The DC resistance tester under UHV power can help many power workers conduct various power tests more conveniently.
The measurement of DC resistance in transformer windings is an important part of its daily testing. By measuring the DC resistance, it is possible to check the quality of the coil, whether the contact position of the tap changer is good, whether the coil or lead is broken, whether the parallel branch is correct, and whether there is a short circuit phenomenon, which is an important data for determining the short-circuit loss. Therefore, this experiment should be conducted after the handover, pre testing, major repair, and replacement of the tap changer. In recent years, with the continuous increase of power system capacity, transformer capacity has also been increasing. The larger the transformer capacity, the higher the voltage level, and the greater the ratio of inductance to resistance. Therefore, the stability time of a large transformer winding DC circuit may be several tens of minutes or even longer.
DC resistance measurement plays an important role in the production, operation, and maintenance of power transformers. In the production and maintenance departments, DC resistance measurement during intermediate testing can be used to check the quality issues of transformer windings, welding, leads, and other processes. DC resistance, as an important component of factory testing, is not only a key indicator for judging transformers, but also a performance parameter for loading during recovery, providing a basis for operation and maintenance. In the operation department, regular measurement of DC resistance can be used to monitor the operation of transformers and eliminate potential hazards. At present, electronic instruments have been used for resistance measurement, but due to the characteristics of power transformers themselves, they cannot meet the requirements of accuracy and automated testing. Therefore, developing a precise, fast, and automated measuring device has important practical significance.
The measurement of transformer DC resistance is an essential test item in the factory test, installation test, handover test, and preventive test of the power department during the manufacturing process of semi-finished and finished transformers.
The DC resistance tester is a new generation of transformer DC resistance testing instrument, which can automatically select the test current according to different types of power transformers and quickly display the test results. The DC resistance tester also has functions such as storage, printing, and discharge indication. Built in storage that does not lose power and can store measurement data for a long time. The use of LCD display enables the instrument to have a good human-machine interface and is the preferred device for DC resistance testing.
The DC resistance of transformer windings is measured using voltage drop method or bridge method. Although the measurement method is simple, there are many factors that affect the measurement accuracy. In order to ensure the accuracy of measurement, the safety of measuring equipment and personnel, and accelerate the testing process. When measuring, the following points should be noted:
(1) For transformers with voltage tap connections, measurements should be taken at all tap positions. (2) When a three-phase transformer has a midpoint lead wire, the resistance of each phase winding should be measured; When there is no midpoint lead out, the inter line resistance can be measured to calculate the resistance of each phase.
(3) The measurement must be carried out under the condition that the winding temperature is stable and the difference between the winding and the ambient temperature is not greater than 3 ℃. When the temperature is stable, the upper oil temperature of the transformer can be used as the winding temperature.
(4) Due to the large inductance of the transformer, it takes a long time for the current to stabilize. In order to measure accurately, the instrument must wait for a stable indication before reading, and measures should be taken if necessary to shorten the stabilization time.
(5) Considering that there are many factors that affect the accuracy of DC resistance measurement, such as the accuracy level of the instrument, the testing wiring method, the temperature measurement accuracy, the connection contact state, and the current stability, it is necessary to recheck the measurement after completion and re measure if there are any doubts to obtain accurate measurement results.
(6) When measuring, the non test winding should be open circuited and not short circuited. When measuring the low-voltage winding, the high-voltage winding will induce high voltage at the moment of power supply opening and closing. Attention should be paid to personal safety.
(7) Due to the large inductance of transformers, the self induced electromotive force is very high at the moment of power on or off. Therefore, special attention should be paid to the operation sequence to prevent damage to the instrument. When connecting the power supply, first connect the power circuit, then connect the voltmeter or ammeter, and finally disconnect the power circuit.
