I have been asked this question many times on site, especially by novice engineers. When they receive a test task sheet and see the words "withstand voltage test", they don't know whether to choose power frequency or ultra-low frequency. Some even directly ask "Can we just use DC for a makeshift solution?" Today, I will clarify the core differences among these three methods.

Power frequency withstand voltage (50Hz AC)

The most traditional method yields the most direct test results and is closest to the actual operating state of the equipment. The problem lies in the fact that the capacitive reactance of capacitive test samples (such as long-distance power cables) is extremely low at 50Hz. To maintain sufficient test voltage, the required power capacity is enormous. To test a 10kV/4km cable, the equipment capacity may require hundreds of kVA, and that test transformer itself is a massive iron block that cannot be moved on site. Therefore, power frequency withstand voltage is mainly used for factory or laboratory tests of resistive, low-capacity equipment, and is not suitable for on-site large-capacity equipment.

DC withstand voltage

The equipment, being small in size and low in price, was once the mainstream choice on site. However, the problem is quite evident: DC voltage accumulates space charges inside XLPE (cross-linked polyethylene) insulation, which is destructive to the insulation in the long run. Moreover, DC voltage withstand testing is far less effective than AC in detecting defects such as water trees inside cables. For this reason, the national preventive testing regulations for the electric power industry have explicitly stipulated that DC high voltage is no longer recommended for cable voltage withstand testing.

0.1Hz ultra-low frequency voltage withstand test

It combines the advantages of the former two - low frequency, small equipment capacity, and easy on-site portability; the test form is AC, and the detection effect is equivalent to power frequency, without generating harmful space charge accumulation. It has strong excitation ability for defects such as water trees and air gaps in cable insulation, meeting the requirements of the national DL/T849.4-2004 standard.

Brief summary: For on-site voltage withstand tests of power cables and large-capacity motors, 0.1Hz ultra-low frequency is the preferred choice; for tests of fixed equipment in laboratories, power frequency can be considered; DC voltage withstand has basically been phased out in cable testing scenarios.