What makes a Premium High Voltage (HV) Coil?

What makes a premium high voltage coil insulation system - Houghton International

There are two fundamentally different insulation systems ranging across almost all types of high voltage (HV) rotating electrical machines.

  • Vacuum Pressure Impregnation (VPI) which requires the main insulation of epoxy, polyester or silicone resin to be introduced to a winding after coil insertion inside a pressure chamber. The impregnated stator is then baked in an oven to cure the resin, thus completing the insulation system. The machine is then fully tested at high voltage to ensure dielectric integrity and compliance to international repair standards, i.e. BS EN ISO 60034.
  • Resin rich insulation technology, for which a high quality epoxy resin insulation is applied and consolidated during the coil manufacturing process. This means the individual coils and the connected winding are fully tested thus guaranteeing the individual dielectric integrity of each HV coil.

There are many differing schools of thought on which system is the most appropriate.

This view depends on experience, personal preference, manufacturing cost, verified technical data, whether the machine is being built new or is undergoing repair and the application, such as pump or alternator.

VPI systems can offer an improved degree of sealant against moisture ingress and, arguably, heat dissipation compared to resin rich coils. However, there are questions against consistency of impregnation throughout the slot portion of a VPI coil giving rise to concerns about Corona Discharge undermining the insulations system’s integrity.

Select a category for more technical details:

Insulation System

High Voltage Coil Insulation System - Houghton InternationalThe insulation system of a HV motor or alternator is critical in terms of its performance, longevity and value as an investment to the asset owner. Selecting the appropriate insulation requires consideration of application, circumstance, atmospheric condition and geographic location.

Houghton International have developed a range of systems that cover the broad scope of requirements faced by our expanding customer base.

Corona Discharge Protection

High Voltage Coil Corona Discharge ProtectionCorona Discharge is an electrical phenomenon created by the ionization of air around a conductor as a side effect of an electric field with a high potential gradient or strength.

In high voltage rotating machines, Corona activity is present at 6600V and above, increasing in intensity and severity as the machine’s potential gradient rises. The protection against Corona Discharge and its adverse debilitating effects on a winding’s insulation is what governs the design concept and features of a premium HV coil.

Every HV coil should posses a conductive outer layer which grounds the coil in the slot. It should also feature a stress grading system which protects the coil as it leaves the slot, where the potential gradient is at its greatest and Corona activity is therefore most intense.

Tan δ / Tip Up

High Voltage Coil Tan Delta Tip Up In-house Testing Facility - Houghton InternationalTan δ and Tip Up is the accepted measure of a premium HV coil. What is measured is the AC power factor involved in the passage of AC leakage current through the insulation wall.

This measures the air gaps or void content in a cured HV coil. This measure is taken using a Schering bridge rectifier, which quantifies the capacitive content of insulation through the slot portion of the coil.

Most standards require only a sample percentage of coils out of an entire coil set to be tested. However, we advise testing the entire coil set in order to ensure dielectric integrity and machine longevity.


High Voltage Coil Fit - Houghton InternationalWhen describing fit we refer to how closely together the coil fits into the stator slot. Any area of the slot section that is not ground leaves an air gap, meaning Corona Discharge activity will occur above 6600V.

Variances in fit promote Corona activity, hence coils should be manufactured to at least 0.25mm across the length of the coil and similarly 0.5mm for skew slot coils from any given slot width.

Hot pressed resin rich slot sections guarantee such a fit, where VPI systems rely on sufficient resin penetration to ensure the main wall is adequately ground.

Shape Consistency

High Voltage Coil Shape Consistency - Houghton InternationalShape consistency is a critical feature because the more fitting the shape of the coil, the less mechanical stresses are placed on it during the insertion process.

Shape consistency is critical because across a significant number of coils, if the shape is out, the engineer can lose the space available, which creates many problems, not least placing further adverse stresses that can increase the likelihood of failure.

Houghton International’s tried and tested methods of controlling shape consistency have underlined our value to the people who work with our products.

Dielectric Integrity

High Voltage Coil Dielectric Integrity - Houghton InternationalRoutine electrical tests are carried out according to BS EN 60034, BS EN 50209 and IEEE 286.

Typical routine testing regimes:

  • Surge comparison or turn to turn test: 2(U+ 1000)
  • Tan δ / Tip Up (minimum 10% of straight coil sides)
    Measured at intervals of 0.2 UN
    Loss tangent maximum increment = 5 x 10-3
  • Hi-pot (AC flash) test 2(UN + 1000)1.2
  • Lamination test
    (when there are multiple conductors /turn) @ 240vAC

Independent Testing Criteria

HiFLEX™ coils were subjected to the following tests by a leading independent Canadian testing laboratory, with satisfactory results:

  • Visual inspection and tap tests on the coils before and after VE test
  • Partial discharge (PD) analysis at 7.3kV before the VE tests, as per IEEE Standard 1434-2000
  • Dissipation factor (DF) measurements as per IEEE Standard 286-2000 from 2kV to 16kV in 2kV steps before the VE test
  • Turn to turn insulation test before VE test as per IEEE Standard 522-2004
  • Voltage endurance test as per IEEE Standard 1043-1996 at 31.7kV, 90°C for 500 hours
  • Dissection and microscopic examination of coil after VE test
  • Insulation thickness measurements of one coil after VE test

Independent Testing Results

Turn Insulation Test complied to:

  • As per IEEE Standard 522-2004
  • 5 successive voltages were applied to the coils with both polarities before the VE test

Voltage Endurance Test

  • Coils were satisfactorily subjected to an accelerated insulation-ageing program
  • As per IEEE Standard 1043-1996 and IEEE Standard 1553-2002
  • Test parameters were 31.7kV at 90°C for 500 hours
  • Both coils passed the 250 hours required by IEEE 1553.
    Coil 1 passed by 161% and coil 2 passed by 182%