Design, Evaluation, Aging, Testing, and Repair

Wednesday, February 8, 2012

|1.3.3|Form-Wound Stators—Roebel Bar Type, |1.4|STATOR WINDING INSULATION SYSTEM FEATURES, |1.4.1|Strand Insulation

1.3.3 Form-Wound Stators—Roebel Bar Type

In large generators, the more the power output, the larger and mechanically stiffer each coil usually is. In stators larger than about 50 MW, the form-wound coil is large enough that there are difficulties in inserting both legs of the coil in the narrow slots in the stator core without risking mechanical damage to the coil during the insertion process. Thus, most large generators today are not made from multiturn coils, but rather from “half-turn” coils, often referred to as Roebel bars. With a Roebel bar construction, only one half of a “coil” is inserted into
the slot at a time, which is considerably easier than inserting two sides of a coil in two slots simultaneously. With the Roebel bar approach, electrical connections to make the “coils” are needed at both ends of the bar (Figure 1.7).


The stator winding insulation system contains several different components and features, which together ensure that electrical shorts do not occur, that the heat from the conductor I2R losses are transmitted to a heat sink, and that the conductors do not vibrate in spite of the magnetic forces. The basic stator insulation system components are the:

  • Strand (or subconductor) insulation
  •  Turn insulation 
  • Groundwall (or ground or earth) insulation

Figures 1.8 and 1.9 show cross sections of random-wound and form-wound coils in a stator slot, and identify the above components. Note that the form-wound stator has two coils per slot; this is typical. Figure 1.10 is a photograph of the cross section of a multiturn coil. In addition to the main insulation components, the insulation system sometimes has high-voltage stress-relief coatings and end-winding support components.
       The following sections describe the purpose of each of these components. The mechanical, thermal, electrical, and environmental stresses that the components are subjected to are also described.

1.4.1 Strand Insulation
In random-wound stators, the strand insulation can function as the turn insulation, although extra sleeving is sometimes applied to boost the turn insulation strength in key areas. Many form-wound machines employ separate strand and turn insulation.

Figure 1.7. Photo of a turbogenerator stator winding using Roebel bars.
The following mainly addresses the strand insulation in form-wound coils and bars. Strand insulation in randomwound machines will be discussed as turn insulation. Section 1.4.8 discusses strand insulation in its role as transposition insulation.
       There are both electrical and mechanical reasons for stranding a conductor in a formwound coil or bar. From a mechanical point of view, a conductor that is big enough to carry

Figure 1.8. Cross section of a random stator winding slot.
the current needed in the coil or bar for a large machine will have a relatively large cross-sectional area. That is, a large conductor cross section is needed to achieve the desired ampacity. Such a large conductor is difficult to bend and form into the required coil/bar shape. A conductor formed from smaller strands (also called subconductors) is easier to bend into the required shape than one large conductor.
       From an electrical point of view, there are reasons to make strands and insulate them from one another. It is well known from electromagnetic theory that if a copper conductor has a large enough cross-sectional area, the current will tend to flow on the periphery of the conductor. This is known as the skin effect. The skin effect gives rise to a skin depth through which most of the current flows. The skin depth of copper is 8.5 mm at 60 Hz. If the conductor has a cross section such that the thickness is greater than 8.5 mm, there is a tendency for the current not to flow through the center of the conductor, which implies that the current is not making use of all the available crossection. This is reflected as an effective AC resistance that is higher than the DC resistance. The higher AC resistance gives rise to a larger I2R loss than if the same cross section had been made from strands that are insulated from one another to prevent the skin effect from occurring. That is, by making the required cross section from strands that are insulated from one another, all the copper cross section is used for current flow, the skin effect is negated, and the losses are reduced.

       In addition, Eddy current losses occur in solid conductors of too large a cross section. In the slots, the main magnetic field is primarily radial, that is, perpendicular to the axial direction. There is also a small circumferential (slot leakage) flux that can induce eddy currents to flow. In the end-winding, an axial magnetic field is caused by the abrupt end of the rotor and stator core. This axial magnetic field can be substantial in synchronous machines that are under- excited. By Ampere’s Law, or the ‘right hand rule’, this axial magnetic field will tend to cause a current to circulate within the cross section of the conductor (Figure 1.11). The larger
the cross sectional area, the greater the magnetic flux that can be encircled by a path on the periphery of the conductor, and the larger the induced current. The result is a greater I2R loss from this circulating current. By reducing the size of the conductors, there is a reduction in stray magnetic field losses, improving efficiency.
       The electrical reasons for stranding require the strands to be insulated from one another. The voltage across the strands is less than a few tens of volts; therefore, the strand insulation can be very thin. The strand insulation is subject to damage during the coil manufacturing process, so it must have good mechanical properties. Since the strand insulation is immediately adjacent to the copper conductors that are carrying the main stator current, which produces the I2R loss, the strand insulation is exposed to the highest temperatures in the stator.


Figure 1.9. Cross sections of slots containing (a) form-wound multiturn coils; (b) directly cooled
Roebel bars.

Figure 1.10. Cross-section of a multiturn coil, with three turns and three strands per turn.

Therefore, the strand insulation must have good thermal properties. Section 3.8 describes in detail the strand insulation materials that are in use. Although manufacturers ensure that strand shorts are not present in a new coil, they may occur during service due to thermal or mechanical aging (see Chapter 8). A few strand shorts in form-wound coils/bars will not cause winding failure, but will increase the stator winding losses and cause local temperature increases due to circulating currents

Figure 1.11. Side view of a generator showing the radial magnetic flux in the air gap and the bulging
flux at the core end, which results in an axial flux.

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