Abstract:
An electric generator is operated under conditions in which rapid off-loading may occur. A generator control unit (GCU) employs a detector to detect rates of change of output of the generator. In response to a rapid change of output, e.g. an off-loading, an overvoltage protection system is activated. Excess energy stored is an excited winding is directed into an impedance circuit thus precluding overvoltage that may have been produced by the excess energy.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention is in the field of electrical generators and, more particularly, electrical generators which operate under rapidly varying load conditions. 
         [0002]    Generators used in aircraft or space vehicles are often operated under variable load conditions. Typically, these generators are provided with regulators that modulate generator field current based on a detected voltage at a point of regulation (a POR). This detected voltage is referred to as a POR voltage. In many applications of these variable load generators, a POR voltage must be maintained within narrowly defined limits. Failure to maintain a POR voltage at a desired level may result in damage to equipment to which the generator supplies power. 
         [0003]    In many aircraft or spacecraft applications, electrical generators are subjected to widely varying and rapidly changing load conditions. When load is rapidly removed from a generator, it is difficult to prevent POR voltage from rising above a desired level. This is because rapid removal of load requires rapid reduction of current in the exciter winding to keep the POR voltage within the desired limit. However, the exciter winding current cannot be reduced to zero instantly due to the inductive nature of the winding. A residual amount of energy in an exciter winding of the generator continues circulating through a freewheeling diode for a short time and this energy contributes to a short-term rise in POR voltage. In the prior art, this residual energy has been discharged into an impedance circuit when POR voltage rises as a result of a rapid off-loading of the generator. One particularly, effective technique for discharging this residual energy is described in U.S. Pat. No. 6,628,104, issued to Yuan Yao et al. on Sep. 30, 2003. 
         [0004]    But, even with availability of sophisticated discharging techniques for residual exciter energy, there still remains a problem in the maintenance of POR voltage during rapid load-off conditions. In the prior art, energy discharging systems have been activated or triggered in response to signals from voltage detectors. When a POR voltage rose above a predetermined level during load reduction, the discharge system would be triggered. But a triggering technique based on measuring POR voltage is inherently limited in the degree of precision with which POR voltage may be controlled. Such a technique requires that triggering not occur at a voltage that is at or below the predetermined POR voltage level. Triggering may only occur after POR voltage exceeds the predetermined POR voltage level. 
         [0005]    After triggering is performed there is an inherent time delay before POR voltage is effectively reduced by discharging the residual energy. During this time delay POR voltage continues to rise. This presents a doubly problematic situation. First of all, as stated above, a triggering voltage threshold must be selected which is higher than the desired POR. Secondly, there must be consideration given to the fact that POR voltage will rise even higher during a time delay after triggering. In the prior art there has never been a generator control system that completely precludes an overvoltage condition from developing during a rapid load-off event. 
         [0006]    Consequently, any equipment driven by the prior-art generators must be robust enough to tolerate a POR overvoltage without being damaged. This of course means that the driven equipment must be built with a certain factor of safety. This translates into undesirable increases in size and weight of the equipment. Excessive size and weight are properties that must be avoided in aircraft and spacecraft equipment. 
         [0007]    As can be seen, it would be desirable to provide electrical generators in which precise control of POR voltage may be maintained during rapid load varying conditions In particular, it would be desirable to provide a control system which precludes an overvoltage at the point of regulation during a rapid load-off event. 
       SUMMARY OF THE INVENTION 
       [0008]    In one aspect of the present invention an electrical generator with a field discharging system comprises a detection unit adapted to trigger operation of the field-discharging system. The detection unit comprises a detector responsive to a rate of change of generator output. 
         [0009]    In another aspect of the present invention a load-off protection system for an electrical generator comprises a first detector responsive to POR voltage of the generator, a second detector responsive to a rate of change of generator output, and a field current discharging system responsive to an activation signal that is produced when the first detector detects a baseline voltage and the second detector produces a do/dt signal when a threshold rate of change of output is detected. 
         [0010]    In still another aspect of the present invention a method for controlling an electrical generator comprising the steps of detecting a rate of change of output of the generator and discharging field current when the rate of change of output reaches a predetermined value. 
         [0011]    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 
         [0012]      FIG. 1  is a block diagram of load-off protection system in accordance with the invention; and 
           [0013]      FIG. 2  is a flow chart of a method preventing load-off overvoltage in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    The following detailed description is of the best currently contemplated modes of carrying out 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. 
         [0015]    Broadly, the present invention may be useful in electrical generators which operate under variable load conditions. More particularly, the present invention may provide a control system for a generator which may preclude an overvoltage condition during a rapid load-off event. The present invention may be particularly useful in vehicles such as aircraft and spacecraft. In such vehicles, avoidance of overvoltage may contribute to low weight and size of equipment driven by the subject electrical generators. 
         [0016]    In contrast to prior-art electrical generators, among other things, the present invention may provide an overvoltage control system which does not depend only on detecting of POR voltage to control overshooting of POR voltage during load-off events. The present invention, instead of only utilizing POR voltage detecting, may utilize detecting of a rate of change of field current or a rate of change of POR voltage to initiate overvoltage control. These desirable improvements of an electrical generator and a method of operating such a generator may be achieved by constructing and operating a generator in an inventive configuration illustrated in  FIG. 1  and an inventive method illustrated in  FIG. 2 . 
         [0017]    Referring now to  FIG. 1 , an electrical generator assembly  10  may comprise a conventional generator  12  and an inventive generator control unit  14  (hereinafter GCU  14 ). The GCU  14  may comprise a field-current modulation switch  16 , an impedance circuit  18 , a bypass switch  20 , a driver  22  and a detection unit  24 . In operation the GCU  14  may provide control of current passing through an exciter of the generator  12 . Field current may pass into and out of the GCU  14  on conductors  26  and  28 . The field current modulation switch  16 , the impedance circuit  18 , the bypass switch  20  and the driver  22  may cooperatively function to vary field current as needed to maintain a desired regulated output voltage from the generator  12 , i.e. a so-called point of regulation voltage or POR voltage, Cooperative functioning of the field current modulation switch  16 , the impedance circuit  18 , the bypass switch  20  and the driver  22  may occur as described in U.S. Pat. No. 6,628,104 which is incorporated herein by reference. 
         [0018]    The detection unit  24  may provide signals to the driver  22  and the bypass switch  20  so that the impedance circuit  18  may be activated as needed to prevent POR overvoltage during variations in load conditions of the generator assembly  10 . The detection unit  24  may comprise first and second POR voltage detectors  30  and  32 , a voltage rate of change detector  34  and a current rate of change detector  36 . The detection unit  24  may also comprise one or more AND gates  38  and an OR gate  40 . In operation the detection unit  24  may produce a signal that activates the impedance circuit  18  during events which may produce a POR overvoltage. 
         [0019]    An impedance-circuit activation signal may be produced by the detection unit  24  under conditions which may be detected by one or more of the detectors  30 - 36 . For example, the detection unit  24  may produce such an activation signal when a POR overvoltage is detected by the first POR voltage detector  30 . Or the unit  24  may produce an activation signal if, as described in U.S. Pat. No. 6,628,104, a Generator Load Contactor is released to remove load from the generator and its command signal (GLC Command) is off. In both of these circumstances the detection unit  24  may be considered to be operating in a conventional manner. 
         [0020]    But, the detection unit  24  may incorporate inventive functionality along with the conventional functionality described above. In that regard, the detection unit  24  may employ the second POR voltage detector  32  in conjunction with the detector  34  and/or the detector  36  to produce the activation signal. The detector  32 , which detects a rate of change of the POR voltage, (dv/dt), may produce a signal when dv/dt reaches or exceeds a predetermined threshold rate of change, i.e. a dv/dt signal. A dv/dt signal may be combined, in one of the AND gates  38 , with a signal from the second POR voltage detector  32 . The second POR voltage detector may have a threshold level lower than threshold level of the first POR voltage detector  30 . A signal may be produced by the detector  32  at all times that the POR voltage meets or exceeds a predetermined baseline threshold (e.g. about 90% to about 100% of a desired POR voltage). Consequently the signal produced by the second POR voltage detector  32  may be referred to as a baseline signal. The baseline POR voltage may be lower than the desired POR voltage of the generator assembly  10 . 
         [0021]    It may be seen that if a condition exists in which POR voltage is equal to or greater than the baseline threshold and there is a dv/dt signal, then an activation signal may be produced. Conversely, presence of a dv/dt signal in the absence of a baseline signal is an insufficient condition for producing an activation signal 
         [0022]    In the above described logical arrangement, an activation signal may be produced whenever a generator load change produces a rapid change in POR voltage, but only if the POR voltage is at or above a baseline threshold. This logical arrangement provides a desired activation signal when load changes occur during steady state operation of the generator assembly  10 . If the generator assembly  10  is in a start-up mode with an associated rapid increase in POR voltage, it may be inappropriate to activate the impedance circuit  18 . Combining signals from the detectors  32  and  34  in one of the AND gates  38  may avoid this anomalous result. 
         [0023]    It may also be seen that when signals from the detectors  32  and  34  are combined as described above, the impedance circuit  18  may be activated at a POR voltage which does not necessarily exceed or even reach a desired POR voltage of the generator assembly  10 . This represents a departure over the prior art wherein POR voltage was required to exceed a desired POR voltage before corrective action could be taken to reduce an overvoltage condition. 
         [0024]    In  FIG. 1 , one of the AND gates  38  may combine signals from the second POR voltage detector  32  and the rate of change of current detector  36 . The detector  36  may produce a signal, a di/dt signal, whenever a rate of change of a detected current reaches or exceeds an established threshold. The detector  36  may detect a rate of change of the field current in the conductors  26  and/28, or alternatively the detector  36  may detect a rate of change of output current of the generator assembly  10 . A combination of the detectors  32  and  36  may operate with the same logical scheme described above with respect to the combination of detectors  32  and  34 , 
         [0025]    The detector unit  24  may operate successfully if it includes either of the detectors  34  or  36 . In other words, a desired control of POR overvoltage may achieved by using only the dv/dt signal combined with the baseline signal. Similarly, the desired control of POR overvoltage may be achieved by using only the di/dt signal combined with the baseline signal. Nevertheless it may be advantageous to incorporate both the detectors  34  and  36  into the detector unit  24 . There may be conditions in which a rate of POR voltage change may be slower than a rate of current change. In such a case the rate of current change detector  36  would be the operative device to activate the impedance circuit  18 . Conversely the current change may be slower than the POR voltage change. Then the detector  34  may be the operative device. 
         [0026]    It may be seen that either or both of the detectors  34  and  36  may detect a rate of change of output of the generator assembly  10 . In that regard either or both of the detectors  34  and  36  may be considered to be rate of output change detectors. Signals produced by either of these rates of output change detectors may be considered to be do/dt signals. 
         [0027]    In an exemplary operation of the inventive control unit  14  the generator  10  may be operated with a desired POR voltage of 230 Vrms. A threshold voltage for the first POR voltage detector may be established at between about 240 Vrms and 250 Vrms. A baseline threshold voltage for the second POR voltage detector  32  may be established at about 210 Vrms and 230 Vrms. A threshold of about 10 to about 50 milliamps/millisecond may be established for the rate of change of current detector  36 . A threshold of about 5 to about 20 volts/millisecond for the detector  34 . At these values the POR voltage of the generator assembly  10  may not exceed 230 Vrms even if the generator assembly  10  may be subjected to off-loading that occurs a period of time as short as about 0.2 millisecond to about 1.0 millisecond. 
         [0028]    In one embodiment of the present invention, a method is provided for controlling an electric generator (e.g. the generator assembly  10 ). In that regard the method may be understood by referring to  FIG. 2 . In  FIG. 2 , a flow chart portrays various aspects of an inventive method  200 . In a step  202  detecting of POR voltage is performed. In a step  204  a signal is produced if the detected POR voltage meets or exceeds an established baseline voltage (e.g. the baseline signal). In a step  206  a rate of change of output of the generator assembly is detected. In a step  208  a signal, (e.g. the do/dt signal) is produced if the rate of change of output reaches or exceeds an established threshold value. In a step  210  the do/dt signal and the baseline signal are combined (e.g. in the AND gate  38 ). In a step  212  an activation signal may be produced in response to combined baseline and do/dt signals. In a step  214  implementation of overvoltage control is performed in response to a signal from the AND gate (e.g. the activation signal). 
         [0029]    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.