METHOD AND DEVICE FOR COMPENSATING A LEAKAGE CURRENT IN AN EMC FILTER

Description

[0001] The invention relates to a method for compensating an operational leakage current caused by a harmonic in a voltage of a power grid through an EMC filter according to the preamble of claim 1.

[0002] Furthermore, the invention relates to a device for carrying out the method according to the preamble of claim 6 and an EMC filter according to claim 13.

[0003] A method and a device of the type mentioned above are known from the publication by Jiang Yan-Shu et al.: "Research on an Inverter Output Filter for Reducing Common-Mode Voltage at Motor Terminals in PWM Drive System", XP032391979. An EMC filter at the output of a PWM inverter for reducing operational leakage currents is described. The EMC filter comprises three impedances connected from phase conductors of a three-phase network to a neutral point. The neutral point is connected to earth potential via another impedance. To compensate for the leakage current, the voltage across this second impedance is measured.

[0004] JP 2010 057268 A relates to a noise filter for compensating a leakage current on the output side of an inverter. The filter comprises three capacitors, each connected at one terminal to a phase conductor and at the other to a neutral point, with the neutral point connected to earth potential via a further capacitor. The voltage across the earth capacitor is evaluated to generate a compensation current.

[0005] WO 03 / 005578 A1 relates to an active EMC filter designed to be used in low-voltage amplifier devices in which a separate amplifier DC power source is used that is not connected to the operating voltage of the system.

[0006] WO 2004 / 001927 A2 concerns an active EMC filter for detecting a current in an earth conductor or a phase conductor as a voltage drop across a capacitor, wherein the capacitor is connected to the earth conductor.

[0007] A method and a device of the type mentioned above are also known from DE 10 2018 102 122 A1. Switching power supplies and electronic drives are frequently used in the operation of machines and systems. To meet the legal requirements for electromagnetic compatibility (EMC), filtering measures against electrical interference are necessary.

[0008] EMC filters are used for this purpose. These EMC filters have radio interference suppression capacitors connected to protective and equipotential bonding conductors, which generate leakage currents. A leakage current is an electric current that flows in an unwanted current path under normal operating conditions (International Electrotechnical Vocabulary - IEV 195-05-15).

[0009] If residual current circuit breakers are used in electrical installations for the protection of property and persons, the leakage currents flowing towards earth or ground potential can trip the residual current circuit breaker - especially if it serves to protect persons.

[0010] DE 10 2018 102 122 A1 describes a device comprising a load circuit that generates a leakage current, an EMC filter, a frequency converter, a motor cable, and a motor. To compensate for the leakage current, it is proposed that during operation the leakage current be detected and a compensating current, which opposes the leakage current, be generated via a leakage current compensator and superimposed on the leakage current. This is intended to reduce the leakage current, in particular to essentially eliminate it.

[0011] The leakage current compensator comprises a device for detecting the leakage current and a device for generating the compensation current, which is directed opposite to the leakage current and is superimposed on the leakage current.

[0012] Using a clamp meter, the currents of three phases are measured, and the leakage current is calculated from the corresponding differences. Consequently, the leakage current IA is the sum of all individual leakage currents from Ifilter + Ifrequency converter + Icable and Imotor of the load circuit.

[0013] Compensation for a single leakage current, e.g. from the EMC filter or the frequency converter, is not possible with the known method or device.

[0014] DE 10 2011 078 304 A1 relates to the reduction of leakage currents in a frequency converter. A method for reducing a parasitic current on the DC side of a rectifier is disclosed, wherein at least one potential of the DC side is grounded via a capacitor. The method comprises the following steps: Determining an expected harmonic in the output current on the DC side; generating a compensating current such that the expected harmonic is at least partially canceled out; superimposing the compensating current with the output current.

[0015] A rectifier is an electrical converter in a load circuit that converts alternating current from an AC voltage source into direct current for an electrical load. From the perspective of the AC voltage source, the load circuit has at least two poles. Current fed into the load circuit via one pole is completely returned to the electrical power source via the remaining poles. To protect people from electric shocks, for example, the earth is excluded as a pole from the load circuit. This is ensured by a residual current device (RCD), which interrupts the load circuit as soon as any current from the load circuit is returned to the AC voltage source via the earth.

[0016] When alternating current is rectified, a rectification harmonic is created in the direct current, which can be dampened by a downstream smoothing capacitor, but not completely removed.

[0017] Furthermore, a rectifier and its downstream components, such as the intermediate circuit of a frequency converter, are typically grounded via a parasitic capacitance. This parasitic capacitance represents a ground fault for the rectification harmonic, so that the rectification harmonic is partially fed back to the AC voltage source as leakage current via the parasitic capacitance and ground.

[0018] If the leakage current caused by the rectification harmonic exceeds a limit current set by the residual current device, the residual current device interrupts the load circuit without there actually being a connection to the earth of the load circuit.

[0019] According to DE 10 2011 078 304 A1, a targeted compensation of the harmonic leakage current is proposed, which does not lead to any restrictions on the function of the residual current device, since fault currents caused by a faulty earthing of the load circuit are not covered by this compensation.

[0020] The equalizing current is determined based on an alternating voltage applied to the rectifier and a parasitic capacitance.

[0021] Another method for compensating for earth leakage currents in electrical equipment is known from DE 103 53 192 A1. A method is proposed for reducing earth leakage currents to a protective conductor, whereby the earth leakage currents flow from live parts of the installation to the protective conductor without an insulation fault, specifically in electrical equipment with a line-commutated converter circuit, for example, frequency converters with a line-side rectifier circuit.

[0022] Compensation is achieved by injecting a current into the protective conductor, whereby this current must be in opposite phase to the earth leakage current so that the addition of the earth leakage current and the injected current results in a minimum, thereby compensating the earth leakage current.

[0023] The compensation current is obtained from a voltage present in the DC intermediate circuit of the electrical equipment, for example the voltage across decoupling capacitors, whereby the compensation current is guided to the protective conductor via an inductor that ensures the phase angle of the compensation current necessary to compensate the earth leakage current.

[0024] However, the design and manufacture of the inductor involves considerable effort.

[0025] DE 10 2008 024 348 A1 relates to a method and a compensation circuit for reducing pulsating earth currents in a mains connection line of a single-phase or multi-phase system. The compensation of a ground current through the earth capacitances is achieved by a controlled current source between the neutral conductor and the protective conductor.

[0026] A first compensation circuit for shifting fault current from a protective conductor to a neutral conductor includes a simulation of the earth capacitances of the large appliance. The neutral point is connected to the protective conductor via a high-resistance resistor, which acts as a DC voltage potential source at an input of an operational amplifier.

[0027] A second compensation circuit for fault current shifting from a protective conductor to a neutral conductor is characterized in that the output of a summation current transformer supplied via the conductors and the neutral conductor is indirectly connected via a load resistor and a resistor R2 to an input of an operational amplifier.

[0028] Furthermore, it is proposed that the controlled current source be controlled by a P-controller, which regulates the current flow through the protective conductor to almost zero in a frequency range sufficiently above the fundamental frequency.

[0029] US Patent 10,139,848 B1 relates to a method for generating a compensation current in an electric vehicle charging system. The system comprises a power source, a current transformer, a leakage current compensation unit, a load, and ground potential. The current transformer is connected to the power source and supplies power to the load. During operation of the current transformer, a common-mode current flows from the load to the ground node of a leakage capacitor. The compensation circuit receives at least one signal, which depends on the common-mode current, and generates a compensation current that is introduced into at least one node of the system. The compensation current has an amplitude opposite to that of the differential-mode current.

[0030] US 2014 / 0292401 A relates to an active voltage-fed "feed-forward" EMI filter comprising a noise signal reduction and current reduction circuit that receives EMI noise generated by a noise source, and a noise voltage compensation circuit that is coupled to the noise detection and current reconstruction circuit during operation. The circuit generates a noise voltage compensation signal based on the EMI noise reconstructed by the noise detection circuit.

[0031] US 2014 / 0071719 A1 concerns a method for noise filtering. A noise filter circuit uses open-loop signal processing to process a signal that causes the noise and generates a signal that is fed back into the system to compensate for the currents caused by the noise.

[0032] DE 11 2007 003 682 T5 relates to an EMC filter for filtering a noise component in a power line. The EMC filter comprises at least one inductor in the power line, a shunt module for ground shunting in at least a specified frequency band, wherein said noise component in the power line comprises a high-pass filter which provides a noise signal representing a noise component in the power line, and a controlled source in response to said noise signal which shunts a current controlled by the noise signal to ground.

[0033] Based on this, the present invention aims to further develop a method and a device of the type mentioned above in such a way that operational leakage currents generated in EMC components upstream and / or in converters can be reliably detected and precisely compensated in a simple manner. Furthermore, systems equipped with these EMC components should be able to be operated safely with residual current devices, whereby fault currents occurring in the event of a fault should neither be detected nor compensated.

[0034] The invention relates to a method for compensating an operational leakage current caused by a harmonic in a voltage of a power grid through an EMC filter according to claim 1, an associated device according to claim 6 and an EMC filter according to claim 13.

[0035] Further embodiments are defined in the dependent claims.

[0036] The EMC filter has at least one grounding capacitor connected to earth potential, through which the operational leakage current IA is diverted to earth potential, whereby the quantity UM proportional to the leakage current IA is measured directly in the current path of the grounding capacitor or is derived from the voltage applied to the grounding capacitor.

[0037] According to an inventive embodiment, a measuring resistor, such as a shunt, is connected in series with the grounding capacitor to form an RC circuit, whereby the quantity proportional to the leakage current is measured across the measuring resistor in the form of a measuring voltage.

[0038] Alternatively, according to an inventive embodiment, the voltage applied to the grounding capacitor is switched to ground potential by means of an RC circuit connected in parallel to the grounding capacitor, consisting of a series circuit of a measuring resistor and a measuring capacitor, wherein the measuring capacitor is connected via an impedance converter to a pole exhibiting the voltage fluctuation and the measuring resistor is connected to ground potential. The quantity proportional to the leakage current is then measured as a measuring voltage across the measuring resistor.

[0039] The device according to the invention is characterized in that the EMC filter has at least one grounding capacitor connected to earth potential, via which the operational leakage current IA is discharged to earth potential, and that a measuring device is arranged directly in a current path of the grounding capacitor or in a current path connected in parallel to the grounding capacitor, with which the quantity UM proportional to the leakage current IA can be determined.

[0040] The invention is based on the concept of evaluating a potential or voltage fluctuation UIN of the power grid, caused by a harmonic and resulting in a leakage current IA, using a differentiator in the form of an RC circuit. The RC circuit comprises a series connection of a measuring capacitor and a measuring resistor in the form of a shunt, the series connection being connected between a pole where the potential or voltage fluctuation is present and ground potential. The quantity UM, proportional to the leakage current IA, is measured across the measuring resistor in the form of a measuring voltage. The measuring voltage UM, which reflects the time course of the leakage current IA, is inverted and controls a voltage-controlled current source to generate the compensation current IK. The compensation current IK, which opposes the leakage current IA, is impressed into the power grid by the current source.

[0041] The method according to the invention enables the reliable, real-time detection and precise compensation of operational leakage currents IA generated in EMC components such as grounding capacitors. This allows systems that must be equipped with such EMC components to meet legal electromagnetic compatibility (EMC) requirements to be operated with residual current devices (RCDs). Furthermore, it ensures that fault currents occurring in the event of a fault, i.e., currents flowing through a given fault location due to an insulation fault (VDE 0100 / 200), are neither detected nor compensated for.

[0042] The leakage currents IA are detected directly at their source, in this case in particular the grounding capacitors, and directly counteracted. The method and device according to the invention have no influence on any upstream protective devices such as residual current circuit breakers.

[0043] By using the device according to the invention in the form of a leakage current capacitor, capacitors with higher capacitances can be used in the EMC filter and / or the drive component such as frequency converters, thereby achieving improved EMC performance. The resulting higher capacitive leakage currents IA can be easily compensated by the leakage current compensators according to the invention.

[0044] Several methods can be used to obtain the measurement voltage UA, which is proportional to the leakage current IA, when compensating the leakage current IA of an EMC filter.

[0045] The measuring resistor, such as a shunt, preferably has a resistance R ≤ 10 Ω and can be connected in the protective conductor path of the grounding capacitor. Preferably, non-inductive precision resistors are used to avoid impairing the filtering effect.

[0046] The capacitance of the measuring capacitor can be approximately 1 / 10 of the capacitance of the grounding capacitor. With a capacitance of, for example, 0.47 µF, this results in an impedance of 170 Ω at a harmonic frequency of 2 kHz. Therefore, a resistance of approximately 10 Ω could be used for the measuring resistor without smoothing the signal waveform.

[0047] The capacitance of the measuring capacitor can also roughly correspond to the capacitance of the grounding capacitor. With a capacitance of 4.7 µF, this would result in an impedance of 17 Ω at a harmonic frequency of 2 kHz. Therefore, a measuring resistor with a resistance of approximately 100 Ω could be used without smoothing the signal waveform. This results in a measuring voltage of 10 mV with a leakage current IA of 1 mA. Theoretically, the capacitor would need to have a voltage rating of 300 V, but not the specifications of a Y-capacitor.

[0048] The embodiments described above are used in particular when the grounding capacitor is freely accessible, such as in an EMC filter with a Y capacitor and a star connection of three X capacitors, where the Y capacitor is the grounding capacitor.

[0049] In a frequency converter, the EMC filter, in the form of a grounding capacitor connected to ground potential via a positive or negative pole, is not accessible from the outside.

[0050] According to a further embodiment, in such a case it is proposed that the DC voltage containing the harmonic causing the leakage current be acquired purely resistively, divided, and fed to an impedance converter, which preferably maps the signal 1:1. The signal is then differentiated using a differentiator in the form of an RC high-pass filter, a series circuit consisting of a measuring resistor and a measuring capacitor.

[0051] The differentiated voltage signal is inverted by means of an inverter and fed to a controllable, preferably voltage-controlled, current source, which generates the compensation current and impresses it into a neutral conductor, a protective conductor and / or into the phases of a three-phase power network via a feed-in network.

[0052] The output signal of the inverter is low-pass filtered by means of a low-pass filter, wherein the low-pass filter has a frequency range of preferably 20 Hz to 2 kHz.

[0053] A preferred embodiment is characterized in that the grounding capacitor or the component containing the grounding capacitor, such as a converter, is connected to the power grid with a time delay by means of a switching element. This ensures that the functions of the leakage current compensator, such as the voltage supply to the circuit, have stabilized before operational leakage occurs.

[0054] Another preferred embodiment is characterized in that the amplitude of the compensation current IK is set once for a specific type of EMC filter, wherein a differential current in phases L1, L2, L3 of the power grid is detected by means of a differential current measurement, and a control signal for the voltage-controlled current source is determined from a comparison of the differential current and a variable current signal specified, preferably by means of a teach function, such that the amplitude of the compensation current IK is set in such a way that the leakage current IA is compensated. This method ensures that the amplitude of the compensation current can be adapted to the amplitudes of leakage currents of different frequency converters.

[0055] The EMC component generating the leakage current, in particular the frequency converter with grounding capacitor, is switched on with a time delay using a switching element such as a contactor, so that the functions and circuits of the leakage current compensator are in a steady state before leakage currents occur.

[0056] The switching on preferably takes place with a time delay T in the range of 200 msec ≤ T ≤ 800 msec, preferably T = 500 msec.

[0057] The leakage current compensator according to the invention can, for example, be integrated directly into the EMC filter, thereby allowing the use of capacitors with higher capacitances and consequently avoiding space-consuming inductors. The EMC filters can thus be made significantly smaller.

[0058] The EMC filter according to the invention comprises a star connection of X-capacitors, each of which is connected with a first terminal to a phase L1, L2, L3 of a three-phase network and with a second terminal to a common star point, as well as at least one Y-capacitor as a grounding capacitor, which is connected with a first terminal to the star point of the star connection of X-capacitors.

[0059] According to the invention, the grounding capacitor is connected to earth potential at its second terminal via a measuring resistor, or a series circuit consisting of a measuring capacitor and a measuring resistor is connected in parallel to the grounding capacitor, preferably via an impedance converter, wherein a measuring voltage UM proportional to the current IA through the grounding capacitor can be derived from the measuring resistor, the measuring resistor is optionally connected to an input of an inverter via a low-pass filter, an output of the inverter is optionally connected via a low-pass filter to the input of a controlled, preferably voltage-controlled, current source for generating a compensation current IK, and an output of the controlled current source is connected to the neutral conductor N, the protective conductor PE and / or to a feed-in network for feeding the compensation current IK into the power grid via the neutral conductor N, the protective conductor PE and / or a feed-in network for feeding into phases L1, L2,L3 of the three-phase power grid is connected.

[0060] Furthermore, the leakage current compensator according to the invention can be directly integrated into a frequency converter and another drive component.

[0061] Further details, advantages and features of the invention will become apparent not only from the claims and the features to be derived therefrom - individually and / or in combination - but also from the following description of preferred embodiments to be derived from the drawings.

[0062] They show: Fig. 1 schematically shows a drive control with a first embodiment of a leakage current compensator according to the invention for compensating a leakage current generated by a mains-side EMC filter, and Fig. 2 schematically shows a drive control with a second embodiment of a leakage current compensator according to the invention for compensating a leakage current generated by an EMC filter arranged in a frequency converter.

[0063] Fig. 1 Figure 10 schematically shows an electrical system 10 with a frequency converter 12 for operating an electric drive 14 on a three-phase network 16. To meet the legal requirements for electromagnetic compatibility (EMC), an EMC filter 18 is connected upstream of the frequency converter 12 on the network side. A leakage current IA caused by harmonics in the voltage of the three-phase network in the EMC filter 18 is reliably detected in real time by means of a leakage current compensator 20 and precisely counter-compensated by means of a compensation current IK.

[0064] The leakage current compensator 20 is connected upstream of the frequency converter 12 on the grid side and comprises a measuring device 22 for detecting the leakage current IA and a device 24 for generating the compensation current IK. The device 24 for generating the compensation current IK comprises a low-pass filter 26, an inverter 28, and a controllable, voltage-controlled current source 30, which is connected on its output side to a feed-in circuit 32 (known per se) for feeding the compensation current IK into phases L1, L2, L3 of the three-phase network 16 or alternatively directly to the neutral conductor N. The feed-in circuit 32 for feeding the compensation current IK can comprise a star connection 34 of filter or X-capacitors, the star point 36 of which is connected to the output of the current source 30, optionally via an intermediate matching circuit.

[0065] The leakage current compensator 20 is supplied with voltage via a voltage transformer 38. A voltage input 40 of the voltage transformer 38 is connected on the input side via connections 42 to the phases L1, L2, L3 of the three-phase network 16, and a voltage output 44 is connected on the output side via connections 46 to a voltage input 48 of the device 24. Furthermore, the device 24 is connected via a connection 49 to a protective conductor PE at earth potential 50.

[0066] According to the invention, the leakage current IA is detected directly in the source circuit of the EMC filter 18. The EMC filter 18 has a line-side filter choke 52 and a load-side filter choke 54 for each phase L1, L2, L3. Furthermore, the EMC filter 18 comprises a star connection of X-capacitors 56, each of which is connected at a first terminal to a phase L1, L2, L3 of the three-phase network 16 between the filter chokes 52, 54, and which is connected at a second terminal to a common star point 58.

[0067] The neutral point 58 of the X-capacitors 56 can be connected via a switching element 60 to a first terminal of a grounding capacitor in the form of the Y-capacitor 62 of the EMC filter 18. A second terminal of the Y-capacitor 62 is connected in series to ground potential 50 via a measuring resistor 64, such as a shunt. The leakage current IA generated by the harmonic of the voltage in the EMC filter 18 flows to ground potential 50 via the Y-capacitor 62. A measuring voltage proportional to the leakage current IA drops across the measuring resistor 64, which is further processed by the device 24. Consequently, the series connection of the Y-capacitor 62 and the measuring resistor 64 forms the measuring device 22 for detecting the leakage current IA. The switching element 60 can be switched by means of an actuator 66, such as a relay coil. The actuator 66 is controlled by a timer 68, which can be integrated into the device 24.

[0068] The frequency converter 12 is connected to a three-phase output 70 of the EMC filter 18, which forms the output of the leakage current compensator. The frequency converter 12 includes a rectifier 72, which is connected to the three-phase network 16 at its input side and generates a rectified DC link voltage UDC+, UDC- from the star voltages in a DC link 74. This DC link voltage is smoothed by a smoothing capacitor 76. From the DC link voltage, an inverter 78 generates an AC voltage in a three-phase motor line 80 with an AC frequency and an RMS value, which is set by means of the frequency converter 12.

[0069] The function of the leakage current compensator 20 according to the invention is explained below. To meet the legal requirements for electromagnetic compatibility (EMC) during the operation of a frequency converter, filtering measures against electrical interference are necessary. According to the first embodiment, the EMC filter 18 is connected upstream of the frequency converter 12 on the mains side in the form of a star connection consisting of the X or filter capacitors 56 and the Y or grounding capacitor 62.

[0070] Due to a non-constant midpoint voltage of the 6-pulse bridge rectifier 72 and / or pulse width modulation in the frequency converter 12, harmonics and consequently operational leakage currents IA can occur, i.e., currents that do not flow exclusively in and out via the mains lines of a residual current device.

[0071] Instead, the circuit is closed via the earth or ground potential 50. In the illustrated embodiment, the leakage current IA flows via the Y or grounding capacitor 62 to earth potential 50. Further leakage currents can be generated by capacitive coupling due to parasitic capacitances 81 between the frequency converter 12, the motor cable 80 and / or the electric machine 14 on the one hand and objects in their vicinity on the other, but these are not considered in the present case.

[0072] The present invention relates to the compensation exclusively of the leakage current IA generated by the EMC filter 18 and flowing to earth potential 50 via the grounding capacitor 62. According to the invention, the leakage current IA is detected directly in the current branch in which the grounding capacitor 62 is arranged, i.e., in the current path that connects the neutral point 58 of the star connection of the filter capacitors 56 to earth potential 50.

[0073] The voltage applied to the neutral point 58 is pulsating, meaning it has a harmonic that causes the leakage current IA. This pulsating voltage causes the leakage current IA to flow in the RC circuit consisting of the grounding capacitor 62 and the measuring resistor 64. The leakage current IA results in a measurement voltage drop across the measuring resistor 64, which is proportional to the leakage current. The measurement voltage UM is detected and, if necessary, further processed in the measuring device 22, in particular by increasing its voltage level. The voltage signal is fed to the low-pass filter 26, which preferably has a frequency range of 20 Hz to 2 kHz, in order to selectively attenuate higher frequencies; because leakage currents IA with a frequency above 2 kHz are not among the leakage currents to be compensated, as these can be attenuated by appropriate filter circuits.

[0074] After low-pass filtering by the low-pass filter 26, the voltage signal is inverted in the inverter 28 and fed to the current source 30. In the current source 30, the compensation current IK is generated from the inverted voltage signal and fed into the individual phases L1, L2, L3 via the feed-in circuit 32. In the illustrated embodiment, the compensation current IK is fed into the star point 36, i.e., a virtual neutral conductor, optionally via an intermediate matching circuit, and coupled into the individual phases L1, L2, L3 via the star capacitors 34.

[0075] In networks with a neutral conductor N, in Fig. 1 As shown in dashed lines, the output of the current source 30 can also be directly connected to the neutral conductor N for feeding in the compensation current IK.

[0076] The current path of the grounding capacitor 62 is switched on with a time delay by means of the timer 68 and the relay 66 when the mains power is connected, preferably with a time delay of T = 500 ms. The time delay is implemented via the timer 68. The reason for the delayed switch-on is that, according to the exemplary embodiment, the device 24 is not supplied independently of the mains power supply, but via the voltage transformer 38. The time-delayed switch-on ensures that both the voltage transformer 38 and the device 24 for compensating the leakage current IA are in a steady state before operational leakage currents IA flow through the grounding capacitor 62. The switching element 60 is arranged between the neutral point 58 and the input of the grounding capacitor 62 to ensure that the grounding capacitor 62 is at earth potential 50 via the measuring resistor 64 when the power is off.

[0077] The circuit arrangement differs from the prior art in that only the leakage current IA generated by the EMC filter 18 is compensated. Consequently, systems that must be equipped with such an EMC filter 18 can be safely operated with residual current devices. Furthermore, the leakage current compensator 20 according to the invention ensures that fault currents occurring in the event of a fault are neither detected nor counter-compensated.

[0078] Fig. 2 shows a second embodiment of a leakage current compensator 82 according to the invention, which is connected upstream of a frequency converter 84 on the mains side, via which an electric drive 86 is controlled.

[0079] The frequency converter 84 has an input rectifier 88, whose output voltage UDC+, UDC- is smoothed by a smoothing capacitor 90, which forms an intermediate circuit 92. An inverter 94 is connected to the smoothing capacitor 90, via which an adjustable AC voltage is provided for controlling the electric drive 86.

[0080] According to the exemplary embodiment, the frequency converter 84 has its own EMC filter 96, which in the illustrated embodiment is designed as a filter capacitor or grounding capacitor 96, which is connected from a positive terminal UDC+ of the intermediate circuit 92 to earth potential 50. During operation of the frequency converter 84, a leakage current IA flows through the filter capacitor 96 to earth potential 50, which can cause a residual current device (not shown) to trip.

[0081] The leakage current compensator 82 according to the invention is designed to compensate the leakage current IA generated by the filter capacitor 96 of the frequency converter 84.

[0082] Since the leakage current IA through the earthing capacitor 96 in the frequency converter 84 cannot be detected due to its design, the invention proposes that the pulsating DC voltage applied to the positive terminal UDC+ or negative terminal UDC- of the intermediate circuit 92, which is the cause of the leakage current IA, be detected by means of a measuring device 100.

[0083] The ripple of the DC voltage results from the non-constant midpoint voltage of the rectifier 88, which in this case is configured as a 6-pulse bridge. A pulsating DC voltage is applied to the positive terminal UDC+ and the negative terminal UDC- of the intermediate circuit 92, whereby the sum of the two voltages UDC+ and UDC- would result in a triangular waveform of 160 Vpp with a frequency of 150 Hz.

[0084] The principle according to the invention consists of first dividing the DC voltage with respect to neutral conductor N or protective conductor PE in the intermediate circuit 92 by means of an ohmic voltage divider comprising a resistor 102 and a series resistor 104, wherein the series resistor is connected at one terminal to either the voltage UDC+ or the voltage UDC- and is connected to earth potential via the resistor 102. The voltage divider consisting of the series resistor 104 and the resistor 102 preferably has a division ratio of 1:100.

[0085] The leakage current IA flowing through the filter capacitor 96 follows the formula IA (t) = C filter x dUDC / dt. The leakage current IA thus follows the time-dependent change of the voltage UDC+ or UDC- of the intermediate circuit 92. In this process, the DC component is lost, and a triangular voltage is transformed into a square wave current.

[0086] Provided that the DC intermediate circuit 92 has a very low internal resistance compared to the capacitive impedance of the filter capacitor 96 for frequencies ≤ 150 Hz, the time course of the leakage current IA can be uniquely reconstructed from the course of the DC voltage. With a change in capacitance, the amplitude of the leakage current IA changes, while the signal shape remains largely the same.

[0087] According to the invention, the DC voltage UDC+ or UDC- is measured purely ohmically against ground potential 50 and divided by the voltage divider consisting of series resistor 104 and resistor 102 by a factor of approximately 100, e.g. 1 MΩ to 10 kΩ.

[0088] This generates a voltage of approximately 3 V DC relative to ground potential 50 with a ripple of approximately 1 V and an internal resistance of approximately 10 kΩ. This voltage is fed to an impedance converter 106 in the form of a transistor or operational amplifier, which preferably replicates the signal 1:1 and provides it with an internal resistance of approximately 100 Ω. The above differentiation formula IA(t) = Cfilter x dUDC / dt is implemented by connecting a capacitor 108 in series with a measuring resistor 110, which is connected to ground potential 50.

[0089] A capacitor 108 with a capacitance of 47 nF / 10 V and a measuring resistor 110, preferably of 100 Ω, achieves an impedance of 2.3 kΩ at 150 Hz. The signal applied to the measuring resistor 110 is inverted by an inverter 112 and, if necessary, after low-pass filtering by a low-pass filter 114, fed to a power stage 116 in the form of a voltage-controlled current source 116. This power stage feeds the compensation current IK into phases L1, L2, L3 of a three-phase network 120 via a feed-in circuit 118. The power source is connected to earth potential 50 of the protective conductor PE via connection 121.

[0090] In a power grid with a neutral conductor N, the compensation current IK can also be fed directly into the neutral conductor N (in Fig. 2 (shown as dashed lines) are fed in.

[0091] The feed-in circuit 118 can be configured as a star connection of three capacitors 122, each connected to a phase L1, L2, L3, and interconnected via a star point 124. The compensation current IK can optionally be fed into the star point 124 via an intermediate matching circuit.

[0092] Due to different technical designs of the filter capacitor 96 in various frequency converters 84, the current mirroring proposed according to the invention can indeed reproduce the correct temporal profile of the leakage current IA, but not the amplitude of the leakage current IA. Consequently, the magnitude of the amplitude of the compensation current IK must be set once for the corresponding frequency converter type or taught using a teach function.

[0093] For this purpose, a comparator 126 is provided, to whose first input 128 a variable teach signal is fed. A second input 132 is connected to a measuring device 134 in the form of a differential current transformer. The differential current ΔI measured by the differential current transformer 134 is fed to the second input 132 of the comparator. In the comparator 126, the teach signal 130 is compared with the differential current ΔI. At an output 136 of the comparator 126, which is connected to an input of the controllable current source 116, a control signal is present according to the result of the comparison. This control signal allows the amplitude of the compensation current IK flowing at the output of the controllable current source 116 to be adjusted.As soon as the differential current ΔI corresponds to the teach signal, a teach function is triggered, which ends the regulation, leaving the gain level of the current source 116 at the set level.

[0094] In this context, it should be noted that in series applications the teach function for amplitude determination can be omitted, since the leakage currents IA of the frequency converters are known and the dimensioning of the amplifier stage can be adjusted accordingly.

[0095] The leakage current compensation device IA is supplied with voltage via a voltage converter 138. This converter is connected on its input side to phases L1, L2, L3 and on its output side to a voltage input 140 of the device 142. Since the leakage current compensation device 142 is not supplied independently of the mains power supply, it must be ensured that the circuit functions are in steady state, i.e., in particular, that the voltage converter is steady before any operational leakage currents occur. Therefore, the mains connection of the frequency converter 84 is time-delayed. For this purpose, the leakage current compensator 82 has a timer 144, via which a switching device 146, such as a relay or contactor, is switched on with a time delay, preferably with a time delay T = 500 ms. A three-phase switch 148 is controlled via the switching device 146, through which the input of the rectifier 88 can be connected to the three-phase network 120.In other words, the time-delayed activation of the switching device 144, such as a contactor, for the power supply of the frequency converter 84 is implemented with a potential-free relay output.

[0096] With the inventive embodiments of leakage current compensators 20, 82 and the inventive method, operational leakage currents IA generated in EMC components such as grounding capacitors can be reliably detected and precisely counter-compensated in real time. Thus, systems that must be equipped with such EMC components to meet legal electromagnetic compatibility (EMC) requirements can be operated with residual current devices. Furthermore, it is ensured that fault currents occurring in the event of a fault, i.e., currents flowing through a given fault location due to an insulation fault (VDE 0100 / 200), are neither detected nor counter-compensated.

[0097] According to the invention, the leakage currents IA are detected directly at their sources, in this case in particular the grounding capacitors 62, 96, and directly counterbalanced. The method and device according to the invention have no effect on any upstream protective devices such as residual current circuit breakers.

[0098] By using the leakage current capacitors according to the invention, capacitors 62, 96 with higher capacitances can be used in the EMC filter 18 and / or the drive component such as frequency converter 84, thereby achieving better EMC performance. The resulting higher capacitive leakage currents IA can be easily compensated by the leakage current compensators according to the invention.

[0099] The leakage current compensator 20 according to the invention can, for example, be integrated directly into the EMC filter 18, thereby avoiding capacitors with higher capacitances and consequently space-consuming inductors. The EMC filters can be made significantly smaller.

[0100] Furthermore, the leakage current compensator 82 according to the invention can be used directly in frequency converters and other drive components.

Claims

1. A method for compensating an operation-related leakage current IA caused by a harmonic in a voltage of a power supply network (16, 120), for example of a three-phase power supply network, by an EMI filter (18, 96), wherein a quantity UM proportional to the leakage current IA is detected and passed to a device (24, 142) for generating a compensation current IK, which generates a compensation current IK which is in the opposite direction to the operation-related leakage current IA and which is superimposed on the operation-related leakage current IA such that the latter is reduced, in particular substantially compensated, wherein the EMI filter (18, 96) has a current path with at least one grounding capacitor (62, 96) connected against ground potential (50) and via which path the operation-related leakage current IA is conducted against ground potential (50), and wherein the quantity UM proportional to the operation-related leakage current IA is measured directly in the current path of the grounding capacitor (62) or is deduced from the voltage applied at the grounding capacitor (96), characterized in that the quantity UM proportional to the operation-related leakage current IA is measured by means of a measuring resistor (64), connected in series to the grounding capacitor (62), in the form of a measured voltage, or in that the voltage applied at the grounding capacitor (96) is connected against ground potential (50) by means of a series connection, in parallel to the grounding capacitor (96), made up of a measuring resistor (110) and a measuring capacitor (108), and in that the quantity UM proportional to the operation-related leakage current IA is measured at the measuring resistor (110) in the form of a measured voltage.

2. The method according to claim 1, characterized in that the voltage applied at the grounding capacitor (96) is divided by means of a preferably high-resistance voltage divider (102, 104) and passed to an impedance converter (106) which outputs the voltage signal, preferably 1:1, in that the voltage signal applied at the output of the impedance converter (106) is differentiated by means of a differentiator in the form of an RC high pass from the series connection of the measuring capacitor (108) and of the measuring resistor (110), and in that the quantity UM proportional to the operation-related leakage current IA is measured at the measuring resistor (110), for example at a shunt resistor, in the form of the measured voltage.

3. The method according to claim 1 or 2, characterized in that the measured voltage is, preferably after low pass filtering, inverted by means of an inverter (28, 112) and passed to a voltage-controlled power source (30, 116) which drives the compensation current IK into a neutral conductor N, a protective earth PE and / or the phases L1, L2, L3 of the power supply network (16, 120).

4. The method according to claim 3, characterized in that an amplitude of the compensation current IK is set once for an EMI filter, wherein a differential current in the phases L1, L2, L3 of the power supply network (120) is detected by means of a differential current measurement (134), wherein a control signal for the voltage-controlled power source (116) is determined from a comparison of the differential current and of a variable current signal, preferably presettable by means of an input function, so that the amplitude of the compensation current IK is set such that the operation-related leakage current IA is compensated.

5. The method according to at least one of the preceding claims, characterized in that the grounding capacitor (96) or a component (84) having the grounding capacitor, for example a frequency converter, is switched in by means of a switching element (146, 148), for example by a contactor, with a time delay, wherein switching is performed preferably with a time delay T in the range 200 ms ≤ T ≤ 800 ms, particularly preferably T = 500 ms.

6. A device (20, 82) for compensating an operation-related leakage current IA caused by a harmonic in a voltage of a power supply network (16, 120), for example of a three-phase power supply network, by an EMI filter (18, 96), wherein a quantity UM proportional to the leakage current IA is detected and passed to a device (24, 142) for generating a compensation current IK and configured to generate a compensation current IK which is in the opposite direction to the operation-related leakage current IA and which is superimposed on the operation-related leakage current IA such that the latter is reduced, in particular substantially compensated, wherein the EMI filter (18, 96) has a current path with at least one grounding capacitor (62, 96) switched against ground potential (50) and which is configured to conduct the operation-related leakage current IA against ground potential (50), and wherein a measuring device (64) for measuring the quantity UM proportional to the operation-related leakage current IA is arranged in the current path of the grounding capacitors (62) or wherein a measuring device (100) is arranged parallel to the grounding capacitor (96) and is designed to deduce the quantity UM proportional to the operation-related leakage current IA from the voltage applied at the grounding capacitor (62, 96), characterized in that the measuring device (64) is designed as a measuring resistor connected in series to the grounding capacitor (62), and in that the measuring resistor (64) is connected to ground potential (50) or in that the measuring device (100) is designed as a series connection, made up of a measuring capacitor (108) and a measuring resistor (110), which is connected in parallel to the grounding capacitor (96), wherein the measuring resistor (110) is connected to ground potential (50).

7. The device according to claim 6, characterized in that the measuring device (100) is a differentiator in the form of an RC high pass from a series connection of the measuring capacitor (108) and of the measuring resistor (110) connected to ground potential, wherein the measuring capacitor is connected to an output of an impedance converter (106) and an input of the impedance converter is linked via a voltage divider (102, 104) to the voltage UDC+, UDC- applied at the grounding capacitor.

8. The device according to at least one of claims 6 to 7, characterized in that the measuring resistor (64, 110) is connected, optionally via a low pass filter (26, 114), to an input of an inverter (28, 112), wherein the low pass filter (26, 114) has a frequency range of preferably 20 Hz to 2 kHz, and / or in that an output of the inverter (28, 112) is connected, optionally via a low pass filter, to an input of a controlled, preferably voltage-controlled, power source (30, 116) for generating the compensation current IK, and in that an output of the controlled power source (30, 116) is, for impressing the compensation current IK into the power supply network, connected to a neutral conductor N, to a protective earth PE and / or to a feed network (32, 118) for feeding into phases L1, L2, L3 of a three-phase power supply network used as the power supply network (16, 120).

9. The device according to at least one of the preceding claims 6 to 8, characterized in that the grounding capacitor (62) is a filter capacitor in the form of a Y-capacitor of the EMI filter (18), wherein the EMI filter (18) furthermore comprises a star connection (56), made up of filter capacitors in the form of X-capacitors which are each connected to a phase L1, L2, L3 of the three-phase power supply network (16) and to a star point (58), and in that the series connection, made up of the grounding capacitor (62) and the measuring resistor (64), is connectable to the star point (58) of the star connection, made up of filter capacitors of the EMI filter, by means of a switching element (60) with a time delay, or in that the grounding capacitor (96) is a filter capacitor of a DC intermediate circuit (92) preferably of a frequency converter (84), wherein the filter capacitor (96) is connected against ground potential (50) by a positive or negative terminal of the DC intermediate circuit (92), and in that the series connection of measuring capacitor (108) and measuring resistor (110)) is connected via the impedance converter (106) to the positive or negative terminal of the DC intermediate circuit (92.

10. The device according to claim 8, characterized in that the device comprises a device (134) for measuring a differential current in a network supply cable, wherein the differential current is applied at a second input (132) of a comparator (126) and the comparator (126) is configured to compare the differential current with a variable signal, preferably presettable by means of an input function, at a first input (128) of the comparator (132), and wherein an output (136) of the comparator (126) is connected to a controllable power source (116), wherein the power source (116) is configured to set, depending on the comparison of the input quantities, an amplification such that the leakage current IA is compensated.

11. The device according to at least one of the preceding claims 6 to 10, characterized in that the component (84) having the grounding capacitor (96), in particular a frequency converter, is configured to be connected to the power supply network (120) by means of a switching element (146, 148), for example a contactor, with a time delay.

12. The device according to claim 9 or 11, characterized in that the EMI filter (18) is integrated in the device (20) with the Y-capacitor as the grounding capacitor and with the star connection of X-capacitors, and / or in that the device is integrated in the frequency converter (84).

13. An EMI filter (18), comprising a star connection (56) of X-capacitors, which are each connected with a first connection to a phase L1, L2, L3 of a three-phase power supply network (16) and with a second connection to a common star point (58), and at least one Y-capacitor as the grounding capacitor (62), which is connected with a first connection to the star point (58) of the star connection of X-capacitors, characterized in that the grounding capacitor (62) is connected to ground potential (50) with its second connection via a measuring resistor (64) or in that a series connection made up of a measuring capacitor and of a measuring resistor is connected in parallel to the grounding capacitor (62), preferably via an impedance converter, wherein the measuring resistor (64) is configured to deduce a measured voltage UM proportional to the current IA through the grounding capacitor (62), in that the measuring resistor (64) is connected, optionally via a low pass filter (26), to an input of an inverter (28), in that an output of the inverter (28) is connected, optionally via a low pass filter, to the input of a controlled, preferably voltage-controlled, power source (30) for generating a compensation current IK, and in that an output of the controlled power source (30) is, for impressing the compensation current IK into the power supply network, connected to a neutral conductor N, to a protective earth PE and / or to a feed network (32) for feeding into the phases L1, L2, L3 of the three-phase power supply network (16).