Protective earth monitoring system and method

Abstract

A method is provided for monitoring the line impedance of a protective earth (“PE”) conductor. The method injects a small stimulating current at a frequency much higher than the grid frequency. This stimulating current circulates in a loop between the grid-connected equipment and the gate source, creating a small voltage drop in the gate conductor, resulting in a measurable common-mode voltage. The method then includes measuring the common-mode voltage and residual current at the point of common coupling (“PCC”). The method then includes monitoring the resistive and reactive portions of the line impedance. For example, the method can monitor the line impedance and report a disconnected PE conductor or a high PE resistance exceeding a given threshold where safety becomes a problem.

Description

Technical Field

[0001] This invention relates to electrical protection circuits for battery-electric vehicles and other applications. Background Technology

[0002] A protective earthing conductor (PE conductor) is a component of a circuit designed to provide a safe path for dissipating current in the event of a fault. The primary function of the PE conductor is to protect people and property from electric shock and to prevent electrical fires. Known PE conductors include copper or aluminum conductors electrically connected to the metal parts of grid-connected equipment. In the event of a short circuit or insulation breakdown, the PE conductor provides a low-resistance path for the fault current to flow safely to ground. This helps ensure that the metal parts of grid-connected equipment remain at a low potential, minimizing the risk of electric shock.

[0003] In battery electric vehicle (BEV) applications, suitable grounding can include a metal chassis or a similar structure with low impedance and high current capacity. In a stationary grid-connected configuration, the BEV (specifically, the onboard charger) is electrically connected to the PE conductor. However, a faulty PE conductor can increase the risk of damage to the BEV and the risk of electric shock to personnel who may come into contact with it. To mitigate this risk, known methods for measuring the resistance of the PE conductor include both passive and active methods. However, despite the existence of these methods, there remains a persistent need for an improved method for monitoring the resistance of the PE conductor. In particular, there remains a persistent need for an improved method and system for real-time monitoring of the PE conductors of a wide range of power grids, including three-phase Y-type electrical systems, three-phase delta-type electrical systems, US split-phase electrical systems, and single-phase electrical systems. Summary of the Invention

[0004] An improved method for monitoring line impedance is provided. The method typically includes: (a) applying a stimulating current to a PE conductor at a frequency higher than the grid voltage; (b) measuring the common-mode voltage at a common coupling point between the grid voltage and the grid-connected equipment; (c) measuring the residual current at the common coupling point; (d) determining the impedance at the common coupling point, the impedance having resistive and reactive components, based on the common-mode voltage and the residual current; and (e) generating an alarm if the measured impedance exceeds a predetermined threshold. For example, the method can monitor the line impedance and report faulty PE conductors exceeding a given threshold where safety becomes a concern.

[0005] In another aspect of the invention, a system is provided. The system includes a PE conductor electrically coupled to a PE terminal of a power source and a PE terminal of an electrical load, such as a power converter in an electric vehicle. The system also includes a PE monitoring circuit for monitoring the impedance of the PE conductor. The PE monitoring circuit includes a stimulation injector configured to apply a stimulation current directly to the PE conductor at a frequency at least an order of magnitude greater than the frequency of the power source. The PE monitoring circuit also includes a measurement circuit configured to measure a common-mode voltage and a residual current at the common coupling point (PCC) of a current-carrying conductor extending between the power source and the electrical load. The PE monitoring circuit also includes a processor configured to calculate the impedance of the PE conductor based on the measured common-mode voltage and the measured residual current. In one embodiment, calculating the impedance of the PE conductor includes performing a vector division at the stimulation frequency, wherein performing the vector division includes dividing a complex common-mode voltage measurement by a complex residual current measurement.

[0006] In these and other embodiments, the method and system inject a small stimulating current (e.g., a sinusoidal current) at a frequency much higher than the grid frequency. For a grid frequency of 50 / 60 Hz, the stimulating current could, for example, include frequencies between 1 kHz and 10 kHz, which do not contribute to power transfer from the grid to the grid-connected device. The stimulating current circulates in a loop between the grid-connected device and the gate source, creating a small voltage drop in the gate conductor, resulting in a common-mode voltage and residual current at the PCC. For example, in a three-phase Y-connection with current-carrying conductors L1, L2, L3, and N, each of these four lines is measured relative to the PE conductor at the PCC. These voltages are summed and divided by four to obtain a vector average voltage, also known as the common-mode voltage. The method and system use vector division at a specified injected stimulating frequency to determine the grid impedance. The method and system monitor the resistive and reactive portions of the grid impedance and can be applied to any grid-connected application, such as three-phase Y-connected electrical systems, three-phase delta-connected electrical systems, US split-phase electrical systems, and single-phase electrical systems.

[0007] These and other features and advantages of the invention will become apparent from the following description of the invention when viewed in conjunction with the accompanying drawings and appended claims. It will be understood that any preferred and / or optional features of the invention may be incorporated individually or in suitable combinations into embodiments of the invention while still falling within the scope of claim 1, even if such combinations are not expressly claimed in the appended claims. Attached Figure Description

[0008] Figure 1 This is a circuit diagram of an electrical protection circuit according to an embodiment of the present invention.

[0009] Figure 2This is a circuit diagram of a current injector circuit used to apply a stimulating current to the PE conductor.

[0010] Figure 3 The diagram shows a gain stage, a high-pass filter, and a voltage divider for common-mode voltage measurement. Figure 1 Electrical protection circuit.

[0011] Figure 4 A circuit for measuring residual current at the common coupling point is shown. Detailed Implementation

[0012] refer to Figure 1 A protective grounding circuit according to one embodiment is shown, and this protective grounding circuit is generally designated as 10. The protective grounding circuit 10 includes a protective grounding conductor 12 electrically connected between a mains (trunk) voltage 14 and a grid-connected device 16. As described below, the protective grounding circuit 10 includes digital and analog circuitry for actively monitoring the fidelity of the protective grounding conductor 12 according to: (a) applying a stimulating current to the protective grounding conductor 12 at a frequency greater than that of the mains voltage 14; (b) measuring the common-mode voltage at a common coupling point between the mains voltage 14 and the grid-connected device 16; (c) measuring the residual current at the common coupling point; (d) determining the impedance at the common coupling point, the impedance having resistive and reactive components, based on the common-mode voltage and the residual current; and (e) generating an alarm if the measured impedance is greater than a predetermined threshold for the protective grounding conductor 12. Each of these operations is discussed separately below.

[0013] Applying a stimulating current to the protective grounding conductor 12 typically involves generating a high-frequency, low-voltage waveform. For example... Figures 1-2As shown, for example, current injector 18 receives a stimulation signal from digital signal processor (DSP) 20. Current injector 18 includes an operational amplifier open-loop gain function for providing injected current to protective ground conductor 12. The injected current includes a frequency at least one order of magnitude larger than the grid frequency (i.e., at least 10 times larger). For a grid frequency of 50 / 60 Hz, for example, the stimulation current may include a sinusoidal waveform with a frequency between 1 kHz and 10 kHz, which does not contribute to power transfer from the grid to grid-connected device 16. The stimulation current may include an amplitude that does not interfere with the system, optionally approximately 500 µARMS (Amperes, RMS). The stimulation current circulates in a loop between gate source 14 and grid-connected device 16, producing a small voltage drop in gate conductor 22, resulting in measurable common-mode voltage and residual current. In the illustrated embodiment, grid conductor 22 includes four current-carrying conductors L1, L2, L3, and N for a three-phase Y-connection. However, in other embodiments, the grid conductor 16 may include fewer current-carrying wires as required by a three-phase delta electrical system, a U.S. split-phase electrical system, and / or a single-phase electrical system.

[0014] For example Figure 1 As shown, the virtual grounding network 19 serves as the return path for the current injector 18. The virtual grounding network 19 provides a path for the stimulating current to return to the current injector 18. Also as... Figure 1 As shown, the current injector 18 is coupled to the virtual grounding network 19 (e.g., virtual grounding node V). gnd And coupled to the protective grounding conductor 12. The virtual grounding network 19 includes multiple capacitors used as an averaging circuit, such that the virtual grounding node (V gnd The potential of the protective grounding conductor 12 is close to that of the protective grounding conductor 12, thereby allowing a low voltage to be injected into the protective grounding conductor 12.

[0015] The method then includes measuring the common-mode voltage at the point of common coupling (PCC). The common-mode voltage represents the voltage (relative to ground) shared by the current-carrying conductors 22 that electrically connect the grid voltage 14 to the grid-connected equipment 16. This differs from the differential voltage in the current-carrying conductors. The point of common coupling (PCC) provides a reference point for analyzing the power flow through the protective grounding circuit 10. Figure 3As shown in the optimal diagram, the common-mode voltage is measured digitally by the DSP20 after the following stages: (a) a voltage divider stage, (b) a high-pass filter stage, and (c) a gain stage. The voltage divider stage limits the residual current drawn by the measurement circuitry and reduces the size of the capacitors otherwise required for common-mode voltage measurement. In this example, the high-pass filter stage removes larger grid frequency components below 400Hz and prevents operational amplifier output saturation. Finally, the gain stage amplifies the resulting signal (to improve resolution) for processing by the DSP20. The resulting frequency-dependent common-mode voltage (as output to the DSP) is the vector average of the four-phase voltages at the PCC relative to ground.

[0016] In parallel with voltage measurement, the method includes measuring residual current in current-carrying conductors L1, L2, L3, and N. In the illustrated embodiment, the residual current at the PCC is measured via a residual current coil 24. The residual current coil 24 is a current transformer with a toroidal core made of magnetic material through which the live wire (L1, L2, L3) and neutral (N) conductors pass. Conductors 22 (L1, L2, L3, N) form a coupled primary winding, each with one turn, while a single coupled secondary winding with more turns is connected to a transimpedance amplifier that converts the secondary winding current into a voltage. The residual current coil 24 senses the current difference between the live and neutral conductors. In a balanced system, the magnetic fields generated by these currents cancel each other out, resulting in almost no induction in the residual current coil 24. However, a stimulating current applied to the protective ground conductor 12 generates a residual current at the PCC, which induces a signal proportional to this residual current at the residual current coil 24.

[0017] Similar to the voltage measurement at the PCC, the current measurement at the PCC undergoes high-pass filtering and amplification. For example... Figure 4 As shown, for example, residual current coil 24 is electrically coupled to transimpedance amplifier 26. Transimpedance amplifier 26 converts the input current signal into a proportional output voltage signal. Amplifier 26 has low input impedance and provides a low-resistance path for the input current. The output of amplifier 26 is processed by differential high-pass filter 28. Differential high-pass filter 28 includes a differential amplifier with non-inverting and inverting inputs, thereby allowing high-frequency components to pass while attenuating low-frequency components. Next, high-pass filter 30 allows signals above a threshold frequency to pass through DSP 20 with minimal attenuation, while attenuating or suppressing signals below that frequency. The current embodiment includes a 400Hz high-pass filter, but other high-pass filters may be included in other embodiments as needed. Finally, the amplitude of the resulting signal is increased at amplifier stage 32 before being output to DSP 20.

[0018] The method then includes determining the impedance at the common coupling point based on the measured common-mode voltage and the measured residual current. The common-mode voltage in the current-carrying conductor decreases due to changes in resistive impedance and reactive impedance. By measuring the residual current and common-mode voltage, the DSP20 uses a specified injection stimulation frequency (f... s The vector division at (V) is used to calculate the grid impedance. This operation typically involves calculating the common-mode voltage (V) using digital logic. cm ) and residual current (i residual The Fourier coefficients of the trigonometric functions of are shown in the following equations (1) and (2):

[0019]

[0020] The Fourier coefficients include the complex common-mode voltage measurement (a+jb) and the complex residual current measurement (c+jd). DSP20 then calculates the uncorrected grid impedance according to the following equation (3):

[0021]

[0022] Then, a calibration correction factor (Z) is applied to the gain, phase, and analog-to-digital conversion delay. corr (fs)), calculate the corrected grid impedance (Z) according to the following equation (4). grid (fs)):

[0023]

[0024] Corrected grid impedance (Z) grid (fs) includes both resistive and reactive components and can be compared to a threshold. For example, the DSP20 can compare the line impedance (i.e., the mains impedance) to a threshold and report a broken PE conductor or high PE resistance if the line impedance exceeds the threshold.

[0025] If the line impedance exceeds the impedance threshold, the DSP can assume that the grounding resistance is excessively high and can generate a corresponding alarm and / or short-circuit or disable the grid-connected device 16, optionally transmitting the alarm to a remote server 34. This invention is well-suited for a wide range of applications, including both on-board chargers for electric vehicles and stand-alone charging stations for electric vehicles. Although described above in conjunction with a three-phase Y connection, this invention can be applied to any grid connection application, such as three-phase Y electrical systems, three-phase delta electrical systems, US split-phase electrical systems, and single-phase electrical systems.

[0026] As a further advantage, the system and method of the present invention measure the actual residual current at the PCC, and therefore do not need to rely on the injected stimulus current when determining the line impedance. For example, a creeping path may induce some injected current and result in a lower common-mode voltage measurement. Alternative methods that do not measure the actual residual current will have lower accuracy in the calculated line impedance. This disadvantage is overcome even when a creeping return path for the residual current exists. If the actual current at the injector frequency is too small to measure the common-mode voltage, the system and method can respond by increasing the DSP signal value (amplitude), thereby driving the common-mode voltage to a measurable level. This technique ensures sufficient current circulates between the grid and grid-connected equipment, regardless of the presence of a possible creeping return path.

[0027] The above description is a description of the current embodiments of the present invention. Various substitutions and changes can be made without departing from the spirit and broader aspects of the invention. This disclosure is presented for illustrative purposes and should not be construed as an exhaustive description of all embodiments of the invention, or as limiting the scope of the claims to the specific elements shown or described in connection with these embodiments. For example, any reference to an element in the singular form using the articles “a,” “an,” “the,” or “the” should not be construed as limiting the element to the singular.

Claims

1. A method comprising: A protective grounding circuit is provided between the grid voltage and the grid-connected equipment, the protective grounding circuit including a protective grounding conductor; An stimulating current is applied to the protective grounding conductor, the stimulating current including a stimulating frequency, the stimulating frequency being greater than the grid frequency; Measure the common-mode voltage and residual current at the common coupling point between the grid voltage and the grid-connected equipment; The impedance at the common coupling point is determined based on the measured common-mode voltage and the measured residual current, and the impedance includes a reactance component and a resistive component. as well as An alarm is generated in response to the measured impedance at the common coupling point exceeding a predetermined threshold.

2. The method according to claim 1, wherein, The grid-connected equipment includes a power converter for electric vehicles.

3. The method according to claim 1, wherein, The grid voltage includes three-phase power supply, split-phase power supply, or single-phase power supply.

4. The method according to claim 1, wherein, The stimulation frequency is at least an order of magnitude greater than the power grid frequency.

5. The method according to claim 1, wherein, The stimulation frequency is at least 1 kHz.

6. The method according to claim 1, wherein, The stimulating current is applied directly to the protective grounding conductor between the grid voltage and the grid-connected equipment.

7. The method according to claim 1, wherein, The measurement of the common-mode voltage is performed by a digital signal processor in a digital logic manner after the high-pass filter stage.

8. The method according to claim 1, wherein, The measurement of the residual current is performed digitally by a digital signal processor after the high-pass filter stage.

9. The method according to claim 1, wherein, The residual current is measured using a current transformer.

10. The method according to claim 9, wherein, The current transformer includes a loop coil that extends around a current-carrying conductor coupled to the grid voltage and each of the grid-connected devices.

11. The method according to claim 1, wherein, Determining the impedance at the common coupling point includes performing a vector division at the stimulation frequency.

12. The method according to claim 11, characterized in that, The vector division includes dividing the complex common-mode voltage measurement by the complex residual current measurement and scaling it according to calibration correction factors for gain, phase, and analog-to-digital conversion delay.

13. The method according to claim 1, further comprising: The power flow to the grid-connected device is terminated in response to the measured impedance at the common coupling point exceeding the predetermined threshold.

14. A system comprising: Multiple current-carrying conductors electrically coupled to a power source and an electrical load, each of the power source and the electrical load including a protective grounding terminal; A protective grounding conductor, which is electrically coupled to the protective grounding terminal of the power supply and the protective grounding terminal of the electrical load; as well as A protective grounding monitoring circuit is used to monitor the impedance of the protective grounding conductor, the protective grounding monitoring circuit comprising: —A stimulation injector configured to apply a stimulation current to the protective grounding conductor, the stimulation current including a stimulation frequency greater than the frequency of the power supply; —A measurement circuit for measuring the common-mode voltage and residual current in the plurality of current-carrying conductors; and —A processor coupled to the output of the measurement circuit, the processor being configured to calculate the impedance of the protective grounding conductor based on the measured common-mode voltage and the measured residual current.

15. The circuit according to claim 11, wherein, The stimulation injector includes circuitry adapted to convert digital signals from the processor into stimulation currents applied directly to the protective grounding conductor, the stimulation frequency being equal to or greater than 1 kHz.

16. The circuit according to claim 11, wherein, The measuring circuit includes a current transformer having a loop coil extending around the plurality of current-carrying conductors.

17. The circuit according to claim 11, wherein, The power supply includes three-phase power supply, split-phase power supply, or single-phase power supply.

18. The circuit according to claim 11, wherein, The electrical load includes electric vehicles.

19. The circuit according to claim 11, wherein, Calculating the impedance of the protective grounding conductor includes performing a vector division at the stimulation frequency, wherein performing the vector division includes dividing a complex common-mode voltage measurement by a complex residual current measurement.

20. The circuit according to claim 11, wherein, The processor is also configured to perform one or both of the following: (a) generate an alarm in response to the calculated impedance exceeding a predetermined threshold; and (b) terminate the electrical power flow to the electrical load.

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