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Partial discharge refers to a non-penetrating electrical discharge that occurs within the insulation system of an electrical device, such as air gaps, oil films, or conductor edges, under the influence of voltage. These tests are commonly performed on equipment like transformers, bushings, and capacitors. Among various testing methods, the pulse current method is widely used today.
The pulse current method works by detecting the charge exchange that happens during partial discharge. This exchange causes a pulsating voltage across the test sample, and a corresponding pulse current flows through the loop connected to it. By measuring this current using a detection impedance, we can identify and quantify the partial discharge signal.
There are three basic circuits used for measuring partial discharge via the electrical method, as shown in Figure 2-19. The first two, (a) and (b), represent direct measurement loops, while (c) illustrates a balanced measurement loop. Each circuit has its own advantages depending on the specific conditions of the test.
In these setups, the measured impedance Zm is part of a four-terminal network. It could be a simple resistor, inductor, or more complex combinations like RC or RL circuits. In some cases, an RLC tuning loop is used, with frequency characteristics matched to the measuring instrument’s operating range. The impedance should also prevent the power supply frequency from interfering with the measurement. A shielded coaxial cable is typically used to connect the impedance to the amplifier unit in the measuring system.
A high-voltage filter is often included to block unwanted interference from the power supply, improving the sensitivity of the measurement. When selecting the test circuit, factors such as the sample’s capacitor current and environmental disturbances are considered. If the capacitor current exceeds the allowable level, or if grounding is fixed, the circuit in (a) is preferred. For lower current levels, the setup in (b) is suitable. When interference is significant, the balanced circuit in (c) is the best choice.
The measuring instrument used in partial discharge testing displays the apparent discharge quantity, usually through an oscilloscope or a digital PC meter. Before each test, the system must be calibrated to ensure accurate results. Calibration can be done directly or indirectly, depending on the setup.
The measurement procedure involves several key steps. First, the test sample must be clean and dry to avoid moisture-related discharges. The test should be conducted at ambient temperature, and after any mechanical, thermal, or electrical stress, the sample should be allowed to rest before testing.
Next, the test circuit itself must be checked for inherent partial discharge. If no discharge occurs at a slightly higher voltage than the test voltage, the circuit is considered acceptable. Otherwise, the source of interference must be identified and eliminated.
Before applying voltage, the test loop must be calibrated to determine the scale factor. Under the calibrated sensitivity, any major interference should be removed. Then, the starting and extinguishing voltages for partial discharge are determined by gradually increasing and decreasing the voltage until the discharge reaches the desired level.
Finally, the actual discharge quantity is measured at the specified test voltage, following the guidelines set by relevant standards. This ensures that the results are both accurate and meaningful for further analysis.