Patent Publication Number: US-2017353120-A1

Title: Precharge apparatus for power conversion system

Description:
BACKGROUND INFORMATION 
     The subject matter disclosed herein relates to power conversion and more specifically to power converters and precharging methods and apparatus therefor. 
     BRIEF DESCRIPTION 
     Various aspects of the present disclosure are now summarized to facilitate a basic understanding of the disclosure, wherein this summary is not an extensive overview of the disclosure, and is intended neither to identify certain elements of the disclosure, nor to delineate the scope thereof. Rather, the primary′ purpose of this summary is to present various concepts of the disclosure in a simplified form prior to the more detailed description that is presented hereinafter. The present disclosure provides power conversion systems, methods and precharge systems to charge a DC bus capacitor, including thyristors or other semiconductor switching devices and reverse diodes coupled in AC circuit paths between AC input lines and a rectifier. A precharge resistor is coupled in one or more of the AC circuit paths. A controller turns all the thyristors off to allow the DC bus capacitor to charge through the precharge resistor, and turns all the thyristors on when the DC bus voltage reaches a non-zero threshold value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-9  are schematic diagrams. 
         FIG. 10  is a flow diagram. 
     
    
    
     DETAILED DESCRIPTION 
     In the drawings, like reference numerals refer to like elements throughout, and the various features are not necessarily drawn to scale. In the following discussion and in the claims, the terms “coupled”, “couple”, “couples” or variants thereof are intended to include indirect or direct electrical connection or combinations thereof. For example, if a first device couples to or is coupled with a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via one or more intervening devices and connections. Several embodiments or implementations are hereinafter described in conjunction with the drawings, wherein like reference numerals are used to refer to like elements throughout, and wherein the various features are not necessarily drawn to scale. 
     Referring initially to  FIGS. 1-3 , a system  100  is shown, including a three phase AC power source  102  with a grid connection and associated feeder lines that provide AC input power to AC input phase lines A, B and C of a motor drive power conversion system  110  to drive a motor load  104 . Disclosed examples include power conversion systems  110 , methods and precharge systems  118 ,  120  to charge a DC bus capacitor CDC at the output of a switching rectifier  114 . The rectifier  114  includes rectifier switching devices S 1 -S 6  individually coupled between a corresponding one of the AC input terminals U, V, W and one of the first and second DC output terminals DC+, DC−. As shown in the examples of  FIGS. 3-9 , the precharge circuitry includes solid state or semiconductor switching devices such as thyristors (e.g., SCRs) Qa, Qb and Qc and reverse diodes Da, Db and Dc coupled in AC circuit paths between the AC input lines A, B, C and the rectifier  114 . A precharge resistor R is coupled in one or more of the AC circuit paths, and a controller  120  turns off all the thyristors Qa, Qb, Qc to allow the DC bus capacitor CDC to charge through the precharge resistor R, and turns the thyristors on when the DC bus voltage VDC reaches a non-zero threshold value TH. The use of solid state precharge switching circuitry advantageously avoids or mitigates the cost, size and weight issues associated with precharging contactors or circuit breakers, and the disclosed examples do not require complicated switching logic in order to facilitate precharging of the DC bus capacitor CDC, as well as the possibility of precharging filter capacitors Cf of the input filter circuit  112 . In this regard, solid state precharge circuitry  118  can be located within the filter circuit  112  as shown in  FIG. 1 , upstream of (e.g., before) the input filter circuit  112  ( FIG. 2 ) or after (e.g., downstream of) the filter circuit  112  ( FIG. 3 ) and various embodiments. The precharging apparatus and techniques of the disclosed examples can be employed to mitigate or reduce inrush current and associated stress or degradation of the power conversion system components, and avoid undesired tripping of the converter  110 . 
     As shown in  FIGS. 1-3 , the power conversion system  110  includes a three phase input filter circuit  112  that delivers AC power to the input terminals U, V and W of the three-phase active front end (AFE) rectifier  114 . The drive  110  includes a feedback circuit  128 , and a control circuit or controller  120 . The rectifier  114  has an input to receive AC input power via the terminals U, V and W, and an output to provide a DC output signal at DC output terminals DC+ and DC−. The feedback circuit  128  in one example provides three phase input voltage values to the controller  120 , and the controller  120  determines the phase angle of the input line to line voltage. An intermediate DC circuit or DC bus is connected between the DC output terminals DC +  and DC− of the rectifier  114  and the input of an inverter  116 . In one example, the DC intermediate circuit includes a capacitor CDC connected between positive and negative DC bus lines DC +  and DC− as shown in  FIG. 1 . 
     The inverter  116  includes a DC input coupled to the output terminals of the rectifier  114  through the intermediate circuit, and an output coupleable to the motor load  104  to provide an AC output signal. In this example, the inverter  116  provides a three-phase output signal to drive the motor load  104 . In other examples, a single-phase AC output signal can be provided by the output of the inverter  116  to drive a load  104 . The AFE rectifier  114  can provide DC input power to any suitable load or loads. In the example of  FIG. 1 , the system  110  includes a single inverter  116  operated by an inverter controller  126  to drive a single motor load  104 . In other examples, the AFE rectifier  114  and the filter circuit  112  can be used in a multi-drive configuration to provide DC input power to multiple loads, such as a plurality of inverters  116  individually driving a separate motor load  104 . Such a system can be provided in a multi-bay enclosure with common DC bus connections to provide power from the rectifier  114  to a plurality of inverters  116 . The power conversion system  110  can include a variety of different input filter circuit topologies or configurations. For example, an inductor-capacitor (LC) or inductor-capacitor-inductor (LCL) input filter circuit can be associated with each AC input phase A/U, B/V, C/W to control the harmonic content of a connected power grid. In other examples, the conversion system can include an AFE rectifier  114  to provide a DC output, without an included inverter or driven motor. 
     As further shown in  FIGS. 4-7 , the system  110  includes a controller  120  with one or more processors  200  and associated electronic memory  202  with program instructions to operate the rectifier  114 , any included inverter  116 , and to also control precharging of the DC bus capacitor CDC. The controller  120  in one example includes a rectifier control component or system  126  that provides rectifier switching control signals  125  to IGBTs or other switches S 1 -S 6  to operate the AFE rectifier  114 . The controller  120  in one example also includes or implements an inverter control component or system to provide inverter switching control signals (not shown) to operate the inverter  116 . The controller  120  operates the motor drive  110  in various operational modes, and obtains measurements of various system operating parameters and signals or values  130  from the feedback system  128 . The controller  120  and the components thereof may be any suitable hardware, processor-executed software, processor-executed firmware, logic, or combinations thereof that are adapted, programmed, or otherwise configured to implement the functions illustrated and described herein. The controller  120  in certain embodiments may be implemented, in whole or in part, as software components executed using one or more processing elements, such as one or more processors  200 , and may be implemented as a set of sub-components or objects including computer executable instructions stored in the non-transitory computer readable electronic memory  202  for operation using computer readable data executing on one or more hardware platforms such as one or more computers including one or more processors, data stores, memory, etc. 
     The components of the controller  120  may be executed on the same computer processor or in distributed fashion in two or more processing components that are operatively coupled with one another to provide the functionality and operation described herein. The controller  120  in one example is configured by execution in the processor  200  of instructions in the memory  202  to implement various motor drive functions as are known, as well as resonance detection and impedance computation functionality via the component  122  provided as a component including processor-executable instructions in the memory  202  in one example. Similarly, the rectifier control functions can be implemented at least in part via processor executable instructions  124  stored in the memory  202  for execution by the processor  200 . In addition, the controller  120  can include various signal conditioning circuits for receiving and converting analog signals into digital signals, and for providing suitable output signals (e.g., rectifier switching control signals  125  and inverter switching control signals  127  ( FIG. 1 ) suitable for operating the various switching devices of the rectifier  114  and the inverter  116 . 
     In the various non-limiting examples of  FIGS. 1-7 , the precharge circuit  118  is coupled between the AC input lines A, B, C and the AC input terminals U, V, W of the rectifier  114 . In certain examples, the precharge circuit  118  is connected to filter capacitors Cf of the filter circuit  112  ( FIGS. 1, 5 and 6 ). In other examples, the precharge circuit  118  is coupled between the AC input lines A, B, C and the filter circuit  112 , and the filter circuit  112  is coupled between the precharge circuit  118  and the AC input terminals U, V, W of the rectifier  114  ( FIGS. 2 and 4 ). In further examples, the filter circuit  112  is coupled between the AC input lines A, B, C and the precharge circuit  118 , and wherein the precharge circuit  118  is coupled between the filter circuit  112  and the AC input terminals U, V, W of the rectifier  114  ( FIGS. 3 and 7 ). 
     As shown in  FIGS. 4-7 , an example LCL filter circuit  112  includes first and second filter inductors L 1  and L 2  connected between the input of the rectifier  114  and the output of the source  102  for each phase A/U, B/V and C/W. The phase A/U of the filter  112  includes a first (rectifier side) inductor represented as an inductance L 1   a  as well as a second (grid side) inductor represented as an inductance L 2   a . Similarly, the filter phase B/V includes first and second inductors represented by inductances L 1   b , L 2   b , and the filter phase C/W includes first and second inductors represented by inductances L 1   c , L 2   c . Between the first and second inductors L 1  and L 2  of each phase line of the filter  112 , a filter capacitor Cf is connected from the line joining the corresponding inductors L 1  and L 2  to a filter neutral N 1 . These are shown in  FIGS. 4-7  as filter capacitors Cfa, Cfb and Cfc. The filter neutral N 1  can, but need not, be connected to the source neutral N. In other examples, the filter capacitors Cf can be connected in a delta configuration. Delta or Y connected filter capacitors in certain examples may include damping resistors or trap filter inductors in series with the capacitors. 
     The feedback circuit or system  128  includes one or more sensors (not shown) to sense or detect one or more electrical conditions in the filter circuit  112 , the rectifier  114  and/or the intermediate DC bus circuit. The feedback circuit  128  provides one or more feedback signals or values  130  (e.g., analog signals and/or converted digital values) to the controller  120  for use in closed loop feedback control of the motor drive  110  generally, as well as for use by the precharge control component  122 . As further shown in  FIGS. 4-7 , the rectifier  114  in one example is a switching rectifier with IGBT type switching devices S 1 -S 6  individually coupled between a corresponding one of the AC input terminals U, V or W and one of the first and second DC output terminals DC+ or DC−. Other semiconductor-based switching devices can be used, including without limitation field effect transistors (FETs), etc. Each rectifier switching device S 1 -S 6  is operated by a corresponding rectifier switching control signal  125  from the rectifier controller  124  of the control circuit  120  to selectively connect or disconnect the corresponding AC input terminal to the corresponding DC output terminal. 
     The precharge circuit  118  includes three AC circuit paths individually coupled between a corresponding one of the AC input lines A, B and C and a corresponding one of the AC input terminals U, V and W. The individual AC circuit paths include a semiconductor switching device (e.g., Qa, Qb and Qc in the three-phase example of  FIG. 4 ). The semiconductor switching devices each operate according to a corresponding precharge control signal (e.g.,  124   a ,  124   b  or  124   c ) to selectively allow current to flow from the corresponding AC input line A, B or C and the corresponding AC input terminal U, V or W. In addition, the individual AC circuit paths of the precharge circuit  118  include a reverse diode (e.g., Da, Db or Dc) with an anode coupled with the corresponding AC input terminal U, V or W, and a cathode coupled with the corresponding AC input line A, B or C. Furthermore, in order to mitigate excessive inrush current to charge the DC bus capacitor CDC and/or the capacitors Cf of the filter circuit  112 , the precharge circuitry  112  includes one or more precharge resistors R. In the examples of  FIGS. 4-7 , a single precharge resistor R is connected in the AC circuit path associated with the input line A and the rectifier input terminal U. In other examples, a single precharge resistor R can be connected in parallel with the corresponding switching device Q and the reverse diode D of another one of the AC phases. In further examples, two or more precharge resistors R can be included in the precharge circuitry  118 , individually connected in two or more corresponding ones of the AC circuit paths. 
     The controller  120  operates in first and second modes, including a first or PRECHARGE mode to charge the DC bus capacitor CDC, as well as a second or NORMAL operating mode during which the active front end rectifier  114  is actively switching to regulate the DC bus voltage VDC across the capacitor CDC. In the first mode, when the DC bus voltage is below a non-zero threshold value TH, the controller  120  provides precharge control signals  124   a ,  124   b  and  124   c  to turn all the semiconductor switching devices Qa, Qb, Qc off. This allows the DC bus capacitor CDC to charge through the precharge resistor R, with current flowing from the AC input line A through the resistor R into the filter  112  in  FIG. 4 , and current returning to one or both of the other AC input lines B and/or C through the corresponding reverse diodes of the precharge circuit  118 . In the example of  FIG. 4 , the precharge current operates to charge up the filter capacitors Cfa, Cfb and Cfc of the filter circuit  112 , in addition to charging up the DC bus capacitor CDC. In this example, the precharge current from the AC input line A conducts through the filter inductors L 2   a  and L 1   a  to the AC input terminal U of the rectifier  114 . This current flows through the diode of the first rectifier switch S 1  the, and charges the DC bus capacitor CDC. The return current flows from the capital DC− rectifier output terminal through one or both of the diodes of S 4  or S 6 , and flows back through the corresponding filter inductors of the B/V and/or C/W AC lines of the filter  112 , and returns through the reverse diodes Db and/or Dc of the precharge circuit  118  to the power source  102 . 
     The controller  120  receives a feedback signal representing the DC bus voltage VDC, and operates in the appropriate first or second mode accordingly. In the second mode, when the voltage VDC is greater than or equal to the non-zero threshold value TH, the controller  120  provides the precharge control signals  124   a ,  124   b  and  124   c  to turn all the semiconductor switching devices Qa, Qb and Qc on. This bypasses or short-circuits the one or more precharge resistor(s), and allows the controller  120  to begin and continue operation of the rectifier  114 . In the illustrated example of  FIG. 4 , the controller  120  also implements the rectifier control component  126 , which provides rectifier switching control signals  125  to turn the rectifier switching devices S 1 -S 6  off in the first mode. In the second mode, the controller  120  provides the rectifier switching control signals  125  to operate the rectifier switching devices S 1 -S 6  to convert AC input power to control the DC bus voltage VDC across the DC bus capacitor CDC. In certain examples, moreover, the controller  120  transitions from the first mode to the second mode by providing the precharge control signals  124   a ,  124   b ,  124   c  to initially turn on all the semiconductor switching devices Qa, Qb and Qc when a voltage of the given AC circuit path is at a peak value in order to minimize inrush In the second mode, the precharge switches Qa, Qb and Qc are turned on and maintained in an on state to allow normal operation of the motor drive  110 . 
     Any suitable semiconductor switching devices Qa, Qb and Qc can be used in the precharge circuit  118 . In the illustrated examples, the switching devices Qa, Qb and Qc are thyristors, such as silicon-controlled rectifiers (SCRs). The thyristors individually include an anode coupled with the corresponding AC input line A, B or C, as well as a cathode coupled with the corresponding AC input terminal U, V or W, and a gate to receive the corresponding precharge control signal  124   a ,  124   b  or  124   c  from the controller  120 . In this example, moreover, the controller  120  provides the precharge control signals  124   a ,  124   b  and  124   c  as constant frequency, constant duty cycle pulse signals to turn all the thyristors Qa, Qb and Qc on in the second mode. In one implementation, the thyristors Qa, Qb and Qc are fully fired in the second mode by applying a relatively high frequency, constant duty cycle pulse signal to each of the thyristors Qa, Qb and Qc, such as a 10% duty cycle at approximately 30 kHz switching frequency in one non-limiting implementation. This serves to maintain the thyristors Qa, Qb and Qc in the fully on condition throughout normal operation in the second mode. In other examples, different types or forms of semiconductor-based switches can be used in the precharge circuitry  118 , including without limitation bipolar transistors, field effect transistors, or other semiconductor devices, etc. 
     In the example of  FIG. 5 , the precharge circuit  118  is coupled in the filter circuit  112  with the filter capacitors Cf. In this implementation, like that of  FIG. 4 , the operation of the precharge circuit  118  allows precharging of the filter capacitors Cf, in addition to precharging the DC bus capacitor CDC. In  FIG. 6 , the precharge circuit  118  is also connected to the filter capacitors Cf within the filter circuit  112 .  FIG. 7  shows another example in which the precharge circuit  118  is disposed between the filter  112  and the rectifier  114 . 
     As seen in  FIGS. 8 and 9 , precharge resistors R can be provided in more than one of the AC phases.  FIG. 8  shows one example in which the precharge circuitry for phases A/U and B/V include precharge resistors Ra and Rb, respectively. In  FIG. 9 , precharge resistors Ra, Rb and Rc are respectively provided in each one of the AC phases. The precharge circuitry can be used in other single or multiphase implementations, including more than three phases (not shown). The precharge circuits  118  in other examples can include reversed SCR and diode connections. For example, an alternate implementation of the precharge circuit  118  can include, for a given phase, and SCR or other semiconductor switching device with an anode connected to the rectifier side and a cathode connected to the input side, along with a diode having an anode connected to the input side and a cathode connected to the rectifier side, where one or more such circuits includes a resistor connected in parallel with the SCR and diode. In such examples, the SCR(s) are fired at the negative peak of the corresponding phase voltage. 
       FIG. 10  illustrates a process or method  1004  charging a DC bus capacitor of a power conversion system  110 . At  1002 , the driver power conversion system  110  is powered up with the precharge switches (e.g., SCRs) all turned off. This first operating mode continues at  1004 , where the DC bus capacitor (e.g., CDC above) charges up through one or more precharge resistors R. The controller  120  determines at  1006  whether the DC bus voltage VDC exceeds a non-zero threshold value TH. If not, operation in the first mode continues with the DC bus capacitor CDC charging up at  1004  through the precharge resistor or resistors R. Once the DC bus voltage is greater than or equal to the threshold (YES at  1006 ), the process  1000  continues in a second mode, with the controller  120  beginning to fire the SCRs at  1008  when the AC voltage of the phase having the precharge resistor is at a peak value or maximum voltage on that phase relative to the other two phases to facilitate reduced inrush current. Operation thereafter continues in the second mode, with the controller  120  maintaining the precharge switches (e.g., SCRs) fully on. As mentioned above, this can be accomplished in one example by providing a constant duty cycle, constant frequency pulse signal to each of the SCRs. 
     In the preceding specification, various embodiments have been described with reference to the accompanying drawings. It will, however, be evident that various modifications and changes may be made thereto, and additional embodiments may be implemented, without departing from the broader scope of the invention as set forth in the claims that follow. The specification and drawings are accordingly to be regarded in an illustrative rather than restrictive sense.