The triple frequency withstand voltage generator is designed to meet the requirements of the triple frequency induced withstand voltage test and partial discharge test in the "Regulations for Preventive Tests of Electrical Equipment". This article introduces the working principle of the triple frequency withstand voltage generator.
The induced withstand voltage test of transformers and instrument transformers is an important test to verify whether the product meets national standards. The longitudinal insulation induction test of interlayer, inter-turn, segment, and phase-to-phase insulation of windings is an important item to verify whether the windings meet the requirements. In order to increase the test voltage of the windings without causing the iron core of the transformer or instrument transformer to be over-saturated and the excitation current to be too large, the test frequency should be increased, generally to 100Hz~400Hz.
There are various methods to obtain a 100Hz~400Hz power supply, among which the more commonly used ones are:
1. Combination of three variable frequency generators (100Hz~400Hz);
2. Combination of three single-phase transformers (150Hz), and the most convenient is the second method.
Combination method of three single-phase transformers: the primary side is star-connected, and the secondary side is open-delta. When the iron core is not saturated, the main magnetic flux it produces must be a flat-top wave. Since the main magnetic flux of the flat-top waveform contains a large component of third harmonic magnetic energy φ3, and the frequency of the third harmonic magnetic flux f3=3f1, the main magnetic flux induces a third harmonic phase potential. Due to this connection, the iron core operates in a saturated state, and the no-load current I is a peaked wave. In addition to the fundamental wave, it also contains higher harmonics. Among the higher harmonics, the amplitude of I is relatively large. Since the secondary side is open, the three-phase fundamental components cancel each other out, leaving only the third harmonic output.
The triple frequency power generator utilizes the saturation characteristics of the magnetic circuit to extract the third harmonic voltage with the largest component among the harmonics as the power supply for the generator. It is used for frequency multiplication and voltage withstand tests on inductive coil-type electrical products such as transformers and instrument transformers, testing the inter-turn, inter-segment, and interlayer insulation strength and withstand voltage level of the coils.
