Title:
ROTATING ELECTRICAL MACHINE WITH MEANS FOR PREVENTING DISCHARGE FROM COIL ENDS
United States Patent 3670192
Abstract:
In order to prevent glow discharge from coil ends of a rotating electrical machine to adjacent grounded machine parts, the coil end is provided on the outside of its insulation with a conducting layer which is connected to ground by a voltage dependent impedance, such as a resistance, whose impedance decreases as the voltage increases.
US Patent References:
Method of diminishing corona and insulation heating at the ends of slots
Calvert - November 1936 - 2061502
Method of diminishing corona and insulation heating at the ends of slots
Calvert - November 1936 - 2061503
Conductor-ventilated generators
Fromm - April 1957 - 2788456
Corona suppression in high voltage coils
Manni - June 1960 - 2939976
Coating for equalizing the potential gradient along the surface of an electric insulation
Virsberg et al. - November 1962 - 3066180
Method of diminishing corona and insulation heating at the ends of slots
Calvert - November 1936 - 2061502
Method of diminishing corona and insulation heating at the ends of slots
Calvert - November 1936 - 2061503
Conductor-ventilated generators
Fromm - April 1957 - 2788456
Corona suppression in high voltage coils
Manni - June 1960 - 2939976
Coating for equalizing the potential gradient along the surface of an electric insulation
Virsberg et al. - November 1962 - 3066180
Inventors:
Andersson, Anders R. (Vasteras, SW)
Virsberg, Lars-goran (Vasteras, SW)
Virsberg, Lars-goran (Vasteras, SW)
Application Number:
05/082955
Publication Date:
06/13/1972
Filing Date:
10/22/1970
View Patent Images:
Export Citation:
Assignee:
Allmanna, Svenska Elektriska Aktiebolaget (Vasteras, SW)
Primary Class:
Other Classes:
174/DIG.020
International Classes:
H02K3/40; H02K3/32; H02H7/085
Field of Search:
310/196 174/12R,127,73R,73SC
US Patent References:
| 3210461 | Electrical stress-grading coatings | October 1965 | Berg et al. | |
| 3354331 | High voltage grading for dynamoelectric machine | November 1967 | Broeker et al. | |
| 3412200 | High voltage cable with potential gradient equalization means | November 1968 | Virsberg et al. | |
| 3487455 | INSULATED HIGH VOLTAGE CONDUCTOR WITH POTENTIAL GRADIENT EQUALIZATION MEANS | December 1969 | Laurell et al. | |
| 3508096 | ARRANGEMENT FOR PREVENTING GLOW DISCHARGES BETWEEN INSULATED CONDUCTORS IN GENERATOR END TURNS | April 1970 | Kull et al. |
Primary Examiner:
Goldberg E. A.
Assistant Examiner:
Witkowski, Stanley J.
Claims:
We claim
1. In a rotating electrical machine with a core furnished with winding slots and insulated coil sides disposed in said slots, each coil side having a relatively strongly insulated first portion, substantially located adjacent to a winding slot and a second, relatively weakly insulated coil end portion spaced from the slot, said machine having a grounded part adjacent to said second portion and spaced from the slot end, means for avoiding glow between said second portion and said grounded machine part during high voltage testing comprising a conducting layer on the surface of said second portion, said layer being electrically spaced from the winding slot, and an impedance element connecting said conducting layer to said grounded machine part, the impedance element being voltage-dependent in a voltage range beyond normal machine voltage in such a way that the impedance decreases as the voltage increases.
2. In a machine according to claim 1, said impedance element comprising a resistive member arranged on the outside of the coil surface.
3. In a machine according to claim 1, at least a part of said impedance element comprising a coating having voltage-dependent surface resistivity and arranged on the coil surface between the conducting layer and the end of the winding slot.
4. In a machine according to claim 1, said conducting layer on one coil continuing into a corresponding layer on a following coil in the current direction.
5. In a machine according to claim 1, said conducting layer being joined at its axially outer end to a layer of a resistive material arranged axially outside this layer and on the same coil, said material being voltage-dependent in such a manner that its surface resistivity decreases as the voltage increases.
1. In a rotating electrical machine with a core furnished with winding slots and insulated coil sides disposed in said slots, each coil side having a relatively strongly insulated first portion, substantially located adjacent to a winding slot and a second, relatively weakly insulated coil end portion spaced from the slot, said machine having a grounded part adjacent to said second portion and spaced from the slot end, means for avoiding glow between said second portion and said grounded machine part during high voltage testing comprising a conducting layer on the surface of said second portion, said layer being electrically spaced from the winding slot, and an impedance element connecting said conducting layer to said grounded machine part, the impedance element being voltage-dependent in a voltage range beyond normal machine voltage in such a way that the impedance decreases as the voltage increases.
2. In a machine according to claim 1, said impedance element comprising a resistive member arranged on the outside of the coil surface.
3. In a machine according to claim 1, at least a part of said impedance element comprising a coating having voltage-dependent surface resistivity and arranged on the coil surface between the conducting layer and the end of the winding slot.
4. In a machine according to claim 1, said conducting layer on one coil continuing into a corresponding layer on a following coil in the current direction.
5. In a machine according to claim 1, said conducting layer being joined at its axially outer end to a layer of a resistive material arranged axially outside this layer and on the same coil, said material being voltage-dependent in such a manner that its surface resistivity decreases as the voltage increases.
Description:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a means of avoiding corona discharges between the coil ends of a rotating electrical machine with coils arranged in winding slots and neighboring grounded machine parts during high test voltage.
2. The Prior Art
During the manufacture of a high voltage machine, voltage tests are carried out on the stator winding at various stages in the manufacture with a view of discovering any faults in the insulation. It is also usual for the customer to request such a test upon delivery of the finished machine, in which the winding is subjected to a test voltage which is about 3.5 times as high as the voltage during normal operation. The high test voltage means that it is often difficult to avoid glow and the formation of creep discharges between the coil ends and neighboring grounded machine parts such as bolt-heads, cooling pipes or bracing components. Such discharges often lead to direct sparking from the coil ends or provoke sparking indirectly by giving rise to uncontrolled transient voltages in the voltage source used for the test. The difficulties are accentuated when testing large water-cooled machines where there are often uninsulated live coil parts.
It is known to counteract glow in a winding slot in a stator by surrounding the part of the coil side lying in the slot with a grounded layer of conducting varnish so that grounding is effected by direct contact between the layer and the grounded stator core.
If the possibility is investigated by avoiding glow at the coil ends in a similar way by providing the coil end insulation with a grounded conducting layer, it is found that this would involve considerable disadvantages. This is because it is rather difficult to achieve a high insulation quality on the coil ends as on the straight coil part lying in the slot. When a high-voltage coil is being insulated, the straight part of the coil is pressed with the help of a special press means so that the impregnating medium solidifies under high pressure and the formation of pockets is avoided. It is not possible by normal methods to effect an equally high degree of compression of the insulation on the other parts of the coil. It is therefore normally taken into account that the voltage which this latter insulation can withstand for a long time is only a fraction of the corresponding insulation strength in the straight coil parts. However, since the surface of the weakly compressed coil insulation, because of the relatively long distance to ground, has a potential only slightly deviating from the potential of the conductor rod inside, there is normally no risk of this insulation playing the same part as a weak link in an otherwise strong chain. However, such will be the case if the weak coil end insulation is coated with an grounded, conducting surface layer.
SUMMARY OF THE INVENTION
The object of the invention is to protect the affected part of the coil ends against glow at high test voltages and sporadically occuring over-voltages by providing these parts with a conducting surface layer which is connected to ground in such a way that no substantial increase of the dielectric stresses during normal operation is caused.
According to the invention, a part of a coil near to any machine part is provided with a conducting layer arranged on the surface of the coil insulation and at a certain distance from the winding slot, said conducting layer being connected to ground through an impedance element, the impedance of which is voltage-dependent in such a way that the impedance decreases as the voltage increases.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in the following with reference to the accompanying schematic drawings in which
FIGS. 1, 2 and 3 show three different embodiments of the invention in tangential view in relation to the rotating machine and
FIG. 4 gives a graphic picture of potential conditions in the embodiment shown in FIG. 3. FIG. 1a shows a section through the free coil part along the line B--B in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the drawings 1 designates an end part of the stator core in a rotating electric machine and 2 an end part of a rotor surrounded by the stator core, the air gap between stator and rotor being designated 3. The dotted line 4 indicates the bottom of a winding slot and the part lying outside the stator core. A coil side arranged in the winding slot is designated 5. In FIG. 1, 6 is an end connection for joining two coil sides. The straight side of the coil provided with compressed insulation comprises between the slot part a short part running axially outside the stator core which, at the line A--A, continues into the weakly insulated part of the coil ends. A machine part in the vicinity of this latter coil part is designated 7. It is arranged at ground potential and may, for example, be a part of a winding support. A part of the coil side 5 in the vicinity of the machine part 7 is provided with a conducting surface layer 9. In FIG. 1 this part is indicated by a small check pattern. In FIG. 1a, 5a designates the copper conductor of the coil side and 5b the surrounding coil insulation. The conducting surface layer 9 is connected to ground by a resistive element 8, the resistance of which is voltage-dependent in such a way that the impedance decreases as the voltage increases.
When the expression "conducting" is used in the present text about a layer arranged on the coil insulation, for example the layer 9, this does not necessarily mean that the layer must consist of a material having low resistivity. It may, for example, consist of a varnish having relatively slight additions of graphite powder.
In the means shown in FIG. 1 the resistor 8 is chosen with such resistance/voltage characteristic that its resistance at a voltage corresponding to the maximum potential to ground of the winding at normal operation is several times greater than the impedance formed by the capacitance between the layer 9 and the conductor of the coil side, for example six times as great. This means that the potential of the conducting layer during normal operation deviates only negligibly from the conductor potential of the coil side.
If, for example during testing, the conductor of the coil is given a potential in relation to ground which is considerably greater than normal operating potential, for example 3.5 times as great, the resistance of the resistor 8 drops to, for example, 10 percent of its previous value. This means that the high test voltage does not cause any essential increase in the potential of the layer 9 in relation to ground and consequently the risk of glow during testing is eliminated. Admittedly the coil insulation will be subjected to the greater part of the test voltage but, in view of the short testing time, this can certainly be permissible.
If the conducting layer 9 has very high surface resistivity, certain potential differences may occur within the layer. If desired, these differences can be reduced by connecting the resistor 8 to the conducting layer at one end at a number of suitably spaced contact points. If the transition between the ends of the conducting layer and the uncoated coil insulation is carried out without field equalizing means, it may be advantageous if the ends of the layer during testing have a somewhat higher potential than the central part of the coating. This is achieved with the device shown in FIG. 1 if the resistivity of the layer 9 is high. If the conducting layer is provided with some field equalizing means (for example similar to that shown in British Pat. No. 842,039) only at one end, it may be an advantage to make the layer with a relatively high surface resistivity and connect the resistor 8 at the same end.
In FIG. 2, 12 is a conducting layer which differs from the corresponding layer 9 in FIG. 1 in that it comprises coil end parts of two different coil sides and also intermediate coil ends parts. A neighboring grounded machine part in FIG. 2 is designated 10. Of course it is also possible in this embodiment to use an impedance element arranged outside the coil surface, for example of the same type as the resistor 8 in FIG. 1. However, it is preferred instead to provide each coil side 5 with a coating 11 arranged on the surface of the coil insulation, the surface resistivity of this coating being voltage-dependent in the same way as the resistance of the resistor 8. A conducting coating 13 is also arranged in known manner to surround the part of each coil side lying in the winding slot and also projects slightly outside the slot. The coating 11 has the function of controlling the electric field, as described in British Pat. No. 842,039, and at the same time provides the effect intended according to the invention and achieved by the resistor 8 in FIG. 1. If the outermost portion of the coil ends is constructed in accordance with FIG. 1, that is, with a means corresponding to 6 in FIG. 1 intended to connect the conductors of the coil sides, this should also be surrounded by the conducting layer 12. It is of course also assumed that the device 6 is provided with insulation. If not, which is often the case, the layer 12 should not be drawn out in the vicinity of the uninsulated part.
When testing windings with increased voltage, it is often required that certain parts of the winding should be subjected to this voltage while the other parts are grounded. This means that during testing a coil can have a potential to ground which is, for example, 3.5 times greater than normal, whereas a coil with its free winding part immediately next to the first coil is at ground potential. In such cases it is advantageous if the conducting coating 12 which is shown in FIG. 2 covers the whole of the relatively weakly insulated, but anyway insulated, portion of the coil ends.
In the embodiment of the invention shown in FIG. 3 the coating A 1 corresponds to the conducting, grounded coating 13 in FIG. 2. B 1 designates a coating of the same voltage-dependent material as the coating 11 in FIG. 2, A 2 is a well conducting layer and B 2 a coating of material having voltage-dependent resistivity. The function is clear from FIG. 4 where the fully drawn curve shows the potential distribution of the part of a winding coil shown in FIG. 3 during normal operation of the machine, and the broken curve during testing with increased voltage. The distance X from the end of the winding slot is indicated along the abscissa and the potential of the coil surface in relation to ground along the ordinate. The potential in relation to ground given to the coil conductors during testing is designated U p , whereas the potential of the conductors during normal operation is designated U d . The well conducting layer A 2 can, by suitable dimensioning (composition, length) of the coating B 1 1, be made to assume an arbitrary potential U m between zero and test voltage during the testing, whereas at operating voltage it assumes approximately the potential of the copper conductor and thus does not cause any increase of the dielectric stress on the coil insulation.
In the embodiments of the invention described in connection with the drawings it is assumed that the voltage-dependent impedance element characteristic of the invention is principally of resistive type. A device according to the invention can be constructed with many different types of voltage-dependent resistive impedance elements and it is even feasible to use voltage-dependent impedance elements of capacitive or inductive type.
1. Field of the Invention
The present invention relates to a means of avoiding corona discharges between the coil ends of a rotating electrical machine with coils arranged in winding slots and neighboring grounded machine parts during high test voltage.
2. The Prior Art
During the manufacture of a high voltage machine, voltage tests are carried out on the stator winding at various stages in the manufacture with a view of discovering any faults in the insulation. It is also usual for the customer to request such a test upon delivery of the finished machine, in which the winding is subjected to a test voltage which is about 3.5 times as high as the voltage during normal operation. The high test voltage means that it is often difficult to avoid glow and the formation of creep discharges between the coil ends and neighboring grounded machine parts such as bolt-heads, cooling pipes or bracing components. Such discharges often lead to direct sparking from the coil ends or provoke sparking indirectly by giving rise to uncontrolled transient voltages in the voltage source used for the test. The difficulties are accentuated when testing large water-cooled machines where there are often uninsulated live coil parts.
It is known to counteract glow in a winding slot in a stator by surrounding the part of the coil side lying in the slot with a grounded layer of conducting varnish so that grounding is effected by direct contact between the layer and the grounded stator core.
If the possibility is investigated by avoiding glow at the coil ends in a similar way by providing the coil end insulation with a grounded conducting layer, it is found that this would involve considerable disadvantages. This is because it is rather difficult to achieve a high insulation quality on the coil ends as on the straight coil part lying in the slot. When a high-voltage coil is being insulated, the straight part of the coil is pressed with the help of a special press means so that the impregnating medium solidifies under high pressure and the formation of pockets is avoided. It is not possible by normal methods to effect an equally high degree of compression of the insulation on the other parts of the coil. It is therefore normally taken into account that the voltage which this latter insulation can withstand for a long time is only a fraction of the corresponding insulation strength in the straight coil parts. However, since the surface of the weakly compressed coil insulation, because of the relatively long distance to ground, has a potential only slightly deviating from the potential of the conductor rod inside, there is normally no risk of this insulation playing the same part as a weak link in an otherwise strong chain. However, such will be the case if the weak coil end insulation is coated with an grounded, conducting surface layer.
SUMMARY OF THE INVENTION
The object of the invention is to protect the affected part of the coil ends against glow at high test voltages and sporadically occuring over-voltages by providing these parts with a conducting surface layer which is connected to ground in such a way that no substantial increase of the dielectric stresses during normal operation is caused.
According to the invention, a part of a coil near to any machine part is provided with a conducting layer arranged on the surface of the coil insulation and at a certain distance from the winding slot, said conducting layer being connected to ground through an impedance element, the impedance of which is voltage-dependent in such a way that the impedance decreases as the voltage increases.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in the following with reference to the accompanying schematic drawings in which
FIGS. 1, 2 and 3 show three different embodiments of the invention in tangential view in relation to the rotating machine and
FIG. 4 gives a graphic picture of potential conditions in the embodiment shown in FIG. 3. FIG. 1a shows a section through the free coil part along the line B--B in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the drawings 1 designates an end part of the stator core in a rotating electric machine and 2 an end part of a rotor surrounded by the stator core, the air gap between stator and rotor being designated 3. The dotted line 4 indicates the bottom of a winding slot and the part lying outside the stator core. A coil side arranged in the winding slot is designated 5. In FIG. 1, 6 is an end connection for joining two coil sides. The straight side of the coil provided with compressed insulation comprises between the slot part a short part running axially outside the stator core which, at the line A--A, continues into the weakly insulated part of the coil ends. A machine part in the vicinity of this latter coil part is designated 7. It is arranged at ground potential and may, for example, be a part of a winding support. A part of the coil side 5 in the vicinity of the machine part 7 is provided with a conducting surface layer 9. In FIG. 1 this part is indicated by a small check pattern. In FIG. 1a, 5a designates the copper conductor of the coil side and 5b the surrounding coil insulation. The conducting surface layer 9 is connected to ground by a resistive element 8, the resistance of which is voltage-dependent in such a way that the impedance decreases as the voltage increases.
When the expression "conducting" is used in the present text about a layer arranged on the coil insulation, for example the layer 9, this does not necessarily mean that the layer must consist of a material having low resistivity. It may, for example, consist of a varnish having relatively slight additions of graphite powder.
In the means shown in FIG. 1 the resistor 8 is chosen with such resistance/voltage characteristic that its resistance at a voltage corresponding to the maximum potential to ground of the winding at normal operation is several times greater than the impedance formed by the capacitance between the layer 9 and the conductor of the coil side, for example six times as great. This means that the potential of the conducting layer during normal operation deviates only negligibly from the conductor potential of the coil side.
If, for example during testing, the conductor of the coil is given a potential in relation to ground which is considerably greater than normal operating potential, for example 3.5 times as great, the resistance of the resistor 8 drops to, for example, 10 percent of its previous value. This means that the high test voltage does not cause any essential increase in the potential of the layer 9 in relation to ground and consequently the risk of glow during testing is eliminated. Admittedly the coil insulation will be subjected to the greater part of the test voltage but, in view of the short testing time, this can certainly be permissible.
If the conducting layer 9 has very high surface resistivity, certain potential differences may occur within the layer. If desired, these differences can be reduced by connecting the resistor 8 to the conducting layer at one end at a number of suitably spaced contact points. If the transition between the ends of the conducting layer and the uncoated coil insulation is carried out without field equalizing means, it may be advantageous if the ends of the layer during testing have a somewhat higher potential than the central part of the coating. This is achieved with the device shown in FIG. 1 if the resistivity of the layer 9 is high. If the conducting layer is provided with some field equalizing means (for example similar to that shown in British Pat. No. 842,039) only at one end, it may be an advantage to make the layer with a relatively high surface resistivity and connect the resistor 8 at the same end.
In FIG. 2, 12 is a conducting layer which differs from the corresponding layer 9 in FIG. 1 in that it comprises coil end parts of two different coil sides and also intermediate coil ends parts. A neighboring grounded machine part in FIG. 2 is designated 10. Of course it is also possible in this embodiment to use an impedance element arranged outside the coil surface, for example of the same type as the resistor 8 in FIG. 1. However, it is preferred instead to provide each coil side 5 with a coating 11 arranged on the surface of the coil insulation, the surface resistivity of this coating being voltage-dependent in the same way as the resistance of the resistor 8. A conducting coating 13 is also arranged in known manner to surround the part of each coil side lying in the winding slot and also projects slightly outside the slot. The coating 11 has the function of controlling the electric field, as described in British Pat. No. 842,039, and at the same time provides the effect intended according to the invention and achieved by the resistor 8 in FIG. 1. If the outermost portion of the coil ends is constructed in accordance with FIG. 1, that is, with a means corresponding to 6 in FIG. 1 intended to connect the conductors of the coil sides, this should also be surrounded by the conducting layer 12. It is of course also assumed that the device 6 is provided with insulation. If not, which is often the case, the layer 12 should not be drawn out in the vicinity of the uninsulated part.
When testing windings with increased voltage, it is often required that certain parts of the winding should be subjected to this voltage while the other parts are grounded. This means that during testing a coil can have a potential to ground which is, for example, 3.5 times greater than normal, whereas a coil with its free winding part immediately next to the first coil is at ground potential. In such cases it is advantageous if the conducting coating 12 which is shown in FIG. 2 covers the whole of the relatively weakly insulated, but anyway insulated, portion of the coil ends.
In the embodiment of the invention shown in FIG. 3 the coating A 1 corresponds to the conducting, grounded coating 13 in FIG. 2. B 1 designates a coating of the same voltage-dependent material as the coating 11 in FIG. 2, A 2 is a well conducting layer and B 2 a coating of material having voltage-dependent resistivity. The function is clear from FIG. 4 where the fully drawn curve shows the potential distribution of the part of a winding coil shown in FIG. 3 during normal operation of the machine, and the broken curve during testing with increased voltage. The distance X from the end of the winding slot is indicated along the abscissa and the potential of the coil surface in relation to ground along the ordinate. The potential in relation to ground given to the coil conductors during testing is designated U p , whereas the potential of the conductors during normal operation is designated U d . The well conducting layer A 2 can, by suitable dimensioning (composition, length) of the coating B 1 1, be made to assume an arbitrary potential U m between zero and test voltage during the testing, whereas at operating voltage it assumes approximately the potential of the copper conductor and thus does not cause any increase of the dielectric stress on the coil insulation.
In the embodiments of the invention described in connection with the drawings it is assumed that the voltage-dependent impedance element characteristic of the invention is principally of resistive type. A device according to the invention can be constructed with many different types of voltage-dependent resistive impedance elements and it is even feasible to use voltage-dependent impedance elements of capacitive or inductive type.