AC withstand voltage test

         When the power switch S1 is closed and the green light is on, it indicates that there is power. Operate the SA "close" button to make the YS live. The moving contact K1, K2, K3, K4 is closed, and the breaking contact K5 is open. At this time, the green light is off, red. The light is on, the voltage regulator has electricity and can be boosted. When the current of the test item is too large or breakdown, the overcurrent relay KOA operates, the breaking contact KOA1 is turned off, the control circuit is cut off, the YS is turned off, and the contacts K1 to K4 are turned on to cut off the power supply of the regulator. If an accident occurs during the boosting process, you need to immediately power off, just press the SA2 trip button to disconnect the power. YP1, YP2 are low voltage measuring coils of test transformers; TV is a standard voltage transformer for measuring the high voltage on the test object; and the function of the capacitive voltage divider is the same as that of TV. The above three measurement methods can be selected according to the measurement requirements. Q is to protect the ball gap; R, the role is to prevent the test article from breaking down, because the current is too large, the test article is enlarged or the test transformer is burned; R1 is to prevent the ball gap discharge and the gap between the ball gap and the test sample. The voltage oscillates and limits the discharge current to burn the ball gap surface. R1 is selected according to the rated current of the high-voltage side of the transformer. When the rated current is 100~300mA, it can be 0.5~1Ω/V. When the rated current is 1A, it can be 0.1~1Ω/v (when the sample capacity is large, take the lower limit) R2 is selected according to Table 2-2.

         The capacity of the test transformer S _T is calculated using equation (2-4)

S _T =wC _x U ₂ *10 ^-9 (kVA)

                 U—voltage (effective value) applied to the test object, kV.

                 When selecting according to the calculation results, the capacity should be as large as S _T as much as possible, because the estimated test current must be smaller than the actual value considering the stray capacitance of the test circuit and equipment to the ground.

         2. Test procedures and attention items

         (1) Determine the test voltage value as required by the “Regulations”.

         (2) Select the test equipment and determine the test wiring.

         (3) When site layout and wiring, it should be noted that there should be sufficient distance between the high voltage and the ground. The high voltage and the test personnel should have sufficient safety distance. The high voltage lead should be firmly connected. The non-tested phase and the equipment casing should be reliably grounded. The regulator should be in the zero position.

         (4) Adjust the protective ball gap. The discharge voltage is 1.1-1.2 times of the test voltage, and a gradual and uniform rise and fall pressure method is adopted in the test.

         (5) Perform boosting. The rate of rise of the test voltage may be arbitrarily before the 40% test voltage; thereafter, it is continuously raised to the test voltage value at a test voltage of 3% per second. The withstand voltage is usually 1 min. At the end of the withstand voltage, it should be evenly reduced to less than 25% of the test voltage within 5s. Disconnect the power supply and ground the test object.

         (6) Inspection after pressure resistance. Immediately after the pressure is applied, the insulation resistance test and the heat test should be performed on the test object.

                      X _C — test product tolerance.

         When the adjustable reactor is used, the X _L can be adjusted to have the same value as the XC value. At this time, the series resonance occurs in the loop, the current in the circuit is I=U/R, the voltage on the sample Uc and the voltage on the reactor U _{L is} equal. which is

U _C =U _L = I X _L =(U/R)X _L =Q _U

         Available in Figure 2-16

         The condition of parallel resonance can be obtained as wL=1/wC; Xc=XL. At this time, although the currents of both branches are large, the total current 1 SO, the voltage on Xc is equal to the power supply voltage. Since the total current is small, the capacity of the test transformer can be greatly reduced. Therefore, it is possible to test a large-capacity test article with a small-capacity test transformer.

         3. Series-parallel resonance test circuit

         When the rated voltage and rated current of the test transformer cannot meet the test requirements, a series-parallel resonant test circuit can be used. As shown in Figure 2-17. At this time, the inductance L in the loop: compensates for the capacitance Cx of the test object, and in parallel constitutes series resonance with the inductor L1, thereby satisfying the voltage and current requirements of the test.

         In Figure 2-18, winding 1 is a low voltage winding; winding 2 is a high voltage winding; winding 3 is a series excitation winding for supplying the next stage of excitation. One end of the high voltage winding 2 of the first test transformer is grounded, the other end of the series winding 3 is supplied to the low voltage winding of the second transformer , and the windings 1 and 2 of the second transformer are each connected to the outer casing of the transformer, and they are all in the first The voltage at the high voltage end of the transformer is grounded. Therefore, the outer casing of the second transformer must be insulated from the ground. The voltage at the high voltage of the second transformer is the sum of the output voltages of the high voltage terminals of the two transformers. The potential of the casing of the third transformer is equal to the voltage of the ground of the high voltage of the second transformer, and the high voltage terminal is grounded to 3 _u 2 . A test voltage of three times the rated voltage of a single test transformer was obtained by series connection.