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Partial discharge refers to a localized electrical discharge that occurs within an insulating material, such as air gaps, oil films, or conductor edges, under the influence of an applied voltage. These discharges do not fully penetrate the insulation and can be an early indicator of potential insulation failure. Partial discharge testing is commonly performed on electrical equipment like transformers, bushings, and capacitors. Among various testing methods, the pulse current method is widely used due to its accuracy and reliability.
The pulse current method works by detecting the charge exchange that occurs during partial discharge, which results in a pulsating voltage across the test sample. This leads to a corresponding pulse current in the loop connected to the sample. By measuring this current through a detection impedance, the partial discharge signal can be captured and analyzed.
There are three basic circuits for measuring partial discharge using the electrical method, as shown in Figure 2-19. The first two (a) and (b) are direct measurement loops, while (c) represents a balanced measurement loop. The choice of circuit depends on the specific conditions of the test setup and the characteristics of the sample being tested.
In these circuits, the measured impedance Zm is a key component of a four-terminal network. It can consist of a single resistor, inductor, or combinations such as RC, RL, or even RLC circuits. The frequency response of the impedance must match the operating frequency of the measuring instrument, and it should also prevent the power supply frequency from interfering with the readings. A high-voltage filter is often included to reduce noise and improve the sensitivity of the measurement.
When selecting the appropriate test circuit, several factors must be considered. If the power frequency capacitor current of the sample exceeds the allowable limits of the impedance, the configuration in Figure 2-19(a) is typically used. For cases where the current is within acceptable limits, Figure 2-19(b) may be more suitable. However, if there is significant interference, the balanced configuration in Figure 2-19(c) is preferred. The type of measuring instrument also plays a role; narrowband instruments are ideal for environments with strong interference, while wideband instruments work well when the background noise is minimal.
The partial discharge measuring instrument is designed to quantify the apparent discharge level, often displayed on an oscilloscope or a digital PC meter. Each time the test setup changes, the apparent discharge must be recalibrated. Calibration can be done directly or indirectly, depending on the requirements of the test.
The PD test procedure involves several steps. First, the test sample must be clean and dry to avoid moisture-related discharges. It should also be kept at ambient temperature and allowed to rest after any prior mechanical, thermal, or electrical stress. Before applying voltage, the test circuit itself should be checked for any existing partial discharge. If no discharge is detected at a slightly higher voltage than the test level, the circuit is considered acceptable.
Next, the test loop must be calibrated to determine the scale factor. This ensures accurate measurements when the sample is connected. Interference should be minimized before and after applying high voltage. Once the setup is ready, the starting and extinction voltages for partial discharge are determined. The starting voltage is the point at which discharge begins, while the extinction voltage is when the discharge stops.
Finally, the partial discharge amount is measured at the specified test voltage. The results are compared against regulatory standards to ensure compliance and identify any potential issues with the insulation system.