Abstract:
A distributed electrical power generating and utilizing system includes an induction generator driven by a prime mover requiring reactive power to operate for providing electrical power on a bus. The bus has a gross load and is also connectable to a utility power grid by a switch. The gross load includes at least a non-linear electrical load component, typically including a variable speed device and associated diode rectifier front end. The bus includes a harmonic filter having a power factor-correcting capacitor integrated therewith for collectively compensating harmonic distortion caused by the non-linear load component and for correcting power factor to compensate for reactive power required by at least the inductive generator.

Description:
TECHNICAL FIELD 
     This disclosure relates to electrical power generation and utilization, and more particularly to distributed electrical power generation and utilization systems. The disclosure relates even more particularly to power quality concerns associated with certain generators and load-types connected to the utility grid. 
     BACKGROUND ART 
     Entities other than the utility power grid now frequently generate electrical power for local or dedicated uses, as well for connection to the utility grid to generate revenues and/or offset costs. Such systems may be referred to as having a dual, or multiple, sourced power bus. 
     In some electric power generating systems, the manner of managing the energy that will operate the electric generator may require auxiliary equipment, such as pumps and fans. An example is a power system which recovers heat, from such sources as geothermal wells, food processing plants or landfills, or the like, utilizing an Organic Rankin Cycle system. 
     For economic efficiency, it is desirable that a low cost generator provide power for all auxiliary equipment, while at the same time providing power which has shape (little or no harmonic distortion), power factor (PF), and frequency that are all suitable for interface with the utility power grid. Synchronous generators are typically more expensive and require additional controls compared with other, cheaper generators such as induction generators which, because of their construction, are less expensive than synchronous generators. However, induction generators have an inherently lower power factor (PF) than what is typically acceptable to utility grids. Moreover, to the extent certain types of non-linear loads, such as variable-speed drives with diode front ends, are associated with the auxiliary equipment, high levels of harmonic distortion may occur in the current. 
     An example of the conditions described in the foregoing paragraph may be seen in the characterization of the “prior art” (FIG. 1 therein) described in U.S. Pat. No. 7,038,329 (hereinafter &#39;329) to S. J. Fredette, et al for “Quality Power From Induction Generator Feeding Variable Speed Motor”, the disclosure of which is incorporated herein by reference to the extent consistent and appropriate. Similar thereto is the following characterization herein of that “prior art” as it is depicted in  FIG. 1  herein. 
     Referring to  FIG. 1  herein of the “prior art”, there is depicted a single line of a typically multi-phase (typically 3-phase) power system in which an induction electric power generator  10 , driven by some form of prime mover  12 , is connected, or connectable, with the utility power grid  14  via power bus  16  including switch  18 , typically a breaker or contactor. Moreover, there is depicted broadly in block form, one or more non-linear loads  20 , typically including variable speed drives with diode rectifier front ends and associated with the auxiliary loads, operatively connected to the power bus  16 . Those non-linear loads, and particularly the variable speed drives with diode rectifier front ends, are the same or similar to elements 11, 12, 13 and 16 of FIG. 1 of the aforementioned &#39;329 patent. Because the induction generator requires reactive power to operate, the reactive power is provided by a power factor correction capacitor (C pfc )  22  in order to maintain the power factor at the interface with the utility grid above limits imposed by accepted standards. The power factor limits are normally above 90%, and mainly above about 95%. The power factor correction capacitor(s)  22  is typically connected in shunt with the non-linear loads  20 . Still further, to address the significant levels of harmonic distortion in the current that may be introduced by the non-linear loads  20 , one or more harmonic filter(s)  24 , is/are connected in series with the non-linear loads  20 . A source inductance (L s )  26  is represented in the power bus  16  as being the inductance of the power grid  14  and any interface transformer at or near the installation site, and an illustrated source resistance  27  is of similar origins. 
     As shown in  FIG. 1  herein, as well as in FIG. 1 of the aforementioned &#39;329 patent, the power factor correction capacitor (C pfc )  22  and the harmonic filter(s)  24  are separate and distinct from one another. Stated another way, one may be said to be external to the other. Indeed, although the harmonic filter  24  may include a filter capacitor or capacitance in combination with one or more inductive impedance elements to provide the requisite filtering of the harmonics, such capacitor is separate from the power factor correction capacitor (C pfc )  22 . 
     The prior art configuration of  FIG. 1  will be further understood in the context of prior art  FIG. 2 , which is substantially the same as  FIG. 1  but configured to illustrate the harmonic filter  24  and the power factor correction capacitor  22  in greater detail and as separate from one another. The AC power grid and the induction power generator are collectively represented by section block  100  containing the AC source  114 , the inductive source impedance  126 , the source resistance  127 , and induction generator  110 . Correspondingly, the current to the various loads, including the non-linear loads, is represented by the current source symbol  120 . Intermediate the AC source  114  and the output load current  120  are separate section blocks  122  and  124 , representing the power factor correction capacitor  22  and the harmonic filter  24 , respectively, of  FIG. 1 . 
     The power factor correction block  122  includes a power factor correction capacitor  22  having a detuning inductor (L det )  30  in series therewith, and is connected across the AC source  114  and across the induction generator  110 . The inductor  30  is required to form a resonant tank so as to limit harmonic currents from flowing to the capacitor(s) and thereby causing excessive heating, which may be life-limiting. 
     Similarly, the harmonic filter section block  124  representing the harmonic filter  24  of  FIG. 1  is also connected across the AC source  114  in a general “T” network including, more specifically, a “bridged-T”. The cross arm of the general T harmonic filter section block  124  includes several inductances arranged or depicted in series, including an input inductance (L in )  32  shown connected at one end to the junction of the source inductance  126  or the source resistance  127  and the detuning inductance  30 , and at the other end to the cross arm of a bridged-T comprising a parallel-bridged arrangement of a further inductance (L 1 )  34  connected in parallel with a series connection of a resistance (R)  36  and a filter inductance (L f )  38 . The cross arm of the general T filter network is completed by the connection at the junction of inductances  34  and  38  of one end of a still further inductance (L 2 )  40 , the other end of which is connected to one side of the non-linear load(s) represented by the current source  120 . A filter capacitor (C f )  42  is connected at one end to the junction between the resistance  36  and the filter inductance  38  and is thereafter connected in shunt with the power source  114  and current source  120  to complete the “vertical arm” of the bridged-T filter. 
     While the afore-described arrangement is effective in maintaining acceptable power factor in the presence of the induction generator and of also reducing or eliminating the harmonic distortion introduced by the non-linear loads, especially as represented by the variable speed drives with diode rectifier front ends, it nonetheless comes at significant parts count and cost of equipment. More specifically, the ratings required of the filter capacitor(s) and the power factor correction capacitor(s) cause them to be relatively large and expensive. Although the insulated gate bipolar transistor switched bridge and associated control circuitry of the aforementioned &#39;329 patent do provide a good quality of power without power factor correction and with little or no harmonic distortion, that circuitry itself comes at a significant cost or expense, such that it may not be a particularly acceptable trade-off. 
     Accordingly, what is needed is an arrangement in which an induction generator and associated non-linear loads are connectable to the utility power grid and operate with an acceptable power factor and minimal harmonic distortion, yet are reliable and cost effective in attaining that result. 
     SUMMARY 
     The present disclosure provides a distributed electrical power generating and utilizing system having an induction generator driven by a prime mover and requiring reactive power to operate for providing electrical power on a bus, the bus having a gross load and also being connectable to a utility power grid by a switch, such as a circuit breaker or contactor; the gross load including at least a non-linear electrical load component; and also connected to the bus is a harmonic filter having a power factor-correcting capacitor integrated therewith for collectively compensating harmonic distortion caused by the non-linear load component and for correcting power factor to compensate for reactive power required by at least the induction generator. The integrated capacitor(s) in the harmonic filter contain(s) series inductance to form a tank circuit to reduce harmful effects of harmonic currents on the capacitor life. Further, the capacitor required of the harmonic filter will, if sized appropriately, also serve as the power factor correction capacitor. Multiple such capacitors may be switched into or out of the circuit to improve dynamic voltage stability at light loading conditions. Typically, a harmonic filter having a power factor-correcting capacitor integrated therewith is provided for each phase of the power generating system. 
     The non-linear load component may typically include variable speed drives with diode rectifier front ends. 
     A process flow routine is disclosed for guiding the design of the harmonic filter with integrated power factor correction. Values for input variables such as power of the generator, power of variable speed drives, power factor of generator, power factor of grid, quality factor, resonance, anti-resonance, and source/grid inductance, resistance and % impedance are selected to calculate the requisite values for the capacitance, inductance and resistance associated with the integrated harmonic filter and power factor-correcting capacitor. 
     The foregoing features and advantages of the present disclosure will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a simplified schematic block diagram of one phase of an exemplary electric power generation system having an induction generator powering at least non-linear loads and providing power to a power utility grid, and depicting a harmonic filter and separate power factor correction capacitor as known in the prior art; 
         FIG. 2  is a further depiction of the prior art system of  FIG. 1 , illustrating the harmonic filter and separate power factor correction capacitor in further detail; 
         FIG. 3  is an illustration similar to  FIG. 2  of one phase of an exemplary electric power generation system having an induction generator powering at least non-linear loads and providing power to a power utility grid, and illustrating the harmonic filter having a power factor-correcting capacitor integrated therewith in accordance with the present disclosure; 
         FIG. 4  is a simplified flow diagram illustrating the process and parameters for guiding the design of the  FIG. 3  harmonic filter having a power factor-correcting capacitor integrated therewith in accordance with the present disclosure; and 
         FIG. 5  is a frequency response graph showing a filter designed with integrated power factor correction capacitor(s) vs. a filter designed with external power factor correction capacitor(s). 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 3 , there is illustrated one phase of an exemplary electric power generation system having an induction generator powering at least non-linear loads and providing power to a power utility grid, and particularly illustrating the harmonic filter having a power factor-correcting capacitor integrated therewith in accordance with the present disclosure. It will be noted that in many respects the system of  FIG. 3  is similar to or the same as that of  FIGS. 1 and 2 . This is particularly so with respect to the inclusion, implied herein but not again shown for the sake of brevity, of an induction generator providing power to the non-linear loads including, for example, variable speed drives with diode rectifier front ends, and being connected, or connectable, with the utility power grid via a power bus. However, the system of  FIG. 3  differs in that the harmonic filter for each phase has the power factor correction capacitor, or capacitors, integrated therewith, and thus avoids at least the cost of a separate capacitor for that latter function, as had been the case with the prior art. 
     Referring to  FIG. 3  in greater detail, the AC power grid and the induction power generator are collectively represented by voltage source  214  and the induction generator is represented by voltage source  210 . Associated with that voltage source are a source inductance  226  and a source resistance  227 . Correspondingly, the current to the various loads, including the non-linear loads, is represented by I 0 , and is depicted as being associated with the non-linear loads represented broadly by block  220  that includes the symbol of a rectifier to represent a variable speed drive and associated diode rectifier front end, similar to  120  in  FIG. 2 . Intermediate the AC voltage source  214  and the output load current  220  is a generalized T filter  224  which includes the more specific bridged-T harmonic filter  224 ′. 
     Although the component configuration of the generalized T filter  224  of  FIG. 3  is substantially the same as that of  FIG. 2 , it now integrates the component(s) and function(s) of both power factor correction and harmonic filter into this singular harmonic filter configuration. More particularly, the cross arm of the generalized T harmonic filter section block  224  includes several inductances arranged or depicted in series, including an input inductance (L in )  232  shown connected at one end in series with the source inductance  226  and source resistance  227  and at the other end to the cross arm of a bridged-T comprising a parallel-bridged arrangement of a further inductance (L 1 )  234  connected in parallel with a series connection of a resistance (R)  236  and a filter inductance (L f )  238 . The cross arm of the generalized T filter network is completed by the connection at the junction of inductances  234  and  238  of one end of a still further inductance (L 2 )  240 , the other end of which is connected to one side of the non-linear load(s) represented by the current source  220 . A filter capacitor (C f )  242  is connected at one end to the junction between the resistance  236  and the filter inductance  238  and is thereafter connected in shunt with the voltage source  214  and the load current source  220  to complete the “vertical arm” of the bridged-T filter. 
     Importantly, the filter capacitor  242  is sized to serve the additional function of power factor correction, such that both the functions of harmonic filtering and power factor correction are integrated into a single capacitor, or bank of capacitors, without the costly requirement of a separate power factor correction capacitor. Principal factors in the sizing of the capacitor(s) are the power, P, of the non-linear loads and reactive power, Q, of the induction generator, etc. This capability is further facilitated by the fact that the integrated capacitor(s)  242  in the harmonic filter  224  already contains series inductance to form a tank circuit to greatly reduce the effects of harmonic currents from either the non-linear loads or the utility grid on the capacitor&#39;(s) life. That series inductance is that depicted as the inductances  232 ,  238 , and  240  in  FIG. 3 . Still further, and depending on the requirement for power factor correction, one or more additional capacitors  242 ′ can be switched, as by switch  250 , into and out of connection directly in parallel with the basic filter capacitor  242  to improve voltage stability on the system at light loading conditions. More specifically, capacitors may be switched out at lighter loads, thereby reducing the reactive current provided by the capacitor and thus preventing voltage instabilities at the grid interface. 
     Referring to  FIG. 4 , a simplified flow diagram illustrates the process and parameters for guiding the design of the  FIG. 3  harmonic filter having a power factor-correcting capacitor integrated therewith in accordance with the present disclosure. The flow diagram is depicted in a very general sense, and utilizes a number of input variables to calculate the component values of the various inductances, capacitance(s) and resistance(s) that make up the general harmonic filter  224 , and especially the bridged-T portion  224 ′ of that filter. Block  460  indicates the use of input variables for generator nameplate power (PF gen ), generator nameplate power factor (PF gen ), required grid power factor (PF grid ), and power to variable speed devices (P vsd ) in the calculation or computation at block  464  of values for the reactive power for the generator and for the grid (Q gen  and Q grid,  respectively). Further input variables, seen in block  468 , include the quality factor, Q, and the resonance and anti-resonance frequencies, designated Res and Anti, respectively. The resonance and anti resonance frequencies are the frequencies at which the filter is designed to get the appropriate attenuation of harmonics. These input variables, in combination with the calculated values of Q gen  and Q grid  from block  464 , are utilized at block  472  to calculate component values for the filter capacitor (C f )  242 , the filter inductance (L f )  238 , the inductance (L 1 )  234 , and the resistance (R)  236 . Still further, input variables, seen in block  476 , include the source inductance (L s )  226 , the source resistance (R s )  227 , and % Z, which is the measure of grid “stiffness” and is an important factor in sizing the filter. These input variables, in combination with the calculated values of C f , L f , L 1  and R from block  472 , are utilized at block  480  to calculate component values for input inductance (L in )  232  and the further inductance (L 2 )  240 . The component values calculated in blocks  472  and  480  are then provided on line  482  to a function block  484  at which is conducted a Bode or frequency response and circuit simulation analysis. If that analysis provides desired results, the process is complete, as indicated by the Finish arrow  486 . If the response analysis is not within the desired range, adjustment is made to one or more of the variables in blocks  468  and/or  476 , as indicated by the Adjust arrow or line  488 . 
     The array of equations that follows is intended to supplement the foregoing general description of the  FIG. 4  flow diagram for guiding the design of the  FIG. 3  harmonic filter having a power factor-correcting capacitor integrated therewith. Those equations are: 
                     P   grid     =       P   gen     -     P   vsd               (     Eq   .           ⁢   1     )                 Q   grid     =       P   grid     ⁢     tan   ⁡     (       cos     -   1       ⁡     (     PF   grid     )       )                 (     Eq   .           ⁢   2     )                 Q   gen     =       P   gen     ⁢     tan   ⁡     (       cos     -   1       ⁡     (     PF   gen     )       )                 (     Eq   .           ⁢   3     )                 C   f     =         Q   gen     -     Q   grid         ω   ⁢           ⁢     V   LL   2                 (     Eq   .           ⁢   4     )                 L   f     =     1       C   f     ⁢     ω   s   2                 (     Eq   .           ⁢   5     )               R   =       Q   s         ω   s     ⁢     C   f                 (     Eq   .           ⁢   6     )                 L   1     =       (           ω   p     ⁢   R   ⁢           ⁢     C   f         Q   p       -   1     )     ⁢     L   f               (     Eq   .           ⁢   7     )                 L   in     =       (         Z   %     ⁢     V   LL         100   ⁢   ω   ⁢     3     ⁢     I   vsd         )     -       L   s     ⁢           ⁢     (         if   ⁢           ⁢     L   in       &lt;   0     ,       then   ⁢           ⁢     L   in       =   0       )                 (     Eq   .           ⁢   8     )                   L   2     =       L   in     ⁢           ⁢     (         if   ⁢           ⁢     L   in       &lt;   0     ,       L   2     =     L   s         )         ,           (     Eq   .           ⁢   9     )               
where the parameters not mentioned earlier but included in the equations include: V LL , which is the line-to-line voltage, which in north America is 480V; Z % , which is the percentage of impedance from a normalized (per unit) set and is the measure of grid “stiffness”, and is an important factor in sizing the filter; √3I vsd , where I vsd  is the current to the variable speed device(s), and the square root of 3 is used because the illustrated system is 3 phase and when you compute power using V LL  it requires the square root of 3. Further still, Q p  and Q s  are the series and parallel quality factors corresponding to the resonance and anti resonance points. ω s  and ω p  are the series and parallel (resonance and anti resonance) frequencies. Note that the series resonance is the higher frequency (lower impedance) portion of  FIG. 5 , while the parallel resonance is the lower frequency (higher impedance).
 
     Referring briefly to  FIG. 5 , there is illustrated a frequency response graph showing a filter designed with integrated power factor correction capacitor(s) vs. a filter designed with external power factor correction capacitor(s). The resonance and anti resonance frequencies are selected such that the filter design yields appropriate attenuation of the harmonics. Most significantly, using the design process described with reference to  FIG. 4  and yielding the integrated harmonic filter and power factor correction functions of  FIG. 3 , it is to be noted in  FIG. 5  that the performance of the system having the disclosed integrated PFC performs very nearly the same as a system for which the PFC is external to (or separate from) the harmonic filter. 
     Although the disclosure has been described and illustrated with respect to the exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made without departing from the spirit and scope of the invention.