Fault detection of partial discharge tester

1. Frequency, voltage, duration, and judgment of partial discharge test power supply

The detection of partial discharge in transformers often uses induction voltage, and the test voltage is generally higher than the rated voltage of the transformer. To prevent iron core saturation, the power frequency is often 150-250Hz. The detection circuits for single-phase and three-phase transformers are shown in Figure a and b.

Partial discharge signals often originate from the high-voltage terminal screen. If the high-voltage bushing does not have a terminal screen, it can be used to couple the signal from capacitor Ck. By connecting the impedance test to the testing instrument and comparing it with the calibrated discharge amount on the testing instrument, the discharge amount of partial discharge can be determined.

During the partial discharge test of the transformer, the neutral point of the tested winding should be grounded and a high voltage terminal voltage should be applied according to the procedure shown in Figure C. The voltage application program includes a test voltage that rises to Z high within 5 seconds. The use of power frequency test power supply cannot induce such high test voltage in the winding. Because the magnetic flux density of the iron core is saturated, the excitation current and ferromagnetic losses will increase sharply, so increasing the power frequency is a feasible solution. At the same time, when measuring partial discharge of power equipment, the test standard includes a pre applied voltage process that is higher than the specified test voltage value for a short period of time. This is because in actual operation, partial discharge is often triggered by overvoltage, and the purpose of pre applied voltage is to artificially create an overvoltage condition to simulate actual operation, in order to observe the level of partial discharge of insulation under specified conditions. For example, the situation where partial discharge occurs under simulated overvoltage and extinguishes during the subsequent approximately 30 minutes of pressurization time.

To determine the level of partial discharge in a transformer, it is necessary to apply a specified voltage and duration of 30 minutes. The discharge amount at the winding wire end of a voltage level of 220kV or higher should generally not exceed the corresponding discharge amount standard, and the starting and extinguishing voltages and their development trend with the applied voltage should be observed.

2. Determination of partial discharge faults in transformers

Partial discharge faults of transformers may occur in any location where electric fields are concentrated or insulation is poor, such as solid insulation materials or bubbles in transformer oil, high-voltage winding electrostatic shielding outgoing lines, high-voltage leads, phase to phase enclosures, and winding turns. Strictly speaking, there are always varying degrees of partial discharges inside transformers. This type of discharge that has not yet penetrated the electrodes, if it involves solid insulation, can leave traces on the insulation in severe cases and eventually develop into breakdown between the electrodes. For severe partial discharge faults, due to the short time it takes for some to develop into breakdown and the often unclear characteristics of oil chromatography analysis, it can be difficult to make timely judgments.

Partial discharge testing includes electrical and ultrasonic methods. The testing should try to follow the pressurization method specified in national standards, so that both the main and longitudinal insulation of the transformer can withstand high voltage, and the discharge defects can be clearly exposed. Ultrasonic method can help determine the location of discharge and is a promising experimental method. However, the performance of current testing instruments is not satisfactory, and it is difficult to determine the discharge amount, which also limits its independent development and use.