The DC withstand voltage test cannot reflect the electric field distribution under actual operating conditions of the equipment, making it difficult to correctly detect internal defects in capacitors.
Under DC voltage, the voltage on the elements of power capacitors 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 is poorly insulated, its resistivity can drop significantly to a fraction of the original. During DC withstand voltage, the voltage borne by well-insulated capacitor elements with high resistivity can be several times higher than that on poorly insulated elements, making it easier for poorly insulated elements to pass the test. Their insulation defects will then be exposed more quickly under operating voltage, developing into faults or leading to accidents.
DC voltage can greatly reduce partial discharge inside the capacitor, which is not conducive to the detection of insulation defects.
Certain insulation weaknesses inside the capacitor or areas with concentrated electric field at the edges of plates may produce partial discharge. Continuous partial discharge is harmful to the insulation of capacitors. 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, for the same composite insulation, partial discharge under DC voltage is greatly reduced. The basic principle is shown in Figure 1. After partial discharge occurs in the air gap, the generated positive and negative ions form a reverse electric field E′, which reduces the resultant field strength in the gap, weakening or even extinguishing the partial discharge. In contrast, with AC voltage, as long as the applied test voltage is higher than the partial discharge inception voltage, at least two partial discharges will 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 situation of power frequency transient voltage rise during operation, without equivalence issues.
Thus, only by applying AC withstand voltage can the manufacturing quality of power capacitors be truly assessed, and insulation defects caused by poor materials or improper processes be effectively detected.
In view of the problems existing in DC withstand voltage tests, the power industry has specially formulated DL/T628-1997 "Ordering Technical Conditions for Collective High-Voltage Shunt Capacitors" to ensure engineering 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 for factory tests and acceptance tests. The "Ordering Technical Conditions for High-Voltage Shunt Capacitors" has also been discussed by the standard committee, which clearly specifies 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.
