How to Select an Impulse Voltage Generator for High Voltage Testing

Introduction

Impulse testing is one of the most demanding procedures in high-voltage equipment testing. Unlike power-frequency withstand tests, an impulse test reproduces the short-duration overvoltage conditions that electrical equipment may experience during lightning strikes or switching operations.

For laboratories and manufacturers, selecting an impulse voltage generator is therefore not simply a matter of choosing the highest available test voltage.

The generator must provide the required impulse voltage, energy, waveform characteristics, measurement accuracy and control capability while remaining suitable for the test object and laboratory configuration.

In practice, several parameters are closely related to the actual test performance.

This article explains the main technical factors that should be considered when selecting an impulse voltage generator for high-voltage testing.

1. Maximum Impulse Voltage: More Than Just a Rated Voltage

The first parameter normally considered is the maximum impulse voltage.

An impulse voltage generator must be capable of producing the required test voltage for the equipment under test. However, selecting a generator only according to the nominal test voltage can lead to an unsuitable system.

The required voltage level depends on the type of equipment, insulation system and applicable test standard.

For example, transformers, bushings, GIS components, insulators and other high-voltage equipment may have different impulse withstand requirements.

More importantly, the generator should provide sufficient voltage margin for waveform adjustment and different test configurations.

A generator rated exactly at the required test voltage may have limited flexibility when the test object introduces additional capacitance or when a different impulse waveform is required.

Selection consideration

When selecting an impulse voltage generator, engineers should consider:

• Required test voltage
• Maximum charging voltage
• Number of stages
• Voltage utilization coefficient
• Test object capacitance
• Required waveform
• Future testing requirements

SUTE's impulse voltage generators cover a wide voltage range, from 20 kV to 7200 kV, allowing the generator configuration to be selected according to different laboratory and high-voltage testing requirements.

2. Energy: The Parameter Behind the Generator's Actual Test Capability

Voltage alone does not determine the capability of an impulse generator.

The energy stored in the impulse capacitors is another important parameter.

During an impulse test, the generator stores electrical energy before the test and releases that energy through the impulse circuit in a very short period.

The available energy affects the generator's ability to maintain the required waveform when connected to a test object with significant capacitance.

A simplified relationship between stored energy and capacitance is:

$$E=\frac{1}{2}CU^2$$

where:

• E is stored energy
• C is capacitance
• U is charging voltage

Because energy increases with the square of voltage, increasing the generator's voltage capability can significantly increase the stored energy.

This is one reason why a high-voltage impulse generator should not be selected only by comparing its maximum voltage rating.

Selection consideration

For a specific test system, engineers should evaluate the relationship between:

generator energy + test object capacitance + required impulse voltage + waveform

rather than treating these parameters independently.

3. Waveform Adjustment: The Generator Must Produce the Required Impulse

Generating a high voltage pulse is only part of the problem.

The waveform must also meet the requirements of the test.

A standard lightning impulse is commonly represented by a 1.2/50 μs waveform, where the first value describes the nominal front time and the second describes the nominal time to half-value.

In practical testing, however, the actual waveform is affected by:

• Test object capacitance
• Generator circuit inductance
• Stray capacitance
• Resistance
• Connection layout
• Voltage divider characteristics

This means that a generator capable of producing a particular voltage does not automatically guarantee the required waveform on every test object.

The impulse circuit must be designed and adjusted according to the actual testing configuration.

Why this matters

For a laboratory performing different types of high-voltage tests, waveform flexibility can be more valuable than simply increasing the maximum voltage rating.

SUTE's impulse voltage generator systems support waveform adjustment and can be configured for different impulse testing requirements.

4. Circuit Inductance: A Small Parameter With a Large Effect

At normal power frequency, several meters of conductor may have little practical influence on a test circuit.

An impulse test is different.

The voltage changes within microseconds, so stray inductance and circuit layout can significantly affect the resulting waveform.

Excessive circuit inductance can cause:

• Slower voltage rise
• Oscillation
• Waveform distortion
• Reduced voltage utilization
• Differences between calculated and measured waveforms

For this reason, low-inductance construction is an important consideration when selecting and configuring an impulse voltage generator.

SUTE's impulse generator design emphasizes low circuit inductance, helping improve energy transfer and waveform control during impulse testing.

5. Voltage Utilization Coefficient: How Much of the Generator's Capability Reaches the Test Object?

The rated voltage of an impulse generator does not necessarily equal the voltage that can be effectively applied to the test object.

The practical output is influenced by the generator configuration, circuit parameters and test object.

This is why the voltage utilization coefficient is an important engineering parameter.

A higher utilization coefficient means that a greater proportion of the generator's available charging voltage can be converted into useful test voltage.

This becomes particularly important at higher voltage levels.

For large impulse generators, improving voltage utilization can affect:

• Required number of generator stages
• Overall equipment size
• Charging voltage
• Stored energy
• Laboratory space
• Operating efficiency

Therefore, two generators with similar nominal voltage ratings may have different practical testing capabilities.

6. Charging System: Stability Before the Test

Before an impulse is generated, the capacitor stages must be charged to the required voltage.

The charging system therefore plays an important role in repeatability and operating safety.

For laboratory applications, the charging system should provide stable and controllable charging rather than simply reaching the target voltage.

Important considerations include:

• Charging voltage stability
• Charging current
• Automatic control
• Stage-to-stage voltage balance
• Charging time
• Protection functions

SUTE's systems use constant-current charging and automatic control, allowing the charging process to be controlled according to the requirements of the impulse generator.

This becomes increasingly important as the number of stages and stored energy increase.

7. Measurement and Control: The Generator Is Only Part of the Test System

An impulse test is not complete simply because the generator produces a high-voltage pulse.

The resulting waveform must be measured, recorded and evaluated.

A complete impulse testing system normally includes:

• Impulse voltage generator
• Voltage divider
• Measuring and recording system
• Triggering system
• Control system
• Waveform analysis

The measuring system must have sufficient bandwidth and accuracy to capture a microsecond-scale transient waveform without significantly changing the waveform itself.

For this reason, the measurement system should be considered during generator selection rather than added as an independent component later.

SUTE also provides Impulse Voltage & Current Measuring and Controlling Systems, which can be integrated into high-voltage impulse testing configurations.

8. Lightning Impulse, Switching Impulse and Chopped Wave Testing

Another important selection factor is the type of impulse test that the system needs to perform.

A laboratory may require more than a standard lightning impulse.

Depending on the application, the system may need to support:

Lightning impulse

Used to evaluate insulation performance under fast transient overvoltage conditions associated with lightning.

Switching impulse

Used to simulate slower transient overvoltages associated with switching operations in high-voltage systems.

Chopped impulse

A controlled interruption of the impulse waveform used for specific insulation tests and diagnostic purposes.

Special impulse waveforms

Some research and specialized testing applications require customized impulse conditions.

Therefore, a generator intended for long-term laboratory use should be selected according to the range of required impulse tests, rather than a single test waveform.

SUTE impulse voltage generators can be configured for full lightning impulse, switching surge, chopped wave and other special-wave tests.

9. Test Object Capacitance Should Be Considered Before Selecting the Generator

One parameter that is often underestimated during equipment selection is the capacitance of the test object.

A transformer, GIS component, bushing or cable system can present a significant capacitive load to the impulse generator.

This additional capacitance changes the behavior of the impulse circuit.

The generator therefore needs to be evaluated together with the actual test object.

For a laboratory testing multiple types of equipment, engineers should consider the expected range of test-object capacitance rather than designing the system around only one product.

This is particularly important when moving from component-level testing to larger high-voltage equipment.

10. How to Select the Right Impulse Voltage Generator

A practical selection process can be summarized as follows.

This approach is more useful than simply comparing equipment by maximum voltage.

11. SUTE Impulse Voltage Generator

SUTE develops impulse voltage generator systems for high-voltage testing applications.

The systems can be configured for different voltage and energy requirements, with available voltage levels extending from 20 kV to 7200 kV.

Key capabilities include:

• Lightning impulse testing
• Switching surge testing
• Chopped wave testing
• Special impulse-wave testing
• Low-inductance impulse circuits
• High voltage utilization
• Waveform adjustment
• Constant-current charging
• Automatic control
• Integrated impulse measurement and control

The equipment can be applied to high-voltage testing of transformers, GIS and other electrical insulation systems.

For larger laboratories and specialized testing facilities, the generator configuration can be developed according to the required test voltage, energy, waveform and test-object characteristics rather than using a fixed standard configuration.

Conclusion

Selecting an impulse voltage generator is fundamentally an engineering problem rather than a simple equipment specification comparison.

Maximum voltage determines the upper testing range, but it does not tell the whole story.

Energy, waveform control, circuit inductance, voltage utilization, charging stability, test-object capacitance and measurement capability all affect the actual performance of an impulse testing system.

For this reason, the generator should be selected together with the complete testing circuit and the intended test application.

A properly configured impulse voltage generator should not only produce a high voltage. It should produce the required transient waveform, at the required energy level, with controlled and measurable test conditions.

That is the basis of reliable impulse testing for modern high-voltage equipment.