Switching Impulse Voltage Withstand Test: Standards, Methods, and Applications

The switching impulse voltage withstand test is a high-voltage dielectric test that verifies the ability of electrical equipment insulation to withstand overvoltages caused by switching operations in power systems. As a mandatory requirement for extra-high-voltage (EHV) and ultra-high-voltage (UHV) equipment such as transformers, gas-insulated switchgear, and insulators, this test follows international standards including IEC 60060-1, IEC 60076-3, and IEC 62271. This article explains the test principle, standard waveform, sample types, test procedures, acceptance criteria, and typical applications of the switching impulse withstand voltage test.

Technical Overview

The switching impulse voltage withstand test (also referred to as the operational impulse withstand test) is a dielectric strength test in which a standard switching impulse voltage waveform is applied to electrical equipment to confirm that its insulation system can safely endure the transient overvoltages generated during switching events. These events include line energization and de-energization, load rejection, fault clearing, capacitor bank switching, and the operation of circuit breakers and disconnectors. In power systems rated 300 kV and above, switching overvoltages rather than lightning surges become the dominant factor governing insulation coordination, which is why the switching impulse test is a key type test and routine test requirement for EHV and UHV equipment.

The standard switching impulse waveform specified in IEC 60060-1 has a virtual front time of 250 μs and a time to half value of 2500 μs, conventionally written as 250/2500 μs. The permitted tolerances are ±3% on the peak value, ±20% on the front time, and ±60% on the time to half value. Compared with the 1.2/50 μs lightning impulse, the switching impulse has a much longer duration and lower frequency content, which stresses internal and external insulation in a fundamentally different manner, making it impossible to substitute one test for the other.

The test is generally performed with negative polarity impulses. This is because the flashover voltage of external air insulation is lower under positive switching impulses, and using negative polarity reduces the risk of irrelevant external flashovers during testing, allowing the internal insulation of the test object to be evaluated effectively. Key standards governing the test include IEC 60060-1 (high-voltage test techniques), IEC 60076-3 and GB/T 1094.3 (power transformers), IEC 62271-1 and IEC 62271-203 (high-voltage switchgear and GIS), IEC 60071 (insulation coordination), and GB/T 16927.1. Typical rated switching impulse withstand levels include approximately 950 kV peak for 330 kV systems, 1175 kV for 500 kV systems, 1550 kV for 750 kV systems, and 1800 kV for 1000 kV UHV systems.

Testing Samples

Power Transformers and Shunt Reactors

Transformers and reactors rated for EHV and UHV systems undergo switching impulse tests as part of type testing or routine testing depending on their voltage class. The impulse is applied to each phase winding terminal in turn while non-tested terminals are solidly grounded, and the dielectric condition of the winding insulation, major insulation, and oil-paper system is verified through voltage and neutral current waveform analysis.

Gas-Insulated Switchgear (GIS) and High-Voltage Switchgear

GIS assemblies, circuit breakers, disconnectors, and metal-enclosed switchgear for rated voltages of 300 kV and above are subjected to switching impulse withstand tests across main circuit insulation, between phases, and across open switching gaps. Both polarities may be applied to verify the withstand capability of SF6 gas insulation and spacer assemblies.

Insulators, Bushings, and Insulating Components

Porcelain, glass, and composite insulators, transformer bushings, wall bushings, and post insulators are tested under switching impulse voltage, frequently in wet conditions for outdoor applications, to evaluate the external insulation performance of the air gap and the insulating surface.

Instrument Transformers and HVDC Equipment

Current transformers, capacitor voltage transformers, voltage transformers, converter transformers, and valve-side insulation components used in HVDC transmission systems also require switching impulse withstand verification, sometimes combined with DC voltage and polarity reversal tests to reflect composite service stresses.

Testing Items

The switching impulse withstand test program typically covers the following items:

• Rated switching impulse withstand voltage verification using the standard 250/2500 μs waveform
• Wet switching impulse withstand tests for external insulation of outdoor equipment
• 50% disruptive discharge (flashover) voltage determination for self-restoring insulation such as air gaps and insulator strings
• Switching impulse tests between phases and across isolating distances for switchgear
• Combined or composite withstand verification when specified by product standards
• Atmospheric correction of test voltage for air density, humidity, and altitude

Testing Methods

The test is carried out using a multi-stage impulse voltage generator (Marx generator) equipped with dedicated front and tail resistors and load capacitors tuned to produce the 250/2500 μs standard switching impulse waveform. The applied voltage is measured with a calibrated capacitive voltage divider and a digital impulse measurement system compliant with IEC 61083, and the test circuit is verified through low-voltage calibration before energizing the full test level.

For equipment with self-restoring insulation, such as GIS and external air insulation, a common withstand criterion is the application of 15 consecutive impulses at the specified withstand voltage with no disruptive discharge occurring inside the equipment, with only a strictly limited number of external flashovers permitted after atmospheric correction. For equipment with non-self-restoring insulation, such as transformer windings, the procedure generally involves one or more calibration impulses at approximately 50% to 75% of the test voltage, followed by a series of full-level negative polarity impulses applied to each phase terminal in turn.

Failure detection relies on the comparison of recorded waveforms. The applied voltage oscillogram and the neutral or winding current traces obtained during calibration shots are compared with those recorded at full test voltage. Any sudden change in waveform shape, reduction in peak voltage, interruption near the wave crest, or abnormal neutral current oscillation indicates an internal insulation breakdown or partial discharge event, and the test object is judged to have failed. Before testing, atmospheric condition corrections are applied for external insulation, and the test area must be cleared to prevent uncontrolled flashover paths.

Applications

The switching impulse withstand test is applied throughout the lifecycle of high-voltage electrical equipment:

• Type testing and routine testing of EHV and UHV transformers, reactors, GIS, switchgear, bushings, and insulators at 330 kV, 500 kV, 750 kV, and 1000 kV voltage levels
• Factory acceptance testing and commissioning verification for major grid infrastructure projects, including HVDC converter stations
• Insulation coordination studies and verification of protective margins according to IEC 60071
• Product certification for domestic and international markets through accredited third-party laboratories
• Research on air gap dielectric characteristics, altitude correction factors, and insulation design optimization for high-altitude installations
• Diagnostic assessment of aged or repaired equipment to confirm remaining dielectric strength before return to service

FAQ

What is the difference between a switching impulse test and a lightning impulse test? The lightning impulse test uses a 1.2/50 μs waveform to simulate fast transient lightning surges, while the switching impulse test uses a 250/2500 μs waveform to simulate slower, longer-duration switching overvoltages. The two tests stress insulation differently and are both required; the switching impulse test becomes the more decisive test for equipment rated above approximately 300 kV.

Why is negative polarity normally used for switching impulse tests? External air insulation exhibits a lower flashover voltage under positive switching impulses. Applying negative polarity reduces the likelihood of external flashovers during the test so that the internal insulation of the test object can be assessed without interference, and it reflects conservative practice established in IEC and GB standards.

When is the switching impulse withstand test required? It is generally required for equipment used in power systems rated 300 kV and above, where switching overvoltages determine insulation levels. Whether the test is a type test or a routine test depends on the product standard and the rated voltage of the equipment.

How is a failed test identified? A failure is identified by comparing voltage and current oscillograms: voltage collapse, waveform distortion, abnormal oscillations, or changes in neutral current compared with the calibration records indicate internal insulation breakdown. For self-restoring insulation, disruptive discharges beyond the permitted number constitute failure.

Which standards apply to the switching impulse withstand test? The principal standards include IEC 60060-1 and GB/T 16927.1 for high-voltage test techniques and measurement, IEC 60076-3 and GB/T 1094.3 for power transformers, IEC 62271-1 and IEC 62271-203 for switchgear and GIS, and IEC 60071 for insulation coordination.