Abstract:
A method employing a lead-unity-lag adjustment on a power generation system is disclosed. The method may include calculating a unity power factor point and adjusting system parameters to shift a power factor angle to substantially match an operating power angle creating a new unity power factor point. The method may then define operation parameters for a high reactance permanent magnet machine based on the adjusted power level.

Description:
GOVERNMENT RIGHTS  
     This invention was made with government support under grant number NAS8-01098 awarded by the NASA. The government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to power generation systems, and more particularly, to a method and system employing a lead-unity-lag power factor operation of a power generation system for a DC power bus. 
     Power generation systems (PGS) play a significant role in the modern aerospace/military industry. This is particularly true in the area of more electric architecture (MEA) for aircraft, spacecraft, and electric hybrid technology in military ground vehicles. The commercial aircraft business is moving toward MEA having no bleed-air environmental control systems (ECS), variable-frequency (VF) power distribution systems, and electrical actuation. A typical example is the Boeing 787 platform. In the future, next-generation commercial aircraft may use MEA. Some military aircraft already utilize MEA for primary and secondary flight controls among other functions. Future space vehicles may require electric power generation systems for thrust vector and flight control actuation. Military ground vehicles have migrated toward hybrid electric technology, where the main propulsion is performed by electric drives. Therefore, substantial demand for increased power generation in that area has emerged. These systems should be more robust and offer greatly reduced operating costs and safety compared to the existing Space Shuttle power systems. 
     These new aerospace and military trends have significantly increased electrical power generation needs. The overall result has been a significant increase in the challenges to accommodate electrical equipment to the new platforms. This has led to increased operating voltages and efforts to reduce system losses, weight, and volume. A new set of electrical power quality and electromagnetic interference (EMI) requirements has been created to satisfy system quality and performance. One of the latest developments of machines under MEA themes is the energy efficient aircraft where electric power and heat management go hand to hand. Therefore, overall system performance improvement and more specifically, power density increase may be necessary for the new-generation hardware. 
     As can be seen, there is a need for a method and system to improve power generation in aircraft. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a method employing a lead-unity-lag power factor adjustment on a power generation system comprises defining standard parameters for the power generation system; determining a power factor angle for the power generation system based on a power level defined in the standard parameters; calculating a unity power factor point based on the standard parameters; defining an operating power angle based on the unity power factor point; adjusting the power generation system standard parameters to shift the power factor angle to substantially match the operating power angle; and defining operation parameters for the power generation system based on-the unity power factor point. 
     In another aspect of the present invention, a method for moving a unity power factor point in a power generation system comprises determining operation parameters for the power generation system; generating a phasor diagram representing operation of the power generation system according to the operation parameters; defining a first vector representing a voltage terminal for the power generation system; defining a first angle based on a distance of the first vector from an originating axis, wherein the first angle represents a power factor angle and wherein the originating axis represents a phase current reference vector for the power generation system; defining a second vector representing an electromagnetic field of the power generation system; defining a second angle from the originating axis, wherein the second angle represents a control angle; defining a third angle between the first vector and the second vector representing a power angle for the power generation system; calculating a reduction in operational power for the power generation system; reducing the power factor angle to cause the first vector to approach the originating axis based on the reduction of the operational power; determining a new unity of power factor point in the power generation system according to the reduced magnitude of the power factor angle; and adjusting the operation parameters for the power generation system according to the new unity of power factor point. 
     In yet another aspect of the present invention, an electric power generation system comprises a three phase bridge; a DC link capacitor bank operatively coupled to the three phase bridge; an EMI filter operatively coupled to the DC link capacitor bank and a DC bus; a contactor disposed in operative contact between the EMI filter and the DC bus; and wherein power flow in the power generation system is operated at a nominal power level for the system based on a unity of power factor point adjusted upward from a zero power point to the nominal power level. These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is diagrammatic illustration of an EPGS topology according to an exemplary embodiment of the present invention; 
         FIG. 2A  is a phasor diagram illustrating qualitative representations of an operation of an EPGS under prior art operating conditions; 
         FIG. 2B  is a phasor diagram illustrating qualitative representations of an operation of an EPGS according to an exemplary embodiment of the present invention; 
         FIG. 3  illustrates a series of steps according to an exemplary embodiment of the present invention; 
         FIG. 4  illustrates a series of steps for controlling an EPGS according to an exemplary embodiment of the present invention; 
         FIG. 5  is a plot depicting a comparative analysis of performance between a conventionally operated EPGS and an EPGS according to an exemplary embodiment of the present invention; and 
         FIG. 6  is an exemplary table of operating parameters for an EPGS according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of the best currently contemplated modes of carrying out exemplary embodiments of the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
     Various inventive features are described below that can each be used independently of one another or in combination with other features. 
     Broadly, embodiments of the present invention generally provide a method and apparatus for moving the unity power factor point of a leading power factor system from zero power point to a power point where the machine operates predominantly. This operating point can thus, become the nominal power of the system. This means, for operation of a system below this new operating power point, the system may operate with a lagging power factor. Operation above of this power point, the system will operate with a leading power factor. Thus, one may improve the power factor and hence, one may also improve the efficiency of the system about the region where the system operates predominantly. 
     Referring to  FIG. 1 , a power topology of an electric power generation system (EPGS) in accordance with one exemplary embodiment of the present invention is shown. An exemplary EPGS used for an MEA application may be a high-reactance permanent magnet machine (HRPMM)  150 . The topology depicts a three-phase bridge  110 , a DC link capacitor bank  120 , an EMI filter  130  for a DC bus, and a contactor  140 . It should be understood that current and voltage measurement devices for control and protection purposes are shown for illustrative purposes. The contactor  140  may be an optional component for all applications. This exemplary topology has bidirectional power flow capability by applying an appropriate voltage to the machine terminals. A synchronous rotation of the HRPMM  150  may be performed for continuous motoring or self-starting. Power generation may actively regulate DC bus voltage to a desired value. 
     One feature of this system provides a short-circuit current at the DC bus during generation to clear a fault. If the DC bus  160  is overloaded, the EPGS  100  may reduce the output voltage linearly to prevent components from overloading. Below certain voltage levels, a pure diode rectification may be used to supply desired current. The reactance of the electric machine  150  may be selected such that the short circuit of the electric power generation system  100  satisfies requirements of a DC bus short circuit current. One typical ratio between the DC bus short circuit current and the electric machine  150  short circuit current may be described as: I DCSC =1.35*I SC , wherein I DCSC  is the DC bus short circuit current and ISC is the system short circuit current. The ratio may vary depending on component selection for the three phase bridge  110  and the electro-magnetic interference (EMI) filter  130 . When a short circuit occurs within the power electronics  110 , the HRPMM  150 , or the interface between the HRPMM  150  and the power electronics  110 , control of the generation process may be instantly discontinued. The failure current may be limited by the HRPMM  150  and may be comparable to the operating current. 
     Referring to  FIGS. 2A and 2B , phasor diagrams of the HRPMM  150  operation in a complex plane may be used for sake of illustration and for providing qualitative assessments.  FIG. 2A  depicts a phasor diagram according to conventional operations of a power generation system.  FIG. 2B  depicts a phasor diagram with an adjusted power of nearly unity power factor according to an exemplary embodiment of the present invention. The phasor diagram of an HRPMM  150  in operation can be created according to the following exemplary equation.
 
 V   T   =E   EMF   −I   M   *Z   S  
 
     The phase current vector, I M , is aligned with the real (Re) axis of the complex plane. The leading power factor control is achieved by maintaining the power factor angle (θ)&lt;0 (negative). That means the machine phase current vector is ahead of the terminal voltage vector. The terminal voltage vector, V T , is decomposed to two components real [V T ] cos(θ) and imaginary [V T ] sin(θ). Another angle α, may be the angle between the electromotive force (EMF) voltage and the phase current I M . The power angle δ defines the angle between the EMF voltage phasor and the terminal voltage phasor. The phasor V S =I M * Z S  represents the internal machine (HRPMM  150 ) voltage drop. 
     In terms of application to the HRPMM  150 , machine shaft power P T  may be expressed as: 
                 P   T     =     3   *         V   T     *     E   EMF     *     sin   ⁡     (   δ   )           X   S           ,         
wherein V T  is the terminal voltage, E EMF  is the HRPMM  150  back EMF voltage, and X S  is the HRPMM  150  reactance.
 
     Expressing output power may be described as: 
     P OUT =P T *η pe *η m , wherein P OUT  is the output power of the HRPMM  150 , P T  is the shaft power, η pe  is the efficiency of power electronics, and η m  is the efficiency of the HRPMM  150 . 
     One expression describing the power angle (δ) may be derived from the HRPMM  150  shaft power (P T ) and the output power (P OUT ) according to the following equation: 
             δ   =       sin     -   1       (         (       P   out         η   pe     *     η   m         )     *     E   EMF     *     X   S         3   *     V   T     *     E   EMF         )           
wherein the variables are described by the aforementioned equations.
 
     In accordance with these equations, one may adjust the power angle (δ) so that the power factor angle (θ) is reduced and the terminal voltage V T  phasor is shifted toward the Re axis of the complex plane. One exemplary result may be seen when comparing  FIG. 2A  to  FIG. 2B  where the power angle (δ) approaches the angle α. Thus, a unity power factor point of the electric power generation system  100  may be adjusted to operate where the system predominantly operates. Thus, in practice, defining the unity power factor point may be achieved by determining the EPGS  100  characteristic parameters. An exemplary table of input conditions and constraints for an EPGS  100  may be seen in  FIG. 6 . 
     Exemplary input conditions as illustrated in the table of  FIG. 6  may include parameters  605  which may include a P Load    610 , a η pe    620 , a η m    630 , a V DC    640 , a SC factor    650 , a V T    660 , a E EMF    670 , a frequency  680 , and a X S    690 . The P Load    610  may represent an output power at a load. The η pe    620  may represent an efficiency of power electronics in the EPGS  100 . The η m    630  may represent an efficiency of the HRPMM  150 . The V DC    640 , may represent an output DC voltage in the EPGS  100 . The SC factor    650  may represent a maximum short circuit DC current above a maximum operating current in the HRPMM  150 . The V T    660  may represent a HRPMM terminal voltage. The E EMF    670  may represent a back EMF voltage of the HRPMM  150 . The frequency  680  may represent the HRPMM  150  electrical operating frequency. The X S    690  may represent the HRPMM  150  reactance. Thus, in one exemplary operation, adjustment of those parameters may be made to achieve a desired unity power factor point as illustrated in the following exemplary methods. 
     Referring to  FIG. 3 , a series of steps illustrate an exemplary method according to the present invention. In step  210 , standard system parameters may be defined. Exemplary parameters may be extracted from a table of values such as that one shown in  FIG. 6 . In step  220 , a unity of power factor operating power point may be defined for the system for a given power level based on the extracted system parameters. In step  230 , a system short-circuit current may be computed. The short-circuit DC current may be described as I scdc =(P load /V DC )*(1+SC factor ). In step  240 , a back EMF (E EMF ) voltage may be computed for the unity power factor power point. One exemplary equation that may be used to calculate the power factor (PF) as a function of the back EMF (E EMF ) may be described as: 
     
       
         
           
             
               PF 
               ⁡ 
               
                 ( 
                 
                   E 
                   EMF 
                 
                 ) 
               
             
             = 
             
               cos 
               ( 
               
                 
                   
                     cos 
                     
                       - 
                       1 
                     
                   
                   ( 
                   
                     
                       
                         V 
                         T 
                       
                       - 
                       
                         
                           E 
                           EMF 
                         
                         * 
                         
                           cos 
                           ⁡ 
                           
                             ( 
                             δ 
                             ) 
                           
                         
                       
                     
                     
                       
                         
                           E 
                           EMF 
                           2 
                         
                         + 
                         
                           V 
                           T 
                           2 
                         
                         - 
                         
                           2 
                           * 
                           
                             E 
                             EMF 
                           
                           * 
                           
                             V 
                             T 
                           
                           * 
                           
                             cos 
                             ⁡ 
                             
                               ( 
                               δ 
                               ) 
                             
                           
                         
                       
                     
                   
                   ) 
                 
                 - 
                 
                   π 
                   2 
                 
               
               ) 
             
           
         
       
     
     wherein the variables are previously described. In step  250 , a system reactance may be computed. One exemplary equation describing the system reactance may be described as X S =E EMF /I sc . In step  260 , the system parameters may be assessed for controllability. In step  270 , a modified HRPMM  150  may be designed based on the parameters obtained from steps  210 - 250 . 
     Referring now to  FIG. 4 , an exemplary method of controlling the lead-lag-unity power factor is shown according to another exemplary embodiment of the present invention. In step  305 , demand power may be computed from a measured bus voltage and current. In step  310 , current demand may be computed from a difference between the computed demand power and nominal power. In step  315 , a position decoder may be used to measure machine rotor position that may be used for reference frame transformations. In step  320 , machine terminal currents may be measured and transformed to a Park vector in the stationary reference frame using the rotor position measured in step  315 . In step  325 , the current Park vector may be transformed from a stationary reference frame to a synchronous reference frame. In step  330 , a voltage command error may be computed. The voltage command error may be computed based on the DC voltage and measured feedback voltage. and DC bus current feedback. In step  335 , the voltage command error in step  330  may be regulated, and the regulated voltage error and the DC bus current feedback may be used to compute current command magnitude. In step  340 , the current command magnitude and angle may be transformed into a vector in the synchronous reference frame. In step  345 , a current command error may be generated from current feedback vector (step  320 ) and current command vector (step  340 ) and regulated. The current regulator outputs may be inverter voltage commands. In step  350 , the inverter voltage command may be transformed back to the stationary reference frame. In step  355 , space vector modulation may be used to transform inverter voltage command to desired machine terminal voltage. 
     Referring to  FIG. 5 , exemplary results showing a comparative analysis of machine current employing conventional operation of an electric power generation system against an exemplary operation of the EPGS  100  as a lead-unity-lag system according to an embodiment of the present invention is illustrated. Taking the lead-unity-lag system current as a percentage of the leading system current, it may be seen that the lead-unity-lag system of the EPGS  100  requires 10% lower current at full load than a system under conventional operation. Since the current at full load determines the rating of the system, this may be a significant efficiency improvement. Also, the rating of the three-phase bridge and machine-electronics may be reduced by 10%. Thus, reduced electric machine size and power electronics may be expected. 
     It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.