Abstract:
A method for controlling a rectifier, connected to an AC power network, of a drive converter which is provided with a load-end pulse-controlled rectifier is disclosed. An electronically controlled switch is electrically connected in parallel to every diode of the rectifier. The switching operations of the switches are produced in a manner synchronized with the conduction phases of the associated network-commutated phase voltages and independently of the phase voltages of the supply network. Every switching operation produced is released upon arrival of the next zero value at the rectifier end. The network-end switching operations are linked with the load-end zero values, thereby allowing switching operations to be carried out in a currentless manner.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a method and circuit for controlling a rectifier, which is connected to an AC power supply system on its input side and a pulse-controlled inverter on its load side. 
   In drive technology, there is frequently a requirement for an uncontrolled, power supply system-commutated rectifier of simple design in order to feed a DC intermediate circuit in a drive converter from a three-phase power supply system, in which case the requirement to draw and/or to supply a current which is as sinusoidal as possible from and to the power supply system is not of importance. Since the converter is not only intended to drive the three-phase motor which is supplied with a variable three-phase voltage and at a variable frequency, but also to brake it a current flow must, however, be possible in both directions. 
   In the braking mode, energy flows from the motor via the load-side inverter into the DC intermediate circuit, and from there into the power supply system. This is not possible in the case of uncontrolled rectifiers which contain only diodes. Electronically switchable braking resistors, which convert the braking energy that is produced to heat, must therefore be arranged in the power supply input for drive converters with diode bridges. 
   However, if the aim is to feed the braking energy back into the supplying power supply system, it is possible to provide a self-commutated pulse-controlled converter with semiconductor switches which can be switched off, in particular insulated-gate bipolar transistors (IGBTs) instead of a rectifier. This self-commutated pulse-controlled converter can be operated as a rectifier or as an inverter. This self-commutated pulse-controlled converter is therefore also referred to as a self-commutated, pulsed, input/output feed unit, or as an active front end (AFE). The power supply system current is virtually sinusoidal as a result of the use of an active front end, that is to say the reactions on the power supply system are minimal. 
   In comparison to an unregulated input-feed unit, the intermediate-circuit voltage can be regulated by means of the self-commutated, pulsed input/output feed unit. The disadvantage is the high costs and the high level of complexity. In the event of stringent requirements relating to the reactions on the power supply system or when a high braking power is produced, there is no other alternative to the use of an input/output feed unit such as this, even if there is no need for a regulated intermediate-circuit voltage. 
   DE 35 39 027 A1 discloses a further embodiment of a drive converter, which has an uncontrolled rectifier on the power supply system side and two diodes (which are electrically connected in series) with electronically controllable switches connected in parallel with them, for each voltage phase. Furthermore, this drive converter has an auxiliary rectifier, which is terminated on the DC-voltage side by a high-impedance burden resistor and, in series with each of its auxiliary diodes, has a current sensor, in particular an optocoupler with a downstream amplifier, which is used to drive an electronically controllable switch. This auxiliary rectifier generates control signals in such a manner that each electronically controllable switch is switched on in synchronism with the on-phases of the associated power supply system-commutated diodes. The rectifier in the drive converter is thus forward-biased bidirectionally for both current directions, and is nevertheless of the power supply system-commutated, uncontrolled type. The advantage of this drive converter is that the rectifier does not require any complex control and regulation electronics. Although the rectifier uses valves which can be switched off, the DC voltage is as high as that in the case of an uncontrolled rectifier. The valves which can be switched off switch only at the power supply system frequency, and after being switched off the voltage across them rises only slowly, so that thyristors which can be switched off are also suitable for use as electronically controllable switches. 
   The publication “Fundamental Frequency Front End Converter (F 3 E)—a DC-link drive converter without electrolytic capacitor” by Kurt Göpfrich, Dr. Carsten Rebbereh and Dr. Lothar Sack, PCIM 2003, Nürnberg, May 2003 discloses a drive converter which has an uncontrolled rectifier on the power supply system side, and a self-commutated pulse-controlled converter on the load side. The two converters are connected directly electrically in parallel on the DC voltage side, that is to say neither an intermediate-circuit capacitor nor an intermediate-circuit inductor is arranged in the so-called DC voltage intermediate circuit. Each diode in the rectifier on the power supply system side is connected electrically in parallel with an electronically controllable switch, in particular an insulated gate bipolar transistor (IGBT). A filter circuit, comprising filter inductors and filter capacitors, is arranged between the power supply system connections and the input connections of the converter on the power supply system side. A comparator circuit is provided for generation of control signals for these electronically controllable switches, and is linked on the input side to the power supply system which feeds it. This comparator circuit and its operation are dealt with in detail in DE 199 13 634 A1, so that it will not be explained in any more detail at this point. 
   The switching operations of the electronically controllable switches for the uncontrolled rectifier on the power supply system side when the power supply system-commutated converter for the drive converter is being controlled on the basis of DE 35 39 027 A1 or the PCIM publication are carried out solely on the basis of the general criterion of the power supply system voltages in the power supply system which feeds them. The direct current flowing between the converter on the power supply system side and the converter on the load side is in this case ignored. This current is impressed on the load side, and may be positive, negative or zero depending on the operating state and time. Furthermore, there is no insurance that, during the commutation of the current from one phase to a subsequent phase, that the switch carrying the current and the switch to which the current is being transferred will switch at the same time. This is caused by the determination of the zero crossing of the associated line-to-line voltage which, for example, is in each case present across a filter capacitor in the input-side filter circuit, and the determination of the time of natural commutation. A plurality of states therefore exist as a function of the type of operation, during the commutation process. For example, a delay in switching on a switch to which the current is being transferred during the commutation process in the case of a negative direct current causes disturbance excitation of the entire system in the drive converter in the event of a line-to-line voltage which is greater than, equal to or less than zero, and leads to voltage spikes across the filter capacitors and the electronically controllable switches. These overvoltages that occur must be taken into account in the design of the drive converter. 
   SUMMARY OF THE INVENTION 
   The invention is based on the object to provide a control method for controlling a rectifier so that such spikes no longer occur. 
   In accordance with the invention, this object is achieved by a control method for a rectifier, which is connected to an AC power supply system, for a drive converter which has a pulse-controlled inverter on the load side, with each diode in this rectifier being electrically connected in parallel with an electronically controllable switch whose switching operations are generated in synchronism with the on-phases of the associated power supply system-commutated diodes and as a function of phase voltages of the feeding power supply system, wherein each generated switching operation is allowed on arrival of a next inverter-side zero vector. 
   Since, according to the method on which the invention is based, the control signals that are produced are passed on only while a load-side zero vector is present, the switching operation takes place only when the intermediate-circuit current is equal to zero. In this operating state, it is irrelevant how the commutation process takes place (overlapping or simultaneously) or whether the zero crossing of a line-to-line voltage has been determined exactly. The switching processes on the power supply system side are linked to the switching states on the load side when using this method according to the invention. 
   In this case, a distinction must be drawn between two situations, specifically a range up to below the full voltage drive for the load-side pulse-controlled inverter and the range with the full voltage drive. In the range up to below the full voltage drive for the load-side pulse-controlled inverter, the zero vectors occur at short time intervals, since the switching frequency of the pulse-controlled inverter is in the kHz range. Two active vectors and one zero vector are always used for approximation of a nominal voltage vector of the load-side pulse-controlled inverter up to the full voltage drive. Three switched-on durations are calculated for a vector representation time for the three vectors. The switched-on duration for the zero vector is advantageously halved, in order that the vector representation time starts and ends with a zero vector. A second equivalent vector representation time can thus be linked with mirror-image symmetry to the first vector representation time. One zero vector therefore exists in each vector representation time. This vector representation time corresponds to half the pulse period duration. If a requirement occurs for switching in the uncontrolled rectifier, it is possible to determine in a very short time whether a zero vector occurs. If zero vectors are present, the switching takes place with the start of the next zero vector, and thus without any current flowing. 
   In the case of full voltage control, zero vectors are ignored, in favor of maximizing the voltage. In consequence, the pauses in which no current flows in the intermediate circuit are missing in some places. In order to avoid any excessive switching delay in the power supply system-side converter when a switching requirement occurs in this case, a zero vector is now inserted according to the invention. Of the two possible zero vectors of the load-side pulse-controlled inverter, that is selected which requires the switching of only one switch pair in the pulse-controlled inverter. A pause during which no current flows in the intermediate circuit in this drive converter therefore also occurs in the full voltage drive range, and the switching in the power supply system-side converter likewise takes place with no current flowing. Since this process takes only a short time this measure has no practical effect on the load. 
   According to another feature of the present invention, the presence of an inverter-side zero vector can be determined as a function of the output voltages from the load-side pulse-controlled inverter. 
   According to another feature of the present invention, the presence of an inverter-side zero vector can be determined as a function of the control signals of the load-side pulse-controlled inverter. 
   According to another feature of the present invention, the full voltage drive operating state is determined as a function of a predetermined drive level. 
   The advantages of the control method according to the invention comprise, owing to the switching processes being carried out with no current flowing, in the power supply system-side electronically controllable switches, a reduction in the load on these switches as a result of the absence of switching losses, and a reduction in the switching-dependent excitations in the entire system. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     In order to explain the invention further, reference is made to the drawing in which a plurality of embodiments of the circuit arrangement according to the invention are illustrated schematically, and in which: 
       FIG. 1  shows an equivalent circuit of a known drive converter without a voltage intermediate circuit, 
       FIG. 2  shows an equivalent circuit of a known control device for generation of control signals for the electronically controllable switches for the rectifier in a known drive converter, 
       FIG. 3  shows a graph of the power supply system voltages in the power supply system which feeds it and of a rectifier voltage that is produced plotted against the time t, 
       FIG. 4  in each case shows a graph of the control signals for the electronically controllable switches for the rectifier in a known drive converter plotted against the time t, 
       FIG. 5  shows a drive converter with a first embodiment of the circuit arrangement according to the invention, while 
       FIG. 6  shows a drive converter with a second embodiment of the circuit arrangement according to the invention, 
       FIG. 7  shows a block diagram of a further embodiment of the circuit arrangement according to the invention, and 
       FIGS. 8 and 9  each show the method according to the invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  shows an equivalent circuit of a drive converter  2 , which is known from the PCIM publication. This known drive converter  2  has a rectifier  4  on the power supply system side, and a self-commutated pulse-controlled converter on the load side. The rectifier  4  and the self-commutated pulse-controlled converter  6  on the load side are directly electrically conductively connected to one another on the DC voltage side by means of a positive and negative busbar  8  and  10 . The DC intermediate circuit therefore no longer contains any intermediate-circuit inductor or intermediate-circuit capacitor. A filter circuit  18  is arranged between the connections U, V, W of a power supply system, which is not illustrated in any more detail, and the input connections  12 ,  14  and  16  on the AC voltage side of the rectifier  4 . This filter circuit  18  has filter inductors L F  and filter capacitors C F , with each of these filter inductors L F  being connected in one power supply system supply line, and with the filter capacitors C F  being connected in delta. The power supply system-side converter  4  has two diodes D 1 , D 2 ; D 3 , D 4  and D 5 , D 6  (which are electrically connected in series) for each voltage phase and are electrically connected so as to produce a three-phase rectifier bridge circuit. A respective electronically controllable switch T 1 , . . . , T 6  is connected electrically in parallel with each diode D 1 , . . . , D 6  in this rectifier  4 . Semiconductor switches which can be switched off, in particular IGBTs, are provided as electronically controllable switches T 1 , . . . , T 6 . 
   The control connections of these electronically controllable switches are linked, in accordance with the PCIM publication, to a control device. Since this control device is preferably formed from comparators, it is referred to as a comparator circuit. The detailed design of this comparator circuit can be found in DE 199 13 634 A1. 
     FIG. 2  shows an equivalent circuit of a further control circuit  20 , which is already known from DE 35 39 027 A1. This control circuit  20  has an auxiliary rectifier  22 , which is terminated on the DC voltage side with a high-impedance burden resistor R B . The inputs  24 ,  26  and  28  of the auxiliary rectifier  22  are electrically conductively connected to the connections U, V and W of a power supply system which feeds it, but which are not illustrated in any more detail. This auxiliary rectifier  22  comprises six auxiliary diodes D 7 , D 8 , D 9 , D 10 , D 11  and D 12 , which are connected in a generally known three-phase bridge circuit. An optocoupler  34 ,  36 ,  38 ,  40 ,  42  and  44  with a transmitting diode is connected as a current sensor between each auxiliary diode D 7 , . . . , D 12  and a respectively associated connection  30  or  32  on the DC voltage side. The phototransistor which is provided for each optocoupler  34 , . . . ,  44  and is driven by the associated transmitting diode is linked to a respective downstream amplifier  46 ,  48 ,  50 ,  52 ,  54  and  56 . The amplifiers  46 , . . . ,  56  in this case each generate a switch-on signal S T1 , S T2 , S T3 , S T4 , S T5  and S T6  for the electronically controllable switches T 1  to T 6  for the rectifier  4  in the drive converter  2  as shown in  FIG. 1 , as a function of the light-emitting transmitting diode. These control signals S T1 , . . . , S T6  are respectively applied to corresponding control connections G T1 , G T2 , G T3 , G T4 , G T5  and G T6 . These control signals S T1  to S T6  are each shown plotted against the time t in a graph in  FIG. 4 . 
   According to DE 35 39 027 A1, the resistance of the burden resistor R B  is designed such that the burden current I aux  which flows when the auxiliary diodes D 7  to D 12  are in the forward-biased state reaches a small value, which is suitable for the transmitting diodes of the optocouplers  34 , . . . ,  44  (for example 10 mA). 
   The phase voltages U U , U V  and U W  of the feeding power supply system are shown in a graph plotted against the time t in  FIG. 3 . As can be seen from this figure, an intersection K 1 , . . . , K 6  of two phase voltages occurs at each of the times t 1  to t 6 . These intersections K 1 , . . . , K 6  are the natural commutation times of an uncontrolled rectifier. A DC voltage U ZK  produced by the rectifier  4  is likewise illustrated, plotted against the time t, in the graph in  FIG. 3 . A line-to-line voltage which corresponds to the two phase voltages U W , U U ; U V , U W ; U U , U V ; U W , U U ; U V , U W  and U U , U V  involved passes through zero at each of these times t 1 , . . . , t 6 . This means that this line-to-line voltage changes its mathematical sign. These line-to-line voltages can be tapped off across the filter capacitors C F  in the filter circuit  18  in the drive converter  2  as shown in  FIG. 1 . 
     FIG. 5  shows a first embodiment of a circuit arrangement  58  according to the invention for a known drive converter as shown in  FIG. 1 . The circuit arrangement  58  has a zero-vector detection device  60  and an enable circuit  62 . The zero-vector detection device  60  is electrically conductively connected on the input side to load-side connections  64 ,  66  and  68  of the load-side pulse-controlled inverter  6 . On the output side, this zero-vector detection device  60  is linked to a control input  70  of the enable circuit  62 . On the input side, this enable circuit  62  is connected to control connections G T1  to G T6  of the control device  20 , which is connected on the input side to the AC power supply system which feeds the drive converter  2 . The control device  20  is designed in a corresponding manner to that shown in  FIG. 2 . The control connections G T1  to G T6  are not illustrated in detail in  FIG. 5 , for clarity reasons. On the output side, the enable circuit  62  is linked to control connections of the electronically controllable switches T 1  to T 6  in the uncontrolled rectifier  4  in the drive converter  2 . Only when the zero-vector detection device  60  has detected a zero vector  U   0  or  U   7  are the control signals S T1  to S T6  which are present on the input side emitted at the outputs of the enable circuit  62  when the next respective zero vector  U   0  or  U   7 , respectively, occurs. In this illustration, the zero-vector detection device  60  on the input side is linked to the load-side connections  64 ,  66  and  68  of the pulse-controlled inverter  6  for the drive converter  2 . 
   The illustration shown in  FIG. 6  differs from the illustration shown in  FIG. 5  in that the zero-vector detection device  60  is now connected to one output of a modulator  72  for the load-side pulse-controlled inverter  6 . Since associated control signals for electronically controllable switches T 7  to T 12 , in particular IGBTs, for the load-side pulse-controlled inverter  6  are generated in this modulator  72  from a nominal voltage vector  u * that is present it is possible to determine simply by means of a level check whether a zero vector  U   0  or  U   7  is or is not present. This zero-vector detection device  60  can advantageously be integrated in the modulator  72 . The enable circuit  62  can likewise be integrated in the control device  20  for the electronically controllable switches T 1  to T 6  for the uncontrolled rectifier  4  in the drive converter  2 . This would reduce the circuit arrangement  58  to a connecting line between the modulator  72  for the load-side pulse-controlled inverter  6  and the control device  20  for the power supply system-side converter  4  for the drive converter  2 . 
     FIG. 7  shows a block diagram of a further embodiment of the circuit arrangement  58  according to the invention. This differs from the embodiment shown in  FIG. 6  in that an apparatus  74  is additionally provided. This apparatus  74  determines whether a zero vector  U   0  or  U   7  has been detected in the vector representation time T P . The vector representation time T P  is likewise produced by the modulator  72  for the load-side pulse-controlled inverter  6 . If no zero vector  U   0  or  U   7  occurs within a vector representation time T P , and if a switching operation K U  is required for the power supply system-side converter  4 , then the apparatus  74  causes the modulator  72  to generate a zero vector  U   0  or  U   7  in the following vector representation time T P . In this case, care must be taken to ensure that, as far as possible, only one switching operation of one switch pair for the pulse-controlled inverter  6  need be carried out. This inserted zero vector  U   0  or  U   7  need be present only until the requested switching operation K U  has been completed in the power supply system-side converter  4 . Switching therefore takes place without any current flowing even when the power supply system-side converter  4  has the full voltage drive. This zero vector  U   0  or  U   7  is therefore inserted in order to prevent an excessive switching delay occurring in the power supply system-side converter. 
     FIGS. 8 and 9  show the method according to the invention in the range below the full voltage drive and in the range of the full voltage drive. According to this method on which the invention is based, and as shown in  FIG. 8 , it is found that a zero vector  U   0  or  U   7  has occurred at the start of a vector representation time T P =½ f S . At the same time, a switching operation K U  for the power supply system-side converter  4  is requested at the start of this vector representation time T P . This requested switching operation K U  is carried out when the next zero vector  U   0  or  U   7  occurs. Since the load-side pulse-controlled inverter  6  implements the zero vector  U   0  or  U   7  while the switching operation K U  is being carried out, no current flows in the intermediate circuit in the drive converter  2 . The switching operation K U  in the power supply system-side converter  4  therefore takes place with no current flowing. 
   According to the method on which the invention is based as shown in  FIG. 9 , no zero vector  U   0  or  U   7  occurs for a requested switching operation K U . The apparatus  74  therefore causes the modulator  72  to generate a zero vector  U   0  or  U   7 . The requested switching operation K U  is carried out with the implementation of this zero vector  U   0  or  U   7  in the next vector representation time T P . This zero vector  U   0  or  U   7  need be present only until the switching operation K U , for example the commutation between the electronically controllable switches T 1  and T 3  for the power supply system-side converter  4 , has been carried out. Since this process takes only a short time, this measure has no practical effect for a load on the outputs  64 ,  66  and  68  of the load-side pulse-controlled inverter  6 . As already mentioned, the one of the two possible zero vectors  U   0  or  U   7  of the pulse-controlled inverter  6  which is selected is that which requires only one switch operation in the pulse-controlled inverter  6 . The insertion of a zero vector  U   0  or  U   7  in the full voltage drive range results in the switching process being carried out in the power supply system-side converter  4  with no current flowing. 
   Since the switching processes in the power supply system-side converter  4  are carried out with no current flowing, switching losses no longer occur, so that the load on the electronically controllable switches T 1  to T 6  for the uncontrolled rectifier  4  is reduced. Furthermore, this reduces the switching-dependent excitations in the entire system.