The Switching Impulse Test (SI) is a critical dielectric test used to verify the insulation strength of high-voltage and extra-high-voltage power transformers under slow-front overvoltage conditions. Unlike lightning impulse tests, which simulate very fast surges, the switching impulse test represents overvoltages caused by system switching operations, such as line energization, fault clearing, or circuit-breaker reclosing.
IEC 60076-3 defines when the Switching Impulse Test is required, how it must be performed, which voltage levels apply, and how results are evaluated. For transformers operating at high system voltages, the SI test is often more critical than the lightning impulse test, because switching overvoltages dominate insulation design at EHV levels.
This article explains the purpose, waveform, procedure, and importance of the Switching Impulse Test in a clear and technically accurate.
Table of Contents
What Is the Switching Impulse Test (SI)?
The Switching Impulse Test is a dielectric test in which a transformer winding is subjected to a slow-front impulse voltage. This impulse has a much longer rise time and duration than a lightning impulse and is designed to simulate overvoltages produced by switching events in high-voltage power systems.
Typical sources of switching overvoltages include:
- Energization of long transmission lines
- Circuit-breaker opening and reclosing
- Load rejection
- Earth-fault clearing
- Ferroresonance and network resonance
These events generate voltage surges with slower rise times but high peak values, which can stress transformer insulation in a different—and often more severe—way than lightning impulses.
Why Switching Impulse Tests Are Needed
As system voltage increases, the relative importance of lightning overvoltages decreases, while switching overvoltages become dominant. This is especially true for EHV systems (≥ 300 kV).
The Switching Impulse Test verifies that the transformer can withstand:
- High peak voltages with long duration
- Non-uniform electric field distribution
- Stress across large insulation distances
- Voltage transfer between windings
- Cumulative dielectric stress from slow-front waves
In many EHV designs, insulation dimensions are determined primarily by switching impulse withstand levels, not lightning impulse levels.
When Is the Switching Impulse Test Required? (IEC 60076-3)
IEC 60076-3 specifies clear rules based on Um (Highest Voltage for Equipment).
Um ≤ 72.5 kV
- Switching impulse test not required
72.5 kV < Um ≤ 170 kV
- Switching impulse test is a special test
- Performed only if specified by the purchaser
Um > 170 kV
- Switching impulse test becomes a routine test
- Mandatory for every transformer
This reflects the increasing importance of slow-front overvoltages at higher system voltages.
Switching Impulse Waveform Characteristics
The switching impulse waveform is fundamentally different from a lightning impulse.
Typical waveform parameters
- Front time: approximately 250 µs
- Time to half-value: approximately 2 500 µs
- Much slower rise than lightning impulse
- Longer stress duration
Because of the long duration, the electric field penetrates deeper into the insulation system, stressing:
- Inter-winding insulation
- Axial insulation
- Insulation to tank and core
- Major oil gaps
This is why switching impulses are considered bulk insulation tests, while lightning impulses focus more on local line-end stresses.
Switching Impulse Test Voltage Levels
IEC 60076-3 provides standardized Switching Impulse (SI) test voltages for each Um value. These values are listed together with LI, LIC, and AC test voltages to maintain insulation coordination.
Examples:
- Um = 245 kV → SI ≈ 700–850 kV
- Um = 420 kV → SI ≈ 950–1 175 kV
- Um = 550 kV → SI up to 1 300 kV
The test voltage must not be lower than the minimum values defined by IEC.
All test voltages must be taken from the same row of the IEC table to ensure coordinated insulation performance.
Purpose of the Switching Impulse Test
The Switching Impulse Test verifies the transformer’s ability to withstand:
- Long-duration overvoltages
- High energy stress
- Voltage distribution along the entire winding
- Stress between windings and to ground
- Cumulative dielectric stress without breakdown
Unlike lightning impulse tests, SI tests are less sensitive to turn-to-turn insulation and more sensitive to overall insulation design.
How the Switching Impulse Test Is Performed
The test procedure follows a structured sequence defined by IEC 60076-3.
1. Test preparation
- Transformer fully assembled
- Non-tested terminals grounded
- Correct tap position selected
- Measurement and recording equipment calibrated
2. Reduced-voltage impulse
- One or more impulses at reduced voltage
- Used to verify polarity, waveform shape, and test connections
3. Full-voltage switching impulses
- Several impulses applied at 100% of the SI test voltage
- Typically applied with both polarities
- The number of impulses is defined by the standard and test agreement
4. Waveform recording
- Peak voltage
- Front time
- Time to half-value
- Polarity
- Any oscillation or distortion
The waveform must closely match the IEC-defined switching impulse shape.
Acceptance Criteria for the SI Test
A transformer passes the Switching Impulse Test when:
- No internal insulation breakdown occurs
- No sudden collapse of voltage is observed
- No abnormal waveform distortion appears
- No abnormal sound or mechanical disturbance is detected
- All recorded impulses show stable and repeatable behavior
IEC is explicit:
Any internal flashover or insulation failure means the test has failed.
External flashovers are generally not permitted unless explicitly agreed.
Handling Test Failures
If a failure occurs during the Switching Impulse Test:
- The cause must be investigated
- External test-circuit issues must be ruled out
- Internal insulation failure requires repair
- After repair, the entire test sequence must be repeated
This strict approach ensures confidence in transformer insulation integrity.
Switching Impulse Test vs Lightning Impulse Test
Although both are impulse tests, they serve different purposes:
- Lightning Impulse (LI)
- Fast-front (1.2/50 µs)
- Focus on line-end and turn insulation
- Switching Impulse (SI)
- Slow-front (250/2500 µs)
- Focus on bulk and inter-winding insulation
For EHV transformers, the SI test is often the governing insulation test.
Why the Switching Impulse Test Is Critical for EHV Transformers
Modern power systems rely on:
- Long transmission lines
- Complex switching schemes
- High system reliability
These conditions increase the frequency and severity of switching overvoltages. The Switching Impulse Test ensures that transformers can survive these stresses repeatedly over decades of operation.
Without proper SI testing, a transformer may pass lightning impulse tests yet still fail in service due to slow-front overvoltage stress.
Simple Summary of the Switching Impulse Test
- The Switching Impulse Test simulates slow-front overvoltages
- It is mandatory for transformers with Um > 170 kV
- The waveform has a long rise time and long duration
- It stresses bulk insulation rather than local line-end insulation
- Any internal breakdown means test failure
- It is essential for EHV transformer reliability
Conclusion
The Switching Impulse Test (SI) is one of the most important dielectric tests for high-voltage and extra-high-voltage power transformers. Defined by IEC 60076-3, it ensures that transformer insulation can withstand the slow-front overvoltages produced by real switching operations in modern power systems.
By applying standardized switching impulse waveforms at carefully defined voltage levels, manufacturers and utilities can verify insulation coordination, prevent in-service failures, and ensure long-term transformer reliability. In today’s EHV networks, the Switching Impulse Test is not optional—it is fundamental.