The DC withstand voltage test cannot reflect the electric field distribution under actual operating conditions, making it difficult to correctly detect internal defects in capacitors.
Under DC voltage, the voltage across power capacitor elements is distributed according to resistance; whereas under AC voltage, it is distributed according to dielectric constant, which reflects the actual operating conditions. The solid dielectric resistivity of all-film or paper-film capacitors can be as high as 1 to 100EΩm. When the insulating film of a certain capacitor element has poor insulation, its resistivity can drop significantly to a fraction of its original value. During DC withstand voltage, the voltage borne by good capacitor elements with high resistivity can be several times higher than that on defective elements, making it easier for defective elements to pass the test. Their insulation defects will then quickly manifest under operating voltage, developing into faults or even accidents.
DC voltage can greatly reduce partial discharge inside the capacitor, which is not conducive to detecting insulation defects.
Certain insulation weaknesses inside the capacitor or areas with concentrated electric field at the edges of plates may cause partial discharge. Continuous partial discharge is harmful to capacitor insulation. Therefore, standards stipulate that the partial discharge level of power capacitors under test voltage shall not exceed 100pC [1].
When voltage is applied, the field strength in oil gaps, especially air gaps [2], within capacitor elements is often higher than that in solid dielectrics, but their breakdown field strength is lower, so partial discharge often occurs first. However, under DC voltage, partial discharge in the same composite insulation is greatly reduced. 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 resultant field strength in the air gap, weakening or even extinguishing the partial discharge. In contrast, under AC voltage, as long as the applied test voltage is higher than the partial discharge inception voltage, at least two partial discharges occur in each half cycle. Therefore, AC withstand voltage is far more sensitive in detecting insulation defects than DC withstand voltage.
The power frequency AC withstand voltage test conforms to the actual waveform of operating voltage and is more consistent with the transient voltage rise at power frequency during operation, without equivalence issues.
It can be seen that applying AC withstand voltage can truly assess the manufacturing quality of power capacitors and more effectively detect insulation defects caused by poor materials or improper processes.
In view of the problems existing in DC withstand voltage tests, the power industry has specially formulated DL/T628-1997 "Technical Conditions for Ordering Collective High-Voltage Shunt Capacitors" to ensure project quality and operational safety. This standard was issued on October 22, 1997, and implemented on March 1, 1998. The standard clearly stipulates that AC withstand voltage tests between poles shall be conducted during factory tests and acceptance checks. The "Technical Conditions for Ordering High-Voltage Shunt Capacitors" has also been discussed by the standard committee, clearly specifying the AC withstand voltage test between poles. Its release is imminent. The urgent task is to promote and implement the new standard within the power industry as soon as possible to ensure early adoption.
