Fabric (woven – knitted – or nonwoven textile or cloth – etc.) – Coated or impregnated woven – knit – or nonwoven fabric which... – Coating or impregnation provides heat or fire protection
Reexamination Certificate
2001-07-02
2003-11-11
Dawson, Robert (Department: 1712)
Fabric (woven, knitted, or nonwoven textile or cloth, etc.)
Coated or impregnated woven, knit, or nonwoven fabric which...
Coating or impregnation provides heat or fire protection
C428S375000, C428S413000, C442S110000, C442S172000, C427S386000, C427S389900, C427S544000
Reexamination Certificate
active
06645886
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention is directed to a corona-shielding band for electrical machines, particularly high-voltage machines.
An electrical machine is essentially composed of a stator, which is constructed of what is referred to as the plate packet wherein insulated windings are inserted into prefabricated channels, as well as of a rotor that rotates in the stator. The sheet metal packet is composed of a specific plurality of individual sheets stacked on top of one another into which the channels are punched. The stator winding is inserted into these channels, and the winding is coated with a specific insulating system dependent on the demands. A typical insulating system for high-voltage machines comprises a principal insulation that is composed of mica bands that are wound around the conductor. In the channel region, the stator winding is provided with an outside corona protection (AGS), which has good electrical conductivities, in order to prevent partial discharges in this region. The outside corona protection is thereby conducted out beyond the plate packet, so that no discharges can occur given the slight spaces relative to the pressure plates and pressure fingers of the plate packet. The windings are also impregnated with an impregnation resin with an impregnation process (VPI process) which resin is then hardened.
The electrical conditions at the channel exit of high-voltage machines correspond to those of a sliding arrangement. One electrode, i.e. the outside corona protection grounded via the plate packet, ends shortly after the plate packet; and the other electrode, i.e. the conductor, in contrast, continues farther. This arrangement yields an increase of the field strength at the insulation surface at the end of the outside corona protection. The tangential field strength E
T
arising therefrom along the surface of the principal insulation leads to corona and sliding discharges when the dielectric strength of the air is exceeded, and these discharges potentially destroy the insulation. In practice, this is the case beginning with operating voltages >5 kV.
Without measures for field control, the potential of the individual surface elements of the principal insulation relative to the end of the outside corona protection is defined by the two capacitances C
1
(capacitance between the individual surface elements and the outside corona protection) and C
0
(capacitance of the insulation between the surface elements and the conductor) and the applied voltage. For the initial voltage U
a
at which the sliding discharges begin, the following derives:
U
a
≈
E
d
C
1
C
0
whereby E
d
is the dielectric field strength of the surrounding medium. With air as ambient medium, C
1
becomes small compared to C
0
and the majority part of the voltage lies between the outside corona protection and the surface elements of the insulation. Given high-voltage machines having an operating voltage >5 kV, a coat, which has a defined, low conductivity, is therefore applied following the outside corona protection, and the surface capacitance C
1
is shorted by this coat. The charging current for the capacitance C
0
then generates a voltage drop as a result whereof the adjacent potential is gradually dismantled. This coat on the principal insulation is referred to as final corona protection (EGS) or potential control.
To this end, the insulations are usually provided with an electrically conductive layer having an adapted, specific electrical impedance. In practice, this ensues with lacquers or insulating bands to which silicon carbide is added as a conductive filler. This filler exhibits semiconducting properties in the doped condition and a highly voltage-dependent conductivity, and this means that the surface resistance of the corona protection layer decreases with increasing field strength and vice versa. This leads to a high conductivity of the final corona protection at the transition to the outside corona protection and to a low conductivity of the final corona protection at the transition to the insulation surface. A steady dismantling of the field strength up to the insulation surface is thus achieved. In practice, corona protection layers have proven themselves whose surface resistance, which is measured according to DIN IEC 167, lies between 800 and 5000 M&OHgr; given 5 kV DC voltage.
Insulating bands, what are referred to as corona shielding bands, are usually employed for manufacturing the corona protection layer. These are composed of a carrier material composed of glass fabric or organic fabric material that is saturated with an epoxy resin (as a binding agent) that contains silicon carbide as an inorganic filler having an adaptive grain size and concentration. Silicon carbide (SiC) is employed as a conductive filler in corona shielding layers because, differing from metallic fillers, it allows the setting of the required, extremely low conductivities in the super-percolated range, and the conductivity therefore does not change significantly given processing-induced, slightly fluctuating filler concentrations. The silicon carbide is usually employed in a doped form in order to set the conductivity, particularly the voltage dependency of the conductivity, to the desired level. In practice, silicon carbide set p-conductively with aluminum has thereby proven itself. Commercially obtainable glycidylethers are employed as epoxy resins. The resins can contain an aminic hardening agent as well as a curing accelerator. Such corona shielding bands are known, for example, from the following publications: German Published Application 30 45 462, German Published Application 42 18 928 and U.S. Pat. No. 3,066,180.
The corona shielding band is wound overlapping around the outside corona protection in the region of the winding that closest to the iron core. Subsequently, the entire winding is then subjected to a VPI process (vacuum pressure impregnation) with an impregnation resin. This means that the corona shielding band that is employed must be compatible with this complex process. Thus, the band dare not contain any constituents that disturb the impregnation process or, respectively, give any such constituents off into the impregnation bath. Moreover, it must be uniformly integrated in the formed material arising after the curing so that partial discharges are avoided.
In technical employment, however, commercially obtainable corona shielding tapes which have silicon carbide as a conductive filler exhibit serious disadvantages. Thus, corona shielding tapes of the same type exhibit a great scatter in dielectric behavior (resistance level) which is dependent on the batch. This can probably be already found in the initial test of the conductivity of test members composed of a pure final corona protection; on the other hand, however, this can only be partly found at the completely insulated and VPI-impregnated windings as a result of corona discharges that occur. The technical manufacturing process of the silicon carbide is suspected as the cause. This is manufactured in a rotary tubular kiln in a reducing atmosphere from silicon carbide and carbon (Acheson process). Oxidic layers having different configurations form on the surface of the SiC particles that are formed, and these oxidic layers have a great influence on the conductivity. Dependent on the quality of the corona shielding, an unacceptable deterioration of the insulation due to increased dielectric. losses frequently results.
The conductive fillers such as lamp black, aluminum powder and silver powder that are usually technically utilized in plastics cannot be used in the present instance since they exhibit too high a specific conductivity.
SUMMARY OF THE INVENTION
An object of the invention is to make corona shielding bands available that can be manufactured in reproducible quality and that effect an optimally slight rise in the dielectric losses in the winding insulations of electrical machines.
This is inventively achieved by corona shielding bands that can be obtained in the following way.
Muhrer Volker
Rogler Wolfgang
Schaefer Klaus
Harness & Dickey & Pierce P.L.C.
Keehan Christopher M
Siemens Aktiengesellschaft
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