The Neutral Terminal Lightning Impulse Test (LIN) is a specific dielectric test defined in IEC 60076-3 for power transformers. Unlike the standard lightning impulse test applied to line terminals, the LIN test is applied directly to the neutral terminal of a winding to verify the adequacy of its insulation under lightning-type overvoltages.
Although the LIN test is not mandatory for all transformers, it becomes essential in certain insulation and grounding configurations. Incorrect assessment of neutral insulation can lead to internal dielectric failure during lightning or fault conditions, even when the line terminal insulation is correctly designed.
This article explains when the Neutral Terminal Lightning Impulse Test is required, why it is necessary, and how IEC 60076-3 defines its application, using clear, professional, and technically accurate language.
Table of Contents
What Is the Neutral Terminal Lightning Impulse Test (LIN)?
The LIN test is a lightning impulse test applied to the neutral terminal of a transformer winding, usually a star-connected winding. The test uses the standard lightning impulse waveform (1.2/50 µs), identical to the waveform used for line terminal LI tests, but at a lower voltage level appropriate to the neutral insulation strength.
The purpose of the LIN test is to verify that the neutral insulation can withstand lightning-type overvoltages that may appear at the neutral point during real system events.
In IEC 60076-3, the LIN test is treated as a special impulse test, applied only when specific technical conditions exist.
Why the Neutral Terminal Can Be Subjected to Lightning Overvoltages
In normal operation, the neutral terminal is often assumed to be close to earth potential. However, IEC 60076-3 recognizes that this assumption is not always valid.
During events such as:
- lightning strikes on connected overhead lines,
- earth faults in the network,
- switching operations,
- unbalanced system conditions,
the neutral terminal can experience transferred overvoltages due to the internal electrical coupling of the winding. If the neutral insulation is insufficient, these overvoltages may cause insulation breakdown even though the line terminals are adequately protected.
The LIN test exists to confirm that the neutral insulation can withstand these stresses.
When Is the Neutral Terminal Lightning Impulse Test Required?
According to IEC 60076-3, the LIN test is required only in specific cases, mainly related to grounding method and insulation design.
1. Neutral Not Solidly Earthed
The LIN test is required when the neutral terminal:
- is not directly connected to earth,
- is connected through an impedance (resistor or reactor),
- may become temporarily floating.
In such configurations, the neutral can rise to a significant voltage during lightning or fault events. IEC therefore requires verification of the neutral insulation by applying a lightning impulse directly to the neutral terminal.
2. Non-Uniform Insulation of the Winding
High-voltage and extra-high-voltage transformers are often designed with non-uniform insulation, meaning:
- full lightning impulse insulation at the line terminals,
- reduced insulation level at the neutral terminal.
This design is acceptable under IEC 60076-3, but only if the reduced neutral insulation is verified. The LIN test confirms that this lower insulation level is still sufficient under lightning conditions.
3. Requirement by Purchaser or Utility
IEC allows the LIN test to be specified by agreement. Many transmission system operators require the LIN test for:
- EHV transformers,
- transformers in areas with high lightning activity,
- critical network transformers where failure risk must be minimized.
In these cases, the LIN test is included as part of the contractual dielectric test program.
When Is the LIN Test Not Required?
The Neutral Terminal Lightning Impulse Test is generally not required when:
- the neutral is solidly and permanently earthed,
- the neutral insulation is designed strictly for earth potential,
- system conditions do not allow significant overvoltage transfer.
This is commonly the case for:
- distribution transformers,
- medium-voltage transformers,
- star-connected windings with solid grounding and uniform insulation.
LIN Test Voltage Level
IEC 60076-3 does not specify a single fixed voltage value for the LIN test. Instead:
- the test voltage depends on the neutral insulation level,
- it is related to the expected overvoltage stress,
- it is usually defined by the purchaser.
The waveform remains the standard 1.2/50 µs lightning impulse.
Acceptance criteria are identical to standard LI testing: no internal flashover, no waveform collapse, and no abnormal oscillations.
How the Neutral Terminal Lightning Impulse Test Is Performed
During the LIN test:
- the lightning impulse is applied to the neutral terminal,
- all line terminals are solidly grounded,
- the impulse generator is adjusted to produce a compliant waveform,
- voltage and time parameters are recorded for every impulse.
The test sequence follows the same principles as a standard lightning impulse test, including reduced-voltage shots followed by full-voltage impulses.
Any evidence of internal insulation failure constitutes a test failure.
Why the Neutral Terminal Lightning Impulse Test Is Important
The LIN test is important because it:
- validates reduced neutral insulation designs,
- prevents hidden dielectric weaknesses,
- reduces the risk of internal transformer failure,
- improves long-term reliability of high-voltage transformers.
In high-voltage networks, neutral insulation failure can be just as severe as line terminal failure. The LIN test ensures this critical insulation region is adequately verified.
Conclusion
The Neutral Terminal Lightning Impulse Test (LIN) is a conditional but essential dielectric test defined in IEC 60076-3. It is required when the neutral terminal may experience lightning-type overvoltages due to non-solid grounding or non-uniform insulation. By applying a standard lightning impulse directly to the neutral terminal, the LIN test verifies that reduced insulation remains safe under real operating conditions.
Correct application of the LIN test plays a key role in insulation coordination and long-term transformer reliability.