Method and apparatus for measuring ground resistance in a battery charging system

Abstract

1. A method for measuring ground resistance in a battery charging system, comprising the steps of injecting an alternating current measurement signal between a first neutral node (N) and a second ground node (PE); detecting a voltage signal representative of a potential difference between the first node (N) and the second node (PE); processing the voltage signal to extract a first signal having a frequency equal to and a phase correlated to a frequency of the measurement signal; and calculating a ground resistance value as a function of a ratio between the measurement signal and a reference value of the first signal.

Description

Technical Field

[0001] The present invention relates to a battery charging system, preferably a method and apparatus for measuring the ground resistance in a traction (automotive) battery.

[0002] Therefore, the present invention is mainly applied to the field of automobiles, particularly to the design and construction of charging systems for batteries.

Background Art

[0003] In fact, in the field of electric vehicles, the charging modes of battery packs are classified into two different macro-categories: in-vehicle chargers and ground chargers.

[0004] As the name indicates, an "in-vehicle charger" is incorporated in the vehicle and includes all the power supplies and control electronics necessary to convert the alternating current from the grid into the direct current required for recharging the battery pack.

[0005] On the other hand, a "ground" charger is a conventional "column" or wall box that directly provides a conversion by supplying direct current to the vehicle.

[0006] Therefore, both categories of battery chargers have significant problems from the perspective of user safety, and it is considered that the battery charger must be equipped with an appropriate protection system. This is because the battery charger has to manage the alternating current provided by the grid and convert it into direct current to recharge the high-voltage battery.

[0007] This is even more important in the construction of a non-insulated in-vehicle charging system where the battery charger has a direct (and non-inductive) electrical connection to the alternating current wiring.

[0008] One of the main research areas related to safety systems is verifying whether a vehicle or column is properly "connected to ground," which in some cases is done by injecting a current signal into the grid, measuring the resulting voltage through appropriate computer calculations, and calculating its ground resistance.

[0009] However, unfortunately, this method is not without its drawbacks. Its effectiveness is closely related to the boundary state, i.e., the noise grid state.

[0010] In power distribution systems, given that numerous users are connected to the same substation with the most diverse uses, it is not uncommon for the frequencies used to generate current signals to already be occupied by significant disturbances.

[0011] In some cases, strict regulatory constraints limit the usable current intensity for measurement to just over 1 mA, making it practically difficult to determine which components of the detected voltage signal correlate with the injected signal and which components are noise. [Overview of the Initiative]

[0012] Therefore, an object of the present invention is to provide a method and apparatus for measuring ground resistance in a battery charging system that can overcome the aforementioned drawbacks of the prior art.

[0013] In particular, the object of the present invention is to provide a highly reliable and versatile method and apparatus for measuring grounding resistance in a battery charging system, which can be adapted to power distribution systems with different noise levels without compromising efficiency.

[0014] The aforementioned objective is achieved by a method and apparatus for measuring ground resistance in a battery charging system having one or more features of the claims of the subsequent claims.

[0015] In particular, this method includes injecting (or generating) an AC measurement signal between a first neutral node and a second ground node of the grid.

[0016] Preferably, the measurement signal has a preset frequency and a preset duration.

[0017] Therefore, a voltage signal representing the potential difference between the first node and the second node is detected.

[0018] Accordingly, according to the present invention, the voltage signal is processed to extract a first signal having a frequency equal to the preset frequency and a phase correlated with the phase of the measurement signal. It should be noted that the term "phase correlated with ~" is intended to specify that the first signal has a phase shift that is constant or zero over time with respect to the measured signal.

[0019] In other words, by knowing the measurement signal or its frequency, it is possible to determine what frequency the voltage signal to be detected should have. Unlike noise components of the same frequency, this is closely related to the measurement signal and therefore maintains the same phase shift over time, thus enabling its extraction.

[0020] At this point, if the voltage signal is known, it is possible to calculate the grounding resistance value as a function of the ratio between the value that identifies the measurement signal and the reference value of the first signal.

[0021] Clearly, since the power distribution system uses alternating current, the resistance must also be calculated taking into account the capacitance and equivalent impedance of the circuit.

[0022] However, according to a further aspect of the present invention, the measurement methods described herein can be used as the basis for a method of protecting a battery charging system, which is also an independent object of the present invention.

[0023] In fact, this protection method includes using the measured / calculated ground resistance value as a basis for comparing it with a reference limit value, and then insulating the converter assembly of the battery charger from the grid if the ground resistance value exceeds the reference limit value.

[0024] Preferably, the reference limit value is set to less than 300 ohms, and more preferably to about 200 ohms.

[0025] A further object of the present invention is an apparatus for measuring the ground resistance in a battery charging system, comprising a processing unit configured to execute the method described so far.

[0026] The dependent claims incorporated herein by reference correspond to various embodiments of the present invention.

Brief Description of the Drawings

[0027] Further features and advantages of the present invention will become more apparent from the following exemplary but non - limiting description of preferred but not exclusive embodiments of a method and an apparatus for measuring the ground resistance in a battery charging system, as shown in the accompanying drawings. [Figure 1] The structure of an in - vehicle charging device provided with a protection device according to the present invention is schematically shown. [Figure 2] The structure of a measuring device for measuring the ground resistance according to the present invention is schematically shown.

Embodiments for Carrying Out the Invention

[0028] Referring to the accompanying drawings, reference numeral 1 generally indicates an apparatus for measuring the ground resistance in a battery charging system according to the present invention.

[0029] Hereinafter, by way of example only, an apparatus 1 for measuring the ground resistance in an in - vehicle charging device 100 will be described. In fact, this description is not intended to limit the use and scope of the present invention.

[0030] Similar explanations are possible for ground charging systems such as columns or wall boxes, but since measuring ground resistance is more important in vehicles, the following explanation prioritizes providing descriptions of applications where the present invention can find greater advantages.

[0031] In this specification, the term "onboard charging device 100" is intended to generally define any charging system for a traction battery pack 6 that can be connected to an AC grid and converted to DC before supplying power to the battery.

[0032] Therefore, the charging device 100 comprises at least one casing C (connected to earth) associated with a connection socket 9 for connecting to a grid G, and the casing C houses a converter assembly 4 configured to convert the alternating current provided by the grid G ​​into a direct current usable for recharging the battery pack 6.

[0033] Therefore, the connection socket 9 is configured to accept both the three-phase L1, L2, L3 and the neutral wire N.

[0034] Preferably, the charging device 100 further comprises at least one electromagnetic disturbance filter element located along the current input line, i.e., between the connection socket 9 and the converter assembly 4.

[0035] More preferably, there are two electromagnetic disturbance filter elements, the first filter element 5 along the (AC) current input line and the second filter element 7 along the (DC) current output line, i.e., between the converter assembly 4 and the battery pack 6.

[0036] In a preferred embodiment, the charging device 100 is non-isolated, that is, it provides a physical (i.e., non-inductive) connection between the battery and the power distribution system.

[0037] Preferably, in this type of device 100, the converter assembly 4 comprises at least one boost module and at least one buck module.

[0038] In a preferred embodiment, the converter assembly 4 comprises the following: - A first conversion stage (or AC-DC converter) configured to convert the alternating current provided by the grid G ​​into direct current, - A charging stage, preferably defined by a capacitor bank, operably positioned downstream of a first conversion stage, and configured to be charged by receiving its output. - A second conversion stage (or DC-DC converter) configured to modulate the level of DC supplied to the battery pack 6.

[0039] In a preferred embodiment, as described above, the charging device 100 is non-isolated, so the second conversion stage is directly connected to the battery pack (i.e., without a conversion / induction stage).

[0040] According to one aspect of the present invention, for another object of the present invention, the ground resistance is associated with the charging device 100.

[0041] The measuring device 1 is configured to detect grounding problems by measuring the resistance.

[0042] The measuring device 1 includes a current generator 2 having variable amplitude and frequency (and possibly phase) configured to generate and inject an AC measurement signal into the grid G ​​between a first neutral node N and a second ground node PE.

[0043] A voltage detection module 3 is also provided, configured to detect a voltage signal representing the potential difference between a first node N and a second node PE.

[0044] In particular, the detection module 3 is preferably configured to detect a voltage signal following the injection or generation of a measurement signal by the current generator 2.

[0045] Furthermore, the detection module 3 is preferably of a variable gain type and is configured to change the gain, and thus change the identification value (e.g., amplitude) of the voltage signal as a function of its noise level.

[0046] In a preferred embodiment, the detection module 3 includes a peak rectifier, preferably a dual half-wave type peak rectifier, configured to generate a rectified signal representing the module of the voltage signal.

[0047] The measuring device 1, which includes the configured processing unit 10, is also configured to identify the peak value of the rectified signal within a predetermined time interval.

[0048] The processing unit 10 is further configured to amplify or attenuate the identification value (e.g., amplitude) of the measurement signal as a function of the peak value.

[0049] Therefore, more preferably, the processing unit 10 is configured to decrease the identification value (e.g., amplitude) as the peak value increases, and vice versa.

[0050] The measuring device 1 further comprises the detection module 3 and an amplifier module 11 associated with the current generator 2.

[0051] The amplifier module 11 is configured to process the voltage signal based on the measurement signal in order to extract a first signal having a frequency corresponding to the measurement signal and a phase correlated therewith.

[0052] It should be noted that the term "phase correlated with ~" is intended to specify that the first signal has a phase shift that is constant or zero over time with respect to the measured signal.

[0053] In a preferred embodiment, the amplifier module 11 is a lock-in amplifier (or phase-sensitive detector) and can be made entirely of hardware components or at least part of an architecture defined by software functions, preferably.

[0054] This lock-in amplifier comprises a multiplier (reference signal and noise signal), followed by a low-pass integral filter with an integration period longer than the period of the extracted signal.

[0055] Since the sine functions are orthogonal, the result of this operation will be null in the following cases, i.e., - If the extracted signal and carrier wave have different frequencies, - If they have the same frequency but are 90° out of phase, the result is null.

[0056] Alternatively, if they are of equal frequency, or if they are out of phase by a certain angle, the result will have a non-zero value and will be constant.

[0057] Conversely, if a signal has equal frequency components but its phase changes continuously, the result will also change continuously.

[0058] Therefore, this makes it possible to identify whether a noise component (with a continuously changing phase) or a component related to the measurement signal (with an equal or phase-shifted phase that correlates with the measurement signal at a constant rate over time, as described above) is detected at a given frequency.

[0059] Alternatively, the amplifier module 11 may be a phase-locked loop that also includes a multiplier and a filter, but is followed by a control oscillator.

[0060] In this regard, it should be noted that the processing unit 10 of the measuring device 1 is also configured to calculate the grounding resistance value as a function of the ratio between a value that identifies the measurement signal and a reference value of the first signal.

[0061] Preferably, the processing unit 10 of the measuring device 1 is configured to calculate the grounding resistance value as a function of the ratio between a value that identifies a parameter representing the measurement signal and a reference value of the same parameter of the first signal.

[0062] In one embodiment, the parameter is defined by the amplitude of the signal. Alternatively, it may be a value of a time-domain signal or yet another parameter.

[0063] More preferably, the processing unit is configured to convert the first signal into the frequency domain, identify the frequency spectrum corresponding to the preset frequency of the measurement signal, and determine that the reference value corresponds to the voltage signal value at the preset frequency.

[0064] Therefore, in other words, the processing unit 10 is configured to perform a method for measuring ground resistance, which is also an (independent) objective of the present invention.

[0065] Therefore, this method includes injecting (or generating) an AC measurement signal between a first neutral node N and a second ground node PE.

[0066] The measurement signal is generated by the current generator 2 of the measuring device 1 and injected between the two nodes described above.

[0067] Second ground Node PE is selectively, - Physical grid nodes, - Having a potential corresponding to the ground potential and the first neutral node N and three phase (L1, L2, L3) It is electrically conductive. like Please note that this could also refer to a constructed virtual node.

[0068] Furthermore, grid noise analysis is preferably performed prior to the generation or injection of phase measurement signals.

[0069] This grid noise analysis step includes detecting a voltage signal representing the grid voltage and identifying a free frequency range in which the voltage signal has a minimum or zero value (e.g., minimum or zero amplitude).

[0070] Therefore, preferably, the preset frequency of the measurement signal is within the free range in order to maximize the reduction of disturbances during measurement.

[0071] Furthermore, preferably, the measurement signal has a limiting average value within a reference time interval.

[0072] In other words, current regulations stipulate that metal components that should not be touched during normal use should not have a contact voltage that can be generated through the human body, with a current that does not exceed the limit of each current cycle (typically considered to be 3.5 mARMS every 20 msec or 16.6 msec, depending on the grid frequency).

[0073] Considering that some of these limits are necessary for the operation of certain electronic components, only the difference between the limit and the usable portion can be used to generate the measurement signal.

[0074] Therefore, in a preferred embodiment, the limiting average value of the measurement signal within a single current cycle can be quantified as approximately 1 mA RMS.

[0075] Preferably, and therefore the limit mean is the limit mean amplitude.

[0076] To make the injected signal more distinguishable from the grid noise, which is typically very high for a large number of loads connected to a single grid substation, the measurement signal injection step preferably includes modulating the identifier value and a predetermined time interval such that the identifier value has a maximum value that conforms to the limit average value.

[0077] In other words, if it becomes necessary to amplify the discriminant values ​​to make the signal more visible, this method includes the (optional additional) possibility of shortening the time interval to a fraction of the grid's current cycles so as to keep the average value below the threshold.

[0078] Preferably, the method then includes detecting a voltage signal representing the potential difference between the first node N and the second node PE.

[0079] Next, this voltage signal is processed to extract a first signal having a frequency equal to the preset frequency and a phase correlated with the phase of the measurement signal.

[0080] It should be noted that the term "phase correlated with ~" is intended to specify that the first signal has a phase shift that is constant or zero over time with respect to the measured signal.

[0081] Therefore, advantageously, by knowing the measurement signal, it is possible to determine what frequency the voltage signal to be detected should have. Unlike noise components of the same frequency, this is closely related to the measurement signal and maintains the same phase shift over time, thus enabling its extraction.

[0082] In a preferred embodiment, this extraction step is performed by amplification and filtering procedures similar to those used in a lock-in amplifier.

[0083] More precisely, this voltage signal processing step is: - Multiplying the voltage signal and the reference signal to obtain a derived signal containing a first frequency component equal to the sum of the frequencies of the voltage signal and the reference signal, and a second frequency component equal to the difference between the frequencies of the voltage signal and the reference signal. - Includes filtering the obtained derived signal to extract only the first or second component of the derived signal.

[0084] Advantageously, this makes it possible to identify the only voltage component that can be measured by injecting the measurement signal in a highly noisy environment, and thus enable the measurement of the ground resistance.

[0085] This grounding resistance is actually calculated as a function of the ratio between the value that identifies the measurement signal and the reference value of the first signal.

[0086] As already stated above, the reference value is defined by the voltage signal value of the measurement signal (evaluated in the frequency domain) at a preset frequency. Preferably, as described above, the reference value corresponds to the voltage signal amplitude of the measurement signal at a preset frequency.

[0087] Furthermore, advantageously, this method and apparatus for measuring ground resistance can be used within a method and apparatus for protecting a charging device, which is also an independent objective of the present invention.

[0088] In fact, by following the measurement method described above, the grounding resistance value can be compared to a reference limit value (preferably less than 300 ohms), and if the grounding resistance value exceeds the reference limit value, the converter assembly 4 of the charging device 1 can be isolated from the grid G.

[0089] Structurally, to accomplish this, the charging device 1 includes a switch assembly 8 mounted on the measuring device 1, which can selectively switch between a closed state in which the socket 9 is electrically connected to the converter assembly 4 and an open state in which the connection is opened and the socket 9 (and therefore the grid G) is electrically released from the converter assembly 4 (and therefore from the battery pack 6).

[0090] This invention achieves its intended purpose and provides significant advantages.

[0091] In fact, by properly processing "noisy" voltage signals (preferably with an amplifier), it is possible to selectively extract only the relevant information, regardless of the grid noise level.

[0092] In other words, this solution detects ground resistance and ensures the potential to enhance system safety against all types of grid and noise conditions.

Claims

1. A method for measuring ground resistance in a battery charging system connected to or connectable to an AC grid (G), the battery charging system comprising a converter assembly (4) configured to convert alternating current provided by the AC grid (G) into direct current usable for charging a battery pack (6), - A step of detecting a voltage signal representing the grid voltage, - A step of identifying the free frequency range in which the voltage signal has a minimum value or zero value, - A step of injecting an AC measurement signal between a first neutral node (N) and a second ground node (PE), wherein the measurement signal has a preset duration and a preset frequency within the free range. - A step of detecting a voltage signal representing the potential difference between the first node (N) and the second node (PE), - A step of processing the voltage signal to extract a first signal having a frequency equal to the preset frequency and a phase correlated with the phase of the measurement signal, - The step of calculating the grounding resistance value as a function of the ratio between an identification value that identifies the measurement signal and a corresponding reference value of the first signal, A method wherein the measurement signal has a limiting average value within each current cycle, and the step of injecting the measurement signal includes modulating the preset duration and the identification value of the measurement signal such that the identification value that identifies the measurement signal has a maximum value that matches the limiting average value.

2. - A step of converting the first signal into the frequency domain, The method according to claim 1, comprising the step of identifying the reference value with a frequency value equal to the preset frequency.

3. The method according to claim 1 or 2, wherein the phase of the first signal is equal to the phase of the measurement signal, or differs from the phase of the measurement signal in a constant manner over time.

4. The processing step described above is: - Multiplying the voltage signal and the reference signal to obtain a derived signal that includes a first frequency component equal to the sum of the frequencies of the voltage signal and the reference signal, and a second frequency component equal to the difference between the frequencies of the voltage signal and the reference signal. - The method according to any one of claims 1 to 3, comprising filtering the obtained derived signal to extract only the first or second component of the derived signal.

5. The method according to any one of claims 1 to 4, wherein the identification value is defined by at least one parameter corresponding to the amplitude of the measurement signal.

6. The grid noise analysis step includes, - Peak noise values ​​are detected within a predetermined time interval. - The method according to any one of claims 1 to 5, wherein the preset duration and the identification value of the measurement signal are modulated according to the peak noise value.

7. The AC grid (G) is a three-phase grid having three phases (L1, L2, L3) and the first neutral node (N), The second grounding node (PE) is selectively, - Physical grid nodes, - A virtual node having a potential corresponding to the ground potential and constructed to be electrically conductive with the first neutral node (N) and the three phases (L1, L2, L3), according to any one of claims 1 to 6.

8. A method for protecting a battery charging system, - A step of measuring the ground resistance in the charging device (100) by performing the measurement method described in any one of claims 1 to 7, - A step of comparing the grounding resistance value with the reference limit value of grounding resistance, A method comprising the step of isolating the converter assembly (4) of the charging device (100) from the AC grid (G) if the grounding resistance value exceeds the reference limit value of the grounding resistance.

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