Abstract:
The present embodiments are directed to locating electrical faults in an electrical circuit, in particular electrical faults in transmission wires of an electrical circuit. Examples of the present embodiments provide a method and apparatus for opening a switch in the electrical circuit to cause an open circuit or discontinuity at the fault; transmitting a signal to be reflected from the open circuit or discontinuity and receiving the signal reflected from the open circuit or discontinuity to determine the location of the fault. Examples are particularly suitable for high voltage systems, for example over 100V.

Description:
BACKGROUND 
       [0001]    The present disclosure is directed to locating electrical faults in an electrical circuit, in particular electrical faults in transmission wires of an electrical circuit. 
         [0002]    Many electrical systems, such as those in an aircraft power distribution system use long transmission wires, often hundreds of metres or kilometres in length, to deliver power from electrical power sources to electrical loads. However, the long transmission wires can experience faults which need to be repaired. Attempts have been made to identify the location of a fault on live transmission wires in an electric circuit using “reflectometry”. This technique involves measuring the time taken for a signal to be reflected from a discontinuity or an open circuit at a fault. Knowing the speed at which the signal travels, the location of the fault may be determined from the time taken for the reflected signal to return from the discontinuity. 
         [0003]    However, it has been found that the “reflectometry” technique is only suitable in particular situations, such as for faults in low voltage applications. The inability to reliably detect the location of a fault in an electrical system leads to time consuming investigations to determine the location of faults requiring the electrical systems to be shut down which is inconvenient. 
       BRIEF DESCRIPTION 
       [0004]    According to a first aspect of the present disclosure there is provided a method of locating a fault in a live electrical circuit having transmission wires, the method comprising opening a switch in the electrical circuit to cause an open circuit or discontinuity at the fault; transmitting a signal to be reflected from the open circuit or discontinuity and receiving the signal reflected from the open circuit or discontinuity to determine the location of the fault. 
         [0005]    By opening a switch in the electrical circuit to provide an open circuit or discontinuity at the fault, the location of faults in electrical systems which have not previously been reliably detectable, such as in high voltage circuits where arcing across the fault prevents a clear open circuit or discontinuity from being generated, may now be detected. Examples are particularly suitable for high voltage systems, for example of 100V or more. 
         [0006]    According to a second aspect of the present disclosure there is provided an apparatus for locating a fault in an electrical circuit having transmission wires, the apparatus comprising a controller for opening a switch in the electrical circuit to cause an open circuit or discontinuity at the fault; transmitting a signal to be reflected by the open circuit or discontinuity; and receiving the signal reflected from the open circuit or discontinuity to determine the location of the fault. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Embodiments of the present disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which: 
           [0008]      FIG. 1  is a flow diagram showing an example of locating a fault in an electrical system according to an embodiment; 
           [0009]      FIG. 2  shows an example of an electrical system according to an embodiment; 
           [0010]      FIG. 3  is a more detailed example of an electrical system according to an embodiment; and 
           [0011]      FIG. 4  is a series of graphs showing the response at various points in the electrical system of  FIG. 3  during the location of an electrical fault. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]      FIG. 1  shows an example of locating a fault in an electrical circuit having transmission wires. At step  10  a switch in the electrical circuit is opened. This leads to the voltage in the electrical circuit falling along with the arc current across the fault until the arc current has fallen sufficiently to quench the arc causing an open circuit or discontinuity at the fault. At step  20  a signal transmitted into the circuit is reflected from the open circuit or discontinuity at the fault. At step  30  the signal reflected from the open circuit or discontinuity is received to determine the location of the fault. For example if the signal is known to travel in the transmission wires at a known speed, the time taken by the reflected signal can be used to determine the distance to and thus the location of the fault. 
         [0013]      FIG. 2  shows an example of an electrical circuit according to an embodiment. The circuit includes a length of electrical wiring  60 . The electrical wiring  60  may be for example be in a vehicle such as an aircraft, ship etc or in a factory, hospital or other facility.  FIG. 2  shows a fault  61  that has occurred in the wiring  60 . The circuit  50  also includes an electrical power source  70  and an electrical load  80  connected to each other by the wiring  60 . A switch  90  is also provided in the electrical wiring  60 . The switch  90  may be part of some associated equipment  91  forming part of the circuit, such as a solid state power controller (SSPC), the power generator  70  or a current transformer for example. 
         [0014]    In order to determine the location of the fault  61  in the electrical wiring  60 , which can extend to considerable lengths such as some kilometres, the switch  90  is opened by a controller in an arc fault location device  100  which leads to the voltage and current in the electrical circuit  50  falling. When the current in the arc across the fault  61  falls, typically to 200-400 mA for a short arc (but to different currents depending on the size of the gap) the arc quenches resulting in an open circuit. The arc quench produces a very fast drop in current in the electrical circuit  50  resulting in a change in impedance of the wiring  60  as the arc has now been replaced by an air gap. A signal from the controller in the arc fault location device  100  provided to the wiring  60  of the electrical circuit  50  is reflected by the change in impedance of the wiring  60  caused by the arc quench at the fault  61 . The reflected signal is received, in this case by the arc fault location device  100 , but it could be received by another component at the same or some other location in the circuit  50 . The time taken for the signal to travel from the arc fault location device  100  to and back from the fault  61  is determined and knowing the speed at which the signal travels in the wiring  60 , the distance to and thus the location of the fault  61  may be calculated. The method of locating the fault  61  may be repeated to determine the location even more precisely. 
         [0015]      FIG. 3  illustrates a typical DC aircraft electrical power distribution circuit  150 , having a Solid State Power Controller (SSPC) device  191  with a switch  190 . The SSPC device  191  of this example has arc fault detection (AFD) capability. Arc fault location capability is provided by an arc fault location device  200  including a reflectometry system  201 .  FIG. 4  shows waveforms of electrical characteristics at various points in the circuit  150 . 
         [0016]    A power source, in this example in the form of an electrical generator  170  with voltage V src  and inherent output impedance Z src  is provided. When used in an aircraft, an operating gas turbine engine may provide mechanical energy which may be used to provide a driving force for the generator  170 . 
         [0017]    The generator  170  is connected to the SSPC  191  by upstream current limiting wiring  171  with inherent inductance L up  and resistance R up . In this example the main switch  190  is provided with a transorb  192  and a diode  193  to suppress any transient electrical signal generated when the switch is opened. 
         [0018]    The wiring  161  between the switch  190  and the electrical load  150  is shown with inherent inductance L dn  and resistance R dn . A fault  161  shown as a physical separation between two separated terminals  162 ,  163  is present in the wiring  160 . A load  180  with inherent inductance L load , resistance R load  and capacitance C load  is also shown. 
         [0019]    The arc fault locating device  200  includes a reflectometry system  201  to transmit a signal into the electrical circuit  150  and to receive the signal reflected from the fault  161 . The fault locating device  200  also has a switch device  202  to open and close the switch  190  and a microprocessor  203  to control the reflectometry system  201  and the switch device  202  and to calculate the distance to and thus the location of the fault  161 . In this example current and voltage sensors  204 ,  205  are also shown which may be used to detect the occurrence of a fault  161 . The microprocessor  203  of the example of  FIG. 3  is arranged to control the function of arc fault detection and location as well as other switch functions which the particular circuit  150  may be arranged to perform depending upon its use. 
         [0020]    Prior to time ( 1 ) in  FIG. 4 , a series arc fault is present in the system. The presence of an arc fault may be detected by any method known to a person skilled in the art, such as by monitoring current and/or voltage of the circuit  150 . Alternatively, the method of locating a fault may be performed periodically without the need to specifically detect that a fault has occurred beforehand. 
         [0021]    At point ( 1 ) in  FIG. 4 , arc fault detection hardware in the SSPC device  191  of  FIG. 3  has had sufficient time to flag a possible arc fault. In this example the presence of an arc fault may be determined by detecting the short current reduction caused by disconnection and then reconnection during arcing. 
         [0022]    At point ( 1 ) in  FIG. 4 , the arc fault location device  200  is enabled. At this point it starts transmitting signals to be reflected by a fault in the circuit when the arcing at the fault has been quenched. 
         [0023]    At point ( 2 ), the series arc fault is confirmed by a further short current reduction and the SSPC switch  190  is opened which leads to the SSPC  191  output voltage falling along with the arc current. At point ( 3 ) which occurs when the arc current is typically around 200-400 mA for short arcs, the arc quenches. Arc quench is characterised by a very fast negative dI/dt which is indicative that an arc fault is present.  FIG. 3  shows a snubber  194  comprising a resistor and a capacitor in series between the output node  195  of the SSPC device  191  and ground  196 . The snubber  194  ensures that the arc quenches at point ( 3 ) in  FIG. 4 . The quenching of the arc is seen by the reflectometry hardware  201  in the arc fault location device  200  as a gross change in the line impedance as the arc has now been replaced by an air gap. 
         [0024]    A signal transmitted by the reflectometry system  201  is reflected from the open circuit or discontinuity at the fault  161  and received to measure the time taken for the signal to travel to the fault and back and determine the location of the fault. The microprocessor  203  in the device  200  calculates the distance from the device  200  to the fault  161  knowing the time taken for the round trip (double the distance between the device  200  and the fault  161 ) and the propagation speed of the signal in the cable using the formula: 
         [0000]      distance to fault=propagation speed×time taken/2
 
         [0025]    At point ( 4 ) the SSPC switch  190  is closed and the SSPC  191  output voltage rises to match the line voltage. At this point, the arc current does not rise because there remains a gap between each side of the electrodes  162 ,  163  shown in  FIG. 3 . At point ( 5 ), the voltage across the electrodes  162 ,  163  has risen sufficiently for arcing to occur between them and the loop current rises. The delay between points ( 4 ) and ( 5 ) is another indication that a series arc was present. At point ( 6 ) an arc has been confirmed and the SSPC switch  190  is tripped (opened). At point ( 7 ) loop current has fallen to zero. This confirmation/perturbation/location method (point ( 1 ) to point ( 5 ) could be run multiple times to improve confidence further. 
         [0026]    It has been found that power quality of 270VDC systems can support an interruption of 50 ms in accordance with Aerospace standard RTCA DO-160G. The interruption time required for the arc fault perturbation methodology to function correctly is approximately 100 microseconds which has an insignificant effect on power quality. 
         [0027]    Generally speaking, reflectometry schemes required more time in order to detect faults which are further away due to an increase in roundtrip time. Assuming a propagation speed of approximately 0.66 speed of light and a maximum cable length of 100 m, the round trip time can be calculated as follows: round trip time=2*distance/speed=2*100/(0.66*3×10̂8)=1 microsecond. Therefore numerous reflectometry signals can be used in order to monitor for cable breaks. For example, numerous reflectometry signals can be used after the arc has been quenched at point ( 3 ) of  FIG. 4  during t_delay. 
         [0028]    Many variations may be made to the examples described above whilst still falling within the scope of the present disclosure. For example, the switch  190  may be an independent component or may be provided as part of an existing device in a circuit, such as an SSPC device or a power generator. As well as locating a fault in wiring in an aircraft, the method disclosed herein may be used locate wiring faults in other situations such as ships, factories, hospitals and other facilities such as in solar or photovoltaic facilities. The signal to be reflected by the open circuit or discontinuity may be transmitted at any suitable time, such as by being repeatedly transmitted from before the switch is opened or as the switch is opened. It may be repeatedly transmitted until after a reflected signal has been received or until or after the switch has been subsequently closed.