27 Aug-2026

Introduction: In the handover and preventive testing of high-voltage electrical equipment, the variable frequency series resonant withstand voltage test system has become the mainstream technical solution for withstand voltage testing of capacitive equipment such as cables, GIS, and transformers, owing to its low power supply requirements, compact size and light weight, pure output voltage waveform, and comprehensive safety protection mechanisms. With the continuous advancement of UHV grid construction and the increasing undergroundization of urban power grids, market demand for high-voltage, high-capacity series resonant systems has grown significantly. This article takes Kedi Zhongwei's KGXZ-3000kVA/650kV variable frequency series resonant withstand voltage test system as an example, providing an in-depth analysis from six dimensions: technical principles, application scope, system specifications, configuration solutions, manufacturing standards, and typical application value.

This article is intended for: Power system testing personnel, substation operation and maintenance engineers, and electrical equipment procurement technical staff.

1. Technical Principles: The Energy Amplification Effect of LC Series Resonance

The core principle of the variable frequency series resonant withstand voltage test system is based on the resonant characteristics of LC series circuits. When the output frequency of the variable frequency power supply matches the natural frequency of the circuit formed by the reactor inductance (L) and the test object capacitance (C), resonance occurs. The inductive reactance of the reactor and the capacitive reactance of the test object cancel each other out (XL=XC), and the total circuit impedance drops to its minimum. At this point, the voltage across the test object can reach Q times the output voltage of the excitation transformer (Q is the quality factor, typically 30–90), achieving the effect of driving high-voltage, high-capacity tests with a small-capacity power supply.

The resonant frequency is determined by both inductance and capacitance: f₀ = 1 / (2π√LC) . By precisely adjusting the output frequency of the variable frequency power supply to approach the resonant frequency of the test object, the required test high voltage can be obtained across the test object. From an energy perspective, under resonant conditions, the magnetic field energy of the reactor and the electric field energy of the test object capacitance compensate for each other. The variable frequency power supply only needs to supply the active power loss portion of the circuit, requiring only 1/Q of the power supply capacity of a conventional test transformer, reducing power supply requirements by over 95% .

This principle is analogous to "swinging on a swing" – as long as the pushing frequency matches the swing's natural frequency, a small amount of force can make the swing go higher and higher. The variable frequency power supply plays the role of the "pusher," precisely adjusting the frequency to bring the circuit into resonance and obtaining a high voltage several times the input voltage across the test object, thereby rigorously verifying the insulation performance of the equipment.

2. Application Scope: Covering 220kV–500kV Voltage Levels

The KGXZ-3000kVA/650kV variable frequency series resonant withstand voltage test system is specifically designed for on-site withstand voltage testing of high-voltage, high-capacity electrical equipment, meeting the following typical application requirements:

1. 220kV Power Cable AC Withstand Voltage Test: Suitable for 220kV/2500mm² XLPE cables up to 1km in length, with capacitance ≤0.22μF, test frequency 30-300Hz, test voltage 216kV. AC withstand voltage testing of XLPE cable main insulation preferentially uses 20Hz-300Hz AC voltage, making the variable frequency series resonant system the standard configuration for AC withstand voltage testing.

2. 220kV Substation Switchgear and Electrical Equipment Withstand Voltage Test: Suitable for AC withstand voltage testing of 220kV switchgear, busbars, and other electrical equipment, with test frequency 30-300Hz, test voltage not exceeding 460kV, test duration 1 minute.

3. 220kV Transformer and Distribution Equipment Withstand Voltage Test: Suitable for AC withstand voltage testing of main transformers and distribution equipment in 220kV substations, with test frequency 45-65Hz, test voltage 360kV.

4. 420kV/500kV Transformer Neutral Point Withstand Voltage Test: Suitable for AC withstand voltage testing of transformer neutral points in 420kV/500kV substations, with test frequency 45-65Hz, test voltage 200kV.

5. 420kV/500kV GIS Equipment Withstand Voltage Test: Suitable for AC withstand voltage testing of GIS equipment (15 bays) and distribution equipment in 420kV/500kV substations, with test frequency 30-300Hz, test voltage 580kV. On-site AC withstand voltage testing is the most rigorous, effective, and direct method for assessing the insulation strength of electrical equipment, playing a critical role in determining whether equipment can be safely and stably commissioned.

3. System Key Specifications

The main technical specifications of the KGXZ-3000kVA/650kV variable frequency series resonant withstand voltage test system are as follows:

Functional Features:

• Supports three operating modes: Manual Test, Automatic Tuning, and Automatic Test

• Equipped with a large-screen display showing key parameters including output voltage (RMS), output frequency, excitation current, excitation voltage, and test duration in real time

• Test parameter settings for voltage, duration, frequency range, and more

• Built-in overvoltage, overcurrent, and overtemperature protection

• Flashover protection: When flashover occurs on the test object, the resonant circuit detunes and the power supply immediately stops output, displaying "Test Failed" and related information on the screen

• Supports data printing functions

4. System Configuration and Component Specifications

The KGXZ-3000kVA/650kV variable frequency series resonant withstand voltage test system consists of four core units – variable frequency power supply, excitation transformer, high-voltage reactors, and capacitive voltage divider – along with accessory systems, all working together to complete high-voltage withstand voltage testing.

4.1 Variable Frequency Control Power Supply (180kW)

The variable frequency control power supply is the control core of the entire system, featuring an integrated design that combines frequency conversion, voltage regulation, control, and protection functions.

The variable frequency power supply uses a high-performance dedicated microprocessor for voltage and frequency control, equipped with dedicated connecting cables and plugs for interconnection with other devices. It features overvoltage, overcurrent, overtemperature, and discharge protection, supporting manual test and automatic test modes.

4.2 Excitation Transformer (180kVA)

The excitation transformer steps up the output voltage of the variable frequency power supply to the appropriate level for the reactors while providing electrical isolation between the primary and secondary sides.

The excitation transformer adopts an oil-immersed structure with Class H insulation and heat resistance, meeting national standards for oil-immersed transformers. Electrostatic shielding is provided between the high-voltage and low-voltage windings and the core, serving both as an excitation transformer and an isolation transformer. Built-in overvoltage protection prevents breakdown counterattack.

4.3 High-Voltage Reactors (750kVA/170kV, 4 Units)

The high-voltage reactors form the LC series resonant circuit with the test object capacitance and are the core components for generating resonant high voltage.

The high-voltage reactors feature a modular design. The four reactors can be flexibly combined according to the type of test object on site – multiple units connected in parallel for high-current, low-voltage cable withstand testing, or in series for high-voltage, low-current GIS and transformer withstand testing.

4.4 Capacitive Voltage Divider (TRF-650/0.001)

The capacitive voltage divider is connected in parallel with the test object to precisely measure the high-voltage resonant voltage and provide feedback to the control system.

The high-voltage and low-voltage arm capacitors use consistent dielectric structures with low temperature coefficients and low angular displacement, ensuring a constant division ratio across the 30-300Hz frequency range and maintaining measurement accuracy across the entire frequency spectrum.

5. Manufacturing Standards and Factory Testing

5.1 Applicable National and Industry Standards

The KGXZ-3000kVA/650kV variable frequency series resonant withstand voltage test system is designed, manufactured, and tested in strict compliance with the following national and industry standards:

• GB 50150: Standard for Handover Test of Electric Equipment Installation Engineering

• GB/T 16927.1-2: High-voltage test techniques

• GB 1094: Power transformers

• GB 10229: Reactors

• GB 7328: Determination of sound levels of transformers and reactors

• GB 4208: Degrees of protection provided by enclosures (IP Code)

• GB 2900: Electrotechnical terminology

• GB 5273: Terminals for transformers, high-voltage apparatus and bushings

• GB 191: Packaging, storage and transportation pictorial markings

• GB 4793: Safety requirements for electronic measuring apparatus

• DL/T 849.6: General technical specifications for special test instruments for electric power – High voltage resonant test devices

• DL/T 474.4: Guide for field insulation testing – AC voltage withstand test

• IEC 358: Coupling capacitors and capacitor dividers

• IEC 1000: Electromagnetic compatibility (EMC)

5.2 Factory Test Items

Before leaving the factory, each component unit and the complete system undergo rigorous factory testing to ensure all specifications fully meet the requirements of the contract and applicable standards:

Variable Frequency Power Supply Factory Tests: Insulation resistance test; withstand voltage test (2000V, 1 minute); load test (temperature rise of all components ≤45K at full load).

Excitation Transformer Factory Tests: DC resistance measurement; turns ratio measurement; no-load current and no-load loss; short-circuit impedance and load loss; insulation resistance test; temperature rise test (≤65K after 60 minutes at rated capacity).

Reactor Factory Tests: DC resistance measurement; inductance measurement; AC withstand voltage test; temperature rise test (≤65K after 60 minutes at rated capacity).

Complete System Tests: Withstand voltage test (1.1 times rated voltage for 1 minute); short-circuit test (with output short-circuited 3 times at 0.5U, 0.8U, and 1.0U – protection device must operate reliably and all units must remain intact); noise test (≤60dB).

6. Typical Application Value

For a 3000kVA/650kV series resonant system, the typical on-site application value is reflected in the following aspects:

1. Low Power Supply Requirements: Conventional power frequency withstand voltage test transformers require enormous power supply capacity for 650kV-level testing. Series resonant technology only requires the power supply to provide the active power loss portion (approximately 1/Q), allowing a standard three-phase 380V site power supply to drive the test.

2. Flexible Equipment Configuration: The four reactors can be flexibly combined in series and parallel configurations, allowing a single system to meet the testing requirements of various equipment including cables (low voltage, high current), GIS (high voltage, low current), and transformers (power frequency withstand voltage).

3. Comprehensive Safety Protection: The flashover protection function cuts off the output within milliseconds when breakdown occurs on the test object, preventing fault point expansion.

4. Pure Waveform Output: Output voltage distortion ≤0.5%, preventing harmonic damage to the test object insulation.

7. About Wuhan Kedi Zhongwei Electric Co., Ltd.

Wuhan Kedi Zhongwei Electric Co., Ltd. is a high-tech enterprise integrating R&D, production, and sales of power testing equipment, instruments, and online power monitoring systems. The company has obtained certifications including High-Tech Enterprise Certificate, ISO9001 Quality Management System Certification, and Member of Hubei Metrology Association. The company holds 6 patents, 8 software copyrights, and 1 registered trademark, and maintains close technical cooperation with Huazhong University of Science and Technology, Wuhan University, Wuhan High Voltage Research Institute, and other research institutions.

The company's products cover 15 major categories with over 300 products, including series resonant test systems, high-voltage test equipment, transformer testing instruments, high-voltage switch testers, instrument transformer calibration instruments, relay protection testers, and more. Among them, the variable frequency series resonant withstand voltage test systems are designed and manufactured in strict compliance with national and industry standards such as GB 50150 and DL/T 849.6, and are widely used in power grids, power generation, renewable energy, railways, petrochemicals, metallurgy, and other fields, with exports to Southeast Asia, the Middle East, Africa, South America, and other overseas markets.

Kedi Zhongwei consistently adheres to the business philosophy of "technology leadership, service first," providing customers with full-cycle services from selection consulting and on-site training to after-sales maintenance.

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• KGXZ-3000kVA/650kV Variable Frequency Series Resonant Withstand Voltage Test System

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• DC High Voltage Generator

• Loop Resistance Tester

• Relay Protection Tester

Tags: #WuhanKediZhongwei #3000kVA650kVSeriesResonant #VariableFrequencySeriesResonant #GISACWithstandTest #HighVoltageCableWithstandTest