GENERATORS: Understanding insulation basics helps promote long service life

By Dr Nancy Frost, Gerome Technologies

Various insulation materials are used in rotating machines, each with specific performance requirements. Understanding how each type of insulation works is critical for the proper construction, operation, and maintenance of generators (and motors). The trend in rotating-machine design and manufacture is to continually require more from insulation because customers want smaller generators with higher outputs. This means insulation systems experience higher electrical stresses and must have a higher thermal conductivity.

Insulation

There are three critical aspects to electrical insulation: design, material, and processing. The material must be suitable for the application, conducive to use in a manufacturing environment, and satisfy the machine’s performance specifications. Many times the requirements for these three areas are at odds with one another, so trade-offs often are made in the selection of insulation materials.

Mica is one of the most important insulating materials. This naturally occurring mineral has a platelet structure that gives it a high dielectric strength, excellent long-term resistance to electrical stress, and a resistance to partial discharge (PD, Fig 1).

For modern insulation systems, mica is processed into small pieces and made into mica paper to facilitate its use in a manufacturing setting. In this form, mica has little mechanical strength; therefore, it is applied to a carrier—such as glass cloth, Dacron®, or film. A binder holds the mica and carrier together.

The binder—polyester, epoxy, or silicone—can be fully cured or b-staged. The latter means the resin, or glue, is only partially cured; it is in a tacky state that is not gooey or flowing at room temperature. Depending on the type of resin, addition of a catalyst also may be necessary.

The small mica flakes form overlapping layers in the insulation, making it harder for a critical electrical failure path to form through the insulation, because it has to weave back and forth between flakes.

Excerpted from “A Brief History of Turbine-Driven Generators” by Clyde V Maughan. Paper is available in the online Resource Center managed by the International Generator Technical Community at http://www.generatortechnicalforum.org.

History lesson on generator insulation and associated failures

About the turn of the 20th century, mica flake was discovered to have remarkable electrical and thermal properties. Insulation systems rely on mica because of its resistance to partial discharge (PD); no manmade product can compare in this regard.

But with shellac, cotton, and other relatively primitive materials incorporated in early generator insulation systems, troubles persisted. By the mid-teen years of the last century, it was discovered that by using a vacuum-pressure cycle, mica/cotton tapes could be impregnated with a hot asphalt compound to obtain a major improvement in electrical duty. But in the late 1920s a tape-migration phenomenon had surfaced (Fig A, circle) and, by the mid-1940s, most stator windings on large Westinghouse generators were experiencing fatal migration.

Thermalastic™, said to be the first modern synthetic groundwall insulation system (1950) was developed by Westinghouse in response to the failures experienced. The epoxy-like polyester material provided the incentive for development of improved groundwall systems by all OEMs—such as GE’s Micapal I™, Micapal II™, and others; Siemens’ Micalastic™; ABB’s Micadur™, etc.

Stress on the groundwall has a dramatically important impact on electrically related problems associated with stator windings. The predominant issue is the phenomenon referred to as partial discharge (PD), sometimes incorrectly called “corona.” The stress gradient within the groundwall results in electrical breakdown in the inevitable, tiny voids in the groundwall. These mini-arcs tend to eat through any insulation system that does not have the PD resistance of mica.

If the outside surface of the groundwall is not adequately grounded, there will be discharges of much more energy. This, in turn, can result in surface PD (Fig B).

In the 1950s, the stress level on the asphalt insulation systems was about 45 volts/mil (vpm). With the advent of hard (polyester-type) systems, the stress level increased to around 54 vpm. By the mid-1960s, improved (epoxy) systems were being used, and the stress levels increased to vpms in the low 60s, creating problems for designers under pressure to produce larger indirectly cooled generators.

In these machines, where the copper thermal losses had to transmit through the groundwall, thinner groundwall insulation became highly valuable. From this pressure, and in recognition that root-cause electrical failures of the groundwall insulation were rare, evolution toward much higher stress levels occurred—today exceeding 90 vpm.

Problems associated with bar windings. Because eddy-current losses would immediately melt a stator bar of sold copper, a stranded design always has been required. Very early (in 1915) a Swiss engineer, Louis Roebel, invented an elegantly simple way to address the problem by transposing the bars.

The standard Roebel transposition effectively compensates for the radial flux density gradient in the slot portion of the winding. However, on large generators of non-coil design, the radial flux gradient in the endwindings becomes sufficiently large to cause problems.

Another approach for endwinding radial flux compensation has been to sub-group the bar strands into bundles of strands; bundles may range from as few as one strand to as many as 14 or more. These “bundles” are maintained throughout the entire phase belt. The bundle design greatly complicates the winding manufacturing process and has been accompanied by numerous service problems (Fig C).

The evolution of large-generator winding insulation and support systems has presented many difficult challenges to generator design engineers and to manufacturing personnel who make these machines. Progress will continue, but with continuing increases in machine ratings and continuing cost pressures, old problems will recur, present problems will continue, and new problems inevitably will develop.

Stator

Mica tape is one of the main components in the stator insulation system, which isolates the current-carrying copper from the magnetic steel of the stator core (Fig 2). The design can be simple or complex depending on the physical size and output of the machine.

Recall that stator bars have three main components: copper, mica, and resin. The copper carries the current, the mica provides primary electrical stress protection, the resin fills voids and serves as a glue to hold everything together.

Challenges: Keeping the stator-bar insulation intact during installation into the stator slot is critical to success. The groundwall insulation must be sufficiently robust to withstand the mechanical stresses during installation and remain electrically sound for a long service life. Other components in the stator groundwall insulation system include end-basket support materials—such as felts and tie cords to hold the stator windings in place.

Magnet wire. The current-carrying component of the stator, the magnet wire, must be electrically insulated from the adjoining turns and from the stator core itself.

Each coil design has a unique electrical stress on the groundwall and magnet wire components within the coil. Example: A machine can have a two-turn coil with small copper cross-section, which facilitates manufacturing of the insulation system; or it can have a higher-aspect-ratio magnet wire in a four-turn coil, which leads to higher electrical stress on the insulation.

Magnet wire material can be enameled or covered (also called “served”) copper, or both. Enameled magnet wire may be single, heavy, or quad build, which refers to the thickness of the enamel applied to the copper. Materials for served magnet wire include Daglas®, mica, and Kapton® (polyimide); all can be put over bare or enameled copper. Served wire typically is used in high stress systems requiring extra protection.

Challenges: Magnet wire insulation must withstand the flexing and manipulation required to insert the stator bar into the slot. This often can be minimized by using stack consolidation to create a coil or bar that will hold its shape during the winding operation.

The consolidation material can consist of a separate tape on the outside of the wire stack in a barber-pole layup or an adhesive layer on the outside of individual wire strands. Another method uses a rigid b-staged material in the center of the stack. The magnet wire coil stack then is formed and squeezed in a heated press to cure the consolidation tape and hold all the copper strands in the desired coil structure.

Groundwall insulation. The design choices for stator construction must be evaluated for the electrical stress each puts on the groundwall insulation. Engineers can vary the number of coil slots, the number of turns in a coil, and the number of conductors per turn to achieve the desired outcome.

Mica and resin are the components of groundwall insulation, which covers the copper magnet wire. Discrete layers of mica tape can be applied to build up sufficient electrical insulation for the groundwall package. Resin holds the layers together and fills any voids. Mica works well in this application because it resists PD, one of the known failure modes of groundwall insulation in high-voltage (HV) applications.

Challenges: The groundwall insulation must be flexible and strong enough to hold together during manufacturing and while the machine is in service. If the mica tape is not applied properly it is easier for a conductive path to form, leading to PD. Proper application begins with consistent lapping and indexing—terms describing the overlapping of the mica tape on itself over the length of the coil—to avoid the formation of voids and possible ground paths between the copper and steel (Fig 3).

Also, it is critical to get the right amount of resin saturation in the tape to prevent faults and premature aging. In general, there are two ways to do this: Push in the resin (VPI process), or squeeze out extra resin after taping (RR process).

Stator slot. As the rotor spins, the stator coils experience large electromagnetic forces that make them want to fly apart. The stator winding kit provides components that hold everything in place when properly installed. Slot insulation materials typically are composites—including a slot liner, bottom filler, mid stick, top packing, and wedging system. The slot liner often is a laminate of Nomex® and films, while the rest of the system usually is glass laminate or composites.

Challenges: Slot insulation materials keep the stator coils in place during operation, and their long-term mechanical strength, plus their electrical performance, are critical. There are established methods for the installation of side and top packing, as well as ripple springs. Where wedges are used to hold the coils in place, they must be installed carefully to avoid damaging the groundwall insulation.

Side and top packing, wedges, and top ripple springs (if used) must fill the slot area without over-stressing the material or damaging the stator ground-wall. Ripple springs, both side and top, are designed for installation to a proscribed compression level—not completely flat.

Rotor

The rotor body typically is machined from a single forging, with slots for the rotor copper cut into the steel. The key concerns for the insulation materials in the rotor are mechanical, because of the high radial centrifugal forces experienced when the rotor spins.

In addition, thermal damage/degradation of the insulation system is a concern because the currents in the copper can lead to high heating losses (I2R losses). Since the coefficients of thermal expansion of the copper and the insulation are not the same, movement during heating/cooling can occur and abrade the insulation.

The design of a rotor must balance the need for cooling with that of sufficient copper cross section to carry the current. More copper carries a higher current, which means more power (energy transfer). But it also generates more heat and, therefore, needs more cooling. Venting provides cooling, but it reduces the amount of copper, so there is less power output. This is the push/pull the design engineer must accommodate.

Insulation. There are a variety of materials and designs for rotor insulation that have performed successfully in the field. These are comprised of composite materials in a special layered construction. This includes primary insulation to electrically isolate the copper from the steel (slot liners). Insulation between individual copper turns can be vented and can be comprised of a variety of materials—such as glass-based composites (with or without film layers)—to improve the dielectric breakdown strength. Fillers also are used in rotors, as in stators, to completely fill the slot—such as top and bottom fillers, as well as creepage blocks and sub-slot covers.

Challenges: Expertise in the fabrication of rotor components is critical because these materials experience high mechanical forces and strong thermal cycling throughout their service lifetimes. CCJ

Dr Nancy Frost is a dielectrics engineer working as business development manager for Gerome Technologies, an insulation fabricator. She has more than two decades of experience as a materials supplier, academic, HV laboratory manager, and developmental engineer for GE and Kodak. Frost has given over 50 presentations and short courses on insulation materials; she is active in IEEE and other professional organizations.