Abstract : Based on insulation structure analysis and practical experience, this paper demonstrates the necessity, urgency, and feasibility of implementing a new standard that replaces DC withstand voltage with AC inter-electrode withstand voltage as soon as possible. Keywords : Power capacitor, AC inter-electrode withstand voltage. Introduction For a long time, the acceptance test after on-site installation of AC power capacitors has relied on the simple and easy-to-implement DC inter-electrode withstand voltage test as a means of verifying the goodness of the main insulation performance. In recent years, paper-insulated or fully insulated power capacitors have become increasingly common, and the capacity of single capacitors has become larger and larger, leading to the emergence of large-capacity aggregated power capacitors. However, on-site acceptance still relies on DC withstand voltage tests, resulting in frequent reports of power capacitor damage after commissioning, especially in aggregated types. The following discussion and opinions address these issues. 1. Necessity of AC Inter-electrode Withstand Voltage Test for Power Capacitors ① DC withstand voltage tests cannot reflect the electric field distribution under actual operating conditions of the equipment, making it difficult to accurately detect internal defects in the capacitor. Under DC voltage, the voltage on the power capacitor element is distributed according to resistance; while under AC voltage, it is distributed according to dielectric constant, reflecting the actual operating conditions. The resistivity of the solid dielectric in all-film or paper-film capacitors can be as high as 1 to 100 EΩm. When the insulation of the insulating film of a capacitor element is poor, its resistivity can drop significantly to a fraction of the original resistivity. During DC withstand voltage testing, the voltage that a good capacitor element with high resistivity can withstand can be several times higher than that of a poor capacitor element. This makes it easier for poorly insulated capacitor elements to pass the test, and their insulation defects will be exposed more quickly under operating voltage, developing into a fault or causing an accident. ② DC voltage can greatly reduce the partial discharge inside the capacitor, which is not conducive to the detection of insulation defects. Some weak points in the insulation inside the capacitor or the parts where the electric field is concentrated at the edge of the plate may generate partial discharge. Continuous partial discharge is harmful to the insulation of the capacitor. Therefore, the standard stipulates that the partial discharge of power capacitors under test voltage shall not exceed 100 pC [1]. When the voltage is applied, the electric field strength in the oil gap, especially the air gap [2] of the capacitor element is often higher than that of the solid dielectric, but its breakdown electric field strength is lower, so partial discharge often occurs first. However, the partial discharge of the same composite insulation will be greatly reduced under the action of DC voltage. The basic principle is shown in Figure 1. After partial discharge occurs in the air gap, the positive and negative ions generated form a reverse electric field E′, which reduces the combined field strength in the air gap, weakening or even extinguishing the partial discharge. However, AC voltage is different. As long as the applied test voltage is higher than the partial discharge initiation voltage, at least two partial discharges will occur within each half-cycle. Therefore, AC withstand voltage is far more sensitive in detecting insulation defects than DC withstand voltage. [align=center] Figure 1 Schematic diagram of the reverse electric field formed after air gap discharge[/align] ③ The power frequency AC withstand voltage test conforms to the actual waveform of the operating voltage and is more consistent with the transient voltage rise that occurs during operation, so there is no equivalence issue. Therefore, applying AC withstand voltage is necessary to truly assess the manufacturing quality of power capacitors and to effectively detect insulation defects caused by poor materials, improper processes, etc. In view of the problems existing in DC withstand voltage testing, the power industry, in order to ensure project quality and operational safety, specifically formulated DL/T628-1997 "Technical Conditions for Ordering High-Voltage Parallel Capacitors," which was published on October 22, 1997, and implemented on March 1, 1998. The standard clearly stipulates that inter-electrode AC withstand voltage testing should be performed during factory testing and acceptance. The "Technical Conditions for Ordering High-Voltage Parallel Capacitors" has also been discussed by the standards committee and clearly stipulates the inter-electrode AC withstand voltage test items. Its publication is imminent, and the urgent task is to quickly publicize the provisions of the new standard within the power industry so that it can be widely implemented as soon as possible. 2. Comparison of AC Withstand Voltage Test Methods Inter-electrode AC withstand voltage testing of power capacitors requires a large-capacity testing device. The following compares several possible methods. ① Conventional Method: This method uses a large-capacity test transformer combined with a compensating reactor (see Figure 2). When testing large-capacity capacitors, many reactors need to be connected in parallel, and it is difficult to avoid a certain degree of "detuning." The required test power supply capacity is large, and the flexibility is insufficient. [align=center]Figure 2 Reactor Compensation Method[/align] ②The significant advantages of resonant withstand voltage test system are: a) Reducing the capacity of the test power supply to 1/Q of the total reactive power of the tested equipment, where Q is the quality factor of the resonant system, generally ≥40. b) The distortion of the sinusoidal waveform of the test voltage is ≤ (0.5～1.0)%. c) The total volume and weight of the test equipment are much smaller than conventional equipment (about 1/5～1/10), which is beneficial for field application. d) When the test specimen breaks down, the system detunes, resulting in less damage to the test specimen. Resonant withstand voltage can be configured in various ways, such as series resonance, parallel resonance, series-parallel resonance, and composite resonance, to achieve withstand voltage for test specimens of various specifications. my country's "Technical Conditions for Ordering High Voltage Parallel Capacitors" stipulates that their standard rated capacities are: 50, 100, 200, 300, 334, 500, 667, 900, 1000, 1200, 1400, 1600, 2000, and 2400 kVA. DL/T628-1997 specifies the preferred rated capacities for aggregate capacitors as follows: single-phase 500, 1000, 1667, 3340, 5000, and 6667 kVA; three-phase 1000, 1200, 1500, 1800, 2400, 3600, and 5000 kVA. An inductor-adjustable resonant transformer capable of independently testing 334 kVA power capacitors, coupled with an inductor-adjustable multi-functional reactor suitable for specifications of 800-1200-1800-2400-3340 kVA, can meet the testing needs of almost all product specifications. The resonant test transformer was used to conduct a simulated commissioning test on a 28.5 μF, 3608 kVA capacitor bank with a voltage of 11.6 kV and Q=41.08. A simulated test was also conducted on a 2400 kVA integrated capacitor bank with Q=36.3. The results of the simulated test were very satisfactory. 3 Conclusions and suggestions a. The inter-electrode AC withstand voltage test can detect insulation defects more realistically and effectively than the DC withstand voltage test. b. The "composite" resonant withstand voltage test system can meet the inter-electrode AC withstand voltage test requirements of large-capacity power capacitors and integrated power capacitor banks, and has sufficient coverage. c. Efforts should be increased to publicize the new standard and to configure the necessary test equipment as soon as possible to carry out inter-electrode AC withstand voltage tests on capacitors to ensure the safe operation of the installed capacitors. References: [1] DL/T628-1997, Technical Conditions for Ordering Integrated High Voltage Parallel Capacitors [2] Zhu Deheng, Yan Zhang. High Voltage Insulation. Beijing: Tsinghua University Press