Gas-insulated switchgear (GIS) is designed around a compact insulation system. The high-voltage conductor, enclosure, spacers, disconnectors and circuit breakers are arranged inside a grounded metal enclosure, with the insulating gas providing the dielectric medium.
This construction brings obvious advantages in substations, particularly where space is limited. But it also creates a particular challenge when the GIS has to be subjected to an impulse voltage test.
Unlike a simple test object, a large GIS installation can behave as a considerable capacitive load connected to a high-frequency impulse circuit. The physical length of the GIS, internal connections, bushings, spacers and external test leads all become part of the test circuit.
For this reason, GIS impulse testing is not simply a matter of applying a high voltage to the equipment. The impulse generator, test object and measuring system have to work together as one system.
Why impulse testing is important for GIS
GIS insulation can be very reliable under normal operating conditions, but small local defects can become critical when the electric field changes rapidly.
Examples include:
- Metallic particles inside the enclosure
- Protrusions on high-voltage conductors
- Damaged or improperly installed spacers
- Abnormal electric-field concentrations
- Assembly-related defects
- Foreign particles or contamination
These defects may not always be obvious during a conventional visual inspection.
Impulse testing applies a short-duration high-voltage transient to the GIS insulation system. The purpose is not simply to reproduce a lightning event in the laboratory. The test provides a controlled way of checking whether the insulation system can withstand specified transient overvoltages.
IEC 62271-203 specifically covers AC gas-insulated metal-enclosed switchgear above 52 kV, and impulse voltage testing is part of the dielectric test requirements for GIS.
There is also an important practical point: impulse testing can be particularly sensitive to certain abnormal electric-field configurations. Earlier editions of IEC 62271-203 note the sensitivity of lightning impulse testing to defects such as damaged electrodes.
The difficulty starts when the GIS becomes a large capacitive load
A conventional impulse generator is relatively straightforward when the test object has a modest capacitance.
GIS is different.
As the length and voltage class of the GIS increase, its equivalent capacitance can become substantial. The test object is no longer just something connected at the end of the generator. Its capacitance interacts with the generator's internal inductance, charging circuit, front resistor, tail resistor, connecting conductors and measurement system.
This interaction directly affects the resulting impulse waveform.
For a lightning impulse test, engineers are normally concerned with parameters such as front time and time to half-value. A standard lightning impulse is commonly represented as 1.2/50 μs.
But producing a suitable waveform at the generator terminals does not automatically mean that the required waveform will be obtained at the GIS under test.
The larger the capacitive load, the more important the complete circuit becomes.
Research on UHV GIS field testing has shown that when the equivalent test capacitance becomes sufficiently large, obtaining a standard lightning impulse waveform becomes increasingly difficult. The choice of test connection, external voltage divider and high-voltage lead arrangement can also affect the measured waveform.
This is one reason why GIS impulse testing should be treated as a system engineering problem, rather than simply a generator specification.
What happens to the impulse waveform?
Consider a simplified impulse test circuit:
Impulse Generator
│
│
HV Connection
│
▼
┌───────────┐
│ GIS │
│ Test │
│ Object │
└───────────┘
│
GroundIn an actual test, the circuit also includes the voltage divider and measuring system.
The waveform observed at the test object is determined by the combined electrical characteristics of these components.
Increasing the effective capacitance of the GIS can change:
- Wavefront time
- Oscillation
- Peak voltage
- Damping
- Wave tail
- Energy distribution
- Measurement response
The connecting conductors also matter.
A long high-voltage connection introduces additional inductance. Under a fast transient, that inductance is not negligible. Reflections and oscillations can therefore appear in the measured waveform.
This is particularly relevant for GIS because the test object itself can be physically large while the required impulse front remains very short.
Lightning impulse and switching impulse are not interchangeable
GIS impulse testing can involve different types of transient voltage depending on the equipment rating and applicable test requirements.
Lightning impulse
Lightning impulse testing is associated with fast transient overvoltages and is commonly represented by the 1.2/50 μs waveform.
For GIS, it is especially useful for evaluating the insulation system under a steep voltage rise.
Switching impulse
Switching impulses have a much longer time scale than lightning impulses. They are associated with switching operations in high-voltage systems and become particularly relevant as system voltage increases.
IEC 62271-203 includes switching impulse voltage testing for applicable GIS voltage classes.
The two tests should therefore not be regarded simply as different voltage levels from the same test.
The waveform itself changes the electrical stress applied to the insulation.
Why a conventional impulse generator can become difficult to use on large GIS
The problem becomes clearer when the test object is a large UHV GIS installation.
A traditional impulse generator generally uses multiple capacitor stages connected through a low-inductance discharge circuit. The generator has to deliver sufficient energy while maintaining the required waveform at the test object.
When the GIS capacitance increases, the generator and load form a different transient circuit.
Several problems can appear:
1. The wavefront becomes too long
The additional capacitance and circuit inductance can slow the voltage rise.
2. Oscillation becomes more noticeable
The generator, connecting leads and GIS capacitance can form an oscillating network.
3. The effective test voltage can differ from the expected value
The external divider and connection arrangement introduce their own influence on the circuit.
4. Physical installation becomes difficult
Large conventional impulse generators require considerable space. This can become a major limitation when testing GIS in a substation or another location where the available test area is limited.
These challenges have been documented in research on field impulse testing of high-voltage GIS. Compact gas-insulated impulse generator designs have also been developed specifically to address the combination of high GIS capacitance and circuit inductance.
The test connection is part of the engineering work
One of the easiest things to underestimate in GIS impulse testing is the connection between the impulse generator and the GIS.
For a slow AC withstand test, a few additional metres of cable may not dramatically change the test.
For a microsecond-scale impulse, the situation is different.
The connection has its own inductance, capacitance and propagation characteristics.
The engineer therefore needs to consider:
- High-voltage lead length
- Connection geometry
- Grounding arrangement
- Voltage-divider position
- GIS configuration
- Test terminal selection
- Equivalent capacitance
- Generator configuration
A published analysis of UHV GIS on-site lightning impulse testing found that the external voltage divider can reduce the measured voltage slightly and increase the wavefront time, while the length of the high-voltage lead can influence oscillation and wavefront characteristics.
This is why a GIS impulse test should be prepared as a complete test circuit before the generator is selected.
Measuring the voltage is another challenge
Generating an impulse is only half of the problem.
The test engineer also needs to know what voltage was actually applied to the GIS.
A high-voltage divider is therefore an essential part of a practical impulse voltage test system.
The divider needs to reproduce the transient voltage accurately enough for the measuring system to determine parameters such as:
- Peak voltage
- Front time
- Time to half-value
- Oscillation
- Polarity
- Waveform deviation
The measurement circuit itself must have sufficient bandwidth and appropriate response characteristics.
This is particularly important for GIS because a waveform that appears acceptable at one point in the circuit may not be identical at another point.
For this reason, the impulse generator, voltage divider and digital measurement system should be considered together rather than specified as completely independent devices.
A GIS impulse test system is more than an impulse generator
For a practical GIS application, the complete system may include:
- Impulse voltage generator
- Charging unit
- Wave-shaping components
- Chopping arrangement where required
- High-voltage connection system
- Voltage divider
- Digital impulse measurement system
- Control and protection system
- Grounding arrangement
The generator determines the available voltage and energy.
The wave-shaping circuit determines how the impulse develops.
The GIS provides the actual capacitive load.
The divider measures the resulting voltage.
The measurement system determines whether the waveform meets the specified test requirements.
This is why Impulse Voltage Test System is a more useful engineering concept than simply Impulse Voltage Generator when discussing large GIS applications.
GIS testing also changes when the test moves to site
Factory testing and on-site testing have different practical conditions.
In a factory, the impulse test system can normally be installed around a dedicated test bay. The generator, divider, grounding system and auxiliary equipment can be permanently arranged for high-voltage testing.
On site, the situation is much less convenient.
The GIS may already be installed in a substation, and the available space around the equipment can be limited. The test system therefore has to be brought to the equipment and connected without significantly changing the test conditions.
This has driven the development of more compact impulse testing equipment for GIS.
For example, SUTE's technical equipment range includes gas-insulated lightning impulse generators intended for high-voltage GIS/GIL applications. The company's product documentation describes a gas-insulated impulse generator concept for lightning and switching impulse testing of 1000 kV-class GIS/GIL, with the objective of addressing the practical limitations of conventional equipment during high-voltage GIS installation testing.
The company's product information also describes a 2400 kV / 240 kJ impulse voltage generator and gas-insulated impulse equipment for GIS/GIL testing.
For a manufacturer, these are not simply higher numbers on a specification sheet. They reflect a basic engineering requirement: the test equipment has to deliver sufficient voltage and energy while maintaining a usable impulse waveform on a large capacitive GIS load.
What should be considered when selecting a GIS impulse test system?
For an engineer planning a GIS impulse test, the first question should not be:
How many kilovolts can the generator produce?
A better starting point is the complete test requirement.
1. GIS rated voltage
The required impulse test level depends on the rated voltage and applicable test specification of the GIS.
2. Equivalent capacitance
The capacitance of the GIS installation has a direct influence on the impulse circuit and should be considered when determining generator requirements.
3. Required impulse waveform
Determine whether the application requires:
- Lightning impulse
- Chopped lightning impulse
- Switching impulse
- Other specified transient waveforms
4. Required energy
A high-voltage generator must have sufficient stored energy to maintain the required test voltage when connected to the actual GIS load.
5. Test location
For factory testing, physical size may be less restrictive.
For field testing, equipment footprint, transport, connection method and installation time become much more important.
6. Measurement system
The voltage divider and digital measurement system should be suitable for the required voltage level and transient waveform.
7. Complete system compatibility
The generator, GIS load, divider and connecting system should be evaluated together.
This last point is often overlooked when equipment is selected purely from a generator nameplate.
The engineering objective is not simply “apply a high voltage”
A successful GIS impulse test requires several conditions to be satisfied simultaneously.
The test system needs to generate the required voltage.
The waveform needs to remain within the applicable requirements.
The measurement system needs to capture the actual transient accurately.
The GIS insulation needs to withstand the applied stress.
And the entire arrangement needs to remain practical for the location where the test is performed.
That is why GIS impulse testing becomes increasingly demanding as voltage level and equipment size increase.
For large GIS installations, the challenge is often not generating a high voltage by itself. The difficult part is generating the right impulse waveform, delivering it efficiently to a large capacitive test object, and measuring the resulting transient reliably.
GIS Impulse Testing and SUTE
SUTE develops and manufactures high-voltage test equipment covering impulse voltage generation, SF6 gas-insulated test transformers, resonant test systems, high-voltage DC generators and automatic measurement and control systems.
For GIS-related testing, this equipment can be configured according to the voltage level, test object, capacitance and testing environment rather than treating the impulse generator as an isolated piece of equipment.
This system-oriented approach is particularly relevant for high-voltage GIS and GIL applications where the test object itself has a significant influence on the impulse circuit.
Related topics:
[Impulse Voltage Test System] · [Impulse Voltage Generator] · [SF6 Gas Testing Transformer] · [High Voltage Testing
Equipment]
FAQ
What is GIS impulse testing?
GIS impulse testing is a high-voltage dielectric test used to verify the insulation withstand capability of gas-insulated metal-enclosed switchgear under specified transient overvoltages.
Why is impulse testing difficult for large GIS?
Large GIS installations can have substantial equivalent capacitance. This capacitance interacts with the impulse generator and connecting circuit, affecting the generated waveform, energy transfer and measurement.
What impulse waveform is used for GIS testing?
Lightning impulse and switching impulse are used according to the applicable GIS test requirements and voltage class. A standard lightning impulse is commonly represented as 1.2/50 μs.
Can a conventional impulse generator test GIS?
It can, depending on the GIS voltage level, capacitance, required waveform and test configuration. For large or high-voltage GIS, the generator and complete test circuit need to be evaluated carefully because load capacitance and circuit inductance can affect the impulse waveform.
Why is a voltage divider needed?
The voltage divider scales the high-voltage transient to a measurable level while preserving the waveform sufficiently for the measurement system to determine the impulse parameters.
Is GIS impulse testing only performed in a factory?
No. Impulse withstand testing can also be performed on site for appropriate GIS applications. Field testing introduces additional challenges involving equipment size, connection arrangement, test location and waveform control. Research has demonstrated on-site impulse testing of UHV GIS installations.