Abstract:
The invention relates to a pulse resistor for a frequency converter in the higher voltage and capacity range. The inventive pulse resistor is characterized by comprising at least two bipolar subsystems ( 24 ) and a resistor element ( 14 ), said subsystems ( 24 ) and said resistor element ( 14 ) being connected in series. The inventive pulse resistor is devoid of the drawbacks of known pulse resistors, it can be finely controlled by a brake current (iB) and can be adapted to any medium voltage by simple means.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a pulse resistor for a converter in the moderately high voltage and power range. 
     Converters having a DC voltage input are increasingly being used for regulated electrical drives and power supply installations in the moderately high voltage ranges. A converter of this type is also known as a voltage-source inverter. The standardized medium voltages 2.3 kV, 3.3 kV, 4.16 kV and 6.9 kV are counted as moderately high voltages. 
       FIG. 1  shows an equivalent circuit of a voltage-source inverter known from the prior art, of which just one load-side three-phase converter  2  is shown for reasons of clarity. Owing to the high voltage range, the converter valves T 1 -T 6  of this load-side three-phase converter  2  each comprise a plurality of turn-off capable semiconductor switches  4  electrically connected in series, across each of which is connected a diode  6  in antiparallel. As each converter valve T 1 -T 6  has three turn-off capable semiconductor switches  4 , this converter topology is also called an on-off converter having a series connection number of Three. Every two converter valves T 1 ,T 2  and T 3 ,T 4  and T 5 ,T 6  respectively form a bridge path  8 , which constitutes a phase module of the on-off converter  2 . Each junction  10  between two converter valves T 1 ,T 2  or T 3 ,T 4  or T 5 ,T 6  forms a terminal L 1  or L 2  or L 3  respectively for connecting a three-phase load, for example a three-phase motor. The three phase modules  8  of the three-phase converter  2  are electrically connected in parallel by two busbars P 0  and N 0 . A DC-link circuit capacitor C ZW  is connected between these two busbars P 0  and N 0 , said capacitor comprising, for example, one or a plurality of capacitors electrically connected in series and/or parallel. A DC voltage U d  lies across this DC-link circuit capacitor C ZW . In this equivalent circuit of an on-off converter having a series connection number of Three, insulated gate bipolar transistors (IGBT) are provided as the turn-off capable semiconductor switches  4 . The series connection number depends on the DC voltage U d  lying across DC-link circuit capacitor C ZW  and on the blocking ability of commercially available IGBTs. 
     With temporary energy recovery in the DC-link circuit capacitor C ZW , the DC voltage U d  lying across the DC-link circuit capacitor C ZW  can increase such that it exceeds a maximum permissible value for this DC voltage. Such a situation occurs in particular during braking of a three-phase motor connected to the terminals L 1 , L 2  and L 3 . Other causes that are generally of short duration, such as rapid fluctuations of the line voltage of a grid supply or load fluctuations, can also produce such overvoltages. The following measures are known for overcoming these problems:
         Connecting a converter with an energy-recovery facility, the converter being electrically connected in parallel with the DC-link circuit capacitor C ZW . The excess energy from the DC-link circuit capacitor C ZW  can thereby be fed back into a grid system that is able to receive power.   Connecting a pulse-controlled resistor across the busbars P 0 ,N 0  of the DC-link circuit, said resistor being used to convert the excess energy of the DC-link circuit capacitor C ZW  into heat.       

       FIG. 2  shows an equivalent circuit of a pulse-controlled resistor, also known as a pulse resistor. This known pulse resistor comprises a final control element  12  and a resistance element  14 . A phase module  8  is used as the final control element  12 , for which the turn-off capable semiconductor switches  4  of the lower converter valve T 8  are not needed. The implementation of the upper converter valve T 7  of this phase module  8  is the same as the implementation of the converter valve T 1  or T 3  or T 5  respectively of the load-side three-phase converter  2  shown in  FIG. 1 . To aid understanding, the turn-off capable semiconductor switches  4  of the lower converter valve T 8  of the final control element  12  of the pulse resistor are not shown explicitly in the equivalent circuit diagram. These can, however, be present in the phase module  8 , but are not actuated with the “brake” function. The resistance element  14  is electrically connected in parallel with the lower converter valve T 8  having the series connection number of Three. This resistance element  14  comprises a resistive and an inductive component  16  and  18 . The inductive component  18  represents its parasitic inductance. This pulse resistor has the following disadvantages for high voltages:
     a) The currents i P  and i N  in the supply lines  20  and  22  of the pulse resistor have a very high rate of current rise di/dt, resulting in emission of electromagnetic interference.   b) The supply lines  20  and  22  must be made physically short and of low inductance in order to limit the voltages arising across the turn-off capable semiconductor switches  4 .   c) This pulse resistor has an on-off response and in the periodic pulsed operation generates a high AC component of the current i P  and i N  in the supply lines  20  and  22 .   d) In order to perform its function, this pulse resistor requires a DC capacitor C ZW  to be physically located as close as possible, i.e. this pulse resistor must be physically positioned immediately beside the DC-link circuit capacitor C ZW .   

     The disadvantages of points a) and b) are particularly troublesome if the pulse-controlled resistor  14  is to be used as an optional add-on to the converter  2 . The disadvantage stated in point c) results in increased ripple on the DC voltage U d  of the DC-link circuit capacitor C ZW  of the one-off converter  2  having the series connection number of Three. This increased ripple has unwanted repercussions for the operation of other converters connected to the busbars P 0 , N 0 . The disadvantage stated in point d) means that this pulse resistor cannot be used with converter topologies that do not comprise a DC-link circuit capacitor C ZW . 
     SUMMARY OF THE INVENTION 
     Hence the object of the invention is to define a pulse resistor that no longer has the stated disadvantages. 
     This object is achieved according to the invention by a pulse resistor for a converter in the moderately high voltage and power range having at least two two-terminal subsystems and a resistance element, these subsystems and the resistance element being electrically connected in series. 
     The fact that at least two two-terminal subsystems are now used instead of turn-off capable semiconductor switches means that the resistance element of the pulse resistor can be connected directly in series with the subsystems that are electrically connected in series. The degree of fine-control of a braking current can be defined by the choice of the number of subsystems. Since the two-terminal subsystems each have a unipolar storage capacitor, this pulse resistor according to the invention no longer needs a DC capacitor. Hence this also removes the condition that this pulse resistor must be physically positioned immediately beside a DC-link circuit capacitor or a load-side converter, i.e. this pulse resistor according to the invention can be connected by two supply lines, for example stranded wires, to a positive and a negative busbar of a load-side converter. 
     Incremental control of a braking current is achieved by switching in and out subsystems of the pulse resistor according to the invention, i.e. the pulse resistor according to the invention no longer has an on-off response. As a result, high AC current components no longer arise in the supply lines of the pulse resistor. 
     In an advantageous embodiment of the pulse resistor, the storage capacitors of the subsystems that are electrically connected in series are designed to be of such a capacitance that an amount of energy stored in parasitic inductances of the supply lines and of the resistance element is small compared with an amount of energy stored in these storage capacitors. This minimizes an overvoltage that results when a braking current is switched off. This condition is achieved by the storage capacitors being designed to have a sufficiently large capacitance. 
     In another advantageous embodiment of the pulse resistor, the storage capacitors of the subsystems that are electrically connected in series are designed to be of such a capacitance that the time constant formed from the resistance element and storage capacitors is small compared with the period of each control state of the subsystems. This prevents, during each switching operation, any unnecessary fluctuation in the voltages across the unipolar storage capacitors of the subsystems that are electrically connected in series. This condition is likewise satisfied by storage capacitors having a sufficiently large capacitance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The invention is explained in greater detail with reference to the drawing, which shows schematically an embodiment of a pulse resistor according to the invention. 
         FIG. 1  shows an equivalent circuit of a load-side converter of a voltage-source inverter known from the prior art, 
         FIG. 2  shows an equivalent circuit of a known pulse-controlled resistor, 
         FIG. 3  shows an equivalent circuit of a pulse resistor according to the invention, and 
         FIGS. 4 and 5  each show a circuit arrangement of a subsystem. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As shown in the equivalent circuit of the pulse resistor according to the invention shown in  FIG. 3 , four subsystems  24  and a resistance element  14  are electrically connected in series. The number of subsystems  24  is shown by way of example in this diagram, i.e. there can be any number of subsystems  24  electrically connected in series. For example, the requirement for a degree of fine-control of a braking current i B  determines the number of subsystems  24  used. This pulse resistor is electrically conductively connected by a supply line  26  and  28  to a busbar P 0  and N 0  of a load-side converter  2 . Specific requirements placed on the supply lines  20  and  22  of the known pulse resistor shown in  FIG. 2  are not placed on these supply lines  26  and  28  in the pulse resistor according to the invention. 
     In order to control the braking current i B , only the individual subsystems  24  need to be switched in or out, said subsystems being all switched in or out simultaneously or switched in or out successively. In the idle state, when the braking current i B  is zero, all the subsystems  24  are in a control state in which the terminal voltages U X21  of the subsystems  24  each assume values that differ from zero irrespective of the direction of the terminal current, and each subsystem  24  receives or releases energy depending on the direction of the terminal current. This control state is denoted by control state II in DE 101 03 031 A1. In order to produce the maximum braking current i Bmax , all the subsystems are driven in one control state in which the terminal voltages U X21  of the subsystems  24  each assume the value zero irrespective of the direction of the terminal current. This control state is denoted as control state I in DE 101 03 031 A1. In order to produce finely graded intermediate values of the braking current (0&lt;i B &lt;i Bmax ), in n subsystems  24  only one to n−1 subsystems  24  are switched in control state I. The remaining subsystems  24  are left in control state II. In accordance with the method disclosed in DE 101 03 031 A1 for balancing the voltages U C  lying across the storage capacitors  40  of the n subsystems  24 , within a series connection of n subsystems  24 , the subsystems  24  having the highest capacitor voltages U C  are each preferably switched into the control state I. 
     In order to prevent these capacitor voltages U C  fluctuating during the switching operations to an unnecessarily high degree, it is advantageous and practical to choose the period of each control state to be small with respect to the time constant formed from the resistive resistance element  14  and the storage capacitors  40  of the n subsystems  24 . This is achieved by storage capacitors  40  of the n subsystems  24  having sufficiently large capacitance. A second option is to select the switching frequency to be sufficiently high. 
     In order to minimize an overvoltage of the storage capacitors  40  of the n subsystems  24  that results when the braking current is switched off, it must be attempted to keep the energy stored in the parasitic inductances  30  and  18  of the supply lines  26 ,  28  and of the resistance element  14  small with respect to the energy stored in the storage capacitors  40  of the n subsystems  24 . This can always be achieved by designing the storage capacitors  40  of the n subsystems  24  to have a sufficiently large capacitance. 
       FIG. 4  shows a simple circuit arrangement disclosed in DE 101 03 031 A1 for the subsystem  24  of the pulse resistor shown in  FIG. 3 . The circuit arrangement shown in  FIG. 5  represents a variant that is fully identical in function. This known two-terminal subsystem  24  comprises two turn-off capable semiconductor switches  32  and  34 , two diodes  36  and  38  and a unipolar storage capacitor  40 . The two turn-off capable semiconductor switches  32  and  34  are electrically connected in series, with this series circuit being electrically connected in parallel with the storage capacitor  40 . One of the two diodes  36  and  38  is electrically connected in parallel with each turn-off capable semiconductor switch  32  and  34  in such a way that this diode are connected in antiparallel with the corresponding turn-off capable semiconductor switch  32  or  34 . The unipolar storage capacitor  40  of the subsystem  24  is composed of either one capacitor or a capacitor bank containing a plurality of such capacitors having a resultant capacitance C 0 . The junction between the emitter of the turn-off capable semiconductor switch  32  and the anode of the diode  36  forms a connecting terminal X 1  of the subsystem  24 . The junction between the two turn-off capable semiconductor switches  32  and  34  and the two diodes  36  and  38  form a second connecting terminal X 2  of the subsystem  24 . 
     In the embodiment of the subsystem  24  shown in  FIG. 5 , this junction forms the first connecting terminal X 1 . The junction between the drain of the turn-off capable semiconductor switch  34  and the cathode of the diode  38  forms the second connecting terminal X 2  of the subsystem  24 . 
     In control state I, the turn-off capable semiconductor switch  32  is switched on, and the turn-off capable semiconductor switch  34  is switched off. In order to obtain control state II, the turn-off capable semiconductor switch  32  is switched off and the turn-off capable semiconductor switch  34  is switched on. In control state I, the terminal voltage U X21  of the system  24  equals zero, whereas in control state II, the terminal voltage U X21  equals the capacitor voltage U C  lying across the storage capacitor  40 . 
     By selecting the number of subsystems  24  that are electrically connected in series of the pulse resistor shown in  FIG. 3 , this pulse resistor according to the invention can be adjusted by simple means to suit any standardized medium voltage. Likewise, the choice of the number of subsystems  24  of the pulse resistor shown in  FIG. 3  predetermines the capacitor voltage U C  lying across each storage capacitor  40 . This capacitor voltage U C  also defines the withstand voltage of the two turn-off capable semiconductor switches  32  and  34 . As shown in  FIGS. 4 and 5 , insulated gate bipolar transistors (IGBT) are used as the turn-off capable semiconductor switches  32  and  34 . MOS field effect transistors, also known as MOSFETs, can also be used. 
     All the aforementioned disadvantages a) to d) can be avoided by this pulse resistor according to the invention. This pulse resistor according to the invention additionally has the following advantages:
         A fine degree of control of a braking current i B  in a plurality of intermediate levels equal to the number of the series-connected subsystems  24 .   Standardized implementation using the subsystems disclosed in DE 101 03 031 A1.       

     The sum total of these properties justifies the larger number of components, in particular for converters in the moderately high voltage and power range.