The withstand voltage test of instrument transformers is mainly divided into two categories: external power frequency withstand voltage test and induced withstand voltage test. External power frequency withstand voltage test is mainly used to assess the main insulation, while induced withstand voltage test is mainly used to assess the longitudinal insulation, and is more commonly used for voltage transformers. Commonly used test equipment in the field includes traditional power frequency withstand voltage devices, variable frequency series resonant withstand voltage devices, PT-specific induced withstand voltage devices, and simplified secondary winding withstand voltage devices. Each type of device has different structural characteristics and is applicable to different test scenarios.
I. Traditional Power Frequency Withstand Voltage Device This device is a mainstream basic equipment and is commonly used for primary main insulation testing of 110kV and below instrument transformers. It has advantages such as simple operation, controllable cost, and standard output waveform. The entire system consists of five parts: a control unit, a high-voltage generation unit, a high-voltage measurement unit, a protection unit, and auxiliary accessories. These units work together to ensure a stable and controllable test process. The control unit, as the core control component of the device, integrates power input, autotransformer, parameter display, and program control functions. It can achieve automatic voltage boosting, constant voltage timing, and automatic voltage reduction to zero. It is also equipped with overcurrent, overvoltage, breakdown protection, and fault alarm functions. The voltage boosting rate can be controlled between 2 and 3 kV/s, meeting the smooth voltage regulation requirements of the standard. Some high-end models adopt DSP platform technology, optimizing the human-machine interface and supporting data storage and remote printing. The high-voltage test transformer, as the core high-voltage generating component, adopts a single-phase boost structure, which can boost the mains voltage (380V/220V) to the high voltage required for the test.
Based on structural materials, current transformers can be divided into three categories: dry-type (epoxy casting) type, which is oil-free, lightweight, maintenance-free, and leak-free, making it the recommended choice for on-site inspection and testing, suitable for 110kV and below transformers; oil-immersed type, with large capacity and excellent heat dissipation and insulation performance, suitable for laboratory and high-capacity, high-voltage testing scenarios; and cascade type, which outputs high voltage through multiple transformers connected in series, meeting the testing requirements of 220kV and above ultra-high voltage transformers. The high-voltage measurement system’s main function is to accurately monitor test voltage and leakage current, preventing misinterpretation of test results due to measurement errors. Its core components include a high-voltage divider and low-voltage side monitoring meters. The high-voltage divider uses the resistive-capacitive voltage divider principle, with an accuracy of no less than 0.5 class, effectively eliminating the capacitive rise effect. Some high-end devices are equipped with a capacitive voltage divider, which can simultaneously achieve voltage division and partial discharge signal extraction, with a measurement accuracy of up to 1.5 effective value class. The high-voltage protection unit ensures the safety of the test equipment and the tested transformer. Its core components include a current-limiting protection resistor, a protective ball gap, and a matching grounding system. The current-limiting protection resistor is configured at 0.1~1Ω/V to limit the short-circuit current when the test specimen breaks down; the protective ball gap can bypass the circuit during overvoltage to prevent overvoltage damage to the test specimen; the grounding system includes grounding clamps, high-voltage discharge rods, etc. Some devices also have zero-position start-up and system detuning (flashover) protection functions to further improve test safety. Auxiliary accessories include high-voltage leads, shielded wires, insulating supports, test wiring fixtures, etc., used to achieve reliable connections between equipment, reduce external interference, and ensure test wiring specifications. Among them, the shielded wire can effectively eliminate stray leakage interference, ensuring measurement accuracy and test safety.
II. Variable Frequency Series Resonant Withstand Voltage Device This device is a high-end adaptable device, mainly for large-capacity, high-voltage test specimens such as 110kV and above transformers, capacitive transformers, and semi-insulated PTs, solving the problems of insufficient capacity and large test current surges of traditional power frequency withstand voltage devices. Its core principle is to adjust the power supply frequency to create a series resonance between the reactor and the tested transformer, thereby obtaining a high test voltage with a small power supply. It features low power capacity requirements, excellent output waveform, and no large capacitive current surges. This effectively avoids short-circuit current burning of fault points during test sample breakdown and eliminates the risk of recovery overvoltage. The device mainly consists of a frequency converter power supply unit, an excitation transformer, a high-voltage reactor, a capacitor divider, and protection and control accessories. The frequency converter power supply unit’s output frequency can be adjusted within the range of 30~300Hz, with a waveform distortion rate not exceeding 1%. Some models support three operating modes: fully automatic, manual, and automatic tuning with manual voltage boost, adaptable to different field scenarios. The excitation transformer is used to boost the frequency converter power supply output voltage to the required excitation voltage of the reactor; its capacity can be customized within the range of 3kVA~160kVA. The high-voltage reactor adopts a modular design, with a lightweight unit for easy on-site handling and assembly. The device’s own quality factor Q≥30 (f=45Hz) ensures stable resonance performance.
III. PT Dedicated Inductive Withstand Voltage Test Device This device is a specialized longitudinal insulation test equipment, primarily used for longitudinal insulation testing of the primary winding of electromagnetic voltage transformers. Using a frequency converter avoids the problem of core saturation caused by power frequency voltage application. The device consists of a partial discharge-free frequency converter, an isolation transformer, an intermediate step-up transformer, a high-voltage divider, and a partial discharge detection unit. The frequency adjustment range is 30~300Hz. It can simultaneously perform withstand voltage tests and partial discharge detection, with partial discharge controlled to ≤10pC and background noise ≤1~2pC. It accurately reflects the longitudinal insulation performance of the tested object and is suitable for full-scale laboratory factory tests and critical on-site PT maintenance tests.
IV. Simplified Secondary Winding Withstand Voltage Test Device Withstand voltage tests between the secondary windings and between the secondary winding and ground of the transformer are relatively simple. A 2kV power frequency withstand voltage tester is used uniformly. In some scenarios, a 2500V megohmmeter can be used to complete the insulation test. It eliminates the need for a complex high-voltage generation and protection system, featuring convenient operation and low cost, making it suitable for rapid on-site testing. Before the test, the secondary winding must be short-circuited to ground to prevent the test voltage from damaging the secondary insulation.
Post time: Apr-22-2026