Method and apparatus for measuring ground resistance in battery charging system

Abstract

A method for measuring ground resistance in a battery charging system connected to an electrical power grid, the method comprises, in each measurement period, the steps of injecting a first AC electrical signal between a first neutral node and a second ground node, detecting a first voltage signal representative of a potential difference between the first node and the second node, and calculating a ground resistance value from the first voltage signal. The method further comprises the step of injecting an information signal representing the calculated ground resistance value (RPE) between the first neutral node and the second ground node. Before the step of injecting the first AC electrical signal, the method also provides the following steps:-detecting one or more information signals (IS) generated by a further battery charging system connected to the electrical power grid; -determining the ground resistance value (RPE) on the basis of the information content of the received information signal; if the number of information signals (IS) is detected to fall below the preset limit value, steps a) to c) are carried out.

Description

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

[0002] The present invention is mainly applied in the automotive field, especially in the design and manufacture of charging systems for batteries.

[0003] In fact, in the context of electric vehicles, the battery pack charging modes are divided into two different relatively macroscopic categories: on-board chargers and ground chargers.

[0004] As the name implies, on-board chargers are integrated into the vehicle and include all the power and control electronics for converting alternating current from the power grid into direct current required for charging the battery pack.

[0005] On the other hand, 'ground' chargers are the well-known'standposts' or wall boxes that directly perform the conversion by supplying direct current to the vehicle.

[0006] Thus, it is clear that these two types of battery chargers, which must manage alternating current from the power grid and convert it into direct current for charging the high-voltage battery, are quite critical in terms of user safety, as they must be equipped with appropriate protection systems.

[0007] One of the main research areas related to the safety system is to verify that the vehicle or the standpost is correctly 'grounded'. In some cases, this is done by injecting a current signal into the power grid, measuring the resulting voltage by means of appropriate computer calculations, and calculating the relevant ground resistance.

[0008] However, this process is not without its drawbacks, as its effectiveness is closely related to the boundary conditions (i.e., the noise of the power grid).

[0009] Considering that the distribution network has a large number of users with the most diverse applications connected to the same substation, it is not uncommon for the frequencies used in generating the current signal to be occupied by very significant interference.

[0010] Considering the strict regulatory constraints that limit the current intensity available for measurement to slightly more than 1 mA, this causes practical difficulties in determining which component of the detected voltage signal is related to the injected signal and which component is instead the noise component in several cases.

[0011] It must also be considered that the quantity and type of loads connected to the power grid are not considered constant parameters, but vary over time, even significantly, which makes the applicant study possible solutions that will make the system robust to these variables. Summary of the Invention

[0012] Accordingly, an object of the present invention is to provide a method for measuring the ground resistance in a battery charging system, which method is capable of overcoming the above-mentioned drawbacks of the prior art.

[0013] Specifically, an object of the present invention is to provide a method and apparatus for measuring the ground resistance in a battery charging system, which battery charging system maintains its efficiency regardless of the type and number of loads connected to the power grid.

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

[0015] Specifically, the method includes: injecting a first alternating current signal between a first neutral node and a second ground node in each measurement cycle.

[0016] The first current signal has a preset duration and a predetermined injection frequency.

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

[0018] At this time, the value of the ground resistance is calculated based on the first voltage signal.

[0019] According to the present invention, the method further includes the step of injecting an information signal representing the calculated ground resistance value between the first neutral node and the second ground node.

[0020] In addition, preferably, before the step of injecting the first alternating current signal, one or more information signals generated by another battery charging system connected to the power grid are detected (from the power grid).

[0021] Preferably, if the number of detected information signals is at least equal to a preset limit value, the value of the ground resistance is determined based on the information content of the received information signals.

[0022] Alternatively, if the number of detected information signals is below the preset limit value, the method calculates the value of the ground resistance by implementing the steps of injecting the first current signal and detecting the first voltage signal described above and then injecting the information signal into the power grid.

[0023] According to another optional aspect of the present invention, there is a step of prohibiting the injection of the first alternating current signal if the number of detected information signals is at least equal to a preset limit value.

[0024] Another object of the present invention is to provide an apparatus for measuring the ground resistance of an on-vehicle charger, which apparatus is preferably configured to implement the above method.

[0025] The dependent claims incorporated herein by reference correspond to different embodiments of the present invention.

[0026] Further features and advantages of the present invention will become more apparent from the indicative and thus non - limiting description of preferred but non - exclusive embodiments of a method and apparatus for measuring the ground resistance in a battery charging system as shown in the accompanying drawings, in which:

[0027] Figure 1 An electric vehicle battery charging facility implementing the method of the present invention is schematically shown.

[0028] Referring to the accompanying drawings, the reference numeral 100 generally identifies a facility for charging an electric vehicle battery, to which a battery charging system equipped with a device for measuring the ground resistance according to the present invention can be connected.

[0029] By way of example only, a method and a device within an on - vehicle charger device will be described hereinafter, however the description is not intended to limit the application and scope of the present invention.

[0030] A similar description can be provided for a ground charging system (e.g., like a pillar or a wall box), but since the measurement of the ground resistance is a more critical parameter within the vehicle, we preferably provide hereinafter a description of an application in which the present invention has more advantages.

[0031] It should be noted that the expression on - vehicle charger device is herein intended to generally define any charging system for a traction battery pack that is capable of connecting to an alternating - current power grid and converting it to direct current before supplying power to the battery.

[0032] To this end, the battery charger includes at least one housing (grounded), which is connected to a power grid connection socket and contains a converter unit configured to convert alternating current from the mains into direct current available for charging the battery pack.

[0033] The connection socket is thus configured to receive a phase line and / or a neutral line.

[0034] The method 1 for measuring the ground resistance, which is the subject of the present invention, involves injecting (i.e., generating) an alternating - current signal between a first neutral node and a second ground node.

[0035] The measurement signal is preferably generated by a current generator.

[0036] Note that the neutral node can alternatively be:

[0037] - A physical node in the power grid (i.e., a node directly connected to the neutral wire of the connection socket);

[0038] - A reconstructed virtual node having a potential corresponding to the ground potential and being in electrical continuity with the phase line.

[0039] Preferably, a power grid noise analysis is also performed before the generation or injection of the phase measurement signal.

[0040] This power grid noise analysis step includes detecting a reference signal representing the power grid voltage and identifying a free frequency range, wherein the voltage signal has a minimum or zero value (e.g., minimum or zero amplitude).

[0041] More precisely, a spectral analysis of the reference signal is performed, and the preset injection frequency of the first current signal is identified as the frequency having the lowest spectral content.

[0042] In a preferred embodiment, instead of performing a spectral analysis on the entire spectrum, a spectral analysis is only alternatively performed on the first low-frequency range or the second high-frequency range.

[0043] Advantageously, this accelerates and optimizes the analysis.

[0044] Preferably, at the start of the method, i.e., when the battery charging device is connected to the power grid, a preset frequency is selected within the second high-frequency range.

[0045] It should be noted that preferably, the first low-frequency range is between 0 Hz and 200 Hz, and more preferably between 5 Hz and 150 Hz.

[0046] The second high-frequency range is preferably between 200 Hz and 600 Hz, and more preferably between 250 Hz and 400 Hz.

[0047] Therefore, the method includes detecting a first voltage signal representing the potential difference between the first node and the second node and calculating the ground resistance value based on the first voltage signal.

[0048] These steps are known per se, have been patented by the applicant under Italian patent application 102020000031625, and are hereby cited as a reference.

[0049] Therefore, the phase line for measuring the ground resistance R_PE will not be further described.

[0050] According to the present invention, the method includes the step of injecting an information signal IS representing the calculated ground resistance value R_PE between the first neutral node and the second ground node.

[0051] In other words, according to the present invention, the method involves using the potential difference between the neutral and the ground wire to inject another signal into the ground wire, which can be read by other devices connected to the power grid.

[0052] Preferably, the information signal is a highly recognizable signal. The possible frequency range of this signal is preferably between 600 hz and 5 khz.

[0053] The amplitude of this information signal is preferably between 100 and 500 mV.

[0054] In this regard, the method preferably includes analyzing the power grid (actively or passively) before the first alternating current signal injection phase.

[0055] More specifically, preferably, the method includes detecting one or more information signals IS generated by an additional battery charging system connected to the power grid.

[0056] Advantageously, this makes it possible to detect whether other charging systems have detected the ground resistance.

[0057] If the number of detected information signals IS is at least equal to a preset limit value, the ground resistance value R_PE is determined based on the information content of the received information signal IS.

[0058] Alternatively, if fewer information signals IS than the preset limit value are detected, the ground resistance value is determined by means of the above steps (injecting the first current signal, detecting the first voltage signal, calculating the ground resistance, and injecting the information signal).

[0059] Preferably, in the case where the number of detected information signals IS is at least equal to the preset limit value, a step of prohibiting the injection of the first signal into the alternating current I_PEM is provided.

[0060] In a preferred embodiment, the preset limit value is between 2 and 10.

[0061] It should be noted that preferably, before the injection of the first alternating current signal I_PEM, there is a network noise analysis step within a selected frequency range.

[0062] This noise analysis step includes obtaining a reference signal representing the mains voltage, performing a spectral analysis of this reference signal, and identifying the injection frequency as the frequency with the lowest spectral content.

[0063] Advantageously, in this way, all devices connected to the power grid will continue to inject current signals into the power grid at different frequencies, thus avoiding interference in each other's measurements.

[0064] Therefore, for a limited number of battery charging devices below the limit value, the ground resistance measurement for each vehicle will be independently carried out by injecting the first alternating current signal at different frequencies.

[0065] For a large number of battery charging devices connected to the power grid, even significantly above the limit value, the measurement will instead be carried out using those already taken by the above devices, avoiding using independent signals for measuring the same quantity to congest the frequency band.

[0066] Preferably, the step of prohibiting the injection of the first signal into the alternating current I_PEM is only executed when the value of the ground resistance calculated by the additional charging system falls within a preset tolerance range.

[0067] Advantageously, this makes the system robust to interference because if one or more of the measured resistances fall outside the tolerance range, it allows individual devices to make independent measurements.

[0068] In a preferred embodiment, the information signal IS has a first contribution representing the value of the ground resistance R_PE and a second contribution representing the state of the battery charger 100.

[0069] Thus, the information signal IS can be a single signal containing a dual information contribution, or equivalently defined by a pair of signals traveling on different channels.

[0070] Herein, the "state of the battery charger 100" means the charging, discharging (vehicle-to-grid), or disconnection (vehicle charging) situation of the battery charger.

[0071] Preferably, the method includes being able to prohibit the injection of the first alternating current signal based on the first and second contributions of the information signal IS generated by an additional charging system connected to the power grid.

[0072] Advantageously, therefore, preferably following the master-slave logic, the activation of the generation of the first signal is given to the additional charger that previously prohibited this function, thereby stopping the injection through the charged battery charger.

[0073] Another object of the present invention is to provide a device for measuring the ground resistance of a battery charger 100, wherein the battery charger device includes a power grid connection socket G and a converter assembly configured to convert alternating current from the power grid into direct current available for charging a battery pack.

[0074] The measuring device includes a current generator having a variable amplitude and frequency, which is configured to generate an alternating current measurement signal between a first neutral node and a second ground node PE and inject it into the power grid, and the alternating current measurement signal corresponds to the first current signal I_PEM.

[0075] A voltage detection module is also provided, which is configured to detect a voltage signal representing the potential difference between the first N node and the second PE node.

[0076] The processing module 2 is also connected to a generator and a detection module and is configured to implement the method that is the subject of the present invention.

[0077] Specifically, the processing module 2 is configured to:

[0078] a) Activate the current generator so as to inject a measurement signal in the alternating current between the first neutral point N node and the second earthing PE node;

[0079] b) Receive a voltage signal from the detection module in response to the injection of the measurement signal;

[0080] c) Calculate the earthing resistance value based on the voltage signal.

[0081] According to the present invention, the processing module 2 (or another component to which it is connected) is configured to inject an information signal representing the calculated earthing resistance value R_PE between the first neutral node and the second earthing node.

[0082] Furthermore, preferably, the processing module 2 is configured to perform the following steps:

[0083] - Detect one or more information signals IS generated by an additional battery charging system connected to the power grid G before the current generator is activated;

[0084] - Count the number of detected information signals IS;

[0085] - Compare the number of detected information signals IS with a preset limit value; and:

[0086] o When the number of detected information signals IS is equal to the preset limit value, determine the earthing resistance R_PE value based on the information content of the received information signal IS;

[0087] o If fewer information signals IS than the preset limit value are detected, perform steps a) to c) and inject the information signal.

[0088] Furthermore, preferably, similar to the method, the processing module 2 is configured to prohibit the activation of the current generator when the number of detected information signals is equal to the preset limit value.

[0089] The present invention achieves its intended purpose and brings important advantages.

[0090] In fact, when the number of devices connected to the power grid increases, the use of the method of disabling the "active" measurement of the earthing resistance means that the measurement efficiency and accuracy are independent of the number of vehicles being recharged.

Claims

1. A method for measuring the ground resistance in a battery charging system connected to an electrical power grid (G), the method comprising the following steps in each measurement cycle: a) Injecting a first alternating current signal (I_PEM) between a first neutral node (N) and a second ground node (PE), the first current signal (I_PEM) having a preset duration and a preset injection frequency; b) Detecting a first voltage signal representing the potential difference between the first node and the second node; c) Calculating a ground resistance value (R_PE) based on the first voltage signal; Characterized in that the method comprises the following steps: d) Injecting an information signal (IS) representing the calculated ground resistance value (R_PE) between the first neutral node (N) and the second ground node (PE), wherein, before the step of injecting the first alternating current signal, the following steps are provided: - Detecting one or more information signals (IS) generated by another battery charging system connected to the grid; - If the number of the detected information signals (IS) is at least equal to a preset limit value, determining the ground resistance value (R_PE) based on the information content of the received information signals (IS); and - When the number of the detected information signals (IS) is less than the preset limit value, performing steps a) to d).

2. The method according to claim 1, comprising the following steps: When the number of the detected information signals (IS) is at least equal to the preset limit value, the injection of the first alternating current signal (I_PEM) is prohibited.

3. The method according to claim 1 or 2, comprising the following steps: Analyzing the grid noise in a frequency range selected before the injection of the first alternating current signal, the step of analyzing the noise comprising: - Detecting a reference signal representing the grid voltage; - Performing a spectral analysis of the reference signal; and - Identifying the injection frequency as the frequency having the lowest spectral content.

4. The method according to any one of the preceding claims, wherein, The step of prohibiting the injection of the first alternating current signal is only performed when the ground resistance value calculated by another charging system falls within a predetermined tolerance interval.

5. The method according to any one of the preceding claims, wherein, The information signal (IS) presents a first contribution representing the ground resistance value (R_PE) and a second contribution representing the state of the battery charging device (100); the step of prohibiting the injection of the first alternating current signal is performed according to the first contribution and the second contribution of the information signal (IS) generated by another charging system connected to the network.

6. A device for measuring the ground resistance for a battery charging device (100), wherein, The battery charging device (100) includes a socket for connecting to an electrical power grid (G) and a converter assembly (not shown), the converter assembly being configured to convert alternating current from the electrical power grid (G) into direct current that can be used to recharge a battery pack, and the measuring device includes: - A current generator having a variable amplitude and frequency, configured to generate an alternating current measurement signal and inject the alternating current measurement signal into the electrical power grid (G) between a first neutral node (N) and a second ground node (PE); - A voltage detection module, configured to detect a voltage signal representing the potential difference between the first node (N) and the second node (PE); and - A processing module (2), configured to: a) Activate the current generator so as to inject an alternating current measurement signal between the first neutral node (N) and the second ground node (PE); b) Receive a voltage signal from the detection module in response to the injection of the measurement signal; c) Calculate the ground resistance value (R_PE) based on the voltage signal; It is characterized in that the processing module (2) is configured to inject an information signal (IS) representing the calculated ground resistance value (R_PE) between the first neutral node and the second ground node; Wherein, the processing module (2) is configured to perform the following steps: - Before activating the current generator, detect one or more information signals (IS) generated by another battery charging system connected to the power grid; - Count the number of the detected information signals (IS); - Compare the number of the detected information signals (IS) with a preset limit value, and: o When the number of the detected information signals (IS) is equal to the preset limit value, determine the ground resistance value (R_PE) based on the information content of the received information signal (IS); o When the number of the detected information signals (IS) is less than the preset limit value, perform steps a) to c).

7. The apparatus according to claim 6, wherein, The processing module (2) is configured to prohibit the activation of the current generator when the number of the detected information signals (IS) is equal to the preset limit value.

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