Usage Method of Oil-immersed Test Transformer_Operation Method is as follows:
1. The wiring schematic diagram for the power frequency withstand voltage test of the test object using GH-YD oil-immersed test transformer is shown in Figure 6.
Figure 6: Wiring diagram for power frequency withstand voltage test of test object
In the figure: R1 – Current limiting resistor RCF – Resistive-capacitive voltage divider RF – Ball gap protection resistor
G – Ball gap CX – Test object
Note: The high voltage tail must be reliably grounded.
In the power frequency withstand voltage test of the oil-immersed test transformer, the current limiting resistor R1 should be selected based on the rated capacity of the test transformer. If the rated output current on the high voltage side is between 100 - 300mA, it can be taken as 0.5 - 1Ω∕V (test voltage); when the rated output current on the high voltage side is above 1A, it can be taken as 1Ω∕V (test voltage). Commonly used water resistor as current limiting resistor, the tube length can be considered as 150KV∕m, and the thickness of the tube should have sufficient heat capacity (water resistance solution preparation method: use distilled water with appropriate amount of copper sulfate to prepare various resistance values).
Ball gap and protection resistor: When the voltage exceeds the set value of the ball gap (generally taken as 110% - 120% of the test voltage), the ball gap discharges, protecting the test object. The ball gap protection resistor can be selected as 1Ω∕V (test voltage).
In the power frequency withstand voltage test, the voltage measured on the low voltage side (instrument voltage) is not very accurate because the test transformer has leakage reactance, and there must be voltage drop or capacitive rise on this leakage reactance, causing the voltage on the test object to be lower or higher than the voltage reflected on the low voltage side voltmeter. During the power frequency withstand voltage test, the voltage on the test object is higher than the output voltage of the test transformer, which is the so-called capacitive rise phenomenon. During the induced withstand voltage test, the leakage reactance of the test transformer must have a voltage drop. In order to accurately measure the voltage applied to the test object, it is common to connect an RCF resistive-capacitive voltage divider on the high voltage side to measure the voltage (see Figure 6).
Precautions for power frequency withstand voltage test operation:
(1) Test personnel should divide the work and clarify the method of mutual contact. There should be a dedicated person to monitor the safety of the site and observe the state of the test object.
(2) The test object should be cleaned and absolutely dry before testing to avoid damage to the test object and errors caused by the test.
(3) For large tests, generally a no-load test should be conducted first. That is, without connecting the test object, increase the voltage to the test voltage, calibrate various meters, and adjust the ball gap.
(4) The voltage increase speed should not be too fast, and sudden pressurization must be prevented. For example, sudden closing when the voltage regulator is not at zero position. Also, the power supply should not be suddenly cut off; generally, the switch should be turned off after the voltage regulator is reduced to zero position.
(5) When the voltage rises to the test voltage, start timing. After 1 minute, only when the voltage is quickly reduced to below 1/3 of the test voltage can the power supply be turned off.
(6) During the voltage increase or withstand voltage test, if any of the following abnormal conditions are found, the voltage should be immediately reduced, the power supply cut off, the test stopped, and the cause investigated: ① The voltmeter pointer swings greatly; ② Insulation burning smell or smoke is found; ③ Abnormal sounds inside the test object.
(7) Before and after the withstand voltage test, the insulation resistance should be measured to check the insulation condition.
2. The wiring schematic diagram for GH-TDM (G) AC/DC test transformer when conducting DC withstand voltage or leakage test on the test object is shown in Figure 7.
Note: For this test, the short-circuit rod "D" should be removed first, as shown in Figure 7.
In the figure: VD – High voltage silicon stack R1 – Current limiting resistor C1 – High voltage filter capacitor
RCF – Resistive-capacitive voltage divider CX – Test object uA – Microammeter with protection
In the leakage test, the current limiting resistor R1 is selected such that at the rated output voltage, the short-circuit current at the output end does not exceed the maximum rectified current of the high voltage silicon stack. If the maximum rectified current of the voltage silicon stack is 100mA and it is used in a 60KV test device, the current limiting resistor is selected as R1=60/0.1=600KΩ. The current limiting resistor should also have sufficient capacity and creepage distance. The high voltage filter capacitor C1 is generally selected as 0.01 - 0.1uF. When the capacitance of the test object is large, C1 can be omitted.
Operation and precautions for leakage test:
(1) Before the test, check whether the test object is de-energized, grounded and discharged, and whether all external connections are clean. Strictly prevent the test voltage from being applied to parts where people are working.
(2) After connecting the test device wiring, it should be rechecked for correctness before applying voltage. Special attention should be paid to checking the safe distance between high voltage equipment and leads, and the ground, and the operator, and whether the outer shell of the test object is reliably grounded. The test should be conducted according to the contents specified in the safety regulations.
(3) For large capacitance equipment, the voltage should be increased slowly to prevent the charging current of the test object from burning out the microammeter. If necessary, the voltage should be increased in stages, and the stable readings of the microammeter at each voltage level should be recorded separately.
(4) During the test, the test object, test device, and microammeter should be closely monitored. Once abnormal phenomena such as breakdown or flashing occur, the voltage should be immediately reduced, the power supply cut off, and the cause investigated and recorded in detail.
(5) After the test, reduce the voltage, cut off the power supply, and then fully discharge the test object and the test device itself.
