1. Introduction

An impulse voltage generator is a high-voltage apparatus that produces pulsed waveforms such as lightning impulse voltages and switching impulse voltages. It is a fundamental piece of equipment in high-voltage test laboratories. Its primary use is to perform impulse voltage tests on electrical power equipment-including lightning impulse full waves, lightning impulse chopped waves, and switching impulse waves-to verify insulation performance. Before high-voltage electrical equipment in power systems is put into service, impulse voltage tests are generally required to assess its insulation behaviour under overvoltage conditions.

With the continuous increase in power system voltage levels and the rapid development of renewable energy equipment, the importance of impulse voltage generators in ensuring safe and reliable power grid operation has become more prominent.

2. Operating Principle

Impulse voltage generators commonly employ the Marx circuit (proposed by Erwin Otto Marx in 1924), the core concept of which is parallel charging and series discharging of multiple capacitor stages.

The basic operating process is as follows: each stage capacitor is connected in parallel via charging resistors and charged to a preset voltage through a rectifier circuit. At this time, the protective resistor is generally about ten times larger than the charging resistor, which both protects the rectifier equipment and ensures relatively uniform charging of each stage capacitor. When the first-stage firing spark gap is triggered by an ignition pulse, the intermediate spark gaps in the remaining stages successively break down due to overvoltage and spark irradiation. After all spark gaps have operated, the stage capacitors are connected in series through the front (wave-front) resistors and discharge into the load capacitor. The total series capacitance becomes C/n and the total voltage becomes nV (where n is the number of stages). Because the load capacitance is relatively small, it charges quickly, and then discharges together with the stage capacitors through the tail (wave-tail) resistors. In this way, a short-duration high-voltage impulse waveform is generated across the load capacitor.

The impulse voltage waveform can be adjusted by changing the values of the front- and tail-resistors; the amplitude is controlled by the charging voltage, and the polarity can be reversed by swapping the rectifier stack connections.

In recent years, with advances in power electronics, the all-solid-state Marx generator has become an important research direction. Unlike conventional gas-spark-gap switches, all-solid-state generators use semiconductor devices such as MOSFETs as the main switches, arranged in a modular multilevel Marx topology with precise control provided by FPGAs. Such new generators can significantly reduce size and weight while improving discharge repetition rates and waveform flexibility.

3. System Composition

A complete impulse voltage generator test system typically consists of the following components:

1. Charging device – includes the charging transformer, high-voltage silicon rectifier, protective resistor, DC resistive voltage divider, and automatic earthing device. The charging transformer steps up the mains frequency voltage, which is then rectified to DC to charge the stage capacitors.
2. Generator main body – comprises multiple pulse capacitors, firing spark gaps, front resistors, tail resistors, charging resistors, and insulating supports. The main body is constructed in a tower or column structure, with the stage capacitors connected in series to achieve a high output voltage. Depending on the usage, it can be classified as outdoor type, indoor fixed type, or indoor movable type.
3. Control equipment – modern control systems are typically centred around a PLC (Programmable Logic Controller) with a digital measurement and control system. They enable functions such as automatic constant-current charging control, voltage and current monitoring, automatic spark-gap distance adjustment, firing triggering, and overcurrent/overvoltage protection.
4. Measuring system – consists of a damped capacitive voltage divider, measurement cables, a digital recorder or oscilloscope, etc., used to accurately record and analyse the impulse voltage waveform.

4. Standard Waveforms and Technical Requirements

The standard impulse voltage waveforms for insulation testing are specified in international and national standards for high-voltage test techniques:

• Lightning impulse full wave: front time T₁ = 1.2 μs (with permitted deviation of ±30%), time to half value T₂ = 50 μs (±20%).
• Switching impulse wave: front time Tcr = 250 μs, time to half value T₂ = 2500 μs.
• Lightning impulse chopped wave: chopped in the range of 2 to 6 μs.
• Steep-front wave: extremely short front time, with steepness up to 1000 kV/μs or more.

The manufacturing and verification of impulse voltage generators primarily follow the following standards:

• DL/T 848.5 – General technical specifications for high-voltage test devices – Part 5: Impulse voltage generators (applicable to rated voltages from 300 kV to 4800 kV and rated energies from 5 kJ to 480 kJ).
• GB/T 16927.1/.2 – High-voltage test techniques series.
• IEC 60060 – International Electrotechnical Commission standards on high-voltage test techniques.
• GB 311.1 – Insulation coordination for high-voltage transmission and transformation equipment.

5. Application Areas

Impulse voltage generators are widely used in the following fields:

1. Type tests and routine tests on power equipment – Transformers, instrument transformers, bushings, insulators, surge arresters, GIS, and other transmission and distribution equipment typically require lightning and switching impulse voltage tests. These include full-wave impulse withstand tests and chopped-wave impulse tests, used to verify the insulation withstand capability of equipment under lightning and switching overvoltages.
2. Insulation research – Used to study the insulation performance of power equipment subjected to atmospheric overvoltages (lightning) and switching overvoltages.
3. Renewable energy equipment testing – With the integration of wind power, photovoltaic, and other renewable sources, a large number of step-up transformers and switchgear require rigorous impulse voltage tests.
4. Scientific research – Also widely applied in areas such as electromagnetic compatibility research, discharge mechanism studies, and pulsed power technology.

6. Development Trends

As power systems move towards ultra-high voltages and greater intelligence, the impulse voltage generator industry shows the following trends:

1. Intelligence and automation – Modern impulse voltage generators are generally equipped with PLC-based automatic control systems, enabling automatic parameter adjustment, automatic triggering, and automatic recording and analysis of test data. The test sequence can be programmed via touchscreen interfaces to achieve a largely automated operation. Some systems have begun to integrate auxiliary diagnostic functions to identify and indicate potential test deviations.
2. Compactness and all-solid-state – New generators based on solid-state switches can significantly reduce volume and weight, improve discharge repetition rates, and increase waveform flexibility. All-solid-state Marx generators, using power electronic devices instead of traditional spark gaps, have become an important direction of development.
3. Digitalisation and connectivity – Modern generators employ digital recorders and high-precision acquisition cards instead of traditional light-beam oscilloscopes, enabling automatic waveform analysis and report generation. Some devices can interface with laboratory information management systems via network connections for data exchange.
4. High precision and high reliability – Through improved sensor technology and signal processing algorithms, measurement accuracy and repeatability have been continuously enhanced. Currently, the charging voltage instability of typical equipment can be controlled within ±1%.
5. Multifunctionality – A single generator can produce various standard and non-standard waveforms, including lightning full waves, chopped waves, switching waves, oscillating waves, and steep-front waves, to suit different application scenarios.

7. Conclusion

As a core fundamental device in the high-voltage testing field, the technical level of the impulse voltage generator directly affects the accuracy and reliability of insulation performance verification for power equipment. From the traditional Marx circuit to modern all-solid-state topologies, and from manual operation to intelligent automatic control, impulse voltage generator technology is undergoing significant transformation. With the ongoing development of ultra-high-voltage power grids, the growth of renewable energy installations, and the advance of digital transformation, impulse voltage generators continue to play an important role in ensuring the safe and reliable operation of power systems.