GB1600239A - Winding carrying laminated cores in electromagnetic machines - Google Patents

Description

(54) IMPROVEMENTS IN AND RELATING TO WINDING CARRYING LAMINATED CORES IN ELECTROMAGNETIC MACHINES (71) We, NORTHERN ENGINEERING IN DUSTRIES LIMITED, of NEI House, Regent Centre, Newcastle-upon-Tyne, NE3 3SB, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following state menu: This invention relates to the laminated stator cores and windings of alternating current dynamo-electric machines.

It is an essential part of the known technology of electrical machines to construct the magnetic core carrying the windings from thin steel laminations insulated from one another to prevent energy loss and heating due to electro-magnetically induced eddy currents. Failure of the interlaminar insulation can, in certain circumstances, give rise to circulating current paths within such a core, which, by linking a sufficient amount of alternating magnetic flux may result in high induced eddy currents flowing therein, and so cause over-heating which in extreme cases may result in catastrophic damage to the machine.

The magnetic cores of large electrical machines are generally connected to earth, such earthing usually being achieved deliberately or fortuitously by mechanical contact between the laminations and the supporting framework. Provided such earthing connections are limited to the outer perimeter only of the core laminations, no short-circuiting current loops are thereby formed embracing significant amounts of magnetic flux as just described.

It is also part of the known art in the design of such machines, however, for the purpose of minimising the risk of forming short circuit current loops within the core in the event of a group of adjacent laminations accidentally becoming inter-connected through failure of the interlaminar insulation at a position nearer to the axis of the machine, for all or nearly all the laminations to be individually insulated from one another and from earth. Such a form of construction has, for example, previously been described in British Patent Application No. 14663/74 (1508793). With any such arrangement, means must nevertheless be provided for maintaining the electrical potential of the core reasonably close to earth potential, and one means of achieving this, as described in the above-mentioned patent, is by the provision of a number of electrodes capacitively coupled to the core laminations, such electrodes being earthed preferably via current limiting resistors. Such resistors serve the important function of limiting the magnitude of any circulating currents which might otherwise flow in the aforementioned circumstances thereby causing severe heating within the core and consequent aggravation of the initial fault.

Such earthing means, in a machine having insulated windings of conventional design, at comparatively high voltages to earth, must nevertheless also be capable of passing to earth any currents flowing into the core, and in particular capacitive currents flowing through the winding insulation, especially for example during a dielectric test at a high alternating voltage, or such currents due to any surge voltage or (in a multi-phase machine) to unbalanced power frequency voltage.

If such capacitive earthing electrodes exist only at discrete intervals along the axial length of the core, current flow through the interlaminar insulation is relied upon in order to transfer such capacitive current from the winding at intermediate axial positions to the nearest earthed electrodes. This current flow must not impair the interlaminar insulation of the core. It must also be possible for such capacitive currents to flow between phases, either via the earthing connections or directly via the interlaminar capacitance of the laminated core itself, in the case of a machine having each layer of laminations sub-divided into a number of separately insulated sectors.

It is a further part of the known technology of design of alternating current machines for high operating voltages, to provide a conductive layer or coating generally of relatively high resistivity at least over the core-slot length of the conductor-bars for the purpose of eliminating discharges in the inevitable small gas spaces between the high voltage conductor insulation and the core, which discharges may otherwise cause gradual erosion of the conductor-bar insulation. Furthermore in conventional design practice a conformable packing material having slightly conductive properties may be used to fill the inter-space and to make direct electrical contact with the core laminations, thereby connecting the conducting layer to earth. However, such material in contact with the core may itself be prejudicial to the integrity of the inter-laminar insulation and may contribute, for example under partial core-fault or other abnormal conditions which give rise to axial voltage gradients in the core, to the development of fault current paths within the core as already described.

It is still another part of the known art in constructing high voltage electrical machines to provide resistive material, sometimes for example in the form of solid blocks of resin impregnated asbestos cloth laminate as described in British Patent Specification No.

1,129,887, or more usually in the form of a resistive tape wrapping or of a painted-on coating of resistive material on or adjacent to the main insulation of a portion of the end winding section of each of the winding conductors, nearest to the core, for the purpose of suppressing surface discharges in the axial direction due to the local high electrical surface stress. Resistive materials so used, hereinafter referred to as "end winding stress control layers", are commonly earthed by arranging that they are in direct electrical contact with the previously mentioned earthed external conductive outer layer applied to the slot portion of the bar.

It is the object of the invention to provide means whereby the capacitive current through the insulation of the winding conductors or the greater part of such current, which normally flows to earth via the magnetic core, is instead diverted to earth directly or indirectly without passing through the core itself.

According to the invention, a stator core of an alternating current dynamo-electric machine comprises laminations separated and mutually insulated by interlaminar insulation, the core having slots transverse to the laminations and conductors extending along the slots, the core as a whole and each lamination being insulated from, and not directly connected to, earth, each conductor being surrounded by inner and outer insulation between which and in contact with which there is a layer of conductive material which extends along the respective slot, and which is in electrical contact with a conductive member which also extends along the slot between the inner and outer insulation, the conductive member being separated from the laminations and electrically earthed and having an electrical resistance less than that of the layer.

Currents originating as dielectric currents in the winding insulation and flowing into the core, under any of the circumstances previously mentioned, whether such currents are in the nature of capacitance currents or dielectric leakage currents or due to partial discharges or sometimes even to dielectric breakdown, are thus interrupted by layers of conductive material of suitably high resistance external to the main high voltage insulation of the winding conductors, such layers being of the type conventionally used as already described for the purpose of suppressing discharges in the interspace between this insulation and the core slot wall, but being in electrical contact with a member of lower resistance which extends in the axial direction and insulated overall from the core, and whereby such currents are separately conducted to earth via external terminals.

In the accompanying drawings Figure 1 and Figures 2a to 2c show details of known machines, and Figure 2d and Figures 3 to 6 show various details of machines incorporating features according to the present invention.

Figure 1 shows, by way of illustration, the full length of a typical conductor bar (100) for a high voltage machine made in accordance with normal manufacturing practice, and having a layer (1) of slightly conductive material extending over the core slot length of the bar, and having at either end thereof end winding stress control layers (2) connected to the layer (1), it being understood that when the bar is placed in the machine the layer (1) will be earthed electrically by contact with the core.

Figure 2a shows to a larger scale a typical cross section of such a conductor bar (100) of conventional construction, comprising a plurality of copper conductors (3) each separately insulated with tape wrapped insulation (4) and having an overall insulating tube (5) comprising usually a bonded mica insulation capable of withstanding the full operating and test voltages applied to the machine windings. One or more such bars is (are) contained in a slot in the laminated core (6), the interspace being filled by a suitably conformable material (7).

Figure 2b shows in more detail part of the insulation (5), slot wall (6), more especially for a high voltage machine in which a conducting coating (1) is applied to the surface of the conductor insulation for the purpose of suppressing surface discharges as already explained, this being surrounded in turn by the conformable packing material or a slot liner (7), the latter however being of electrically weak insulating material which therefore does not entirely prevent the flow of the capacitive current through the HV insulation, although by puncturing at randomly dispersed points it may cause such current to flow non-uniformly into particular laminations of the core and the material (7) may in consequence also suffer gradual erosion due to arcing or partial discharges produce within it and in any residual gas space surrounding it.

Figure 2c shows an arrangement similar to that of Figure 2b, in which the conductive material (1) and conformable packing material (7) are combined and comprise a wrapping of suitable textile or other porous material impregnated with a slightly conducting substance, and thus provide a suitable degree of electrical conduction both axially along the surface of the insulation and radially between the insulation and the slot wall, so as to pass capacitive currents in both these directions without any electrical discharges, this material also effectively filling the otherwise electrically stressed gas spaces created for example by slight misplacement of some of the laminations of the core along the axial length of the slots.

Figure 2d shows a typical arrangement of the same portion of the machine, but illustrating the present invention. In this case the slightly conductive layer (1) (not differing essentially from such layers as shown in Figures 2b or 2c and as used in conventional manufacturing practice) is over-wrapped or surrounded by additional insulation (8) capable of withstanding at least a few hundred volts, so as to isolate it from the slot wall. The construction shown requires a conductor (which is omitted for clarity) such as conductor 9 described below with reference to Figure 3. Conformable packing material (7) may or may not be provided between this and the slot wall.

Figure 3 shows an arrangement of conductor bars (100) each generally as represented in Figures 1 and 2, but illustrating one method by which the layer (1) may be connected electrically to any appropriate earthing point or measuring circuit. Figure 3 shows additionally provision for earthing independently, if so desired, the end winding stress control layers (2) at either end of the slot portion of the bar (100), if such stress control layers are provided.

If a slightly conductive layer (1) (having a resistivity of the same order as used in current manufacturing practice) is used to conduct to either end the total capacitive current along the full length of each conductor bar (100), an excessive axial voltage drop could arise, especially if the bar carried the fault current i resulting from a breakdown of the main conductor insulation. The conductive layer (1) is therefore reinforced in the axial direction by one or more low-resistance elongate conductors in the form of strips (9) of metallic foil, fine wire, or other lowresistance conducting material. These lower resistance conductors (9) are preferably located, as shown in Figures 4a and 4b, in such a position that they could not, even in the event of a puncture of the insulation (8) come into direct contact with the core laminations (6). Thus, the risk of a heavy current loop arising within the core (6) is very greatly reduced or is eliminated. For example, where there are two conductor bars (100) in one slot as shown in Figure 3, the respective conductors (9) on both of these may be adjacent to a central spacer (10). The conductors (9) are bound to their respective bars (100) by the overall insulation (8). If necessary the spacers (10) between the bars (100) may be very slightly recessed to allow for the extra thickness of the conductors (9).

As an alternative arrangement, shown in Figure 4b, additional axial conductors (11) may be attached to each spacer (10), in which case these would make contact either with the subsidiary reinforcing conductors (9) (omitted for clarity) on the conductor bars (100) or directly as shown with the layers (1), via spring contacts (12) provided at intervals along the length of each bar (100). In this case the strips (11), are used to make connections to external earthing terminals (13) (Figure 3). In either of the methods shown in Figure 4a or Figure 4b, connections from each bar (100) are made to the terminals (13), which are preferably mounted close to the connecting points to strips (9) or (11) on each bar (100) or spacer (10), and from these terminals connections are made as appropriate to earth, or to a suitable measuring circuit as hereinafter described.

By suitable choice of resistivity of the layer (1) and of the conductors (9) or (11), the effective average resistance value between the outer surface of the main insulation and earth can be kept sufficiently low to prevent significant voltage drop under any operating or test condition between any part of the layer and the earthing terminal. If necessary, however, the reinforcing earthing conductors may be duplicated on opposite edges of each bar, provided that both such conductors are suitably insulated from the core.

While for the purpose of this description the type of material and the surface resistivity to be chosen for the layer (1) have been assumed to be similar to those conventionally used on the outer surface of the insulation of winding conductors within the core slots in large high voltage machines, it is to be understood that the designer has a free choice in these respects, and there are no fixed limits for the resistivity value which may be varied over a very wide range. The upper limit is determined by the requirement of conducting capacitance current through the insulation (5) to the conductor (9) without appreciable voltage drop. A minimum limit is determined by the need to avoid appreciable eddy current loss and heating due to magnetic flux cutting the conductor slots, as would be likely to occur for example if both the layers (1) and the conductors (9) were formed from any type of metal foil.

It will be understood also that despite a low effective resistance from all parts of the layer (1) to earth, an EMF will nevertheless be induced along the axial length of this layer and reinforcing conductors, and that because (in contrast to the conventional arrangement as shown in Figure 1) this layer is not in direct contact with earthed core laminations, this EMF will result in significant induced potentials to earth still being developed in these layers at points along the length of the bar remote from the point or points of earthing. These potentials must therefore be sustained by the external overall insulation (8) between the layer (1) and the core.

Whilst, except possibly in extremely large generators, this induced voltage will not be of sufficient magnitude to constitute any serious insulation problem or in itself to cause an appreciable capacitive current to flow into the core, it may nevertheless be found advantageous to limit the magnitude of such voltages by subdividing the axial reinforcing conductors into two or more separate sections along the length of the bar and by earthing each of these separately. Figure 5a shows how this may be done in the case of subdivision into two sections earthed at either end of the core of the machine only; Figure Sb shows a similar arrangement with subdivision into a number of separate sections. In the latter case connecting leads to terminals (13) at intermediate axial positions may be taken for example through radial ventilating ducts in the core. In any of these arrangements, induced voltage differences will exist between adjacent ends of the axially subdivided reinforcing conductors and some axial separation between them must consequently be provided to withstand these potential differences. In Figure 5a this voltage difference is equal to the full axial EMF, e, along one conductor bar although the maximum voltages to the core are only half this value. In Figure 5b, for 4 separate sections the EMF'S are approximately equal to e/4: or generally for N separate sections the voltage across the intermediate gaps will be e/N. At each such position, continuity of the layer (1) on the conductor insulation surface must be maintained, but to avoid an undesirably high local power dissipation in these parts of the layer (1), the designer may find it necessary to use a high resistivity material (14) at these positions. For example the higher resistivity coating material as employed for end winding stress control layers (2), may be used at these positions provided that its surface resistivity is not high enough to result in its overheating by winding capacitance currents or to produce thereby potentials to earth in the conducting layer sufficient to over-stress the external overall insulation (8).

Both the electro-magnetic and capacitive voltages are readily calculable (assuming the resistivities of the coating materials to be known), and it should be within the competence of a competent designer, therefore, to ensure that acceptable ri2 losses and insulation voltages at these positions are not exceeded. Figure 6 shows, by way of illustration one way of bringing out and of selecting for the purpose of measurement, connections from separate subsections of the layer (1) and from the stress control layers (2) of a group of conductor bars. The selector switches shown in this diagram should be of the "makebefore-break' type. Whatever the intended purpose of measuring the currents from these different regions either separately or collectively, for example that of monitoring leakage current to earth during a test or during normal operation of the machine, facilities may usefully be provided, by means of suitably accessible linkage terminals, for isolating any of the individual earthing layers, should occasion arise, for the purpose of identifying a faulty conductor bar or section thereof. Thus it may for example be advantageous to measure separately the total capacitive and dielectric loss components of current from the slot length insulation, and those from the end winding stress control layers, in order to determine the influence of the latter on the apparent dielectric loss in the insulation.

Various forms of instrument may be employed for measuring and/or continuous monitoring of the dielectric currents, if such measurements are desired, depending on the type of information required. For example a simple ammeter or milliammeter, a current recorder, a dielectric bridge circuit, or a discharge analyser may be used.

If provision is made, as suggested above, for making separate connections to individual sections of the layer (1) and stress control layers (2) on each different bar, it will be obvious that numerous different connections and/or switching arrangements may be de vised, of which that shown in Figure 6 is only an example. In the particular arrangement shown, the total dielectric current from both layers (1) and (2) of the whole group of bars (or of all bars in the machine), or that from any one of the six sub-sets (of which two in this case comprise the stress control layers at either end of the machine) which are brought out to separate switches, may be observed separately. Alternatively the currents from any one or more such sub-sets may be individually excluded from the measurement, in order for example to identify by elimination the one responsible for any observed abnormally high overall dielectric loss or leakage current. If similar identification is then required within any one sub-set, this may be done, as already suggested, by arranging to insert a measuring instrument into one or more of the individual conductors within that sub-set, provided that precautions are taken to avoid any circuit being open-circuited while the machine is excited at a high voltage.

Another measurement function which is difficult to achieve in machines of conventional design, but for which advantage may be taken of the connections brought out from the layers (1) and the conductors (9) of individual conductor bars is that of observing, at different positions in the winding, transient overvoltages due for example to lightning surges or to switching operations.

This may be done by earthing the conductor (9) of any selected bar, preferably with a short connecting lead, through a suitable capacitor (shunted if necessary by a resistor to by-pass power frequency current) so as to form, with the capacitance of the winding insulation, a capacitive divider of suitable ratio, and recording with an oscillograph the voltage across this capacitor.

WHAT WE CLAIM IS: 1. A stator core of an alternating current dynamo-electric machine comprising laminations separated and mutually insulated by interlaminar insulation, the core having slots transverse to the laminations and conductors extending along the slots, the core as a whole and each lamination being insulated from, and not directly connected to, earth, each conductor being surrounded by inner and outer insulation between which and in contact with which there is a layer of conductive material which extends along the respective slot, and which is in electrical contact with a conductive member which also extends along the slot between the inner and outer insulation. the conductive member being separated from the laminations and electrically earthed and having an electrical resistance less than that of the layer.

2. As claimed in claim 1, wherein means are connected to the conductive member for measuring leakage or dielectric or other spurious currents arising in the layer of conductive material.

3. As claimed in claim 1 or 2, wherein the conductive member is electrically connected to a further conductive member by a connection piece extending through the outer insulation.

4. As claimed in claim 1, 2 or 3 wherein the layer of conductive material is divided into sections joined by sections of higher resistivity material.

5. A stator core as claimed in claim 1 substantially as hereinbefore described with reference to Figure 2d of the accompanying drawings.

6. A stator core as claimed in claim 1, substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

7. A stator core as claimed in claim 1, substantially as hereinbefore described with reference to Figure 4a of the accompanying drawings.

8. A stator core as claimed in claim 1, substantially as hereinbefore described with reference to Figure 4b of the accompanying drawings.

9. A stator core as claimed in Figure 1, substantially as hereinbefore described with reference to Figure 5a of the accompanying drawings.

10. A stator core as claimed in Figure 1, substantially as hereinbefore described with reference to Figure Sb of the accompanying drawings.

11. A stator core as claimed in Figure 1, substantially as hereinbefore described with reference to Figure 6 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. vised, of which that shown in Figure 6 is only an example. In the particular arrangement shown, the total dielectric current from both layers (1) and (2) of the whole group of bars (or of all bars in the machine), or that from any one of the six sub-sets (of which two in this case comprise the stress control layers at either end of the machine) which are brought out to separate switches, may be observed separately. Alternatively the currents from any one or more such sub-sets may be individually excluded from the measurement, in order for example to identify by elimination the one responsible for any observed abnormally high overall dielectric loss or leakage current. If similar identification is then required within any one sub-set, this may be done, as already suggested, by arranging to insert a measuring instrument into one or more of the individual conductors within that sub-set, provided that precautions are taken to avoid any circuit being open-circuited while the machine is excited at a high voltage. Another measurement function which is difficult to achieve in machines of conventional design, but for which advantage may be taken of the connections brought out from the layers (1) and the conductors (9) of individual conductor bars is that of observing, at different positions in the winding, transient overvoltages due for example to lightning surges or to switching operations. This may be done by earthing the conductor (9) of any selected bar, preferably with a short connecting lead, through a suitable capacitor (shunted if necessary by a resistor to by-pass power frequency current) so as to form, with the capacitance of the winding insulation, a capacitive divider of suitable ratio, and recording with an oscillograph the voltage across this capacitor. WHAT WE CLAIM IS:

1. A stator core of an alternating current dynamo-electric machine comprising laminations separated and mutually insulated by interlaminar insulation, the core having slots transverse to the laminations and conductors extending along the slots, the core as a whole and each lamination being insulated from, and not directly connected to, earth, each conductor being surrounded by inner and outer insulation between which and in contact with which there is a layer of conductive material which extends along the respective slot, and which is in electrical contact with a conductive member which also extends along the slot between the inner and outer insulation. the conductive member being separated from the laminations and electrically earthed and having an electrical resistance less than that of the layer.

2. As claimed in claim 1, wherein means are connected to the conductive member for measuring leakage or dielectric or other spurious currents arising in the layer of conductive material.

3. As claimed in claim 1 or 2, wherein the conductive member is electrically connected to a further conductive member by a connection piece extending through the outer insulation.

4. As claimed in claim 1, 2 or 3 wherein the layer of conductive material is divided into sections joined by sections of higher resistivity material.

5. A stator core as claimed in claim 1 substantially as hereinbefore described with reference to Figure 2d of the accompanying drawings.

6. A stator core as claimed in claim 1, substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

7. A stator core as claimed in claim 1, substantially as hereinbefore described with reference to Figure 4a of the accompanying drawings.

8. A stator core as claimed in claim 1, substantially as hereinbefore described with reference to Figure 4b of the accompanying drawings.

9. A stator core as claimed in Figure 1, substantially as hereinbefore described with reference to Figure 5a of the accompanying drawings.

10. A stator core as claimed in Figure 1, substantially as hereinbefore described with reference to Figure Sb of the accompanying drawings.

11. A stator core as claimed in Figure 1, substantially as hereinbefore described with reference to Figure 6 of the accompanying drawings.