, the difference between the DC resistance values of the coils of each phase should not be greater than 4% of the three-phase average.

The difference between them should not be greater than 2% of the three-phase average value. Compared with the last measurement value, the change should not be greater than the last measurement value.

2% of the value.
2.2 For comparison and analysis, the conversion must be performed at the same temperature. If the temperature is different, the resistance value at the time of conversion to 20 ° C is calculated as follows: R20 ° C = RtK, K = (T + 20)) / (T + t)
Where R20 ℃ ———— DC resistance value at 20 ℃ (Ω)
Rt —— DC resistance at t ℃ (Ω)
T ———— Constant (234.5 for copper wire and 225 for aluminum wire)
t ———— When measuring the temperature, in order to determine the phase where the defect is located, for a three-phase transformer without a neutral point, the measured line-to-line resistance needs to be converted into each phase.

Resistance. Set the measurable line resistance of the three-phase transformer as Rab, Rbc, Rac, and the resistance of each phase is Ra, Rb, Rc.

When the transformer coil is Z-connected, 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 a delta connection and a, y, b, and c are x, Ra = (Rac- R

P) -RabRbc / (Rac-RP)

RB = (Rab-RP) -RacRbc / (Rab-RP)
RC = (Rbc-RP) -Rabrac / (Rbc-RP)
When the transformer coil is △ -connected, and a is connected to ｚ, b is connected to x, and c is connected to y,
Ra = (Rab-RP) -RacRbc / (Rab-RP)
RB = (Rbc-RP) -Rabrac / (Rbc-RP)
RC = (Rac-RP) -RabRbc / (Rac-RP)
In the formula, RP = (Rab + Rbc + Rca) / 2, if the three phases are balanced, the phase resistance is equal to 1.5 times the line resistance.

3 Example analysis It can be seen from the actual measurement results that the factors that cause the transformer coil resistance to exceed the specification requirements are many.

Insufficient bridge accuracy, incorrect measurement wiring, excessive lead resistance and wiring resistance, short transformer charging time, and inadequate bridge voltage

In the transformer itself, there are mainly poor contact of the tap, poor welding of the coil or lead, breakage, and connection of the bushing guide rod to the lead.

Bad contact, short circuit between coil turns, layers, and phases. For delta-connected transformers, if

When the defect is reflected in only one phase, the phase resistance can be obtained according to the following simplified formula for analysis and determination, that is, when Rab = Rbc ≠ Rac,

Then Rb = Rc ≠ Ra. The measurement results of several common failure phenomena are analyzed as follows:
Failure phenomenon (compared with the test value under normal conditions)
Analysis result Ｙ-type wiring △ -type wiring The resistance value between one line does not change, and the resistance value between two lines cannot be measured (the resistance value is very large)
The resistance between the two wires is 1.5 times higher than the normal value, the resistance between one wire is 3 times the normal value, the phase coil is broken, the resistance between the wires is unchanged, and the resistance between the two wires is reduced to a normal value (0.5 to 1). ) Times the resistance between the two wires increases to (1 to 3) times the normal value, and the resistance between one wire decreases to (0 to 1) times the normal value. The resistance between the two wires increases. The resistance between the wires does not change. The resistance between the two wires increases. The one-phase lead is in poor contact with the conductive rod. The resistance between the three wires cannot be measured (the resistance is very large).
One line resistance is equal to 3 times the normal value, and the resistance between two lines cannot be measured (the resistance is very large)
When the two-phase coil breaks, the three inter-wire resistances are all reduced to (0.5 to 1) times the normal value, and one of them has a much lower resistance value. The three inter-wire resistance values are all reduced to (0 to 1) times the normal value. Two of them have much lower resistance values. Two-phase coils have short-to-turn short circuits. The resistance between three wires is larger than normal. One of them has a much larger resistance. The resistance between three wires is larger than normal. And one of them has a much larger resistance value

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