Frequency converter

Abstract

Frequency converter (100), comprising: - a DC intermediate circuit (1), wherein the DC intermediate circuit (1) has a positive intermediate circuit branch (2) at which a positive intermediate circuit potential (ZK+) is present during operation of the frequency converter (100), and a negative intermediate circuit branch (3) at which a negative intermediate circuit potential (ZK-) is present during operation of the frequency converter (100), and - an inverter (4), wherein the inverter is fed from the DC voltage intermediate circuit (1), characterized in that the frequency converter (100) further comprises - a differentiating element (5) which is looped between the positive intermediate circuit branch (2) or the negative intermediate circuit branch (3) and protective earth (PE), and - an earth fault detection unit (6) which is designed to evaluate a test voltage (UP) generated by means of the differentiating element (5) in order to detect an earth fault (33).

Description

EP 3 485 549 B1 discloses a frequency converter according to the preamble of claim 1.US 2008 / 0 084 215 A1 discloses a circuit and a method for insulation monitoring of inverters.The invention is based on the object of providing a frequency converter which enables the most reliable possible ground fault detection.The frequency converter according to the invention has a conventional DC voltage intermediate circuit, wherein the DC voltage intermediate circuit has, as usual, a positive intermediate circuit branch at which a positive intermediate circuit potential is present during operation of the frequency converter, and a negative intermediate circuit branch at which a negative intermediate circuit potential is present during operation of the frequency converter. The difference between the positive intermediate circuit potential and the negative intermediate circuit potential is usually referred to as the intermediate circuit voltage. The intermediate circuit voltage is typically a DC voltage with a level between 400 V and 1000 V, for example.The frequency converter according to the invention further comprises a conventional inverter which is fed from the DC voltage intermediate circuit. The inverter can be designed to generate one or more approximately sinusoidal alternating voltages from the intermediate circuit voltage by means of pulse width modulation.The frequency converter according to the invention further comprises an electrical component in the form of a differentiating element which is looped in between the positive intermediate circuit branch or the negative intermediate circuit branch and (protective) ground (protective earth, PE).The frequency converter according to the invention further comprises a ground fault detection unit, for example in the form of a microprocessor and a suitable periphery, which is designed to evaluate a test voltage generated by means of the differentiating element for detecting a ground fault. A ground fault is in particular a low-impedance galvanic connection between one or more inverter outputs and protective ground. The ground fault can also be referred to as phase ground fault. In this respect, reference is also made to the relevant technical literature.In one embodiment, the ground fault detection unit is designed to detect a ground fault if an amount of the test voltage and / or a time derivative of the test voltage exceeds a threshold value. The threshold value can be determined in this case by calculation and / or empirical and / or measurement and can depend, for example, on properties of the differentiating element and / or a voltage level used, etc.In one embodiment, the ground fault detection unit is designed to measure a potential difference between the positive intermediate circuit potential and the negative intermediate circuit potential and to set the threshold value as a function of the measured potential difference. For example, the threshold value can be set in such a way that there is a linear relationship between the threshold value and the measured potential difference. The threshold value can be, for example, a fraction, in particular half, of the measured potential difference.In one embodiment, the ground fault detection unit is designed to compare or compare a temporal profile of the test voltage with a temporal profile of switching states of the inverter and to deactivate the inverter only if there is a temporal dependence between the profile of the switching states of the inverter and the test voltage. The temporal profile of the switching states of the inverter corresponds here to a pulse width modulation (PWM) carried out by means of the inverter, which is used to generate the phase voltages. In this respect, reference is also made to the relevant technical literature.In one embodiment, the differentiating element has an, idealised, infinitely high resistance for a DC voltage. In other words, the differentiating element does not allow a (measurable or relevant) current flow for a DC voltage.In one embodiment, the differentiating element has a capacitor and a resistance voltage divider, wherein the capacitor and the resistance voltage divider are connected in series between the positive intermediate circuit branch or the negative intermediate circuit branch and the protective ground, and the test voltage is present at a center tap of the resistance voltage divider.In one embodiment, the frequency converter further comprises: a first Y capacitor, which is looped in between the positive intermediate circuit branch and protective earth, and a second Y capacitor, which is looped in between the negative intermediate circuit branch and protective earth.The invention will be described in detail below with reference to the drawings. The following shows: FIG. 1 shows a simplified schematic circuit diagram of a frequency converter having a differentiating element and a ground fault detection unit which is designed to evaluate a test voltage generated by means of the differentiating element for detecting a ground fault, and FIG. 2 shows a circuit diagram of a differentiating element and a ground fault detection unit according to a further embodiment.FIG. 1 shows a simplified schematic circuit diagram of a frequency converter 100 with a conventional DC voltage intermediate circuit 1, wherein the DC voltage intermediate circuit 1 has a positive intermediate circuit branch 2 at which a positive intermediate circuit potential ZK+ is present during operation of the frequency converter 100, and a negative intermediate circuit branch 3 at which a negative intermediate circuit potential ZK- is present during operation of the frequency converter 100.The frequency converter 100 further comprises a conventional inverter 4, wherein the inverter 4 is fed from the DC voltage intermediate circuit 1. The inverter 4 has, for example, conventionally three half-bridge branches for three associated phases, wherein for reasons of clarity only a single half-bridge branch of an associated phase with switching means 13 and 14 connected in series is shown. In this respect, reference is also made to the relevant technical literature.FIG. 1 shows, by way of example, a (phase) ground fault between an output or a phase of the inverter branch 13, 14 and protective earth PE.The frequency converter 100 further comprises a differentiating element 5, which is looped in between the positive intermediate circuit branch 2 and the protective earth PE. An electrically conductive housing or chassis of the frequency converter 100 is likewise electrically connected to ground protection PE.The frequency converter 100 further comprises a ground fault detection unit 6 with a sigma-delta converter 15, a comparator 16 and an evaluation unit 17, wherein the ground fault detection unit 6 is configured to evaluate a test voltage UP generated by means of the differentiating element 5 for detecting a ground fault. For reasons of simpler illustration, the sigma-delta converter 15 is shown outside the ground fault detection unit 6, but is part of the ground fault detection unit 6.The ground fault detection unit 6 is designed to detect a ground fault depending on whether an amount of the test voltage UP exceeds a threshold value or a threshold voltage US, which is detected by means of the comparator 16.The sigma-delta converter 15 of the ground fault detection unit 6 is designed to measure a potential difference between the positive intermediate circuit potential ZK+ and the negative intermediate circuit potential ZK-, i.e. to measure the intermediate circuit voltage. The ground fault detection unit 6 adjusts the threshold value or the threshold voltage US as a function of the measured potential difference. The threshold value or the threshold voltage UScan correspond, for example, to half the measured intermediate circuit voltage.The ground fault detection unit 6 or its evaluation unit 17 compares a temporal profile of the test voltage UP or an output voltage UK of the comparator 16 derived therefrom with a temporal profile of switching states of the inverter 4 and deactivates the inverter 4 if there is a temporal dependence between the profile of the switching states of the inverter 4 and the test voltage UP or the output voltage UK of the comparator 16. For example, in the case of a potential jump of the test voltage UP by more than half of the intermediate circuit voltage at the time of a switching state change of the inverter 4, the inverter 4 can be deactivated.The differentiating element 5 has a capacitor 7 and a resistance voltage divider 8, 9, wherein the capacitor 7 and the resistance voltage divider 8, 9 are connected in series between the positive intermediate circuit branch 2 and the protective ground PE and the test voltage UP is present at a center tap 10 of the resistance voltage divider 8, 9. The differentiating element 5 has, in an idealised manner, an infinitely high resistance or an infinitely high impedance for a DC voltage.The frequency converter 100 further comprises a first Y capacitor 11 looped in between the positive intermediate circuit branch 2 and the protective earth PE, and a second Y capacitor 12 looped in between the negative intermediate circuit branch 3 and the protective earth PE.FIG. 2 shows a circuit diagram of a differentiating element 5 and a ground fault detection unit 6 according to a further embodiment. In the embodiment shown in FIG. 2, the differentiating element 5 is looped in between the negative intermediate circuit branch 3 and the protective earth PE. The inverter 4 is not shown in FIG. 4, but corresponds to the inverter 4 shown in FIG. 1.The embodiment shown in FIG. 2 makes it possible to detect the capacitive ground fault current through both switching means 13 and 14 of the respective inverter branch.In order to be able to detect both directions of the ground fault current, first the measured signal UP must be raised by half the supply voltage UV, here by way of example by 2.5 V. This can be achieved with a voltage divider 18, 19, back-up capacitors 20 and 21, and a buffer capacitor 32. The voltage divider 18, 19 is fed from a voltage supply 34 which generates the supply voltage UV.The desired common error signal is realized via two comparators 22 and 23 with open collector outputs in the form of an OR combination. A pull-up resistor is divided 24, 25 and at the same time the current limitation for an optocoupler transmitting diode 26 which transmits a corresponding signal in a potential-isolated manner to a galvanically isolated side of a controller, which is not shown, when a ground fault is detected.The thresholds High-Ref and Low-Ref for the comparators 22, 23 can be generated here, for example, independently of the intermediate circuit voltage, via a voltage divider 27, 28, 29 buffered by associated buffer capacitors 30 and 31. Low-Ref is, for example, 2 V and high-Ref 3 V. If the thresholds high-Ref and low-Ref are intended to be parameterizable, then a PWM signal could be transmitted by means of optocouplers, an analog voltage could be generated from this by means of the RC element, and these could then be applied once positively and once negatively to the offset voltage of 2.5 V. At least 2 operational amplifiers would again be required for this purpose.If a (phase) ground fault occurs in frequency converters, this leads to the destruction of the inverter after a relatively short time, depending on the intermediate circuit voltage and PWM frequency of the inverter, if the inverter is not deactivated or switched off in good time.Conventionally, for example, for detecting a ground fault, intermediate circuit potentials are measured with respect to PE and the intermediate circuit voltage is calculated therefrom. Each measuring chain has, for example, a resistance of 20 MOh to PE, so that an impedance of 10 MOh to PE or (protection) ground arises as an effective parallel circuit.If this resistance is to be increased to at least 25 MOh, for example, on account of specific requirements, the intermediate circuit voltage would have to be implemented as a galvanically isolated voltage measurement. However, this lacks the PE reference and a phase ground fault would in principle no longer be detectable. However, this phase-ground fault leads, in the case of large Cy capacitances, to device failure after a few seconds when switching is repeated, as a result of overloading of the respectively loaded switching means of the bridge branch.The invention is based on the following considerations.Characteristic of a ground fault of an output phase are sudden changes in the intermediate circuit potentials ZK+ and ZK- with respect to PE in the cycle of the switching state changes of the corresponding inverter branch, i.e. with the inverter PWM frequency.In the case of a ground fault of an output phase, the intermediate circuit potentials change with respect to PE in a step-like manner with the PWM frequency and the step height corresponds to the intermediate circuit voltage. The step height can thus be measured by means of the differentiating element 5 and compared, for example, against half the intermediate circuit voltage as a threshold value or threshold voltage US or switch-off criterion. Since this is a pure AC measurement, this type of earth fault measurement is not detrimental to a required high DC impedance.According to the invention, the differentiating element 5 or its components 7, 8, 9 differentiates the change of the intermediate circuit potential from PE. By means of an optional capacitor 32 (see FIG. 2 ) which is connected in parallel with the resistor 9, it is additionally possible to set a noise resistance.The comparison of the output voltage UKof the comparator 16 in the evaluation unit 17 with a temporal profile of switching states of the inverter 4 or of the inverter branches 13, 14 enables a determination as to whether the ground fault is caused by the inverter 4 or originates, for example, from other subscribers of an intermediate circuit composite which are likewise electrically coupled to the intermediate circuit 1. If the output voltage UKof the comparator 16 changes synchronously with a switching state change of the inverter 4, a ground fault of the inverter 4 can be inferred. Otherwise, a ground fault of another subscriber of the intermediate circuit network can be deduced.Since, in the case of a short circuit to ground of an output phase due to the differentiating element 5, the switching means 13, 14 are only subject to capacitive loading, the power loss occurs exactly with the switching operation and is only a few microseconds long.The solution according to the invention does not require any separation of the power path, is only capacitive (i.e. not visible in the case of a DC leakage current measurement) and evaluates only the change in the intermediate circuit potentials. If this change is then still compared with the PWM switching operations of the inverter 4, it is possible to detect whether the ground fault is caused internally or externally by another device on the intermediate circuit composite. If the phase ground connection originates from another device in the intermediate circuit connection, it does not have to be switched off.According to the invention, an intermediate circuit potential or both intermediate circuit potentials is measured in a differentiating manner, i.e. by means of capacitive coupling. The criterion for the presence of a ground fault is then the magnitude of the potential edge of the test voltage UP resulting from the differentiation in relation to the intermediate circuit voltage. A shutdown criterion is the time assignment of the potential edge(s) of the test voltage UP to the PWM of the inverter 4.

Claims

Frequency converter (100), comprising: - a DC link (1), wherein the DC link (1) comprises a positive DC link branch (2) at which a positive DC link potential (ZK+) is present during operation of the frequency converter (100), and a negative DC link branch (3) at which a negative DC link potential (ZK-) is present during operation of the frequency converter (100), and - an inverter (4), wherein the inverter is fed from the DC link (1), characterized in that the frequency converter (100) further comprises - a differentiating element (5) which is looped in between the positive DC link branch (2) or the negative DC link branch (3) and protective ground (PE), and - a ground fault detection unit (6) which is designed to detect ground faults, a test voltage (UP) generated by means of the differentiating element (5) for detecting a ground fault (33).Frequency converter (100) according to Claim 1, characterized in that - the ground fault detection unit (6) is designed to detect a ground fault (33) if an amount of the test voltage (UP) and / or a time derivative of the test voltage (UP) exceeds a threshold value (US).Frequency converter (100) according to Claim 2, characterized in that - the earth fault detection unit (6) is designed to measure a potential difference between the positive intermediate circuit potential (ZK+) and the negative intermediate circuit potential (ZK-) and to set the threshold value (US) as a function of the measured potential difference.Frequency converter (100) according to one of the preceding claims, characterized in that - the earth fault detection unit (6) is designed to compare a temporal profile of the test voltage (UP) with a temporal profile of switching states of the inverter (4) and, if there is a temporal dependence between the profile of the switching states of the inverter (4) and the test voltage (UP), to deactivate the inverter (4).Frequency converter (100) according to one of the preceding claims, characterized in that - the differentiating element (5) has an infinitely high resistance for a DC voltage.Frequency converter (100) according to one of the preceding claims, characterized in that - the differentiating element (5) has a capacitor (7) and a resistance voltage divider (8, 9), wherein the capacitor (7) and the resistance voltage divider (8, 9) are connected in series between the positive intermediate circuit branch (2) or the negative intermediate circuit branch (3) and the protective earth (PE), and the test voltage (UP) is present at a centre tap (10) of the resistance voltage divider (8, 9).Frequency converter (100) according to one of the preceding claims, characterized in that the frequency converter (100) further comprises - a first Y capacitor (11), which is looped in between the positive intermediate circuit branch (2) and protective earth (PE), and - a second Y capacitor (12), which is looped in between the negative intermediate circuit branch (3) and protective earth (PE).

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