Patent Application
20240353465
This application claims the benefit of European Patent Application No. EP 23169015.7, filed on Apr. 20, 2023, which is hereby incorporated by reference in its entirety.
The present embodiments relate to an arrangement having an insulation monitor for an IT system and a discharge resistor for discharging a ground discharge capacitance of the IT system.
Electrical networks without a galvanic connection between active conductors and grounded parts are referred to as IT networks or IT systems (French: “Isolé Terre” [isolated ground]) or also as an isolated network. In IT networks, insulation monitors are used to increase installation availability. For this purpose, a maximum effective capacitance or (ground) discharge capacitance and a discharge resistance may be set in insulation monitors.
IT systems are used in mostly small-scale industrial networks and in hospitals. In these networks, the neutral point is not grounded. Therefore, a simple ground fault does not immediately lead to a network failure and may be rectified during operation if necessary. The insulation monitoring device then indicates the fault caused by a reactive current at the location of the fault. The magnitude of the reactive current depends on the ground discharge capacitance of the IT network. During operation of converters and power supplies in the IT network, the capacitances and discharge currents acting with respect to ground (PE) may be low.
DE722348C discloses, for example, an insulation monitoring and ground fault indicator device for AC systems, in which at least one of the phases and a neutral point are connected in a common point and grounded via resistors of any phase, but of approximately the same order of magnitude as the absolute values of the normal complex discharge resistance of the installation to be monitored with respect to ground. The current flowing in the common ground line is used to detect ground faults or, if the network is sound, as an approximate measure of the complex discharge resistance of the installation with respect to ground.
A more contemporary variant exists, for example, with the Bender Isometer 685 insulation monitoring device (https://www.bender.de/fileadmin/content/Products/m/d/iso685-x-P_D00170_M_XXDE.pdf). By way of example, the device is capable of monitoring capacitances up to 1000 μF and insulation resistances >10kΩ.
It is common for devices that are intended for both the TN network (e.g., grounding at the power source and the electrical operating means (TN-C system, TN-C-S system, TN-S system)) and the IT network to disconnect a portion of the filter capacitors acting as protective earth (PE) during operation in the IT network. This reduces the capacitance acting as protective earth PE, but still allows basic interference suppression of the device. High-impedance resistors are connected in parallel via the capacitances acting as protective earth PE. High-impedance resistors are used for the defined discharge of the specified capacitances. In order to avoid dangerous voltages for maintenance personnel, a defined discharge (e.g., less than 60 V) is to be provided. In 24 V networks, there are also capacitances and discharge resistors as PE. In this case, these may be used for the defined discharge of the capacitances after a surge pulse.
The scope of the present invention is defined solely by the appended claims and is not affected to any degree by the statements within this summary.
It is a challenge, for example, from the point of view of an installation operator, to achieve as large resistance values as possible and as small capacitances as possible. Further, it is often desirable to monitor as many power supplies and devices as possible using an insulation monitor so that comparatively fewer insulation monitors are sufficient for monitoring the insulation of the entire installation.
The present embodiments may obviate one or more of the drawbacks or limitations in the related art. For example, arrangements with an insulation monitor configured such that further optimization is achieved with regard to the aforementioned challenge are provided.
Specifically, the arrangement includes two Z diodes that are arranged back-to-back and connected in series with a discharge resistor. The two Z diodes each have a reverse voltage above a test voltage of the insulation monitor. According to the present embodiments, the discharge resistor is replaced by a series circuit that is composed of Z diodes that are connected back-to-back, and the discharge resistor. The defined discharging of the capacitors to a voltage below 60 V or after surge pulses may still be provided. The discharge resistance is to be adjusted where necessary. The fundamental consideration that the resistance for the insulation monitor is hidden as a result of the connection of Z diodes of the present embodiments, which are arranged back-to-back, in series with the discharge resistor is provided. During a test cycle, the insulation monitor generates a test voltage having a polarity that is periodically reversed. This test voltage may be approximately 10 V. In terms of the reverse voltage, the Z diodes are selected to be above this voltage value so that the desired effect of hiding the discharge resistor is provided, for example.
Specifically, the arrangement may have two Z diodes that are arranged back-to-back and connected in series with the discharge resistor. The two Z diodes each have a reverse voltage above a test voltage of the insulation monitor.
One development of the present embodiments makes provision for an interference suppression capacitor to be arranged in parallel with the series circuit that is composed of the discharge resistor and the two Z diodes that are arranged back-to-back.
Such interference suppression capacitors are used for the basic interference suppression of an IT system in order to divert higher-frequency interference components from the system. This diverting is well suited for the parallel arrangement of the series circuit of the present embodiments consisting of the discharge resistor and the two Z diodes that are arranged back-to-back, such that the entire ground discharge capacitance may be discharged by the series circuit up to the reverse voltage of the Z diodes.
One development of the present embodiments makes provision for an additional discharge resistor to be arranged in parallel with the two Z diodes that are arranged back-to-back, and in series with the discharge resistor. The additional discharge resistor has a higher resistance value than the discharge resistor that is arranged in series with the two Z diodes that are arranged back-to-back.
The advantage gained by the present embodiments in this development is that the insulation monitor ISG does not “see” the lower-impedance discharge resistor in the series arrangement to the pair of Z diodes that are arranged back-to-back, but sees only the comparatively higher-impedance additional discharge resistor. In this way, a virtually gradual discharge occurs such that voltages above the reverse voltage of the Z diodes are discharged more quickly and voltages below the reverse voltage are discharged more slowly.
Functionally same components are sometimes provided with same reference signs. Functionally same components are sometimes not identified in all figures and are also not explained separately again for each individual figure. It may be assumed, in principle, that these components in the different illustrations each have a substantially same function.
The embodiments shown schematically as a circuit diagram in each of
The IT system IST is supplied with power using alternating current (AC phases L1, L2, L3). A common-mode choke CMC is used to limit the current in the IT system IST. The individual AC phases are each connected to a filter capacitor FTC in order to divert undesired frequencies to ground. This connection is interrupted when the arrangement ARG is operated in accordance with the present embodiments in the IT network. The alternating current is subsequently rectified by a B6 rectifier BSE. DC link capacitors CZK enable the removal of unwanted frequencies after rectification.
A load that is connected, for example, as a motor MTR with a shielded motor cable SCC to a circuit breaker IVT is connected at the output of the arrangement ARG.
The circuit diagram shows ground discharge capacitances TCP. These ground discharge capacitances TCP illustrate in this case the capacitance of the IT system IST with respect to ground PE and may also include interference suppression capacitors SPC. For the safe discharge of the ground discharge capacitances TCP, a discharge resistor DCR is provided on each of the DC lines (e.g., in parallel with the ground discharge capacitance TCP). In order that the insulation monitor ISG does not concomitantly measure these discharge resistors DCR during a test run with a test voltage TSV with alternating polarity, it is provided according to the present embodiments that in each case two Z diodes ZDI that are arranged back-to-back are connected in series SCT with the discharge resistor DCR, where the Z diodes ZDI each have a reverse voltage RBV above a test voltage TSV of the insulation monitor ISG.
The arrangement ARG, which is illustrated in simplified form in
For each component of the converter CVT, a respective ground discharge capacitance TCP is provided with a parallel-connected discharge resistor DCR. According to the present embodiments, in each case, two Z diodes ZDI that are arranged back-to-back are connected in series SCT with the discharge resistor DCR. The Z diodes ZDI each have a reverse voltage RBV above a test voltage TSV of the insulation monitor ISG. To avoid undetermined potentials between the series circuit SCT of the pair of Z diodes and the discharge resistor DCR, an additional discharge resistor DRA is arranged in series with the discharge resistor DCR. The additional discharge resistor DRA has a higher resistance value than the discharge resistor DCR arranged in series with the two Z diodes ZDI that are arranged back-to-back. The insulation monitor as a result only sees the higher resistance value of the additional discharge resistor DRA in each case. As a result of this, a rapid discharge is provided by the lower-resistance discharge resistor DCR.
The respective discharge resistor may be adjusted as required. According to the present embodiments, the resistance is “blanked out” for the insulation monitor ISG, or the resistance measured by the insulation monitor ISG is significantly increased compared to the actual discharge resistance. The adjustable measurement voltages of the insulation monitor ISG are typically between 10 V-60 V depending on the mode. This test voltage TSV is applied during the measurement with alternating polarity. The following table shows the measurement results with different combinations in DC power mode (e.g., measurement voltage of the Bender Isometer 685=50 V, configuration according to
The measurement results show that the resistance measured using the insulation monitor increases significantly through the use of the Z diodes.
Irrespective of the grammatical gender of a specific term, persons with male, female, or other gender identity are also included in this document.
The elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present invention. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims may, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent. Such new combinations are to be understood as forming a part of the present specification.
While the present invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.
| Number | Date | Country | Kind |
|---|---|---|---|
| 23169015.7 | Apr 2023 | EP | regional |
