The induced overvoltage withstand test device is a high-voltage testing equipment specifically designed for AC withstand voltage tests on iron-core electrical equipment such as transformers and instrument transformers (voltage transformers PT, current transformers CT).

Unlike the conventional applied voltage test (directly applying high voltage to the winding), the induced overvoltage test applies voltage to the low-voltage side of the equipment, utilizing the principle of electromagnetic induction to induce a high voltage on the high-voltage side (or between turns), thereby assessing the equipment's main insulation (winding to ground, between windings) and longitudinal insulation (between turns, layers, and sections).

1. Core Function and Necessity

Why is "induced" withstand voltage needed instead of directly applying high voltage?

Assessing longitudinal insulation: For transformers with graded insulation (where the neutral point of the high-voltage winding has a lower insulation level), if voltage is applied directly from the high-voltage terminal to ground, the neutral point would break down. Only by inducing voltage from the low-voltage side can the potential of each point on the high-voltage winding rise according to the distribution law, while also subjecting the turns to high voltage, thus comprehensively assessing turn-to-turn insulation.

Full insulation testing: For fully insulated equipment, induced overvoltage testing can simultaneously verify main insulation and longitudinal insulation, making it more efficient.

Simulating operating conditions: The high-voltage waveform and electric field distribution produced by induction are closer to the actual operating state of the equipment.

2. Working Principle

The core of this device is a variable-frequency power supply and an excitation transformer (or directly utilizing the excitation characteristics of the test object itself).

Variable-frequency voltage boosting:

According to the transformer induced electromotive force formula

 it can still be maintained within the rated range, avoiding core saturation.

Induction process:

The device applies a voltage higher than the rated value (usually 2 times the rated voltage) to the low-voltage winding of the test transformer (such as the low-voltage side or medium-voltage side).

Through electromagnetic induction, a corresponding multiple of high voltage (such as 2 times the rated high voltage) is induced across the high-voltage winding.

At the same time, due to the fixed turns ratio, the voltage between turns also increases proportionally, thereby assessing the turn-to-turn insulation.

3. System Composition

A complete induced overvoltage withstand test device typically includes the following parts:

Variable-frequency power supply cabinet: The core component. Converts industrial frequency 50Hz power into a sinusoidal power supply with adjustable frequency (100Hz-400Hz) and adjustable voltage. It has voltage stabilization and frequency stabilization functions.

Excitation transformer (optional): If the low-voltage side voltage of the test object is low, it is necessary to boost the voltage through an excitation transformer before inputting it to the low-voltage winding of the test object; if the rated voltage of the low-voltage side of the test object is high (such as 35kV side), sometimes it can be directly driven by the variable-frequency power supply.

Compensation reactor: Since the test object (transformer/instrument transformer) is a capacitive or inductive load, in order to reduce the power supply capacity requirement, reactors are often connected in series or parallel for reactive power compensation, making the circuit close to resonance.

Voltage divider: Used to accurately measure the test voltage on the high-voltage side (capacitive voltage divider or resistive voltage divider).

Control and protection system: Automatically controls the voltage boosting process, has overcurrent, overvoltage, flashover, detuning and other protection functions, and can automatically record test curves.

4. Main Technical Indicators

Output frequency: Usually adjustable in the range of 45Hz - 300Hz (commonly 100Hz, 150Hz, 200Hz).

Output voltage: Depending on the test object's level, it can range from a few kV to several hundred kV.

Output capacity: From tens of kVA to several thousand kVA, depending on the excitation current and stray capacitance current of the test object.

Waveform distortion rate: The output sine wave distortion rate is required to be small (usually < 1%) to avoid affecting the judgment of test results.

Duration:

When the test frequency

t=120×(rated frequency/test frequency) seconds, but at least not less than 15 seconds.

For example: when testing at 150Hz (triple frequency), the time should be

5. Operation Procedure

Wiring inspection:

Short-circuit and ground the non-tested phases of the test object.

Connect the variable-frequency power supply, excitation transformer, voltage divider, and the low-voltage winding of the test object.

Key point: It must be confirmed whether the neutral point of the test object is grounded (for graded insulation transformers, the neutral point needs to be floating during neutral point withstand voltage test, and grounded during main insulation withstand voltage test, depending on the test plan).

Parameter setting:

Input the parameters of the test object (rated voltage, transformation ratio, frequency requirements).

Set the target test voltage value (usually 2 times the rated voltage or according to regulations).

Set the automatically calculated withstand voltage time.

Pre-test:

Slowly raise the voltage to a lower value to check the wiring polarity, phase sequence, and instrument readings are normal.

Formal voltage boosting:

Start the variable-frequency power supply, the frequency automatically tracks and rises to the set value (such as 150Hz).

The voltage smoothly rises to the target value.

Start timing and maintain the specified time.

Voltage reduction and discharge:

After the time is up, automatically and quickly reduce the voltage uniformly to zero.

Cut off the power supply and fully discharge the test object.

Result judgment:

If there is no breakdown sound, no violent fluctuation of the voltmeter pointer, no abnormal sudden change in the ammeter during the test, and the withstand voltage time meets the requirements, it is judged as qualified.

6. Precautions

Frequency selection: The frequency should not be too high, otherwise it will cause excessive uneven voltage distribution between turns of the winding; it should not be too low either, otherwise it will cause core saturation. Generally, 100Hz - 200Hz is recommended.

Neutral point protection: For graded insulation transformers, during induced overvoltage testing, the insulation level of the neutral point is low. It must be protected by an auxiliary transformer or by temporarily raising the potential of the neutral point, or only the phase-to-ground induced overvoltage test is performed without assessing the neutral point (needs to be combined with the applied voltage test).

Partial discharge monitoring: Modern induced overvoltage tests usually require simultaneous partial discharge (PD) monitoring. Because under high voltage, tiny defects inside the insulation will produce partial discharge, which is an important indicator for judging the health of the insulation.

Core heating: Although increasing the frequency avoids saturation, long-term high-frequency operation may still cause heating of the core or structural parts, so the test time must be strictly controlled.

Resonance risk: The test circuit may form series or parallel resonance, leading to voltage runaway, which must be quickly cut off by the protection system.

7. Application Scenarios

Transformer factory tests: Routine tests that must be performed before each power transformer leaves the factory.

Acceptance tests: Before a newly installed transformer is put into operation, to verify whether the insulation was damaged during transportation and installation.

Post-overhaul tests: After transformer cover lifting inspection, winding replacement, or treatment, to verify the repair quality.

Instrument transformer testing: The induced overvoltage test of electromagnetic voltage transformers (PT) is the only effective method to assess their turn-to-turn insulation.

Summary

The induced overvoltage withstand test device is key equipment that uses variable-frequency technology to solve the core saturation problem, thereby achieving high-intensity assessment of the turn-to-turn insulation and main insulation of transformers and instrument transformers. It is the last line of defense to ensure the integrity of internal insulation of high-voltage electrical equipment and prevent turn-to-turn short circuit explosion accidents during operation. It complements the applied voltage withstand test and together they form a complete system for transformer insulation testing.