Abstract:
Methods and systems are disclosed for reducing alternating current ripples in direct current electrical power generation systems with one or more regulated permanent magnet machines. Ripple suppression is achieved, in one aspect, by modulating the control current of a regulated permanent magnet machine.

Description:
TECHNICAL FIELD 
     The invention relates generally to power generation and, in particular, to regulating voltage in power generation systems with a regulated permanent magnet machine. 
     BACKGROUND OF THE ART 
     The output impedance of permanent magnet generators varies as a function of feeder length and internal construction and is generally not perfectly balanced. More specifically the output impedance of regulated permanent magnet machines (PMMs) is more sensitive to the feeder length and internal construction due to very low machine inductances. Unbalanced output impedance results in a ripple on the direct current bus. The ripple has a frequency which is related to the frequency of the generator (alternator). The direct current (DC) bus ripple can be controlled by increasing the size of the DC bus capacitor. However, this approach results in weight increase of the electric power generation system. U.S. Pat. Nos. 5,218,520 and 7,099,165 disclose approaches for controlling DC bus ripple by modulating generator output. Room for improvement exists. 
     SUMMARY 
     According to an aspect, there is provided a ripple suppression circuit for regulating a direct current (DC) power converted from an alternating current (AC) power produced by a regulated permanent magnet generator (PMG) having a control winding. The DC power has voltage or current ripple. The ripple suppression circuit comprises: a synchronization unit for deriving a synchronization signal; a synchronous compensator for determining, using the synchronization signal, a compensation signal to at least partly suppress the voltage or current ripple; a voltage regulator control unit for producing a current modulation signal for modulating a control current in the control winding in response to the compensation signal in order to adjust the AC power and hence also to regulate the DC power. 
     According to another aspect, there is provided a ripple suppression circuit for regulating a direct current (DC) power converted from an alternating current (AC) power produced by a regulated permanent magnet generator (PMG) having a control winding. The DC power has voltage or current ripple. The ripple suppression circuit comprises: means for deriving a synchronization signal; means for determining, using the synchronization signal, a compensation signal in a synchronous reference frame to at least partly suppress the voltage or current ripple; and means for producing a current modulation signal for modulating current in the control winding in response to the compensation signal in order to adjust the alternating current power and hence also to regulate the DC power. 
     According to another aspect, there is provided a method for regulating a direct current (DC) power converted from an alternating current (AC) power produced by a regulated permanent magnet generator (PMG) having a control winding. The DC power has voltage or current ripple. The method comprises: deriving a synchronization signal; determining, using the synchronization signal, a compensation signal in a synchronous reference frame to at least partly suppress the voltage or current ripple; and producing a current modulation signal for modulating a control current in the control winding in response to the compensation signal in order to adjust the alternating current power and hence also to regulate the DC power. 
     According to another aspect, there is provided a direct current electric generator comprising: a regulated permanent magnet generator (PMG) drivingly connected to a prime mover to produce alternating current power, the regulated PMG comprising control windings through which a control current has an effect on the alternating current power; a rectifier arranged to convert the alternating current power into a direct current power having a voltage or current ripple; and a ripple suppression circuit for regulating the direct current power. The ripple suppression circuit comprises: a synchronization unit for deriving a synchronization signal; a synchronous compensator for determining, using the synchronization signal, a compensation signal to at least partly suppress the voltage or current ripple; and a voltage regulator control unit for producing a current modulation signal for modulating current in the control winding in response to the compensation signal in order to adjust the alternating current power and hence also to regulate the DC power. 
     Further details of these and other aspects will be apparent from the detailed description and figures included below. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying figures, in which: 
         FIG. 1  is a block diagram showing a direct current electric generator including an active ripple suppression circuit, according to an embodiment described herein; 
         FIG. 2  is a block diagram showing the direct current electric generator including an active ripple suppression circuit of  FIG. 1  in more detail; 
         FIG. 3  is a block diagram showing a direct current electric generator including an active ripple suppression circuit, according to another embodiment described herein; 
         FIG. 4  is a block diagram showing an alternative embodiment to the one described in  FIG. 6 , wherein the 2 nd  and 6 th  harmonics are suppressed; and 
         FIG. 5  is a block diagram of a method for reducing a ripple in a direct current power according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Pending U.S. patent application Ser. No. 11/533,548, filed Sep. 20, 2006 and entitled Modulation Control of Power Generation System, describes a power generation system and method preferably employing one or more alternators of the general type described in U.S. Pat. No. 7,262,539. The system employs using a control winding in the alternator&#39;s stator to vary a saturation level of a portion of the stator to thereby modulate power output in a desired manner. The present description is directed to reducing ripple, and is described generally with reference to such a system. 
       FIG. 1  shows a generator system  100  including a ripple suppression circuit  218  in accordance with the present teachings for reducing ripples in the generated electrical power  152 . Ripple suppression is achieved by varying how a portion of an alternator  110  is saturated, as will be described herein in reference to  FIGS. 1 to 4 . The alternator  110  is drivingly connected to a prime mover  112 , such as a gas turbine engine, a windmill or water turbine, or other suitable source of mechanical power. The alternator  110  may be a permanent magnet electric machine, also referred to as a regulated permanent magnet generator (PMG), including power windings  22  and control windings  24 , single or multiphase. The stator of alternator  11  may be provided in accordance with U.S. Pat. No. 7,262,539. 
     Rotation of the rotor relative to the stator of the alternator induces an AC power  156  in the alternator power winding(s)  22 . The AC power  156  is rectified using a rectifier  114  to produce DC power  152 . The rectifier  114  typically uses a bridge-type configuration known in the art. As described in co-pending U.S. patent application Ser. No. 11/533,548, in use the alternator output DC level may be modulated, by varying current levels supplied to the control winding(s)  24  to vary stator saturation, to thereby provide an alternator output which, when appropriately conditioned, may be provided to a load as either DC or AC having a selected desired frequency or frequencies. In this example, the output is provided to the load directly as DC, without further conditioning or inversion. Further details of the system may be found in co-pending U.S. patent application Ser. No. 11/533,548, and thus only aspects of such system relevant to the present description will be addressed hereinbelow. 
     The resultant DC power  152  generally presents an AC ripple, also referred to as a voltage or current ripple, which could be smoothed by passive techniques such as filtering, for example. However, the AC ripple is suppressed herein using an active ripple suppression circuit  218 . The ripple suppression circuit  218  uses the known ripple pattern having a phase and a frequency which are synchronized with the ripple to be suppressed using a feedback on the alternator phase. Accordingly, the ripple suppression circuit  218  comprises a synchronization unit  124  which derives a synchronization signal  154 . In one embodiment ( FIGS. 1 and 2 ), the synchronization unit  124  reads and uses the position of the rotor of the PMG in real time to determine synchronization signal  154 . In another embodiment ( FIGS. 3 and 4 ), synchronization unit  124  reads and uses phase voltages of the stator of the PMG in real time to determine synchronization signal  154 . 
     As described further in co-pending U.S. patent application Ser. No. 11/533,548, control of the generated DC power  152  is achieved by varying control current  150  provided by the voltage regulator  120  to the control windings  24  of the alternator  110 , varying stator saturation level such that the alternator AC output power  156  induced in the power windings  22  varies proportionally to the control current  150  in the control windings  24 , as described above. That is, as the control current  150  in the control windings  24  is increased, the absolute value of the alternator output AC power in the power windings  22  is increased in amplitude proportionally according to the principles discussed above. By varying the control current  150  provided to the control windings  24  in a desired pattern and at a level sufficient to saturate at least a portion of the stator, at a desired frequency and phase, the absolute value of the amplitude of the AC power  156  in the power windings  22  of the alternators will vary according to the same general pattern and frequency. 
     The control current  150  is modulated as controlled by the voltage regulation control circuit  134  and according to the determined alternator phase and known ripple pattern, such that, once the AC power  156  from the power windings  22  of the alternator  110  is rectified from AC to DC by the rectifier  114 , the ripple on the DC output power  152  is suppressed or at least reduced. High frequency filtering may be applied to the rectified DC power signal to eliminate any ripple remaining in the rectified power. 
     In accordance with the present teachings, ripple suppression circuit  218  further comprises a synchronous compensator  194  which in turn comprises a quadrature generator  130  and harmonic compensation unit  132 . Quadrature generator  130  receives the synchronization signal  154  and generates a first signal  160  and a second signal  162  each having a frequency corresponding to the ripple frequency. The second signal  162  is the phase quadrature of the first signal  160 . A harmonic compensation unit  132  receives the first and second signals  160 ,  162  and produces a compensation signal  170  having a frequency corresponding to the ripple frequency and having a phase corresponding to the ripple phase. 
     A voltage regulation control circuit  134  controls the voltage regulator  120  according to the compensation signal  170  and with proportional integral control loops with feedback on the control current as read by a current transducer  136  and on the output DC power voltage as read by a voltage transducer  138 . 
     Signals  160  and  162  are in-phase and phase quadrature components respectively of the harmonic to be eliminated. The in-phase reference is essentially a sinusoidal signal having the same frequency as the ripple of DC power  152 . The quadrature reference is essentially a cosinusoidal signal having the same frequency as the in-phase reference. The voltage transducer  138  connected to the DC output bus reads the voltage of the DC power  152 . The read voltage  166  comprises a DC level and the ripple. 
     The harmonic compensation unit  132  includes two paths. In the first path, the first in-phase signal  160  is input to the first input port of multiplier  164  while the voltage  166  read by the voltage transducer  138  is input to the second input port of multiplier  164 . Multiplier  164  multiplies the first in-phase signal  160  and the read voltage  166  placing the result on line  172 . This signal contains a DC component reflecting the amplitude and phase of the ripple and is fed into the conditioning unit  176 . 
     The purpose of the conditioning unit  176  is to eliminate all AC components from the signal on line  172  leaving only the DC component reflecting the phase and amplitude of the ripple. It is a further purpose of the conditioning unit  176  to enhance this component to a desired amplitude level. In one embodiment, the conditioning unit  176  comprises an integrator. However it should be noted that conditioning unit  176  along with conditioning unit  178  could also comprise a low pass filter followed by a proportional integrator unit. 
     Conditioning unit  176  produces its output on line  180  which feeds the first input port of multiplier  184 . Multiplier  184  takes as its second input the first in-phase signal  160 . 
     Similarly, in the second path of the harmonic compensator  132 , the second phase quadrature signal  162  is input into the first input port of a multiplier  168 , while the voltage  166  read by the voltage transducer  138  is input into second input port of multiplier  168  which in turn produces its output signal on line  174 . Similar to the first path, the signal on line  174  enters a conditioning unit  178 , the purpose of which is to eliminate all AC components of the output of multiplier  166  and enhance the amplitude level of the desired signal. The conditioning unit  178  produces its output, which is a DC signal reflecting the conditioned amplitude and phase of the ripple on the generated DC power  152 . 
     The signal on line  180  is essentially the DC component of the product of the amplitude of the ripple and the sine of the ripple phase. Likewise, the signal on line  182  is essentially the DC component of the product of the amplitude of the ripple and the cosine of the ripple phase. Taken together, these two DC components form a vector representing the ripple. 
     The signal on line  182  is then input into the first input port of multiplier  186 . The second phase quadrature signal  162  feeds the second input port of multiplier  186  which in turn produces its output on line  190 . Summer  192  combines the signals existing on lines  188  and  190  producing a single compensation signal  170  which is equal in frequency, and related in phase and amplitude to the undesired ripple to be suppressed on the generated DC power  152 . 
     The output of summer  192 , i.e., the compensation signal  170 , serves as an input signal to the voltage regulation control circuit  134  to vary the control current  150  such that the ripple on the generated DC power  152  is suppressed. 
     It is noted that in the embodiment shown in  FIG. 4 , multiple harmonic frequencies (the 2 nd  and 6 th  harmonics) in the ripple are suppressed by providing a multiple channel compensation arrangement (i.e., synchronization unit  127 ) in which each channel responds to a different ripple frequency. Each channel comprises components substantially as shown in  FIG. 1 , comprising synchronous compensators  194  including a harmonic compensation unit  132  and a quadrature generator  130 . Each channel receives a different synchronization signal  154  which corresponds to a different ripple frequency to be suppressed. All channels are then added for modulation control of the variable current source. 
     Now turning to  FIG. 2 , a block diagram shows the direct current electric generator including an active ripple suppression circuit of  FIG. 1  in more detail. Generally, the description of  FIG. 1  applies to  FIG. 2 . The additional components will be well understood by those skilled in the art and will not be further described herein. 
     Now turning to  FIG. 3 , a block diagram shows a direct current electric generator including an active ripple suppression circuit, according to another embodiment described herein. In this embodiment, the phases VA and VB are sampled off the phases of the AC power output from stator of regulated PMG  110 . Phases VA and VB are input to synchronization unit  125  which comprises a zero cross detector and an angle generator which operate in a manner well know to those skilled in the art. It is to be noted that  FIG. 3  also introduces current transducer  137  which sample the current on the DC bus for inputting to multipliers synchronous compensator  194 . 
     Now turning to  FIG. 5 , a method is shown for regulating a DC power on a DC bus. The DC power being converted from an AC power (i.e., for reducing a ripple in a DC power) is described. AC power is produced by a regulated permanent magnet generator (PMG) having a control winding. The method  700  comprises: measuring the voltage or current ripple on the dc bus in a stationary reference frame (step  702 ); transforming the components of voltage or current ripple into synchronous reference frame by using synchronization signal derived from the generator stator phase voltages or rotor position (step  704 ); determining the components of the compensating signal in a synchronous reference frame that minimizes the voltage or current ripple on the DC bus by integrating the components of the voltage or current ripple in a synchronous reference frame (step  706 ); transforming the components of the compensating signal into a stationary reference frame (step  708 ); and modulating the control current in the alternator control winding in response to the compensating signal (step  710 ). (The step of modulating the control current in the alternator control winding in step  710  is otherwise performed in accordance with the teachings of co-pending U.S. patent application Ser. No. 11/533,548). 
     The above description is meant to be exemplary only, and one skilled in the art will recognize that certain changes may be made to the embodiments described without departing from the scope of the appended claims.