The core purpose of the withstand voltage test is to evaluate the electrical strength of the main insulation (winding-to-ground and phase-to-phase) and the longitudinal insulation (phase-to-phase, layer-to-layer, and section-to-section) of electromagnetic power equipment such as transformers and voltage transformers.
For fully insulated transformers, the power frequency withstand voltage test can only meet the requirements for evaluating the main insulation. To conduct more rigorous testing on the longitudinal insulation, the test voltage needs to be increased, but this operation will cause the test voltage to far exceed the rated voltage of the winding, thereby causing the magnetic flux in the core to saturate and resulting in a significant increase in excitation current.
According to the law of electromagnetic induction
U = 4.44 × f × N × B × A
, in the induced electromotive force U
Maintain a constant level while increasing the frequency f
Can reduce the magnetic flux density within the core B
This is done to avoid the problem of core saturation. Based on this principle, the test method that raises the test frequency to 150Hz (three times the power frequency) has been included in the provisions of international standards and national standards.
The applicability of the three times frequency induction withstand voltage test device is determined by its core principle: The device can output a high-voltage power supply of 150Hz, while conducting high-voltage tests on longitudinal insulation and main insulation of the windings, effectively avoiding the phenomenon of core magnetic saturation.
Core applicable range of the three times frequency induction withstand voltage test device (by equipment type)
This device is mainly applicable to various electromagnetic power equipment that need to undergo induction withstand voltage tests. Specifically:
Power transformers
Power transformers are the main application objects of this device.
Oil-immersed / dry-type transformers: Whether it is a distribution transformer or a large power transformer, this test is required to complete the insulation performance test.
Test locations:
Longitudinal insulation assessment: Covers inter-turn, inter-layer, and inter-section insulation. This is the core function of this test that cannot be replaced.
Main insulation assessment: Applies voltage by induction to test the insulation between the winding and ground, and between phases.
Grade-insulated transformers: For transformers with graded insulation (such as a lower neutral point insulation level), the external power frequency withstand voltage test cannot be directly carried out. Therefore, an induction withstand voltage test (usually combined with partial discharge measurement) must be adopted. With the support of the three times frequency device, the voltage at each point of the winding to ground can reach the preset test requirements.
Test types: Including factory tests, type tests, handover acceptance tests, and after major repair tests.
Voltage transformers
Electromagnetic voltage transformers: Its structure is similar to a small transformer and needs to undergo induction withstand voltage tests to assess the longitudinal insulation and main insulation performance.
Capacitive voltage transformers: The intermediate transformer part in the equipment also needs to carry out this test.
Other special transformers
Inductors: Especially parallel inductors with iron cores, their insulation structure is similar to that of transformers, and this device is required to carry out the test.
Special transformers: Including rectifier transformers, traction transformers, etc. types.
Post time: Jan-17-2026