In high-powered electrical generators, it is customary to produce slots in the surface of the stator core into which electrical coils are placed. Typically, two stator coils are located in each stator slot. The coils are energized during generator operation.
The stator core is generally maintained at ground potential whereas the coils or windings in the stator slots have a relatively high potential. To protect against arcing between the two coils located in a single stator slot and between the stator core and the coils, it is customary to cover each of the coils with a conducting surface, referred to as an outer electrode. The outer electrode prevents gap discharges between the top and bottom coils as well as between the coils and the stator core. The outer electrode protects the coils from physical abrasion which would otherwise result from arcing. FIG. 1 provides a perspective view of the top 102 and bottom 104 coils at the slot exit 106 of a stator core 100. An outer electrode 108 is applied to each coil 102, 104 for the full length of the slot portion 106 of the stator core 100 and terminates at a position on the coil 102, 104 outside the stator slot 106. The outer electrode 108 is applied over a layer of insulating material 131 that is typically applied to the coils 102, 104.
As shown in FIG. 1, it is common for either one or both of the stator coils 102, 104 to bend at some location after exiting the stator core 100. This creates the problem whereby a portion of one of the coils, either the top 102 or the bottom 104 depending upon which coil is bent, is itself not covered by an outer electrode 108 and in close proximity to a portion of the opposite coil which is covered by an outer electrode having a very high voltage. This same problem can occur when two stator coils after exiting from two separate stator slots, cross in close proximity to one another. For stator coils of very high voltage rating, for example in excess of 18 kVrms, the difference in voltage potential between the outer electrode 108 of one stator coil and the coil endings of the opposite stator coil may become so large that arcing between the two coils could occur.
These problem situations are illustrated in FIGS. 2A and 2B. FIG. 2A provides a top view of the top 102 and bottom 104 coils exiting from the same stator slot. As shown, the top 102 and bottom 104 coils bend in opposite directions near the end of the outer electrode 108. The portion of the bottom coil 104 which corresponds to the area shown in cross section 110 is closely situated to the outer electrode 108 of the top coil 102 but is not itself covered by an outer electrode. The same holds for the bottom of the top coil 102. As noted above, the relative close proximity of two points with relatively high potential differences presents the possibility of arcing. FIG. 2B provides a view of two coils 111 which after exiting from two different stator slots, cross in close proximity to each other. Similar to the situation described above, a portion 110 of each coil 111 is placed in close proximity to a portion of the other coil which is at a very high voltage. Generally, in both of the above described situations a tapered electrode is applied to the coils. A tapered electrode is applied to the areas shown in cross section 110 to shield the un-shielded portions of either coil that is closely situated to a shielded portion on the opposite coil.
FIG. 2C provides a perspective view of a bottom coil 104 that has been prepared with a prior art tapered electrode 112. Although the tapered electrode is shown and described with reference to the bottom coil 104, it should be noted that it is necessary to apply the tapered electrode to both coils that may cross in close proximity to one another. The tapered electrode 112 is wrapped around the coil 104 and overlaps the end of the conducting outer electrode 108 so as to maintain electrical conductivity between the two. The tapered electrode 112 has a shorter width at one corner 114 of the coil 104 than opposite corner 116 of the coil 104. The electrode 112 can be said to be "tapered" between the two corners portions 114, 116 of differing length. It should be noted that the shape of the tapered electrode 112 on the top surface of the coil generally matches the shape of the cross sectioned area 110 shown in FIG. 2A. Thus, the tapered electrode 112 is geometrically shaped so that portions of a top 102 and bottom 104 electrode which are in close proximity to each other are electrically shielded. The tapered design insures that no portion of any one stator coil that is covered by the potentially high voltage outer electrode 108 is in close proximity to a stator coil that is not also shielded.
FIG. 3 provides a detailed view of the geometry of a prior art tapered electrode 112. In order to provide a more useful perspective, the tapered electrode 112 is shown unraveled and flattened. The tapered electrode 112 has a base portion 118 with a tab portion 120 extending therefrom. The tab portion 120 terminates in a straight line 122 between two sharp points 128, 130. The tab portion 120 gradually decreases in width as the distance from the base portion 118 of the electrode increases. Two tapered sides 124,126 are formed between the base 118 of the electrode and the terminating straight line 122.
In FIG. 4A, the prior art tapered electrode 112 of FIG. 3 is shown applied to a stator coil 104. A tapered side 124 is shown running along the side of the coil 104 and terminating at a sharp point 128 at the intersection of the top and side surfaces. FIG. 4B provides a view of the bottom of the same coil 104 prepared with a tapered electrode 112. Similar to the prior figure, a tapered side 126 runs along the bottom of the coil 104 and terminates at a sharp point 130 at the intersection of two coil sides.
FIG. 5 depicts the current flow into the prior art tapered electrode 112. Again, the tapered electrode 112 is shown in an unraveled and flattened form for purposes of illustration. Arrows indicate current flow into the tapered electrode 112. As shown, the geometry of the tapered electrode 112 effects the flow of current into the electrode 112. In particular, the current flow concentrates at the two sharp points 128, 130 created at the intersection of the straight line 122 and the tapered sides 124, 126. The I.sup.2 R loss is exaggerated at these points 128, 130 due to the high concentration of current. Such points 128, 130 are commonly referred to as "hot spots."
As shown in FIG. 6, a voltage grading electrode 132 is applied to the coils 102, 104 over the outer electrode, tapered electrode (not shown) and insulation 131. The voltage grading electrode 132 consists of a nonlinear resistive coating of paint or tape which provides a voltage current characteristic that is non-linear and prevents an abrupt change in voltage at the slot exit region. The voltage grading electrode 132 thereby reduces the level of voltage stress on the coils at the slot exit.
In those areas of the grading electrode that envelop the hot spots of the tapered electrode, the grading electrode experiences very high temperatures. The temperatures can become so high that the grading electrode material may burn and deteriorate the voltage grading properties of the grading electrode. Thus, the concentration of flow to the sharp points of the tapered electrode detrimentally affects the structure and performance of the coils.
It is therefore desirable to provide a tapered electrode and method of grading high voltage stator coils that minimizes the concentration of current flowing to isolated points on the tapered electrode.