WO2015124884A1

WO2015124884A1 - Method for limiting current in a circuit

Info

Publication number: WO2015124884A1
Application number: PCT/GB2014/050465
Authority: WO
Inventors: Peter James Handy; Julian Peter Mayes
Original assignee: GE Aviation Systems Ltd
Current assignee: GE Aviation Systems Ltd
Priority date: 2014-02-18
Filing date: 2014-02-18
Publication date: 2015-08-27
Anticipated expiration: 2016-08-18
Also published as: GB2545293A

Abstract

A method for limiting current in an electrical circuit (10) having transmission wires (18, 20) for power transmission includes determining whether an unexpected operating condition exists along the transmission wires and limiting the current to prevent the damage caused by an unexpected operating condition. If the unexpected operating condition persists, the method may disable the electrical circuit.

Description

METHOD FOR LFMITING CURRENT IN A CIRCUIT

BACKGROUND OF THE INVENTION

Electrical systems, such as those found in an aircraft power distribution system, employ electrical bus bars and miles of wiring for delivering power from electrical power sources to electrical loads. In the event of an unexpected electrical condition or electrical fault, high currents may be transmitted, resulting in unexpected operations of the power distribution system.

BRIEF DESCRIPTION OF THE INVENTION

In one embodiment, a method for limiting current in a circuit having a power source electrically coupled with a solid state power controller (SSPC), the SSPC configured to operate in a first conducting state and a second non-conducting state and further coupled with an electrical load via transmission wire having an inductance, and a flywheel diode configured across the transmission wire. The method includes a first sensing of a current along the transmission wire while the SSPC is operated in the first conducting state, determining when the first sensing of the current satisfies a first current threshold, reducing the power transmission along the transmission wire by operating the SSPC in the second non-conducting state and allowing electrical energy stored in the inductance to generate current through the flywheel diode, a second sensing of a current along the transmission wire, determining if the second sensing of the current satisfies a second current threshold, and increasing the power transmission along the transmission wire by operating the SSPC in the first conducting state.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic circuit diagram of a power distribution system in accordance with one embodiment of the invention. FIG. 2 is a schematic circuit diagram of the power distribution system illustrating current flow of the system when the switchable component is closed, in accordance with the first embodiment of the invention.

FIG. 3 is a schematic circuit diagram of the power distribution system illustrating current flow of the system at the instance the switchable component is opened, in accordance with the first embodiment of the invention.

FIG. 4 is a series of graphs showing the response of the power distribution system, in accordance with the first embodiment of the invention.

DESCRIPTION OF EMBODFMENTS OF THE INVENTION The described embodiments of the present invention are directed to an electrical power distribution system, which may be used, for example, in an aircraft. While this description is primarily directed toward a power distribution system for an aircraft, it is also applicable to any environment using an electrical system for transmitting power from a power source to an electrical load. FIG. 1 illustrates an exemplary schematic circuit diagram of a power distribution system 10, such as an exemplary power distribution system in an aircraft, comprising a power source, for example, a generator 12, an electrical switch, such as a solid state power switch (SSPC) 14, and an electrical load 16. As shown, the power distribution system 10 further comprises electrical interconnects, cables, cable junctions, or bus bars, illustrated as a first electrical transmission wire 22 electrically coupling the generator 12 with the SSPC 14, and a second electrical transmission wire 28 electrically coupling the SSPC 14 with the electrical load 16. As shown, the first transmission wire 22 and/or the generator 12 may define a variable or predetermined first set of electrical characteristics upstream from the SSPC 14, for example, a first inductance 24 and first resistance 26. Likewise, the second transmission wire 28 may define a variable or predetermined second set of electrical characteristics, for example, a second inductance 30 and a second resistance 32. The SSPC 14 may include a controllable switching component 34, a transient-voltage- suppression device configured across the switching component 34, such as a transorb 36, and a diode, such as a flywheel diode 38, biased from, for example, electrical ground to the power line, downstream from the switching component 34. As shown, the power distribution system 10 may also include a third resistance 40 and a first capacitance 42 configured upstream from the switching component 34 and across the first transmission wire 22 input, a third inductance 44 downstream from the switching component 34, and a fourth resistance 46 and a second capacitance 48 configured downstream from the switching component 34 and across the second transmission wire 28 output. The SSPC 14 may further include a current sensor 50 positioned downstream from the switching component 34 and flywheel diode 38, and capable of sensing and/or measuring the electrical current characteristics of the current flowing through the power distribution system 10. It is envisioned the switching component 34 is controllable to operate in an open (non-conducting) state that prevents electrical transmission via the switching component 34, and a closed (conducting) state that allows electrical transmission via the switching component 34.

One example of the SSPC 14 may comprise a silicon carbide (SiC) or Gallium Nitride (GaN) based, high bandwidth power switch. SiC or GaN may be selected based on their solid state material construction, their ability to handle large power levels in smaller and lighter form factors, and their high speed switching ability to perform electrical operations very quickly.

The switching component 34 of the SSPC 14 is envisioned to comprise a field-effect transistor (FET). Additionally, while a transorb 36 is described, any transient- voltage-suppression devices may be used which, for example, allowing current to flow from upstream of the switching component 34 to downstream of the switching component 34, when exposed to a transient voltage greater than a breakdown voltage for the transorb 36. It is envisioned the breakdown voltage for the transorb 36 is less than the voltage generated by the generator 12. Example current characteristics measurable by the current sensor 50 may include, but are not limited to, instantaneous current, average current, or rate of change in current. While the current sensor 50 is illustrated measuring the current characteristics at the SSPC 14, other measurement locations are envisioned so long as the measurements are captured downstream from the switching component 34 and flywheel diode 38. While the current sensor 50 is described as "sensing" and/or "measuring" the electrical current of the power distribution system 10, it is envisioned that sensing and/or measuring may include the determination of a value indicative or related to the electrical current characteristics, and not the actual current values.

The SSPC 14 may further include a controller 52 having an input to receive the sensed current measurement from the current sensor 50, and capable of generating and providing a control signal 54 output to the switching component 34. The controller 52 may additionally include memory 56, the memory 56 may include random access memory (RAM), read-only memory (ROM), flash memory, or one or more different types of portable electronic memory, such as discs, DVDs, CD-ROMs, etc., or any suitable combination of these types of memory. The controller 52 may be operably coupled with the memory 56 such that any suitable controller programs or computational code which may be stored in said memory 56 may be executed on the controller 52. It is envisioned the control signal 54 is capable of controlling the switching component 34, and thus, controlling the operation of the SSPC 14.

While the controller 52 is illustrated as a subcomponent of the SSPC 14, alternative configurations are contemplated wherein the controller 52 may provide control of the SSPC 14 from a remote location. For example, one controller 52, provided away from the SSPC 14, may be configured to provide control for one or more SSPCs 14 or sets of SSPCs 14. Furthermore, embodiments are envisioned wherein the memory 56 may be may be separate from the controller 52, but may be in communication with the controller 52 such that it may be accessed by the controller 52. For example, it is contemplated that the suitable controller programs stored in the memory 56 may be updated through the wireless communication link, or from a common memory storage system.

During operation, in an aircraft embodiment for example, an operating gas turbine engine may provide mechanical energy to provide a driving force for the generator 12, which outputs electricity in response. The generator 12, in turn, provides the generated power to the SSPC 14 via the first transmission wire 22, which in turn, is controllable by the controller 52 to deliver the power to the electrical loads 16, via the second transmission wire 28. Additional power sources for providing power to the electrical loads 16, such as emergency power sources, ram air turbine systems, starter/generators, Auxiliary Power Units (APUs), fuel cells, or batteries, are envisioned. It will be understood that while one embodiment of the invention is described in an aircraft environment, the invention is not so limited and has general application to electrical power systems in non-aircraft applications, such as other mobile applications and non-mobile industrial, commercial, and residential applications.

FIG. 2 illustrates operation of the power distribution system 10 when the controller 52 controls the switching component 34 to operate in a closed state. In this illustration, a first current flow, shown as an arrow 58, shows the current flowing from the generator 12, through the first transmission wire 22, through the SSPC 14 (via the switching component 34 in a closed state), through the second transmission wire 28, to the electrical load 16, and back to the generator 12 along the closed circuit.

FIG. 3, alternatively, illustrates operation of the power distribution system 10 at the instantaneous time when the controller 52 controls the switching component 34 to toggle to an open state. This illustration demonstrates a second current flow, shown as a summation of arrows 60, and may comprise of current stored in the electric field of the second capacitance 48 and current stored in the magnetic fields of the second inductance 30 and third inductance 44, as each respective inductance attempts to resist the change in current caused by the toggling of the switching component 34 to an open state. The second current flow 60 may additionally comprise of current stored in the electric field of the first capacitance 42, and first inductance 24, the current traversing around the switching component 34, through the transorb 36, so long as voltage difference across the transorb 36 is greater than the transorb 36 breakdown voltage. Any current of the second current flow 60 completes a closed circuit through the flywheel diode 38 downstream of the switching component 34 in an open state. It is envisioned that, at the instantaneous time when the controller 52 toggles the switching component to an open state, the second current flow 60 generated by any of the first, second, and third inductances 24, 30, 44, and any of the first or second capacitances 42, 48, will decay over time. If the switching component 34 is left in the open state for a long enough period of time, the second current flow will decay until no current flows in the power distribution system. During operation of the power distribution system 10, unexpectedly high currents travelling through at least one of the transmission wires 22, 28, SSPC 14, and/or switching component 34 may cause system 10 failure or an over-temperature condition in the system 10. One non-limiting example of an unexpectedly high current, or "over-current" condition, may be two to three times the maximum expected current for the system 10. Higher over-current conditions and/or limits are envisioned.

Over-current conditions may occur due to electrical arcing or electrical shorts in an environment where, for example, physical defects in an electrical connection cause a permanent or temporary loss in transmission capabilities, or a sudden transmission of high levels of current. While electrical arcing and/or electrical shorts are described, additional causes of over-current conditions are envisioned, such as lightning strikes, current rush during starting conditions, etc.

A method for operating the power distribution system 10 to limit the transmission of current through the system 10 is described herein. First, the current sensor 50 provides a first sensing of current along the second transmission wire 28 of the system 10 under normal operation (i.e. the switching component 34 in a closed state), and this first sensing of current is provided to the controller 52. It is envisioned that the current sensor 50 may, for example, provide the first sensing of current at timed increments, continuously, or when polled by the controller 52. The controller 52 then determines if the first sensing of the current satisfies a first current threshold. This determination may, for example, compare the first sensed current against a first current threshold indicative of an over-current condition. It is envisioned the first current threshold may be predetermined and stored in the memory operating conditions, an absolute current value, or a percentage of the switch rating, for example, such as two hundred percent higher than the last 2 seconds average current. Alternative first current thresholds are envisioned, such as a first current threshold indicative of, or related to, a predetermined, relative, or estimated first thermal threshold of the switching component 34 or SSPC 14, for instance, a maximum operating temperature of the FET.

Based upon a determination that the first sensing of the current satisfies the first current threshold, for instance, if the first sensed current is greater than the first current threshold, the controller 52 provides a control signal 54 to the switching component 34 to control the component 34 to operate in an open state. By operating the switching component 34 in the open, non-conducting state, the power transmission along the transmission wires 22, 28 is reduced to only the current flow explained above and indicated by arrow 60.

Next, the current sensor 50 provides a second sensing along the second transmission wire 28 while the power distribution system 10 is under the reduced power transmission operation, and this second sensing is provided to the controller 52. It is envisioned that the current sensor 50 may, for example, provide the second sensing of current at timed increment or increments after the first sensing, continuously, or when polled by the controller 52. The controller 52 then determines if the second sensing of the current satisfies a second current threshold. This determination may, for example, compare the second sensed current against a second current threshold indicative that the over-current condition has been relieved or removed. Stated another way, the determination may determine that the over-current condition no longer exists in the power distribution system 10. This determination may be indicated by, for example, a second sensed current is less than the second current threshold. It is envisioned the second current threshold may also be predetermined and stored in the memory 56 of the controller 52, or in another example, may be a current threshold relative to normal operating conditions, such as fifty percent of rated current or an absolute current value, or a percentage of a rated current or absolute current value over a period of time, such as the last 2 seconds average current.

Alternative second current thresholds are envisioned, such as a second current threshold indicative of, or related to, a predetermined, relative, or estimated second thermal threshold of the switching component 34 or SSPC 14. One non-limiting example of a second thermal threshold of the switching component 34 may include a temperature below which the FET will operate as expected, or such that the FET is satisfactorily cooled after an over-temperature or over-current condition.

If the controller 52 determines that the second sensing of current satisfies the second current threshold, the controller 52 provides a control signal 54 to the switching component 34 to control the component 52 to operate in the closed, conducting state.

It is envisioned that the above-mentioned steps may occur repeatedly until the unexpectedly high currents are no longer present in the power distribution system 10, or until the controller 52 controls the switching component 34 of the SSPC 14 to operate in a non-conducting state due to the heating of switch 34. For example, the controller 52 may count the number of repetitions of the above-mentioned steps, or count the number of repetitions for a given period of time, such as a moving time window of the last two seconds. The controller 52 may further compare the number of repetitions with power distribution system 10 failure threshold or a predetermined number of repetitions. For example, five or ten repetitions may satisfy a system 10 failure threshold such that the controller 52 may determine that the failures are likely to continue to occur, or for example, that a arcing fault or electrical short exists in the system 10. Alternatively, the controller 52 may determine that a number of successive repetitions in the moving time window may satisfy the system 10 failure threshold. In any of these scenarios, the controller 52 may disable the power distribution system 10, temporarily or permanently, until the failure can be addressed, for example, by maintenance personnel. Alternative system 10 failure thresholds, or counting of repetitions indicative of failures, are envisioned. While only a single generator 12, SSPC 14, first transmission wire 22, second transmission wire 28, electrical load 16, and controller 52 are illustrated for ease of understanding, alternate power distribution systems 10 are envisioned having one or more of the aforementioned components 12, 16, 22, 28, 14, 52 configured to define a robust power distribution system 10, or network of systems 10. For example, alternative configurations are envisioned having more than one electrical load 16 coupled to each SSPC 14, more than one set of transmission wires 22, 28 configured in series or parallel, or more than one SSPC 14 configured to selectively couple multiple sets of transmission wires 22 to additional portions of the power distribution system 10. Additionally, embodiments are envisioned wherein, for example, one controller 52 remotely controls operation of a plurality of SSPCs 14.

Furthermore, it is contemplated that the first set of electrical characteristics, the second set of electrical characteristics, and the third inductance are selected such that the desired power transmission response, or electrical responses of the power distribution system 10, may be designed to, for example, operate without power interruption during the method, or such that the method operates faster than the reset time for the electrical load. Additionally, the third inductance 44 may be an inherent inductance defined by the SSPC 14, or it may be designed to be at least a minimal inductance, as needed, to generate a minimal current, as needed, when the switching component 34 is opened. In another instance, the third inductance 44 is optional where the second inductance 30 is capable of maintaining a minimal current, as designed, when the switching component 34 is opened.

One embodiment of the power distribution system 10 operation may be further understood with reference to the time-aligned graphs presented in FIG. 4. As illustrated, a first graph 70 showing an example voltage on the second transmission wire 28 ("voltage out"), a second graph 72 showing an example voltage at the upstream location of the transorb 36 ("voltage in"), a third graph 74 showing an example current signal relative to the first current threshold 76 and second current threshold 78, as measured by the current sensor 50 ("current out"), a fourth graph 80 showing an example current at the upstream location of the transorb 36 ("current in"), a fifth graph 82 showing an example current across the flywheel diode 38 ("diode current"), and a sixth graph 84 showing an example current across the transorb 36 ("transorb current"). The graphs 70, 72, 74, 80, 82, 84 additionally illustrate sequentially corresponding time values of a first time instance 86, and a second time instance 88, a third time instance 90, a fourth time instance 92, and a fifth time instance 94. It is understood that the first, second, third, fourth, fifth, and sixth graphs 70, 72, 74, 80, 82, 84 are intended for illustrative purposes demonstrating the power distribution system 10 responses, as described, and are not envisioned to accurately represent voltage, current, or timing aspects.

At the start of the method, the power distribution system 10 is operating with the switching component in a closed conducting state. At this time, the voltage in and voltage out are positive, as is the current in and the current out. Also shown, there is no diode current in the fifth graph 82, nor is there transorb current in the sixth graph 84 (due to current through the switching component in a closed state). However, as illustrated, voltage out is rising in the first graph 70, the current out is rising in the third graph 74, and the current in is rising in the fourth graph 80.

At the first time instance 86 the current out satisfies the first current threshold 76, as shown in the third graph 74. When this occurs, the switching component 34 is toggled to an open state. Between the first time instance 86 and the second time instance 88, the voltage out in the first graph 70 goes to zero. Additionally, between these time instances 86, 88, the voltage in of the second graph 72 jumps up, as the first inductance 24 attempts to dissipate its stored energy, until the transorb 36 starts to conduct current, as shown in the sixth graph 84. The transorb current in the sixth graph 84 also corresponds to the current in shown in the fourth graph 80, as all current in is traversing via the transorb 36 in this period. Likewise, the second and third inductances 30, 44 dissipate their stored energy through the circuit. In this sense the first, second, and third inductors 24, 30, 44 are selected in order to provide a minimal amount of current, as needed, after the switching component 34 has been toggled to an open state. In this sense, while the inductors 24, 30, 44 attempt to provide a minimal amount of current, the current out, as shown in the third graph 74, is reduced below the previous current levels (prior to opening the switchable component 34), and will continue to reduce as the inductors 24, 30, 44 continue to dissipate their stored energy. During this period, as current flows through and into the components downstream of the switching component 34, the current out completes a loop via the only remaining closed circuit via the flywheel diode 38. As shown in the fifth graph 82, the diode current increases as long as the current in flows.

Additionally, between the first time instance 86 and the second time instance 88, the transorb current of the sixth graph 84, and correspondingly the current in of the fourth graph 80, decay as the first inductance 24 dissipates its stored energy. At the second time instance 88, the transorb current of the sixth graph 84, and correspondingly the current in of the fourth graph 80, have decayed to zero.

Between the second time instance 88 and the third time instance 90, current is still flowing via the closed downstream loop, as represented by the diode current in the fifth graph 82 and the current out in the third graph 74. The diode current is shown falling due to the resistive losses of, for example, the second resistance 32 and load 16.

At the third time instance 90, the current out satisfies the second current threshold 78, as shown in the third graph 74. When this occurs, the switching component 34 is toggled to a closed state. With the generator 12 returned to the conducting loop, the current in begins to rise between the third time instance 90 and the fourth time instance 92, as shown in the fourth graph 80, as the first inductor 24 is charged. Additionally, the diode current in the fifth graph 82 drops as the first current flow 58 becomes available. During this time the current out continues to fall until the diode current falls to zero and the inductor 24 is charged. At the fourth time instance 92, the diode current falls to zero in the fifth graph 82, and thus, the current out of the third graph 74 begins to increase again as the voltage out and voltage in, respectively of the first and second graphs 70, 72, return to their expected line voltages. As illustrated, between the fourth time instance 92 and the fifth time instance 94, the voltage out is again shown rising in the first graph 70, the current out is again shown rising in the third graph 74, and the current in is again shown rising in the fourth graph 80, until the current out in the third graph 74 again reaches the first current threshold 76, repeating the switching cycle.

Alternative power distribution system 10 responses are envisioned. For instance, if the current out of the third graph 74 satisfies the second current threshold 78 prior to the transorb current of the sixth graph 84 decaying to zero, the period between the second time instance 88 and the third time instance 90 may not need to be represented, and correspondingly, the period between the third time instance 90 and fourth time instance 92 may be reduced as the inductors 24, 30, 44 may not need to be charged for as long.

Many other possible embodiments and configurations in addition to that shown in the above figures are contemplated by the present disclosure. Additionally, the design and placement of the various components may be rearranged such that a number of different in-line configurations could be realized. A beneficial effect of the embodiments is a method for limiting current in a circuit. One advantage that may be realized in the above embodiments is that the above described embodiments provide for reducing system current in response to an over- current condition, which may limit the over-current condition to a limited time. Over- current conditions may be indicative of a parallel or arcing fault in the system, which may generate intense localized heat from resistive losses at the fault, which could further lead to an electrical fire, smoke, melting of or damage to components, or catastrophic failures of the electrical system or greater structure, such as an aircraft. Thus, by limiting the current in the system, and correspondingly, limiting the time the system is operating in an over-current condition, the likelihood of thermal runaway due to a fault is reduced.

Another advantage of the above-described embodiments is the method allows for detection and prevention of an over-temperature condition at the switching component of the SSPC, which is likely to be a point of thermal failure during an over-current or over-temperature condition. The method thus allows for increased protection of the switching component and SSPC due to the current limiting technique described above. Furthermore, the above-described method may be applied to a plurality of SSPCs to ensure thermal failure protection across each of the SSPCs, or if failure occurs, further protection from thermal damage, electrical fire, smoke, etc. due to the disabling of the SSPC.

Yet another advantage of the above-described embodiments is by repeating the method a number of times prior to disabling the circuit, the method is capable of validating a persistent over-current or over-temperature condition as opposed to a temporary condition, and disabling the circuit permanently until corrective actions can be taken. For example, the method may be run multiple times in order to confirm the presence of an over-current or over-temperature condition before taking appropriate actions, and thus may improve power quality and availability by reducing nuisance tripping events. The method may also employ filtering techniques to further reduce erroneous false-positive fault indications. Additionally, the method allows for uninterrupted electrical load operation in the event that a false-positive fault is indicated, but not confirmed.

The above described embodiments, thus, provide for increased safety for an aircraft electrical power distribution system and hence improve the overall safety of the aircraft and air travel. Furthermore, by disabling the electrical circuits in the event of repeated over-current and/or over-temperature conditions reduces or eliminates any additional maintenance time and/or costs associated with having to replace electrical components damaged or destroyed due to the over-current and/or over-temperature fault.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent stmctural elements with insubstantial differences from the literal languages of the claims.

Claims

CLAIMS:

1. A method for limiting current in a circuit having a power source electrically coupled with a solid state power controller (SSPC), the SSPC configured to operate in a first conducting state and a second non-conducting state and further coupled with an electrical load via a transmission wire having an inductance, and a flywheel diode configured across the transmission wire, the method comprising: a) a first sensing of a current along the transmission wire while the SSPC is operated in the first conducting state; b) determining when the first sensing of the current satisfies a first current threshold; c) reducing a power transmission along the transmission wire by operating the SSPC in the second non-conducting state and allowing electrical energy stored in the inductance to generate current through the flywheel diode; d) a second sensing of a current along the transmission wire; e) determining if the second sensing of the current satisfies a second current threshold; and f) increasing the power transmission along the transmission wire by operating the SSPC in the first conducting state.

2. The method of claim 1, wherein determining if the first sensing of the current satisfies a first current threshold comprises comparing the first sensed current with the first current threshold and wherein determining if the second sensing of the current satisfies a second current threshold comprises comparing the second sensed current with the second current threshold.

3. The method of claim 2, wherein determining if the first sensing of the current satisfies a first current threshold comprises determining if the first sensing of the current is greater than the first current threshold and wherein determining if the second sensing of the current satisfies a second current threshold comprises determining if the sensed current is less than the second current threshold.

4. The method of any preceding claim, wherein determining if the first sensing of the current satisfies a first current threshold further comprises determining if an over- current condition exists.

5. The method of any preceding claim, wherein determining if the first sensing of the current satisfies a first current threshold further comprises determining if an over- temperature condition exists.

6. The method of claim 5, wherein the over-temperature condition is defined by a SSPC thermal profile.

7. The method of any preceding claim, further comprising selecting the inductance based on a desired power transmission response during reducing the power transmission.

8. The method of any preceding claim, wherein determining if the first sensing of the current satisfies a first current threshold further comprises a first comparing of the first sensing of the current to a SSPC thermal profile.

9. The method of any preceding claim, further comprising repeating a) - f).

10. The method of claim 9, further comprising counting a number of repetitions of a) - f).

11. The method of claim 10, further comprising disabling the circuit upon a determination that the number of repetitions satisfies a predetermined number.

12. The method of claim 11, wherein disabling the circuit further comprises at least one of temporarily or permanently disabling the circuit.

13. The method of any of claims 10 to 12, wherein counting the number of repetitions further comprises counting the number of repetitions of a) - f) in a moving time window.

14. The method of claim 13, further comprising disabling the circuit upon a determination that the number of repetitions in a moving time window satisfies a system failure threshold.

15. The method of claim 14, wherein disabling the circuit further comprises at least one of temporarily or permanently disabling the circuit.