A high-voltage equipment can pass a power-frequency withstand test and still have an insulation weakness when exposed to a fast transient voltage.
This is one of the main reasons why impulse testing is used in the development and verification of high-voltage electrical equipment.
A transformer, GIS, cable, bushing or insulator does not operate only under a steady voltage. During its service life, the insulation system may also be exposed to transient overvoltages caused by lightning, switching operations or other disturbances in the power system.
These events are short in duration, but the voltage can rise very quickly.
For the engineer responsible for insulation design, the question is therefore not only:
Can the equipment withstand its rated operating voltage?
It is also:
Can the insulation system withstand the transient voltage stress for which the equipment has been designed?
That is the purpose of impulse testing.
Why Is Power-Frequency Withstand Testing Not Enough?
Power-frequency withstand testing remains one of the basic tests for high-voltage equipment.
It allows engineers to verify whether the insulation can withstand a specified AC voltage for a defined period.
But an impulse test creates a very different electrical condition.
The voltage changes much faster, and the voltage distribution inside the equipment can also be different from that under power-frequency conditions.
This matters particularly in equipment containing complex insulation structures.
Take a power transformer as an example.
Under normal operating conditions, the voltage distribution along a winding follows one electrical condition. When a steep impulse reaches the winding, however, the initial voltage distribution can be strongly influenced by the capacitance and inductance of the winding and insulation structure.
The result is that some parts of the insulation can experience a much higher local electrical stress than would be expected from the normal operating voltage alone.
This is why passing an AC withstand test does not automatically mean that the equipment has been fully verified against impulse voltage.
The two tests answer different engineering questions.
What Does an Impulse Test Actually Verify?
Impulse testing is essentially an examination of the insulation system under a rapidly changing voltage.
The test can provide information about whether the insulation structure has sufficient withstand capability when exposed to transient electrical stress.
Depending on the equipment and test requirement, engineers may be concerned with:
- internal insulation
- external insulation
- clearances
- creepage paths
- winding insulation
- terminal insulation
- insulation interfaces
- electric-field concentration
- transient voltage distribution
A high-voltage insulation system is not simply a matter of choosing a material with a sufficiently high dielectric strength.
The geometry, distance, field distribution and interaction between different components all matter.
Impulse testing gives the engineer another way to verify whether the complete insulation design behaves as expected.
Lightning Impulse and Switching Impulse Are Different
The term impulse testing covers more than one type of waveform.
For high-voltage equipment, the required test may involve a lightning impulse, switching impulse, chopped wave or another specified waveform.
Lightning Impulse Test
Lightning impulse testing is used to examine insulation performance under a fast transient voltage.
The concern is not simply the maximum voltage. The shape and rate of the voltage change are also important because they influence how the voltage is distributed through the insulation system.
Switching Impulse Test
High-voltage switching operations can also produce transient overvoltages.
Switching impulse testing provides another type of insulation verification and is particularly relevant when the equipment's insulation system needs to be evaluated under this slower transient condition.
Chopped-Wave Test
A chopped impulse introduces a rapid interruption of the impulse waveform.
This creates another form of electrical stress and can be used when the applicable test program requires chopped-wave testing.
Special Waveforms
Not every test laboratory works with only one standard waveform.
Research institutions and manufacturers developing specialized high-voltage equipment may need additional waveform configurations.
SUTE's lightning impulse voltage generator is designed for full lightning impulse, switching surge, chopped-wave, shock wave and other special waveform tests.
Which High-Voltage Equipment Needs Impulse Testing?
Impulse testing is not limited to transformers.
The actual requirement depends on the equipment type, voltage level, insulation structure and applicable test program.
Common applications include:
Power Transformers
Transformer windings contain complicated insulation systems.
An impulse applied to the winding can produce a voltage distribution that is different from the normal power-frequency condition.
For transformer manufacturers, impulse testing is therefore an important part of insulation verification.
GIS and GIL
GIS and GIL use compact insulation structures in which the arrangement of conductors, enclosure and insulating medium is highly integrated.
At higher voltage levels, the test system itself becomes an engineering consideration.
SUTE's SF6 Gas Insulation Lightning Impulse Voltage Generator developed for lightning-wave and switching-surge testing of 1000 kV GIS or GIL. The documented system uses a gas-insulated configuration and can also be combined with a gas-insulated test transformer to form a multifunctional test platform.
Power Cables
Cable insulation can also be affected by transient voltage.
Impulse testing can therefore be used as part of the overall verification of cable insulation and associated components.
For a cable manufacturer, the important issue is not simply whether the insulation survives a steady test voltage, but whether the complete insulation system has sufficient transient withstand capability.
Bushings and Insulators
Bushings and insulators provide critical insulation and mechanical support in high-voltage equipment.
Their insulation performance needs to be considered under different electrical stresses, including impulse conditions where required.
Switchgear and Other High-Voltage Components
Switchgear, electrical components and other high-voltage products may also be subjected to transient electrical stresses during operation.
The exact impulse test requirement depends on the equipment design and the applicable test specification.
Why Does the Impulse Waveform Matter?
When engineers select an impulse voltage generator, the maximum voltage is only one part of the specification.
The generated waveform is equally important.
Consider a test system rated for a high voltage.
If the generator cannot produce the required waveform under the actual test-object conditions, a high nominal voltage rating does not solve the problem.
Several factors need to be considered together:
Test voltage
Can the generator reach the required voltage level?
Waveform
Can it generate the required lightning, switching or chopped impulse?
Test-object capacitance
How will the capacitance of the equipment under test affect the impulse circuit?
Circuit inductance
Will the test circuit allow the required waveform to be generated without excessive distortion?
Energy
Does the system have sufficient stored energy for the intended test?
Repeatability
Can the required waveform be reproduced consistently?
These are practical engineering questions when selecting an impulse voltage test system.
Why Large Test Objects Are More Challenging
A small laboratory test object and a large high-voltage product do not necessarily present the same testing conditions.
Large equipment can have considerable capacitance.
Once the test object is connected to the impulse generator, the test object becomes part of the electrical circuit.
The generator, connecting conductors, measuring system and test object interact with each other.
This can affect the resulting impulse waveform.
For this reason, a test system should not be selected only by looking at:
Maximum impulse voltage = XXX kV
The engineer also needs to understand the expected load and waveform requirements.
SUTE's impulse voltage generator documentation specifically identifies low circuit inductance as one of the main technical characteristics, together with the ability to generate standard waveforms under large-capacity loads.
This is particularly relevant when the test object is a large transformer, GIS/GIL system or other high-capacitance equipment.
What Happens If an Insulation Weakness Is Not Found?
This is where impulse testing becomes more than a laboratory procedure.
Imagine a transformer that passes its routine electrical tests and is delivered to the customer.
If a weakness in the insulation structure has not been identified, the problem may only become visible after the equipment is exposed to an abnormal transient in service.
At that point, the consequences can be much greater than finding the problem during factory testing.
The possible consequences include:
- insulation breakdown
- internal flashover
- equipment damage
- unplanned outage
- repair costs
- delayed commissioning
- investigation of the manufacturing process
Finding an insulation weakness during development or factory testing gives the manufacturer an opportunity to correct the design.
That is one of the practical values of impulse testing.
Testing is not only about proving that a product works. It is also about finding weaknesses before the product enters service.
Impulse Testing as a Design Verification Tool
For a high-voltage equipment manufacturer, impulse testing should not always be viewed as the final step before shipment.
It can also be useful during product development.
Suppose an engineering team is developing a new transformer or insulation structure.
The team may have already completed:
- insulation design
- electric-field analysis
- material selection
- dimensional verification
- AC withstand testing
- partial discharge testing
The product may look satisfactory.
Impulse testing adds another layer of evidence.
If the result is not as expected, engineers can investigate:
- insulation arrangement
- local field concentration
- clearances
- winding structure
- terminal design
- connection arrangement
- waveform behavior
This can make the impulse test a useful part of the design-development cycle rather than simply a compliance exercise.
What Should You Look at When Choosing an Impulse Voltage Test System?
For a testing laboratory or equipment manufacturer, the following questions are more useful than simply asking for the maximum voltage.
1. What equipment will be tested?
Is it a transformer, GIS, GIL, cable, insulator, bushing or another high-voltage component?
2. What voltage level is required?
The generator configuration needs to match the actual test voltage and provide an appropriate operating margin.
3. Which waveform is required?
Lightning impulse, switching impulse, chopped wave or another special waveform?
4. What is the electrical characteristic of the test object?
Large-capacitance test objects can place different demands on the impulse generator.
5. How important is waveform adjustment?
A system should be capable of producing the required waveform under the actual test conditions.
6. What level of automation is required?
For repeated laboratory testing, automated charging, triggering, measurement and grounding can improve operating efficiency and test consistency.
SUTE's impulse voltage and current measuring and controlling system uses constant-current charging based on PID calculation and includes functions such as automatic voltage tracking, automatic grounding, preset impulse times and adjustable charging intervals. The impulse voltage generator can be triggered manually, automatically or by an alarm signal.
SUTE Impulse Voltage Generator Systems
SUTE manufactures impulse testing equipment for different high-voltage test requirements.
Its documented Lightning Impulse Voltage Generator range is 20 kV to 7200 kV and is designed for full lightning impulse, switching surge, chopped-wave, shock-wave and other special waveform tests.
characteristics relevant to engineering applications:
- complete voltage-level coverage
- low circuit inductance
- waveform generation under large-capacity loads
- high voltage utilization coefficient
- convenient waveform adjustment
- automatic constant-current charging
- high automation
- high anti-interference capability
SUTE offers SF6 gas-insulated lightning impulse voltage generators specifically designed for GIS and GIL applications. These systems enable lightning and switching impulse testing on 1000 kV GIS or GIL equipment, and can be integrated with SF6 gas-insulated test transformers to create multifunctional testing platforms.
For laboratories requiring impulse current testing rather than impulse voltage testing, SUTE also manufactures impulse current generators. Including zinc oxide arrester valve-plate testing and lightning protection research, with selectable impulse-current waveforms including 1/20 μs, 4/10 μs, 8/20 μs, 10/350 μs, 10/1000 μs, 18/40 μs and 30/80 μs.
Impulse Voltage Testing vs. Impulse Current Testing
These two terms are sometimes used together, but they represent different test requirements.
Impulse Voltage Testing
The test object is subjected to a controlled impulse voltage.
The focus is primarily on insulation withstand performance under transient voltage stress.
Typical applications include transformer, GIS, GIL, cable, insulator and other high-voltage insulation tests.
Impulse Current Testing
The test object is subjected to a controlled impulse current.
The focus is on the equipment's ability to withstand impulse-current stress.
For example, SUTE's impulse current generator is used for impulse-current testing of zinc oxide arrester valve plates and lightning protection research.
Therefore, when planning a high-voltage test laboratory, it is important to distinguish between the two.
A Practical Way to Think About Impulse Testing
For engineers, the simplest way to understand the purpose of impulse testing is this:
Power-frequency testing asks:
Can the insulation withstand the specified operating-frequency voltage?
Impulse testing asks:
What happens when the insulation is subjected to a rapidly changing transient voltage?
These are not the same question.
A high-voltage product needs an insulation system that can perform under the electrical conditions it is designed to encounter.
That is why impulse testing remains an important part of high-voltage equipment verification.
Conclusion
A high-voltage equipment may perform perfectly under normal operating voltage and still contain an insulation weakness that only becomes apparent under a transient.
Impulse testing provides another level of verification by applying controlled transient voltage conditions to the equipment.
For manufacturers of transformers, GIS, GIL, cables, bushings, insulators and other high-voltage equipment, the test is not simply about reaching a particular voltage.
The quality of the waveform, test circuit, test-object characteristics, measurement system and overall test configuration all influence the result.
The right impulse voltage test system therefore needs to be selected according to the actual equipment and testing requirements—not simply by its maximum voltage rating.
For high-voltage testing laboratories and manufacturers, impulse testing is ultimately a way to answer a practical engineering question:
Will the insulation system remain reliable when the electrical stress is no longer steady?
FAQ
What is high-voltage impulse testing?
High-voltage impulse testing applies a controlled transient voltage to electrical equipment to evaluate the withstand capability of its insulation system.
Why is impulse testing necessary if the equipment has already passed an AC withstand test?
AC withstand testing and impulse testing represent different electrical stresses. Passing one does not automatically demonstrate performance under the other.
What is the difference between lightning impulse and switching impulse?
They represent different transient voltage conditions and therefore place different stresses on the insulation system. The appropriate waveform depends on the equipment and test requirement.
What equipment can be tested with an impulse voltage generator?
Typical applications include transformers, GIS, GIL, power cables, bushings, insulators, switchgear and other high-voltage electrical equipment, depending on the applicable test requirements.
Does the maximum voltage determine which impulse generator I should choose?
No. Voltage level is important, but engineers should also consider waveform requirements, test-object capacitance, circuit inductance, energy, measurement and automation requirements.
What impulse voltage generator voltage range does SUTE provide?
Its lightning impulse voltage generator range is 20 kV–7200 kV.