Patent Abstract:
A generator control circuit is disclosed that includes a first circuit ( 40 ) monitoring a generator voltage and producing a first output when the voltage is below a first level, a second circuit ( 48 ) monitoring a generator current change rate and producing a second output when the current change rate is above a second level, and a third circuit ( 13, 28 ) operatively connected to the first circuit ( 40 ) and the second circuit ( 48 ) for decreasing a power level supplied to the generator when a plurality of conditions are satisfied, the plurality of conditions including the voltage being below the first level and the current change rate being above the second level. A method of controlling a generator is also disclosed.

Full Description:
FIELD OF THE INVENTION 
   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 voltage regulation is impaired. 
   BACKGROUND OF THE INVENTION 
   The output voltage of a generator may be regulated by comparing the voltage at a point of regulation (POR) with a reference voltage and using a voltage regulator to maintain the output voltage at a desired level. One failure mode for such a generator control may be referred to as a “loss of voltage sensing” failure mode. In this failure mode, the voltage regulator detects a voltage of 0 (or a very low voltage) and attempts to increase the generator output to raise the voltage back to the desired level. However, since in fact no feedback is being provided to the voltage regulator, the voltage regulator will continue to increase the generator output, and the generator output voltage will quickly reach dangerously high levels. This may damage equipment connected to the generator. The cause of this failure mode may be, for example, a loose wire providing the voltage feedback to the voltage regulator or a short in the voltage regulating circuit. 
   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 used on aircraft. Such variable frequency electrical systems are becoming increasingly popular because of their overall lighter weight and increased efficiency. 
   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. 
   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 generator from the power supply 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. 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 a generator. 
   SUMMARY OF THE INVENTION 
   These problems and others are addressed by the present invention which comprises, in a first embodiment, a generator control circuit that includes a first circuit monitoring a generator voltage and producing a first output when the voltage is below 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. A third circuit is operatively connected to the first circuit and the second circuit and decreases power supplied to the generator when a plurality of conditions are satisfied, including the voltage being below the first level and the current change rate being above the second level. 
   Another aspect of the invention comprises a method of protecting a generator that involves monitoring a generator voltage, 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 generator voltage being below a first level and the rate of generator current change being above a second level. 
   A further 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, 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 v is less than a first value and a second determination as to whether di/dt is greater than a second value. If each of the number of determinations is true, a level of power supplied to the generator is reduced. 
   Another aspect of the invention comprises a control circuit for a generator that has a field winding and an output and includes a field transistor having a gate connected between the generator field winding and ground. A field controller is connected to the field transistor gate for controlling current flow through the field winding. A detector is provided for determining a generator output voltage and a current change rate in a generator current and producing a first output signal when the voltage is less than a first level and a second output signal when the current change rate is above a second level. A generator controller is provided that is operatively connected to the detector, the generator controller decreasing current flow through the generator field winding upon receipt of the first output signal and the second output signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
       FIG. 1  illustrates, partially schematically, a generator control circuit and detection circuits according to an embodiment of the present invention; 
       FIGS. 2   a – 2   d  are graphs illustrating current and voltage levels that result when a simulated loss of voltage sensing fault occurs in a system using the protection circuits of  FIG. 1 ; and 
       FIG. 3  is a flow chart illustrating the steps followed in performing the method of an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   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 . 
   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 loss of voltage sensing, and this turns off or reduces the output of power supply  12 . 
   Three monitoring circuits provide input to an AND gate  38  on line  36  in this embodiment; when all three 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 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 . 
   The present inventor has recognized that a rapid increase in generator current at a time when the generator output voltage is below a nominal level is indicative of a loss of voltage sensing fault. Therefore, di/dt monitoring circuit  48  senses for a rapid change in current levels. Normal system operation may produce a low generator output voltage, such as when the generator operates in current limiting mode or at generator start up, and these conditions should not trigger a fault protection response. Rapid current change when low voltage is detected, however, is a good predictor of a fault that must be addressed. 
   To prevent normal low voltage conditions from triggering a fault protection sequence, an additional high input to AND gate  38  is 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 below a nominal level, such as 5 volts, for example. 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 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 system. 
   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 first 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 less than 5V. 
   The di/dt monitoring circuit  48  receives a current input on fifth 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. Fifth line  84  is connected to the inverting input of a second op amp  86 . Fifth line  84  includes a fifth resistor R 5  (12 kΩ) and is connected to ground at a point between fifth resistor R 5  and second op amp  86  via a sixth resistor R 6  and a first capacitor C 1  (0.22 μF) arranged in parallel. First capacitor C 1  filters the incoming signal, and its value should be chosen so that normal noise on fifth line  84  does not trigger di/dt detection circuit  48 . Fifth line  84  further includes a second capacitor C 2  (0.01 μF) and seventh resistor R 7  (50 kΩ) between fifth resistor R 5  and the inverting input of second op amp  86 . The value of second capacitor C 2  determines the sensitivity of the di/dt detector. The non-inverting input of second op amp  86  is connected to a 12V power source by a sixth line  88  through an eighth resistor R 8 , and sixth line  88  is connected to ground through a tenth resistor R 10  (100 kΩ). Seventh line  90  provides feedback from the output of second op amp  86  to the inverting input of second op amp  86  through a ninth resistor R 9  (150 kΩ), and the output of second op amp  86  on eighth line  92 , which is proportional to the derivative of the current on fifth line  84 , is conditioned by second logic conditioning circuit  94  to produce a logically high output on di/dt output line  50  when a current change of a given magnitude is detected. 
   GLC monitoring circuit  52  provides a logically high output on line  54  when the generator line contactor is closed. 
   In normal operation, current flow through exciter winding  10  is controlled by field transistor  16  which in turn is controlled by PWM control  18 . Generator output voltage is sensed by a generator controller (not shown) and compared with a reference voltage. Adjustments are made to the exciter current based on the sensed generator output voltage. During normal operation, the signal on generator output detection line  42  is low because the output of the generator, detected on first line  62  and second line  64 , is more than about 5V. Changes in the derivative of the current when the generator voltage is above about 5V are generally not indicative of a loss of voltage sensing fault. The signal on GLC line  54  is high because the generator line contactor is closed. However, if voltage sensing is lost, the voltage detected on first line  62  and second line  64  will drop to 0 (or below, for example, 5V) which will cause the voltage controller to rapidly increase current to the exciter winding  10  and provide a positive signal on di/dt output line  50 . With all three inputs to AND gate  38  high, the shutdown procedure described above will occur. 
     FIGS. 2   a – 2   d  illustrate voltage and current levels during a simulated loss of voltage sensing 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 solid line in  FIG. 2b . 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, a drop in generator output voltage is seen, which causes the output of voltage detection circuit  40  to go high. At this point all three 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 206 volts (about 145V rms) for this 115V rms system. As will be appreciated from the above, the fault protection system of this embodiment of the present invention responds quickly to faults that in a previous system would have led rapidly to an overvoltage condition and thus provides improved protection for both the generator and equipment connected thereto. 
     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 less than a predetermined nominal level such as 5 V, for example. If the sensed voltage is greater than this nominal level, the process returns to step  100 . If the sensed voltage is less than the nominal level, flag A is set to high at step  106 . 
   Concurrently with steps  100  through  106 , steps  108  through  114  are performed. At step  108  a flag B is set to low and a determination is made as to whether the generator line contactor is closed at a step  110 . If it is not closed, the process returns to step  108 . If the GLC is closed, a determination is made at step  112  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  108 . If the current is changing at more than this rate, flag B is set to high at step  114 . 
   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  108  respectively. If both flags are high and the generator line contactor is closed, generator excitation power is disabled at step  124 . 
   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 Classification (CPC): 7