Patent Application
20120206148
The invention relates to a method for determining a maximum leakage current in unearthed power supply networks with a plurality of live conductors.
Furthermore, the invention relates to a device for determining a maximum leakage current in unearthed power supply networks with a plurality of live conductors.
When designing a power supply system, in addition to the performance of the power network, operational and personal safety must be ensured. A protective concept for a faultless operation therefore comprises protective measures which ensure continuous power supply with simultaneous personal and plant protection.
Unearthed mains have proven themselves with regards to operational safety and the minimisation of the risk of unavailability of the power supply. In unearthed mains, there are no active conductors directly connected to earth potential, so that in the event of a (unipolar) earth fault, in the absence of a return wire, a short circuit current, which triggers a fuse and leads to an interruption of operation, cannot flow as it can in earthed systems. An unearthed network can be operated further safely in spite of this earth fault.
Completely insulated, unearthed mains furthermore offer the best possible protection for people, in case they should touch a live conductor, as due to the ideally infinitely large impedance value between conductor and earth, no closed circuit with a current flowing via the (earthed) person can result.
In practice, however, the mains have finite ohmic and capacitive resistances—a complex-valued network leakage impedance—with respect to earth, so that in the event that a conductor becomes connected to earth potential by means of touching an (earthed) person, a closed circuit results via this network leakage impedance and the human body, in which closed circuit potentially lethal leakage currents can flow. The leakage current is principally defined by the capacitance formed by the conductor arrangement and earth and thus has a predominantly capacitive component.
Particularly in hospitals with their multiplicity of medical electrical devices, people come into direct contact with live parts, in that for example the patient is connected via electrodes or probes to a device of this type during an examination or during an operative intervention. An additional current flow, which is brought about unintentionally, could impair the action of the medical measure and, in the worst case scenario, have fatal consequences for the patient. It is for this reason that unearthed mains in hospitals are subject to particular regulations which require a constant checking of the maximum leakage current THC (total hazard current). A monitoring device of this type is also designated as a line isolation monitor (LIM).
In this case, the current which would flow in the event that a person touches a live conductor is designated as the maximum leakage current. In accordance with the definition, this current arises from the division of the maximum occurring voltage between conductor and earth (network voltage ULE,max) and network leakage impedance (ZE): THC=ULE,max/ZE.
The two values ULE,max and ZE required for determining the maximum leakage current can be determined independently of one another. Whilst the determination of the network voltage for the most part results from a conventional voltage measurement by means of a resistor network and does not place any great demands on measurement technology, the determination of the complex-valued network impedance ZE shapes up to be complex, as a suitable test or measurement signal for impedance measurement has a small amplitude and is overlaid by the network voltage and further interference pulses during the continuous monitoring of a conductor system. A fundamental problem therefore consists in the separation of the measurement signal from the line-to-line voltage, whereby the selection of a suitable frequency of the measurement signal can make a decisive contribution to a reliable determination of the network leakage impedance and therefore of the leakage current.
A method is known from the published document U.S. Pat. No. 4,206,398, in which, for determining the leakage current, a measurement signal is impressed into the mains, which, although it has the network frequency, is additionally phase-modulated. Due to the continuous phase modulation of the measurement signal, this can be separated as a non-stationary signal portion from the total voltage by means of the filtering of the useful portion (network voltage). In a closed-loop control circuit, this test voltage is used as a control variable, it being possible to set the impressed current in such a manner that the test voltage corresponds to the network voltage. The stationary end value of the current then corresponds to the leakage current to be determined.
In the digital line isolation monitor (LIM) described in the published document U.S. Pat. No. 5,450,328, a sinusoidal measurement current is impressed into the mains for determining the leakage impedance, its value of the network frequency can be calculated with known modelling of the leakage impedance. After filtering out and measuring the test voltage, which falls as a consequence of the measurement current across the leakage impedance, the value of the leakage impedance can be calculated for the network voltage with known modelling of the leakage impedance.
In the published document U.S. Pat. No. 4,472,676, a system for measuring the leakage impedance is disclosed, in which the test signal fed into the conductor network only has a small frequency deviation from the network frequency. By evaluating the interference pattern of the total voltage resulting by means of the overlaying of test voltage and network voltage, it is possible in the case of known impressed measurement current to measure a voltage drop proportional to the leakage impedance.
The previously mentioned solution approaches show various ways in which the measurement signal in the frequency spectrum can be extracted from the aggregate signal. By means of a phase modulation of a test signal with the same frequency as the network, by means of a test signal of clearly deviating frequency with the option of setting the frequency manually or by means of a test signal with only small frequency deviation and subsequent evaluation of the interference signal.
Common to all three solution approaches is the fact that the frequency of the measurement signal is always constant. As, however, in the practical operation of a conductor network, interference signals, which lie in the frequency range of these fixed measurement frequencies, can also arise to a greater extent during the measurement, inaccurate determinations of the maximum leakage current and thus, in the case of the monitoring of a conductor network, the triggering of false alarms can occur. By way of example for this, mention may be made of the generation of interference frequencies remote from the network frequency by means of the increasing use of frequency converters in the hospital sector or the mutual influence on the network frequency by means of two unearthed mains installed in parallel.
The present invention is therefore based on the object of developing a method and also a device for determining the maximum leakage current, which increases the reliability in the case of the determination of the leakage current and thus decreases the frequency of the triggering of false alarms in the case of the monitoring of unearthed power supply networks.
This object according to the invention is achieved with the characteristics of a method for determining a maximum leakage current in unearthed power supply networks (2) with a plurality of live conductors (L1, L2) in that the method comprises the method steps:—feeding of a measurement signal UP with a measurement frequency fPuls into the conductor arrangement (2, 10),—setting the measurement frequency fPuls (54),—measuring a line-to-line voltage UL-E for each conductor (L1, L2, 56),—determining a maximum occurring line-to-line voltage UL-Emax from the measured line-to-line voltages UL-E (56),—determining a voltage UZe across a measurement resistance RM for calculating the leakage impedance ZE (62),—calculating a leakage impedance ZE (66),—calculating a maximum leakage current THC (68),—repeating the previous steps, characterised in that the measurement frequency fPuls can be changed during the measurement and the setting of the measurement frequency fPuls takes place automatically by means of an interference frequency analysis (54). Additional, particularly beneficial, embodiments of the invention are provided in accordance with the following subsidiary methods for determining a maximum leakage current in unearthed power supply networks (2) with a plurality of live conductors (L1, L2).
In accordance with a second method of determining a maximum leakage current embodiment of the invention, the first embodiment is modified so that the interference frequency analysis (54) comprises the following automatically running method steps:—setting of the measurement signal value UP=0 volt (70),—setting of a centre frequency fG
The method according to the invention therefore has the advantage compared to the prior art of taking account of interference pulses arising in the mains during the determination of the leakage impedance and diverting to other interference-free measurement frequencies. In this case, the determination of a suitable substantially interference-free measurement frequency takes place without manual interventions by means of an interference frequency analysis initiated and executed in the method automatically. The measurement voltages drawn on for determining the maximum leakage current are therefore incorporated into the calculations in an undistorted manner and lead to a reliable value of the leakage current. The risk of false warnings due to an imprecise determination of the leakage current is reduced considerably. By means of the automatic setting of the measurement frequency, it is furthermore possible to operate a plurality of monitoring devices (LIMs) in unearthed networks which are managed in parallel or even to operate a plurality of monitoring devices in the same network in a safety redundant manner.
By means of the automatically running interference frequency analysis, a value is determined for the measurement frequency fPuls, for which the interference amplitudes are minimal. In an advantageous manner, the interference frequency analysis comprises the following method steps: setting of the measurement signal value UP=0 volt, setting of a centre frequency fG
In the case of deactivated measurement voltage UP, a search is made in the settable frequency range of the signal generator in the pass-range of a band-pass filter with tunable centre frequency fG
In a further advantageous configuration, differential voltages ULnm between the conductor pairs n, m are initially determined from the measurement of the line-to-line voltages UL-E, following that, the respective spectral components of these differential voltages ULnm at the point fPuls are calculated and the method is continued with a return jump to setting the measurement frequency fPuls, in the event that |ULnm|(fPuls)>0 is true for one of the differential voltages.
During the measurement of the leakage impedance, an interference component, which is caused by a connected consumer, can be detected for the measurement frequency fPuls by means of this evaluation of the differential voltages ULnm. In cases in which an interference amplitude has been detected at the place of the measurement frequency fPuls, a return jump is brought about for setting the measurement frequency, that is to say a renewed execution of the interference frequency analysis is brought about.
Preferably, after the determining of the voltage UZe, a threshold value query UZe<UZe
This threshold value query is used for the detection of further external interference sources at the measurement frequency fPuls connected in series, which would lead to a distortion of the voltage drop UZe required for calculating the leakage impedance ZE. If the voltage UZe determined exceeds a predetermined measurement range threshold value, then the method is likewise continued with a return jump to setting the measurement frequency, that is to say a search is made in accordance with the interference frequency analysis for a measurement frequency which is less susceptible to interference and, if necessary, a switch is made to this measurement frequency.
In a preferred configuration, the determining of the voltage UZe takes place across the measurement resistance RM with applied measurement voltage UP by means of the measurement of the line-to-line voltages UL-E, addition of the measured line-to-line voltages UL-E and subsequent band-pass filtering of the aggregate signal for obtaining a voltage component giving the voltage UZe at the frequency fPuls.
Starting from the individual line-to-line voltages UL-E already measured for determining the maximum occurring line-to-line voltage UL-Emax, these are initially supplied to an adder. The addition of the line-to-line/partial voltages UL-E in this case corresponds to the parallel connection of the coupling branches, so that the (filtered) aggregate signal can be drawn upon as the actual measurement signal for determining the leakage impedance. The subsequent band-pass filtering of the aggregate signal using the measurement frequency fPuls as centre frequency fG
In this case, the centre frequency of the band-pass filter for filtering out the voltage UZe is automatically set to the measurement frequency fPuls of the signal generator. This is advantageous, as the band-pass filter centre frequency fG
The calculation of the leakage impedance ZE takes place in accordance with the relationship ZE=(UP*RM)/UZE−(RM+RA). Advantageously, the leakage impedance ZE can therefore be expressed as a function of the known values UP, RM and RA, as well as the determined value UZe.
The subsequent determination of the maximum leakage current THC takes place in accordance with the formula THC=UL-Emax/ZE0, ZE0 being the leakage impedance at the network frequency, which arises in a frequency-corrected manner from the value ZE calculated at the measurement frequency fPuls. As the value of the leakage current at the network frequency is definitive, but the value of the leakage impedance was determined at a measurement frequency fPuls deviating from the network frequency, the value of the leakage impedance at the network frequency is to be used for the determining of the leakage current. This can be attained in the case of known modelling of the leakage impedance as parallel connection of an ohmic and a capacitive component from the leakage impedance at the measurement frequency fPuls.
In a further preferred configuration, an initialisation and a self test is carried out at the start of the method. In these method steps, a calibration and a test of measurement-relevant parameters takes place, in order to counteract possibly occurring environmental effects and particularly temperature influences.
Expediently, after the initialisation a timer can be started in connection with the self test with the function of restarting the method running repeatedly with a self test, as soon as the timer has run out or to continue with the renewed measurement of the line-to-line voltages in the case of a timer which has not yet run out.
Preferably, the measurement signal UP is generated with a pulse generator from a rectangular signal characteristic. The generation of a rectangular signal characteristic in connection with a subsequent filtering for generating a desired sinusoidal signal component constitutes a cost-effective option for signal generation.
The object according to the invention is also achieved with the characteristics of a device for determining a maximum leakage current in unearthed power supply networks (2) with a plurality of live conductors (L1, L2) according to a thirteenth embodiment of the invention in that the device consists of the a signal generator (10) for feeding in a measurement signal UP with a measurement frequency fPuls, a network coupling circuit (12) between signal generator (10) and conductor arrangement (2), a voltage measuring circuit (14) for measuring the line-to-line voltages UL-E, an impedance measuring circuit (16) for determining the leakage impedance Ze by means of the determining of a voltage UZe, wherein the voltage measuring circuit (14) is connected at the input-side to the network coupling circuit (12) and at the output-side via analogue/digital converter (24) to an arithmetic unit (18) for digital signal processing, which arithmetic unit has a storage unit (34) and also controls a display unit (42) and a digital interface (40), characterised in that for setting the measurement frequency fPuls of the signal generator (10), a signal generator control apparatus (36), by means of which the measurement frequency fPuls can be changed during the measurement, is connected to the signal generator (10). Additional, particularly beneficial, embodiments of the invention are provided in accordance with the following subsidiary devices for determining a maximum leakage current in unearthed power supply networks (2) with a plurality of live conductors (L1, L2).
In accordance with a fourteenth device of determining a maximum leakage current embodiment of the invention, the thirteenth embodiment is modified so that for each conductor, the network coupling circuit (12) has coupling resistances RA and, for picking up of the line-to-line voltages UL-E, measurement resistances RM, which are arranged in series connection with the signal generator (10) and the respective conductor (L1, L2). In accordance with a fifteenth device of determining a maximum leakage current embodiment of the invention, the thirteenth embodiment or the fourteenth embodiment is modified so that at the input side, the voltage measuring circuit (14) detects a voltage UL-E for every conductor (L1, L2) across the measurement resistance RM and has an amplification circuit (21, 22), as well as a difference amplifier (26) for every conductor pair n, m for determining the differential voltage ULnm, and at the output side forwards the measured line-to-line voltages UL-E and also the differential voltage ULnm to the analogue/digital converter. In accordance with a sixteenth device of determining a maximum leakage current embodiment of the invention, the thirteenth embodiment, the fourteenth embodiment, or the fifteenth embodiment is modified so that the impedance measuring circuit (16) has an adder (28), to which the line-to-line voltages UL-E provided by the voltage measuring circuit (14) are supplied and has a band-pass filter for filtering out the voltage UZe from the aggregate signal of the adder (28), the centre frequency fG
The realisation of the fundamental idea according to the invention, namely to draw on a measurement frequency which is as interference-free as possible for determining the maximum leakage current THC and to determine this measurement frequency automatically, preferably consists in providing a signal generator control apparatus for setting the measurement frequency. The signal generator control apparatus constitutes a functional interface between signal generator and arithmetic unit and can preferably be a constituent of the arithmetic unit. The measurement frequency fPuls of the measurement signal UP which the signal generator outputs can be changed during the measurement and in particular during the interference frequency analysis by means of the signal generator control apparatus. The arithmetic unit itself in this case executes the overwhelming part of the digital signal processing.
For each conductor, the network coupling circuit has coupling resistances RA and, for detecting the line-to-line voltages UL-E, measurement resistances RM, which are arranged in series connection with the signal generator and the respective conductor. This configuration of the network coupling allows the pick up of the line-to-line voltages UL-E including the voltage UZe required for determining the leakage impedance ZE.
In a further expedient configuration, the voltage measuring circuit is realised in such a manner that at the input side, this detects a voltage UL-E for every conductor across the measurement resistance RM and has an amplification circuit, as well as a difference amplifier for every conductor pair n, m for determining the differential voltage ULnm, and at the output side forwards the measured line-to-line voltages UL-E and also the differential voltages ULnm to the analogue/digital converter.
The voltage measuring circuit is the link between the network coupling circuit, which enables physical access to the voltages, and the arithmetic unit, which by means of the analogue to digital converter records the line-to-line voltages UL-E detected in this manner for digital processing. The differential voltages ULnm between two conductors n, m supplied to the arithmetic unit likewise via the digital/analogue converter furthermore enable the detection of an interference voltage caused by a connected consumer.
It also proves to be an advantage that the impedance measuring circuit has an adder, to which the line-to-line voltages UL-E provided by the voltage measuring circuit are supplied and has a band-pass filter for filtering out the voltage UZe from the aggregate signal of the adder, the centre frequency fG
The impedance measuring circuit provides the voltage UZe required for calculating the leakage impedance ZE, which voltage results due to the applied measurement voltage UP across the measurement resistances RM. To this end, the impedance measuring circuit comprises an adder for summing the line-to-line voltages UL-E provided by the voltage measuring circuit. The aggregate signal of the adder is subsequently subjected to a band-pass filtering with the measurement frequency fPuls as centre frequency fG
The band-pass filter control apparatus proves particularly advantageous for setting the centre frequency of the band-pass filter, by means of which the selected measurement frequency fPuls can be supplied to the band-pass filter as centre frequency fG
Preferably, the analogue/digital converter, the signal generator control apparatus and the band pass filter control apparatus are integrated into the arithmetic unit in each case. In this case, the analogue/digital converter is realised as distinct physical units, the signal generator control apparatus and the band-pass filter control apparatus by contrast constitute functional units of the arithmetic unit.
Further configuration features result from the following description and the drawings which explain a preferred embodiment of the invention with reference to examples. In the figures:
In the following, the background of the invention including the relevant circuit values is initially presented in
The signal generator 10 is at earth potential 3 on one side and feeds a measurement signal UP, which contains the measurement frequency fPuls, into the wiring network L1, L2 via the network coupling circuit 12.
The network coupling circuit 12 consists for every conductor L1, L2 of a coupling branch, which in each case has a coupling resistance RA1, RA2 and, for the picking up of the line-to-line voltages UL-E1, UL-E2, a measurement resistance RM1, RM2 and is arranged in series connection with the signal generator and the respective conductor.
The voltage measuring circuit 14 taps off the line-to-line voltages UL-E1 and UL-E2 by means of the measurement resistances RM1 and RM2 and forwards them in each case via an amplification circuit 21, 22 to the analogue/digital converter 24 and also to the impedance measuring circuit 16. The voltage measuring circuit 14 furthermore has a difference amplifier 26 for determining a differential voltage UL12 from the two line-to-line voltages UL-E1 and UL-E2, which is likewise forwarded to an analogue/digital converter 24.
In the impedance measuring circuit 16, the two line-to-line voltages UL-E1 and UL-E2 are initially combined in an adder 28, in order to detect both currents flowing in the parallel circuit of the conductor-based coupling branches or the voltages resulting therefrom. Subsequently, the aggregate signal passes through a band-pass filter 30, which filters out the signal components at the measurement frequency fPuls. The centre frequency of the band-pass filter fG
The determination of a suitable measurement frequency fPuls which is as interference-free as possible takes place completely automatically according to the invention. The digital signal processing of the signals received via the analogue/digital converter 24, including data storage 34, takes place in the arithmetic unit 18. In the present exemplary embodiment, the analogue/digital converter 24, a signal generator control apparatus 36 and a band-pass filter control apparatus 38 are integrated into the arithmetic unit 18. The signal generator control apparatus 36 forwards the measurement frequency fPuls determined in the arithmetic unit as a result of the interference frequency analysis to the signal generator 10. Synchronously to that, the band-pass filter control apparatus 38 controls the centre frequency fG
A digital interface 40 and also an optical display 42 of the determined maximum leakage current or also acoustic warning apparatuses (not shown here) in the event of exceeding of the maximum acceptable leakage current are provided as further interfaces.
A flow chart of the method according to the invention for determining a maximum leakage current for the unearthed power supply network 2 shown in
The method begins after the start in step 50 with the initialisation of the system and carries out a self test in step 52. Here, important measurement-relevant parameters are tested and calibrated, in order to counteract possibly occurring environmental effects and particularly temperature influences. In addition, a timer (in combination with step 69) can be set, following the running out of which, the course of the method, which is constantly repeating in a loop, is continued with a self test at step 52.
After the self test, in step 54 an interference frequency analysis, executed by the arithmetic unit 18, which delivers a measurement frequency fPuls at which the interference pulses are minimal as a result, follows; see
In step 56, the measurement of the two line-to-line voltages UL-E1 and UL-E2 and, derived therefrom, the determination of the differential voltage UL12 and also the determination of the maximum occurring line-to-line voltage ULEmax. Whilst the maximum occurring line-to-line voltage ULEmax is integrated directly into the determination of the maximum leakage current (see step 68), the differential voltage UL12 is used during ongoing measurement at the position of the measurement frequency fPuls to detect an interference frequency component which is caused by a connected consumer. The measurement voltage UP itself cannot be determined in the determined differential voltage UL12, as the measurement current driven by the signal generator 10 flows in equal parts through the measurement resistances RM1 and RM2 into the coupling branches of the network coupling circuit 12 and the components are mutually compensated by subtraction 26. A possible interference amplitude at the place of the measurement frequency fPuls can be discovered by means of a discrete Fourier transformation (DFT) of the temporal progression of the differential voltage UL12 (step 58). If an interference of this type is detected, according to step 60, |UL12|(fPuls)>0 is therefore true, so the method is continued with a return jump to setting the measurement frequency, that is to say with renewed execution of the interference frequency analysis. If no interference of this type can be determined, |UL12|(fPuls)=0 is therefore true, so in step 62, the voltage UZe is determined for calculating the leakage impedance ZE. Before the voltage UZe for impedance determination is drawn on in step 66, a threshold value query takes place in step 64. By means of the test, whether the determined voltage UZe is smaller than a predetermined maximum measurement range threshold value UZe
At the start of the interference frequency analysis, the measurement signal UP is set to zero in step 70, in order to detect external interference signals exclusively. The lower threshold frequency of the settable frequency range fPuls
