Background Synchronous compensators have traditionally been designed with horison- tal shafts and for voltages not exceeding 25 kV. Such machines are fixed to the floor of the machine room to avoid displacements, mainly due to forces in case of short-circuits, which can be considerable.

By designing the synchronous compensator as a rotating electric machine comprising at least one winding with solid insulation enclosing the electric field, such a machine can be made for such high voltages, up to 800 kV, that it can be directly connected to any mains. Examples of synchronous compensators of this type are described in WO 97/45922 and WO 98/34312 as well in the simultane- ously filed Swedish patent application SE 0001272-4. As is explained in these publications, such machines provide a number of advantages. They may be made with considerable size and weight. As an example it could be mentioned that in a synchronous compensator for 300 kV and 200 MVAr the weight of the stator is typically 450 tons and the weight of the rotor typically 200 tons. The diameter of the stator is typically 6300 mm and its length 4000 mm. This leads to a new situa- tion concerning mechanical design and transport. For a conventional synchronous compensator for 200 MVAr and about 20 kV, its stator and rotor would typically be built in a factory and thereafter be transported as completed units to the place of installation. Weights and sizes, which would be typical for a machine according to the invention, would make that difficult or impossible, as shown by the example mentioned above.

The aim of the present invention is to solve those problems for large syn- chronous compensators of the type described.

Disclosure of the invention This said aim is obtained with a synchronous compensator of the kind stated above, with the characterizing features of claim 1.

On a synchronous compensator according to the invention, the gravity forces will not have any radial component on the rotor. This in turn allows the rotor to be designed without any kind of through shaft, which is a considerable simplifi- cation of the machine design. The machine can then be made totally shorter, and machine components such as rotor shaft, hub and rotor center can be omitted.

Fewer components also means advantages in the maintenance of the machine.

Making the rotor completely without a shaft also simplifies supply and outlet of cooling medium, such as coling water, at the center of one end of the rotor, and distribution of it is simplified by utilizing a central room extending along the central axis of the rotor. On a horisontal rotor, the rotor shaft protrudes at each end to make room for bearings, as said above. At a synchronous compensator according to the invention, there is no such need, thus allowing the machine to be made shorter in its axial direction.

In one advantageous embodiment of a synchronous compensator accord- ing to the invention, the bearing integrated with the rotor body is a supporting roller bearing, minimizing the losses in the bearing.

In another advantageous embodiment of a synchronous compensator according to the invention, the bearing integrated with the rotor body is located within the axial extent of the rotor, which allows a design with small space re- quiremeht. still another advantageous embodiment of a synchronous compensator according to the invention, only a simple guide bearing is needed at the upper end of the rotor, since this bearing does not carry any part of the rotor weight, but only serves to keep the orientation of the rotor. If the synchronous compensator is en- closed in a room, the guide bearing can preferentially be fixed to the roof of the room.

A synchronous compensator according to the invention can suitably be lo- cated in a room, where the stator and frame of the machinery are designed to form

as integrated parts of the room. The room is advantageously adapted to the exter- nal shape of the stator, which is then supported and fixed by its frame to the walls of the room. In this way, the machine can be integrated with the surrounding room or building, preferably excavated in rock, to let the building serve as part of the machinery and let the stator frame be affixed to the wall around its circumference.

The resulting very solid fixation of the stator is an important advantage, especially since very large forces may appear at short circuits in machines of the actual very large sizes.

According to still another advantageous further development the machine can be designed for location in a building above ground that cannot withstand forces, by reinforcing the frame and the fastening of the upper guide bearing being fixed to the frame.

According to still another advantageous embodiment of the synchronous compensator according to the invention, the excitation means of the rotor is lo- cated at the upper end of the rotor. Possible other control equipment may also ad- vantageously be located at the upper end of the rotor.

According to still another advantageous embodiment of the synchronous compensator according to the invention, the winding is made from a flexible cable comprising an electrically conducting core surrounded by an insulating system with two semiconductive layers and an intermediate solid insulation. By this means, the electric field will be substantially enclosed within the winding. The potential on the outer semiconducting layer can then be held at ground potential, with obvious ad- vantages resulting therefrom.

Description of figures For further explanation of the invention, one embodiment of the synchro- nous compensator according to the invention chosen as an example, will now be described in greater detail with reference to the following drawings, on which Figure 1 shows a cross section of a part of a building enclosing a synchro- nous compensator according to the invention, Figure 2 shows a horisontal cross section of the building of figure 1, with stator and rotor,

w Figure 3 and 4 show views similar to that of figure 1, of two alternative embodi- ments of the rotor and its bearing for a synchronous compensator according to the invention, and Figure 5 shows a cross section of the high voltage cable used for the wind- ing of the machine.

Description of embodiments Figure 1 shows part of a building, preferably excavated in rock 2, in which a synchronous compensator according to the invention is enclosed.

The stator 6 of the machine is assembled at site, simplifying transport, since the complete machines in question are very large as mentioned above. Dur- ing assembly of the plates of the stator 6 the roof 4 of the building is preferably removed, permitting successive piling of stator plates from above. No turning over of the stator is needed, as in case of assembling the stator of horizontal machines.

The stator winding is formed of high voltage cable, illustrated at 29 in fig- ure 1. In this way the synchronous compensator according to the invention can be directly connected to mains of voltage up to 800 kV, and the synchronous com- pensator according to the invention can be constructed for powers in the range 10 -1000 MVAr. Details of the cable design will be further described in the context of figure 5.

After assembly of the stator 6 at site in the building, the rotor 8 is lowered axially from above into the stator 6 The rotor 8, which lacks a continuous through shaft and is provided with a cylindrical room extending along its center line, is mounted in a bearing 10 at its lower end, said bearing being integrated with the ro- tor body and located directly on the building foundation to support the rotor.

The bearing 10 is preferably an axial roller bearing. With this kind of bear- ing, the mechanical losses in the bearing will be very low, much lower than what is possible for horizontal machines.

The wall 14 of the building is advantageously adapted to the external shape of the stator 6. The stator 6 is mounted in an outer frame 18, which in turn is fixed to the wall 14 of the building by means of props or braces 20 located around the outer periphery of the frame 18 as shown in figure 2. A strong fastening of the stator 6 along its periphery is of great importance for withstanding the considerable forces, which can occur at this type of machines.

To simplify transport the frame 18 is divided into three parts 24, the as- sembling of which is schematically illustrated at 26.

At a synchronous compensator according to the invention supply and out- let of the cooling water to and from the rotor are simplified, and occur preferentially at the bearing 10 at the lower end of the rotor, since there is no rotor shaft. The water cooling systems of the rotor 8 and stator 6 are indicated at 28 in figure 1.

One type of water supply system which could be used on a synchronous compen- sator according to the invention is described in patent US, A, 3 652 880.

Slip rings 22 for static excitation of the machine and control equipment are advantageously located at the upper end of the rotor 8. The machine can also be arranged for brushless excitation by rectifiers. To get the required space for such equipment, the roof of the building is slightly raised as shown with broken lines in figure 1.

In figures 3 and 4 are shown views similar to the one in figure 1 of two al- ternative embodiments of the rotor with its bearing and similar components, having the same reference numbers as corresponding parts in figure 1. In figure 3 is thus shown a vertical machine with the rotor 8 located inside the stator 6. A suitable bearing 7 is shown, such as a roller bearing, integrated with the rotor body and lo- cated on a suitable foundation 9 to support the rotor 8. In the embodiment shown in figure 3 the bearing 7 is located with its attachment within the axial extent of the rotor 8, which is advantageous to reduce the space requirement.

In figure 4 is shown a similar embodiment. In this embodiment the rotor 8 and bearing 11 are so designed that the bearing is located below the rotor 8. In certain applications it is desired to design the rotor with as a large weight as pos- sible to let it serve as a flywheel. The rotor design shown in figure 4 is advanta- geous in such cases.

The cable 46 which constitutes the windings of the machine is shown in cross section in figure 5 and is a high voltage cable of substantially the same type as used for power distribution, i. e. XLPE-cable. The high voltage cable 46 com- prises a conductive core of a number of strands 36. The conductor is surrounded by an insulating system with two semiconductive layers 32, 34 located on each side of a solid insulate 33. The cable is flexible and the semiconductive layers 32, 34 will be substantially equipotential surfaces, which means that the electric field will be enclosed and the external surface of the cable substantially at ground

potential, a very important advantage when the cable forms a winding on the iron core of an electric machine. The solid insulation 33 and its surrounding semicon- ductive layers 32, 34 are made with an electric insulating resistance exceeding 3kV/mm, preferably exceeding 5kV/mm. The cable will in this way be well suited for use in a stator core for high voltages, while controlling the electric field and avoiding the risk of destructive electric discharges, PD.