Methods for locating grounding faults in IT systems

Abstract

This invention relates to a method for locating ground faults in an IT system. A method for locating ground faults in an IT system (1) having a power transmission network (5), rotating electric machines (7, 8), and converters (9, 10), wherein the converters (9, 10) have an inverter (17) connected to the electric machines (7, 8). In this method, the common-mode voltage characteristics of the common-mode voltage of the converters (9, 10) or the power transmission network (5) are detected, and the common-mode voltage characteristics are compared with the output voltage characteristics of the output voltage of the inverter (17). In the event of a ground fault in the IT system (1), based on the comparison between the common-mode voltage characteristics and the output voltage characteristics, it is inferred whether a machine area (21, 22) downstream of the converters (9, 10) in the IT system (1) is the cause of the ground fault, the machine area including the electric machines (7, 8) and the connecting lines (19, 20) between the electric machines (7, 8) and the converters (9, 10).

Description

[0001] This application is a divisional application of the PCT application No. PCT / EP2018 / 067617, filed on June 29, 2018, entitled "Ground fault localization in IT systems", the Chinese national phase of which has the application number 201880049940.6, the whole content of which is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application relates to a method for localizing a ground fault in an IT system having a power supply network, a rotating electrical machine and a converter, which is connected on the input side to the power supply network and on the output side to the rotating electrical machine. BACKGROUND

[0003] In IT systems for power supply (safety ground lead systems), in the event of a simple ground fault, the IT system is in principle allowed to continue operating. This is also valid in IT systems having rotating electrical machines, which are connected to the power supply network of the IT system via a converter, respectively, in the event of a ground fault occurring in the power supply network. What is raised in this case is only the fundamental frequency common mode voltage of all connected consumers.

[0004] However, if a ground fault occurs in the machine area of the IT system, which has rotating electrical machines and connection lines between the electrical machines and the associated converters, the entire IT system and all connected consumers are subjected to the output voltage of the converter at the pulse frequency. This can lead to an overload of the consumers on the IT system. In addition, the electromagnetic radiation of the IT system can be so great that it is not permitted and interferes with neighboring electrical equipment. This problem exists both in AC IT systems and in DC IT systems.

[0005] In general, a ground fault occurring in the machine area of the IT system is identified by an isolation monitor, which detects whether an impermissible common mode current is present. Alternatively, a ground fault can also be discovered by monitoring the network voltage of the IT system with respect to the ground potential and, when the network voltage contains a pulse frequency component, it is concluded that a ground fault has occurred in the machine area of the IT system. However, these methods do not determine in which machine area of the IT system the ground fault has occurred. Therefore, when a ground fault occurring in a machine area is identified, in general all consumers on the IT system are switched off.

[0006] EP 2439827 A2 discloses a method for identifying a ground fault in a power grid having a large impedance value, which has a rectifier, an inverter and a DC voltage intermediate circuit connected to the rectifier and the inverter. Here, the voltage between the potential of the DC voltage intermediate circuit and the ground potential is detected and used for identifying a ground fault.

[0007] US 2017 / 131340 A1 discloses a fault identification system for identifying a ground fault in a converter system. The fault identification system comprises a detection device having a coupling capacitor connected to a DC voltage intermediate circuit of the converter system in order to detect a time variable voltage which indicates a magnetically coupled ground fault.

[0008] WO 2017 / 032992 A1 discloses an energy distribution system having a plurality of converter systems. In order to determine a location of a ground fault in the energy distribution system, a frequency spectrum of a zero sequence system current of the converter system is evaluated.

[0009] US 2008 / 084215 A1 discloses a method for isolation monitoring in a converter application, wherein an isolation fault is determined by a comparison of a voltage measured in a test configuration or a variable derived therefrom with a reference value.

[0010] US 2013 / 170077 A1 discloses a method for operating a fault current protection switch for a frequency converter. Here, a signal for a common mode voltage is detected in a network line device through which the frequency converter receives a network current. In order to identify a fault current, the detected signal is compared with an expected signal which is derived from switching information about a switching process in the frequency converter.

[0011] DE 102010030079 A1 discloses a method and a device for monitoring an isolation impedance in an ungrounded electrical network having a DC voltage intermediate circuit and at least one inverter connected thereto for controlling an electrical consumer. Here, a voltage to be monitored is determined when the electrical consumer is operated, which voltage indicates a voltage fluctuation of a supply voltage potential of the DC voltage intermediate circuit with respect to a reference potential. In order to detect an isolation fault, a frequency spectrum amplitude of the voltage to be monitored is compared with a reference value. SUMMARY

[0012] It is a basic object of the present invention to provide an improved method for locating a ground fault in an IT system, which method in particular enables an identification of a machine area in which the ground fault occurs.

[0013] The method according to the application is used for locating a ground fault in an IT system having a power supply network, a rotating electrical machine and a converter, wherein the converter is connected on the input side to the power supply network and on the output side to the rotating electrical machine, and the converter has a DC voltage intermediate circuit and an inverter connected to the DC voltage intermediate circuit and to the electrical machine. In the method, a common-mode voltage characteristic of the common-mode voltage of the converter or of the power supply network is detected, the common-mode voltage characteristic is compared with an output voltage characteristic of the output voltage of the inverter, and in the event of a ground fault in the IT system, it is inferred from the comparison of the common-mode voltage characteristic with the output voltage characteristic whether a machine region of the IT system, which is downstream of the converter and which has the electrical machine and the connecting line between the electrical machine and the converter, is the cause of the ground fault.

[0014] The inverter is understood here and hereinafter always as an inverter having electronic switching units controlled by pulse width modulation.

[0015] The application makes use of the recognition that, in the event of a ground fault in a machine region of the IT system, the output voltage of the inverter connected to this machine region influences the common-mode voltage of the converter having this inverter and of the power supply network. Thus, by comparing the output voltage characteristic of the output voltage of the inverter with the respective common-mode voltage characteristic of the common-mode voltage of the converter or of the power supply network, it is determined whether a ground fault has occurred in the machine region connected to this inverter.

[0016] By monitoring in this way all the converters of the IT system, which are connected to the respective machine region, it is possible to determine in which one of these machine regions a ground fault has occurred. This enables, inter alia, the machine region in which a ground fault has occurred to be switched off selectively, so that the IT system can continue to operate when a ground fault occurs in one machine region. The method according to the application is therefore particularly advantageous for use in an IT system having a plurality of rotating electrical machines, which are connected to the power supply network of the IT system by means of a converter each. The expression that the IT system has a rotating electrical machine and a converter connected to it is therefore not to be understood as meaning that the IT system has only one rotating electrical machine and one converter connected to it, but rather means that the IT system has at least one rotating electrical machine, which is connected to the power supply network of the IT system by means of a converter.

[0017] The method according to the application can be implemented in particular with little outlay and at low cost, since the implementation of the method does not require components or devices located in the power path of the rotating electrical machine.

[0018] The first method of the application proposes to detect the voltage pulse of the voltage between the intermediate circuit potential of the DC voltage intermediate circuit of the converter and the ground potential or of the grid voltage of the power supply network relative to the ground potential as a common mode voltage characteristic, to use the switching operation of the phase of the inverter as an output voltage characteristic, and to deduce that the machine region is the cause of the ground fault when the voltage pulse and the switching operation occur simultaneously.

[0019] The method makes use of the insight that in the event of a ground fault in a machine region of the IT system, the voltage on the DC voltage intermediate circuit of the converter connected to the machine region and the grid voltage of the power supply network of the IT system, respectively relative to the ground potential, are impressed with the pulsed output voltage of the inverter of the converter. Thus, by the coincidence in time of the voltage pulse of the voltage on the DC voltage intermediate circuit or of the grid voltage and the switching operation of the phase of the inverter connected to the machine region, a ground fault in the machine region of the IT system can be identified.

[0020] One variant of the first method proposes to preset a threshold duration and to define the voltage pulse as occurring simultaneously with the switching operation when the time interval between the voltage pulse and the switching operation is less than the threshold duration. For example, a threshold duration of less than 10 μβ is preset.

[0021] This variant takes into account that the voltage pulse follows the switching operation with a slight time delay. Furthermore, measurement errors can also be compensated by the threshold duration when determining the temporal correlation of the voltage pulse and the switching operation.

[0022] The first method proposes to preset a count duration, for example of approximately 1 second, and a number threshold, for example of approximately 1000, and to count the number of simultaneous voltage pulses and switching operations within the count duration, and to deduce that the machine region is the cause of the ground fault when the count number exceeds the number threshold.

[0023] This variant of the first method takes into account that the switching operation is in the form of a pulse and that the voltage pulse caused by the switching operation is therefore also in the form of a pulse. A ground fault in the machine region thus causes a plurality of voltage pulses in short time intervals, which are temporally correlated with the switching operation of the inverter connected to the machine region. Thus, when the pulse pattern of the voltage pulses and the switching operations largely coincides, it can be deduced particularly reliably that the machine region is the cause of the ground fault. This coincidence is quantified by the number threshold of the simultaneous voltage pulses and switching operations within the count duration.

[0024] A further variant of the first method proposes to determine the switching operation of the phase of the inverter by measuring the output voltage of this phase relative to the intermediate circuit potential of the DC voltage intermediate circuit or from the control signal for controlling this phase.

[0025] In these solutions, the switching operation of the phase of the inverter is determined by measuring the output voltage of the phase or from a drive signal for driving the phase. Detecting the switching operation by measuring the output voltage enables a reliable determination of the actually performed switching operation. In contrast, determining the switching operation from the drive signal has the advantage that it is not necessary to measure the output voltage of the phase.

[0026] The second method of the application proposes that the spectrum of the network voltage of the power transmission network with respect to the ground potential is detected as a common-mode voltage characteristic, the output fundamental frequency of the output voltage of the inverter is used as an output voltage characteristic, and it is concluded that the machine area is the cause of the ground fault when the spectrum of the network voltage has a spectral portion that coincides with the output fundamental frequency. In the case of a multiphase power transmission network, the sum of all phase voltages or one phase voltage of the power transmission network with respect to the ground potential is used as the network voltage.

[0027] The second method makes use of the insight that in the case of a ground fault in the machine area of the IT system, the voltage fundamental frequency of the output voltage of the inverter is transmitted into the power transmission network of the IT grid, so that the phase voltage of the power transmission network with respect to the ground potential contains not only a spectral portion in the case of its operating frequency (for example 50 Hz or 60 Hz), but also a spectral portion in the case of the output fundamental frequency of the machine area with the ground fault. Thus, a ground fault in the machine area of the IT system can also be identified in that the spectrum of the network voltage of the power transmission network has a spectral portion that coincides with the output fundamental frequency of the output voltage of the inverter connected to the machine area.

[0028] The solution of the second method proposes that the rated fundamental frequency of the output voltage of the inverter is used as the output fundamental frequency or that the output fundamental frequency is measured.

[0029] The solution of measuring the output fundamental frequency enables a reliable determination of the output fundamental frequency. In contrast, the solution of using the rated fundamental frequency of the output voltage of the inverter as the output fundamental frequency has the advantage that it is not necessary to measure the output fundamental frequency.

[0030] Another embodiment of the second method proposes that the spectrum of the network voltage of the power transmission network with respect to the ground potential is detected as a common-mode voltage characteristic, the switching frequency of the output voltage of the inverter is used as an output voltage characteristic, and it is concluded that the machine area is the cause of the ground fault when the spectrum of the network voltage has a spectral portion that coincides with the switching frequency of the output voltage of the inverter.

[0031] In this embodiment, when the grid voltage spectrum includes both the spectrum of the output voltage at its rated base frequency and the spectrum of the converter at its switching frequency, it is concluded that the machine area is the cause of the ground fault. The advantage of this spectrum evaluation is that it can clearly identify the machine area as the cause of the ground fault even when the output voltages of two converters have the same rated base frequency but different switching frequencies, or the same switching frequency but different rated base frequencies.

[0032] Another implementation of the second method proposes that, when it is determined that the machine area is the cause of the grounding fault, the converter control unit isolates the drive and control of the converter, or the upper-level system control unit of the IT system isolates the converter.

[0033] These implementations achieve the following: when it is determined that the machine area is the cause of the ground fault, the machine area is disconnected by the control unit of the associated converter, or by blocking the associated converter through the upper-level system control unit of the IT system. Attached Figure Description

[0034] The above-described features, characteristics, and advantages of the present invention, as well as their implementation, become clearer and more readily understood in conjunction with the following description of embodiments, which are illustrated in conjunction with the accompanying drawings.

[0035] Figure 1 This is a diagram of an IT system. Detailed Implementation

[0036] The only accompanying drawing schematically shows an IT system 1, which has a power source 3, a power transmission network 5, and two rotating motors 7 and 8 connected to the power transmission network 5 via converters 9 and 10, respectively.

[0037] The power transmission network 5 of the IT system 1 shown in the attached diagram is a three-phase AC power grid. Each motor 7, 8 is, for example, a three-phase motor.

[0038] Each converter 9, 10 is connected to the power transmission network 5 on the input side and to the motors 7, 8 on the output side. Furthermore, each converter 9, 10 is implemented as a converter having a three-phase rectifier 11 on the input side, a DC voltage intermediate circuit 13 with intermediate circuit capacitors 15, and an inverter 17 on the output side. The three-phase rectifier 11 is connected to the power transmission network 5 and the DC voltage intermediate circuit 13. The inverter 17 is connected to the DC voltage intermediate circuit 13 and the corresponding motors 7, 8.

[0039] The first motor 7 is connected to the first converter 9 via the first connection line 19. The first motor 7 and the first connection line 19 define the first machine area 21 of the IT system 1.

[0040] The second motor 8 is connected to the second converter 10 via the second connection line 20. The second motor 8 and the second connection line 20 define the second machine area 22 of the IT system 1.

[0041] The method according to the invention is used to identify grounding faults in machine areas 21 and 22, and to detect which machine area 21 or 22 the grounding fault occurs in, so as to selectively disconnect that machine area 21 or 22, and thus allow the remaining IT system 1 to continue operating. In the accompanying drawings, a grounding fault in the second machine area 22 is indicated by a lightning bolt symbol.

[0042] A first embodiment of the first method proposes that, for each converter 9, 10, a voltage pulse is detected between the intermediate circuit potential of the DC voltage intermediate circuit 13 and the ground potential, and the voltage pulse is compared with the switching operation of a phase of the inverter 17 of the corresponding converter 9, 10. When a voltage pulse detected for one of the converters 9, 10 occurs simultaneously with a switching operation, it is deduced that the machine area 21, 22 connected to that converter 9, 10 is the cause of a ground fault.

[0043] Here, for example, a threshold duration is preset, and when the time interval between the voltage pulse and the switching operation is less than this threshold duration, the voltage pulse is defined as occurring simultaneously with the switching operation. For example, a threshold duration of less than 10 μs is preset.

[0044] In addition, a preset counting duration (e.g., approximately one second) and a number threshold (e.g., approximately 1000) are used, and the evaluation count of simultaneous voltage pulses and switching operations is detected within the counting duration. When the evaluation count exceeds the number threshold for the converters 9 and 10 connected to machine areas 21 and 22, it is inferred that machine areas 21 and 22 are the cause of a ground fault.

[0045] For example, the switching operation of that phase of the inverter 17 of the converters 9 and 10 can be determined by measuring the output voltage of the intermediate circuit potential of the DC voltage intermediate circuit of such a phase relative to the converter 9, 10, or derived from the drive signal used to drive that phase.

[0046] The main difference between the second embodiment of the first method and the first embodiment is that, instead of detecting voltage pulses on the DC voltage intermediate circuit 13 of converters 9 and 10, voltage pulses of the grid voltage relative to the ground potential are detected and compared with the switching operation of a phase of inverter 17 of each converter 9 and 10. When a voltage pulse detected for one of converters 9 and 10 occurs simultaneously with a switching operation, it is deduced that the machine areas 21 and 22 connected to that converter 9 or 10 are the cause of a ground fault. The design scheme of this embodiment corresponds to the design scheme of the first embodiment in terms of defining the simultaneous occurrence of voltage pulses and switching operations and determining the switching operation of a phase of inverter 17.

[0047] In one embodiment of the second method, the spectrum of the grid voltage relative to the ground potential of the transmission network is detected and compared with the output fundamental frequency and / or switching frequency of the output voltage of the inverter 17 of each converter 9, 10. When the output voltages of the inverter 17 of converters 9, 10 have different output fundamental frequencies, and the detected spectrum has a spectral portion consistent with the output fundamental frequency of the output voltage of the inverter 17 of converters 9, 10 connected to machine areas 21, 22, it is concluded that machine area 21, 22 is the cause of a ground fault. When the output voltages of the inverter 17 of converters 9, 10 have the same output fundamental frequency but different switching frequencies, and the detected spectrum has a spectral portion consistent with the switching frequency of the output voltage of the inverter 17 of converters 9, 10 connected to machine areas 21, 22, it is inferred that machine area 21, 22 is the cause of a ground fault.

[0048] In the case of the multiphase transmission network 5 shown in the figure, for example, the sum of all phase voltages of the transmission network 5 relative to the ground potential or a single phase voltage can be used as the grid voltage.

[0049] For example, the rated base frequency of the output voltage of inverter 17 can be used as the output base frequency of the output voltage of inverter 17, or the output base frequency can be measured.

[0050] In all the above embodiments, for example, the control units 23 and 24 of the converters 9 and 10 connected to the machine areas 21 and 22 that are identified as having grounding faults are disconnected by means of the following: the control units 23 and 24 of the converters 9 and 10 connected to the machine areas 21 and 22 block the drive and control of the converters 9 and 10, or the upper-level system control unit 25 of the IT system 1 blocks the converters 9 and 10 connected to the machine areas 21 and 22.

[0051] All the above embodiments of the method can also be similarly applied to the IT system 1 of the power transmission network 5 being a DC power grid.

[0052] Although the details of the invention have been set forth and described in detail through preferred embodiments, the invention is not limited to the disclosed examples, and those skilled in the art can derive other variations therefrom without departing from the scope of protection of the invention.

Claims

1. A method for locating a ground fault in an IT system (1) having a power transmission network (5), rotating electrical machines (7, 8) and a power converter (9, 10), wherein, The converter (9, 10) is connected on the input side to the power supply network (5) and on the output side to a rotating electrical machine (7, 8), and has a DC voltage intermediate circuit (13) and an inverter (17) connected to the DC voltage intermediate circuit (13) and to the rotating electrical machine (7, 8), characterized in that - the spectrum of the network voltage of the power supply network (5) relative to the ground potential is detected, - and in the event of a ground fault in the IT system (1), it is concluded that the machine region (21, 22) of the IT system (1) downstream of the converter (9, 10) is the cause of the ground fault when the spectrum of the network voltage has a spectral portion that coincides with the output fundamental frequency of the output voltage of the inverter (17), the machine region having the rotating electrical machine (7, 8) and the connecting line (19, 20) between the rotating electrical machine (7, 8) and the converter (9, 10).

2. The method of claim 1, wherein, In the case of a multiphase power supply network (5), the sum of all phase voltages or one phase voltage of the power supply network (5) relative to the ground potential is used as the network voltage.

3. The method according to claim 1 or 2, characterized in that, The rated fundamental frequency of the output voltage of the inverter (17) is used as the output fundamental frequency.

4. The method according to claim 1 or 2, characterized in that, The output fundamental frequency is measured.

5. The method according to claim 1 or 2, characterized in that, It is concluded that the machine region (21, 22) is the cause of the ground fault when the spectrum of the network voltage has a spectral portion that coincides with the switching frequency of the output voltage of the inverter (17).

6. The method of claim 1 or 2, wherein, When it is concluded that the machine region (21, 22) is the cause of the ground fault, a control unit (23, 24) of the converter (9, 10) blocks the driving of the converter (9, 10).

7. The method according to claim 1 or 2, characterized in that, When it is concluded that the machine region (21, 22) is the cause of the ground fault, a superordinate system control unit (25) of the IT system (1) blocks the converter (9, 10).

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