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.
Further Information
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Insulation System
The 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
Corona discharge is an electrical phenomenon created by the ionisation 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
Tan 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.
Fit
When 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
Shape 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
Routine 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 UN) + 1000
- Tan δ / Tip Up
- Hi-pot (AC and DC flash) test (2 UN + 1000)1.2
- Lamination test
- Partial Discharge test
- Voltage Endurance test
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