Method and device for compensating leakage currents in an electrical power converter

Abstract

A method for compensating leakage currents in a protective conductor of an electrical power converter comprises determining a differential current depending on a phase conductor current in the phase c

Description

The invention relates to a method for reducing leakage currents in a protective conductor of an electrical power converter, for example a battery charging circuit, which, in addition to the protective conductor, comprises a neutral conductor and at least one phase conductor. A further subject-matter of the invention is a device to carry out a method of this type. The number of registrations of electric vehicles is increasing annually. The efficient charging of the traction battery represents a key technology for the further success of e-vehicles on the market. In this connection, increasing attention has recently been focused on concepts of transformerless on-board charging devices. For a given, substantial leakage capacitance of the battery and the connected high-voltage system, particularly in the case of transformerless charging devices, the occurrence of high leakage currents to ground and through the protective converter must be prevented. Particularly in battery charging circuits in which power converters without galvanic isolation, i.e. without an isolating transformer, are connected to a power supply, leakage currents caused by the power converter can occur in the protective conductor of the power supply. If the leakage current exceeds a predefined value, this normally results in the triggering of a residual current device (RCD) and the interruption of the current flow. The residual current device cannot distinguish between operationally related leakage currents of this type, i.e. caused, for example, by a power converter, and leakage currents which are caused by an insulation fault. Unwanted shutdowns therefore occur which restrict the availability of the power supply or the power converter. In this respect, it is known from the prior art to measure the charging currents within the charging electronics by means of a measuring transformer and to feed an inverted signal derived therefrom by means of a current source into at least one conductor of the respective phase or the corresponding neutral conductor so that the leakage current and the fed-in compensation current cancel one another and no overall leakage current occurs on the protective conductor. Figure 1 shows a corresponding schematic view of the characteristics of the leakage currents with ideal compensation. Charging electronics 212 are supplied with electrical energy from an AC voltage source 202 via a phase conductor L and a neutral conductor N. A measuring transformer 210 which measures the currents flowing into the charging electronics is arranged at the input of the charging electronics 212. A possible difference in the currents can be caused, for example, by capacitive coupling to ground. The differential current is supplied to a compensation circuit 208 which correspondingly impresses opposite-phase currents into the conductors so that only a residual current possibly remaining as a result of tolerances and other non-ideal characteristics of the components of the compensation circuit 208 or of the measuring transformer 210 reaches the differential current sensor of a guard circuit 206 and the input filter 204. Leakage currents are indicated by way of example by the arrows arranged next to the lines, wherein the dotted arrows designate residual leakage currents. The present invention seeks to propose an improved method and a corresponding device for compensating leakage currents. This may be achieved by the method indicated in claim 1 and the device indicated in claim 7. Advantageous embodiments and developments are indicated in the respective dependent claims. According to a first aspect of the invention, a method for compensating leakage currents in a protective conductor of an electrical power converter, in particular of a battery charging circuit, which, in addition to the protective conductor, comprises a neutral conductor and at least one phase conductor, comprises initially determining a differential current depending on a phase conductor current in the phase conductor(s) and a neutral conductor current in the neutral conductor, for example by means of a first differential current sensor. The method further comprises generating and feeding a compensation current into the phase conductor(s) and / or into the neutral conductor, by means of a first compensation circuit. The compensation current compensates a leakage current caused by the differential current. In the method according to an aspect of the invention, the leakage current is not measured directly in the protective conductor, but is instead determined indirectly by determining a differential current corresponding to the leakage current. This differential current is determined as the difference between the currents in the phase conductor and the neutral conductor. A compensation current is generated and fed into the phase conductor(s) and / or into the neutral conductor depending on the determined differential current. The measurement and the compensation of the leakage current can therefore be performed without a galvanic connection to the protective conductor, as a result of which a galvanic isolation in the corresponding compensation circuit, for example by means of a transformer, is not required. This means that the compensation circuit can be designed as compact and lightweight. As described above, a residual leakage current can remain despite the aforementioned compensation. According to an aspect of the invention, the method therefore further comprises capturing a signal representing a remaining residual leakage current by means of a second differential current sensor associated with a guard circuit. According to an aspect of the invention, this second differential current sensor is the current sensor present in a guard circuit that is always prescribed for safety reasons, for example an RCD circuit. The measured residual leakage current is used to generate a further compensation signal. A P, PI or PID controller can essentially be used to adjust the differential current measured by the second differential current sensor to zero. The invention exploits the realization that the remaining residual leakage current is small due to the compensation carried out in the first compensation circuit, so that the required dynamics of the additional compensation can be less than in the first compensation circuit. The stability of the overall system can further be more easily ensured due to the two-stage structure. The additional compensation can therefore equalize tolerances of the first compensation circuit, i.e. greater tolerances are permissible in the first compensation circuit, thereby simplifying its design and reducing costs. Since the differential current sensor is explicitly provided to measure DC leakage currents also, a direct connection of a controller to the differential current sensor is not possible, since a connection of this type would result in DC currents also being compensated in an impermissible manner. It must further be ensured that the function of a monitoring circuit for the protective conductor is not impaired. Monitoring circuits of this type evaluate a low-frequency test signal, for example an artificially generated leakage current in the range between 300 and 400 Hz, in order to monitor the state of the protective conductor. This test signal must not be compensated by the second compensation circuit. According to an aspect of the invention, the method therefore further comprises converting the signal representing the residual leakage current to the frequency domain, and generating a further compensation signal which is suitable for compensating at least parts of the residual leakage current in a frequency-selective manner. Dominant spectral components in the differential current can be identified through the conversion to the frequency domain, and the compensation current can be generated according to these spectral components. It has proven particularly advantageous if a frequency spectrum of the determined differential current is determined in a frequency range from 20 Hz to 300 kHz. A Fourier transform, for example, in particular a fast Fourier transform (FFT), can be used to determine the frequency spectrum. Alternatively, the frequency spectrum can be determined by means of a P-Burg algorithm or a trigonometric algorithm. The further compensation signal is generated in such a way that at least DC currents and currents having the frequency of the test signal are not taken into account or compensated. The further compensation signal is then converted once more to the time domain and is supplied to the first compensation circuit as a further control signal or to the second compensation circuit. A residual compensation current corresponding to the further compensation signal is then fed from the first or second compensation circuit into the phase conductor(s) and / or into the neutral conductor. The residual compensation current can be fed in, for example, by means of a digital-to-analog converter or by means of an amplifier, in particular by means of a rail-to-rail (R2R) amplifier or a class D amplifier. In one or more embodiments, the further compensation signal is generated in such a way that, in particular, the fundamental wave of the network frequency and low harmonics, for example the first to fifth harmonic, are taken into account by the controller. Different control parameters can be predefined separately for each frequency. The system can also activate further frequencies through its own analysis of the spectrum, so that the smallest possible leakage current is achieved. One advantageous embodiment provides that the determined differential current is converted by means of an analog-to-digital converter into a digital differential current. The provision of a digital differential current enables the digital processing of the determined differential current, for example in a microcontroller or digital filter. In this way, the installation space required for the compensation circuit can be further decreased and the susceptibility of the compensation circuit to interference can be reduced. According to one advantageous embodiment of the invention, the compensation current is generated depending on the determined differential current and a predefined phase shift. The amount of the compensation current is particularly preferably identical to the amount of the differential current. The predefined phase shift is preferably 180°. The compensation current is preferably fed via a capacitive coupling into the phase conductor and / or the neutral conductor so that a galvanic coupling to the protective conductor is not required. According to an alternative, preferred embodiment, the compensation current is fed via an inductive coupling into the phase conductor and / or the neutral conductor. According to a further, alternative preferred embodiment, the compensation current is fed via a galvanic coupling into the phase conductor and / or the neutral conductor. According to a second aspect of the invention, a device for reducing leakage currents in a protective conductor of a power supply of an electric power converter, for example a battery charging circuit, which, in addition to the protective conductor, has a neutral conductor and a phase conductor, comprises a first differential current sensor which measures a differential current depending on a phase conductor current in the phase conductor and a neutral conductor current in the neutral conductor. The device further has a first compensation circuit for feeding a compensation current into the phase conductor and / or into the neutral conductor, said compensation current compensating a leakage current caused by the differential current. The device according to an aspect of the invention is characterized in that it is configured to capture a signal representing a remaining residual leakage current, for example by means of a second differential current sensor associated with a guard circuit, and in that it is further configured to convert the signal representing the residual leakage current to the frequency domain. The device further comprises a controller which operates in the frequency domain and is configured to generate a compensation signal in a frequency-selective manner in such a way that at least DC current signals and signals having the frequency of a test signal are not taken into account. In one or more embodiments, the controller is parameterized in such a way that, in particular, the fundamental wave of the network frequency and low harmonics, for example the first to fifth harmonic, are taken into account by the controller. Different control parameters can be predefined separately for each frequency. Compensation signals supplied by the controller in the frequency domain are then converted into signals in the time domain and are supplied as a control signal to the first or to a second compensation circuit, for example via a digital-to-analog converter, to which a digital control signal in the time domain is supplied. Due to the frequency-selective control, the method according to an aspect of the invention enables a compensation - within certain limits - of the phase response of the sensor. As a result, either a lower-cost sensor with poorer measurement characteristics is used or, if a more precise and therefore more expensive sensor is retained, the performance of the compensation is improved. In one or more embodiments, the device comprises means for the capacitively coupled feeding of the compensation current into the phase conductor and / or the neutral conductor, so that a galvanic coupling to the protective conductor is not required. In one or more embodiments, the device comprises means for the inductively coupled feeding of the compensation current into the phase conductor and / or the neutral conductor. In one or more embodiments, the device comprises means for the galvanically coupled feeding of the compensation signal into the phase conductor and / or the neutral conductor. A further aspect of the invention relates to a charging device for charging an electrical energy store having a power supply and a device described above. According to one advantageous embodiment of the charging device, it is provided that said charging device is designed as a galvanically non-isolated charging device. A control device which implements at least parts of the method, in particular converting the signal representing the residual leakage current to the frequency domain, generating the compensation signal which is suitable for compensating for at least parts of the residual leakage current in a frequency-selective manner, and converting the compensation signal to the time domain, comprises a microprocessor, a volatile and non-volatile memory, and one or more measurement interfaces and / or data interfaces which are interconnected via one or more data lines or data buses for communication purposes. The non-volatile memory contains computer program instructions which, when they are executed by the microprocessor of the control device, configure said control device to carry out at least parts of the method according to the invention. A computer program product implementing the method according to an aspect of the invention contains commands which, when executed by a microprocessor of a control device, prompt said control device to execute one or more embodiments of the method according to the invention. The computer program product can be stored on a computer-readable medium or data carrier. The medium or the data carrier can be physically embodied, for example as a hard disk, CD, DVD, flash memory or the like, but the medium or data carrier can also comprise a modulated electrical, electromagnetic or optical signal which can be received by a computer by means a corresponding receiver and can be stored in the memory of the computer. Further details and advantages of the invention will be explained below on the basis of the exemplary embodiments shown in the figures, in which: Fig. 1 shows a schematic view of a device known from the prior art for reducing leakage currents in a protective conductor of a battery charging circuit, Fig. 2 shows a flow diagram of an exemplary embodiment of a method according to the invention for reducing leakage currents, Fig. 3 shows a schematic view of a first embodiment of an example of a device according to the invention for reducing leakage currents in a protective conductor of a battery charging circuit, Fig. 4 shows a schematic view of a second embodiment of an example of a device according to the invention for reducing leakage currents in a protective conductor of a battery charging circuit, Fig. 5 shows a block diagram of components of an example of a device according to the invention, and Fig. 6 shows an example of a block diagram of a control device of the device according to the invention configured to carry out at least parts of the method according to the invention. Identical or similar elements can be denoted with the same reference signs in the figures. Figure 1 has already been described above and will not therefore be explained again. Figure 2 shows a flow diagram of an exemplary embodiment of a method 100 according to the invention for reducing leakage currents. In step 102, a differential current is determined depending on a phase conductor current in a phase conductor L and on a neutral conductor current in an associated neutral conductor N in a first differential current sensor 210. In step 104, a corresponding compensation current is generated and fed in step 106 from a first compensation circuit 208 into the phase conductor L and / or into the neutral conductor N. In step 108, a remaining residual leakage current is measured by means of a second differential current sensor 206 associated with a guard circuit 206, and, in step 110, a signal representing the residual leakage current is converted to the frequency domain. In step 112, a compensation signal is generated which is suitable for compensating at least parts of the residual leakage current in a frequency-selective manner. In step 114, the compensation signal is converted to the time domain and, in step 116, is supplied to the first compensation circuit 208 or to a second compensation circuit 207 which feeds a residual compensation current corresponding to the compensation signal into the phase conductor L and / or into the neutral conductor N. Apart from technically related delays, steps 108 to 118 are carried out essentially simultaneously and in parallel with steps 102 to 106 and are executed in a continuously repeated manner. The delays can be compensated at least partially through corresponding adjustment of the compensation signals. Self-test steps which are carried out before the method described above is carried out and which can comprise function tests of the individual elements and a calibration are not shown in the figure. Figure 3 shows a schematic view of a first embodiment of an example of a device 200 according to the invention for reducing leakage currents in a protective conductor of a battery charging circuit. The elements shown in Figure 3 and their arrangement in relation to one another correspond to Figure 1. In contrast to the known arrangement, a signal captured by a differential current sensor of the guard circuit 206 and corresponding to a residual leakage current is supplied to the first compensation circuit 208. The first compensation circuit 208 can then also compensate a remaining residual leakage current which - ideally measured closer to the input filter 204 - possibly remains following the feed-in of the signals from the measuring transformer 210. The greatest possible compensation of leakage currents by the device 200 according to this embodiment of the invention is indicated by the currents that are not present between the two grounding points which are connected by means of the protective conductor not shown in the figure. Figure 4 shows a schematic view of a second embodiment of an example of a device 200 according to the invention for reducing leakage currents in a protective conductor of a battery charging circuit. The elements shown in Figure 4 and their arrangement in relation to one another correspond largely to Figure 1. In contrast to the known arrangement, a signal captured by a differential current sensor of the guard circuit 206 and corresponding to a residual leakage current is supplied to a second compensation circuit 207. This second compensation circuit 207 can correspond in terms of its function to the first compensation circuit, but can be designed with other components due to the possibly less stringent requirements for the dynamics of the residual compensation current that is to be fed in. As previously in Figure 3, the greatest possible compensation of leakage currents by the device 200 according to this embodiment of the invention is indicated in this figure by the currents that are not present between the two grounding points which are connected by means of the protective conductor not shown in the figure. Figure 5 shows a block diagram of components of an example of a device according to the invention. A differential current sensor of a guard circuit 206 supplies a signal which represents a differential current and which simultaneously represents a leakage current and which is supplied to a block 220 in order to convert the signal from the time domain to the frequency domain. The block 220 for converting the signal from the time domain to the frequency domain outputs a representation of the signal representing the differential current in the frequency domain, i.e. a spectral representation of the signal, which is supplied to one or more controllers 222a, 222b, 222c which output compensation signals at different frequencies. It should be noted that the one or more controllers can be implemented in a correspondingly programmed and parameterized microprocessor, and that the representation in the figure is to be understood only as an example. The signals of the one or more controllers are supplied to a block 224 in order to convert the signal from the frequency domain to the time domain. The output signal of said block 224, which can be present as a digital signal and which corresponds to a compensation signal for the residual leakage current, is supplied to a digital-to-analog converter 226 and is fed from a current source 228 into the phase conductor and / or into the neutral conductor. Figure 6 shows an example of a block diagram of a control device 300 of the device 200 according to an embodiment of the invention configured to carry out at least parts of the method according to an embodiment of the invention. The control device 300 comprises a microprocessor 302, a volatile memory 304 and a non-volatile memory 306, and also one or more measurement interfaces and / or data interfaces 308 which are interconnected via one or more data lines or data buses 310 for communication purposes. The current drain or charging of the electrochemical energy store can be measured via the one or more measurement interfaces. Information can be output to a user via the one or more data interfaces. The nonvolatile memory 306 contains computer program instructions which, when they are executed by the microprocessor 302 of the control device 300, configure said control device to carry out at least parts of the method according to embodiments of the invention.

Claims

1. A method for compensating leakage currents in a protective conductor of an electrical power converter which, in addition to the protective conductor, comprises a neutral conductor and at least one phase conductor, comprising:- determining a differential current depending on a phase conductor current in the at least one phase conductor and on a neutral conductor current in the neutral conductor in a first differential current sensor,- generating and feeding a compensation current into the at least one phase conductor and / or into the neutral conductor, said compensation current compensating a leakage current caused by the differential current, by means of a first compensation circuit,wherein the method further comprises:- capturing a signal representing a remaining residual leakage current by means of a second differential current sensor associated with a guard circuit,- converting the signal representing the residual leakage current to the frequency domain,- generating a compensation signal which is suitable for compensating at least parts of the residual leakage current in a frequency-selective manner,- converting the compensation signal to the time domain,- supplying the compensation signal converted to the time domain to the first compensation circuit or to a second compensation circuit, and- feeding a residual compensation current corresponding to the compensation signal into the at least one phase conductor and / or into the neutral conductor, by means of the first compensation circuit or a second compensation circuit.

2. The method as claimed in claim 1, wherein a frequency spectrum of the residual leakage current is determined.

3. The method as claimed in claim 2, wherein the residual compensation current is generated depending on the determined frequency spectrum and a predefined phase shift.

4. The method as claimed in any one of the preceding claims, wherein the compensation current and / or the residual compensation current is / are fed in via a capacitive coupling.

5. The method as claimed in any one of claims 1 to 3, wherein the compensation current and / or the residual compensation current is / are fed in via an inductive coupling.

6. The method as claimed in any one of claims 1 to 3, wherein the compensation current and / or the residual compensation current is / are fed in via a galvanic coupling.

7. A device for compensating leakage currents in a protective conductor of an electrical power converter which, in addition to the protective conductor, comprises a neutral conductor and at least one phase conductor, wherein the device has a determining device for determining a differential current depending on a phase conductor current in the at least one phase conductor and on a neutral conductor current in the neutral conductor, and a first compensation circuit for feeding a compensation current into the at least one phase conductor and / or into the neutral conductor, said compensation current compensating a leakage current caused by the differential current, wherein a differential current sensor of a guard circuit is configured to capture a signal representing a remaining residual leakage current, wherein the device comprises a circuit for converting the signal representing the residual leakage current to the frequency domain, one or more controllers for generating a compensation signal which is suitable for compensating at least parts of the residual leakage current in a frequency-selective manner, and a circuit for converting the compensation signal to the time domain, and wherein the device is configured to feed a residual compensation current corresponding to the compensation signal by means of the first compensation circuit or the second compensation circuit into the at least one phase conductor and / or into the neutral conductor.

8. A charging device for charging an electrical energy store, having a power supply and a device as claimed in claim 7.

9. The charging device as claimed in claim 8, wherein the charging device is designed as a galvanically non-isolated charging device.

10. A computer program product comprising commands which, when the program is executed by a microprocessor of a control device, prompt said control device to carry out at least parts of the method as claimed in one or more of claims 1 to 6.

11. A computer-readable medium, on which the computer program as claimed in claim 10 is stored.