09,22,2026 16 views
For AC withstand voltage tests of test objects with large capacitance such as long cable lines, capacitors, large generators, and transformers, larger capacity testing equipment and power supplies are required, which are often difficult to achieve on site. In this case, the problem of insufficient capacity of the test equipment can be solved by using series, parallel resonance, or series parallel resonance (also known as series parallel compensation) methods according to the specific situation.
(1) Series resonance (voltage resonance) method. When the rated voltage of the test transformer cannot meet the required test voltage, but the current can meet the test current of the tested object, series resonance can be used to solve the problem of insufficient test voltage.
(2) Parallel resonance (current resonance) method. When the rated voltage of the test transformer can meet the requirements of the test voltage, but the current cannot reach the required test current of the tested object, parallel resonance can be used to compensate for the current and solve the problem of insufficient capacity of the test power supply.
(3) Series parallel resonance method. In addition to the series and parallel resonance mentioned above, when the rated voltage and current of the test transformer cannot meet the test requirements, series and parallel resonance lines can be used simultaneously, also known as series parallel compensation method.
In a circuit composed of an inductor coil (which can be simulated by an inductor L series resistor R) and a capacitor element (with a capacitance of C) connected in series, voltage resonance occurs when the inductance is equal to the capacitance. In depth analysis is as follows:
(1) When L and C are constant, the frequency f of the power supply is exactly equal to the natural oscillation frequency of the circuit, that is, f=1/(2 π√ LC).
(2) When the power frequency is constant, adjust the inductance L to make L=1/[(2 π f) 2C].
(3) When the power frequency is constant, adjust the electrical capacity C to make C=1/[(2 π f) 2L]. In a circuit composed of an inductor coil (which can be simulated by an inductor L series resistor R) and a capacitor element (with a capacitance of C) connected in parallel, current resonance occurs when one of the following conditions is met.
(1) Power frequency f=1/2 π√ 1/LC-R/L
(2) Adjust the electrical capacity to make C=L/[R2+(2 π fL) 2]
(3) When 2 π fCR ≤ 1, adjusting inductance L may also cause current resonance.
When conducting series resonant withstand voltage tests on transformers, attention should be paid to:
(1) We should tune first and then boost the voltage. When using a series resonance test device, the frequency of the test voltage should not be lower than 40Hz, and the withstand time under full voltage should be 60s. During the test, the inductance or frequency should be tuned to find the resonance point at a lower excitation voltage. When the voltage on the test object reaches a high level, that is, the resonance point of the test circuit, the voltage can be increased for the test.
(2) The quality factor Q value of the resonant test circuit is related to the test equipment, the dryness and cleanliness of the insulation surface of the test sample, and the diameter and length of the high-voltage lead. Therefore, the test should be conducted in clear weather conditions. The insulation surface of the experimental equipment and test samples should be dry and clean. Try to shorten the length of high-voltage leads as much as possible and use large-diameter high-voltage leads to reduce corona losses. Improve the quality factor Q value of the test circuit.
Possible reasons for insulation failure of power equipment during voltage withstand test include:
(1) The insulation performance deteriorates. If moisture enters the transformer oil, solid insulation becomes damp, insulation aging, etc., it can lead to a decrease in insulation performance and may fail the withstand voltage test.
(2) The experimental method and voltage measurement method are incorrect. For example, during transformer testing, if the non tested winding is not short circuited to ground, the non tested winding may discharge to ground and be misjudged as unqualified. For example, when testing a large capacity sample, the voltage is still measured on the low voltage side. Due to the capacitance rise effect, the actual voltage applied to the sample exceeds the test voltage, causing the sample to break down and be misjudged as unqualified.
(3) Failure to properly consider atmospheric conditions that affect insulation properties. Due to the influence of air pressure, temperature, and humidity on spark discharge voltage and breakdown voltage, failure to consider these factors may lead to the conclusion that the equipment is unqualified.
