CURRENT MEASURING CIRCUIT FOR A CONVERTER, CONVERTER CIRCUIT AND CONVERTER

Abstract

To enable early detection of AC insulation faults, or to permit easier, faster and more accurate measurement of small currents in inverters, a current measuring circuit for a converter is provided. The current measuring circuit has a positive supply rail, a negative supply rail, a reference potential rail and a current measuring device that contains a Y-capacitor and a current sensor connected in series therewith. The load current can be decoupled from the measurement current through the at least one Y-capacitor, so that at high load currents even small fault currents can be acquired reliably with the current sensor.

Claims

1. A current measuring circuit for a converter wherein the current measuring circuit comprises: a positive supply rail, a negative supply rail, a reference potential rail, and a current measuring device, wherein the current measuring device is connected to each supply rail and to the reference potential rail, and wherein the current measuring device is configured to measure an electrical fault current between a respective supply rail and the reference potential rail.

2. The current measuring circuit according to claim 1, wherein the current measuring device contains at least one current measuring arrangement that is connected either to the positive supply rail or to the negative supply rail and to the reference potential rail, and wherein the current measuring arrangement contains a current sensor and a measuring capacitor connected in series therewith.

3. The current measuring circuit according to claim 2, wherein the current sensor is connected to one of the supply rails and to the measuring capacitor, and wherein the measuring capacitor is connected to the current sensor and to the reference potential rail.

4. The current measuring circuit according to claim 2, wherein the current measuring device contains a comparator that is configured and connected to detect an overcurrent through the current measuring arrangement.

5. The current measuring circuit according to claim 2, wherein the current measuring device contains a circuit for measuring a current value that is connected to convert an analog fault current flowing through the measuring arrangement into a digital fault current signal that is indicative of a magnitude of the fault current.

6. The current measuring circuit according to claim 1, wherein a DC link capacitor is connected between the positive and the negative supply rails.

7. A converter circuit for a converter, wherein the converter circuit comprises: a current measuring circuit according to claim 1, and at least one switching device, wherein the switching device is connected between the supply rails and has an output rail.

8. The converter circuit according to claim 7, wherein a DC link capacitor is connected between the positive and the negative supply rails, and wherein the DC link capacitor is arranged closer to the switching device than any current measuring arrangement.

9. The converter circuit according to claim 7, wherein the converter is configured as an inverter.

10. A current measuring apparatus for a converter wherein the current measuring apparatus comprises a current measuring circuit according to claim 1.

11. A converter comprising a current measuring circuit according to claim 1.

12. A converter comprising a converter circuit according to claim 7.

13. A converter comprising a current measuring apparatus according to claim 9.

14. The converter according to claim 11, wherein the converter is configured as an inverter.

15. A current measuring method for ascertaining a fault current caused by an AC insulation fault in an electrical circuit connected to a converter, comprising: measuring a current flowing through a measuring capacitor connected between a supply rail of the converter and a reference potential rail of the converter, and assuming an AC insulation fault if a flow of current that exceeds a predefined threshold value is measured through the measuring capacitor during a switching process.

16. The current measuring method according to claim 15, further comprising: configuring the converter as an inverter, measuring the current flowing through the measuring capacitor during at least one of a positive half-wave or during a negative half-wave, assuming an AC insulation fault if, during a switching process, a flow of current is measured through the measuring capacitor that exceeds a predefined threshold value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Exemplary embodiments are explained in more detail below with reference to the appended schematic drawings. Here:

[0038] The FIGURE shows an exemplary embodiment of an electrical circuit with an AC fault.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] Reference will be made below to the FIGURE, which shows an exemplary embodiment of an electrical circuit 10. The electrical circuit 10 comprises a supplier 12 that comprises a voltage source 14. The voltage source 14 is preferably a DC voltage source. The supplier 12 makes a positive supply rail 16, a negative supply rail 18, and the reference potential rail 20 available. The reference potential rail 20 lies at a reference potential. The reference potential can be ground. The reference potential preferably lies in the middle between the potentials of the positive and negative supply rails 16, 18.

[0040] The supplier 12 can furthermore comprise an output capacitor 22. The output capacitor 22 is connected between the positive supply rail 16 and the negative supply rail 18.

[0041] The supplier 12 can comprise a capacitance 24 between its lines and a housing lying at reference potential. The capacitance 24 is modelled here as a capacitor. The capacitance 24 can—although it does not have to—be configured as a component of the supplier. The supplier 12 is only shown here to better explain the mode of operation, and can take a wide range of forms that are known per se.

[0042] The electrical circuit 10 comprises a converter 26. The converter 26 can be configured as an inverter. The converter 26 contains at least one phase branch 28 with a switching device 30. The switching device 30 comprises two power switches that can be operated in push-pull mode: a first power switch 31 and a second power switch 32. The switching device 30 further comprises an output rail 34. The output rail 34 is arranged between the two power switches 31, 32.

[0043] The converter 26 further comprises a DC link capacitor 36 that is connected between the positive supply rail 16 and the negative supply rail 18, and is preferably arranged close to or immediately adjacent to the switching device 30.

[0044] The converter 26 further comprises a current measuring circuit 38. The current measuring circuit 38 is connected to both the supply rails 16, 18 and to the reference potential rail 20. The current measuring circuit 38 comprises a current measuring device 39 that is configured to measure a current.

[0045] The current measuring device 39 comprises a first current measuring arrangement 40 and a second current measuring arrangement 42. The first current measuring circuit 40 is connected to the positive supply rail 16 and to the reference potential rail 20. The second current measuring device 42 is connected to the negative supply rail 18 and to the reference potential rail 20.

[0046] Each current measuring arrangement 40, 42 contains a current sensor 44 and a measuring capacitor 46 connected in series therewith. The current sensor 44 is connected to the respective supply rail 16, 18, while the measuring capacitor 46 is connected to the reference potential rail 20.

[0047] The measuring capacitors 46 of each current measuring arrangement 40, 42 are connected as Y-capacitors between the supply rails 16, 18 and the reference potential rail 20. The measuring capacitors 46 can, depending on the particular case, also be part of a set of interference suppression capacitors.

[0048] The current sensor 44 can contain a shunt resistor for the current measurement. The shunt resistor is connected in series with the measuring capacitor 46. The current sensor 44 can in addition contain a comparator connected to the shunt resistor for measuring an overcurrent in the respective current measuring arrangement 40, 42. The current sensor 44 can optionally contain an A/D converter for the exact quantification of the overcurrent.

[0049] It is assumed below that the electrical circuit 10 has an AC insulation fault 48 that allows unwanted current to flow, and is modelled here by a resistor 50. Other types of AC insulation fault 48 can, however, also be detected. The AC insulation fault 48 can, for example, occur in a supply line system at one or a plurality of terminal devices or in the terminal devices themselves. A fault current is detected as follows

[0050] When the power switch 31 is switched on, the positive pole of the voltage source 14 is pushed by the AC insulation fault 48 toward the reference potential, for example ground. When the power switch 32 is switched on, the negative pole of the voltage source 14 is pushed by the AC insulation fault 48 toward the reference potential, ground in this case.

[0051] This change to the supply voltage in comparison with the reference potential causes a flow of charge carriers through each measuring capacitor 46. This measuring current is then used to ascertain the AC insulation fault 48.

[0052] In contrast to the known apparatuses and methods, additional AC insulation faults that only lead to low fault currents can be recognized with the measures described herein. Early detection is thus possible. In addition, through the direct measurement of the current of the measuring capacitors, the measuring current can be decoupled from the actual load current, so that measurements can be taken more easily, quickly and accurately.

[0053] To enable early detection of AC insulation faults (48), or to permit easier, faster and more accurate measurement of small currents in inverters (26), a current measuring circuit (38) for a converter (26) is proposed. The current measuring circuit (38) has a positive supply rail (16), a negative supply rail (18), a reference potential rail (20) and a current measuring device (39 that contains a Y-capacitor (46) and a current sensor (44) connected in series therewith. The load current can be decoupled from the measurement current through the at least one Y-capacitor (46), so that even at high load currents even small fault currents can be acquired reliably with the current sensor (44).

[0054] While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.

LIST OF REFERENCE SIGNS

[0055] 10 Electrical circuit [0056] 12 Supplier [0057] 14 Voltage source [0058] 16 Positive supply rail [0059] 18 Negative supply rail [0060] 20 Reference potential rail [0061] 22 Output capacitor [0062] 24 Capacitance [0063] 26 Converter [0064] 28 Phase branch [0065] 30 Switching device [0066] 31 First power switch [0067] 32 Second power switch [0068] 34 Output rail [0069] 36 DC link capacitor [0070] 38 Current measuring circuit [0071] 39 Current measuring device [0072] 40 First current measuring arrangement [0073] 42 Second current measuring arrangement [0074] 44 Current sensor [0075] 46 Measuring capacitor [0076] 48 AC insulation fault [0077] 50 Resistor