The working principle of the oil immersed AC Hipot Test System is based on electromagnetic induction, and its core goal is to raise the input low voltage (usually a few hundred volts) to a very high voltage (up to several hundred kilovolts or even higher), which is used for high-voltage insulation strength tests (such as power frequency withstand voltage tests, partial discharge tests, etc.) on electrical equipment (such as cables, switchgear, insulators, transformers, etc.).
Working principle (electromagnetic induction):
Establishing an alternating magnetic field: When an adjustable low-voltage AC power supply (U1) is applied to the primary winding (N1 turns), according to Ampere's law, an alternating current (I1) will be generated in the winding. This alternating current excites an alternating main magnetic flux (Φ) of the same frequency in the iron core.
Magnetic flux transmission: This alternating main magnetic flux (Φ) is almost completely coupled to the secondary winding (N2 turns) through a low magnetic resistance path formed by the iron core.
Induced electromotive force: According to Faraday's law of electromagnetic induction, the alternating magnetic flux (Φ) passing through the secondary winding will induce an electromotive force (E2) in the secondary winding. The magnitude of induced electromotive force is directly proportional to the rate of change of magnetic flux.
Voltage increase: Due to the fact that the number of turns (N2) in the secondary winding is much greater than the number of turns (N1) in the primary winding, according to the basic formula of the transformer:
U2/U1 ≈ N2/N1=K (transformation ratio)
Therefore, the voltage output from the secondary winding (U2) is much higher than the voltage input from the primary winding (U1). For example, if the transformation ratio K=100 and the input is 1kV, the theoretical output is 100kV.
No load and load: In high-voltage testing, transformers are usually in two states:
No load: Secondary open circuit (without connecting the test sample). At this point, the secondary current is zero, and the primary current is only a small no-load current (used to establish magnetic flux). The output voltage is close to the theoretical calculation value.
Load: Secondary connection to the tested equipment (such as cables, insulators, GIS, etc.). At this point, the secondary generates a load current (I2), and according to energy conservation (ignoring losses), U1 * I1 ≈ U2 * I2. The primary current (I1) will correspondingly increase to balance the secondary current. The output voltage (U2) will slightly decrease due to the internal impedance (mainly leakage reactance) of the transformer.