Claims:1. A method for series arc detection, said method comprising the steps of:
sampling a current signal;
sampling a high frequency signal;
calculating width W1 of gap in high frequency signal;
calculating width W2 of shoulder in current signal;
measuring cycle-to-cycle variation in the width of W1, W2;
subtracting W1 with threshold value of high frequency signal;
subtracting W2 with threshold value of current signal;
incrementing a flag if difference exceeds the threshold value;
comparing the flag with a second threshold value; and
issuing trip signal if flag is greater than the second threshold.

2. The method as claimed in claim 1, wherein the signals are sampled at Nyquist criterion.

3. The method as claimed in claim 1, wherein the width W1, W2 are measured in time by multiplying number of samples with sample time.

4. The method as claimed in claim 1, wherein the flag count indicates the number of disturbances for a time period.

5. The method as claimed in claim 1, wherein the second threshold value is trip threshold value.

6. The method as claimed in claim 1, wherein when the flag is greater than the second threshold, the condition is a persistent series arc condition.

7. The method as claimed in claim 2, wherein a microcontroller samples the signals at Nyquist criterion.

8. The method as claimed in claim 7, wherein the microcontroller comprises analogue-to-digital converters, timers, general purpose input/out unit and the like.

9. The method as claimed in claim 1, wherein a high frequency current sensor senses the current flowing through power line.

10. The method as claimed in claim 1, wherein the current signal is from a current transformer filter.

11. The method as claimed in claim 1, wherein the high frequency signal is from a high frequency current transformer filter, or a high pass filter, or a band pass filter.

12. The method as claimed in claim 7, wherein the current signal and high frequency signal are fed into separate analogue-to-digital converters channels for sampling.

13. The method as claimed in claim 2, wherein sampling rate must be at least twice the highest analogue frequency component.

14. The method as claimed in claim 1, wherein the trip signal is issued by trip mechanism.

15. The method as claimed in claim 1, wherein arc fault detection is for arc currents ranging from about 2.5A to about 63A.

16. The method as claimed in claim 1, wherein arc detection is improved by comparing current magnitude and variation in width of gaps in high frequency signal.

17. The method as claimed in claim 16, wherein gaps in high frequency signal occur at zero crossing.

18. The method as claimed in claim 17, where the high frequency signal disappears as the arc is extinguished at zero crossing.

19. The method as claimed in claim 11, wherein high frequency components, or transients present in current are separated by high frequency current transformer, or high pass filter, or band pass filter.

20. The method as claimed in claim 1, wherein the threshold values are based on rated AC line current by continuously sensing the current from current transformer.
, Description:FIELD OF THE INVENTION
[001] The subject matter of the present invention, in general, pertains to arc fault detection, and more particularly to an arc fault detection method recognising pattern in HF noise.

BACKGROUND OF INVENTION
[002] An arc fault is a high power discharge of electricity between two or more conductors. In particular, it is a condition of luminous and hot discharge of electricity in air, gas existing between two conductors. This discharge translates into heat, which can break down the wire's insulation and possibly trigger an electrical fire. These arc faults can range in power from a few amps up to thousands of amps high and are highly variable in terms of strength and duration. Common causes of arc faults include faulty connections due to corrosion and faulty initial installation. Arc faults are one of the leading causes for residential electrical fires.

[003] These arc faults can be either series or parallel. Examples of a series arc are a broken wire where the ends of the broken wire are close enough to cause arcing, or a relatively poor electrical connection. Parallel arcs occur between conductors of different potential including, for example, a power conductor and a ground. Unlike a parallel arc fault, series arc faults do not usually create an increase in current since the fault is in series with the load. In fact, a series arc fault may result in a slight reduction in load current and not be detected by the normal overload and overcurrent protection of conventional protection devices. Even the parallel arc, which can draw current in excess of normal rated current in a circuit, produces currents which can be sporadic enough to yield RMS values less than that needed to produce a thermal trip, or at least delay operation. Effects of the arc voltage and line impedance often prevent the parallel arc from reaching current levels sufficient to actuate the instantaneous trip function.

[004] Both safe and unsafe series arcs occur in power circuits, such as electrical power distribution systems (or electrical distribution systems). One example of safe series arcs occurs in the commutator brushes of direct current (DC) and universal motors. In order to minimize brush heating and erosion, universal motors are designed to minimize the net duration and thereby the total energy dissipated by commutation arcing. The materials and physical shape of the commutator brushes are chosen, in order that they are minimally affected by the arcing. Hence, the series arcs that occur in universal motors are intended and perform a constructive purpose.

[005] Electrical arcs develop temperatures well above the ignition level of most common flammable materials and, therefore, pose a significant fire hazard. Under certain conditions, reflected in particular characteristics of the electrical disturbance produced, the likelihood of the arc persisting and starting a fire is much higher. Two types of dangerous arcing that are likely to occur in homes are momentary, high-energy arcs caused by high-current faults and persistent, low-current “contact” arcing.

[006] A high-current fault is caused by an inadvertent direct connection between line and neutral or line and ground, will generally draw current up to or beyond the rated capacity of the circuit, arc explosively as the contacts are physically made and broken, dim lights and other loads indicating an excessive load is being drawn, and/or (assuming the circuit is properly protected by a circuit breaker) trip the breaker, thereby interrupting the current to the arc. Because such “line faults” are short-lived, the temperature rise in the feed conductors is limited and the fire danger results primarily from the explosive expulsion of minute glowing globules of copper from the contact area which may ignite nearby flammable materials. Even if a fire begins, however, the high visibility of the fault and the likely presence of someone nearby (who provided the physical impetus to bring the conductors together) sharply mitigates the potential for an uncontrolled fire.

[007] Contact arcing occurs at connections in series with a load. As such, the maximum current in the arc is limited to the load current and, therefore, may be substantially below the overcurrent or “trip” rating of an associated circuit breaker. Contact arcing is a complex physical phenomenon that may be induced by loose connections, oxidized contacts, foreign non-conducting material interfering with the conduction path, differences in contact materials, contact shapes, and other factors. Under certain conditions, such arcing may become persistent and present a substantial fire hazard.

[008] Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which heats and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.

[009] Two types of wiring protection that are standard are thermal breakers and arc fault circuit breakers. Thermal breakers require an overload condition long enough that a heating element in the breaker trips the breaker off. In contrast, arc fault circuit breakers use magnetic or other means to detect increases in current draw much more quickly. Without such protection, visually detecting arc faults in defective wiring is very difficult, as the arc fault occurs in a very small area. A problem with arc fault circuit breaker is they are more likely to produce false positives due to normal circuit behaviours appearing to be arc faults. For instance, lightning strikes on the outside of an aircraft mimic arc faults in their voltage and current profiles. Research has been able to largely eliminate such false positives, however, providing the ability to quickly identify and locate repairs that need to be done.

[0010] Arc fault detection devices are devices that protect specifically against arc faults. They automatically trip a circuit when they detect dangerous electric arcs. They are compact, modular devices that are easily installed in electrical panels along with other protective equipment and designed to protect an electric, instantly detecting any dangerous arc.

[0011] An arc fault circuit interrupter is an advanced circuit breaker that breaks the circuit when it detects a dangerous electric arc in the circuit it protects to prevent electrical fires. It selectively distinguishes between a harmless arc incidental to normal operation of switches, plugs, and brushed motors, versus a potentially dangerous arc that can occur, for example, in a lamp cord which has a broken conductor.

[0012] Reference is made to US 7463037 B2, wherein a method for arc fault detection that includes automatically generating an output signal responsive to an input signal is disclosed. The input signal is indicative of an arc fault. The output signal is configured to cause an electrical circuit to open. The output signal is generated responsive to a derived signal based upon a first threshold having a first amplitude.

[0013] Reference is also made to US 8023235 B2, wherein an electrical fault detection device for use in a branch of a power circuit that utilizes signals from an AC line current sensor coupled to an electrical distribution line having a primary and neutral lines, a line high-frequency sensor coupled to the electrical distribution line, a differential current sensor coupled to the primary and neutral lines, and a ground fault current sensor coupled to the primary and neutral lines is disclosed. A signal conditioner receives the signals outputted by AC current line current sensor, the line high frequency sensor, the differential current sensor and the ground fault current sensor and generates a signal indicative of the load current associated with a branch of the power circuit. Output of the signal conditioner is sampled and processed by a processing resource. The processing resource has stored therein data representing a plurality of time-versus-current curves that define a plurality of regions in which tripping may or may not occur. One region has time data and current data that define a time-duration for a particular current magnitude for which no tripping will occur. Another region has time data and current data that define a time-duration of a particular current magnitude for which tripping will occur. Processing resource processes sampled signal to determine the region to which the processed time data and current data correspond, and generates a signal to initiate tripping if the sampled signal yields a time duration for a particular current magnitude that corresponds to a region for which tripping must occur.

[0014] Reference is also made to US 8089737 B2, wherein an arc fault circuit interrupter includes separable contacts, an operating mechanism and an arc fault detector structured to detect an arc fault condition operatively associated with the contacts is disclosed. The arc fault detector includes a tuned current sensor structured to sense broadband noise of a current flowing through the contacts, a compression circuit including an input of the sensed broadband noise and an output. The compression circuit compresses the dynamic range of the sensed broadband noise. A minimum detector includes an input of the compression circuit output and an output of the minimum value of the minimum detector input. A processor includes a number of inputs and an output. One of the inputs is the output of the minimum value of the minimum detector. A trip mechanism cooperates with the output of the processor and the operating mechanism to trip open the contacts responsive to the detected arc fault condition.

[0015] Significantly, in US’037 and US’235, the received signal strength indication, RSSI is measured and large variation in the parameter is characterised as an arcing condition, while in US’737, the broadband noise is passed through a demodulating logarithmic amplifier and a minimum detection circuit and various statistical methods are implemented for identifying an arc whereas we sample current, HF filter output and calculate cycle to cycle variation in the shoulders and gaps in HF noise respectively. In contrast, in the subject invention, the current and high frequency filter output are sampled to calculate the cycle-to-cycle variation in the shoulders and gaps present in the high frequency noise, respectively, to achieve the desired objectives.

[0016] While, the above mentioned prior art documents describe arc fault detectors or arc fault interrupters and method of arc fault detection. Such devices are found to have inherent problems that are multi-fold owing to the fact that in series arcing, the fault may be due to lose contact, physical damage to the cable etc., thereby the current is limited by the load impedance and impedance offered by the arc and this condition goes undetected as the conventional circuit breakers cannot detect it as overload or as short circuit. Series arcs if not interrupted can lead to overheating of the cable and can subsequently lead to an electrical fire. Therefore, with a view to mitigate such threats, there is a need for a method to reliably detect a series arc and interrupt the circuit.

[0017] The subject invention pertains to an arc fault detection method that recognises a pattern in high frequency noise. In particular, to a method for series arc detection by analysing the gaps in high frequency noise induced in current during arcing.

SUMMARY OF THE INVENTION
[0018] The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the present invention. It is not intended to identify the key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concept of the invention in a simplified form as a prelude to a more detailed description of the invention presented later.

[0019] Another object of the present invention is to detect low current series arc faults.

[0020] Another object of the present invention is to differentiate an arc fault condition from intentional switching.

[0021] An object of the present invention is to provide a method to reliably detect a series arc to interrupt a circuit.

[0022] Another object of the present invention is to provide an arc fault detection method that recognises a pattern in high frequency noise.

[0023] Another object of the present invention is to provide a method for series arc detection by analysing the gaps in high frequency noise induced in current during arcing.

[0024] Accordingly, in one aspect of the present invention, the output from the high frequency filter and Current Transformer CT is fed to separate ADC channels of the microcontroller wherein the signal is sampled on Nyquist criterion. The width of the gap in HF noise, W1 is calculated as well as the corresponding width of the shoulder in the current waveform, W2 is calculated. The width is measured in time by multiplying the number of samples with the sample time. The cycle to cycle variation in width W1, W2 is measured and compared with respective threshold value. If the difference value exceeds the set threshold, a flag variable is incremented. The flag variable is a direct indication of the number of disturbances in a limited period. Once the entire buffer is analysed in this fashion the flag variable value is compared with a second threshold value (i.e. trip threshold). If the flag variable value is greater than the threshold, the condition is considered as a persistent series arc condition and a trip signal is issued.

[0025] Briefly, a method for series arc detection by analysing gaps in high frequency noise induced in current during arcing is disclosed. The output from the high frequency band pass filter and CT are fed to the microcontroller where the signal is sampled. The width of the gap in high frequency noise W1 is calculated as also the corresponding width of the shoulder of the current waveform, W2 is calculated and the cycle-to-cycle variation in width W1, W2 is measured and compared with respective threshold value. If the difference exceeds the set threshold, a flag is incremented that indicates the number of disturbances in a limited period. If the flag variable value is greater than the threshold, the condition is considered as a persistent series arc condition and a trip signal is issued.

[0026] Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The above and other aspects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which:

[0027] Figure 1 illustrates the current waveform and high frequency noise generated during arcing according to one implementation of the present invention.

[0028] Figure 2 illustrates schematic of the series arc detection and protection circuit according to one implementation of the present invention.

[0029] Figure 3 illustrates a flow chart of the method for detecting series arc detection according to one implementation of the present invention.

[0030] Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure. Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures.

DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0031] The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary.

[0032] Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.

[0033] The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.

[0034] It is to be understood that the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise.

[0035] By the term “substantially” it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide.

[0036] Features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments.

[0037] It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or component but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0038] The subject invention lies in providing a method for series arc detection by analysing the gaps in high frequency noise induced in current during arcing.

[0039] In the present invention, a method for detecting series arc by analysing the gaps in high frequency noise induced in current during arcing is disclosed. The output from a high frequency filter and CT are fed to the microcontroller where the signal is sampled. The width of the gap in high frequency noise, W1 is calculated as also the corresponding width of the shoulder of the current waveform, W2

[0040] The cycle-to-cycle variation in width of W1 and W2 are measured and compared with their respective threshold values. If the difference exceeds the set threshold, a flag is incremented that counts the number of disturbances in a limited period. If the flag variable value is greater than the threshold, the condition is considered as a persistent series arc condition and a trip signal is issued.

[0041] In one implementation, a method to detect low current series arc faults is provided for.

[0042] In one implementation, a method to differentiate an arc fault condition from intentional switching is provided for.

[0043] In one implementation, a method to reliably detect a series arc to interrupt a circuit is provided for.

[0044] In one implementation, an arc fault detection method that recognises patterns in high frequency noise is provided for.

[0045] In one implementation, a method for series arc detection by analysing the gaps in high frequency noise induced in current during arcing is provided for.

[0046] In one implementation, the output from the high frequency/band pass filter and CT are fed into separate ADC channels of the microcontroller where the signal is sampled on Nyquist criterion. The width of the gap in high frequency noise W1 is calculated as also the corresponding width of the shoulder of the current waveform, W2 is calculated. The width is measured in time by multiplying the number of samples with the sample time. The cycle to cycle variation in width W1, W2 is measured and compared with respective threshold value. If the difference value exceeds the set threshold, a flag variable is incremented. The flag variable is a direct indication of the number of disturbances in a limited period. Once the entire buffer is analysed in this fashion the flag variable value is compared with a second threshold value (i.e. trip threshold). If the flag variable value is greater than the threshold, the condition is considered as a persistent series arc condition and a trip signal is issued.

[0047] Arcing generates a high frequency noise during current conduction. The spectrum of this noise can range from few hertz to about 1GHz. The high frequency noise is only present during the conduction of current while the arc is sustained, and the noise disappears as soon as the arc is extinguished (near zero crossing) as illustrated in Figure 1.

[0048] The randomness of gaps in high frequency noise during arcing forms the basis of the arc detection logic. A series arc detection and protection scheme is illustrated in Figure 2. The source and load is connected via power line 7 through a circuit breaker 2. The detection unit 5 is a microcontroller based system that contains peripherals like ADCs and timers. A high frequency current sensor/current sensor 3 having a high bandwidth senses the load current flowing through the power line 7. The HF signal and current signal are sampled based on the “Nyquist criterion” which says that the sampling rate must be at least 2fmax, or twice the highest analogue frequency component. A sense of fundamental component of current is taken from a current sensor 4 and passed through a low pass filter 9 to remove any high frequency noise during normal or arcing condition. The output of low pass filter is sampled at high frequency for computation of shoulder width and simultaneously the gaps in the high frequency noise is computed during normal and arcing conditions at zero crossing. The method for series arc detection is performed by a microcontroller-based system comprising peripherals like ADCs, timers, General Purpose Input/Out unit, etc. Upon detecting an arc fault, the trip signal is issued through GPI/O (General Purpose Input/Output) module of the microcontroller to the trip mechanism 8 that signals the circuit breaker 2 to isolate the circuit.

[0049] In the event of a series arc, high frequency components as well as transients may be present in load current that can be separated through high frequency CT or high pass/band pass filter.

[0050] The flow chart indicating the process steps for series arc detection is illustrated in Figure 3. The output from the high frequency CT/high pass/band pass filter and CT are sampled at high frequency for accurate computation of detection parameters. The signals are sampled on the basis of Nyquist criterion. The width of the gap in high frequency noise W1 is calculated as also the corresponding width of the shoulder of the current waveform, W2 is calculated. The width is measured in time by multiplying the number of samples with the sample time. The cycle to cycle variation in width W1, W2 is measured and compared with respective threshold values. If the value (difference) exceeds the set threshold, a flag counter is incremented. The flag count is a direct indication of the number of disturbances in a specified period. Once the entire buffer is analysed in this fashion the flag count value is compared with a second threshold value (i.e. tripping threshold). If the flag count value is greater than the second threshold, the condition is considered to be a persistent series arc condition and a trip signal is issued by the trip mechanism.

[0051] As per standard IEC62606 and UL1699, arc fault detection should occur within specified time for arc currents ranging from 2.5A up to 63A. Therefore, threshold selection is based on the rated AC line current flowing through the system by continuously sensing the current from CT. Also, the sensitivity to series arc detection is improved by observing the current magnitude and variation in width of the gaps in high frequency noise.

[0052] Hence, the proposed method for detecting arc faults by observing the current magnitude and variation in widths of the gaps in high frequency noise is able to detect low current series arc faults. This method can differentiate between an arc fault condition and an intentional switching.

[0053] Although a superior method for series arc detection by analysing gaps in high frequency noise induced in current during arcing has been described in language specific to structural features and/or methods, it is to be understood that the embodiments disclosed in the above section are not necessarily limited to the specific features or methods described therein. Rather, the specific features are disclosed as examples of implementations of the arc fault detection method by recognizing a pattern in high frequency noise.