When performing high-voltage tests on large cross-section, long-distance cables, the issue often lies not with insufficient voltage from power frequency equipment but with its capacity and size making it difficult to handle on-site. Cables can be equivalently modeled as a large capacitance, which generates significant capacitive current under power frequency conditions. The test transformer must therefore provide corresponding reactive power capacity.

According to the capacitive current formula I=2πfCU, when the capacitance C and test voltage U of the specimen remain constant, the current I is directly proportional to the frequency f. At 0.1Hz, the current is only one-fifth hundredth of that at 50Hz. Theoretically, under the same voltage and capacitance, the required test capacity can be significantly reduced. The HZDP manual also explicitly states that the equipment capacity of ultra-low frequency technology is theoretically approximately one-fifth hundredth of that of the power frequency, making the equipment lighter and reducing the pressure on on-site wiring and transportation.

The ultra-low frequency withstand voltage test still requires observing the insulation condition according to the AC testing logic. The equipment outputs a low-frequency AC sinusoidal wave, not a DC voltage. Compared to DC withstand voltage tests, ultra-low frequency AC more closely mimics the electric field conditions of AC operation and reduces the potential adverse effects of DC testing on certain insulation structures.

However, ultra-low frequency cannot directly replace power frequency in all scenarios. The final test voltage, test duration, and equivalent conversion must be determined based on the type of equipment under test and applicable regulations. For instance, the duration for power cable交接试验 (handover tests) and预防性试验 (preventive tests) differs, while motor tests also require calculations of peak values using rated voltage and equivalent coefficients. On-site personnel must not rely solely on the "0.1Hz" label but also verify whether the specimen capacity, test standards, and equipment rated load capacity are compatible.