Abstract:
A rotating asynchronous converter for connection of AC network with equal or different frequencies employs a first stator connected to a first AC network with a first frequency and a second stator connected to a second AC network with a second frequency, and a rotor which rotates in response to the first and second frequencies. The converter has at least one winding formed of a cable, including a conductor and a magnetically permeable, electric field confining insulating covering surrounding the conductor.

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention relates to a rotating asynchronous converter. 
   The present invention also relates to a generator device. 
   BACKGROUND OF THE INVENTION 
   In a number of situations exchange of power must be performed between AC networks with different or at least not synchronous frequencies. The most frequent cases are the following:
         1. Connection of not synchronous three phase networks with equal rating frequencies, e.g. between eastern and western Europe.   2. Connection of three phase networks with different frequencies, most usually 50 Hz/60 Hz (e.g. Japan, Latin America).   3. Connection of a three phase network and a low frequency, one/two phase network for railway supply, in Europe 50 Hz/16.2/3 Hz, in USA 60 Hz/25 Hz.   4. The use of rotating asynchronous converters as a series compensation in long distance AC transmission.       

   Today, the connection is performed with the aid of power electronics and DC intermediate link. In the above mentioned cases 2 and 3 the connection can further be performed with the aid of matrix converters. In case of synchronous, but different frequencies in the above mentioned cases 2 and 3 the connection can further be performed with the aid of rotating converters comprising mechanically connected synchronous machines. 
   In the article, “Investigation and use of asynchronized machines in power systems”, Electric Technology USSR, No. 4, pp. 90-99, 1985, by N. I. Blotskii, there is disclosed an asynchronized machine used for interconnection of power systems, or their parts, which have different rated frequencies, or the same rated frequencies, but differing in the degree of accuracy with which it must be maintained. The structure of the asynchronized machine is disclosed in FIG.  1 . The asynchronized machine includes an electric machine  1  which is a machine with a conventional three-phase stator and either a non-salient-pole symmetrical rotor or a salient-pole or non-salient-pole electrically asymmetrical rotor, the phase leads being connected to slip rings; an exciter  2  which is a cycloconverter or reversing controlled rectifier, the cycloconverter supply  3  or  4 , a regulator  5  forming the control law required for the rotor ring voltages and the main machine rotor angle and speed  6 , voltage  7  and current  9  sensors of the stator and rotor. 
   In the article, “Performance Characteristics of a Wide Range Induction type Frequency Converter”, IEEMA Journal, Vol. 125, No. 9, pp. 21-34, Sep. 1995, by G. A. Ghoneem, there is disclosed an induction-type frequency converter as a variable frequency source for speed control drives of induction motors. In  FIG. 2  there is disclosed a schematic diagram of the induction-type frequency converter. The induction-type frequency converter consists of two mechanically and electrically coupled wound rotor induction machines A, B. The stator windings of one of them (A) are connected to 3-phase supply at line frequency (Vi, Fi), while the stator windings of the other machine (B) represent the variable frequency output (Vo, Fo). The rotor windings  10 ,  12  of the two machines are connected together with special arrangement. The converter is driven by a variable speed primemover  14 , a DC motor can be used. 
   Static converters have drawbacks such as relatively low efficiency (ca 95%) owing to the losses in the semiconductors, harmonics which have to be compensated with the aid of filters. The use of DC intermediate links leads to the use of special converter transformers with very complex design. The fillers are leading to a great need of space for the total assembly. Conventional rotating converters are not designed for high voltages, so a transformer is needed at each side for the connection to the AC network. The efficiency then becomes comparable to or even lower than the efficiency of a static converter. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to solve the above mentioned problems and to provide a rotating asynchronous converter for connection of AC networks with equal or different frequencies. This object is achieved by providing a rotating asynchronous converter. 
   Accordingly, the converter comprises a first stator connected to a first AC network with a first frequency f 1 , and a second stator connected to a second AC network with a second frequency f 2 . The converter also comprises a rotor means which rotates in dependence of the first and second frequencies f 1 , f 2 . At least one of the stators each comprise at least one winding, wherein each winding comprises at least one current-carrying conductor, and each winding comprises an insulation system, which comprises on the one hand at least two semiconducting layers, wherein each layer constitutes substantially an equipotential surface, and on the other hand between them is arranged a solid insulation. 
   According to another embodiment of the converter, it comprises a first stator connected to a first AC network with a first frequency f 1 , and a second stator connected to a second AC network with a second frequency f 2 . The converter also comprises a rotor means which rotates in dependence of said fist and second frequencies f 1 , f 2 . The stators each comprise at least one winding, wherein each winding comprises a cable comprising at least one current-carrying conductor, each conductor comprises a number of strands, around said conductor is arranged an inner semiconducting layer, around said inner semiconducting layer is arranged an insulating layer of solid insulation, and around said insulating layer is arranged an outer semi-conductor layer. 
   According to another embodiment of the converter, it comprises a first stator connected to a first AC network with a first frequency f 1 , and a second stator connected to a second AC network with a second frequency f 2 . The converter also comprises a rotor means which rotates in dependence of said first and second frequencies f 1 , f 2 . The stators each comprises at least one winding, wherein each winding comprises at least one correct-carrying conductor. Each winding also comprises an insulation system, which in respect of its thermal and electrical properties permits a voltage level in said rotating asynchronous converter exceeding 36 kV. 
   A very important advantage of the present invention is that it is possible to achieve a connection of two not synchronous networks without the further use of transformers or any other equipment. Another advantage is the high efficiency, which is expected to be 99%. 
   By designing the insulation system, which suitably is solid, so that it in thermal and electrical view is dimensioned for voltages exceeding 36 kV, the system can be connected to high voltage power networks without the use of intermediate step-down-transformers, whereby is achieved the above referenced advantages. Such a system is preferably, but not necessarily, designed in such a way that it comprises the features of the rotating asynchronous converter. 
   Another object of the invention is to solve the above mentioned problems and to provide a generator device with variable rotational speed. This object is achieved by providing a generator device. 
   Accordingly, the generator device comprises a stator connected to an AC network with a frequency f 2 , a first cylindrical rotor connected to a turbine, which rotates with a frequency f 1 . The generator device also comprises a rotor means which rotates in dependence of the frequencies f 1 , f 2 . The stator and the first cylindrical rotor each comprises at least one winding, wherein each winding comprises at least one current-carrying conductor, and each winding comprises an insulation system, which comprises on the one hand at least two semiconducting layers, wherein each layer constitutes substantially an equipotential surface, and on the other hand between them is arranged a solid insulation. 
   According to another embodiment of the generator device, it comprises a stator connected to an AC network with a frequency f 2 , and a first cylindrical rotor connected to a turbine, which rotates with a frequency f 1 . The generator device also comprises a rotor means which rotates in dependence of the frequencies f 1 , f 2 . The stator and the first cylindrical rotor each comprises at least one winding, wherein each winding comprises a cable comprising at least one current-carrying conductor, each conductor comprises a number of strands, around said conductor is arranged an inner semiconducting layer, around said inner semiconducting layer is arranged an insulating layer of solid insulation, and around said insulating layer is arranged an outer semiconducting layer. 
   The above mentioned and other preferable embodiments of the present invention are specified in the dependent claims. 
   In a certain aspect of the present invention it relates to the use of the invented asynchronous converter in specific applications such as those specified in claims  38 - 41 , in which applications the advantages of the invented device are particularly prominent. 
   Embodiments of the invention will now be described with a reference to the accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic diagram of an asynchronized machine used for interconnection of power system according to the state of the art; 
       FIG. 2  shows a schematic diagram of an induction-type frequency converter as a variable frequency source according to the state of the art; 
       FIG. 3  shows the parts included in the current modified standard cable; 
       FIG. 4  shows a first embodiment of a rotating asynchronous converter according to the present invention; 
       FIG. 5  shows a second embodiment of the rotating asynchronous converter according to the present invention; 
       FIG. 6  shows a first embodiment of a generator device according to the present invention; and 
       FIG. 7  shows a second embodiment of the generator device according to the present invention. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   A preferred embodiment of the improved cable is shown in FIG.  3 . The cable  20  is described in the figure as comprising a current-carrying conductor  22  which comprises both transposed non-insulated  22 A and insulated  22 B strands. There is an extruded inner semiconducting casing  24  which, in turn, is surrounded by an extruded insulation layer  26 . This layer is surrounded by an external semiconducting layer  28 . The cable used as a winding in the preferred embodiment has no metal shield and no external sheath. 
   Preferably, at least two of these layers, and most preferably all of them, has equal thermal expansion coefficients. Hereby is achieved the crucial advantage that in case of thermal motion in the winding, one avoids defects, cracks or the like. 
     FIG. 4  shows a first embodiment of a rotating asynchronous converter  30  according to the present invention. The rotating asynchronous converter  30  is used for connection of AC networks with equal or different frequencies. The converter  30  comprises a first stator  32  connected to a first AC network (not disclosed) with a first frequency f 1 , and a second stator  34  connected to a second AC network (not disclosed) with a second frequency f 2 . In the disclosed embodiment the stators  32 ,  34  are three phase stators  32 ,  34  comprising three windings each, wherein each winding comprises at least one current-carrying conductor, and each winding comprises an insulation system, which comprises on the one hand at least two semiconducting layers, wherein each layer constitutes substantially an equipotential surface, and on the other hand between them is arranged a solid insulation. The windings can also be formed of a cable of the type disclosed in FIG.  3 . The converter  30  also comprises a rotor means  36  which rotates in dependence of the first and second frequencies f 1 , f 2 . In the disclosed embodiment the rotor means  36  comprises two electrically and mechanically connected three phase rotors  36   1 ,  36   2 , which are concentrically arranged in respect of said stators  32 ,  34 . The converter  30  also comprises an auxiliary device  38  connected to said rotors  36   1 ,  36   2  for starting up of the rotors  36   1 ,  36   2  to a suitable rotation speed before connection of said converter  30  to said AC networks. Each rotor  36   1 ,  36   2  comprises a low voltage winding (not disclosed). When the first stator  32  is connected to a three phase AC network with the frequency f 1  and the second stator  34  is connected to a three phase AC network with the frequency f 2 , the rotors  36   1 ,  36   2  will rotate with the frequency (f 1 −f 2 )/2 and the stator current has the frequency (f 1 +f 2 )/2. The efficiency with such a converter will be very high (˜99%) for small frequency differences due to the fact that all power is transmitted as in a transformer. Assuming f 1 &lt;f 2 , a proportion 
           f   1     -     f   2         f   2           
of the power is transmitted mechanically and the remainder 
         f   1       f   2           
of the power is transmitted by transformer action. Mechanical power is only consumed to maintain the rotation.
 
   In  FIG. 5  there is disclosed a second embodiment of the rotating asynchronous converter  40  according to the present invention. The rotating asynchronous converter  40  is also used for connection of AC networks with equal or different frequencies. The converter  40  comprises a first stator  42  connected to a first AC network (not disclosed) with a first frequency f 1 , and a second stator  44  connected to a second AC network (not disclosed) with a second frequency f 2 . In the disclosed embodiment the stators  42 ,  44  are three phase stators  42 ,  44  comprising three windings each, wherein each winding can be of the type described in connection to FIG.  4 . The converter  40  also comprises a rotor means  46  which rotates in dependence of the first and second frequencies f 1 , f 2 . In the disclosed embodiment the rotor means  46  comprises only one rotor  46  concentrically arranged in respect of said stators  42 ,  44 . Said rotor  46  also comprises a first loop of wire  48  and a second loop of wire  50 , wherein said loops of wire  48 ,  50  are connected to each other and are arranged opposite each other on said rotor  46 . The loops of wire  48 ,  50  are also separated by two sectors  52   1 ,  52   2 , wherein each sector  52   1 ,  52   2  has an angular width of α. The converter  40  also comprises an auxiliary device (not disclosed) connected to said rotor  46  for starting up of the rotor  46  to a suitable rotational speed before connection of said converter  40  to said AC networks. To compensate for the frequency difference Δf, the rotor  46  only needs to rotate with the frequency 
           f   R     =         π   -   α     π     ⁢           ·           ⁢       Δ   ⁢           ⁢   f     4         ,       
 
wherein Δf=|f 1 −f 2 |. For α=π/4 this means 
           f   R     =       3   ⁢           ⁢   Δ   ⁢           ⁢   f     16       ,       
 
a very low rotational frequency. The main advantages with this embodiment are the low rotational frequency and the use of only one rotor.
 
   In  FIG. 6  there is disclosed a first embodiment of a generator device  60  with variable rotational speed according to the present invention. The generator device  60  comprises a stator  62  connected to an AC network (not disclosed) with a frequency f 2  and a first cylindrical rotor  64  connected to a turbine  66 , which rotates with a frequency f 1 . The generator device  60  comprises also a rotor means  68  which rotates in dependence of the frequencies f 1 , f 2 . The stator  62  and said first cylindrical rotor  64  each comprises at least one winding (not disclosed). Each winding comprises at least one current-carrying conductor, and each winding comprises an insulation system, which comprises on the one hand at least two semiconducting layers, wherein each layer constitutes substantially an equipotential surface, and on the other hand between them is arranged a solid insulation. Each winding can in another embodiment also comprise a cable of the type disclosed in FIG.  3 . The rotor means  68  comprises two electrically and mechanically connected rotors  68   1 ,  68   2 , which rotors  68   1 ,  68   2  are hollow and arranged concentrically around said stator  62  and said cylindrical rotor  64 . The stator  62  in the disclosed embodiment has a cylindrical shape. The rotors  68   1 ,  68   2  each comprises a low voltage winding (not disclosed) and they are rotating with the frequency (f 1 −f 2 )/2 when said generator device is in operation. The frequency of the rotor current will be (f 1 +f 2 )/2 when the generator device  60  is in operation. This generator device  60  is now disconnected from the power frequency and can be operated with the frequency as an optimizeable parameter. This generator device  60  will also give a better efficiency and power matching than a conventional generator. 
   In  FIG. 7  there is disclosed a second embodiment of the generator device  70  according to the present invention. The generator device  70  comprises a stator  72  connected to an AC network (not disclosed) with a frequency f 2  and a first cylindrical rotor  74  connected to a turbine  76 , which rotates with a frequency f 1 . The generator device  70  also comprises a rotor means  78  which rotates in dependence of the frequencies f 1 , f 2 . The stator  72  and said first cylindrical rotor  74  each comprises at least one winding (not disclosed). The winding can be of the types which were mentioned in the description in connection to FIG.  6 . The rotor means  78  comprises a first rotor  78   1  and a second rotor  78   2 , which rotors  78   1 ,  78   2  are electrically and mechanically connected to each other. The first rotor  78   1  is hollow and arranged concentrically around said first cylindrical rotor  74  and said second rotor  78   2  is cylindrical and surrounded by the stator  72 . The first and second rotors  78   1 ,  78   2  of said rotor means  78  each comprises a low voltage winding and said rotors  78   1 ,  78   2  are rotating with the frequency (f 1 −f 2 )/2 when said generator device  70  is in operation. The stator  72  is hollow and arranged around said second rotor  78   2 . This generator device  70  works in the same way and has the same advantages as the generator device  60  disclosed in FIG.  6 . 
   The disclosed embodiments only show connection of three phase networks, but the invention is also applicable for connection of a three phase network, wherein one stator has a one/two phase application. The invention can also be used for connection of a three phase network and a one/two phase network, wherein one stator having a three phase application is connected via a Scott-connection or another symmetrical connection to a one/two phase network. The invention is also applicable to more than two stators and rotor parts to connect more than two AC networks. The only condition is that only two not synchronous networks are connected. 
   The invention is not limited to the embodiments described in the foregoing. It will be obvious that many different modifications are possible within the scope of the following claims.