System and method for detecting a grounded neutral fault in a three-phase power system

Abstract

Systems and methods detect a grounded neutral fault in a three-phase power system. Drive resistors (120, 122, 124) having different resistance values are coupled to three phase conductors (102, 104, 106) of the three-phase power system. A first current transformer (130) has a primary winding configured to receive drive currents from the drive resistors (120, 122, 124). A neutral conductor (114) passes through the first current transformer (130) and a second current transformer (140) as a coupling element between the first current transformer (130) and the second current transformer (140). The second current transformer (140) allows the three phase conductors (102, 104, 106) to pass therethrough. The ground fault detection circuit (150) detects a current imbalance in a secondary winding of the second current transformer (140). The trip mechanism (160) triggers when the detected current imbalance exceeds a predetermined threshold.

Description

202503222SYSTEM AND METHOD FOR DETECTING A GROUNDED NEUTRAL FAULT IN A THREE-PHASE POWER SYSTEMTechnical Field

[0001] This application relates to the electrical power system protection and monitoring and, more particularly, to ground fault detection in three-phase power distribution systems.Background

[0002] Three-phase electrical power systems are widely used in industrial, commercial, and residential applications to distribute electrical power efficiently. These systems typically include three phase conductors carrying alternating current at different phases, along with a neutral conductor that provides a return path for unbalanced currents. Proper functioning of the neutral conductor is critical for maintaining system balance and ensuring safe operation of connected equipment.

[0003] In many three-phase power systems, the neutral conductor is intentionally grounded at various points to provide a reference potential and enhance system safety. However, under certain fault conditions, the neutral conductor can become inadvertently grounded at locations other than intended grounding points. Such grounded neutral faults can create safety hazards, cause equipment damage, and lead to improper operation of protective devices.

[0004] Ground fault detection has long been recognized as an important aspect of electrical system protection. Traditional ground fault detection methods typically focus on detecting faults between phase conductors and ground, or between phase conductors and the equipment grounding conductor. These conventional approaches generally monitor the vector sum of currents in the phase conductors, with any imbalance indicating a ground fault condition.

[0005] Current transformers are commonly employed in ground fault detection systems to sense current imbalances. In typical configurations, phase conductors pass through a current transformer, and any net current flow through the transformer indicates an imbalance that may represent a ground fault. The secondary winding of such current transformers produces a signal proportional to the detected imbalance, which can then be processed by protection circuits.202503222

[0006] Various electronic circuits and signal processing techniques have been developed to analyze current imbalance signals and determine when protective action should be taken. These circuits typically compare detected imbalance levels to predetermined thresholds and initiate protective measures, such as circuit breaker tripping, when fault conditions are identified.Summary

[0007] In accordance with one embodiment of the disclosure, there is provided a dual transformer approach for electrical protection systems. The approach provides techniques for detecting grounded neutral faults in three-phase power systems using a dual current transformer configuration with drive resistors having different resistance values. The dual transformer approach provides broad protection, universal application, technical excellence, and market differentiation. In particular, the approach covers a broad range of possible electrical configurations, may be used in many electrical systems worldwide, addresses the complex and technically challenging problem of phase-agnostic protection, and is superior to conventional systems that only work with specific phase configurations.

[0008] One aspect is a system for detecting a grounded neutral fault in a three-phase power system, comprising multiple drive resistors, a first current transformer, a neutral conductor, a second current transformer, a ground fault detection circuit, and a trip mechanism. The drive resistors have different resistance values coupled to three phase conductors of the three-phase power system. The first current transformer has a primary winding configured to receive drive currents from the drive resistors. The neutral conductor passes through the first current transformer and the second current transformer as a coupling element between the first current transformer and the second current transformer. The second current transformer allows the three phase conductors to pass therethrough. The ground fault detection circuit detects a current imbalance in a secondary winding of the second current transformer. The trip mechanism triggers when the detected current imbalance exceeds a predetermined threshold.

[0009] Another aspect is a method for detecting a grounded neutral fault in a three-phase power system. Multiple drive resistors have different resistance values coupled to three phase conductors of the three-phase power system. Drive currents from the drive resistors couple through a primary winding of a first current transformer. A neutral conductor passes through the first current transformer and a202503222second current transformer as a coupling element between the first current transformer and the second current transformer. The three phase conductors pass through the second current transformer. A current imbalance is detected in a secondary winding of the second current transformer using a ground fault detection circuit. A trip mechanism is triggered when the detected current imbalance exceeds a predetermined threshold.

[0010] The above described features and advantages, as well as others, will become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings. While it would be desirable to provide one or more of these or other advantageous features, the teachings disclosed herein extend to those embodiments which fall within the scope of the appended claims, regardless of whether they accomplish one or more of the above-mentioned advantages.Brief Description of the Drawings

[0011] For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects.

[0012] FIG. 1 is a circuit diagram of a three-phase GFCI device in an example implementation that is operable to employ techniques described herein.

[0013] FIG. 2 depicts an example operation for the three-phase GFCI device of FIG.1.

[0014] FIGs. 3A and 3B are, respectively, a circuit diagram of a first embodiment of the three-phase GFCI device in accordance with the described techniques and a sampling of output signals of the first embodiment.

[0015] FIGs. 4A and 4B are, respectively, a circuit diagram of a second embodiment of the three-phase GFCI device in accordance with the described techniques and a sampling of output signals of the second embodiment.

[0016] FIGs. 5A through 5I are circuit diagrams of other embodiments of the GFCI device in accordance with the described techniques, which include two-phase configurations and single-phase configurations.202503222Detailed Description

[0017] Various technologies that pertain to systems and methods that facilitate dual transformer configuration will now be described with reference to the drawings, where like reference numerals represent like elements throughout. The drawings discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are byway of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged apparatus. It is to be understood that functionality that is described as being carried out by certain system elements may be performed by multiple elements. Similarly, for instance, an element may be configured to perform functionality that is described as being carried out by multiple elements. The numerous innovative teachings of the present application will be described with reference to exemplary non-limiting embodiments.

[0018] The present ground fault circuit interrupter (GFCI) devices address the technical challenges associated with grounded neutral detection in three-phase power systems. Conventional ground fault detection solutions have been developed for single-phase and two-phase power systems using various approaches including higher frequency oscillation circuits, drive circuits, and impedance detection circuits. However, the electrical protection industry has lacked cost-effective three-phase GFCI circuit breakers, with existing three-phase GFCI devices being limited to external units that are excessively large, expensive, and mounted outside of load centers or electrical panels.

[0019] The techniques described herein solve two fundamental technical challenges in meeting the requirements specified in LIL943, Section 6.7 High-resistance ground fault, and specifically Section 6.7.4 Grounded neutral test. First, the disclosure provides techniques for generating sufficient drive current in a first current transformer to overcome closed loop conductor resistance of up to 2 Ohms, as specified in LIL943 Figure 6.7.4.1 Supplementary high-resistance test circuit and Table 6.7.4.1 Combination 6 for 14 AWG and 12 AWG conductors used in 15A and 20A rated electrical circuits. This drive current generation must function effectively across the specified power source voltage range of 120 AC +10% / - 15%, resulting in a worst-case minimum power source voltage of 102VAC. Second, the disclosure addresses the challenge of generating appropriate phases of drive current coupled202503222into the first current transformer to overcome or supplement ground fault current leaking from any phase to the load. The techniques described herein provide protection across all possible power source configurations, including scenarios with missing one or two phases, ensuring protection whenever at least one phase of power is supplied to the load. These configurations encompass all three-phase configurations, all two-phase configurations including the special case 120 / 240VAC system configuration, and all single-phase configurations.

[0020] The technical solution employs a dual current transformer architecture with drive resistors having calculated resistance ratios. Through extensive SPICE based simulations, the optimal resistance values were determined to satisfy LIL943 Section 6.7 High-resistance ground fault requirements across all power configuration scenarios.

[0021] Referring to FIG. 1, there is shown a circuit diagram of a three-phase GFCI device 100 in an example implementation that is operable to employ the techniques described herein. The three-phase GFCI device 100 includes three phase conductors A, B, and C 102, 104, 106 that receive power from a power source 108 and deliver power to a load 110. The three phase conductors include a first phase conductor A 102, a second phase conductor B 104, and a third phase conductor C 106. Each phase conductor A, B, C 102, 104, 106 is equipped with a respective switch SW 112 that provides air gap contacts which open and close to control the delivery of three-phase power to an electrical circuit branch or load. Each switch SW 112 is positioned in series with its respective conductor 102, 104, 106 to enable complete disconnection of power when a ground fault condition is detected.

[0022] The three-phase GFCI device 100 includes a neutral conductor 114 that provides a return path for current from the load back to the power source 108. The neutral conductor 114 is connected to ground 116 at the power source 108, which is a standard configuration in electrical distribution systems.

[0023] A grounded neutral drive circuit 118 is coupled to the phase conductors A, B, and C 102, 104, 106. The grounded neutral drive circuit 118 includes three drive resistors 120, 122, 124 with calculated resistance values. The three drive resistors include a first drive resistor RA 120 connected to phase conductor A 102, a second drive resistor RB 122 connected to phase conductor B 104, and a third drive resistor Rc 124 connected to phase conductor C 106. Note that the arrangement order of connection of each resistor to each phase conductor is not important. The drive202503222resistors 120, 122, 124 have different resistance values coupled to three phase conductors of the three-phase power system. The first drive resistor RA 120 has a first resistor value, the second resistor RB 122 has a second resistor value, and the third resistor Rc 124 has a third resistor value. These resistance values maintain specific ratios determined through simulation analysis to provide optimal drive current characteristics across all power source configurations. For some embodiments, the third resistor value of the third resistor Rc 124 is at least 8 times greater than the first resistor value of the first resistor RA 120. For some embodiments, the second resistor value of the second resistor RB 122 is at least 4 times greater than the first resistor value of the first resistor RA 120. For some embodiments, the second resistor value is at least 4 times greater than the first resistor value, and the third resistor value is at least 8 times greater than the first resistor value (or at least twice greater than the second resistor value). For some embodiments, a maximum resistance of the resistor value of any drive resistor 120, 122, 124 is less than or equal to about 100 kQ. For example, the first drive resistor RA 120 may be a resistance value of about 12kQ, the second drive resistor RB 122 may be a resistance value of about 50kQ, and the third drive resistor Rc 124 may be a resistance value of about 100kQ.

[0024] The grounded neutral drive circuit 118 also includes a first current transformer T1 130 having a multi-turn T1 winding and a single-turn T1 winding. One terminal of the multi-turn T1 winding is connected to all three drive resistors RA 120, RB 122, and Rc 124, while the other terminal is connected to the neutral conductor 114. For some embodiments, the first current transformer T1 130 has a turns ratio of approximately 1:1000, with the neutral conductor 114 serving as a single-turn primary winding passing through the transformer core, and a multi-turn secondary winding of about 1000 turns. This turns ratio provides current transformation where drive currents from the drive resistors are coupled into the multi-turn secondary winding and induce corresponding current in the single-turn primary winding formed by the neutral conductor 114. The drive currents from all three drive resistors RA 120, RB 122, and Rc 124 are combined and coupled into the multi-turn winding of the first current transformer T1 130. The combined drive current flows through the multi-turn winding and induces a corresponding current in the single-turn winding of the first current transformer T1 130 formed by the neutral conductor 114.

[0025] The three-phase GFCI device 100 includes a second current transformer T2 140, which serves as the ground fault sensing transformer. The second current transformer T2 140 has a single transformer core through which all four conductors202503222pass: the three phase conductors A 102, B 104, C 106, and the neutral conductor 114. Each conductor passing through the core constitutes a single-turn primary winding, with all conductors oriented in the same direction through the core. The single-turn T2 windings correspond to the three phase conductors A 102, B 104, C 106 and the neutral conductor 114, all passing through the transformer core in the same direction. Specifically, a first single-turn winding is connected to the first phase conductor A 102, a second single-turn winding is connected to the second phase conductor B 104, a third single-turn winding is connected to the third phase conductor C 106, and a fourth single-turn winding is connected to the neutral conductor 114. The second current transformer T2 140 also includes the multi-turn T2 winding that generates a signal proportional to any current imbalance detected in the primary conductors 102, 104, 106. For some embodiments, the multi-turn T2 winding of the second current transformer T2 140 may include about 3000 turns.

[0026] The three-phase GFCI device 100 also includes a ground fault detection circuit 150 coupled to the multi-turn T2 winding of the second current transformer T2. The ground fault detection circuit 150 processes signals indicative of ground fault conditions. The ground fault detection circuit 150 analyzes the induced current from the second current transformer T2 140 to detect both conventional ground faults and grounded neutral fault conditions. The ground fault detection circuit 150 processes the signal from the multi-turn secondary winding of the second current transformer T2 140. Under normal conditions, the vector sum of currents in all four conductors (three phases plus neutral) equals zero, resulting in no net magnetic flux and no induced signal in the secondary winding. When a ground fault occurs, current imbalance creates net magnetic flux in the transformer core, inducing a proportional signal in the multi-turn secondary winding that is detected and processed by the ground fault detection circuit 150.

[0027] A trip mechanism 160 is disposed between each of the three phase conductors A 102, B 104, C 106 and is coupled to the ground fault detection circuit 150, via the grounded neutral drive circuit 118 and the second current transformer T2 140. One terminal of the trip mechanism 160 is coupled to the first current transformer T1 130 via the ground fault detection circuit 150, and the other terminal is coupled to the neutral conductor 114. The trip mechanism 160 receives a trip signal from the ground fault detection circuit 150 and actuates the switches SW 112 to open the air gap contacts. The trip mechanism 160 thereby removes power from the202503222electrical circuit branch or load when a fault condition exceeds a predetermined threshold.

[0028] The above description applies to Wye (Y) configuration systems having a neutral conductor and Delta configuration systems which lack a natural neutral conductor. Delta configuration systems may have a derived neutral point where the drive resistors may be coupled between respective phase conductors and a derived neutral point.

[0029] Referring to FIG. 2, there is shown an example operation 200 for the three-phase GFCI device 100, representing the operational sequence and functional relationships between the major components. The operation 200 demonstrates how the grounded neutral detection technique is implemented in practice. The operation 200 begins with coupling (210) the drive resistors 120, 122, 124 to their respective phase conductors A 102, B 104, and C 106. The drive resistors RA, RB, and Rc 120, 122, 124 are coupled to first, second, and third phase conductors A, B, and C 102, 104, 106, respectively. The drive resistors RA, RB, and Rc 120, 122, 124 have different resistance values. For some embodiments, the drive resistors include the first resistor 120 having the first resistor value, the second resistor 122 having the second resistor value, and the third resistor 124 having the third resistor value. For some embodiments, the third resistor value may be at least 8 times greater than the first resistor value, and / or the second resistor value may be at least 4 times greater than the first resistor value. For some embodiments, the maximum resistance of any drive resistor 120, 122, 124 may be less than or equal to about 100 kQ. The coupling (210) establishes the connection between the phase voltages and the drive resistor network that generates the drive currents necessary for grounded neutral detection.

[0030] The operation 200 continues with coupling (220) drive currents 204 from the drive resistors RA, RB, and Rc 120, 122, 124 to the first current transformer T1 130. For some embodiments, the first current transformer T1 130 includes a multi-turn winding coupled to the drive resistors 120, 122, 124 and the neutral conductor 114, and the first current transformer T1 further includes a single-turn winding coupled to the second current transformer T2 140 and the neutral conductor 114. The drive currents are generated by the drive resistors RA, RB, and Rc 120, 122, 124 and are coupled to the multi-turn T 1 winding of the first current transformer T1 130. These drive currents have amplitude ratios determined by the resistance values of the drive202503222resistors 120, 122, 124, with the current amplitudes being inversely proportional to their respective resistance values.

[0031] In response to coupling (210) the drive resistors and coupling (220) the drive currents, the operation 200 proceeds with passing (230) the neutral conductor 114 through the first current transformer T1 130 and the second current transformer T2 140 as a coupling element between the first and second current transformers. The second current transformer T2 140 includes multiple single-turn windings coupled to the three phase conductors 102, 104, 106, the neutral conductor 114, and the first current transformer T1 130, and the second current transformer T2 140 further includes a multi-turn winding coupled to the ground fault detection circuit 150. The first current transformer T1 130 acts as a test signal generator, and the second current transformer T2 140 acts as a fault detector. For some embodiments, the first current transformer T1 130 and the second current transformer T2 140 are magnetically coupled through the neutral conductor 114. The first current transformer T1 130 functions as a test signal generator specifically for detecting grounded neutral fault conditions. When a grounded neutral fault exists (i.e. , when the neutral conductor to the load is inadvertently connected to ground), the drive currents generated by the drive resistors and coupled through the first current transformer T 1 130 create a closed conductive loop through both transformers. This drive current is coupled by the neutral conductor into the second current transformer T2 140, creating a detectable current imbalance that indicates the grounded neutral fault condition.

[0032] The operation 200 then passes (240) the three phase conductors 102, 104, 106 through the second current transformer T2 140. In this manner, the neutral conductor 114 and the three phase conductors A, B, C 102, 104, 106 are routed through the second current transformer T2 140. The neutral conductor 114 passes through both transformers 130, 140 as a single-turn winding, while the three phase conductors pass only through the second current transformer T2 140 as single-turn windings, all in the same direction to maintain proper polarity for current sensing.

[0033] In response to passing (230) the neutral conductor and passing (240) the three phase conductors, the operation 200 continues with detection (250) of the current imbalance where the ground fault detection circuit 150 monitors the output of the multi-turn T2 winding of the second current transformer T2 140. Under normal operating conditions, the total current going to the load equals the total current202503222returning from the load, resulting in zero magnetic field in the core of the second current transformer T2 140. However, when a ground fault or grounded neutral fault occurs, a current imbalance is detected as an induced current in the multi-turn T2 winding. For some embodiments, the ground fault detection circuit 150 detects (250) the current imbalance by comparing (262) the detected current imbalance to a predetermined threshold.

[0034] In response to detecting (250) the current imbalance, the operation 200 triggers (260) the trip mechanism 160. For some embodiments, the trip mechanism is triggered (260) by comparing (262) the detected current imbalance to the predetermined threshold. When the current coupled from the multi-turn winding of the second current transformer T2 140 exceeds a predetermined amplitude of the predetermined threshold, the ground fault detection circuit 150 transmits a trip signal to the trip mechanism 160. For example, the trip mechanism 160 may energize a solenoid to open the air gap contacts SW 112, thereby removing power from the electrical circuit branch or load. For some embodiments, the predetermined threshold may be about 5 mA ground fault current. For some embodiments, the ground fault detection circuit 150 includes signal conditioning components such as amplifiers and filters, analog-to-digital conversion circuitry, and processing logic that implements the fault detection algorithm. The circuit may include a comparator or microcontroller unit (MCU) with internal ADC capability that samples the amplified signal and compares peak amplitude values to the predetermined threshold of approximately 170 mV, which corresponds to the 5 mA ground fault detection requirement.

[0035] FIGS. 3A and 3B are, respectively, a first circuit diagram 300 of a first embodiment of the three-phase GFCI device 100 in accordance with the described techniques and a second sampling of output signals 350 of the first embodiment. These figures demonstrate the operation of the three-phase GFCI device 100 under a specific three-phase power source configuration designated as the ABC configuration. It can also be referred to as BCA or CAB configuration depending upon your preference of which phase to base your reference. The first circuit diagram 300 represents a particular configuration of the circuit diagram of the three-phase GFCI device 100 shown in FIG. 1, in which the first drive resistor RA 120, the second drive resistor RB 122, the third drive resistor Rc 124, the first current transformer T1 130, and the second current transformer T2 140 are identified for ease of correspondence with the first circuit diagram 300.202503222

[0036] FIG. 3A illustrates the first embodiment operating under the ABC three-phase configuration, where Phase A 302 serves as the reference phase at 0°, Phase B 304 lags Phase A by 120°, and Phase C 306 lags Phase A by 240° (equivalently, Phase C leads Phase A by 120°). This represents one of the two three-phase power source configurations encountered in electrical distribution systems. The first circuit diagram 300 shows the three-phase GFCI device 100 operating at the minimum specified power source voltage of 102VAC, which corresponds to the worst-case voltage condition of 120VAC -15% as specified in LIL943 testing requirements.

[0037] The first embodiment demonstrates the drive current generation capability under the ABC configuration when ground faults occur on each of the three phases individually. The drive currents generated by the drive resistors RA 120, RB 122, and Rc 124 are coupled into the first current transformer T1 130, with the resulting combined drive current passing through the neutral conductor and into the second current transformer T2 140 for ground fault detection.

[0038] FIG. 3B presents the second sampling of output signals 350 for the first embodiment, specifically showing the ground fault detection circuit 150 output signals when ground faults are simulated on Phase A 302 (ABC 102VAC GF A), Phase B 304 (ABC 102VAC GF B), and Phase C 306 (ABC 102VAC GF C). These output signal samples demonstrate that the ground fault detection circuit 150 successfully generates output signals exceeding the predetermined threshold of approximately 170 mV, which corresponds to the 5 mA ground fault detection requirement. The signal samples validate that the specific resistance ratios of the drive resistors RA 120, RB 122, Rc 124 provide sufficient drive current to detect ground faults from 6 mA to 240 mA on any phase under the ABC configuration at the minimum operating voltage of 102VAC.

[0039] The output signals in FIG. 3B confirm that the first embodiment maintains consistent ground fault detection sensitivity across all three phases, demonstrating the effectiveness of the drive resistor ratio design in providing balanced protection regardless of which phase experiences a ground fault condition.

[0040] FIGS. 4A and 4B are, respectively, a second circuit diagram 400 of a second embodiment of the three-phase GFCI device in accordance with the described techniques and a second sampling of output signals 450 of the second embodiment. These figures demonstrate the operation of the three-phase GFCI device 100 under a different three-phase power source configuration where two of the phases are202503222swapped designated as the ACB configuration. It can also be referred to as CBA or BAC configuration depending upon your preference of which phase to base your reference.

[0041] FIG. 4A illustrates the second embodiment operating under the ACB three-phase configuration (Phase B and Phase C are swapped), where Phase A 402 serves as the reference phase at 0°, Phase C 406 lags Phase A by 120°, and Phase B 404 lags Phase A by 240° (equivalently, Phase B leads Phase A by 120°). This represents the second of the two three-phase power source configurations encountered in electrical distribution systems, demonstrating the versatility of the described techniques across different phase rotation sequences. Similar to the first embodiment, the circuit diagram 400 shows the three-phase GFCI device 100 operating at the minimum specified power source voltage of 102VAC to validate worst-case operating conditions.

[0042] The second embodiment demonstrates that the same drive resistor configuration with RA 120, RB 122, and Rc 124 maintains effective ground fault detection capability regardless of the three-phase power source phase rotation. The drive currents generated under the ACB configuration are coupled into the first current transformer T1 130 through the same electrical connections, but the phase relationships of the drive currents differ due to the altered phase rotation sequence.

[0043] FIG. 4B presents the second sampling of output signals 450 for the second embodiment, specifically showing the ground fault detection circuit 150 output signals when ground faults are simulated on Phase A 402 (ACB 102VAC GF A), Phase B 404 (ACB 102VAC GF B), and Phase C 406 (ACB 102VAC GF C) under the ACB configuration. These output signal samples demonstrate that the ground fault detection circuit continues to generate output signals exceeding the predetermined threshold of approximately 170 mV across all phases, confirming that the 5 mA ground fault detection requirement is satisfied regardless of phase rotation. The signal samples validate that the specific resistance ratios of the drive resistors RA 120, RB 122, Rc 124 provide sufficient drive current to detect ground faults from 6 mA to 240 mA on any phase under the ACB configuration at the minimum operating voltage of 102VAC.

[0044] The output signals 450 in FIG. 4B validate that the drive resistor ratio design provides consistent ground fault detection performance across different three-phase configurations. This demonstrates the robustness of the disclosed technique in real-202503222world electrical installations where phase rotation may vary between different power source connections.

[0045] FIGS. 5A through 51 are circuit diagrams 500-580 representing the operation of the three-phase GFCI device 100 under various two-phase and single-phase power source configurations. These figures help to validate the versatility and robustness of the described techniques across all possible electrical supply scenarios that may be encountered in real-world installations.

[0046] FIGs. 5A and 5B illustrate the three-phase GFCI device 100 operating under a two-phase AB configuration and a two-phase BA configuration (Phase A and Phase B are swapped), respectively, where only Phase A 502 and Phase B 504 and Phase A 512 and Phase B 514 are energized while Phase C is not connected to a power source. The third embodiment 500 and the fourth embodiment 510 represent common scenarios in electrical installations where not all three phases are utilized. The circuit operates at 102VAC to demonstrate worst-case voltage conditions. The corresponding output signals for ground faults on Phase A (AB 102VAC GF A, BA 102VAC GF A) and Phase B (AB 102VAC GF B, BA 102VAC GF B) confirm that the ground fault detection circuit maintains sensitivity exceeding the 170 mV threshold for both active phases and demonstrate continued effective ground fault detection capability across the AB and BA phase combinations. Also illustrated is a two-phase configuration in which the electrical power system only has 2 Phases A and B that are separated in phase by 180 degrees, and the third Phase C is disconnected inside the GFCI device 500.

[0047] FIGs. 5C and 5D illustrate the three-phase GFCI device 100 operating under a two-phase AC configuration and a two-phase CA configuration, respectively. For the fifth embodiment 520 and the sixth embodiment 530, only Phase A and Phase C are energized while Phase B is not connected. The corresponding output signals for ground faults are presented on Phase A (AC 102VAC GF A, CA 102VAC GF A) and Phase C (AC 102VAC GF C, CA 102VAC GF C), validating ground fault detection performance for the AC and CA phase combinations. Also illustrated is a two-phase configuration in which the electrical power system only has 2 Phases A and C that are separated in phase by 180 degrees, and the third Phase B is disconnected inside the GFCI device 520.

[0048] FIGs. 5E and 5F illustrate the three-phase GFCI device 100 operating under a two-phase BC configuration and a two-phase CB configuration, respectively. For202503222the seventh embodiment 540 and the eighth embodiment 550, only Phase B and Phase C are energized while Phase A is not connected. The corresponding output signals for ground faults are presented on Phase B (BC 102VAC GF B, CB 102VAC GF B) and Phase C (BC 102VAC GF C, CB 102VAC GF C), validating ground fault detection performance for the BC and CB phase combinations. Also illustrated is a two-phase configuration in which the electrical power system only has 2 Phases B and C that are separated in phase by 180 degrees, and the third Phase A is disconnected inside the GFCI device 540.

[0049] FIGs. 5G, 5H, and 5I illustrate the three-phase GFCI device 100 operating under a single-phase A configuration, single-phase B configuration, and single-phase C configuration, respectively. For each of these embodiments 560, 570, 580, only one phase is energized while the other two phases are disconnected. The ninth embodiment 560 of FIG. 5G represents the corresponding output signal for a ground fault on Phase A (A 102VAC GF A). The tenth embodiment 570 of FIG. 5H represents the corresponding output signal for a ground fault on Phase B (B 102VAC GF B). The eleventh embodiment 580 of FIG. 5I represents the corresponding output signal for a ground fault on Phase C (C 102VAC GF C). These embodiments 560, 570, 580 demonstrate that the device 100 maintains ground fault detection capability even when operating in single-phase mode with only one active phase conductor.

[0050] The system and operation represent ground fault detection in a three-phase electrical system using the grounded neutral drive technique. The operation provides a systematic approach for implementing the ground fault detection capabilities of the three-phase GFCI device 100 and can be executed by the ground fault detection circuit 150 in conjunction with the other components of the system.

[0051] The operation is applicable across all power source configurations including three-phase ABC and ACB configurations, two-phase AB, BC, and AC configurations, and single-phase A configuration, providing consistent ground fault detection performance regardless of the number of active phases or their phase relationships.

[0052] There are two challenges or problems solved by the system and method described above in meeting the requirements described in LIL943, Section 6.7 High-resistance ground fault, and specifically Section 6.7.4 Grounded neutral test. First, generating enough drive current in the first transformer to overcome a closed loop conductor resistance of up to 2 Ohms as described in LIL943 Figure 6.7.4.1202503222Supplementary high-resistance test circuit and Table 6.7.4.1 Combination 6 for 14 AWG and 12 AWG used for 15A and 20A rated electrical circuits. This must be achieved for a power source voltage of 120VAC +10% I -15% which results in a worst-case minimum power source voltage of 102VAC and thus a minimum drive current into the first transformer. Second, generating acceptable phases of drive current coupled into the first transformer to overcome or supplement ground fault current leaking from any phase to the load. This must be achieved for all possible power source configurations including missing one phase or two phases (disconnected) to provide protection anytime at least one phase of power is supplied to the load. The configurations include any three-phase configuration, any two-phase configuration, and any single-phase configuration.

[0053] There is described above a full ground fault detection device 100 with grounded neutral detection capability which includes: three conductors labeled A, B, and C; three air gap contacts labeled SW that open and close to provide three-phase power to an electrical circuit branch or load where each air gap is in series with one conductor; one neutral conductor; a grounded neutral drive circuit that includes three resistors RA equal to 12k Ohms, RB equal to 50k Ohms, and RC equal to 100k Ohms each connected to conductor A, B, and C respectively (arrangement order not important), and a first current transformer T 1 with one terminal of a secondary multiturn winding of approximately 1000 turns connected to all three resistors RA, RB, and RC and the other terminal connected to the neutral conductor, and with a primary single-turn winding consisting of the neutral conductor; a second current transformer T2 (ground fault sensing transformer) with 4 primary single-turn windings consisting of the three conductors A, B, and C, and the neutral conductor and with a secondary multi-turn winding of approximately 3000 turns; a ground fault detection circuit that is coupled to the secondary multi-turn winding of the second current transformer T2; a trip mechanism that is disposed between each of the three conductors A, B, and C and the neutral conductor, and is coupled to the ground fault detection circuit.

[0054] A grounded neutral drive circuit consisting of drive currents from each phase with specific ratio of amplitudes set by specific ratio of resistor values of RA, RB, and Rc is coupled to a multi-turn winding of a first current transformer T 1 toroidal in shape that is coupled to a second current transformer T2 also toroidal in shape by passing the neutral conductor through both transformers. The three hot or energized conductors A, B, and C all pass through the second current transformer T2 as well202503222and in the same direction as the neutral conductor to sense current going to and returning from the electrical circuit branch or load. The second current transformer T2 has a multi-turn winding that is coupled into a ground fault detection circuit which is used to detect ground fault leakage current. When the total current going to the load equals the total current returning from the load, the magnetic field in the core of the second current transformer T2 is zero. However, if current is leaking from one of the phases to the load through an alternate path such as a ground conductor or other conductive path back to the power source (i.e. a ground fault), then there exists an imbalance in the magnetic field in the core of the second current transformer T2 which induces a current in the multi-turn winding coupled to the ground fault detection circuit. The ground fault detection circuit asserts a trip signal to a trip mechanism that energizes a solenoid which opens air gap contacts SW removing power from the electrical circuit branch or load when the current coupled from the multi-turn winding of the second current transformer T2 exceeds a predetermined amplitude. When a grounded neutral fault exists, i.e. the neutral conductor to the load is connected to a ground conductor by mistake, a closed conductive loop is created through both transformers. Note that the neutral conductor is connected to ground somewhere in the load center or panel or power source. Hence, the drive current generated by the first current transformer T1 of the grounded neutral drive circuit is coupled by the neutral conductor into the second current transformer T2 which creates an imbalance in the magnetic field in the core of the second transformer T2 which induces a current in the multi-turn winding coupled to the ground fault detection circuit. As mentioned above, the ground fault detection circuit asserts a trip signal to a trip mechanism that energizes a solenoid which opens air gap contacts SW removing power from the electrical circuit branch or load when the current coupled from the multi-turn winding of the second current transformer T2 exceeds a predetermined amplitude.

[0055] The values of RA, B, and Rc that set the drive current into the multi-turn winding of the first current transformer T1 of the grounded neutral drive circuit were determined using SPICE based simulations by varying these resistor values to solve all of the problems I challenges mentioned above to meet I satisfy the requirements described in LIL943, Section 6.7 High-resistance ground fault. Ultimately, to satisfy these requirements it was determined that the resistance value of Rc needs to be approximately 8 times greater than the resistor value of RA, and the resistance value of RB needs to be approximately 4 times greater than the resistance value of RA. TO202503222meet the drive requirements for a single-phase power source configuration, it was determined that the maximum resistance value for any of the resistors setting the drive current should be less than or equal to approximately 100k Ohms using a first current transformer T 1 with certain characteristics, such as those represented by FIGs. 3B and 4B of the drawings, which were modeled in the SPICE based simulation model. So, achieving the ratio of resistor values described above, the resistor value of RA is chosen to be approximately 12k Ohms, the resistor value of RB is chosen to be approximately 50k Ohms, and the resistor value of Rc is chosen to be 100k Ohms to minimize overall drive current which minimizes total power consumption. The resistors RA, RB, and Rc are also interchangeable.

[0056] FIGs. 3A, 3B, 4A, 4B, and 5A-5I represent the simulation results for each power source configuration and ground fault leakage current on each phase out to the load. In the simulation schematic diagram resistors R1, R2, and R3 are each coupled to one phase of the power source and are the current drive setting resistors RA, RB, and Rc respectively. The sum of the drive currents resulting from three resistors is coupled into the multi-turn winding, approximately 1000 turns, of the first current transformer T 1 labeled TX1 in the simulation schematic diagram. The other terminal lead of the multi-turn winding of the first current transformer T 1 labeled TX1 in the simulation schematic diagram is connected to the neutral conductor. The neutral conductor is also coupled to i.e. passes through as a single turn, the first current transformer, T1 labeled TX1 in the simulation schematic diagram and in series is coupled to i.e. passes through as a single turn, the second current transformer T2 labeled TX3 in the simulation schematic diagram, at which point the neutral conductor is made available to the electrical circuit branch or load. Each of the conductors from each of the three phases of the power source is coupled to i.e. pass through as single turns, the second current transformer T2 labeled TX3 in the simulation schematic diagram and then made available to the electrical circuit branch or load. The multi-turn winding, approximately 3000 turns, of the second current transformer T2 labeled as TX3 in the simulation schematic diagram is coupled to the ground fault detection circuit that consists of a low pass filter made up of L1, L2, C3, and C4, a shunt resistor R8 to convert current to a voltage, an amplifier, and an MCU. The MCU contains the ground fault detection algorithm that samples the output signal of the amplifier GF_OUT with an internal ADC and issues a trip signal within the time constraints described in LIL943 Section 6.7.1.1 if the peak amplitude of the signal exceeds a predetermined value, which is set to approximately 170 mV which202503222corresponds to a 5 mA ground fault. The Supplementary high-resistance test circuit as described in LIL943 Section 6.7.4 Figure 6.7.4.1 is implemented in the simulation schematic diagram using combination 6 for 14 AWG or 12 AWG electrical circuit branch wire as described in LIL943 Section 6.7.4 Table 6.7.4.1. The resistance of the neutral conductor to the electrical circuit branch or load represented by RN equal to 0.4 Ohms is R6 in the simulation schematic diagram, and the resistance of a ground conductor represented by RG equal to 1.6 Ohms is R5 in the simulation schematic diagram. The ground fault current setting resistor RB is R7 in the simulation schematic diagram and is varied from 1 mega-Ohm to 500 Ohms, and is optionally disposed between each power phase conductor made available to the electrical circuit branch load and the node where the neutral conductor and a ground conductor is connected which represents the point at which the grounded neutral mis wire occurs. An iterative process of varying the resistor values of R1 , R2, and R3 (RA, RB, and Ro) and simulating ground fault detection for all the power source configurations was used to determine the appropriate resistor values of R1 , R2, and R3 (RA, RB, and Ro) to solve all of the problems I challenges mentioned above to meet I satisfy the requirements described in LIL943, Section 6.7 High-resistance ground fault.

[0057] For the final simulation results, the data indicates the GF_OUT peak signal level (min, max, and peak to peak) for ground fault current ranging from 0 mA rms up to 204 mA rms (500 Ohms with Phase voltage of 102 VAC) to ensure peak signal is greater than or equal to 170 mV.

[0058] Those skilled in the art will recognize that, for simplicity and clarity, the full structure and operation of all data processing systems suitable for use with the present disclosure are not being depicted or described herein. Also, none of the various features or processes described herein should be considered essential to any or all embodiments, except as described herein. Various features may be omitted or duplicated in various embodiments. Various processes described may be omitted, repeated, performed sequentially, concurrently, or in a different order.Various features and processes described herein can be combined in still other embodiments as may be described in the claims.

[0059] It is important to note that while the disclosure includes a description in the context of a fully functional system, those skilled in the art will appreciate that at least portions of the mechanism of the present disclosure are capable of being distributed in the form of instructions contained within a machine-usable, computer-usable, or202503222computer-readable medium in any of a variety of forms, and that the present disclosure applies equally regardless of the particular type of instruction or signal bearing medium or storage medium utilized to actually carry out the distribution. Examples of machine usable / readable or computer usable / readable mediums include: nonvolatile, hard-coded type mediums such as read only memories (ROMs) or erasable, electrically programmable read only memories (EEPROMs), and user-recordable type mediums such as floppy disks, hard disk drives and compact disk read only memories (CD-ROMs) or digital versatile disks (DVDs).

[0060] Although an example embodiment of the present disclosure has been described in detail, those skilled in the art will understand that various changes, substitutions, variations, and improvements disclosed herein may be made without departing from the spirit and scope of the disclosure in its broadest form.

Claims

202503222Claims1. A system for detecting a grounded neutral fault in a three-phase power system, comprising:a plurality of drive resistors having different resistance values coupled to three phase conductors of the three-phase power system;a first current transformer having a primary winding configured to receive drive currents from the plurality of drive resistors;a neutral conductor passing through the first current transformer and a second current transformer as a coupling element between the first current transformer and the second current transformer;the second current transformer configured to allow the three phase conductors to pass therethrough;a ground fault detection circuit configured to detect a current imbalance in a secondary winding of the second current transformer; anda trip mechanism configured to trigger when the detected current imbalance exceeds a predetermined threshold.

2. The system according to claim 1, wherein:the plurality of drive resistors includes a first resistor having a first resistor value, a second resistor having a second resistor value, and a third resistor having a third resistor value;the third resistor value being at least 8 times greater than the first resistor value; andthe second resistor value being at least 4 times greater than the first resistor value.

3. The system according to any of the claims 1 to 2, wherein a maximum resistance of any drive resistor is less than or equal to about 100 kQ.

4. The system according to any of the claims 1 to 3, wherein the first current transformer comprises a multi-turn winding coupled to the plurality of drive resistors and a neutral conductor and a single-turn winding coupled to the second current transformer and the neutral conductor.

5. The system according to any of the claims 1 to 4, wherein the second current transformer comprises a plurality of single-turn windings coupled to the three phase conductors, the neutral conductor, and the first current transformer, and the202503222second current transformer further comprises a multi-turn winding coupled to the ground fault detection circuit.

6. The system according to any of the claims 1 to 5, wherein the first current transformer and the second current transformer are magnetically coupled through the neutral conductor.

7. The system according to any of the claims 1 to 6, wherein the ground fault detection circuit is coupled to the second current transformer and the trip mechanism, and the ground fault detection circuit compares the detected current imbalance to the predetermined threshold.

8. The system according to any of the claims 1 to 7, wherein the ground fault detection circuit converts current coupled to the second current transformer to a voltage representative of the detected current imbalance, and the ground fault detection circuit compares the voltage representative of the detected current imbalance to the predetermined threshold.

9. A method for detecting a grounded neutral fault in a three-phase power system, comprising:coupling a plurality of drive resistors having different resistance values to three phase conductors of the three-phase power system;coupling drive currents from the plurality of drive resistors through a primary winding of a first current transformer;passing a neutral conductor through the first current transformer and a second current transformer as a coupling element between the first current transformer and the second current transformer;passing the three phase conductors through the second current transformer; detecting a current imbalance in a secondary winding of the second current transformer using a ground fault detection circuit; andtriggering a trip mechanism when the detected current imbalance exceeds a predetermined threshold.

10. The method according to claim 9, wherein:the plurality of drive resistors includes a first resistor having a first resistor value, a second resistor having a second resistor value, and a third resistor having a third resistor value;the third resistor value being at least 8 times greater than the first resistor value; and202503222the second resistor value being at least 4 times greater than the first resistor value.

11. The method according to any of claims 9 to 10, wherein a maximum resistance of any drive resistor is less than or equal to about 100 kQ.

12. The method according to any of claims 9 to 11, wherein the first current transformer comprises a multi-turn winding coupled to the plurality of drive resistors and the neutral conductor and a single-turn winding coupled to the second current transformer and the neutral conductor.

13. The method according to any of claims 9 to 12, wherein the second current transformer comprises a plurality of single-turn windings coupled to the three phase conductors, the neutral conductor, and the first current transformer, and the second current transformer further comprises a multi-turn winding coupled to a ground fault detection circuit.

14. The method according to any of claims 9 to 13, wherein the first current transformer and the second current transformer are magnetically coupled through the neutral conductor.

15. The method according to any of claims 9 to 14, wherein detecting the current imbalance in the secondary winding of the second current transformer includes comparing the detected current imbalance to the predetermined threshold by the ground fault detection circuit, which is coupled to the second current transformer and the trip mechanism.

16. The method according to any of claims 9 to 15, wherein detecting the current imbalance in the secondary winding of the second current transformer includes converting the detected current imbalance to a voltage representative of the detected current imbalance and comparing the converted voltage representative of the detected current imbalance to the predetermined threshold by the ground fault detection circuit, which is coupled to the second current transformer and the trip mechanism.

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