The purpose of measuring the DC winding resistance of the transformer windings is to check the welding quality of the winding joints and whether the windings have short circuits between the windings; whether the contact positions of the voltage tap switches are good and whether the actual positions of the taps match; whether there is a break in the lead wires or not. Stranded wires and windings have broken stocks. When the transformer is overhauled or after changing the tap position, or after the outlet fault is short-circuited, the DC winding resistance of the winding together with the bushing needs to be measured. Kingrun has series high precision DC winding resistance tester such as JYR-10C, JYR-20S/40S,also has portable winding resistance tester JYR-9310.
The measurement method is as follows:
(1) Current, voltage meter method. Also known as voltage drop method, the principle is to pass a direct current in the measured resistance, measure the voltage drop on the resistance, according to Ohm's law to calculate the measured resistance value. Since the internal resistance of the ammeter and voltmeter will affect the measurement results, the way they are connected to the measurement circuit should be carefully considered.
(2) Balanced bridge method. It is a method to measure the DC windingresistance using the principle of bridge balance. The commonly used balanced bridge has two kinds of single-arm and double-arm bridges. When measuring the DC winding resistance of the transformer, it should be performed after the transformer is powered off and the high voltage lead removed. For large-scale large-capacity power transformers, the charging time constant τ of the rl series circuit is very large, making each measurement takes a long time to wait for the current and voltmeter indication to be stable, so the working efficiency is very low, and special instruments (such as constant current) are often used. Power supply instead of the power supply in the test, which can greatly reduce the test time. The standard for measuring the DC winding resistance of transformer coils is: For transformers above 1600 kva, the difference between the resistances of each phase winding should not exceed 2% of the three-phase average value, and there should be no winding of the neutral point lead-out line. The difference between lines should not be greater than three-phase The average value of 1%, for 1600kva and below transformers, the difference between the phases is generally not more than 4% of the three-phase average, the difference between the lines is generally not more than 2% of the three-phase average, compared with the previous part of the measured value, its Change should not exceed 2%.