TECHNICAL FIELD 
       [0001]    The present invention concerns transfer of electric power. Especially the transfer of electric power comprises High Voltage Direct Current (HVDC) lines. In particular the invention concerns tapping of electric power from an HVDC transmission line. 
       BACKGROUND OF THE INVENTION 
       [0002]    HVDC is very cost efficient to transmit bulk power from distant power plants to consumers. One drawback has been the difficulty to connect loads along the line. With an ac-transmission system such connection of a load along the line may be brought about by the use of a transformer. For an HVDC transmission system such connection along the line requires a converter station having the same capacity as the receiving station. This capacity requirement would apply also for a small tap of power. Furthermore, disturbances in the ac net at the tap might, at least temporarily, interrupt the main HVDC transmission. This has in some cases led to a HVDC transmission alternative being excluded. 
         [0003]    From U.S. Pat. No. 5,187,651 (Ekstrom) a system for discharging electrical power from a HVDC line to a local network is previously known. The system comprises a first converter connected in series with the HVDC line. The first converter and a second converter constitute an ac network containing a first transformer. The second converter and a third converter constitutes a dc network for feeding a local ac network via a second transformer. 
         [0004]    From WO 9515605 (Asplund) a device for tapping electric power from a HVDC transmission line is previously known. The known system comprises a first converter connected in series with the HVDC transmission line. The first converter and a second converter located on ground potential constitute a high frequency ac network containing a first and second transformer and capacitor means for potential-separation. The second converter and a third converter constitutes a dc network for providing an ac current to a local network. 
       SUMMARY OF THE INVENTION 
       [0005]    A primary object of the present invention is to seek ways to improve the tapping of electric power from a HVDC transmission system. 
         [0006]    This object is achieved according to the invention by a control apparatus characterized by the features in the independent claim  1  or by a method characterized by the steps in the independent claim  3 . Embodiments of the invention are described in the dependent claims. 
         [0007]    A current source converter (CSC) and a voltage source converter (VSC) differ from each other in that the polarity of the dc side of a CSC changes with the transmission direction, while the polarity on the dc side of a VSC is the same irrespective of the transmission direction. An 
         [0008]    HVDC transmission line is often operating in one transmission direction only. This means that the polarity of the transmission lines is always the same. If, however, the transmission direction should change and the HVDC transmission line comprises current source converters, CSC, also the polarity of the transmission lines would change. This polarity change will also affect the operation of any tapping arrangement on the dc lines. 
         [0009]    According to the invention a tapping arrangement comprises an ac network containing VSC connected to the transmission line, and a switching arrangement to disconnect the ac network in case of polarity change on the dc transmission line. The use of VSC is very attractive since it offers full control over the reactive load as well as the possibility to change the direction of transmission. The switching arrangement comprises in an embodiment of the invention mechanically operated switches. In another embodiment the switching arrangement comprises semiconductors. In still a further embodiment the switching arrangement comprises a diode arrangement. These arrangements may preferably be used where there is a main direction of power transmission over the dc network. 
         [0010]    In yet another embodiment of the invention the switching arrangement also comprises switches for reconnecting the ac network to the dc transmission line with changed polarity. By these arrangements the ac network may be connected almost continuously irrespective of the power transfer direction on the HVDC transmission line. In a further embodiment of the invention the ac network comprises a first and a second converter connected between a first and second HVDC transmission line. In this embodiment the midpoint is connected to earth. In yet another embodiment the tapping arrangement comprises one VSC per phase of the local ac network to be provided with power from the dc transmission line. 
         [0011]    In a first aspect of the invention the object is achieved by an apparatus for tapping electric energy from an HVDC power transmission system comprising at least one voltage source converter (VSC), wherein the apparatus contains an intermediate ac network containing the VSC and a switching arrangement for disconnecting the intermediate ac network in dependence on the transmission direction of the HVDC power transmission system. In an embodiment of the invention the ac network comprises a plurality of VSC. In another embodiment the apparatus comprises a plurality of intermediate ac networks, each feeding power to one phase of a local ac network. 
         [0012]    In a second aspect of the invention the object is achieved by a method for tapping electric energy from an HVDC power transmission system by connecting an ac network containing VSC, wherein the method comprises providing a switching arrangement between the HVDC power transmission system and the ac network, sensing the transmission direction of the HVDC power transmission system, disconnecting the ac network from the HVDC power transmission system as a response of a changed transmission direction. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Other features and advantages of the present invention will become more apparent to a person skilled in the art from the following detailed description in conjunction with the appended drawings in which: 
           [0014]      FIG. 1  is a principal circuit of a tapping arrangement according the invention, 
           [0015]      FIG. 2  is another principal circuit of a tapping arrangement containing one VSC converter station, 
           [0016]      FIG. 3  is an embodiment of a tapping arrangement circuit according the invention, and 
           [0017]      FIG. 4  is a principal layout of a tapping arrangement according the invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0018]    An HVDC power transmission system with a tapping arrangement according to the invention is shown in  FIG. 1 . The HVDC power transmission system comprises a first converter station  1  with a first bridge is and a second bridge  1   b,  and a second converter station  2  with a first bridge  2   a  and a second bridge  2   b.  The first and second converter stations are connected to each other by a first transmission line  3  and a second transmission line  4 . In the embodiment shown the HVDC power transmission system comprises two transmission lines and a common grounding. The invention may, however, be applied on any configuration of an HVDC power transmission system. 
         [0019]    A tapping arrangement  5  is connectable between the first and second transmission line. The tapping arrangement comprises a first VSC containing converter bridge  6   a  and a second VSC containing bridge  6   b,  both having a common grounding point  7 . The tapping arrangement also comprises a first switching arrangement containing a first switch  8   a  and a second switch  8   b,  and a second switching arrangement containing a third switch  9   b  and a forth switch  9   a.  In an operation mode the tapping arrangement is connectable to the first transmission lines via a first conductor  3   a  or a third conductor  3   b,  and to the second transmission line via a forth conductor  4   a  or a second conductor  4   b.  For control of the switching arrangement and the converters the tapping arrangement comprises computer means capable of effectuating by a computer program the control of the tapping arrangement. The tapping arrangement also comprises memory means for storing computer programs and control parameters. 
         [0020]    In a first mode of operation a first branch is formed by the first conductor  3   a,  the first switch  8   a,  the first and second converter bridge, the second switch  8   b  and the forth conductor  4   b.  In a second mode of operation a second path is formed by the third conductor  3   b,  the third switch  9   b,  the first and second converter bridge, the forth switch  9   a  and the second conductor  4   b.    
         [0021]    Since the HVDC power transmission system comprising line commutating converters changes polarity with transmission direction any tapping system must follow the change of polarity. A tapping system containing voltage source converters always has the same polarity. Should the power direction be changed in the HVDC the VSC will have to be disconnected and reconnected with the opposite polarity. Most bulk power HVDC schemes never change transmission direction. Still Voltage Source Converters, VSC, has some interesting properties that makes it very attractive for tapping in HVDC transmissions. 
         [0022]    A less resource demanding option of tapping system is shown in  FIG. 2 . The VSC is connected to one pole only of a two pole HVDC power transmission system. The switching arrangement in the embodiment shown makes the VSC being reconnectable to the opposite pole when the power direction change. In this case there will float some ground current between the tap and the rectifier station. By using the same reference numbers as in  FIG. 1  the switching arrangement comprises only one switch  8 , a first conductor  3   a  fro connection to the first transmission line and a second conductor  4   b  for connection to the second transmission line. 
         [0023]    In a first mode of operation a first branch is formed by the first conductor  3   a,  the first switch  8 , the converter bridge and the ground connection  7 . In a second mode of operation a second path is formed by the second conductor  4   b,  the switch  8 , the converter bridge and the ground connection  7 . In the embodiment shown is also indicated an intermediate ac network  20  for tapping the power to a local ac network. 
         [0024]    A further configuration of a tapping arrangement is shown I  FIG. 3 . In this embodiment the tapping arrangement is connected between a transmission line of a HVDC power transmission system and ground. The tapping arrangement comprises a plurality of series connected ac networks containing voltage source converters  6   a,    6   b,    6   c,  one for each phase of a local ac network (not shown). Hence each phase tapping arrangement comprises a reactor  12 , and a first filter  13 , a transformer  14  and a second filter  15 . This tapping arrangement forms together an intermediate ac network  20 . The tapping arrangement is connected to the transmission line with a circuit breaker  10 . For the sake of clarity of the embodiment only the first phase tapping arrangement has reference numbers, since all three phase tapping arrangement are equally erected. 
         [0025]    In order to minimize the amount of expensive semiconductors the plurality of VSC operates with series connected phases. When tapping of power from an HVDC power transmission system problems will arise since the operation of the HVDC rectifier will force the voltage below zero in order to extinguish the dc current if there is a dc line fault. As the VSC has diodes in anti-parallel to the self-extinguishable semiconducting elements (IGBT), this will imply that the voltage at the tap cannot be brought below zero. In order to solve this problem each phase tapping arrangement comprises a diode  11 . 
         [0026]    The diodes are located one per phase and will prevent current to flow from the tap into the dc line when the dc line has low voltage. This will have the effect that faults in the ac net at the tap will have no other influence on the HVDC than that the inverter will take over the power in the tap. Voltage wise there is no interference. 
         [0027]    According to an embodiment of the invention the tapping arrangement is built as an outdoor station as shown in  FIG. 4 . All reference numbers are the same as in  FIG. 3 . 
         [0028]    The following advantages can be noted:
       Low cost by using only one string of semiconductors from pole to ground.   Low cost and simplified local work by using easily transportable outdoor equipment and no valve hall.   No serious interference from the tap to the HVDC possible.   VSC can operate against black ac networks or very weak ac networks and keep the ac voltage constant which is very important in remote areas.       
 
         [0033]    Although favorable the scope of the invention must not be limited by the embodiments presented but contain also embodiments obvious to a person skilled in the art.
An apparatus for tapping electric energy from an HVDC power transmission system includes at least one voltage source converter. The apparatus includes an intermediate ac network containing the voltage source converter, and a switching arrangement for disconnecting the intermediate ac network in dependence on the transmission direction of the HVDC power transmission system.