Abstract:
A method of detecting a parallel source condition includes calculating a reactive power, comparing the reactive power to a predetermined threshold, and determining a parallel source condition in response to the reactive power exceeding the predetermined threshold.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This application relates to reactive power, and more particularly to using negative reactive power to detect a sustained parallel source condition. 
         [0002]    A vehicle, such as an aircraft, may contain multiple electrical generators coupled to a load. While it is possible to configure electrical generators to operate in parallel, such a parallel configuration can involve complex control algorithms. In an electrical system that includes multiple electrical generators not configured to operate in parallel, a fault condition, such as a contactor erroneously closing, may occur that causes the generators to be connected in parallel, resulting in a sustained parallel source (“SPS”) condition. An SPS condition may also be referred to as a sustained unlike sources in parallel (“SUSP”) or inadvertent parallel (“IP”) condition. An SPS condition can have undesirable effects, such as motoring, which is when a first generator provides current to a second generator causing the second generator to act as a load and consume power instead of generating current, which can potentially damage the second generator. 
         [0003]    Some AC circuits, such as those having inductor loads, dissipate zero power, but still appear to dissipate power as they can provide a voltage drop and can draw current. The power that appears to be delivered to such a load is known as “apparent power.” Apparent power is a vector sum of real power and reactive power. Reactive power is measured in Volt-Amps-Reactive (“VAR”). 
       SUMMARY OF THE INVENTION 
       [0004]    A method of detecting a parallel source condition includes calculating a reactive power, comparing the reactive power to a predetermined threshold, and determining a parallel source condition in response to the reactive power exceeding the predetermined threshold. 
         [0005]    A parallel source condition detection system includes a first generator coupled to a load, a second generator coupled to the load, and a controller. The controller is operable to determine a reactive power associated with at least one of the first generator and the second generator, to compare the reactive power to a predetermined threshold, and to determine a parallel source condition in response to the reactive power exceeding the predetermined threshold. 
         [0006]    These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  schematically illustrates a vehicle system. 
           [0008]      FIG. 2  schematically illustrates a method of detecting a sustained parallel source. 
           [0009]      FIG. 3  illustrates a reactive power when a first generator is operating at a first rotational speed and a second generator is operating at a second rotational speed that is less than the first rotational speed. 
           [0010]      FIG. 4  illustrates a reactive power when the first generator is operating a third rotational speed and the second generator is operating at a fourth rotational speed that is less than the third rotational speed. 
           [0011]      FIG. 5  illustrates the reactive power of the first generator and the second generator from  FIG. 3  after being filtered. 
           [0012]      FIG. 6  illustrates the reactive power for the first generator and the second generator from  FIG. 4  after being filtered. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0013]      FIG. 1  schematically illustrates an aircraft  10  that includes a first generator  12  associated with a first turbine engine  13  and a second generator  14  associated with a second turbine engine  15 . Although the aircraft  10  has been schematically illustrated to include a single generator  12 ,  14  associated with each turbine engine  13 ,  15 , it is understood that a location of the generators  12 ,  14  is not limited to just these positions and that the generators  12 ,  14  could be applied to other locations. In one example both generators  12 ,  14  are associated with a single turbine engine. In one example at least one of the generators  12 ,  14  is associated with an aircraft Auxiliary Power Unit (“APU”). In one example the source of power being protected is an External Power (“EP”) cart instead of a generator. 
         [0014]    Each generator  12 ,  14  is coupled to a load  16  via an electrical bus network  19   a,    19   b.  A first controller  18   a  is operable to process signals from the first generator  12  and is operable to control a flow of current from the first generator  12  to the load  16  via bus network  19   a.  The second controller  18   b  is operable to process signals from the second generator  14  and is operable to control a flow of current from the second generator  14  to the load  16  via bus network  19   b.  In one example each of the controllers  18   a,    18   b  includes a digital signal processor (“DSP”) to assist in signal processing. Each controller  18   a,    18   b  is operable to measure a current and a voltage of its associated generator  12 ,  14  in order to calculate a reactive power associated with the associated generator  12 ,  14 . 
         [0015]      FIG. 2  schematically illustrates a method  100  of detecting a sustained parallel source condition. For each of the generators  12 ,  14 , the respective controller  18   a,    18   b  measures a voltage  20   a - c  and a current  22   a - c  for each phase of its associated generator  12 ,  14 . Although a three phase system is illustrated in  FIG. 2 , it is understood that other quantities of phases could be used. The controller calculates a reactive power for each phase of current associated with the generators  12 ,  14  (step  102 ) using a plurality of summers  24 ,  26 ,  28 ,  30  and multipliers  32 ,  34 ,  36 . The controller then calculates a three-phase average of the reactive power values (step  104 ) and filters the average reactive power (step  106 ) to produce a filtered reactive power. In one example the filtering step  106  includes applying a first filter to obtain a first filtered reactive power and applying a second filter to obtain a second filtered reactive power, with the first filter and the second filter being first order filters. In one example each of the first order filters are 50 Hertz first order filters. In one example the filtering step  106  includes applying a single second order filter. However, it is understood that other types and quantities of filters could be used. For example, third order or fourth order filters could be used. 
         [0016]    The filtered reactive power is then compared to a predetermined threshold (step  112 ), and if the filtered reactive power exceeds the predetermined threshold, the controller provides a SPS fault condition notification (step  114 ). In one example the threshold is a negative threshold such as −50 kVAR, and to exceed the threshold the filtered reactive power would be less than −50 kVAR (such as −60 kVAR). Of course, other thresholds could be used. 
         [0017]    Other steps may then be performed, such as actuating at least one contactor to open and resolve the parallel source condition. In one example the notification step  114  includes notifying a microprocessor or controller associated with the aircraft  10  that a SPS condition has occurred. Although an aircraft  10  has been schematically illustrated and described, it is understood that the method  100  is not limited to aircrafts and could be applied to other systems that include generators. 
         [0018]    A threshold may be selected so that the controllers  18   a,    18   b  can detect an SPS condition without falsely indicating a fault downstream of the generators  12 ,  14  as an SPS condition. For example, a feeder fault may occur somewhere in the bus network  19   a,    19   b  that does not correspond to a SPS condition. Because feeder faults do not cause negative reactive power on all three phases of current of sufficient magnitude simultaneously, the threshold can be chosen so that the controllers  18   a,    18   b  will not provide an SPS notification in response to a feeder fault. 
         [0019]      FIG. 3  illustrates a first reactive power  50   a  corresponding to the first generator  12  and a second reactive power  52   a  corresponding to the second generator  14  when the engine  13  is operating at a first rotational speed and the engine  15  is operating at a second rotational speed that is less than the first rotational speed. As shown in  FIG. 3  the first reactive power  50   a  is positive and the second reactive power  52   a  is negative, possibly indicating that the first generator and second generator have become coupled in parallel, and that the second generator  14  is being “motored” by the first generator  12  and is undesirably acting as a load on the first generator  12 . 
         [0020]      FIG. 4  illustrates the first reactive power  50   b  and the second reactive power  52   b  when the engine  13  is operating at a third rotational speed and the engine  15  is operating at a fourth rotational speed that is less than the third rotational speed. As shown in  FIG. 4 , the reactive power  50   b,    52   b  oscillates more at the third and fourth rotational speeds than at the first and second rotational speeds. 
         [0021]      FIG. 5  illustrates the reactive power values of  FIG. 3  after being filtered. As described above, in step  106  the reactive power values  50   a,    52   a  may be filtered.  FIG. 5  accordingly illustrates a filtered reactive power  54   a  corresponding to the first reactive power  50   a,  and a filtered reactive power  56   a  corresponding to the second reactive power  52   a.  Since the filtered reactive power  56   a  is negative for a period of time (indicating a negative reactive power), and exceeds a threshold  60  (step  112 ), a SPS fault condition would be indicated (step  114 ).  FIG. 6  illustrates the reactive power values of  FIG. 4  after being filtered.  FIG. 6  illustrates a filtered reactive power  54   b  corresponding to the first reactive power  50   b  and a filtered reactive power  56 b corresponding to the second reactive power  52   b.  Since the filtered reactive power  56   b  is negative for a period of time (indicating a negative reactive power), and exceeds a threshold  62  (step  112 ), a SPS fault condition would be indicated (step  114 ). 
         [0022]    Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.