What problems need to be paid attention to in the measurement of transformer DC resistance measuring

The purpose of testing the DC resistance of a transformer:

1. Check the welding quality of the winding joint, and whether the winding has an inter-turn short circuit;

2. Whether the contact between the positions of the on-load tap-changer is good, and whether the actual position of the tap-changer is consistent with the indicated position;

3. Whether the outgoing line is broken;

4. For three-phase power transformers, calculate the unbalance rate of the three-phase DC resistance according to the test results to determine whether it meets the design or relevant standards.

The DC resistance of the transformer is measured with a DC resistance tester.

Measuring DC resistance is an important item in transformer testing. By measuring the DC resistance, it is possible to check whether the conductive circuit of the device has defects such as poor contact, poor welding, coil failure, and wiring error. In the actual measurement of small and medium-sized transformers, the DC bridge method is mostly used.

When the resistance value of the measured coil is greater than 1, it is generally measured with a single-arm bridge, and when the resistance value is less than 1, it is measured with a double-arm bridge. When using a double-arm bridge for wiring, the potential pile head of the bridge should be close to the resistance to be measured, and the current pile head should be connected to the potential pile head.

Before measuring, the resistance of the coil to be tested should be estimated, the bridge ratio knob of the DC resistance tester should be placed in the appropriate position, the untested coil should be shorted to ground, and then the power switch should be turned on. Turn on charging. After the power supply is sufficient, press the galvanometer switch to quickly adjust the measuring arm, so that the pointer of the current detector moves to the neutral line in the middle of the galvanometer scale, and fine-tunes it to the value when the pointer is stable and the recording resistance stops at the zero position.

At this time, the resistance value of the coil to be measured = the resistance value of the multiplier measuring arm. After the measurement, turn on the galvanometer button and then release the power switch.

In the measurement process, in addition to strictly following the electrical safety regulations and equipment test procedures, the following points should also be noted:

1) When the temperature of the coil is stable, the temperature difference between the upper and lower parts of the transformer tank shall not exceed 3;

2) Due to the inductance of the transformer coil, the charging current is unstable during measurement. It is necessary to count after the current is stabilized, and if necessary, shorten the charging time;

3) The contact resistance of the conductor in the test circuit should be reduced as much as possible. The tap joints of the transformer in operation are often affected by dirt such as oil film, so that the contact is poor. Generally, it is necessary to switch several times before measuring to avoid misjudgment;

4) When measuring the low-voltage side of the transformer, all personnel should keep a safe distance from the high-voltage side;

5) During the test, the test wire clamp must be fixed firmly. One of the large transformers applied a large current during the test, and if the wiring is not reliable enough, it is easy to produce arcs; Secondly, if the clip falls off during the test, it is easy to damage the instrument (high voltage).

6) The transformer DC resistance tester should avoid strong vibration, direct sunlight and magnetic field during transportation, storage and work. When storing and keeping the instrument, attention should be paid to the ambient temperature and humidity. It should be placed in a dry, ventilated place without corrosive gases.

Do not allow moisture, rain and sun exposure. It should be handled with care and care.

Significance of measuring DC resistance:

Since the internal structure of the transformer is sealed by an insulating medium after the installation is completed, the significance of measuring the DC resistance is:

Check the welding quality of the wires and leads inside the winding, and when the welding is not good, it can be judged by the resistance of the three phases.

Check that the tap-changer contacts well at all positions.

Check the winding or lead-out for breaks.

Check the correctness of the parallel branch, whether there is one or several disconnections in the winding wound by several parallel wires.

Check whether there is a short circuit between the layers and turns.

Methods for measuring DC resistance:

1. The equipment to be tested is connected with a special test line with the instrument, which is connected as shown in the figure below, and at the same time ensures that the clamping of each test fixture is firm and reliable.

2. Turn on the power switch of the 10A DC resistance tester, the instrument displays "Welcome to use", and then enter the "Range Selection" interface, at this time, use the "Up, Down" keys to move the cursor, select the test range you need to set, and press the "Confirm" button to start the measurement.

3. When the instrument enters the measurement state, the LCD screen displays the charging simulation bar, and after the charging is completed, the LCD screen displays the value, which is the DC resistance value of the device under test, and the value will be saved and locked by pressing the "Confirm" button. When the screen displays "Out of Range" for a long time, press the "Back" button to return to the previous menu, select a higher test range to retest, or check whether the test loop is open.

4. The instrument enters the data lock interface, press the "Print" button and the data will be printed, and the instrument will be automatically discharged after the data is printed, and return to the upper menu after discharge.

Respondent: Huatian Power.

The purpose of measuring the DC resistance of a transformer winding is:

1) Check the welding quality of the winding; Transformer.

2) Check whether the contact between the various positions of the tap changer is good;

3) Check whether there is any break in the winding or lead wire;

4) Check the correctness of the parallel branch, whether there is one or several broken wires in the winding winding formed by several parallel wires;

5) Check whether there is a short circuit between the layers and turns.

The necessity of measuring the DC resistance of AC windings live.

Hello, dear. The answer to your question is as follows: Transformer DC Resistance Tester Measurement:

After connecting the test cable, turn off the power switch, the power indicator turns on, press the "Start and Stop" button, and the meter head displays "e0000". Select the resistance range according to the measured value, press the gear switch, the gear indicator light is on, and the stable value displayed on the four and a half LCD screens is the measured resistance value. When reading a numeric value, pay attention to the units (m k) in the selected range.

When you measure large inductance samples, such as power transformers, test circuits other than coils, do not need to measure whether they are short circuited to avoid electromagnetic interference.

The purpose of measuring the DC resistance of transformer windings is: to check the welding quality of the winding joint and whether the winding has inter-turn short circuit; Whether the contact between the various positions of the voltage tap-changer is good and whether the actual position of the tap-changer is consistent; Whether the lead wire is broken, whether there are broken strands in the multi-strand wire and winding, etc. The DC resistance of the windings along with the bushing needs to be measured during an overhaul of a transformer, or after a tap position, or after an outlet fault short circuit.

The measurement method is as follows.

1) Current and voltmeter method. Also known as the voltage drop method, its principle is to measure the voltage drop on the resistor by passing a DC current into the measured resistance, and the measured resistance value can be calculated according to Ohm's law. Since the internal resistance of ammeters and voltmeters has an impact on the measurement results, the way in which they are fed into the measurement circuit should be carefully considered.

2) Balanced bridge method. It is a method that uses the principle of bridge balancing to measure DC resistance, and there are two commonly used balance bridges: single-arm and double-arm bridges. When measuring the DC resistance of a transformer, it should be done after the transformer is powered off and the high-voltage leads are removed.

For large-scale large-capacity power transformer, because the charging time constant of RL series circuit is very large, each measurement takes a long time to wait for the current, voltmeter indication is stable, thus the work efficiency is very low, often use special instruments (such as constant current power supply) to replace the power supply in the test, so that the test time can be greatly shortened. The standard for measuring the DC resistance of transformer coils is: for transformers above 1600kva, the difference between the winding resistance of each phase should not be greater than 2% of the average value of the three phases, the difference between the lines should not be greater than 1% of the average value of the three phases for the windings without neutral point lead wires, and for the transformers of 1600kva and below, the difference between the phases is generally not more than 4% of the average value of the three phases, and the difference between the lines is generally not more than 2% of the average value of the three phases, and the change should not be greater than 2% compared with the previous values measured in the same parts.

Measurements are made with a DC bridge.

The DC resistance of the winding of the small power transformer can be directly measured with the resistance level of the multimeter.

The windings of high-power transformers can be measured with bridges. In order to prevent the influence of self-induced electromotive force of the transformer winding, the reading should be read after the reading is stable; The high-power high-voltage transformer should also be connected with a high-power resistor in parallel on the winding under test to absorb the back EMF when the measurement circuit is disconnected and avoid ignition damage to the instrument.

The DC resistance of the transformer refers to the DC resistance value of each phase winding, and the purpose of measuring the DC resistance is to check whether there is an inter-turn short circuit inside the three-phase winding. Because if there is an interphase short circuit inside the transformer, the short-circuit current value is very large, it is easy to burn the transformer, and the phenomenon of fault manifestation is also obvious, and the appearance is easy to judge; However, if there is a short circuit between the windings of one of the phases, the short-circuit current value is very small, and the gas protection of the transformer will act as a trip, but it is difficult to see whether the transformer itself is faulty from the appearance of the transformer. By measuring the DC resistance of each phase, it is easy to determine whether there is an inter-turn short circuit by comparing the resistance values of the three phases

The resistance value varies greatly, and there is a great possibility of inter-turn short-circuit faults; If it is substantially similar, the possibility of such a failure can be ruled out.

Because according to the structure of the transformer, the windings are almost insulated by the insulating medium of the insulated wire itself, if there are defects in the insulation treatment, and the transformer load is large, the weak part of the insulation is easy to cause inter-turn short circuit. Therefore, the purpose of measuring DC resistance is to determine whether the transformer has an inter-turn short circuit and facilitate fault handling.