System, method and apparatus for leakage current detection for a two-stage ac / ac power converter

Abstract

A system, a method and an apparatus (200) are provided for detecting leakage-current detection in a two-stage AC / AC power converter A controlled low-impedance (202) leakage path is established using a capacitor connected in parallel with a high-value bleed resistor Rg (304) that prevents charge buildup while introducing negligible influence at switching frequencies. A single Hall- effect current sensor H (204) is connected in series within this path to capture the total high- frequency leakage current. A ADC (208) samples the sensor (204) output and applies two dedicated digital band-pass filters. Using RMS values of the respective filtered signals, a microcontroller (206) calculates stage-specific fundamental-frequency RMS leakage currents by applying a modulation-index-dependent proportionality factor derived from the pole-voltage harmonic spectrum, thus achieving precise identification and differentiation of rectifier-side and inverter- side leakage faults using only one current sensor (204).

Description

SYSTEM, METHOD AND APPARATUS FOR LEAKAGE CURRENT DETECTION FOR A TWO-STAGE AC / AC POWER CONVERTERTECHNICAL FIELD

[0001] The present disclosure relates generally to the field of power converters. Particularly, the present disclosure relates to a system, a method and an apparatus for detecting leakage currents in two-stage AC / AC power converters.BACKGROUND

[0002] To identify leakage currents within an electrical system, Zero Phase Current Transformers (ZCTs) are typically employed. The ZCTs operate on the principle that the sum of differential currents within an electrical system is always equal to zero. For example, in a three-phase electrical system, the equation ia+ib+ic=0 holds true. In the here mentioned equation, ia, ib, and icrepresent the currents in each of the three phases. Any deviation from this sum indicates a leakage, resulting in a non-zero output from the Current Transformer (CT).

[0003] Conventionally, zero-phase current Transformers (ZCTs) are used on each side of an electrical system for the detection of leakage currents. For example, FIGURE 1 illustrates an exemplary circuit diagram of a power converter with conventional Zero Phase Current Transformers (ZCTs) for the detection of leakage currents. As illustrated in FIG. 1, a grid (101) is shown, which may be a three-phase or single-phase Alternating Current (AC) voltage supply.

[0004] As shown in FIG. 1, the power converter (100) includes a contactor (111) on the input side and the output side of the power converter (100). The contactor (111) switch works as an isolator between the grid (101) and the power converter (100) in the input side and between power converter (100) and the load 109 at the output side. The contactor (111) ensures safe disconnection of the power converter (100) from the grid (101) and the load (109) in case of any fault.

[0005] The power converter (100) further includes an active rectifier 105, a DC link (113), and an inverter 107. The active rectifier (105) is a circuit that converts the input Alternating Current (AC) voltage to a Direct Current (DC) voltage while maintaining unity power factor. The active rectifier (105) uses Active Semiconductor Switches, for example, Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) or Insulated Gate Bipolar Transistor (IGBT), and the like to control the DC voltage and maintain unity power factor.

[0006] The DC link 113 is a capacitor bank that helps maintain a steady DC voltage at DC terminals. The inverter 107 converts DC voltage to AC voltage. The inverter 107 also uses active switches to control the AC voltage. The power converter (100) further includes Zero Phase Current Transformers (ZCTs) (103) at the input side and the output side of the power converter (100).

[0007] Conventionally, Zero Phase Current Transformers (ZCTs) (103) are used for detecting leakage currents in electrical systems. ZCTs (103) operate on the principle that the sum of differential currents within the electrical system always equals zero. Any deviation from this sum indicates a leakage, resulting in a non-zero output from the Current Transformer (CT). However, a drawback of this method is the requirement of 2 sizable ZCTs, especially for larger systems, to accommodate all phase wires (or two wires in single-phase systems).

[0008] Moreover, given that the currents to be detected are typically in the milliamp range, these ZCTs must offer high resolution, thus leading to increased deployment and maintenance costs. When leakage detection is necessary on both the input and output sides of a system, at least two ZCTs are mandatory. Additionally, when semiconductor switches are involved in power conversion within the electrical system, the presence of harmonics can distort the CT readings, potentially resulting in false trips. This issue often arises due to the bandwidth limitations of the ZCTs, leading to core saturation and the failure to cancel out harmonic currents. Further, the design of traditional ZCTs may limit their ability to accurately measure higher frequency components, such as harmonics, which are prevalent in modern power electronics.

[0009] Therefore, in view of the above-described drawbacks, there exists a need for an improved system that efficiently detects leakage currents without essentially necessitating Zero Phase Current Transformers (ZCTs).OBJECTS

[0010] A few of the objects of the invention described as a part of the present disclosure are as follows.

[0011] It is the principal object of the present invention to provide a single, compact hall effect sensor for the detection of leakage currents on both the input side and the output side of an electrical system.

[0012] It is an object of the present invention to provide an apparatus for establishing a low-impedance pathway in the electrical system that is necessary to ensure the effective flow of the leakage currents to the ground.

[0013] It is another object of the present invention to provide digital harmonic filters (also known as notch filters) that enable the identification of the side (the input side or the output side) of the electrical system on which the leakage currents occur.

[0014] It is another object of the present invention to provide a leakage-current detection system for a two-stage AC / AC power converter that uses a single current-sensing element to detect leakage currents originating from both the rectifier stage and the inverter stage.

[0015] It is another object of the present invention to establish a defined low-impedance path for high-frequency leakage currents between the DC-link negative terminal and the chassis ground, enabling accurate sensing of switching-harmonic leakage components.

[0016] It is another object of the present invention to enable stage-wise discrimination of leakage currents by digitally separating harmonic components associated with the switching frequency of the rectifier stage from those of the inverter stage.

[0017] It is another object of the present invention to enable accurate estimation of fundamentalfrequency leakage current for each converter stage by using proportionality factors derived from the modulation index and harmonic characteristics of the converter.

[0018] It is another object of the present invention to reduce the number of sensors required for leakage detection in multi-stage power converters, thereby minimizing cost, wiring complexity, and physical size.

[0019] It is another object of the present invention to improve immunity to false leakage-fault detection caused by harmonic interference, switching-frequency overlap, or converter-induced distortions.

[0020] It is another object of the present invention to provide a leakage detection architecture that is compatible with different converter topologies, switching schemes, and modulation strategies.

[0021] It is another object of the present invention to deliver a system capable of generating independent leakage-fault signals for the rectifier and inverter stages so that protective actions can be applied selectively and accurately.

[0022] It is another object of the present invention to offer a sensing and detection approach that can be implemented using a microcontroller and digital filters without requiring high-resolution or high-bandwidth current transformers.

[0023] It is another object of the present invention to enhance diagnostic capability, operational safety, and reliability of two-stage AC / AC power converters used in industrial drives, power supplies, and high-power conversion systems.

[0024] These and other objects and advantages of the embodiments of the present invention will become readily apparent from the following detailed description read in conjunction with the accompanying drawings.SUMMARY

[0025] According to an embodiment of the present subject matter, disclosed herein is a leakagecurrent detection system for a two-stage AC / AC power converter, the system comprising: an AC / DC rectifier stage, a DC-link Capacitor Cdc (306), and a DC / AC inverter stage associated with the two-stage AC / AC power converter; a low-impedance path to chassis ground is established by connecting a capacitor Cg between the negative terminal of the DC-link capacitor (Cdc) and the chassis ground, to facilitate optimum detection of high-frequency leakage current; a high-value resistor (Rg) placed in parallel with the capacitor Cg to prevent charge accumulation when the two- stage AC / AC power converter is idle; a Hall Effect sensor (H) is placed in series with said low-impedance path; a microcontroller configured to: implement an analog to digital converter (ADC) component to receive output signal from the Hall Effect sensor (H); implement a harmonic filter component with digital notch filters and bandpass filters (BPFs) to isolate frequency components corresponding to leakage from the rectifier and inverter sides; detect and distinguish leakage from input and output sides to stop false tripping due to harmonic overlap, and to ensure tripping only when the fundamental value of individual RMS leakage currents exceeds the threshold.

[0026] In an embodiment of the present subject matter, the harmonic filter component includes two digital Band Pass Filters namely an ‘inverter BPF’ (or BPFi) and ‘a rectifier BPF (or BPFr) by implementing SOGI filters. Each of the two digital filter’s bandwidth includes its first sidebands (fc± 2fm) where fmis the line (fundamental) frequency and fcis the carrier or switching frequency. The switching frequency of the inverter fciis selected as the center frequency of the passband corresponding to the unipolar sine PWM scheme for the inverter BPF (BPFi). The switching frequency of the inverter fcris selected as the center frequency of the passband corresponding to the unipolar sine PWM scheme. Rectifier BPF (BPFr).

[0027] In an embodiment of the present subject matter, RMS values of filtered signals correspond to leakage components from each converter stage.

[0028] In an embodiment of the present subject matter, leakage estimation is made using modulation index m in both rectifier mrand inverter mithe relation between switching harmonic amplitudes and fundamental leakage currents is derived.

[0029] In an embodiment of the present subject matter, the switching frequency of the rectifier stage differ from the switching frequency of the inverter stage to prevent spectral overlap.

[0030] In an embodiment of the present subject matter, a method for detecting leakage-current for a two-stage AC / AC power converter, is disclosed. The method comprises: configuring an AC / DC rectifier stage, a DC-link Capacitor Cdc, and a DC / AC inverter stage associated with the two-stage AC / AC power converter; establishing a low-impedance path to chassis ground by connecting a capacitor Cg between the negative terminal of the DC-link capacitor (Cdc) and the chassis ground, to facilitate optimum detection of high-frequency leakage current; connecting a high-value resistor (Rg) in parallel with the capacitor Cg to prevent charge accumulation when the two-stage AC / AC power converter is idle; connecting a Hall Effect sensor (H) in series with said low-impedance path; configuring a microcontroller configured to: implement an analog to digital converter (ADC) component that receives output signal from the Hall Effect sensor (H); implement a harmonic filter component with digital notch filters and bandpass filters (BPFs) to isolate frequency components corresponding to leakage from the rectifier and inverter sides; detect and distinguish leakage from input and output sides to stop false tripping due to harmonic overlap, and to ensure tripping only when the fundamental value of individual RMS leakage currents exceeds the threshold.

[0031] In an embodiment of the present subject matter, an apparatus for a leakage current detection system is disclosed. The apparatus comprises a low-impedance path to chassis ground is established by connecting a capacitor Cg between the negative terminal of the DC-link capacitor (Cdc) and the chassis ground, to facilitate optimum detection of high-frequency leakage current; a high-value resistor (Rg) placed in parallel with the capacitor Cg to prevent charge accumulation when the two-stage AC / AC power converter is idle; a Hall Effect sensor (H) is placed in series with said low-impedance path; a microcontroller configured to: implement an analog to digital converter (ADC) component to receive output signal from the Hall Effect sensor (H); implement a harmonic filter component with digital notch filters and bandpass filters (BPFs) to isolate frequency components corresponding to leakage from the rectifier and inverter sides; detect and distinguish leakage from input and output sides to stop false tripping due to harmonic overlap, and to ensure tripping only when the fundamental value of individual RMS leakage currents exceeds the threshold.

[0032] Other objects and advantages of the embodiments herein will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The figures mentioned in this section are intended to disclose exemplary embodiments of the claimed system and method. Further, the components / modules and steps of a process are assigned reference numerals that are used throughout the description to indicate the respective components and steps. Other objects, features, and advantages of the present invention will be apparent from the following description when read with reference to the accompanying drawings.

[0034] FIG. 1 (Prior Art) illustrates an exemplary circuit diagram of a power converter having conventional Zero Phase Current Transformers (ZCTs) for the detection of leakage currents;

[0035] FIG. 2 illustrates an exemplary circuit diagram of the power converter system including an apparatus for the detection of leakage currents.

[0036] FIG.3 illustrates an exemplary embodiment of a three-phase to three-phase AC / AC powerconversion system incorporating the leakage-current sensing arrangement of the present invention.

[0037] FIG.4 illustrates a typical response of the SOGI Filter with K=0.01 and a natural frequency of 8 Kilo Hertz (kHz).

[0038] FIG. 5 illustrates a flowchart illustrating a method for the detection of leakage currents.

[0039] Like reference numerals refer to like parts throughout the description of several views of the drawings.DETAILED DESCRIPTION OF THE INVENTION

[0040] To forestall the drawbacks and disadvantages associated with the prior art and to provide additional advantages, a system, a method and an apparatus are disclosed to provide a leakagecurrent detection for a two-stage AC / AC power converter that uses a single current-sensing element to detect leakage currents originating from both the rectifier stage and the inverter stage, and to establish a defined low-impedance path for high-frequency leakage currents between the DC-link negative terminal and the chassis ground to enable accurate sensing of switchingharmonic leakage components.

[0041] The following description, with reference to the accompanying drawings, illustrates exemplary embodiments of the present invention. These embodiments are described to enable a complete understanding of the invention, and it will be apparent that variations and modifications may be made within the scope of the appended claims.

[0042] The various embodiments of the invention are described in detail below with reference to FIGs. 2 through 6.

[0043] FIG. 2 illustrates an exemplary circuit diagram of the power converter system including an apparatus (200) for the detection of leakage currents. The circuit comprises a grid, a pair of contactors, an active rectifier, an inverter, a DC link, a load, and an apparatus (200). The apparatus (200) comprises a low impedance (202) network, a Hall sensor H (204), a microcontroller unit (microcontroller / controller) (206) having at least one Analog-to-Digital Converter (ADC) (208) and a harmonic filter (210).

[0044] The grid may be a three-phase or single-phase AC voltage supply to provide voltage supply to the power converter circuit. One of the contactors is provided on the input side of the circuit, while the other contactor is provided on the output side of the power converter. The contactor at the input side is a switch that works as an isolator between the grid and the power converter. The contactor ensures safe disconnection of the converter from the grid in case of any faults. Similarly, the contactor at the output side is a switch that works as an isolator between load and the power converter. The load may be a motor, or any other load powered by Alternating Current (AC).

[0045] The active rectifier is a circuit that converts the input Alternating Current (AC) voltage to a Direct Current (DC) voltage. The active rectifier uses active semiconductor switches, for example, Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) or Insulated Gate Bipolar Transistor (IGBT), and the like, to control the DC voltage. The DC link is a capacitor bank that helps maintain a steady DC voltage at the DC terminals. The inverter is a circuit that converts the DC voltage to AC voltage. The inverter also uses active switches to control the AC voltage.

[0046] According to one embodiment of the present invention, the power converter circuit further comprises an apparatus (200) for detecting the leakage currents. The leakage currents may be an unwanted electrical flow that may pose a safety risk. The leakage currents typically include a fundamental component, which may be a main current flow, and a harmonic component, an extra current that occurs at multiples of the main frequency, often caused by non-linear loads.

[0047] The DC-link has a DC-link capacitor (C_dc), and a DC / AC inverter stage configured to supply power to the load such as an electric motor or an inductive-resistive system. The low-impedance (202) path for high-frequency leakage currents is established between the negative terminal of the DC-link capacitor (C_dc) and the chassis ground. This low-impedance (202) path is formed by a capacitor (Cg) (302) electrically connected between the DC-link negative terminal and the chassis. The capacitance value of the capacitor (Cg) (302) is selected such that its impedance at the switching frequencies of the rectifier stage and the inverter stage is sufficiently low to provide a controlled return path for high-frequency leakage components generated during converter operation.

[0048] To prevent undesired accumulation of charge on the capacitor (Cg) (302) during periods in which the power converter is de-energized or otherwise in an idle state, the high-value resistor (Rg) (304) is connected in parallel with the capacitor. The resistor Rg (304) provides a discharge path that maintains the DC potential at safe levels without materially affecting the high-frequency impedance characteristics of the leakage-current path.

[0049] The Hall-effect current sensor H (204) is disposed in series with the low-impedance (202) path formed by the capacitor (Cg) (302). The Hall-effect sensor (204) is configured to sense current flowing from the DC-link negative terminal toward the chassis ground, thereby enabling measurement of both rectifier-side and inverter-side leakage currents through a single sensing element.

[0050] The system further includes the microcontroller-based signal-processing unit (206). In one embodiment, the microcontroller (206) comprises an analog-to-digital converter (ADC) (208) configured to sample the output signal of the Hall-effect sensor H (204), thereby producing a digital representation of the leakage-current waveform. The microcontroller (206) additionally comprises one or more digital-filtering elements implemented through firmware or software instructions stored in a non-transitory memory.

[0051] The harmonic-filtering component that incorporates a combination of digital notch filters and digital band-pass filters (BPFs). The notch filters are configured to suppress frequency components not associated with switching harmonics of interest, while the band-pass filters are tuned to center frequencies corresponding respectively to the switching frequency of the rectifier stage and the switching frequency of the inverter stage. By employing filters centered at distinct switching frequencies, the system is capable of independently isolating harmonic components uniquely associated with each stage of the converter.

[0052] Using the filtered outputs, the microcontroller (206) is configured to detect and distinguish leakage currents originating from the input side (rectifier stage) and from the output side (inverter stage). This selective extraction of stage-specific leakage signatures enables the system to discriminate between normal high-frequency switching behavior and genuine insulation faults. As a result, the system avoids false tripping events that may arise due to harmonic overlap or switching-induced distortions and initiates protective tripping only when the fundamental RMS value of the individual leakage currents exceeds a predetermined safety threshold.

[0053] Through this arrangement, the disclosed embodiment enables accurate, stage-selective leakage-current detection using a single current-sensing element, thereby simplifying the system architecture while improving reliability and diagnostic capability in multi-stage AC / AC powerconversion applications.

[0054] The low-impedance (202) network is a combination of passive components between the inverter and ground. The low-impedance (202) network provides a low-impedance (202) pathway to the leakage currents in the power converter. The low-impedance (202) network ensures that the leakage current in the power converter effectively flows to the ground. This leakage current consists of both fundamental and harmonic components. The establishment of the low-impedance (202) pathway is to facilitate the effective flow of leakage current to the ground. By doing so, the accuracy and reliability of the leakage current detection are significantly enhanced.

[0055] Further, by running a ground wire through the current sensor(204), data related to the leakage currents is collected and sent to the ADC (208) of the MCU. This allows the apparatus (200) to process and analyze the leakage current levels. The harmonic filter (210) (also known as the notch filter (210)) determines the amount of leakage current on the input side and the output side. This distribution of the leakage current on the input side and the output side is important because even if the total leakage current is within safe limits, it is necessary to know where the problem is occurring to prevent erroneous tripping. It is essential to choose distinct switching frequencies for the rectifier and the inverter, ensuring that the gap between them is large enough to prevent any overlap between the fundamental switching harmonics and their sidebands.

[0056] In an embodiment, the harmonic filter (210) may be a digital harmonic filter (210). The ground current detected by the current sensor(204) is processed through the digital harmonic filter (210). The digital harmonic filter (210) segregates the detected leakage currents into an input side leakage current component and an output side leakage component based on their harmonic content. Generally, the input side and the output side may have different leakage current components. In anembodiment, two digital bandpass filters are implemented in the microcontroller (206). The two digital bandpass filters include an Inverter BPF (BPFi) and Rectifier BPF (BPFr).

[0057] In an embodiment, a Second Order Generalized Integrator (SOGI) is used to implement the bandpass filters. Therefore, only one current sensor(204) serves to detect leakage currents on the input side and the output side of the converter. The microcontroller (MCU) (206) provides a trip command to the contactor to disconnect from the power converter in case of a fault. The trip command may be given to the contactor on the input side, the output side, both the input side and the output side depending upon the segregated detected leakage currents of the input side and the output side.

[0058] FIG.3 illustrates an exemplary embodiment of a three-phase to three-phase AC / AC powerconversion system (300) incorporating the leakage-current sensing arrangement of the present invention.

[0059] On the input side, three-phase AC supply voltages Va, Vb, and Vc are applied through corresponding line inductors La, Lb, and Lc. These inductors are typical input inductors and are not included within the leakage detection mechanism as per the embodiments of the present invention.

[0060] The inductors feed an active rectifier constructed using six bidirectional switching devices Q1-Q6. Each of the switches Q1-Q6 may be implemented using insulated-gate bipolar transistors (IGBTs) equipped with antiparallel diodes, forming a conventional three-phase, two-level PWM rectifier topology. The rectifier produces a regulated DC output that charges the DC-link capacitor Cdc (306).

[0061] The DC-link comprises a positive terminal, connected to the inverter switches SI, S3, S5, and a negative terminal, shown as node O, which forms the reference point for the leakage-current sensing arrangement. The inverter is also implemented as a three-phase, two-level PWM bridge consisting of switching devices S1-S6, which synthesize the three-phase motor voltages Viu, Viv, and Viw used to energize the motor M (308).

[0062] A leakage-fault condition on the AC input side is represented by resistor Rlk, which models an insulation breakdown path from one of the phases to ground. Under such a condition, switching harmonics and common-mode voltages generated by both the rectifier and the inverter can drive leakage current through unintended ground paths.

[0063] Unipolar Pulse Width Modulation (PWM) is a switching technique primarily used in single-phase and three-phase full-bridge inverter circuits to synthesize an AC output voltage from a DC source. In a unipolar PWM, each leg (or half-bridge) of the inverter is modulated by comparing a sinusoidal reference signal with a high-frequency triangular carrier. The two legs use reference signals that are 180° out of phase with each other.

[0064] In accordance with the invention, a defined and controlled leakage-current path is created between the DC-link negative terminal O and the chassis ground. This path is established by a low-impedance (202) network consisting of a capacitor Cg (302) connected in parallel with a high-resistance discharge resistor Rg (304). The capacitor Cg (302) is selected such that its reactance at the switching frequencies of the rectifier and inverter is significantly lower than the impedance of typical insulation-fault paths. This ensures that high-frequency leakage currents preferentially flow through the low-impedance (202) network rather than through parasitic or unpredictable ground paths.

[0065] The resistor Rg (304), having a resistance far greater than the impedance of Cg (302), prevents the DC-link negative terminal from accumulating charge when the converter is idle, while having negligible influence on the high-frequency current flowing during normal operation or fault conditions.

[0066] When the Hall-effect current sensor H (204) is placed in series with the low-impedance (202) network, the total leakage current flowing through the capacitor / resistor combination passes through the sensor (204). Said sensor (204) is chosen to have sufficient bandwidth to capture switching-frequency harmonics produced by both converter stages. The output of the Hall-effect sensor (204) therefore represents a composite leakage-current signal containing spectral components corresponding to the switching frequency of the rectifier and the switching frequency of the inverter, together with their respective sidebands.

[0067] The sensed current signal is supplied to the microcontroller (206), which digitizes the signal through an analog-to-digital converter. Within the microcontroller (206), two digital band-pass filters (BPFs) are implemented. Of these two BPFs, a first band-pass filter is tuned to the switching frequency of the active rectifier, while a second band-pass filter is tuned to the switching frequency of the inverter. Because the passbands of these filters exclude one another’s switching frequencies, the controller is able to isolate the switching-harmonic leakage component attributable to each stage of the converter. It is essential to choose distinct switching frequencies for the rectifier andthe inverter, ensuring that the gap between them is large enough to prevent any overlap between the fundamental switching harmonics and their sidebands.

[0068] For each filtered leakage-current component, the microcontroller (206) computes an RMS value and applies a stage-specific proportionality relation derived from the modulation index of the corresponding converter stage. By this process, the controller determines the fundamental RMS leakage current associated independently with the rectifier and inverter. When the estimated leakage current for either stage exceeds a predefined threshold, the controller generates a leakagefault indication used to command appropriate protective action, such as tripping a contactor or shutting down the converter.

[0069] This arrangement enables a single Hall-effect sensor (204) to detect and differentiate leakage currents originating from both stages of a two-stage AC / AC converter, thereby significantly reducing hardware complexity, eliminating the need for multiple zero-phase current transformers, and improving the accuracy and reliability of leakage-fault detection.

[0070] The low-impedance (202) pathway ensures that high-frequency leakage currents generated by the switching actions of the rectifier and inverter stages are diverted through a predictable and controlled path rather than through parasitic ground return routes. By directing the high-frequency components of the leakage current through this designated pathway, the system enables accurate and repeatable sensing of the leakage current using the Hall-effect sensor (204).

[0071] The leakage current measured by the Hall-effect sensor (204) contains harmonic components originating from both the rectifier stage and the inverter stage. Since each converter stage produces switching harmonics centred around its own switching frequency and associated sidebands, the sensed leakage current exhibits harmonic signatures that are unique to each stage. To distinguish these contributions, the sensed current is processed by the controller using digital filtering techniques configured to isolate the switching-frequency components associated with the rectifier and inverter, respectively. Through this harmonic-based segregation, the controller is able to determine the leakage current attributable to each of the two converter stages independently.

[0072] In one implementation, the digital filtering is performed using a Second-Order Generalized Integrator (SOGI) filter structure. In the continuous-time domain, the transfer function of a SOGI filter may be expressed as:Kω_nsH(s) = - — — - -+ Kωns + ωn²

[0073] where K denotes the filter gain, which influences the notch depth and the bandwidth of the filter in the frequency domain, and ωnrepresents the natural frequency of the filter in radians per second. When designing the SOGI filter, the gain K and natural frequency ωnare selected such that the filter exhibits minimal attenuation at the frequencies of interest, namely, the switching frequencies and their sidebands, while sufficiently rejecting frequency components outside the target bands. This configuration allows the controller to extract clean harmonic components corresponding to the rectifier and inverter switching actions, thereby supporting accurate estimation of stage-specific leakage currents.

[0074] FIG.4 illustrates a typical response of the SOGI Filter with K=0.01 and a natural frequency of 8 Kilo Hertz (kHz).

[0075] FIG. 5 illustrates a flowchart illustrating a method (500) for the detection of leakage currents. The method illustrated herein may be executed by the MCU (210). Referring to FIG. 5, the method begins, at (step 502) wherein ground current Ig is sensed through a current sensor (204) or hall effect sensor (204). The output of current sensor (204) is given to the ADC (208) coupled to the microcontroller (206). In an embodiment, two digital bandpass filters are implemented in the microcontroller (206). The two digital bandpass filters include an Inverter Band Pass Filter (BPFi) and a Rectifier Band Pass Filter (BPFr). In an embodiment, for BPFi, twice the switching frequency of the inverter is taken as passband frequency because of the unipolar Pulse width modulation (PWM) scheme. For BPFr, the switching frequency of the boost power factor correction circuit is taken as the passband frequency. In an embodiment, the second Order Generalized Integrator (SOGI) is used to implement the BPFi and the BPFr.

[0076] Further, it is determined whether the output of the BPFi exceeds the current detected by the current sensor(204), and then at (step 504) the MCU determines, that the leakage current at the input side has exceeded a specified limit or a predefined threshold. Subsequently, at (step 508) a trip command is provided to contractor on the input side. Further, it is determined at (step 506) whether the output of the BPFr exceeds the current detected by the current sensor (204), and then the MCU determines that the leakage current at the output side has exceeded a specified limit. Subsequently, a trip command is provided to the contractor on the output side at (step 508).

[0077] Thus, during a leakage fault, an insulation breakdown can be modelled as a leakage resistance to ground, and the invention estimates the fundamental component of the resulting leakage current by exploiting the relationship between switching-harmonic leakage and the fundamental pole-voltage waveform. Because the low-impedance (202) pathway diverts high-frequency leakage components through the Hall -effect sensor (204), the sensed current predominantly contains the first switching harmonic and its sidebands generated by the rectifier and inverter stages. The converter’s switching behavior causes these harmonics to vary with the instantaneous duty ratio, which can be analytically expressed using Fourier coefficients and expanded using Bessel functions. From this relationship, a proportionality factor is obtained that links the amplitude of the switching-harmonic leakage current to the amplitude of the fundamental leakage current, the factor being dependent on the modulation index of the converter stage. By applying this proportionality to the RMS output of the stage-specific band-pass filters, the controller computes accurate estimates of the fundamental leakage current for both the rectifier and inverter without directly measuring the fundamental component.

[0078] During a leakage fault, any insulation breakdown happens resulting in current leaking to the ground. This leakage can be modelled using a resistor Rlk.

[0079] The creation of a low impedance (202) path for the high frequency current ensures that the majority of the high frequency current flows through it. This current is sensed and is filtered for the first switching harmonics and its sidebands. In order to estimate the fundamental current from this sensed current we need to find the relation between these two currents.

[0080] The derivation is given below in detail.Amplitude of harmonics of three phase converter pole voltages wrt to DC negative can be written as:1 + m sin(ωt)D(V=- m = modulation indexω =fundamental line frequencyFor a fixed duty ratio, the kthFourier coefficient (over one period Tcof a pulse of width D Tcis1CD TCCk(D) = — e-jkωtdtTc Jo= (1 / πk)e-jπkDsin(πkD), k ≠ 0Since the duty ratio is time-varying, side bands appear at frequencies kωc+ nω. Where k, n ∈ I Thus, the coefficient Ckbecomes time-dependent:1Ck(t) = (1 / πk)e-jπkD(t)sin(πkD(t))71KSince D (t) contains a single sinusoidal at co, we expand the time-dependent factor into harmonics of co using the Bessel / Jacobi-Anger expansion. The key identity used isgj z sin 0 W '"’Jn(z)ejnθ-jnkD(t)=^e-jnk / 2^e- =e~J^k / 2 ^“__ooJn(inkm) e~ina>ltSimplifying and using the Bessel expansion, the complex amplitude of the spectral line at frequencyωk,n= kωc+ nω, (k, n ∈ I)where h( ) is the Bessel function of the first kind.For k = 0, C0(D(t)) = D(t).1 + m sin(ωt)OrVao(t) = Vac ■ D(t) = Vdc2Hence, the DC component of the pole voltage Vao(t) is Vdc / 2 and the fundamental component is mVdc / 2. For k = 1 and n = 0, the peak amplitude at switching frequency is given asC1,0= (Vdc / π) J0(πm / 2)C1,0= (Vdc / π) J0(πm / 2)and for the first sidebands:C1,1= (Vdc / π) J1(πm / 2), C1,-1= (Vdc / π) J-1(πm / 2)C1'! “ 7T71I 2 )’C1'-! “ 7T I 2 )Since J-1(x) = -J1(x), and| -i |=| Ci, i|Here total rms of the amplitude of switching frequency and its side bands will beVswlk= √(C²1,0+ 2C²1,1)Vswikn 64Vdc ( Qm)4n V11024The leakage (switching) current which is output of the band pass filter can be given aIswlk= Vswlk / RlkKlkand since fundamental pole voltage is given as Vfund pk= mVdc / 2, the ratio becomes Vswlk / Vfund≈ (2 / mπ)(1 - (πm)⁴ / 1024)Vfund\ 1024 )nmlfundlk =~T. ( im)4\Iswik2I11024 / Since / SW;kis already a known quantity now and m is the modulation index which can be easily estimated from the converter operations, leakage current can be easily estimated.For Rectifier:Rms fundamental leakage current Irec fund,lkwill be estimated asIrec fund,lk= πmr / (√2 · (1 - (πmr)⁴ / 1024)) · Iswlk recV2 1 - 1024 ) where,mris modulation index of rectifier / SW;krecis the output of the bandpassfilterFor Inverter:Rms fundamental leakage current Iinv fund,lkwill be estimated asnmiIinv fund,lk= πmi / (√2 · (1 - (πmi)⁴ / 1024)) · Iswlk inv1024 Jwhere,miis modulation index of rectifierIswLKis the output of the bandpassfilterinvADVANTAGES OF THE INVENTION

[0081] The leakage-current detection system described herein provides several significant advantages over conventional multi-sensor approaches used in two-stage AC / AC power converters. Because the system employs a single Hall-effect current sensor (204) positioned along a defined low-impedance path, the overall architecture is substantially simplified while maintaining comprehensive monitoring of leakage originating from both the rectifier and inverter stages.

[0082] First, the use of a single sensing element reduces system size, cost, and wiring complexity, thereby enabling more compact converter designs and lowering manufacturing effort. Second, the implementation of stage-specific digital filtering including band-pass filters tuned to respective switching frequencies renders the system substantially immune to harmonic-induced false tripping, as leakage components associated with each converter stage are isolated and evaluated independently. Third, the filter architecture and processing strategy may be tuned and adapted for different converter topologies, switching frequencies, and modulation schemes, allowing the invention to be integrated across a wide range of AC / AC power-conversion platforms. Finally, by reducing the number of sensing components and defining a controlled leakage-current path, the invention provides a simplified mechanical arrangement and enhanced diagnostic accuracy, thereby improving reliability, maintainability, and fault-classification capability in practical deployment.

[0083] The technical advantages envisaged by the present disclosure include the establishment of a low-impedance pathway that facilitates effective leakage current flow to the ground, thereby enhancing the accuracy and reliability of leakage detection. The current sensor(204) envisaged by the present disclosure processes the ground current through a digital harmonic filter (210), thereby allowing for the segregation of input and output leakage current based on the harmonic content of the ground current.

[0084] Further, utilizing only one Hall effect sensor (204) or the current sensor (204) reduces the overall footprint of the detection circuit, thus making the apparatus (200) suitable for applications constrained by limited space, for example, electrical panels or compact devices. Furthermore, a single current sensor (204) is simpler to install than multiple sensors, which, in turn, leads to reduced installation time and complexity, thereby immensely benefiting retrofit installations. Furthermore, with the use of only one current sensor (204), costs associated with procurement,wiring, and componentry are significantly lowered. Furthermore, fewer components imply less potential for failure, which, in turn, reduces maintenance costs and replacement costs.

[0085] The apparatus (200) envisaged by the present invention incorporates harmonic filters that help distinguish between harmful leakage currents and normal fluctuations caused by harmonics in the circuit. The apparatus (200) efficiently differentiates between actual faults and harmless variations, thereby reducing the likelihood of unnecessary disconnections. Also, by minimizing false trips, the apparatus (200) enhances operational reliability.

Claims

CLAIMSWe Claim:

1. A leakage-current detection system for a two-stage AC / AC power converter, the system comprising:(i) an AC / DC rectifier stage, a DC-link Capacitor Cdc (306), and a DC / AC inverter stage associated with the two-stage AC / AC power converter;characterized in that(ii) a low-impedance (202) path to chassis ground is established by connecting a capacitor Cg (302) between the negative terminal of the DC-link capacitor (Cdc)(306) and the chassis ground, to facilitate optimum detection of high-frequency leakage current; (iii) a high-value resistor (Rg) (304) placed in parallel with the capacitor Cg (302) to prevent charge accumulation when the two-stage AC / AC power converter is idle;(iv) a Hall Effect sensor H (204) is placed in series with said low-impedance (202) path; (v) a microcontroller (206) configured to:implement an analog to digital converter (ADC) (208) component to receive output signal from the Hall Effect sensor H (204);implement a harmonic filter component (210) with digital notch filters and bandpass filters (BPFs) to isolate frequency components corresponding to leakage from the rectifier and inverter sides;detect and distinguish leakage from input and output sides to stop false tripping due to harmonic overlap, and to ensure tripping only when the fundamental value of individual RMS leakage currents exceeds the threshold.

2. The system as claimed in claim 1, wherein the harmonic filter component (210) includes two digital Band Pass Filters namely an ‘inverter BPF’ (or BPFi) and ‘a rectifier BPF (or BPFr) by implementing SOGI filters.

3. The system as claimed in claim 2, wherein each of the two digital filter’s bandwidth includes its first sidebands (fc± 2fm)), where fmis the line (fundamental) frequency and fcis the carrier or switching frequency.

4. The system as claimed in claim 2, wherein switching frequency of the inverter fciis selected as the center frequency of the passband corresponding to the unipolar sine PWM scheme for the inverter BPF (BPFi).

5. The system as claimed in claim 2, wherein switching frequency of the inverter fcris selected as the center frequency of the passband corresponding to the unipolar sine PWM scheme. Rectifier BPF (BPFr).

6. The system as claimed in claim 1, wherein RMS values of filtered signals correspond to leakage components from each converter stage.

7. The system as claimed in claim 1, wherein leakage estimation is made using modulation index m in both rectifier mrand inverter mithe relation between switching harmonic amplitudes and fundamental leakage currents is derived.

8. The system as claimed in claim 1, wherein the switching frequency of the rectifier stage differ from the switching frequency of the inverter stage to prevent spectral overlap.

9. A method for detecting leakage-current for a two-stage AC / AC power converter, the method comprising:(i) configuring an AC / DC rectifier stage, a DC-link Capacitor Cdc (306), and a DC / AC inverter stage associated with the two-stage AC / AC power converter; characterized in that(ii) establishing a low-impedance path to chassis ground by connecting a capacitor Cg (302) between the negative terminal of the DC-link capacitor (Cdc) and the chassis ground, to facilitate optimum detection of high-frequency leakage current;(iii) connecting a high-value resistor (Rg) (304) in parallel with the capacitor Cg (302) to prevent charge accumulation when the two-stage AC / AC power converter is idle;(iv) connecting a Hall Effect sensor H (204) in series with said low-impedance path; (v) configuring a microcontroller (206) configured to:implement an analog to digital converter (ADC) (208) component that receives output signal from the Hall Effect sensor H (204);implement a harmonic filter component (210) with digital notch filters and bandpass filters (BPFs) to isolate frequency components corresponding to leakage from the rectifier and inverter sides;detect and distinguish leakage from input and output sides to stop false tripping due to harmonic overlap, and to ensure tripping only when the fundamental value of individual RMS leakage currents exceeds the threshold.

10. The method as claimed in claim 1, wherein the harmonic filter component (210) includes two digital Band Pass Filters namely an ‘inverter BPF’ (or BPFi) and ‘a rectifier BPF (or BPFr) by implementing SOGI filters.

11. The method as claimed in claim 10, wherein each of the two digital filter’s bandwidth includes its first sidebands (fc± 2fm)), where fmis the line (fundamental) frequency and fcis the carrier or switching frequency.

12. The method as claimed in claim 10, wherein switching frequency of the inverter fciis selected as the center frequency of the passband corresponding to the unipolar sine PWM scheme for the inverter BPF (BPFi).

13. The method as claimed in claim 10, wherein switching frequency of the inverter fcris selected as the center frequency of the passband corresponding to the unipolar sine PWM scheme. Rectifier BPF (BPFr).

14. The method as claimed in claim 9, wherein RMS values of filtered signals correspond to leakage components from each converter stage.

15. The method as claimed in claim 9, wherein leakage estimation is made using modulation index m in both rectifier mrand inverter mithe relation between switching harmonic amplitudes and fundamental leakage currents is derived.

16. The method as claimed in claim 9, wherein the switching frequency of the rectifier stage differ from the switching frequency of the inverter stage to prevent spectral overlap.

17. An apparatus (200) for a leakage current detection system comprising:a low-impedance path to chassis ground established by connecting a capacitor Cg (302) between the negative terminal of the DC-link capacitor (Cdc) and the chassis ground, to facilitate optimum detection of high-frequency leakage current;a high-value resistor (Rg) placed in parallel with the capacitor Cg (302) to prevent charge accumulation when the two-stage AC / AC power converter is idle;a Hall Effect sensor H (204) is placed in series with said low-impedance path; and a microcontroller (206) configured to:implement an analog to digital converter (ADC) (208) component to receive output signal from the Hall Effect sensor H (204);implement a harmonic filter component (210) with digital notch filters and bandpass filters (BPFs) to isolate frequency components corresponding to leakage from the rectifier and inverter sides;detect and distinguish leakage from input and output sides to stop false tripping due to harmonic overlap, and to ensure tripping only when the fundamental value of individual RMS leakage currents exceeds the threshold.

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