Title of Invention

METHOD FOR MEASURING THE INSULATION RESISTANCE IN AN IT NETWORK

Abstract The invention describes a method for measuring the insulation resistance in an IT network. The latter typically has a DC voltage intermediate circuit and at least one self-commutated converter having at least one first and one first and one second power switch. A measuring arrangement for measuring the intermediate circuit voltage relative to ground potential comprising a voltage divider and two assigned potential measuring devices is likewise part of the IT network. The method has an offline and an online measurement, wherein, during the offline measurement, all of the first or second power switches are closed and the potentials Up and Um, respectively, and also the intermediate circuit voltage are measured and the insulation resistance Rf is determined therefrom. During the online measurement the two potentials Up and Um are measured and the temporal profile of the measuring is suitably evaluated.
Full Text FORM 2
THE PATENT ACT 1970 (39 of 1970)
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See Section 10, and rule 13]
1. TITLE OF INVENTION
METHOD FOR MEASURING THE INSULATION RESISTANCE IN AN IT NETWORK

2. APPLICANT(S)
a) Name
b) Nationality
c) Address

SEMIKRON ELEKTRONIK GMBH & CO. KG
GERMAN Company
POSTFACH 82 02 51,
902 53 NUERNBERG,
GERMANY

3. PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed : -

Description
The invention describes a measuring method for determining the insulation resistance in a terminated, fully insulated power supply network, a so-called IT network. IT networks of this type find application in vehicles, by way of example.
An IT network is generally understood to be a network configuration appertaining to electrical engineering in which the feeding power source is not earthed as usual. Consequently, in the event of a first fault case, by way of example an insulation fault, no closed electric circuit is established nor can any dangerous flow through the body take place here. Equally, in the event of a first fault, the operation of the IT network does not have to be stopped, which brings about a higher protection against failure. What is disadvantageous here, however, is that said first fault case is also not identified as long as the insulation to earth is not measured by an insulation monitoring device. An IT network in accordance with the prior art is the starting point of this invention and has at least one power source, preferably a generator, at least one rectifier, an intermediate circuit having at least one capacitor, and also at least one converter and also a measuring arrangement for the intermediate circuit voltage.
On account of the above-described properties of IT networks, monitoring of the insulation at the beginning of operation and also during operation is particularly preferred. In this respect, EP 0 751396 Bl describes a method requiring a considerable additional outlay on circuitry over and above that present in the IT network.
Fundamentally known fault cases in an IT network are various changes in the insulation resistance, which all have to be reliably identified by suitable monitoring. Said fault cases include by way of example the ageing of the insulation, which entails a temporally slow change in the insulation resistance that is often even symmetrical
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over all the phases. A likewise known fault case stems by way of example from damage to insulation, which brings about a temporally more rapidly proceeding change in the insulation resistance usually of only one phase.
The invention is based on the object of presenting a method for measuring the insulation resistance in an IT network, wherein the measurement is intended to reliably identify symmetrical changes as well as asymmetrical changes and wherein the required additional outlay on circuitry is intended to be as low as possible.
The object is achieved according to the invention by means of the measures of the features of Claim 1. Preferred embodiments are described in the subclaims.
The inventive concept is based on an IT network, preferably having a generator or some other power source, having a DC voltage intermediate circuit fed by said power source and having at least one self-commutated converter. Said converter has at least one first and one second power switch and a centre tap.
Furthermore, the IT network has a measuring arrangement for measuring the intermediate circuit voltage. Said measuring arrangement preferably has a symmetrically formed voltage divider and two assigned measuring devices for potential measurement of the two intermediate circuit potentials relative to ground potential.
The method according to the invention for measuring the insulation resistance comprises an "offline" and an "online" measurement. During the offline measurement by way of example at the beginning of operation, all of the first or second power switches of all the converters are closed. The two potentials of the intermediate circuit are measured in this switching state. In the event of an asymmetry of the measured potentials and also with knowledge of the measuring device, in the specific case of the measuring resistances, an insulation fault is reliably identified. This fault can be localized by selectively opening the various power
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switches. It is likewise possible to test the function of the measuring arrangement itself by means of this offline measurement. If all of the switches are open, the two potentials of the intermediate circuit must be identical in magnitude. When one switch is closed, the measured value must assume a known temporal profile. The transient phenomenon of the potential measurements that results from the parasitic load capacitance present is advantageously used for functional checking of the insulation measuring device, whereby no additional hardware outlay is required for such a simulation of an insulation fault.
During the online measurement during operation of the IT network, once again the two potentials are measured and the temporal profile of this measurement is evaluated. In a first configuration, here the two potentials measured are summed, this sum is subsequently Fourier-transformed and the change in the frequency spectrum is evaluated in terms of its temporal profile.
In a second configuration of the online measurement, the two potentials measured are summed, the magnitude of this sum is suitably averaged and this average value is evaluated in terms of its temporal profile.
The inventive solution is explained further on the basis of the exemplary embodiments and Figures 1 to 6.
Figure 1 shows an IT network in accordance with the prior art.
Figure 2 shows an inverter and a motor in an IT network with a first exemplary insulation fault.
Figure 3 shows a generator with rectifier and a measuring devices in an IT network with further exemplary insulation faults.
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Figure 4 shows potential profiles during an online measurement without insulation faults.
Figure 5 shows the temporal profile of the magnitude of the sum of the measured values of an online measurement with a first insulation fault.
Figure 6 shows the temporal profile of the magnitude of the sum of the measured values of an online measurement with a second insulation fault.
Figure 1 shows an IT network in accordance with the prior art such as may find application in a road vehicle, for example. This IT network has a generator (10) for generating power, downstream of which is connected an active rectifier (20) for supplying the DC voltage intermediate circuit (30). Said intermediate circuit (30) advantageously also has at least one capacitor (32) for storing energy. A DC-DC voltage converter (40) with a battery (60) is connected to the intermediate circuit (30). Two inverters (50 a/b) with a respective motor (70 a/b) are here likewise connected to the intermediate circuit (30). A measuring device (80) is also connected for the purpose of measuring the intermediate circuit voltage.
Figure 2 shows an inverter (50) and a motor (70) in an IT network with a first exemplary insulation fault (90a). In this case, the DC voltage intermediate circuit (30) feeds a self-commutated inverter (50), wherein the latter has three first power switches (52 a/b/c). Each power switch comprises an antiparallel arrangement of at least one IGBT (insulated gate bipolar transistor) with at least one freewheeling diode (56). The inverter likewise has three second power switches (54 a/b/c) likewise formed with IGBTs and freewheeling diodes (56). The three first (52 a/b/c) and second (54 a/b/c) power switches form the bridge circuit of the three-phase inverter (50). The motor (70) is arranged at the output of the three phases. The impedances of said motor are represented in the form of a series connection of a non-reactive resistance and an inductance per phase.
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The resistance (90a) here symbolizes a first insulation fault. The value (Rf) of said resistance (90a) is determined by means of the method according to the invention.
Figure 3 shows further possible variants of insulation faults using the example of a generator (10) with rectifier (20) as part of an IT network. The illustration shows the internal resistances and inductances of the generator (10), and also parasitic capacitors (92 a/b/c) of the order of magnitude of tenths of a Nan farad which, during operation of the IT network, contribute to the impedance thereof relative to ground potential.
The measuring devices (80, cf. Figure 1) already contained in the IT network under consideration, for the regulation thereof, in accordance with the prior art is likewise illustrated. Said measuring device comprises a first (82) and a second (84) voltage divider. These voltage dividers are formed in symmetrical fashion and are connected to the assigned intermediate circuit potential and also to the ground potential. Consequently, a first potential (Uzkp) of the intermediate circuit (30) is dropped across the first voltage divider (82) and the second potential (Uzkm) is dropped across the second voltage divider (84). The voltage dividers are formed in such a way that the respective first resistor (820, 840) is of the order of magnitude (Rl) of megohms and the respective second resistor (822, 842) is of the order of magnitude (R2) of kilohms. The tap for measuring the assigned first (Up) and second (Urn) potential relative to ground potential is arranged between the two resistors in each case.
The illustration furthermore shows a plurality of resistances (90b/c/d/e), which represent different insulation faults (Rf) and can occur individually or else in groupings. By way of example, a temporally slow change in the insulation of the IT network can be described by a uniform reduction of all the resistance values of the resistances (90 c/d/e).
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By contrast, a reduction of the resistance value (Rf) of the resistance (90b) corresponds to a deterioration of the insulation of the first potential of the intermediate circuit (30).
For the offline measurement and determination of the insulation resistance by means of a microcontroller that is part of the measuring device (80) described, all of the first (52 a/b/c) or all of the second (52 a/b/c) switches of all the converters (cf. Figures 1 and 2) of the IT network under consideration are closed. By virtue of this circuitry configuration, all of the lines of the IT network under consideration are tested in their entirety by a measurement with regard to their insulation resistance and the insulation resistance of the entire IT network is thus determined. This measurement requires exclusively circuit parts of a measurement device (80) which in accordance with the prior art are contained anyway in the IT network under consideration that was the starting point of this invention.
It is particularly preferred for the two parts of the measuring device (80) to be formed symmetrically with respect to the midpoint with connection to the ground potential, whereby the measurement for example of an insulation fault (90a) as illustrated in Figure 2 is carried out as follows.
The intermediate circuit voltage (Uzk) can be expressed independently of the resistance value (Rf) of the insulation fault as follows:
Uzk = (((Rl + R2) • Up)/R2) + (((Rl + R2) • Um)/R2)
Where Uzk = intermediate circuit voltage
Rl = resistance value of the resistors (820, 840)
R2 = resistance value of the resistors (822, 842)
Up = potential measured by the measuring device (860)
Um = potential measured by the measuring device (862)
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The smaller the resistance value (Rf) of the insulation fault (90a), the smaller the ratio of Up and Uzk becomes, too. In the event of an earth fault and thus a minimum value of Rf = 0, Up is thus also equal to zero. Transformation of the above equation and elimination of Um yields:
Up = Uzk • (R2/(R1 + R2))(Rf/(Rl + R2 • Rf))
with Rl » R2 this equation can be simplified as:
Up = Uzk • (R2/R1) . (1/((R1/Rf) + 2)))
If R2/R1 is then expediently chosen such that the measurement range of an A/D converter connected downstream is fully utilized, a minimum value to be detected for Rf of the order of magnitude of 10 kQ results given a measuring accuracy of 1% and a measurement impedance of 1 MQ, by way of example. This resolution is sufficient for the standard-conforming requirements made of insulation monitoring.
In order to localize the insulation fault, it is possible for individual first or second switches to be switched on, then for the measurement described to be repeated and thus for the region in which the insulation fault is present to be determined.
In contrast to the offline measurement described above, the online measurement is effected during normal operation of the IT network under consideration. An analytical calculation of the resistance value (Rf) is not practicable by means of a microcontroller, such as is part of a described measuring device in accordance with the prior art, on account of the variation and complexity for example of the impedances (92 a/b/c) in the IT network under consideration. The first embodiment of the method according to the invention for online measurement is described by way of example with reference to Figures 3 and 4.
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In the ideal case of an IT network and during operation without insulation faults, at every instant of operation the potentials Up and Um are symmetrical with respect to the ground potential and their magnitudes are therefore identical. A summation therefore yields the value (SU) of approximately zero (cf. Figure 4). In this case, on account of a change in a measured value (Up or Um) of the assigned measuring device (860 or 862), an insulation fault would shift the value (EU) of summation away from the zero line, and the insulation fault (90b, cf. Figure 3) would thus be identified as such.
A Fourier transformation of the summation value (2U) yields, without insulation faults whilst taking account of all the impedances, a frequency spectrum which can be calculated as such in the microcontroller and be stored for comparison with new calculations. The outlay for storing the measurements already carried out is advantageously reduced by the normalization of the values to be stored with the respectively set fundamental component of the inverter output voltage. Any insulation fault that occurs asymmetrically over three phases leads to a change in said frequency spectrum which can be identified as such by comparison with the stored values and by suitable evaluation. These changes in the frequency spectrum may concern both the frequencies that occur and also the amplitudes thereof.
A fully symmetrical change in the insulation resistance of all three phases, that is to say a symmetrical change in the resistance value of the resistances (90c/d/e, cf. Figure 3), is not possible with the first embodiment of the online measurement as described here.
The second embodiment of the method according to the invention for online measurement is described by way of example with reference to Figures 3, 5 and 6. Here the measured values of the potential measurement are added, the magnitude of the summation value (£U) is formed and the average of said magnitude with respect to time is formed in a suitable manner, and stored in the microcontroller. Figure 5 shows the profile prior to averaging and also the average value for the case where an
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arbitrary one of the three resistances (90 c/d/e) from Figure 3 has an exemplary value of 80 kW. In comparison with this, Figure 6 illustrates the profile wherein two arbitrary resistances from among the three resistances (90 c/d/e) from Figure 3 have a value of 160 kW in each case and the insulation likewise as in Figure 5 thus has a total resistance of 80 kW.
A comparison of the present values with those stored in the microcontroller and a suitable evaluation of this comparison again leads to a detection of an insulation fault of the IT network.
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WE CLAIMS:
1. Method for measuring the insulation resistance in an IT network, having a DC
voltage intermediate circuit (30) and at least one self-commutated inverter
(50 a/b) having at least one first (52 a/b/c) and one second power switch
(54 a/b/c), having a measuring arrangement (80) for measuring the
intermediate circuit voltage relative to ground potential (100) having a
voltage divider (82, 84) and two assigned potential measuring devices (860,
862), comprising an offline and an online measurement,
wherein, during the offline measurement, all of the first (52 a/b/c) or second power switches (54 a/b/c) are closed, the potentials Up and Um, respectively, and also the intermediate circuit voltage Uzk are measured, and the insulation resistance Rf is determined therefrom,
wherein during the online measurement, the two potentials Up and Um are measured and the temporal profile of the measurements is suitably evaluated.
2. Method according to Claim 1, wherein
during the online measurement, the two potentials measured are summed, this sum is Fourier-transformed and the change in the frequency spectrum is evaluated in terms of its temporal profile.
3. Method according to Claim 1, wherein
during the online measurement, the two potentials measured are summed, the magnitude of the sum is suitably averaged and this average value is evaluated in terms of its temporal profile.
4. Method according to Claim 1, wherein
during the offline measurement, the transient phenomenon of the potential measurements that results from the parasitic load capacitance present is used for functional checking of the insulation measuring device.
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5. Method according to Claim 1, wherein
the voltage divider (82, 84) is formed in symmetrical fashion.
6. Method according to Claim 1, wherein
the power switches (52 a/b/c, 54 a/b/c) are formed as IGBTs (insulated gate bipolar transistor) with antiparallel-connected freewheeling diodes (56).
Dated this 25th day of June, 2007


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ABSTRACT
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The invention describes a method for measuring the insulation resistance in an IT network. The latter typically has a DC voltage intermediate circuit and at least one self-commutated converter having at least one first and one second power switch. A measuring arrangement for measuring the intermediate circuit voltage relative to ground potential comprising a voltage divider and two assigned potential measuring devices is likewise part of the IT network. The method has an offline and an online measurement, wherein, during the offline measurement, all of the first or second power switches are closed and the potentials Up and Urn,, respectively, and also the intermediate circuit voltage are measured and the insulation resistance Rf is determined therefrom. During the online measurement the two potentials Up and Um are measured and the temporal profile of the measurements is suitably evaluated.


Documents:

1217-mum-2007-abstract(granted)-(2-3-2011).pdf

1217-mum-2007-abstract.doc

1217-mum-2007-abstract.pdf

1217-MUM-2007-CANCELLED PAGES(7-2-2011).pdf

1217-MUM-2007-CLAIMS(AMENDED)-(7-2-2011).pdf

1217-mum-2007-claims(granted)-(2-3-2011).pdf

1217-MUM-2007-CLAIMS(MARKED COPY)-(7-2-2011).pdf

1217-mum-2007-claims.doc

1217-mum-2007-claims.pdf

1217-MUM-2007-CORRESPONDENCE(18-3-2010).pdf

1217-mum-2007-correspondence(20-8-2007).pdf

1217-mum-2007-correspondence(ipo)-(3-3-2011).pdf

1217-mum-2007-correspondence-received.pdf

1217-mum-2007-description (complete).pdf

1217-mum-2007-description(granted)-(2-3-2011).pdf

1217-mum-2007-drawing(25-6-2007).pdf

1217-MUM-2007-DRAWING(7-2-2011).pdf

1217-mum-2007-drawing(granted)-(2-3-2011).pdf

1217-mum-2007-drawings.pdf

1217-mum-2007-form 1(20-8-2007).pdf

1217-mum-2007-form 2(granted)-(2-3-2011).pdf

1217-mum-2007-form 2(title page)-(25-6-2007).pdf

1217-mum-2007-form 2(title page)-(granted)-(2-3-2011).pdf

1217-mum-2007-form 3(25-6-2007).pdf

1217-MUM-2007-FORM 3(7-2-2011).pdf

1217-mum-2007-form-1.pdf

1217-mum-2007-form-18.pdf

1217-mum-2007-form-2.doc

1217-mum-2007-form-2.pdf

1217-mum-2007-form-26.pdf

1217-mum-2007-form-3.pdf

1217-mum-2007-form-5.pdf

1217-MUM-2007-PETITION UNDER RULE 137(7-2-2011).pdf

1217-MUM-2007-REPLY TO EXAMINATION REPORT(7-2-2011).pdf

abstract1.jpg


Patent Number 246527
Indian Patent Application Number 1217/MUM/2007
PG Journal Number 09/2011
Publication Date 04-Mar-2011
Grant Date 02-Mar-2011
Date of Filing 25-Jun-2007
Name of Patentee SEMIKRON ELEKTRONIK GMBH & CO. KG
Applicant Address POSTFACH 820251, 90253 NUERNBERG
Inventors:
# Inventor's Name Inventor's Address
1 KLAUS BACKHAUS GALLASSTRASSE 34, 90768 FUERTH
PCT International Classification Number G01R 27/18, G01R 31/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 102006031663.0-35 2006-07-08 Germany