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Stator Insulating Materials: The Importance of Semi-Conductive Materials & Their Applications

A Review of Insulating Materials


Large rotating machines contain many different types of insulating materials, providing specific protection to maintain efficiency and prevent damage during normal operation, upset conditions, and idle states.

One type of specifically formulated insulation, semi-conductive insulation, serves a structural purpose, but most importantly, corona / partial discharge protection.

Stator showed with stator bars exiting the stator core. Lashings and spacers secure the bars to each other. A uniform coat of paint serves as a visual indicator of problems that may exist: Hot Spots, Partial Discharge Residue, and Contamination.

Insulation Degradation

The insulating components’ life is determined by the severity of thermal, mechanical, electrical, and environmental stresses that the insulation is exposed to over time.

The integrity of insulating components in large rotating machines can be altered by exposure to many factors that are not present in a perfect world:

  • High Temperatures
  • Vibration
  • Thermal Cycling
  • Electrical Stress
  • Mechanical Stress
  • Moisture
  • Chemical Exposure
  • Contamination

Each of these factors can lead to or increase the severity of corona / partial discharge. Partial discharge produces ozone, salts, and carbon deposits.

The damaging cycle of these chemicals contributes to insulation degradation, which increases partial discharge activity — further reducing the insulation components’ integrity, eventually leading to dielectric failure.

Voids and empty spaces simulate the function of a capacitor, building charge until partial discharge occurs.

Corona / Partial Discharge

Corona / partial discharge in rotating equipment is a condition created by a strong electric field ionizing gas molecules (e.g., Air). Partial discharge may occur anywhere a gas-filled void or gap is exposed to high electric fields.

Most stators rated at 4kV or higher require special materials to protect insulating components from the damaging effects of partial discharge.

Partial discharge can degrade insulating components over time but usually does not cause immediate failure. Damage caused by partial discharge is permanent. Identifying where partial discharge is occurring is critical, such that repairs can be made before extensive damage is done.

Corona / partial discharge degradation can be found in areas:

  • End Windings
  • Internal Ground-Wall Insulation
  • Slot Discharge
  • Surface Discharge
Contamination of the stator end windings
Insulating components: Ground wall insulation, Top Filler, Top Ripple Spring, Stator Wedge/Slider Semi-Conductive Components: Bottom Filler*, Middle Filler*, Side Filler * Not always semi-conductive

End Windings:

Partial discharge can occur in the end windings of the stator bars. Moisture/humidity, contamination, and insufficient spacing between stator bars are examples of contributing factors leading to partial discharge in the end windings.


Internal To Ground-Wall Insulation:

Internal partial discharge is due to voids between stator bar conductors and the ground-wall insulation and voids in the ground-wall insulation. These conditions are created during the manufacture of the stator bars. Over time, internal partial discharge increases the size of voids and will eventually lead to dielectric failure.


Slot Discharge:

Slot discharge occurs in the space between the stator bar and the stator core laminations. This type of discharge can be caused by loose stator bars, insufficient semi-conductive coating on the slot portion of the stator bar, and insufficient discharge path from the stator bar semi-conductive coating to the stator core laminations.

Semi-conductive bottom, middle, and side fillers are used to fill this space and tightly secure the slot’s stator bar, reducing the possibility of vibration erosion of the groundwall insulation. Semi-conductive side ripple springs are also used to secure the stator bar tightly against the slot laminations. If the slot contents shrink over time, a ripple spring will compensate, keeping the bar tight in the slot side to side. Ripple spring will also compensate for uneven space between the stator bar and stator core laminations.

The conductivity of these materials is important such that charge is dissipated efficiently, but conduction between laminations is not created. When installed properly, these semi-conductive materials provide a path to ground (stator core) and dissipate excessive charge buildup without damaging the ground-wall insulation and stator core.


Surface Discharge:

Surface discharge can occur both in the stator slot and where the stator bar exits the stator core. Factors contributing to surface discharge include Moisture/humidity, dirt/contamination, and high electric field strength fluctuations at the end of the stator slots. Hydrogen-cooled generators running with reduced hydrogen pressure may experience an increase in partial discharge throughout the machine.

Semi-conductive side ripple spring installed on the right side of the stator bars.

A semi-conductive coating on stator bars (on the slot portion) and a stress grading layer (silicon carbide), after the stator bar exits the stator slot, can prevent surface discharge in these areas. The resistivity of the stress grading layer decreases as the electric field increases. This relationship is non-linear.


Semi-Conductive Material Properties:

Whether using a coating, a flexible, or a rigid semi-conductive material, each plays a key role in preventing damage to stator bar ground-wall insulation and core iron due to corona/partial discharge in large rotating machines. Key properties critical to rigid semi-conductive materials protecting over time are:

  • Surface Resistivity
  • Thermal Class
  • Compressive Strength
  • Impact Resistance

Semi-conductive materials are used to prevent slot and surface partial discharge. The locations of these components are shown in the figure to the right.

Surface Resistivity:

It is most important that these semi-conductive materials have consistent and uniform surface resistivity between 500 and 50,000 Ohm/sq. (ideally 1,000 to 10,000 Ohm/sq.)

Surface resistivity lot-to-lot consistency can be verified through QA testing. It is equally critical that a semiconductive material can exhibit consistent surface resistivity performance across multiple lots to ensure the reliability of the partial discharge protection each time the material is used in the rotating machine.


Resistivity using ASTM D 257 pressure electrode method
 TargetC109 QA Data
# of Tested Lots56
Average, Ohms/Sq1,000 – 10,0004,128


Thermal Rating:

Thermal rating is most important for maintaining mechanical properties and surface resistivity long-term through thermal cycling and upset conditions.

Thermal endurance test data comparing different materials is shown below. A product that maintains stable surface resistivity at elevated temperatures will ensure optimal partial discharge protection, even in demanding temperature conditions.


Target Resistivity using ASTM D 257 pressure electrode method*
Std, Dev.4122,481Std, Dev.9,671
Std, Dev.4122,481Std, Dev.9,671

*Units are in Ohms/square

Compressive Strength


These rigid semi-conductive insulating components are exposed to compressive forces during the installation of the stator bars, during the wedging process, and throughout the insulation system’s life. Semi-conductive materials with higher compressive strength provide dimensional stability over time, ensuring the bars remain tight in the slot and that the gaps they fill, to prevent partial discharge, remain the same as when initially installed.


Impact Resistance

Bottom, middle, and side fillers are subject to impact forces during installation. Impact strength is critical to prevent/reduce damage during installation and repairs. Flat side fillers are exposed to edge driving forces during installation until the bar is tight in the slot. The chart below shows the relative edgewise impact force of various materials.

We fabricate custom installation tooling to compress and drive the ripple spring between the stator and stator core iron.

The Gund Company stocks several options of semi-conductive materials. The table below summarizes available materials, properties, and application location(s).


Product GradeC109C209GPSCGGBE
Standard thicknesses, in (mm).020″ – .375″ (.5 – 9.5).008″ – .375″ (.2 – 9.5).015″ – .250″ (.2 – 1.0).008″ – .039″ (.2 – 1.0)
ConstructionSemi-Conductive Glass PolyesterSemi-Conductive Glass EpoxySemi-Conductive Glass PhenolicSemi-Conductive Glass Epoxy
Application LocationBottom, Middle FillerSide, Bottom, Middle FillerSide, Bottom, Middle FillerSide, Bottom, Middle Filler
Thermal ClassClass H – 180 °CClass H – 180 °CClass F – 155 °CClass F – 155 °C
Surface Resistivity, Ohm/Sq500-50,000500-50,000200-20,000500-30,000
Compressive Strength, psi (MPa)49,000 (338)58,000 (400)48,000 (331)43,500 (300)
Izod Impact Strength, LW, ft-lbs/in121311.89.2
Flexural Strength psi (MPa)
Lengthwise30,000 (207)57,000 (393)34,000 (234)43,500 (300)
Crosswise30,000 (228)80,000 (552)25,000 (172)43,500 (300)
Material properties are important. Several options are available that allow for cost reduction in semi-conductive materials.
When important properties such as surface resistivity and compressive strength are similar, deciding factors can be thermal class, cost, edgewise impact, and availability.


Stator insulation systems for rotating machines consist of a mixture of different materials, each performing a different function. These materials play a key role in preventing damage to stator bar ground-wall insulation during normal operation, upset conditions, and idle states. It is important to select stator insulating and semi-conductive components carefully. Dimensional stability and surface resistivity over time is critical and can determine the life of the insulating components. Proper installation of these components is equally important to ensure each component’s function is effective in its role.


  • Handbook of Large Turbo-Generator Operation And Maintenance, Second Edition, Klempner-Kerzenbaum
  • Rotating Electrical Machines & Power Systems, Second Edition, Patric-Fardo
  • Electrical Insulation For Rotating Machines, Stone-Boulter-Culbert-Dhirani
  • High Voltage Insulation Failure Mechanisms, IEEE International Symposium on Electrical Insulation, Vancouver, BC, 2008
  • Corona Protection in Rotating High Voltage Machines, INDUCTICA 2006 Conference, Berlin

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The Gund Company is a vertically integrated manufacturer and fabricator of engineered material solutions. Since 1951, we have listened to our customers and learned about their industries’ demanding operating environments. We are AS9100D Certified and ITAR Compliant. Our custom-fabricated parts are manufactured according to ISO 9001:2015 certified quality systems.

We understand the challenges of material selection and the demanding operating environment of your application. Our Application Engineering Team takes a consultative approach to understand your requirements. By relying on our material specialists, our customers gain valuable insight into improving component designs to increase efficiency and functionality while reducing cost. In addition to helping with material selection, we challenge ourselves to optimize production for material yield or fabrication efficiency. As a lean enterprise, we focus on continuous improvement and finding the most cost-effective, efficient solutions for our customers.

Please contact us today if we can help answer material property questions or provide an application-specific quote. Thank you for the opportunity to earn your business.

Stator showed with stator bars exiting the stator core. Lashings and spacers secure the bars to each other. A uniform coat of paint serves as a visual indicator of problems that may exist: Hot Spots, Partial Discharge Residue, and Contamination.