Insulated conductor for high voltage windingThe present invention relates in a first aspect to an insulated conductor for a high voltage winding in an electrical machine.
A second aspect of the invention relates to a rotating electric machine or a static electric machine comprising an insulated conductor of the above-mentioned type.
The present invention is applicable to rotating electric machines such as synchronous machines or asynchronous machines, and to static electric machines as power transformers and power reactors. The invention can also be used in other electrical machines, such as two-way fed machines, and in asynchronous static converter stages, external pole machines and synchronous flow machines (flow machines), provided that the windings thereof comprise insulated electrical conductors of the type described in the introduction and are preferably at high voltage. "high voltage" here means a voltage of more than 10 kV. For an insulated conductor for a high voltage winding according to the invention, a typical operating range may be 1 to 800 kV.
In order to be able to explain and describe the electrical machine, a brief description of the rotating electrical machine will first be given, exemplified on the basis of a synchronous machine. The first part of the description relates substantially to the magnetic circuit of such a machine and how it is built according to conventional techniques. Since the magnetic circuit referred to in most cases is located in the stator, the magnetic circuit will generally be described hereinafter as a stator with a laminated core, the windings of which will be referred to as stator windings, and the slots in the laminated core of the windings will be referred to as stator slots or simply slots.
The stator windings are located in slots in the lamination core, the slots typically having a rectangular or trapezoidal cross-section like a rectangle or trapezoid. Each winding phase includes a plurality of series-connected coil groups connected in series, and each coil group includes a plurality of series-connected coils connected in series. The different parts of the coil refer to the coil side of the part positioned inside the stator and the end coil ends of the part located outside the stator. A coil comprises one or more conductors placed together in the height and/or width direction.
Between each conductor there is a thin layer of insulation, such as epoxy/fiberglass.
The coil is insulated by the slot with coil insulation, that is, the insulation is used to withstand the rated voltage of the machine to ground. As insulating material, various plastics, lacquers and glass fibre materials can be used. Usually, so-called mica tapes are used, which are mixtures of mica with hard plastic, specially produced to provide resistance to partial discharges that rapidly break down the insulation. The insulation is applied to the coil by winding a mica tape over the coil in several layers. The insulation is impregnated and the coil side is then coated with a graphite based paint to improve contact with the surrounding stator connected to ground potential.
The conductor area of the winding is determined by the current intensity in question and the cooling method used. The conductor and coil are typically formed in a rectangular shape to maximize the amount of conductor material in the slot. A typical coil is formed by so-called Roebel bars, some of which may be made hollow for coolant. The Roebel bar comprises a plurality of rectangular, parallel-connected copper conductors which are shifted 360 degrees along the slot. Ringland rods with 540 degree transformation and other transformations also appeared. The transformation is performed to avoid the occurrence of circulating currents, which are generated in the cross section of the conductor material, as seen by the magnetic field.
For mechanical and electrical reasons, motors cannot be manufactured in any size. Motor power is basically determined by three factors:
-conductor area of the winding. At normal temperature, for example, copper has a thickness of 3-3.5A/mm2Is measured.
Maximum magnetic flux density (flux) in the stator and rotor material.
Maximum electric field strength in the insulating material, so-called dielectric strength.
The polyphase ac winding is designed as a single-layer or double-layer winding. In the case of a single layer winding, there is only one coil side per slot, whereas in the case of a double layer winding there are two coil sides per slot. The double-layer windings are usually designed as rhombohedral windings, while the single-layer windings concerned in this connection can be designed as rhombohedral windings or as concentric windings. In the case of a diamond winding, only one coil span (or possibly two coil spans) is present, while the parallel winding is designed as a concentric winding, i.e. with a significantly varying coil span. The coil span is the arc distance between two coil sides belonging to the same coil, depending on the number of pole pitches or inter-slot pitches involved. Typically, different variations of the chord, such as short pitch, are used to give the winding the desired performance.
Windings of the type that basically describe how the coils are in the slots, i.e. the coil sides, are connected together outside the stator, i.e. at the ends of the end windings.
Outside the stator lamination, the coil is free of an applied conductive ground potential layer. The end winding heads are usually provided with an E-field control in the form of a so-called corona protection paint for converting the radial field into an axial field, which means that the insulation on the end winding heads occurs at a high potential relative to ground. This sometimes produces a corona in the end winding end region, which can be damaging. The so-called field control points at the end winding ends present problems for rotating electric machines.
Typically, all large motors are designed with double layer windings and equivalently large coils. Each coil is arranged with one side in one of the layers and the other side in the other layer. This means that all coils cross each other in the end winding ends. If more than two layers are used, these crossovers make the winding work difficult and damage the end winding ends.
It is generally known that the connection of the synchronous motor/generator to the power grid must be made via a so-called step-up transformer of the E/YD connection, since the voltage of the power grid is usually in a higher level than the voltage of the rotating electric machine. Together with the synchronous machine, this transformer thus forms an integral part of the power plant. This transformer causes additional costs and also has the disadvantage of reducing the overall efficiency of the system. The step-up transformer can thus be dispensed with if it is possible to manufacture a motor of relatively high voltage.
During the last decades, the need for rotating electrical machines with higher voltages than previously possible has increased. The maximum voltage level which can be reached with good coil productivity for synchronous machines according to the state of the art is about 25-30 kV.
Some attempts to a new approach to the design of synchronous generators are described, inter alia, in the article entitled "water and oil cooled turbine generator TVM-300" by j.elektrotechnika, 11.1.1970, pages 6-8, in the "stator of generator" by US-4,429,244, and in the soviet Patent document CCCP Patent 955369.
The water and oil cooled synchronous generator described in j.elektrotechnika is intended for voltages up to 20 kV. The article describes a new insulation system consisting of oil/paper insulation, which makes it possible to completely immerse the stator in oil. The oil can then be used as a coolant and at the same time as insulation. To prevent oil in the stator from leaking out to the rotor, a dielectric oil isolating ring is provided at the inner surface of the core. The stator winding is made of a conductor with an oval hollow shape, filled with oil and paper insulation. The coil side with insulation is fixed in the groove made rectangular in cross section by means of wedges. The oil serves as a coolant both in the hollow conductor and in the bore in the stator wall. However, such cooling systems require a large amount of oil and electrical connections at the coil ends. Thick insulation also results in an increase in the bending radius of the conductor, which in turn increases the size of the coil overhang.
The above-mentioned US patent relates to a stator part of a synchronous machine comprising a laminated core with trapezoidal slots for the stator windings. The slots are tapered because the required insulation of the stator windings is weakened towards the inside of the rotor where the winding portion arranged closest to the neutral point is located. Here, the stator part comprises a dielectric oil insulating cylinder closest to the inner surface of the core. This part may increase the excitation requirements relative to a motor without this ring. The stator windings are made of oil-saturated cables having the same diameter for each oil layer. The layers are separated from each other by means of spacers in the grooves and secured by wedges. The winding is characterized in that it comprises two so-called half-windings connected in series. One of the two half-windings is arranged and centered in the insulating sleeve. The conductors of the stator winding are cooled by the surrounding oil. A disadvantage with so much oil in the system is that there is a risk of leakage and a significant cleaning effort may result from a fault condition. The part of the insulating sheath located outside the slot has a cylindrical portion and a tapered termination reinforced with a current-carrying layer, the purpose of the tapered termination being to control the electric field strength in the region where the cable enters the end winding.
It is evident from CCCP 955369 that in another attempt to increase the voltage rating of a synchronous machine, the oil-cooled stator winding comprises a conventional high-voltage cable having the same dimensions for all layers. The cables are placed in stator slots formed in a circular shape, with openings radially arranged in correspondence with the cross-sectional area of the cables and the space required for the fixation and cooling. The different radially arranged layers of the winding are surrounded by an insulating tube and fixed therein. Insulating spacer elements secure the tubes in the stator slots. Because of the oil cooling, an inner dielectric ring is also required here to seal off the oil coolant from the inner air gap. The above-mentioned disadvantages of oil in the system also apply to this construction. This structure presents a very narrow, radial waist between the different stator slots, which means a large amount of slot leakage flux that significantly affects the excitation requirements of the electrical machine.
From the report in 1984 from the Electric Power Research Institute EPRI EL-3391, the generator concept was reviewed in order to achieve high voltages for rotating Electric machines in order to be able to connect the machine to the Power grid without an intermediate transformer. Such a scheme provides good efficiency gains and financial advantages as judged by the survey. The main reason that it was thought possible to start developing generators directly connected to the power grid in 1984 was that superconducting rotors have been developed at this time. The considerable excitation capacity of the superconducting field makes it possible to use an air gap winding with sufficient thickness to withstand the electrical stresses. By combining the most promising concept of designing a magnetic circuit according to the project with a winding, a so-called monolithic cylindrical armature, a concept in which two conductor cylinders are put in three insulating cases and the entire structure is fixed to one core without teeth, it is considered that a rotating electric machine for high voltage can be directly connected to the power grid. This solution means that the main insulation must be made thick enough to cope with net-to-net and net-to-ground potentials. After reviewing all the techniques known at the time, it was considered that the insulation system necessary to manage to increase to higher voltages was that which was typically used for power transformers and which included dielectric fluid impregnated cellulose pressboards. A significant disadvantage of the proposed solution is that, in addition to the need for a superconducting rotor, a very thick insulation is required, which increases the size of the machine. The end coil ends must be insulated and cooled with oil or freon in order to control the electric field in the ends. The entire machine is hermetically sealed to prevent the liquid dielectric medium from absorbing moisture from the atmosphere.
When manufacturing rotating electric machines according to the state of the art, the winding with the conductor and insulation system is manufactured in several steps, and therefore has to be wound before being installed on the magnetic circuit. After the winding has been mounted on the magnetic circuit, impregnation for preparing the insulation system is carried out.
The aim of the invention is to be able to produce a rotating electric machine for high voltages without any complicated winding and without impregnation of the insulation system after the winding has been installed.
In order to increase the power of the rotating electric machine, it is known to increase the current in the ac coil. This is achieved by optimizing the amount of conductive material, i.e. by the close packing of the rectangular conductors in the rectangular rotor slots. The aim is to cope with the resulting temperature increase by increasing the amount of insulating material and using a more temperature resistant and thus more expensive insulating material. High temperatures and field loads on the insulation also cause problems with respect to insulation lifetime. In thick-walled insulation layers for high-voltage equipment, such as in impregnated mica tape layers, partial discharges, PD, cause serious problems. In the manufacture of these insulator layers, voids, pores, etc. are easily generated, wherein internal corona discharge is generated when the insulator is subjected to high electric field strength. These corona discharges gradually degrade the material and may lead to electrical breakdown through the insulation.
The invention is based on the recognition that the power of a rotating electric machine can be increased in a technically and economically sensible manner, which must be achieved by ensuring that the insulation is not broken down by said phenomenon. This is achieved according to the invention by using as the insulating layer an extruded layer of, for example, a suitable solid insulating material, such as a thermoplastic resin, a cross-linked thermoplastic resin, a rubber such as silicone rubber, or the like, manufactured in this way so that the risk of voids and voids is minimal. It is also important that the insulation comprises an inner layer with semiconducting properties surrounding the conductor and that the insulation also carries at least one further outer layer with semiconducting properties surrounding the insulation. By semiconducting properties is meant herein a material having a conductivity that is much lower than that of an electrical conductor, but not as low as that of an insulator. By using only insulating layers which can be manufactured with a minimum of defects, and furthermore providing insulation for the inner and outer conductive layers, a reduction of the thermal and electrical load can be ensured. The insulating portions with at least one adjacent conductive layer should have substantially the same coefficient of thermal expansion. Under temperature gradients, defects caused by differential temperature expansion in the insulation and surrounding layers should not be generated. The electrical load on the material is reduced due to the fact that the conductive layer surrounding the insulation will constitute an equipotential surface and the electric field in the insulating part will be distributed fairly evenly over the entire thickness of the insulation. The outer conductive layer may be connected to a selected potential, such as ground. This means that for such a cable the outer housing of the winding over its entire length can be kept at e.g. ground potential. The outer layers of the windings may also be cut at appropriate locations along the length of the conductors and each cut length of conductor may be connected directly to a selected potential. Other layers, housings, etc. may be arranged around the outer conductive layer, such as a metal shield and a protective sheath.
Further knowledge gained in connection with the present invention is that the increased current load leads to problems with concentration of the electrical (E) field at the corners of the coil cross-section and this also brings large local loads on the insulation there. Also the magnetic (B) field in the stator teeth will be concentrated at the corners. This means that magnetic saturation occurs locally and the resulting voltage/current flow waveform will also be distorted without making full use of the core. Furthermore, eddy current losses, which are caused by induced eddy currents in the conductor, due to the geometry of the conductor in relation to the B-field, will bring about further disadvantages when increasing the current strength. A further improvement of the invention is achieved by making the coil and the slot in which the coil is placed substantially circular, rather than rectangular. By making the coils circular in cross-section, they will be surrounded by a constant B-field without concentration where magnetic saturation can occur. Furthermore, the E-field in the coil will be evenly distributed over the entire cross-section and the local load on the insulation is significantly reduced. Furthermore, it is easier to position the circular coils in the slots in such a way that the number of coil sides per coil group can be increased and voltage rises can occur without having to increase the current in the conductors. The reason for this is that the cooling of the conductor is assisted on the one hand by the lower current density and thus by the lower temperature gradient across the insulation and on the other hand by the circular grooves which bring about a more uniform temperature distribution over the entire cross-section. Further improvements can also be achieved by making the conductor from smaller parts, so-called strands. The strands may be insulated from each other and a small number of strands may be left uninsulated and in contact with the inner conductor layer to ensure that they are at the same potential as the conductor.
An advantage of using a rotating electric machine according to the invention is that the machine can be run over-load for a significantly longer period of time than usual such machines without being damaged. This is a result of the thermal load that is limited in the composition and insulation of the machine. For example, it is possible to load the motor with a 100% overload for a period of more than 15 minutes, even up to two hours.
According to one embodiment of the invention the magnetic circuit of the rotating electric machine comprises a winding through a cable with one or more extruded insulated conductors with solid insulation with a conductive layer at both the conductor and the housing. The outer conductive layer may be connected to ground potential. In order to be able to deal with the problems that arise in the case of direct connection of rotating electrical machines to all types of high-voltage power networks, the electrical machine according to the invention has a number of features that are distinguished from the state of the art.
As mentioned above, the winding of the rotating electric machine may be manufactured from a cable with one or more extruded insulated conductors with solid insulation with a conductive layer at both the conductor and the housing.
Some typical examples of insulating materials are thermoplastics like LDPE (low density polyethylene), HDPE (high density polyethylene), PP (polypropylene), PB (polybutylene), PMP (polymethylpentene), or crosslinked materials like XLPE (crosslinked polyethylene), or rubber insulation like EPR (ethylene propylene rubber) or silicone rubber.
A possibility to further develop the conductor consisting of strands is that it is possible to insulate the strands with respect to each other, in order to thus reduce the amount of eddy current losses in the conductor. One or more strands may be left uninsulated to ensure that the conductive layer surrounding the conductor is at the same potential as the conductor.
It is known that high voltage cables for transmission of electrical energy are composed of a conductor with a solid extruded insulation with inner and outer conducting parts. In the process of transmitting electric energy, it is required that the insulation should be defect-free. The starting point at which the insulation should be defect-free has been long during the transfer of electrical energy. When using high voltage cables for power transmission, the aim is not to maximize the current through the cable, since there is no restriction on the transmission cable space.
The conductor insulation for the rotating electric machine can be applied in some other way than by means of extrusion, for example by spraying or the like. It is important, however, that the insulation should not be defective throughout its cross-section and should have similar thermal properties. In combination with insulation applied to the conductor, insulation may be provided to the conductive layer.
Preferably, cables with a circular cross-section are used. In particular, cables with different cross-sections can be used in order to obtain a better packing density. To build up a voltage in the rotating electric machine, the cable is arranged in several successive turns in a slot in the core. The windings can be designed as multi-layer concentric cable windings to reduce the number of end-to-end crossings. The cable can be made with a conical insulation in order to utilize the core in a better way, in which case the shape of the slots can be adapted to the conical insulation of the winding.
A significant advantage of the rotating electric machine according to the invention is that the E-field is close to zero in the end region outside the outer conductive layer and that the electric field does not need to be controlled for the outer housing at ground potential. This means that there is no field concentration either within the sheet, in the end region or in the transition region between them.
The invention also relates to a method of manufacturing a magnetic circuit, and in particular a winding. The manufacturing method comprises placing the winding in the slots by passing the cable through an opening in the internal slot of the core. The cable is flexible and can be bent, and this allows a length of cable to be placed in the coil in several turns. The end winding ends will then comprise the bending zone in the cable. The cables may be spliced in such a way that their performance remains constant over the entire cable length. This approach brings a lot of simplification compared to the existing state. The so-called Roebel bar is inflexible and must be formed into a desired shape. Impregnation of the coils is also a very complex and expensive technique for manufacturing rotating electric machines today.
This is achieved by means of an insulated conductor for a high voltage winding in a rotating electric machine as defined in claim 1, and also by means of a rotating electric machine comprising an insulated conductor of the above-mentioned type according to claim 7. The high voltage cable according to the invention comprises one or more strands surrounded by a first conductor layer. This first conductor layer is in turn surrounded by a first insulating layer, which is surrounded by a second conductor layer. This second conductor layer is grounded at least at two different points along the high voltage cable, i.e. at the inlet and outlet of the stator. The second conductive layer has a resistivity that on the one hand minimizes electrical losses in the second conductive layer, and on the other hand helps the voltage induced in the second conductive layer to minimize the risk of glow discharges.
By means of the high-voltage cable according to the invention, as described above, a high-voltage cable is obtained in which electrical losses caused by induced voltages in the outer conductive layer can be avoided. A high voltage cable is also obtained in which the risk of electrical discharges is minimized. In addition, this is achieved by means of a cable which is simple to manufacture.
The invention will now be explained in more detail in the following description of preferred embodiments with reference to the drawings.
Fig. 1 shows a cross-section of a high voltage cable according to the invention;
FIG. 2 shows a basic diagram explaining what affects the voltage between the conductive surface and ground;
fig. 3 shows the potential on the conductive surface in relation to the distance between the grounding points.
Fig. 1 shows a cross-section of a high voltage cable 10 according to the invention. The high voltage cable 10 shown comprises an electrical conductor consisting of one or more strand parts 12, for example of copper (Cu), having a circular cross-section. These strand parts 12 are arranged in the center of the high voltage cable 10. Around the strands 12 is a first conductive layer 14 and around the first conductive layer 14 is a first insulating layer 16, for example XLPE insulation. Surrounding the first insulating layer 16 is a second conductive layer 18.
Fig. 2 shows a basic diagram explaining what affects the voltage between the conductive surface and ground. A voltage U generated between the second conductive layer 18 and groundsCan be expressed as follows:
wherein U ismaxIs the result of capacitive currents in the surface, and where UindIs the voltage induced by the magnetic flux. To avoid surface discharges, UsMust be < 250V, preferably Us＜130-150V。
In principle, it is assumed that the stator ends are grounded UindNo problem is caused. Thus U iss≈UmaxWherein the maximum value U at the middle of the conductormaxIs given by
Wherein f = frequency; c1= lateral capacitance per length unit; u shapef= relative ground voltage; ρ s = resistivity of the conductive layer 18; a. thes= cross-sectional area of the conductive layer 18 and l = length of the stator.
One way to prevent losses caused by induced voltages in the second conductive layer 18 is to increase its resistance. Since the thickness of the layers cannot be reduced for technical reasons related to the manufacture of cables and stators, the electrical resistance can be increased by selecting a coating or compound with a higher electrical resistivity.
If the resistivity increases too much, the voltage on the second conductive layer 18 halfway between the grounding points (i.e. inside the stator) is so high that there is a risk of glow discharge and thus corrosion of the conductive layers and insulation.
The resistivity ps of the second conductive layer 18 should therefore lie in the interval:
ρmin＜ρs＜ρmax (2)
where ρ isminPower loss due to allowable eddy current loss is determined, and Uind·ρmaxThe resulting resistive losses are determined by the requirement that there be no glow discharge.
Experiments have shown that the resistivity ps of the second conductive layer 18 should be between 10-500 ohm-cm. To obtain good results for all sizes of machine ρ s should be between 10-100 ohm-cm.
Fig. 3 shows the potential on the conductive surface in relation to the distance between the grounding points.
As an example of a suitable conductive layer 18, EPDM is mixed with carbon black. By varying the type of base polymer, and/or varying the type of carbon black and/or the properties of the carbon black, the resistivity can be determined.
The following are a number of examples of different resistivity values obtained using various mixtures of base polymer and carbon black. Carbon Black based Polymer carbon Black amount volume resistivity
Type% ohm-cm ethylene vinyl acetate EC carbon black about 15350-400 copolymer/nitrile rubber "" P carbon black about 3770-10 "" additional conductive carbon black, type I about 3540-50 "" additional conductive carbon black, type II about 3330-60 butyl grafted polyethylene "" about 257-10 ethylene butyl propylene acetylene carbon black about 3540-50 acid salt copolymer "" P carbon black about 385-10 ethylene propylene rubber additional conductive carbon black about 35200-
The invention is not limited to the embodiments shown. Several variants are possible within the scope of the dependent claims.