Capacitor voltage divideris widely used in high-voltage withstand voltage test measurement equipment due to its good voltage withstand capability. The capacitor voltage divider is a device that provides a lower voltage based on the principle of capacitive voltage division. It is composed of multiple series-connected capacitor elements housed in a porcelain shell, with a medium-voltage tap. The operational reliability of the capacitor voltage divider largely depends on the reliability of its insulation. Partial discharge test is an important means to detect the insulation reliability of the capacitor voltage divider. By analyzing the hazards of partial discharge and the partial discharge detection test circuit, several corresponding measures are proposed to suppress interference in response to the susceptibility of partial discharge detection to interference.
I. Interference problems encountered in partial discharge test of capacitor voltage divider:
Using the JDF-251 partial discharge detector to conduct partial discharge test on the capacitor voltage divider, the test is based on the national standard GB/T19749-2005 requiring the partial discharge level of the capacitor voltage divider to be no more than 5pC. Although the series connection method recommended in Figure 2 has the advantage of improving test sensitivity, for capacitor voltage dividers with capacitance up to 20000PF, before using the JDF-251 partial discharge detector for partial discharge detection, system calibration must be performed without applying voltage. When calibrating large capacitance, the amplifier gain of the partial discharge detector must be increased, but this operation also amplifies external interference. After calibration, even without applying voltage, the background displayed by the partial discharge detector has reached about 5pC. In actual testing with voltage applied, the measured partial discharge amount reached 20 pC. Investigating the cause of interference, there are large vacuum drying equipment and cranes around the laboratory, often with dozens of high-power motors running. Additionally, in the laboratory, each shift needs to test more than ten products, and due to space constraints, products are densely placed, and many devices require grounding. There are also fixed interferences from the power grid. During voltage-applied measurement, the partial discharge signal of the test object is often masked by interference signals or cannot be identified. Sometimes a product needs to be tested several times for analysis and judgment, seriously reducing work efficiency.
II. Common test methods:
According to the national standard GB/T7354-2003, three test methods are recommended in the partial discharge measurement standard.
(1) Standard test circuit, also known as parallel method, suitable for test objects that must be grounded. Its disadvantage is that the stray capacitance of the high-voltage line to ground is connected in parallel with Cx, which reduces test sensitivity. See Figure 1.
(2) Series test circuit, which requires the low-voltage end of the test object to be floating relative to ground. Its advantage is that the transformer inlet
capacitance, stray capacitance of high-voltage line to ground, and coupling capacitor Ck are connected in parallel, which is beneficial to improve test sensitivity. See Figure 2.
(3) Balanced method test circuit: requires two test objects to be close, at least with capacitance of the same order of magnitude. Its advantage is that under strong external interference, it can achieve better interference suppression effect, and can do large capacitance tests. The disadvantage is that two similar test objects are required, and when discharge occurs, it is necessary to determine which test object is discharging.
III. Technical measures to eliminate interference and parameter calculation:
(1) Using the balanced method anti-interference circuit wiring method, its advantage is that under strong external interference, it can achieve better interference suppression effect, and can do large capacitance tests. However, this method requires that the test power supply capacity can meet the test requirements.
(2) Grounding interference: Improper grounding of the circuit, such as in a grounding grid system with two or more grounding points, various high-frequency signals can be coupled into the test circuit through the grounding wire, causing interference. This type of interference is generally unrelated to the level of test voltage. The test circuit adopts reliable single-point grounding, and the test circuit system is designed as a single-point grounding structure. The grounding wire uses a 30mm wide braided copper strip. The host is grounded, the grounding resistance should be small, and the grounding point should be separated from the general laboratory ground grid and power grid neutral line. Single-point grounding can reduce this interference.
(3) Floating potential interference: The induced floating potential discharge from ungrounded metal objects near the test circuit is also a common interference. Its characteristic is that it increases with the increase of test voltage, but its waveform is generally easier to identify. The countermeasures to eliminate it are: first, keep the products to be tested as far away from the test object as possible; second, ground the products to be tested.
(4) Corona discharge and contact discharge interference at various connections: Corona discharge occurs at conductive parts of the test circuit at high potential, such as the flange of the test object, metal caps, test transformer, ends of coupling capacitors, and sharp parts of high-voltage leads. All connection points should have good contact, especially the high-voltage end should not leave sharp contact points. The moving high-voltage wire should be as thick as possible to prevent corona, using a corrugated pipe with a diameter greater than 200mm. For example, poor contact at connections can also cause contact discharge interference. The characteristic of this interference is that it increases with the increase of test voltage. To eliminate this interference, anti-corona measures (such as anti-corona rings) are adopted at the high-voltage end, the high-voltage lead uses a corona-free conductive round tube, and good contact at all connection parts is ensured.
(5) Master the zero position of the test voltage. The typical waveform of partial discharge inside the test object is usually symmetrical, located on the positive rising segment of the sine wave, symmetrically superimposed on the elliptical baseline. Some interference (such as corona discharge at high potential and ground potential tips) signals are at the peak of the sine wave. Identifying the zero position of the test voltage on the elliptical baseline also helps with waveform recognition.
