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Views: 0 Author: Site Editor Publish Time: 2026-10-22 Origin: Site
How can a transformer designed for an 11 kV supply have a 75 kV insulation rating?
The numbers describe different duties. One concerns normal operation. The other concerns withstand capability under a specified test.
Understanding transformer insulation level helps buyers read technical schedules without confusing operating voltage, impulse strength, and thermal class. This guide uses LI75 AC35 as a notation example. It is not a universal requirement for every transformer with the same voltage rating.
LI75 identifies a lightning-impulse withstand level of 75 kV peak.
AC35 identifies an AC power-frequency withstand level of 35 kV RMS under specified test conditions.
Neither figure authorizes continuous operation at that voltage.
Confirm the applicable winding, terminals, standard, and test requirements before approving the specification.
Transformer insulation must withstand more than the voltage present during normal service. Its electrical design also accounts for specified overvoltage stresses.
Insulation level expresses assigned dielectric withstand requirements. These requirements are associated with defined tests, not an unlimited promise of protection.
For an 11 kV cast-resin transformer, keep the rated winding voltage and dielectric withstand requirements as separate specification fields. A higher test number does not change the transformation ratio.
LI refers to lightning impulse. The impulse test uses a specified short-duration voltage waveform to assess the relevant insulation withstand capability.
AC refers here to power-frequency withstand. It assesses insulation under a defined alternating-voltage test, with its own duration and connection arrangement.
The two tests are not interchangeable. Passing one does not establish compliance with every requirement covered by the other.
The compact notation LI75AC35 combines two withstand values. For clarity, read each part with its unit and test basis.
Marking | Meaning in this example | What it does not mean |
|---|---|---|
LI75 | Lightning-impulse withstand level of 75 kV peak | A 75 kV continuous operating-voltage rating |
AC35 | Power-frequency AC withstand level of 35 kV RMS under specified conditions | Permission to run the transformer continuously at 35 kV |
Peak and RMS are different ways of expressing voltage. The impulse value describes a waveform’s peak. RMS is used for the AC withstand value in this example.
Do not compare 75 and 35 as though they measure the same duty. Their waveforms and test conditions differ.
Likewise, do not apply a simple conversion factor to turn an AC rating into an impulse rating. Both requirements must be established independently.
The notation alone does not describe every test detail. The approved standard and technical schedule must supply the relevant conditions.
The rated winding voltage defines an operating characteristic used in the transformer design. It belongs with the voltage ratio and other nameplate data.
Highest voltage for equipment, often written Um, is a separate reference used in insulation coordination. It should be identified explicitly rather than guessed from a withstand figure.
LI and AC values then identify particular dielectric withstand requirements. They do not replace either operating-voltage information or the assigned Um.
This distinction prevents a common purchasing error: choosing equipment from the largest kV number in a quotation.
If the operating voltage is wrong, an impressive impulse rating does not make the transformer suitable. If the insulation requirements are wrong, matching the operating voltage alone is not enough.
Class F and Class H concern thermal endurance. They do not specify lightning-impulse or AC withstand voltage.
Insulation resistance is also a different quantity, normally expressed in resistance units. A resistance result cannot be substituted for an LI or AC rating.
Keep these fields separate when preparing a technical comparison.
A transformer contains multiple insulation relationships. The requirements may differ between HV and LV windings and between line and neutral terminals.
A single unqualified “insulation level” entry can therefore leave important questions unanswered.
Ask which winding and terminals each value covers. Where several groups of ratings are separated by a slash, confirm their order and meaning. Do not assume every slash separates LI from AC.
For example, a technical schedule may group the HV requirements separately from the LV requirements. Combining them into one unexplained number loses useful information.
Do not automatically assign the line-terminal withstand value to every neutral terminal.
The applicable requirements depend on the winding insulation arrangement and system design. Request a clear terminal-specific schedule where relevant.
This is a specification question for the manufacturer and project engineer, not a reason to alter connections on site.
The voltage designation alone does not define the entire application.
The applicable standard and agreed specification matter. So do the system’s overvoltage exposure, grounding arrangement, and coordinated protection.
Installation conditions can also affect the assessment. Altitude, contamination, and external connection arrangements may require additional review.
Do not borrow a withstand combination from another project’s table simply because the nominal voltages match.
LI75 AC35 is the example discussed here. It should not be presented as the mandatory combination for every 11 kV transformer or every market.
Nor does specifying a higher impulse level remove the need for surge-protection coordination. Withstand capability and protective-device performance have different roles.
The project engineer should establish the required combination for the actual system.
A useful specification identifies the operating voltages, assigned highest voltage for equipment, and required withstand values separately.
State the relevant winding and terminals. Include the governing standard and edition, then identify any agreed project-specific requirements.
For broader custom duties, use a complete transformer specification alongside the insulation schedule. Capacity, frequency, impedance, cooling, and installation conditions still need their own definitions.
A withstand rating is not the same as a record showing how compliance was verified.
Confirm the required test classification, evidence, reporting, and any witnessing arrangements. Establish whether the evidence concerns the supplied unit or an applicable design test.
Do not infer that every unit receives every possible dielectric test merely because the rating appears in its specification.
Likewise, do not repeat high-voltage factory tests at the installation site without an approved procedure. The test scope and conditions require qualified technical review.
The goal is a clear requirement with matching evidence, not the longest list of tests.
Transformer insulation level describes specified dielectric withstand duties, not an alternative operating voltage. Read LI75 as a peak impulse value and AC35 as an RMS power-frequency withstand value, each under its defined conditions.
Then identify the winding, terminals, applicable standard, and required verification. SNTOOM can discuss cast-resin transformer configurations and project-specific insulation requirements before the technical schedule is finalized.
Need help clarifying a transformer’s insulation requirements?
Not exactly. BIL expresses impulse withstand capability. A complete insulation specification can also include power-frequency withstand and other applicable requirements. A BIL value alone does not describe every dielectric duty.
No. It identifies an impulse withstand level under specified conditions. Continuous operation must follow the transformer’s approved voltage ratings and other service requirements.
No. Dielectric withstand and load capacity are separate design characteristics. A higher withstand value does not authorize additional loading.
No. It does not authorize an installation team to apply that voltage. Any site test needs an approved procedure for the equipment, its condition, and the applicable requirements.
Not automatically. Select the combination required by the insulation-coordination study and purchase specification. Higher values should have a justified project purpose rather than replacing proper engineering review.
