Patent Document

FIELD OF THE INVENTION  
       [0001]     The present application is directed to a method and apparatus for controlling a generator, and more specifically, toward a method and apparatus for decreasing the power supplied to a generator when current flowing through a generator field winding ceases to be controlled.  
       BACKGROUND OF THE INVENTION  
       [0002]     Both AC and DC generators may be controlled by regulating the current flowing through a generator field winding. This current is generally supplied though a controlled transistor, frequently a MOSFET. By controlling the field transistor, the generator field current is modulated based on demand to maintain the generator output voltage at a desired level.  
         [0003]     One failure mode for a generator may be referred to as an “out of control” failure mode. In this failure mode, a generator control unit fails to control the generator excitation current, and the generator output voltage quickly reaches dangerously high levels. This may damage equipment connected to the generator. The cause of this failure mode may be, for example, a shorted field transistor, or a short of any components that are connected in parallel with the field transistor, or a connector pin short. Because the field switch is a semiconductor device, this is not an uncommon failure mode.  
         [0004]     When a short circuit occurs, the field is not controlled and becomes fully excited. As a result, the output voltage of the generator rises quickly to a very high level until the generator becomes saturated. This failure mode can cause problems in constant frequency electrical systems. However, the severity of this failure mode is even greater in variable frequency electrical systems such as those sometimes found on aircraft. Such variable frequency electrical systems are becoming increasingly popular because of their overall lighter weight and increased efficiency.  
         [0005]     In a variable frequency system, the generator may operate at frequencies nearly twice as high as the frequencies used in constant frequency systems. The higher the frequency at which a generator operates, the shorter the time it will take to reach an overvoltage condition. Therefore, effective protection against this failure mode in variable frequency systems is becoming an important concern.  
         [0006]     Some systems now require generators that limit overvoltage to about 150 V rms for 115V AC electrical systems and to 300V rms for 230V AC electrical systems. A conventional approach to overvoltage protection is to monitor voltage levels and disconnect the field winding when an overvoltage is detected. This approach, however, is too slow to provide effective protection for the above failure mode, especially in a variable frequency system.  
         [0007]      FIGS. 4   a  and  4   b  illustrate a simulated response to an overvoltage condition by a conventional voltage protection circuit used with a generator operating at 115V, 700 Hz with a light load of 7.5 kW.  FIG. 4   a  is a graph of generator field current having a nominal level of 0.4 amps.  FIG. 4   b  illustrates the average voltage level of the three-phase power supply. A fault occurs at time t 1  which leads to an increase in the current level to about 3.2 amps at time t 2 . At time t 3 , the generator field voltage peaks at 248.5 V or 176 V rms, well above the 10V rms limit often required in applications employing variable frequency generators. It is therefore desirable to provide a method and apparatus for addressing this failure mode in a manner that limits generator overvoltage and protects components connected to the generator.  
       SUMMARY OF THE INVENTION  
       [0008]     These problems and others are addressed by the present invention which comprises, in a first aspect, a generator control circuit that includes a first circuit monitoring a generator voltage change rate and producing a first output when the voltage change rate is above a first level and a second circuit monitoring a generator current change rate and producing a second output when the current change rate is above a second level. These circuits are connected to a third circuit which decreases the power supplied to the generator when a plurality of conditions are satisfied, the plurality of conditions including the voltage change rate being above the first level and the current change rate being above the second level.  
         [0009]     Another aspect of the invention comprises a method of protecting a generator that involves monitoring a rate of generator voltage change, monitoring a rate of generator current change, and reducing the generator output when a plurality of conditions are satisfied. The plurality of conditions include the rate of generator voltage change exceeding a first level and the rate of generator current change exceeding a second level.  
         [0010]     An additional aspect of the invention comprises a method of protecting a generator having a field winding connected to a power supply that involves detecting a generator voltage v, detecting a generator current i, differentiating the generator voltage v to obtain a quantity dv/dt and differentiating the generator current i to obtain a quantity di/dt. A number of determinations are then made including a first determination as to whether dv/dt is greater than a first value and a second determination as to whether di/dt is greater than a second value. If each of these determinations is true, the level of power supplied to the generator is reduced.  
         [0011]     A further aspect of the invention comprises a control circuit for a generator having a field winding and an output. The circuit includes a field transistor having a gate connected between the generator field winding and ground and a field controller connected to the field transistor gate for controlling current flow through the field winding. A detector monitors a voltage change rate in the generator output and a current change rate in a generator field winding or load current and produces a first output signal when the voltage change rate is above a first level and a second output signal when the current change rate is above a second level. A generator controller is operatively connected to the detector and decreases current flow through the generator field winding upon receipt of the first output signal and the second output signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     These features and aspects of the invention will be better understood after a reading of the following detailed description in connection with the below drawings wherein:  
         [0013]      FIG. 1  illustrates, partially schematically, a generator control circuit and detection circuits according to an embodiment of the present invention;  
         [0014]      FIGS. 2   a - 2   d  are graphs illustrating current and voltage levels that result when a simulated generator out of control fault occurs in a system using the protection circuits of  FIG. 1 ;  
         [0015]      FIG. 3  is a flow chart illustrating the steps followed in performing the method of an embodiment of the present invention; and  
         [0016]      FIGS. 4   a  and  4   b  are graphs illustrating current and voltage levels that result when a simulated generator out of control fault occurs in a system using a conventional overvoltage protection system.  
     
    
     DETAILED DESCRIPTION  
       [0017]     Referring now to the drawings, wherein the showings are for purposes of illustrating preferred embodiments of the invention only and not for the purpose of limiting same,  FIG. 1  illustrates a control circuit for a generator (not shown) having an exciter winding  10  connected to a power supply  12  controlled by a switch  13 , (preferably a semiconductor switch) by a line  14  and to ground via a field transistor  16 . The conductive state of field transistor  16 , which is may be, for example, a MOSFET, is regulated by pulse width modulation (PWM) controller  18  via PWM gating  20  to control the output of a generator in a well known manner. A field dumping line  22  connects to a node  24  between exciter winding  10  and field transistor  16 . When field transistor  16  is not conducting, current flows from node  24  back to line  14  via freewheeling diode  26 . Field dumping control  28  controls transistor  30  to determine a current flow path from node  24  to freewheeling diode  26 . When transistor  30  is conducting, current flows through transistor  30  to the freewheeling diode  26 , bypassing resistor  32 . When transistor  30  is not conducting, current flows through and is dissipated by resistor  32 .  
         [0018]     The state of power supply switch  13  and field dumping control  28  is determined by the logical state of node  34  on line  36 . The signal on line  36  is normally low when the generator is operating in a normal manner. The signal goes high, as described hereinafter, upon the detection of a fault indicative of a failure or bypass of field transistor  16 , and this turns off or reduces the output of power supply  12 .  
         [0019]     Four monitoring circuits provide input to an AND gate  38  on line  36  in this embodiment; when all four inputs to gate  38  are logically high, AND gate  38  outputs a logically high signal on line  36 . The monitoring circuits include a generator output voltage detection circuit  40  providing an output on voltage detection output line  42 , a dv/dt monitoring circuit  44  producing an output on dv/dt output line  46 , a di/dt monitoring circuit  48  producing an output signal on di/dt output line  50  and a generator line contactor (GLC) monitoring circuit  52  producing an output on GLC line  54 .  
         [0020]     The present inventor has recognized that rapid increases in both generator current and generator voltage are generally indicative of a generator out of control fault. Therefore, dv/dt monitoring circuit  44  and di/dt monitoring circuit  48  sense for rapid changes in both voltage and current levels. Normal system operation may occasionally produce an increase in one or the other of these values, but concurrent increases are generally associated with a fault condition. Because the rate of current and voltage change is sensed rather than the absolute value of these quantities, faults can be detected and corrective action taken before the generator output reaches a dangerous level.  
         [0021]     The present inventor has also recognized that under certain transient conditions, dv/dt or di/dt or both may increase when a fault is not present. This may occur, for example, at generator power up and/or when a load is applied to the generator. To prevent those conditions from triggering a fault protection sequence, two additional logically high inputs to AND gate  38  are required before the fault protection system is triggered. First, output voltage detection circuit  40  monitors the generator output voltage and outputs a logically high signal on voltage detection output line  42  only when the voltage is above a nominal level, such as 5 volts, for example, to prevent the signaling of a fault condition on start up before output voltage has reached a stable level. Likewise, GLC monitoring circuit  52  produces a high output on GLC output line  54  only when the GLC is closed to prevent the triggering of a failure mode when the GLC is open. Thus, when logically high signals appear on voltage detection output line  42 , on dv/dt output line  46 , on di/dt output line  50  and on GLC line  54 , AND gate  38  produces a logically high signal on line  36 , which signal is latched to a high level by latch  56 , and triggers a shutdown of the generator.  
         [0022]     In one embodiment of the invention, output voltage detection circuit  44  comprises a first operational amplifier  60  connected to +12V and −12V power supplies and having a first line  62  connected to the output of a generator (not shown) at the point of regulation (POR) and to the inverting input of first op amp  60  through a first resistor R 1 , and a second line  64  connected to the output of the generator and the non-inverting input of op amp  60  through a second resistor R 2  where first and second resistors R 1  and R 2  each have a resistance of, for example, 75 kΩ. (Resistance and capacitance values provided herein are for the purpose of illustrating a suitable example of a protection circuit according to an embodiment of the present invention and are not intended to limit the invention to the use of resistors and capacitors having these values.) A third line  66  provides feedback from the output of the first op amp  60  to the inverting input thereof through a third resistor R 3  (1.43 kΩ), while second line  64  is connected to ground thorough a fourth resistor R 4  (1.43 kΩ). The output of first op amp  60  on fourth line  68  is conditioned by first logic conditioning circuit  70  which outputs a specific signal, assumed to be logically high for this example, on line  42  when the generator output voltage on first line  62  and on second line  64  is at least 5V.  
         [0023]     The dv/dt detection circuit  44  receives as an input the output voltage of first op amp  60  on a fifth line  72  connected to the inverting input of a second op amp  74 . Fifth line  72  includes a fifth resistor R 5  (10 kΩ) and is connected to ground at a point between fifth resistor R 5  and first op amp  60  via a sixth resistor R 6  (2MΩ) and a first capacitor C 1  (0.22 μF) arranged in parallel. First capacitor C 1  filters the incoming signal, and its value is chosen so that normal noise on fifth line  72  does not trigger dv/dt detection circuit  44 . Fifth line  72  further includes a second capacitor C 2  (0.01 μF.) and a seventh resistor R 7  (50 kΩ) between fifth resistor R 5  and the inverting input of second op amp  74 . The value of second capacitor C 2  determines the sensitivity of the dv/dt detector. The non-inverting input of second op amp  74  is connected to a 12V power source by sixth line  76  through an eighth resistor R 8 , and sixth line  76  is connected to ground through a tenth resistor R 10  (100 kΩ). Seventh line  78  provides feedback from the output of second op amp  74  to the inverting input of second op amp  74  through a ninth resistor R 9  (150 kΩ), and the output of second op amp  74  on eighth line  80 , which is proportional to the derivative of the voltage on first line  62 , is conditioned by second logic conditioning circuit  82  to produce a logically high output on dv/dt output line  46  when a voltage change is detected.  
         [0024]     The di/dt monitoring circuit  48  receives a current input on ninth line  84 , which current is normally the generator field winding current. However, in systems where some minimum load is always connected to the generator, the load current can be monitored instead. Ninth line  84  is connected to the inverting input of a third op amp  86 . Ninth line  84  includes an eleventh resistor R 11  (12 kΩ) and is connected to ground at a point between eleventh resistor R 11  and third op amp  86  via a twelfth resistor R 12  (2MΩ) and a third capacitor C 3  (0.22 μF) arranged in parallel. Third capacitor C 3  filters the incoming signal, and its value is chosen so that normal noise on ninth line  84  does not trigger di/dt detection circuit  48 . Ninth line  84  further includes a fourth capacitor C 4  (0.01 μF) and thirteenth resistor R 13  (50 kΩ) between eleventh resistor R 11  and the inverting input of third op amp  86 . The value of third capacitor C 3  determines the sensitivity of the di/dt detector. The non-inverting input of third op amp  86  is connected to a 12V power source by a tenth line  88  through a fourteenth resistor R 14 , and tenth line  88  is connected to ground through a sixteenth resistor R 16  (100 kΩ). Eleventh line  90  provides feedback from the output of third op amp  86  to the inverting input of third op amp  86  through a fifteenth resistor R 15  (150 kΩ), and the output of third op amp  86  on twelfth line  92 , which is proportional to the derivative of the current on ninth line  84 , is conditioned by third logic conditioning circuit  94  which produces a logically high output on di/dt output line  50  when a current change of a given magnitude is detected.  
         [0025]     GLC monitoring circuit  52  provides a logically high output on line  54  when the generator line contactor is closed.  
         [0026]     In normal operation, current flow through exciter winding  10  is controlled by field transistor  16  which in turn is controlled by PWM control  18 . During normal operation, the signal on generator output detection line  42  is high because the output of the generator is more than about 5V. Likewise, the signal on GLC line  54  is high because the generator line contactor is closed. However, under normal operating conditions, the outputs of dv/dt monitoring circuit  44  and di/dt monitoring circuit  48  are logically low because rapid voltage and current changes do not normally occur during generator operation absent a fault condition. However, transient changes could potentially change the current or voltage level in a manner that causes the signal on either dv/dt output line  46  or di/dt output line  50  high for a brief period. The present inventor has found, however, that monitoring both these lines and triggering a fault protection process only when both signals go high, provides reliable generator fault protection.  
         [0027]      FIGS. 2   a - 2   d  illustrate voltage and current levels during a simulated generator out of control fault and show how such a fault is handled by the protective circuitry of one embodiment of the present invention. As illustrated in  FIG. 2   c , generator field current begins to increase at time t 1  which produces a positive di/dt as illustrated by the dotted line in  FIG. 2   b . This positive di/dt is detected by di/dt monitoring circuit  48  and causes the signal on di/dt output line  46  to go high. At time t 2 , about 0.1 msec after the fault occurs, an increase in generator output voltage is seen, and this voltage change is detected as illustrated by the solid line in  FIG. 2   b  which causes the output of dv/dt monitoring circuit  44  to go high at time t 2 . At this point all four inputs to AND gate  38  are high, and the output of AND gate  38  becomes logically high at this time as illustrated in  FIG. 2   a . This high signal activates field dumping control  28  and opens switch  13  to disconnect power supply  12  from line  14  and the exciter winding  10 . Field current peaks at time t 3 , as illustrated in  FIG. 2   c , and generator output voltage peaks at time t 4  at a level of about 208 volts (about 148V rms) for this 115V system. As will be appreciated from the above, the fault protection system of this embodiment of the present invention responds quickly to faults and thus provides improved protection for both the generator and equipment connected thereto.  
         [0028]      FIG. 3  illustrates the logical steps followed by the fault protection system of an embodiment of the present invention. At a step  100 , a first flag A is set to low and the generator output voltage is sensed at step  102 . A determination is made at step  104  as to whether the sensed generator output voltage is greater than a predetermined nominal level such as 5 V, for example. If the sensed voltage is not greater than this nominal level, the process returns to step  100 . If the sensed voltage is greater than the nominal level, a determination is made at a step  106  as to whether the voltage level is changing more quickly than a certain rate. If the voltage level is not changing at such a rate, the process returns to step  100 . If the voltage is changing at a rate greater than a predetermined rate, flag A is set to high at step  108 .  
         [0029]     Concurrently with steps  100  through  108 , steps  110  through  116  are performed. At step  110  a flag B is set to low and a current level is sensed at a step  112 . A determination is made at step  114  as to whether the current is changing more quickly than a certain rate. If such a current rate change is not detected, the process returns to step  110 . If the current is changing more quickly than this rate, flag B is set to high at step  116 .  
         [0030]     A determination is made at step  120  as to whether both flag A and flag B are high. If both flags are not high, flags A and B are set to low and the two sensing processes described above repeat from steps  100  and  110 , respectively. If both flags are high, a determination is made at a step  122  as to whether a generator line contactor is closed. If the generator line contactor is open, flags A and B are set to low and the process repeats from steps  100  and  110  discussed above. If both flags are high and the generator line contactor is closed, generator excitation power is disabled at step  124 .  
         [0031]     The invention has been described in terms of a preferred embodiment; however obvious modifications and additions comprises a part of the present invention to the extent they fall within the scope of the several claims appended hereto.

Technology Category: h