Patent Publication Number: US-11652421-B2

Title: Method and apparatus to mitigate DC bus over-voltages on common AC bus systems utilizing DC and AC drives

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of, and claims priority to, and the benefit of, U.S. patent application Ser. No. 17/037,979, entitled “METHOD AND APPARATUS TO MITIGATE DC BUS OVER-VOLTAGES ON COMMON AC BUS SYSTEMS UTILIZING DC &amp; AC DRIVES”, and filed on Sep. 30, 2020, the entirety of which is hereby incorporated by reference. 
    
    
     BACKGROUND INFORMATION 
     The subject matter disclosed herein relates to power systems with a shared AC bus. 
     BRIEF DESCRIPTION 
     In one aspect, a system is provided having a shared multiphase AC bus, an AC drive, a multiphase line interface filter, and a further drive. The AC drive has a multiphase AC input; a diode rectifier, an input filter, and an inverter. The multiphase line interface filter includes phase circuits are coupled to respective phase lines of the shared multiphase AC bus and include an inductor, a resistor and a capacitor. The individual inductors have a first terminal coupled to the respective phase line of the shared multiphase AC bus, and a second terminal coupled to a respective phase of the multiphase AC input. The individual resistors have a first terminal coupled to the respective phase of the multiphase AC input, and a second terminal, and the individual capacitors have a first terminal coupled to the second terminal of the resistor, and a second terminal, with the second terminals of the capacitors of the individual phase circuits being coupled together. The further drive has a further AC input coupled to the shared multiphase AC bus, and an SCR rectifier coupled to the further AC input. 
     A system is provided in another aspect, which includes a shared multiphase AC bus, an AC drive, a further drive, and a further line interface filter. The AC drive has a multiphase AC input, a diode rectifier, an input filter, and an inverter. The further drive has a further AC input coupled to the shared multiphase AC bus, and an SCR rectifier coupled to the further AC input. The further line interface filter has further phase circuits coupled to respective phase lines of the shared multiphase AC bus. The individual further phase circuits include a further resistor coupled to the respective phase of the further multiphase AC input, and a second terminal, as well as a further capacitor having a first terminal coupled to the second terminal of the further resistor, and a second terminal, where the second terminals of the further capacitors are coupled together. 
     In another aspect, a method includes determining a filter inductance as, 3% to 5% of a per-phase equivalent inductance of an input reactor based on the total KVA rating and rated voltage of the drive or group of drives, or 5% to 8% of a per-phase equivalent inductance of an isolation transformer. The method also includes determining a filter capacitance as 5 to 15 times a per-phase equivalent capacitance of the drive or group of drives, and determining a filter resistance as greater than or equal to two times a damping ratio times a square root of a ratio of the filter inductance to the filter capacitance, where the damping ratio is greater than or equal to 1.0 and less than or equal to 2.0. The method further includes coupling a multiphase line interface filter coupled between a drive or group of drives and a shared multiphase AC bus, where the multiphase line interface filter includes phase circuits coupled to respective ones of the phase lines of the shared multiphase AC bus. The individual phase circuits include an inductor, a resistor and a capacitor. The inductor has the filter inductance, a first terminal coupled to the respective phase line of the shared multiphase AC bus, and a second terminal coupled to a respective phase of a multiphase AC input of the drive or group of drives. Thee resistor has the filter resistance, a first terminal coupled to the respective phase of the multiphase AC input, and a second terminal, and the capacitor has the filter capacitance, a first terminal coupled to the second terminal of the resistor, and a second terminal, where the second terminals of the capacitors of the individual phase circuits are coupled together. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of a power system with a shared AC bus and a line interface filter coupled to a drive with a diode front end rectifier. 
         FIG.  2    is a schematic diagram of another power system with a shared AC bus and a line interface filter coupled to a drive with an SCR rectifier. 
         FIG.  3    is a schematic diagram of another power system with a shared AC bus and a line interface filter coupled to a drive with an isolation transformer and an SCR rectifier. 
         FIG.  4    is a flow diagram of a method of coupling a drive to a shared AC bus. 
         FIG.  5    is a schematic diagram of a power system with a shared AC bus and a line interface filter coupled to a group of drives having active front end rectifiers. 
         FIG.  6    is a schematic system diagram of a shared AC bus system. 
         FIG.  7    is a schematic system diagram of another shared AC bus system. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    shows a power system  100  with a shared AC bus  102  with one or more further drives  106 , as well as a line interface filter  110  (LIF) coupled to a low voltage AC variable frequency drive  120  (e.g., LV AC VFD) a multiphase AC input  103 , and a three-phase AC output that drives a motor load  104 . The shared multiphase AC bus  102  is a three-phase bus with phase lines a, b and c. The illustrated systems have three-phase AC connections. In other examples, more than three phases can be used. The line interface filter  110  is coupled between the shared multiphase AC bus  102  and the multiphase AC input  103  of the AC drive  120 . The shared AC bus  102  is coupled to one or more other or further drives shown schematically in  FIG.  1   . In various examples, the other drive or drives  106  includes one or more low voltage DC drives (LV DC) and/or one or more medium voltage AC variable frequency drives (e.g., MV AC VFD) with one or more associated isolation transformers. The further drive or drives  106  have one or more corresponding further AC inputs (e.g.,  203  in  FIGS.  2  and  3    below) coupled to the shared multiphase AC bus  102 , as well as a silicon controlled rectifier (SCR) rectifier circuit coupled to the further AC input. 
     In practice, connection of a drive or drives  106  having an SCR rectifier to a shared AC bus  102  without the line interface filter  110  can cause undesirable overvoltage tripping of the LV AC VFD drive  120 . Firing of the SCRs in the other drive or drives  106  creates notches in the corresponding phase voltages of the shared bus  102 . However, it is desirable to use a single large capacity (e.g., large mega volt-amp or MVA) low voltage distributed shared common AC bus for multiple loads in a factory or facility, without the traditional approach of using transformers to separate ac drives, dc drives or motor loads, or other load types from one another, in order to reduce cost. Connecting a low voltage DC drive to the shared AC bus can cause AC line notching power quality problems, due to 6-pulse SCR phase control, potentially leading to DC bus overvoltage trip events in a connected low voltage AC variable frequency drive  120 . 
     Certain SCR rectifier-based other drives  106  are themselves designed with AC line feedback voltage filtering to ignore their own line notches and those of others, so that the LV DC drives all function properly on a common bus. However, a low voltage AC variable frequency drive  120  or other AC drive with a diode front end rectifier can be affected by operation of an SCR-based rectifier in the other drive or drives  106  when connected to the common or shared AC bus  102 . 
     The multiphase line interface filter  110  includes three phase circuits coupled to respective phase lines a, b and c of the shared multiphase AC bus  102 . The individual phase circuits include an inductor  111 , a resistor  114  and a capacitor  117 . The individual inductors  111  each have a first terminal  112  and a second terminal  113 . The first terminal  112  of each respective inductor  111  is coupled to the respective shared bus phase line a, b or c, and the second terminal  113  is coupled to a respective phase of the multiphase AC input  103 . The inductors  111  of the individual phase circuits of the multiphase line interface filter  110  have an inductance L f  of 3% to 5% of a per-phase source impedance X based on the KVA rating and rated voltage of the single LVAC drive  120 . 
     The individual resistors  114  of the multiphase line interface filter  110  include a first terminal  115  and a second terminal  116 . The first terminal  115  of each respective resistor  114  is coupled to the respective phase of the multiphase AC input  103 . The individual capacitors  117  have a first terminal  118  coupled to the second terminal  116  of the resistor  114 , and a second terminal  119 . The second terminals  119  of the capacitors  117  of the individual phase circuits are coupled together. 
     The AC drive  120  includes a multiphase input filter  121 , such as an electromagnetic interference (EMI) filter with capacitors  122  and  126 , a diode front end rectifier  130 , and an inverter  132 . The input filter  121  is coupled between the multiphase AC input  103  and the diode rectifier  130 , and the diode rectifier  130  is coupled between the input filter  121  and the inverter  132 . The input filter capacitors  122  have first terminals coupled to respective phases of the multiphase AC input  103 , and second terminals coupled to one another. The fourth filter capacitor  126  is coupled between the second terminals of the capacitors  122  and a ground or other reference voltage node  129 . The input filter  121  has a per-phase equivalent capacitance C eq . 
     The diode rectifier  130  includes rectifier switching devices coupled between the multiphase AC input  103  and a DC bus, for example, having a DC bus capacitor (not shown). The output inverter  132  includes inverter switching devices coupled between the DC bus circuit and the AC output to provide AC output signals to drive the motor load  104 . AC drive  120  includes a controller that operates the inverter  132  by providing inverter switching control signals. Operation of an SCR-based rectifier in the other drive or drives  106  on the shared AC bus  102  can cause loss of volt—second area from deep notches near the sinewave peak, and reduce the AC drive DC bus voltage by 5%. This condition may still allow proper operation of the inverter  132  and the AC motor load  104  without DC bus under-voltage tripping. 
     However, AC variable frequency drive DC bus overvoltage issues can arise on a low voltage shared AC bus from second order effects in a low voltage DC drive. DC drive non-ideal characteristics are applied to every device on the AC line and include SCR snubber high voltage commutation spikes and DC drive SCR snubber high frequency ring between DC drives that may interact with the AC variable frequency drive line side  121  EMI filter network resonant frequency to develop amplified voltage. The amplified voltage, along with LVDC drive high voltage commutation spikes are rectified by the 6-pulse diode bridge of diode rectifier  130  can charge up the AC drive DC bus capacitance to random and often unexplained overvoltage trip shutdown levels. The example multiphase line interface filter  110  and further described methods and apparatus provide an inventive method &amp; apparatus topology that is a pre-engineered and pre-analyzed product to mitigate the non-ideal DC drive characteristics reflected onto the shared common AC bus  102 . In one example, the multiphase line interface filter  110  damps out voltage oscillations and mitigates DC drive voltage spikes at the AC drive input side of the LV AC VFD  120  to prevent undesirable DC bus overvoltage shutdown events. In other examples described below, a line interface filter (e.g., filter  210  in  FIGS.  2 ,  3 ,  6  and  7    below) can be applied at the problem source, such as a low voltage DC drive or a medium voltage drive, which include SCR-based rectifiers. Certain examples, moreover, apply the line interface filter apparatus to medium voltage topology AC line side using phase control front end converters. The described examples provide a solution to mitigate or eliminate exhaustive overvoltage trip engineering failure analysis for every new shared AC bus system configuration. 
     The capacitors  117  of the individual phase circuits of the multiphase line interface filter  110  have a capacitance C f  of 5 to 15 times the per-phase equivalent capacitance C eq  of the AC drive  120 . The resistors  114  of the individual phase circuits have a resistance R f  greater than or equal to two times a damping ratio ζ times a square root of a ratio of the inductance L f  to the capacitance C f . The damping ratio ζ is greater than or equal to 1.0 and less than or equal to 2.0, such as about 1.2 in one example. In the illustrated example, the resistors  114  of the individual phase circuits of the multiphase line interface filter  110  each include a tap T to set the corresponding resistance Rf. 
     The multiphase line interface filter  110  provides an overdamped second order filter that suppresses second order effects caused by SCR-based rectifiers in the other drive or drives  106  to mitigate or avoid undesired DC bus overvoltage trip events in the protected low voltage variable frequency drive  120 . A single line interface filter  110  of this construction can be provided at the line input side of individual low voltage variable frequency drives  120  connected to a shared bus  102 , alone or in combination with a further line interface filter  210  at the line interface of the individual other drives as shown in  FIG.  6    below. In another system configuration, a single protective line interface filter  110  can be coupled between the shared AC bus  102  and the multiphase AC inputs  103  of a group of protected low voltage variable frequency drives  120 , as shown in  FIG.  7    below. 
     The line interface filter  110  in  FIG.  1    damps out the voltage oscillation at the AC drive multiphase AC input  103 . The line interface filter  110  includes the resistors  110  (R f ), capacitors  117  (C f ) and the input filter  121  has equivalent per-phase capacitors  122  of capacitance C eq , along with the inductors  111  (L f ) to form an RLC circuit to mitigate the voltage oscillation. The transfer function can be derived as below: 
     
       
         
           
             
               
                 
                   V 
                   o 
                 
                 ⁡ 
                 
                   ( 
                   s 
                   ) 
                 
               
               
                 
                   V 
                   i 
                 
                 ⁡ 
                 
                   ( 
                   s 
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             = 
             
               
                 
                   
                     R 
                     f 
                   
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                     C 
                     f 
                   
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                 + 
                 1 
               
               
                 
                   
                     R 
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                     L 
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                     eq 
                   
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                       ( 
                       
                         
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     When selecting C f  sufficiently high (e.g., C f &gt;(5˜15)Ceq is usually sufficient) and properly selecting Rf as indicated below, the dynamic behavior of the voltage Vo will be dominated by the components Lf, Rf and Cf (the dynamic oscillating power or current exchange is mainly between L f  and Cf and the impact of Ceq will be minimum. With this condition, the above transfer function can be reduced to the following second order system: 
     
       
         
           
             
               
                 
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     The system ω and damping factor ζ can be derived as follows: 
     
       
         
           
             
               
                 
                   
                     
                       
                         V 
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     When specifying damping ratio, the filter resistance Rf can be calculated as: 
     
       
         
           
             
               
                 R 
                 f 
               
               = 
               
                 2 
                 ⁢ 
                 ζ 
                 ⁢ 
                 
                   
                     
                       L 
                       f 
                     
                     
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                 for 
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             , 
             
               
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                 f 
               
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     Once the line reactor Lf is known, the filter capacitance Cf can be selected first to swamp out the effect of EMI capacitance Ce, and then choose damping ratio ζ to determine the resistance Rf: 
     To damp out voltage oscillation and reduce the peak voltage, a minimum “ζ”=0.707 for 2nd order system would be required. However, an overdamping value “ζ”=1.2 is selected to minimize peak overshoot voltage to a line notch step response besides eliminating oscillations so that the VFD DC bus does not peak charge to the overshoot. 
     With this resistance value calculated, and assuming a non-inductive resistor winding type is used, then the per unit step responses of the system with different C values is obtained. A filter with ζ=1.2 for Rf assuming Cf=8.4*Ce=4.7uF can effectively damp out the oscillation and significantly reduce the peak voltage with only 20% overshoot. For ζ=1.2 for Rf, Lf=450uH, Ce=0.56uF, when without a filter, peak voltage can be almost 3× the steady state voltage when multiple low voltage DC drives are on the same AC line, since the space between line notches could vary or almost synchronize between SCR DC drive firing angle α=30 to 90 degree. As a result, the first line notch response does not decay and the second nearby notch double pumps up the line step response. 
     The multiphase line interface filter  110  in one example includes a standard low cost and readily available AC line reactor (Lf) of approximately 3% (e.g., 0.03 pu) or more of the per-phase source impedance (X) based on the single LVAC drive  120  VA rating (VAr) and rated voltage (Vr). Filter inductance can then be calculated as Lf=(0.03 pu*(Vr) 2 )/(VAr*2*π*f_utility). 
     The multiphase line interface filter  110  in this example also includes a pre-designed assembly having the resistors  114 , where Rf is pre-selected value for the variable frequency drive hp size, and fixed capacitors  117  (Cf) with a value suitable for use with multiple AC variable frequency drive sizes. In the illustrated example, Cf is selected to be five or more times larger than the EMI filter per-phase equivalent capacitance Ce. Since the AC variable frequency drive capacitance Ce capacitor is the same value for a large range of drive horsepower sizes from 1 Hp to 250 Hp, this allows a single Cf capacitor to be implemented in an assembly. Moreover, the illustrated example uses tapped resistors  114  having taps T to set the filter resistance Rf. This allows a single assembly to be installed with inductors  111  and suitable tap settings to accommodate many different protected LV AC VFD drives  120 . In one example, the per-phase inductance Lf is selected as 3% to 5% impedance based on the AC bus voltage and the rated current of the drive  120 . The drive equivalent inherent per-phase capacitance identified as from drive internal EMI filter capacitance Ce (e.g., Ceq=Ce). 
       FIG.  2    shows anther power system configuration  200  with the shared AC bus  102  and a further line interface filter  210  coupled to a low voltage DC drive  220  (LV DC) with an SCR rectifier  222  and output inductors  228  to drive a load  204 . The further drive  220  has a further AC input  203  coupled to the shared multiphase AC bus  102 , and the SCR rectifier  222  is coupled to the further AC input  203 . The system  200  shows a utility source impedance as inductors  211  of inductances Xs, first terminals  212  coupled to the shared bus  102 , and second terminals  213  coupled to the line interface filter and the drive  220  at a point of common coupling PCC. The shared AC bus  102  in this example has AC phase voltages e a , e b  and e c , and the point of common coupling has AC phase voltages V a , V b  and V c , In this example, the inductors  211  represent the is the low voltage utility source AC impedance and the further inductors  214  (X T ) are the input inductors of the DC drive, such as existing % Z AC line reactance (e.g., L T =X T /(2*π*f) to reduce shared AC bus line notch depth. 
     The LV DC drive  220  has an SCR-based rectifier circuit  222  with upper and lower SCRs  224  for each phase line. The SCRs have associated snubber networks that individually include a snubber resistor  225  and a snubber capacitor  226 . In operation, during SCR commutation, two lines are shorted together so that the resulting AC bus notch depth is a voltage divider between the impedances X T  and X S . The line-to-line equivalent circuit input impedance immediately following SCR commutation turn-off Req=Rsn/3, Ceq=3 Csn. In one example, moreover, the per-phase equivalent inductance Lf for the line interface filter  210  is set to an existing LV DC drive inductance L T . The further line interface filter  210  includes further phase circuits coupled between the shared multiphase AC bus  102  and the further AC input  203 , for example, at the point of common coupling PCC of the respective phase of the further multiphase AC input  203  as shown in  FIG.  2   . The multiphase line interface filter  210  operates with the existing inductors  214  to form per-phase RLC filter circuits that protect the shared AC bus  102  from notches created by commutation of the SCR rectifier  222 . The inductors  214  of the individual phase circuits of the multiphase line interface filter  210  have an inductance X T  of 3% to 5% of a per-phase source impedance X. based on the KVA rating and rated voltage of the single LV DC drive  220 . 
     The capacitors  117  of the individual phase circuits of the multiphase line interface filter  210  have a capacitance C f  of 5 to 15 times a per-phase equivalent capacitance C eq  of the LV DC drive  220 , and the resistors  114  of the individual phase circuits of the multiphase line interface filter  210  have a resistance Rf greater than or equal to two times a damping ratio ζ times a square root of a ratio of the inductance L T  to the capacitance C f , where the damping ratio ζ is greater than or equal to 1.0 and less than or equal to 2.0, such as about 1.2 in one example. In the illustrated example, the equivalent capacitance of the DC drive  220  is based on the capacitance Csn of the snubber capacitors  226 . In one example, the capacitance C f  is set to 5×Ceq=15×Csn. As in the above example protective line interface filter  110 , the guarding line interface filter  210  includes tapped filter resistors  114  with individual taps T to set the corresponding resistance R F . 
     The added inductors  214  (X T ) and the shared AC bus source impedance Xs form a voltage divider that sets the notch depth at the point of common coupling PCC. In addition, if Rsn-Csn snubbers are under-damped, commutation voltage spikes, up to 2x peak sinewave voltage peak, can occur along with high frequency follow on ringing. Further, at low magnitude dc load current, dc bus ripple current interacts with the snubber circuit impedance Rsn-Csn and the line impedance Xs to form a highly oscillatory waveform at the PCC for all the drives coupled to the shared AC bus  102 . The line interface filter adds the resistors  114  and capacitors  117  in each phase to provide an overdamped second order filter that suppresses second order effects caused by the SCR-based rectifier circuit  222  to mitigate or avoid undesired DC bus overvoltage trip events in the protected other drives  106  of  FIG.  2   . 
       FIG.  3    shows another power system  300  with a shared AC bus  102  and a line interface filter  210  coupled to a medium voltage AC drive  320  (MV AC) with the impedance of an isolation transformer  330  and cascaded SCR rectifiers  322 . The transformer  330  includes a three-phase, delta-connected, primary circuit P with a per-phase impedance X T , as well as delta-connected, phase shifted, secondary circuits S1 (+20 degrees), S2 (−20 degrees) and S3 (0 degrees). Each three-phase secondary circuit is coupled to a six-pulse (6P) SCR rectifier  322 , and the DC outputs of the individual 6P rectifiers  322  are coupled in series with one another to collectively provide a DC output. The DC output from the SCR rectifiers  322  is provided through DC link inductors  324  (Ldc) to an output inverter  332  that drives the motor load  104 . The filter  210  is as described above in connection with  FIG.  2    and can be used as part of a medium voltage shared bus  102  for connection at the point of common coupling PCC. The multiphase line interface filter  210  guards the shared bus  102  against the notches associated with operation of the 18 pulse DC converter with the three secondary phase shifted SCR 6-pulse bridge circuits  322 . As described above, the individual SCR rectifiers  322  include respective Rsn Csn snubbers, the X S  is medium voltage utility source AC impedance and X T  is 18 pulse isolation transformer % Z leakage reactance refereed to the primary side (LT=X T /(2*π*f) to reduce shared medium voltage AC bus line notch depth. The primary leakage reactance (X T ) and the shared AC bus source impedance Xs form a voltage divider that sets the notch depth at the point of common coupling PCC. During SCR commutation two lines are shorted together so that AC bus notch depth is a voltage divider between the impedances X T  and X S . The medium voltage line-to-line equivalent circuit input impedance immediately following SCR commutation turn-off is the result of the snubber capacitors of the three bridges  322  operating with each 6-pulse bridge (e.g., Req=Rsn/3, Ceq=3 Csn)*3 or Ceq=9*Csn. In one example, the per-phase filter capacitance Cf is calculated as [ 5 ×Ceq] or [ 45 ×Csn]. 
       FIG.  4    shows a method  400  of coupling a drive or group of drives  120 ,  220 ,  320  to a shared multiphase AC bus  102 . The method  400  begins at  402  and includes determining the line interface filter component value solution for one or more of a single LV AC drive (e.g.,  FIG.  1    above), a group of LV AC drives (e.g.,  FIG.  5    below), a single LV DC drive (e.g.,  FIG.  2    above), or a single MV 18-pulse AC drive ( FIG.  3   ). At  404 , the method  400  includes determining the filter inductance L f  as one of 3% to 5% of a per-phase source impedance X based on the KVA rating and rated voltage of a single LVDC drive  220  (e.g.,  FIG.  2   ), 3% to 5% of a per-phase equivalent inductance of an input reactor based on the total KVA rating and rated voltage of a single LV AC  120  drive or group of LVAC drives  120  (e.g.,  FIG.  1    above and  5  below), and 5% to 8% of a per-phase equivalent inductance X based on the primary referred leakage reactance of an isolation transformer  330  (e.g.,  FIG.  3    above). The per-phase equivalent capacitance Ceq of the drive or group of drives is determined at  406  in one example, and the method  400  continues at  408  with determining the per-phase filter capacitance C f  as 5 to 15 times a per-phase equivalent capacitance C eq  of the drive or group of drives. 
     The damping ratio ζ is determined at  410  of 1.0 or more and 2.0 or less (e.g., ζ=1.2). The method  400  continues at  412  with determining the filter resistance R f  as greater than or equal to two times the damping ratio times a square root of a ratio of the filter inductance L f  to the filter capacitance C f . At  414  in one example, the filter resistance R f  is set at  414  to the calculated value from  412  or the next higher standard resistor value. The capacitor current is calculated in one example at  416  according to the system AC source voltage at the fundamental frequency f 0 : Io=V p /(R f2 +(1/(2πf 0  C f ) 2 ) 1/2 , where V p  is the rms phase voltage of the AC voltage source coupled to the shared AC bus  102 . At  418 , the method further includes determining the capacitor RMS current I crms −(2 to 3)*Io. At  420 , the method includes determining the capacitor peak current I cpk =ΔV pk /(R f   2 +L f /C f ) 1/2 , where ΔV p k is peak voltage reference at max notch depth. The capacitor current rating is sized at  422  based on I crms  and I cpk . 
     The method  400  continues at  424  with calculating the power P 0  dissipated by each filter resistor  114  at the fundamental frequency f 0 :P 0 =V p I O , and considering the power dissipated by the voltage harmonic components. For a system (e.g.,  FIG.  2    above) with a 6 pulse SCR-rectifier based DC drive, the selected the filter resistor power rating P r  is given as P r =(2-3)*P 0 . At  426 , the example method  400  further includes setting the resistor value R f  by setting the resistor taps T. The method  400  continues at  428  with coupling the multiphase line interface filter  110  between the drive or group of drives and the shared multiphase AC bus  102 , where the multiphase line interface filter  110  includes phase circuits coupled to respective ones of the phase lines a, b, c. The individual phase circuits include the inductor  111  having the filter inductance L f , a first terminal  112  coupled to the respective phase line a, b, c of the shared multiphase AC bus  102 , and a second terminal  113  coupled to a respective phase of a multiphase AC input  103  of the drive or group of drives  120 ,  220 ,  320 . The individual phase circuits also includes a resistor  114  having the filter resistance Rf, a first terminal  115  coupled to the respective phase of the multiphase AC input  103 , and a second terminal  116 , and a capacitor  117  having the filter capacitance Cf, a first terminal  118  coupled to the second terminal  116  of the resistor  114 , and a second terminal  119 , where the second terminals  119  of the capacitors  117  of the individual phase circuits are coupled together. In one example, coupling the multiphase line interface filter  110  at  428  includes coupling the first terminal  115  of the resistors  114  of the individual phase circuits to the point of common coupling PCC of the respective phase of the multiphase AC input  203 . 
       FIG.  5    shows a power system  500  with a shared AC bus  102  and a single line interface filter  110  coupled between the shared AC bus  102  and a group of LV AC VFD drives  120 , each having an active a diode front end rectifier. The line interface filter  110  is generally as described above in connection with  FIG.  1   . In this example, the group drive inductance Lf for the line interface filter  110  is selected as 3% or 5% impedance based on the rated AC bus voltage and on the sum of the drive rated currents of the group of drives  120 . In this example, moreover, the total drive inherent capacitance is determined as the sum of the internal EMI filter capacitances on a per phase basis. For an integer number ‘n’ grouped drives  120 , the equivalent per-phase capacitance Ceq=Ce1+Ce2+ . . . +Cen. 
       FIG.  6    shows a shared AC bus system  600  having individualized line interface filters  110  and  220  respectively associated on an individual basis with the drives  120  having diode rectifiers (e.g., LV AC VFDs) and those that have SCR-based rectifier circuits (e.g., LV DC drives  220  of  FIG.  2    above, and/or medium voltage AC drives such as MV AC VFDs  320  and isolation transformers  330  as in  FIG.  3    above). The example system  600  includes a medium voltage AC source  601  coupled to the MV VFD drives  320  through a isolation transformer  330  via a medium voltage shared bus and associated line interface filters  210 , a low voltage AC source  602 , and a switch  604  (SW1) that couples a transformer  606  to either the low voltage source  602  or the medium voltage source  601 .  FIG.  7    shows another shared AC bus system  700  having a shared low voltage AC bus  102 , the sources  601  and  602 , the switch  604  and the transformer  606  described above. In this example, a single protective line interface filter  110  is coupled between the shared AC bus  102  and the multiphase AC inputs  103  of a group of “N” protected low voltage variable frequency drives  120 . 
     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.