Abstract:
An AC-AC converter system includes transformer arrangements and HVDC converter units on primary and secondary sides of the system, respectively. The system exhibits first and second three-phase AC networks, and the converter units are interconnected via a DC connection. By integrating at least part of two transformer arrangements in one transformer unit, a cost efficient transformer configuration can be achieved.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to an AC-AC converter High Voltage Direct Current (HVDC) system and more particularly to an AC-AC converter system comprising transformer arrangements and converter units. 
       BACKGROUND OF THE INVENTION 
       [0002]    The advantages of HVDC components have been commercially exploited since 1954 when the first HVDC transmission was commissioned, the so-called HVDC Classic. Mercury-arc valves were eventually replaced with high power thyristors and DC transmissions have reached several GW, over +1-800 kV, and distances around 1000 kilometers. In 1997, a new breed of HVDC converter stations and HVDC transmissions were introduced, the so-called HVDC Light. 
         [0003]    HVDC converter bridges and lines or cables can be arranged into a number of configurations for effective utilization. In a Back-to-Back configuration two HVDC converters are connected more or less directly to each other on the DC side, with the purpose of e.g. interconnecting two asynchronous AC power networks, or to regulate the flow of power in an AC power network. Back-to-back DC links are used in for example Japan, Brazil and Argentina for interconnections between power system networks of different frequencies (50 and 60 Hz). 
         [0004]    The integral part of an HVDC power converter is the valve or valve arm. It may be non-controllable if constructed from one or more power diodes in series or controllable if constructed from one or more thyristors in series.  FIG. 1   a  schematically shows the electric circuit network for a conventional six-pulse converter unit  5 . The standard bridge or converter valve group  10  in HVDC Classic is defined as a double-way connection comprising six valves  20  or valve arms which are connected to one or more physical transformer units  30  as illustrated in  FIG. 1   a.  Electric power flowing between the HVDC valve group and the AC system is three phase. When electric power flows into the DC valve group from the AC system then it is considered a rectifier. If power flows from the DC valve group into the AC system, it is an inverter. Each valve consists of many series connected thyristors in thyristor modules.  FIG. 1   a  represents the electric circuit network depiction for the six pulse valve group configuration.  FIG. 1   b  is the graphical symbol of a  6  pulse converter unit. 
         [0005]    Today nearly all HVDC power converters with thyristor valves are assembled in a converter bridge of twelve pulse configuration.  FIG. 2   a  demonstrates a twelve pulse converter with two three phase converter transformers  31 ,  32  with one DC side winding as an ungrounded star connection  31  and the other a delta configuration  32 . Consequently the AC voltages applied to each six pulse valve group  10  which make up the twelve pulse valve group  40  have a phase difference of 30 degrees which is utilized to cancel the AC side 5th and 7th harmonic currents and DC side 6th harmonic voltage, thus resulting in a significant saving in harmonic filters.  FIG. 2   a  also shows the outline  50  around each of the three groups of four valves in a single vertical stack. These are known as “quadrivalves” and are assembled as one valve structure by stacking four valves in series. Since the voltage rating of thyristors is several kV, a 500 kV quadrivalve may have hundreds of individual thyristors connected in series groups of valve or thyristor modules.  FIG. 2   b  is the graphical symbol of a 12 pulse converter unit. 
         [0006]      FIG. 3  is a scheme over a conventional back-to-back 12 pulse AC-AC converter, comprising an AC inlet  50   a,  an AC outlet  50   b,  two 12-pulse HVDC converter units  40  which are assumed to be arranged in a back-to-back configuration. The converter units are controlled by a control unit (not shown). Each converter unit comprises two six-pulse valve groups in series according to  FIG. 2   a.  The inlet valve groups being connected to the AC inlet via separate inlet transformers  30   a,  and the outlet valve groups being connected to the AC outlet via separate outlet transformers  30   b.    
         [0007]    The converter transformers of a 12-pulse HVDC converter setup which together form a transformer arrangement can take different configurations, see  FIGS. 4   a - d.  The configurations shown therein comprise one single 12-pulse group, which is used as a basic building block for HVDC converter systems. The 12-pulse group can be applied from neutral to pole (single-pole configurations), with two 12-pulse groups to poles of different voltage polarity (bipolar configurations) or with the neutral point in the middle of the converter with mid-point grounding, leading to a six-pulse group between neutral and pole. The transformers used in the different configurations are three-phase three-winding transformers ( FIG. 4   a ), three-phase two-winding transformers ( FIG. 4   b ), single-phase three-winding transformers ( FIG. 4   c ), and single-phase two-winding transformers ( FIG. 4   d ). 
         [0008]    HVDC systems are expensive that there is a desire to be able to choose the most cost efficient HVDC transformer configuration for a given HVDC system topology, taking into consideration the need of spare HVDC transformers. 
       SUMMARY OF THE INVENTION 
       [0009]    An object of the present invention is to provide a cost efficient HVDC transformer configuration for a transmission system with a DC voltage range above 25 kV. 
         [0010]    The invention is based on the insight that the geographical closeness of transformers on the different sides in a back-to-back AC-AC converter system, such as transformers in an HVDC converter system, or other systems with a geographical closeness between different physical transformer units, creates the possibility of integrating different transformer functions into one single transformer unit. 
         [0011]    According to the invention, there is provided an AC-AC converter system as defined in the independent claim. Thus, there is provided an AC-AC converter system comprising at least one first transformer arrangement with a primary side connectable to a first three-phase AC network and with a secondary side, at least one first HVDC converter unit connected to the secondary side of the first transformer arrangement, at least one second transformer arrangement with a primary side connectable to a second three-phase AC network and with a secondary side, at least one second HVDC converter unit connected to the secondary side of the second transformer arrangement, wherein the first and second HVDC converter units are interconnected by means of a DC connection, the system being characterized in that at least part of two transformer arrangements are integrated in one transformer unit. 
         [0012]    In a preferred embodiment, the transformer unit comprises a two-phase transformer. The expression “two-phase transformer” should in this description be construed broadly and thus includes transformers with two phases from the same three-phase network as well as transformers with two phases from two different AC networks. By providing an AC-AC converter system with two-phase transformers, the total number of transformer units needed to perform the transformer function in an HVDC system is typically reduced, particularly when taking into consideration spare transformer units. 
         [0013]    The converter units are preferably 12-pulse converter units and each of the transformer arrangements is connected to one of the 12-pulse converter units. 
         [0014]    In one embodiment, the first and second HVDC converter units are interconnected back-to-back by means of the DC connection. This allows integration of transformer functions of the first and second AC networks. 
         [0015]    In yet a preferred embodiment, the two-phase transformer is a two-phase three-winding transformer. 
         [0016]    In one embodiment, part of two first transformer arrangements of the at least one first transformer arrangement are integrated in one transformer unit. This removes limitations in the equipment, such as maximum voltage and current of converter, transport limitations of transformers etc, in a monopolar or bipolar electrical energy transmission system. 
         [0017]    Other preferred embodiments are defined by the dependent claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The invention is now described, by way of example, with reference to the accompanying drawings, in which: 
           [0019]      FIG. 1   a  schematically shows the electric circuit network for a conventional six-pulse valve group converter unit. 
           [0020]      FIG. 1   b  shows the graphical symbol of a 6-pulse converter unit according to  FIG. 1   a.    
           [0021]      FIG. 2   a  schematically shows the electric circuit network for a conventional 12-pulse valve group converter unit. 
           [0022]      FIG. 2   b  shows the graphical symbol of a 12-pulse converter unit according to  FIG. 2   a.    
           [0023]      FIG. 3  is a scheme over a conventional back-to-back 12 pulse AC-AC converter system. 
           [0024]      FIGS. 4   a - 4   d  show different HVDC transformer arrangement configurations. 
           [0025]      FIG. 5  is a scheme of an AC-AC converter system according to the invention. 
           [0026]      FIG. 5   a  is a detailed scheme of a transformer function comprised in the AC-AC converter system of  FIG. 5 . 
           [0027]      FIGS. 6-9  show different embodiments of the implementation of integration of different transformer functions into one transformer unit. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    In the following, a detailed description of different embodiments of an AC-AC converter system according to the invention will be given. In this context, the expression “transformer arrangement” should be interpreted as a group of transformer connectable to the same AC network. Also, the expression “transformer unit” should be interpreted as a physical unit or tank in which one or more transformers are provided. “AC” will be used as an abbreviation for “Alternating current” and “DC” as an abbreviation for “Direct current”. Thus, the expression “DC connection” is an electrical connection between two locations wherein electrical energy is transferred by means of direct current. 
         [0029]      FIG. 5  is a scheme over a back-to-back 12 pulse AC-AC converter system according to the invention, generally designated  500 , using the general transformer configuration of  FIG. 4   c.  The system comprises an AC inlet  550   a,  an AC outlet  550   b,  and two 12-pulse HVDC converter units  540   a,    540   b  arranged in a back-to-back configuration. The converter units are controlled by a control unit (not shown). Each converter unit comprises two six-pulse valve groups in series according to  FIG. 2   a.    
         [0030]    The inlet converter unit  540   a  is connected to the AC inlet  550   a  via inlet transformer functions provided by a first transformer arrangement and the outlet converter unit  540   b  is connected to the AC outlet via outlet transformer functions provided by a second transformer arrangement. These transformer functions or arrangements correspond to the single-phase, three-winding configuration shown in  FIG. 4   c.  However, the geographical closeness of the transformer functions on each side of the back-to-back converter system  500  creates the possibility of integrating the transformer functions of the two different sides of the converter into one single transformer unit—a two-phase transformer (as the AC networks to which they are connected are separate from each other). An especially attractive configuration is two-phase three-winding transformers, as the number of transformer units needed to serve the transformer function is just three transformers. In  FIG. 5 , it is shown how the transformer functions are integrated into three two-phase three-winding transformers, designated  530   a,    530   b,  and  530   c.    
         [0031]    In one embodiment, the two-phase transformers are realized as two individual active parts, i.e., core and windings, in the same tank. Thus, two separate transformer cores are provided within the same transformer casing. Referring to  FIG. 5   a,  showing one single-phase, three-winding core enclosed by a circle, in one embodiment, two such single-phase, three-winding cores are co-located in one transformer tank. 
         [0032]    In an alternative embodiment, all windings are provided on one single physical transformer core on a core form transformer. In this case, part of the core is used as a return path for the magnetic flux. One example of a solution with a two-phase transformer on the same transformer core would be two single phase core designs each with one wound limb and two unwound return limbs for magnetic flux, being transformed into one single core design with two wound limbs, two unwound core limbs for return flux and an unwound center limb for return flux. The core type could be envisaged to be similar to a three-phase core with unwound outer core limbs without having windings on the center core limb. A single core design reduces labor needed to produce the core and the core and windings combined. 
         [0033]    Advantages of an AC-AC converter system according to the invention with three-phase, three-winding transformers as compared to a conventional system appear from the table below. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Transformer configuration 
               
             
          
           
               
                   
                   
                 3-phase 
                 2-phase 
               
               
                   
                   
                 3-winding 
                 3-winding 
               
               
                   
                   
               
             
          
           
               
                   
                 Number of active transformers 
                 2 
                 3 
               
               
                   
                 Number of spare transformers 
                 1-2 
                 1 
               
               
                   
                 Spare rated power as percentage  
                 50-100% 
                 33% 
               
               
                   
                 of active power 
                   
                   
               
               
                   
                 Total number of windings 
                 27-36 
                 24 
               
               
                   
                   
               
             
          
         
       
     
         [0034]      FIG. 6  is a scheme over an alternative back-to-back 12 pulse AC-AC converter system according to the invention, generally designated  600 , using the transformer configuration of  FIG. 4   d.  The system comprises an AC inlet  650   a,  an AC outlet  650   b,  and two 12-pulse HVDC converter units  640   a,    640   b  arranged in a back-to-back configuration. The converter units are controlled by a control unit (not shown). Each converter unit comprises two six-pulse valve groups in series according to  FIG. 2   a.    
         [0035]    The inlet converter unit  640   a  is connected to the AC inlet  650   a  via inlet transformer functions and the outlet converter unit  640   b  is connected to the AC outlet via outlet transformer functions. These transformer functions correspond to the single-phase, two-winding configuration shown in  FIG. 4   d.  Also in this embodiment, the geographical closeness of the transformer functions creates the possibility of integrating transformer functions of the two different sides of the converter into one single transformer unit. In  FIG. 6 , it is shown how the transformer functions of one single-phase, two-winding function on the two sides are integrated into one two-phase, two-winding transformer  630   a.  The other transformer functions are correspondingly integrated into two-phase two-winding transformer units, resulting in six transformer units  630   a - 630   f.    
         [0036]    Reference is now made to  FIG. 7 , wherein a bipolar system, generally designated  700 , is shown. The system comprises four 12-pulse converter units  740   a - d,  two provided on an AC inlet side 750a and two on an AC outlet side  750   b.  The two sides are interconnected by means of a neutral line  760 , a positive line  762  and a negative line  764 . This system is conventionally used for long distance transmission of electrical energy. This means that the transformer functions on the two sides: the AC inlet side and the AC outlet side, are not provided in geographical closeness to each other. However, the fact that the two converter units  740   a,    740   b  on the AC inlet side  750   a  and correspondingly the two converter units  740   c,    740   d  on the AC outlet side  750   b  are provided in geographical closeness to each other makes the inventive idea applicable also to this kind of system. 
         [0037]    In this embodiment the same transformer function of the two poles in a bipole are integrated into the same transformer unit. Thus, one example is that the transformer functions encircled in  FIG. 7  are integrated into same physical transformer unit  730   a.  The other transformer functions are integrated in the same way, resulting in a total of twelve transformer units: six on the AC inlet side  750   a  and six on the AC outlet side  750   b.  This removes limitations in the equipment, such as maximum voltage and current of converter, transport limitations of transformers etc. 
         [0038]    Reference is now made to  FIG. 8 , wherein a monopole system, generally designated  800 , is shown. The system comprises four 12-pulse converter units  840   a - d,  two provided on an AC inlet side  850   a  and two on an AC outlet side  850   b.  The two sides are interconnected by means of a positive line  860  and a neutral line  862 . This system is conventionally used for long distance transmission of electrical energy. 
         [0039]    The inlet converter units  840   a,    840   b  are connected to the AC inlet side  850   a  via separate inlet transformer functions and the outlet converter units  840   c,    840   d  are connected to the AC outlet side  850   b  via separate outlet transformer functions. The transformer configurations correspond to those shown in  FIG. 4   d.    
         [0040]    In this case, the transformer functions provided on one side, i.e., either the AC inlet side  850   a  or the AC outlet side  850   b,  are provided in the same geographical location and close to each other. 
         [0041]    In this embodiment the transformer integrates the same transformer function of the two poles in a bipole into the same transformer unit. Thus, one example is that the transformer functions encircled in  FIG. 8  are integrated into same physical unit  830   a.  The other transformer functions are integrated in the same way, resulting in a total of twelve transformer units: six on the AC inlet side  850   a  and six on the AC outlet side  750   b.  This could remove limitations in the equipment, such as maximum voltage and current of converter, transport limitations of transformers etc. 
         [0042]    The inventive idea is also applicable to converter units connected in parallel. One such example is shown in  FIG. 9 , wherein a monopole system, generally designated  900 , is shown. The system comprises three 12-pulse converter units  940   a - c,  two  940   a,    940   b  provided in parallel on an AC inlet side  950   a  and one  940   c  on an AC outlet side  950   b.  The two sides are interconnected by means of a positive line  960  and a neutral line  962 . This system is conventionally used for long distance transmission of electrical energy. 
         [0043]    The inlet converter units  940   a,    940   b  are connected to the AC inlet side  950   a  via separate inlet transformer functions and the outlet converter unit  940   c  is connected to the AC outlet side  950   b  via separate outlet transformer functions. The transformer configurations correspond to those shown in  FIG. 4   d.    
         [0044]    In this case, the transformer functions provided on one the AC inlet side  950   a  are provided in the same geographical location and close to each other. 
         [0045]    In this embodiment the transformer integrates the same transformer functions of the two parallel legs on the AC inlet side  950  into the same transformer unit. Thus, one example is that the transformer functions encircled in  FIG. 9  are integrated into same physical unit  930   a.  The other transformer functions are integrated in the same way, resulting in a total of six transformer units on the AC inlet side. This could remove limitations in the equipment, such as maximum voltage and current of converter, transport limitations of transformers etc. 
         [0046]    In the described embodiments of HVDC systems, LCC type converters used in HVDC Classic has been shown. It will be appreciated that the inventive idea is also applicable to HVDC Light, wherein VSC converters are used. 
         [0047]    Although the exemplary embodiments have made reference to HVDC applications, it will be realized that the inventive idea is applicable to any AC-AC converter system with converters arranged back-to-back. In this context, the term “back-to-back” should be interpreted as any configuration wherein parts of converters from the two sides are in the same geographical location and close to each other.