Abstract:
Disclosed are a submodule structure formed of an energy storage element, a first turn-off device, a second turn-off device, a third turn-off device, a freewheeling diode, a series resistor, and diodes respectively in antiparallel connection with the turn-off devices, and a converter completely or partially formed of the submodules. Also disclosed are a relevant protection unit and a control method for the converter. The converter can be locked when a direct current (DC) fault occurs to prevent an alternating current (AC) system from injecting a fault current into a DC network, so that a transient fault of the DC network can be removed without tripping an AC line switch, thereby rapidly restarting the system. A charging resistor is comprised in the submodule so that a charging resistor disposed at an AC side of the converter can be reduced and even may not be disposed.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to the field of power and electronics, and in particular, to a submodule, a protection unit, and a voltage source in multilevel convertor and a control method thereof. 
         [0003]    2. Description of Related Art 
         [0004]    A modularized multilevel converter is a new converter applicable to high voltage applications and attracting much attention in recent years. In the modularized multilevel converter, submodules are cascaded, where the state of each submodule is separately controlled to enable an alternating voltage outputted by the converter to approach a sine wave, thereby reducing a harmonic content in the output voltage. The modularized multilevel converter solves the series average-voltage problem existing in a two-level voltage source converter and has wide application prospects. 
         [0005]    In the “distributed energy stores and converter circuit” of Marquardt Rainer, a modularized multilevel converter (MMC) was first mentioned (patent application publication No.: DE10103031A), where a submodule of the converter is formed of a half-bridge and a capacitor connected in parallel and two levels, a capacitor voltage and a 0 voltage, can be generated through control at an output port of the submodule. In 2010, the Trans Bay project, a flexible direct current (DC) transmission project first adopting this topological structure all over the world and undertaken by the Siemens corporation was successfully put into operation, which proves the feasibility of engineering applications of the topological structure of this converter. 
         [0006]    On the basis of the topological structure of the modularized multilevel converter. the ABB corporation has modified the structure and proposed a cascade two-level modularized multilevel topological structure (patent application publication No.: US20100328977A1), where this converter differs from the foregoing modularized multilevel converter that connection of the submodules is reversed. 
         [0007]    The disadvantages of the two modularized multilevel converters are that, when a fault occurs in a DC network, an alternating current (AC) network can provide a fault current to a fault point through a diode of the submodule, resulting in over-currents at AC and DC sides and at a converter valve, so the DC fault must be removed by tripping an line switch. When a transient fault occurs in the DC network, AC line switches need to be tripped for all of the foregoing two modularized multilevel converters connected to the DC network, so that it takes a long time to restore electricity transmission. 
       SUMMARY OF THE INVENTION 
     Technical Problem 
       [0008]    The objectives of the present invention are to provide a submodule, where a converter can be locked when a DC fault occurs to prevent an AC system from injecting a fault current into a DC network, so that a transient fault of the DC network can be removed without tripping an AC line switch, thereby rapidly restarting the system. In addition, further provided are a protection unit, a converter corresponding to the submodule, and a control method. 
       Technical Solution 
       [0009]    In order to achieve the above objectives, the present invention adopts the following technical solutions: 
       Advantageous Effect 
       [0010]    Through the above technical solutions, the beneficial effects of the present invention are as follows: 
         [0011]    (1) when a fault occurs in a DC network, the converter is locked to prevent an AC network from injecting a fault current into a fault point; 
         [0012]    (2) when a transient fault occurs at a DC side, the fault is removed without tripping an AC line switch; and 
         [0013]    (3) no DC breaker is required for a two-terminal or multi-terminal DC system formed of the converter provided by the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a topological structure diagram of an embodiment of a submodule of the present invention. 
           [0015]      FIG. 2  is a topological structure diagram of an embodiment of a submodule of the present invention. 
           [0016]      FIG. 3  is a topological structure diagram of an embodiment of a submodule of the present invention. 
           [0017]      FIG. 4  is a topological structure diagram of an embodiment of a submodule of the present invention. 
           [0018]      FIG. 5  is a topological structure diagram of a converter completely formed of submodules provided by the present invention. 
           [0019]      FIG. 6  is two topological structure diagrams of an additional submodule in the present invention. 
           [0020]      FIG. 7  is a topological structure diagram of a converter partially formed of submodules provided by the present invention. 
           [0021]      FIG. 8  is a schematic diagram of an embodiment of a control method for the converter of the present invention. 
           [0022]      FIG. 9  is a schematic diagram of an embodiment of a control method for the converter of the present invention. 
           [0023]      FIG. 10  is four topological structure diagrams of a protection unit for a submodule in the present invention. 
           [0024]      FIG. 11  is a schematic diagram of a connection manner of a protection unit for a submodule in the present invention and the submodule. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    The technical solutions of the present invention are described in detail below in combination with accompanying drawings and specific embodiments. 
         [0026]      FIG. 1  to  FIG. 4  are topological structure diagrams of preferred embodiments of a submodule provided by the present invention.  FIG. 1  and  FIG. 2  show a situation where no resistor is contained in the freewheeling diode branch.  FIG. 3  and  FIG. 4  show a situation where a resistor is contained in the freewheeling diode branch. 
         [0027]    As shown in  FIG. 1  and  FIG. 2 , the submodule comprises turn-off devices  1 ,  3 ,  5  in antiparallel connection with diodes and an energy storage element  8 , where the turn-off device  1  is in antiparallel connection with the diode  2 , the turn-off device  3  is in antiparallel connection with the diode  4 , and the turn-off device  5  is in antiparallel connection with the diode  6 . Each of the turn-off devices  1 ,  3 ,  5  may be a single controlled switch device (for example, a fully controlled device such as an IGBT, an IGCT, a MOSFET or a GTO, where in the embodiments provided herein, the IGBT is taken as an example) and may also be of a structure formed of at least two controlled switch devices connected in series. 
         [0028]      FIG. 1  shows a submodule  10 . An emitter of the turn-off device  1  is connected to a collector of the turn-off device  3 , with the connection point being used as a terminal X 1  of the submodule  10 . A collector of the turn-off device  1  is connected to an emitter of the turn-off device  3  through the energy storage element  8 . The collector of the turn-off device  1  is also connected to a cathode of a diode  7 . An anode of the diode  7  is connected to a collector of the turn-off device  5 , with the connection point being used as a terminal X 2  of the submodule  10 . An emitter of the turn-off device  5  is connected to the emitter of the turn-off device  3 . 
         [0029]      FIG. 2  shows a submodule  11 . An emitter of a turn-off device  5  is connected to a cathode of a diode  7 , with the connection point being used as a terminal X 1  of the submodule  11 . A collector of the turn-off device  5  is connected to an anode of the diode  7  through the energy storage element  8 . The collector of the turn-off device  5  is also connected to a collector of the turn-off device  3 . An emitter of the turn-off device  3  is connected to a collector of the turn-off device  1 , with the connection point being used as a terminal X 2  of the submodule  11 . An emitter of the turn-off device  1  is connected to the anode of the diode  7 . 
         [0030]    As shown in  FIG. 3  and  FIG. 4 , the submodule comprises turn-off devices  1 ,  3 ,  5  in antiparallel connection with diodes and an energy storage element C, where the turn-off device  1  is in antiparallel connection with the diode  2 , the turn-off device  3  is in antiparallel connection with the diode  4 , and the turn-off device  5  is in antiparallel connection with the diode  6 . Each of the turn-off devices  1 ,  3 ,  5  may be a single controlled switch device (for example, a fully controlled device such as an IGBT, an IGCT, a MOSFET or a GTO, where in the embodiments provided herein, the IGBT is taken as an example) and may also be of a structure formed of at least two controlled switch devices connected in series. 
         [0031]      FIG. 3  shows a submodule  10 ′. A collector of the turn-off device  1  is connected to an emitter of the turn-off device  3 , with the connection point being used as a terminal X 1  of the submodule  10 ′. An emitter of the turn-off device  1  is connected to a collector of the turn-off device  3  through the energy storage element C. The collector of the turn-off device  1  is also connected to a series resistor R and the other end of the series resistor is connected to a cathode of a diode  7 . An anode of the diode  7  is connected to a collector of the turn-off device  5 , with the connection point being used as a terminal X 2  of the submodule  10 . The collector of the turn-off device  5  is connected to the collector of the turn-off device  3 . Locations of the series resistor R and the diode  7  can be exchanged as long as it can be ensured that the anode of the diode  7  is connected to the terminal X 2  directly or through the series resistor R. 
         [0032]      FIG. 4  shows a submodule  11 ′, which is obtained by changing the topological structure of the submodule shown in  FIG. 3  in the following manner: locations of the terminal X 1  and the terminal X 2  in are exchanged, locations of the collector and the emitter of each turn-off device are exchanged, and locations of the anode and the cathode of each diode are exchanged. The collector of the turn-off device  5  is connected to the cathode of the diode  7 , with the connection point being used as a terminal X 1  of the submodule  11 . The emitter of the turn-off device  5  is connected to one end of the series resistor R through the energy storage element C and the other end of the series resistor R is connected to the anode of the diode  7 . The collector of the turn-off device  5  is also connected to the collector of the turn-off device  3 . The emitter of the turn-off device  3  is connected to the collector of the turn-off device  1 , with the connection point being used as a terminal X 2  of the submodule  11 . The collector of the turn-off device  1  is connected to the one end of the series resistor R. Locations of the series resistor R and the diode  7  can be exchanged as long as it can be ensured that the cathode of the diode  7  is connected to the terminal X 1  directly or through the series resistor R. 
         [0033]    It should be noted that, only equivalent elements for the turn-off devices, the resistor, and the freewheeling diode are described in the embodiments of the present invention. That is to say, the turn-off devices, the resistor, and the freewheeling diode can each be formed by cascading multiple elements. For example, an equivalent resistor may be formed of multiple resistors connected in series or in parallel, an equivalent freewheeling diode ma be formed of multiple freewheeling diodes connected in series or in parallel, and so on. 
         [0034]    It should be noted that, in the embodiments described in  FIG. 3  and  FIG. 4 , the series resistor is an equivalent representation, that is, the locations and the number of resistors and freewheeling diodes are not limited and the resistors and the freewheeling diodes can be arranged alternately. 
         [0035]      FIG. 5  shows a preferred embodiment of a converter of the present invention. Each submodule in the converter is one provided by the present invention. The converter comprises at least one phase unit. The specific number of phase units can be determined according to the number of AC terminals of an AC system. Each of the phase units comprises an upper bridge arm  100  and a lower bridge arm  101 . Each of the upper bridge arm and the lower bridge arm comprises at least two submodules  10  and at least one reactor  20  connected to each other in series. The number of submodules and reactors comprised in the upper bridge arm may be the same as or different from the number of submodules and reactors comprised in the lower bridge arm. Each submodule  10  has two terminals X 1  and X 2 . All of the submodules  10  in the same bridge arm (the upper bridge arm or the lower bridge arm) are connected in the same direction and connection directions of the submodules in the upper bridge arm and the lower bridge arm are opposite to each other, as shown in  FIG. 3 . One end of the upper bridge arm  100  is used as a first DC terminal P of the phase unit to be connected to a DC network. One end of the lower bridge arm  101  is used as a second DC terminal N of the phase unit to be connected to the DC network. The other ends of the upper bridge arm  100  and the lower bridge arm  101  are jointly used as an AC terminal A of the phase unit to be connected to an AC network. It should be noted that, for the upper bridge arm  100  or the lower bridge arm  101 , a series location of the submodules  10  and the reactors  20  is not limited and because one reactor can be formed of multiple reactors connected in series, the number of reactors is not limited as long as a total reactance value in a certain bridge arm meets a requirement corresponding to the bridge arm. 
         [0036]    It should be noted that, the submodule  10  in  FIG. 3  may also be replaced with any one of the four submodules provided above. 
         [0037]      FIG. 6  is two topological structure diagrams of an additional submodule in the present invention. The cost of the converter can be reduced by replacing the submodules in the converter shown in  FIG. 5  with the additional submodule. The additional submodule comprises turn-off devices  1 ,  3  in antiparallel connection with diodes and an energy storage element C, where the turn-off device  1  is in antiparallel connection with the diode  2  and the turn-off device  3  is in antiparallel connection with the diode  4 . Each of the turn-off devices  1 ,  3  may be a single controlled switch device (for example, a fully controlled device such as an IGBT, an IGCT, a MOSFET or a GTO, where in the embodiments provided herein, the IGBT is taken as an example) and may also be of a structure formed of at least two controlled switch devices connected in series.  FIG. 6( a )  shows a submodule  12 . A collector of the turn-off device  1  is connected to an emitter of the turn-off device  3 , with the connection point being used as a terminal X 1  of the submodule  12 . An emitter of the turn-off device  1  is connected to a collector of the turn-off device  3  through the energy storage element C. The collector of the turn-off device  3  is used as a terminal X 2  of the submodule  12 .  FIG. 6( b )  shows a submodule  13 . A collector of the turn-off device  3  is connected to an emitter of the turn-off device  1 , with the connection point being used as a terminal X 2  of the submodule  13 . An emitter of the turn-off device  1  is connected to a collector of the turn-off device  3  through the energy storage element C. The collector of the turn-off device  3  is used as a terminal X 1  of the submodule  12 . 
         [0038]      FIG. 7  shows a preferred embodiment of a converter of the present invention, where one of the submodules in the lower bridge arm of the converter shown in  FIG. 5  is replaced with the submodule  13 . The number of turn-off devices is reduced, thereby saving the cost of the converter. It should be noted that, the converter obtained after replacement should comprise at least one submodule provided by the present invention, and then any number of submodules of the present invention at any location in the converter shown in  FIG. 5  can be replaced with the additional submodule. 
         [0039]    The present invention further provides a control method for the converter as described above, where the converter is controlled by controlling an operation state of each submodule in the converter. The control content of the control method is described below by taking the submodules  10 ,  11  provided in  FIG. 1  and  FIG. 2  of the present invention as examples. The control methods for the converters formed by the submodules  10 ′,  11 ′ in  FIG. 3  and  FIG. 4  are similar and are not described again. 
         [0040]      FIG. 8( a )  and  FIG. 8( d )  are schematic diagrams of two current directions in a state  1  respectively,  FIG. 8( b )  and  FIG. 8( e )  are schematic diagrams of two current directions in a state  2  respectively, and  FIG. 8( c )  and  FIG. 8( f )  are schematic diagrams of two current directions in a state  3  respectively. 
         [0041]    The submodule  10  is controlled to operate in the three operation states. In the state  1 , the turn-off devices  1 ,  5  are turned on, the turn-off device  3  is turned off, and the energy storage element C is connected to the bridge arm through the diode  2  and the diode  6  (see  FIG. 8( a ) ) or the energy storage element C is connected to the bridge arm through the turn-off devices  5 ,  1  (see  FIG. 8( d ) ), so that an output voltage (that is, a voltage of the terminal X 1  relative to terminal X 2 ) of the submodule  10  is a voltage across the energy storage element C. In the state  2 , the turn-off devices  3 ,  5  are turned on and the turn-off device  1  is turned off, so that a current can flow through the turn-off device  3  and the diode  6  (see  FIG. 8( b ) ) or the turn-off device  5  and the diode  4  (see  FIG. 8( e ) ), the energy storage element C is bypassed, and an output voltage of the submodule  10  is 0. In the state  3 , the turn-off devices  1 ,  3 ,  5  are all turned off, so that when a current flows from the terminal X 1  to the terminal X 2 , the diode  2  and the diode  6  are turned on, the energy storage element C is connected to the bridge arm through the terminal X 1  and the terminal X 2 , and an output voltage of the submodule  10  is a voltage across the energy storage element C (see  FIG. 8( c ) ); and when a current flows from the terminal X 2  to the terminal X 1 , the diode  7  and the diode  4  are turned on, the energy storage element C is reversely connected to the bridge arm through the terminal X 1  and the terminal X 2  (see  FIG. 8( f ) ), and an output voltage of the submodule  10  is a negative number of a voltage across the energy storage element C plus a voltage across the resistor. When the submodule operates in the state  3 , the output voltage of the submodule  10  and the current flowing in the submodule  10  are in the opposite directions, so a fault current can be restrained and is eventually 0. The addition of the series resistor R accelerates the attenuation of the fault current. 
         [0042]      FIG. 9( a )  and  FIG. 9( d )  are schematic diagrams of two current directions in a state  1  respectively,  FIG. 9( b )  and  FIG. 9( e )  are schematic diagrams of two current directions in a state  2  respectively, and  FIG. 9( c )  and  FIG. 9( f )  are schematic diagrams of two current directions in a state  3  respectively. 
         [0043]    The submodule  1  is controlled to operate in the three operation states. In the state  1 , the turn-off devices  1 ,  5  are turned on, the turn-off device  3  is turned off, and the energy storage element C is connected to the bridge arm through the diode  6  and the diode  2  (see  FIG. 9( a ) ) or the energy storage element C is connected to the bridge arm through the turn-off devices  1 ,  5  (see  FIG. 9( d ) ), so that an output voltage (that is, a voltage of the terminal X 1  relative to terminal X 2 ) of the submodule  11  is a voltage across the energy storage element C. In the state  2 , the turn-off devices  3 ,  5  are turned on and the turn-off device  1  is turned off, so that a current can flow through the diode  6  and the turn-off device  3  (see  FIG. 9( b ) ) or the diode  4  and the turn-off device  5  (see  FIG. 9( e ) ), the energy storage element C is bypassed, and an output voltage of the submodule  11  is 0. In the state  3 , the turn-off devices  1 ,  3 ,  5  are all turned off, so that when a current flows from the terminal X 1  to the terminal X 2 , the diode  6  and the diode  2  are turned on, the energy storage element C is connected to the bridge arm through the terminal X 1  and the terminal X 2 , and an output voltage of the submodule  11  is a voltage across the energy storage element C (see  FIG. 9( c ) ); and when a current flows from the terminal X 2  to the terminal X 1 , the diode  4  and the diode  7  are turned on, the energy storage element C is reversely connected to the bridge arm through the terminal X 1  and the terminal X 2  (see  FIG. 9( f ) ), and an output voltage of the submodule  11  is a negative number of a voltage across the energy storage element C plus a voltage across the resistor. When the submodule operates in the state  3 , the output voltage of the submodule  11  and the current flowing in the submodule  11  are in the opposite directions, so a fault current can be restrained and is eventually 0. The addition of the series resistor R accelerates the attenuation of the fault current. 
         [0044]    When a ground fault occurs in the DC network, the converter is locked so that the submodules  10  or  11  and possibly disposed additional submodule  12 ,  13  in the converter all operate in the state  3 , thereby restraining the current of a bridge arm on the failure and eventually reducing it to 0. As a result, the AC network cannot provide a fault current to a fault point. When a transient fault occurs at the DC side, the fault can be removed without tripping an AC line switch, and a two-terminal or multi-terminal DC system formed of the converter provided by the present invention can have good ability of removing the fault at the DC side without a DC breaker. 
         [0045]    In addition, the present invention further provides a protection unit. The protection unit may be used in the submodule provided by the present invention and may also be used for protecting other types of full-bridge or half-bridge submodules. The protection unit may be of four structures.  FIG. 10( a )  shows a protection unit formed of a single thyristor.  FIG. 10( b )  shows a protection unit formed of a single high-speed switch.  FIG. 10( c )  shows a protection unit formed of a thyristor and a high-speed switch connected to each other in parallel.  FIG. 10( d )  shows a protection unit formed of antiparallel thyristors and a high-speed switch connected to each other in parallel. 
         [0046]      FIG. 10( a )  shows a protection unit  21  formed of a single thyristor, where a cathode of the thyristor is used as a terminal X 3  of the protection unit  21  and an anode of the thyristor is used as a terminal X 4  of the protection unit  21 , so that when an overcurrent occurs in a submodule, the protection unit  21  can be quickly turned on for shunting, thereby protecting the submodule.  FIG. 10( b )  shows a protection unit  22  formed of a single high-speed switch, where one end of the high-speed switch is used as a terminal X 3  of the protection unit and the other end of the high-speed switch is used as a terminal X 4  of the protection unit, so that when a fault occurs in a submodule, the faulty submodule can be bypassed and if the bridge arm where the faulty submodule is located has a redundant submodule, the converter can continue to operate.  FIG. 10( c )  shows a protection unit  23  formed of as thyristor and a high-speed switch connected to each other in parallel, where a cathode of the thyristor is used as a terminal X 3  of the protection unit, an anode of the thyristor is used as a terminal X 4  of the protection unit, one end of the high-speed switch is connected to the cathode of the thyristor, and the other end of the high-speed switch is connected to the anode of the thyristor, thereby achieving overcurrent protection and active bypassing for a submodule.  FIG. 10( d )  shows a protection unit  24  formed of antiparallel thyristors and a high-speed switch connected to each other in parallel, where one end of the antiparallel thyristors  2 ′ and  3 ′ is used as a terminal X 3  of the protection unit, the other end of the antiparallel thyristors  2 ′ and  3 ′ is used as a terminal X 4  of the protection unit, one end of the high-speed switch  1 ′ is connected to the terminal X 3 , and the other end of the high-speed switch  1 ′ is connected to the terminal X 4 . 
         [0047]      FIG. 11  is a schematic diagram of a connection manner of the protection unit  23  and the submodule  10 . The terminal X 3  of the protection unit  23  is connected to the terminal X 1  of the submodule  10  and the terminal X 4  of the protection unit  23  is connected to the terminal X 2  of the submodule  10 . It should be noted that, the protection unit  23  in  FIG. 9  can be replaced with the protection unit  21 , the protection unit  22 , or the protection unit  24  and the submodule  10  may be replaced with the submodule  11 . 
         [0048]    When a ground fault occurs in the DC network, the converter is locked so that the submodules  10  or  11  in the converter all operate in the state  3 , thereby restraining the current of the bridge arm on the fault and eventually reducing it to 0. As a result, the AC network cannot provide a fault current to a fault point. When a transient fault occurs at the DC side, the fault can be removed without tripping an AC line switch, and a two-terminal or multi-terminal DC system formed of the converter provided by the present invention can have good ability of removing the fault at the DC side without a DC breaker. 
         [0049]    The above embodiments are only intended to describe technical ideas of the present invention and are not intended to limit the scope of the present invention. All changes made according to the technical ideas of the present invention on the basis of the technical solutions fall within the scope of the present invention.