I. Objectives and Basic Principles
The induced voltage withstand test is a key item for evaluating the insulation strength of transformers. Its main purpose is to verify the bearing capacity of the main insulation (winding-to-ground and winding-to-winding) and the longitudinal insulation (phase-to-phase, layer-to-layer, and section-to-section) of the transformer under voltages higher than the operating voltage. The test is conducted by applying a specific frequency and waveform of alternating current voltage to the low-voltage side of the transformer, and inducing the specified high voltage on the high-voltage side to simulate the stress conditions such as the increase of power frequency voltage and operational overvoltage that may occur during actual operation.
II. Main Test Scheme
According to the waveform and generation method of the test voltage, the induced voltage withstand test is mainly divided into two schemes, as follows:
(1) Power Frequency/Breathing Frequency AC Induced Voltage Withstand Test
This scheme is the standard method for testing the longitudinal insulation and main insulation of graded insulation transformers. It has a wide range of application and strong standardization.
1. Core Principle
To avoid magnetic saturation of the core at 2 times or more of the rated voltage, the test power supply frequency must be increased accordingly (such as 100Hz, 150Hz, 200Hz) to ensure the stability of the core’s working state during the test and the accuracy and reliability of the test results.
2. Applicable Objects
This scheme is applicable to the factory, commissioning, and major overhaul tests of various power transformers and transformers-in-a-box, especially suitable for the phase-by-phase tests of graded insulation transformers, which can comprehensively evaluate the insulation performance of the equipment under different operating conditions.
3. Test Procedure
(1) Wiring: The test power supply is usually connected to the low-voltage winding of the tested transformer, the non-tested windings are short-circuited and grounded or connected to a suitable potential, the high-voltage winding is open-circuited or grounded through a measuring device to ensure that the wiring is correct and there are no safety hazards.
(2) Voltage Rise: Use a variable frequency power supply device to rise the voltage steadily from zero to the target test voltage to avoid shock voltage rise that may cause damage to the equipment; for graded insulation transformers, it is necessary to precisely control the potential of the high-voltage terminals and the neutral point to prevent local insulation from exceeding the allowable stress.
(3) Voltage Withstand Time: Maintain the specified test voltage for a specified period of time. The test time is calculated as t = 120 seconds × (rated frequency / test frequency), and should be no less than 15 seconds to ensure that the insulation is fully tested under the specified stress.
(4) Monitoring and Recording: Monitor the test voltage and current waveforms throughout the process and record the local discharge quantity as required, to keep track of the insulation status of the equipment during the test and provide a basis for the judgment of the test results.
4. Acceptance Criteria
If there is no insulation breakdown, the local discharge does not exceed the standard, and the test voltage and current waveforms do not change abnormally within the specified time, the test is considered qualified.
(2) Operational Wave Induced Voltage Withstand Test
This scheme is mainly used for on-site assessment of the ability of large transformers to withstand operational overvoltage and is adapted to the special needs of on-site tests. It can more closely simulate the overvoltage conditions that the equipment may encounter during actual operation.
1. Core Principle
Discharge to the low-voltage winding generates an operational impulse voltage with a standard waveform on the high-voltage side to simulate the overvoltage generated by system switching, accurately assessing the insulation withstand capability of the equipment under instantaneous overvoltage.
2. Applicable Objects
Large power transformers with a voltage rating of 220kV and above, especially when there is no breathing frequency test condition on-site, this is the preferred scheme for on-site tests.
3. Test Procedure
(1) Waveform Calibration: Conduct an impulse test below 50% of the test voltage to determine the ratio of charging voltage to output high voltage and adjust it to a standard waveform (wavehead time ≥ 100μs, wave tail time ≥ 1000μs, zero-crossing time ≥ 200μs) to ensure that the test waveform meets the specification requirements.
(2) Pre-test: Conduct an impulse test at 75% of the test voltage once, record the reference waveforms of voltage and current, and use them as a benchmark for comparing the test results.
(3) Formal Test: Conduct the formal test at 100% of the rated test voltage, usually conducting 3 impulse tests, comprehensively assessing the insulation stability of the equipment under rated overvoltage stress.
4. Criteria for qualification
Compare the voltage and current waveforms under full voltage with the reference waveforms under reduced voltage. If the waveform shape, oscillation frequency, zero-crossing points, etc. do not undergo significant distortion, and the amplitude ratio is normal, then the test is deemed to have passed.
III. List of Instrument and Equipment Configuration
A complete induction withstand voltage test system consists of power supply, measurement, protection and auxiliary equipment. The configuration and requirements of each part of the equipment are as follows:
(1) Core power supply equipment
1. Multi-frequency induction withstand voltage device: As the core of the AC test, it should be a modern device using power electronic frequency conversion technology. Its output frequency should be adjustable within the range of 50-200Hz, and the output capacity should be selected according to the capacity of the tested transformer and the test voltage (usually 3kVA to 100kVA or above). The device should have low waveform distortion rate (≤3%), touch screen control, automatic timing and multiple protection functions to ensure a stable, safe and efficient test process.
2. No-discharge frequency resonant test system: For higher voltage levels or tests requiring more precise measurement of partial discharge, this system is recommended. It adjusts the frequency to cause resonance between the inductor in the circuit and the capacitance of the tested item, thereby obtaining a high voltage on the test item. The advantages of this system include good output waveform (sinusoidal wave distortion rate <3%), small required power supply capacity, and low partial discharge level (≤10pC). The system mainly consists of a frequency converter power supply, excitation transformer, resonant inductor, and capacitance divider.
3. Operating impulse voltage generator: Used to generate standard operating impulse waves, it mainly includes high-voltage energy storage capacitors, wave head/wave tail resistors, trigger ball gap, and DC charging unit, capable of precisely outputting impulse waveforms in accordance with the specifications.
(2) Measurement and monitoring equipment
1. High-voltage measurement system: Must adopt high-precision measurement methods. It is recommended to use high-precision capacitance divider combined with digital peak voltage meter or high-voltage oscilloscope for measurement. The overall measurement error of the system should not exceed 3%, ensuring the accuracy of the test voltage measurement.
2. Current monitoring device: Use precision shunt resistors and oscilloscope to measure the current in the test circuit. In the operation impulse test, the comparison of the “damage current” waveform is the key basis for judging insulation faults, enabling the timely detection of potential insulation defects.
3. Partial discharge detection system: If the test standard or relevant requirements require monitoring partial discharge, a system should be configured, including no-discharge coupling capacitor, high-frequency current sensor (HFCT), and digital partial discharge detector. It is used to monitor, locate and record the partial discharge signals during the test process, providing detailed data for insulation performance assessment.
4. Waveform recording and analyzer: Use multi-channel digital storage oscilloscope to synchronously record voltage and current waveforms and have functions of waveform superposition and comparison analysis, facilitating the review and analysis of waveform data after the test and precise determination of the test results.
(3) Protection and auxiliary equipment
1. Comprehensive protection control unit: Integrates overvoltage protection, overcurrent protection, flashover protection, zero-position start-up protection, and emergency shutdown functions, providing comprehensive safety protection during the test process and avoiding accidental accidents.
2. Compensation reactor: Used to compensate the capacitive current of the tested transformer, improving the output capacity of the power supply device, ensuring voltage stability during the test and meeting the test load requirements.
3. Special high-voltage connection components: Including voltage equalizing cover, shielding wire, insulating pillar, etc., to reduce corona interference, avoiding interference with the accuracy of test measurement data, and ensuring the insulation safety of the high-voltage connection parts.
4. Grounding system: Equipped with heavy-duty grounding wire and grounding rod to ensure reliable grounding of all equipment and the casing of the tested transformer, with grounding resistance meeting safety requirements, eliminating the risk of electric shock and other safety hazards.
IV. Safety Measures and Test Preparation To ensure the safe and orderly conduct of the test and to safeguard the safety of personnel and equipment, adequate preparations must be made before the test and the following safety measures must be strictly followed:
1. Qualifications and Plan: The test personnel must have the necessary qualifications and be familiar with the test procedures and safety regulations. Before the test, a detailed operation manual must be formulated based on current standards such as GB 1094.3, DL/T 474.6, and DL/T 2486, clearly specifying the test steps, safety precautions, and emergency response plans.
2. Safety Isolation: Ensure that the tested transformer is completely isolated from all energized systems, with each side of the winding short-circuited and reliably grounded for discharge, completely eliminating the risk of live electricity. Install safety barriers in the test area, hang warning signs, have a dedicated person for supervision, and prohibit irrelevant personnel from entering the test area.
3. Equipment Status Confirmation: Before the test, the transformer must have passed conventional tests such as insulation resistance, absorption ratio, dielectric loss, and oil chromatography analysis, confirming that there is no severe moisture absorption or defects, ensuring that the tested equipment is in a qualified state, and avoiding accidents caused by equipment defects during the test.
4. Equipment Inspection: Check that the grounding of all test equipment is firm, the instruments and meters are within the valid calibration period, the connections are correct and in good contact, and check for safety hazards in the equipment and connections to ensure the normal operation of the test equipment.
5. Environmental Control: The test should be conducted under good weather conditions, with the relative humidity of the environment not exceeding 80%; the operating environment temperature of the equipment is generally -10°C to +45°C, to avoid adverse environmental conditions affecting the safety of the test and the accuracy of the test results.
6. Step-by-step Operation: During the test, strictly follow the operation sequence of reducing voltage – disconnecting power – grounding. The voltage increase must start from zero and be carried out slowly. Prohibit impulse closing, to prevent improper operation from causing harm to the equipment and personnel.
Post time: Feb-24-2026