METHOD AND ELECTRICAL CIRCUIT ARRANGEMENT FOR DETERMINING A BRANCH INSULATION RESISTANCE AND A BRANCH LEAKAGE CAPACITY IN AN UNGROUNDED POWER SUPPLY SYSTEM

Description

[0001] The invention relates to a method and an electrical circuit arrangement for determining a branch insulation resistance and a branch leakage capacitance of a line branch to be monitored in a branched, ungrounded power supply system with active conductors by generating a measuring voltage by means of a controllable measuring voltage source, feeding the measuring voltage at a central point of the power supply system into the active conductors by means of a coupling circuit and detecting a differential current in the line branch to be monitored caused by the measuring voltage by means of a current sensor.

[0002] When increased demands are placed on the operational, fire, and contact safety of electrical systems, an ungrounded power system is used, also known as an isolated network (isolé terre - IT) or IT (power supply) system. In this type of power system, the active components are separated from the earth potential - from earth. The advantage of these networks is that in the event of an insulation fault, such as a ground fault or a short-circuit to frame, the function of the connected electrical loads is not impaired, since the ideally infinite impedance value between an active conductor of the network and earth prevents a closed circuit from forming.

[0003] This inherent safety of the unearthed power supply system can thus ensure a continuous power supply to the loads fed by the unearthed power supply system even if a first insulation fault occurs.

[0004] The resistance of the unearthed power supply system to earth (insulation resistance - in the event of a fault also insulation fault resistance or fault resistance) is therefore constantly monitored, since a possible further fault on another active conductor would create a fault loop and the resulting fault current, in conjunction with an overcurrent protection device, would result in the system being switched off and operational downtime.

[0005] Provided that the insulation condition of the unearthed power supply system is continuously monitored by an insulation monitoring device, the unearthed power system can continue to operate without any prescribed time limit even after a first fault has occurred.

[0006] Insulation monitoring devices are used to monitor insulation resistance. State-of-the-art insulation monitoring devices complying with the IEC 61557-8 product standard determine the (total) insulation resistance of the entire IT system to earth. The insulation monitoring device is connected between the active conductors on the one hand and earth on the other and superimposes a measuring signal on the network. If an insulation fault occurs, the measuring circuit between the network and earth closes across the insulation fault, resulting in a measuring current corresponding to the insulation fault. This measuring current causes a corresponding voltage drop across a measuring resistor in the insulation monitoring device, which is evaluated by the electronics and triggers an alarm if a preset limit is exceeded.

[0007] Insulation monitoring devices designed in accordance with standards take into account ohmic leakages distributed symmetrically across all active conductors as insulation faults, as well as asymmetrical insulation faults occurring, for example, on a single active conductor.

[0008] In addition to determining insulation resistance, insulation monitoring devices available on the market also have the functionality to determine the (total) (network) leakage capacitance of the ungrounded power system to earth. Analogous to insulation resistance, which represents the resistive component of the leakage impedance, the leakage capacitance represents the capacitive component of the leakage impedance to earth. The (total) leakage capacitance is the sum of all leakage capacitances in the entire IT system to earth.

[0009] Typically, the determination of the insulation resistance and, if applicable, the leakage capacitance is carried out using a centrally located insulation monitoring device and relates to the entire unearthed power supply system.

[0010] In order to be able to make more differentiated statements about the insulation condition of the branched, unearthed power supply system and to be able to locate insulation faults more quickly if necessary, it is therefore advisable to obtain information about the branch insulation resistance or branch leakage capacitance effective in this branch of the line selectively for a specific line branch (line outgoing branch) of the power supply system.

[0011] Various solutions for determining partial insulation resistance and partial leakage capacitance are known from the prior art. For example, patent DE10 2017 217 473 B3 describes a method that requires continuous, metrological determination of the total insulation resistance and the total system leakage capacitance using a centrally located, enhanced insulation monitoring device to determine partial insulation resistance and partial system leakage capacitance.

[0012] In the electrical protective device disclosed in the published patent application DE10 2014 223 287 A1, a second fault in a line branch can be selectively detected, but this requires a communication channel between the feed-in point and the measuring current transformer arranged in the line branch to be monitored.

[0013] Another option for monitoring subsystem insulation resistance is described in published patent application DE10 2013 218 836 A1. However, the measuring current is not fed in centrally, but rather a decentralized measuring current limited to the respective line branch flows. Further prior art can be found in documents DE102011108716 and EP0593007.

[0014] The above-mentioned solutions known from the state of the art therefore result in the following requirements for a two-selective R / C determination, the implementation of which, however, often requires technically complex measures: a) Communication of determined parameters from other line branches or the central measuring device b) Voltage measurement either centrally, connected with a communication, or decentralized in each line branch c) A configuration of the entire network topology in the evaluation devices.

[0015] In particular, the possibility of communication between a centrally feeding measuring device and the line branch to be monitored is often associated with considerable circuitry complexity.

[0016] The present invention is therefore based on the object of specifying a method and an electrical circuit arrangement for determining the branch insulation resistance and the branch leakage capacitance of a line branch in a branched, ungrounded power supply system, which does not depend on a continuous communication connection between the central measuring voltage feed and a current sensor arranged in the line branch to be monitored.

[0017] This object is achieved with reference to a method in conjunction with the features of the preamble in claim 1 by the characterizing features of claim 1.

[0018] The present invention is based on the basic idea of ​​forming the generated and centrally fed measuring voltage over each generator period with a characteristic signal shape defined by the frequency composition, wherein the possible signal shapes are known to the decentrally arranged current sensor and are interpreted by it.

[0019] Based on this (frequency) modulation of the measuring voltage, information is transmitted from the central measuring voltage feed point to the current sensor located in the line branch to be monitored. Based on this information, the current sensor can determine the resistance value with which the coupling circuit feeds the measuring voltage during the respective generator period, without the need for an additional communication channel.

[0020] In conjunction with the detection of the differential current caused by the measuring voltage and the data stored in the current sensor, the current sensor has all the information needed to calculate the branch insulation resistance and the branch leakage capacitance.

[0021] The measuring voltage is formed alternately over one generator period from a first frequency mixture and a second frequency mixture.

[0022] The first frequency mix is ​​fed in (during one generator period) via a first coupling resistor using a coupling circuit that can be switched using a switching unit, and the second frequency mix is ​​fed in (in the subsequent generator period) via a second coupling resistor. The two frequency mixes are fed in alternately over one generator period.

[0023] The feeding of the measuring voltage with the signal shape of the first frequency mixture and thus the detection of the differential current caused thereby and detected by the current sensor is consequently associated with the connection of the first coupling resistor; the feeding of the second frequency mixture is correspondingly associated with the connection of the second coupling resistor.

[0024] The current sensor's data memory stores the value of the first coupling resistor and the value of the second coupling resistor, as well as the spectral composition of the first frequency mix and the spectral composition of the second frequency mix. This current sensor configuration allows the current sensor to know not only the two coupling resistors but also the amplitudes of the voltage components that make up the measurement voltage.

[0025] The current sensor carries out a frequency analysis of the differential current over two consecutive generator periods in a computing unit in order to determine which of the two frequency mixtures is currently being fed in.

[0026] The currently switched coupling resistance is determined from the assignment of the detected frequency mixture to the respective coupling resistance known to the current sensor.

[0027] The branch insulation resistance and the branch leakage capacitance are then calculated in the computing unit from the measured differential current, the measuring voltage, the frequencies contained in the (first and second) frequency mixture and the (first and second) coupling resistance.

[0028] On the basis of the quantities effective in two consecutive generator periods and known through measurement and configuration, a system of equations can be set up from which the branch insulation resistance and the branch leakage capacitance are determined as the required quantities.

[0029] According to the invention, the computing unit of the current sensor thus has all the information available from which the branch insulation resistance and the branch leakage capacitance can be calculated independently of the other network components.

[0030] In conjunction with a central supply of the measuring voltage, the branch insulation resistance and the branch leakage capacitance are determined by the inventive frequency shaping of the measuring voltage and the inventively configured current sensor without any further communication connection or data connection to a centrally arranged insulation monitoring device or centrally arranged test generator (an insulation fault location device).

[0031] In a further embodiment, the first frequency mixture and the second frequency mixture each contain at least two frequencies (frequency components), wherein the first frequency mixture differs from the second frequency mixture in at least one frequency.

[0032] The first frequency composite must differ from the second frequency composite in at least one frequency to ensure unambiguous assignment to the currently applied coupling resistance. Furthermore, a sufficient number of equations must be available to solve the system of equations determining the branch insulation resistance and the branch leakage capacitance. This is achieved if both the first frequency composite and the second frequency composite each consist of at least two frequencies for shaping and alternating the measurement voltage.

[0033] To increase robustness against interference effects, it is also possible to compose the frequency mixtures from more than two frequencies.

[0034] Furthermore, the frequencies contained in the frequency mixtures are chosen so that their periods are contained as an integer multiple in the generator period.

[0035] Preferably, the measuring voltage is a measuring pulse voltage of an insulation monitoring device or a test voltage of an insulation fault location device.

[0036] In combination with a central insulation monitoring device or an insulation fault location device, the measuring voltage can be shaped by the composition of the frequency mixture in such a way that this measuring voltage meets the requirements of a measuring signal to be generated by the insulation monitoring device and thus a continuous measurement of the branch insulation resistance and the branch leakage capacitance is possible during the insulation monitoring of the entire network or the insulation fault location.

[0037] Advantageously, the differential current is detected by means of a current sensor designed as a measuring current transformer.

[0038] The object underlying the invention is also achieved by an electrical circuit arrangement according to claim 6.

[0039] The claimed structural features of the electrical circuit arrangement according to the invention carry out the corresponding method steps of the method according to the invention for determining the branch insulation resistance and the branch leakage capacitance.

[0040] Thus, the technical effects achieved with the process and the resulting advantages also apply equally to the electrical circuit arrangement.

[0041] Further advantageous design features emerge from the following description and the drawings, which explain a preferred embodiment of the invention using examples.

[0042] They show: Fig. 1 a branched, ungrounded power supply system with an electrical circuit arrangement according to the invention, Fig. 2 a course of the measuring voltage and Fig. 3 an equivalent circuit diagram for calculating the branch insulation resistance and the branch leakage capacitance.

[0043] Fig. 1 shows a branched, ungrounded power supply system 2 with an electrical circuit arrangement 10 according to the invention. The electrical circuit arrangement 10 is shown by way of example in the application environment of a single-phase, ungrounded AC power supply system 2 with the active conductors L1 and L2 (not part of the invention). Likewise, the branched, ungrounded power supply system 2 can also be a multiphase AC power supply system or a DC power supply system as a further application environment.

[0044] The power supply system 2 is divided in the illustration into a line branch 4 to be monitored and a residual power supply system 5 (main system with further line branches without the line branch 4 under consideration).

[0045] Each active conductor L1, L2 of the line branch 4 to be monitored has a partial insulation resistance Rzi and a partial leakage capacitance Czi with respect to earth PE.

[0046] The residual power supply system 5 has the partial insulation resistances Rni and the partial leakage capacitances Cni with respect to earth PE.

[0047] The power supply system 2 is fed by a power source 3 with the supply voltage Uv.

[0048] The circuit arrangement 10 according to the invention consists of a controllable measuring voltage source 12 and a coupling circuit 14, which apply a measuring voltage Um to the power supply system 2 at a central point, as well as a current sensor 20 arranged in the line branch 4 to be monitored.

[0049] The coupling circuit 14 is designed to be switchable by means of a switching unit 16 – shown here with a symmetrical coupling separately for each active conductor L1, L2 – so that when the switching unit 16 is controlled via a control line S, either a first coupling resistor Ra1 or a second coupling resistor Ra2 can be synchronously connected to the active conductors L1, L2. In this embodiment, the second coupling resistor Ra2 is formed by connecting the first coupling resistor Ra1 in series (summing) with another resistor.

[0050] A current sensor 20 is arranged in the line branch 4 to be monitored, which detects and evaluates the differential current Iz, which is proportional to the measuring voltage Um. For this purpose, the current sensor 20 has a computing unit 22 and a data memory 26. The current sensor 20 is preferably designed as a measuring current transformer 24 (current transformer with a toroidal core).

[0051] Fig. 2 shows a curve of the measuring voltage Um.

[0052] The first frequency mixture F1 is generated alternately over a generator period P and the second frequency mixture F2 is generated in the following generator period P, wherein the generated first frequency mixture F1 is fed centrally into the power supply system 2 via the first coupling resistor Ra1 and the second frequency mixture F2 is fed centrally into the power supply system 2 via the second coupling resistor Ra2.

[0053] By way of example, the first frequency mixture F1 and the second frequency mixture F2 are each shown superimposed from two frequency components, wherein the first frequency mixture F1 differs from the second frequency mixture F2 in at least one frequency.

[0054] The periods of the frequencies contained in the frequency mixtures F1 and F2 are selected such that they fit, as an integer multiple, into a generator period P. The measuring voltage Um is coupled to the power supply system 2 via Ra1 in the form of the first frequency mixture F1 for the duration of a generator period P. After the end of this generator period P, the switching unit 16 switches to the value of the second coupling resistor Ra2, and the frequency mixture F2 is applied as the measuring voltage Um for the duration of the subsequent generator period P.

[0055] The length of the generator period P is advantageously adapted to the grid parameters of the power supply system 2 (total insulation resistance, total leakage capacitance), whereby the periods of the frequencies contained in the frequency mixtures F1 and F2 can be scaled accordingly.

[0056] The differential current Iz detected by the current sensor 20 in the line branch 4 to be monitored is evaluated in the computing unit 22 in conjunction with the data memory 26. For this purpose, the computing unit 22 performs a frequency analysis of the differential current Iz, for example based on a Fourier transformation, over two consecutive generator periods P in order to determine which frequency mixture F1, F2 is currently being fed in during the respective generator period P. From the assignment of the detected frequency mixture F1, F2 to the respective coupling resistor Ra1, Ra2, which is known to the current sensor 20, the currently switched coupling resistor - first coupling resistor Ra1 or second coupling resistor Ra2 - is thus determined.

[0057] Fig. 3 shows an equivalent circuit diagram for calculating the branch insulation resistance Rz and the branch leakage capacitance Cz.

[0058] For this purpose, the Fig. 1The partial insulation resistances Rzi and the partial leakage capacitances Czi of the individual active conductors L1, L2 of the line branch 4 to be monitored are simplified to form a branch insulation resistance Rz and a branch leakage capacitance Cz. Similarly, the partial insulation resistances Rni and the partial leakage capacitances Cni of the remaining power supply system 5 are combined to form the residual insulation resistance Rn and the residual leakage capacitance Cn, as well as the coupling resistances.

[0059] The complex resistance (impedance) Zz of the line branch 4 to be monitored results from the parallel connection of the branch insulation resistance Rz and the branch leakage capacitance Cz.

[0060] Likewise, the impedance Zn of the residual power supply system 5 results from the parallel connection of the residual insulation resistance Rn and the residual leakage capacitance Cn.

[0061] Using the laws applicable to linear networks and applying Kirchhoff's rules, a system of equations can be established for the four unknown quantities: residual insulation resistance Rn, branch insulation resistance Rz, as well as the residual leakage capacitance Cn and branch leakage capacitance Cz. The differential current Iz is measured by current sensor 20, whereas the measuring voltage Um, its frequency composition, and the respectively actively switched first and second coupling resistors Ra1, Ra2 are known to current sensor 20 through configuration.

[0062] This system of equations can be solved with four combinations formed by the first coupling resistance Ra1, the second coupling resistance Ra2 and for two known frequencies ω1, ω2 contained in the frequency mixtures F1, F2.

[0063] In order to obtain a more reliable assignment of the frequency mixtures F1, F2 to the coupling resistors Ra1, Ra2, it may be advantageous to use more than the two necessary frequencies ω1, ω2.

[0064] From the system of equations, the quantities residual insulation resistance Rn and residual leakage capacitance Cn as well as, in particular, the required branch insulation resistance Rz and the required branch leakage capacitance Cz can be determined analytically.

Claims

1. A method for determining a branch insulation resistance (Rz) and a branch leakage capacitance (Cz) of a line branch (4) to be monitored in a branched, ungrounded power supply system (2) having active conductors (L1, L2), the method comprising the following steps: generating a measuring voltage (Um) by means of a controllable measuring voltage source (12); feeding the measuring voltage (Um) in each active conductor (L1, L2) at a central location of the power supply system (2) by means of a coupling circuit (14); registering, by means of a current sensor (20), a residual current (Iz) caused by the measuring voltage (Um) in the line branch (4) to be monitored, characterized in that the measuring voltage (Um) is formed alternatingly via a generator period (P) from a first frequency composition (F1) and a second frequency composition (F2), respectively, the first frequency composition (F1) is fed via a first coupling resistance (Ra1) by means of a coupling circuit (14) switchable using a switching unit (16) and the second frequency composition (F2) is fed via a second coupling resistance (Ra2); the value of the first coupling resistance (Ra1) and the value of the second coupling resistance (Ra2) and the spectral composition of the first frequency composition (F1) and the spectral composition of the second frequency composition (F2) are stored in a data storage (26) of the current sensor (20); the current sensor (20) executes a frequency analysis of the residual current (Iz) in a computing unit (22) over two subsequent generator periods (P) in order to identify which frequency composition (F1, F2) is fed presently; the coupling resistance (Ra1, Ra2) presently switched is determined from the assignment of the identified frequency composition (F1, F2) to the corresponding coupling resistance (Ra1, Ra2) in the computing unit (22); the branch insulation resistance (Rz) and the branch leakage capacitance (Cz) are computed in the computing unit from the measured residual current (Iz), the measuring voltage (Um), the frequencies (ω1, ω2) contained in the frequency composition (F1, F2), and the coupling resistance (Ra1, Ra2).

2. The method according to claim 1, characterized in that the first frequency composition (F1) and the second frequency composition (F2) each contain at least two frequencies (ω1, ω2), the first frequency composition (F1) differing from the second frequency composition (F2) in at least one frequency.

3. The method according to claim 1 or 2, characterized in that the frequencies contained in the frequency compositions (F1, F2) are chosen such that their periods are contained as integral multiples in the generator period (P).

4. The method according any one of the claims 1 to 3, characterized din that the measuring voltage (Um) is a measuring-pulse voltage of an insulation monitoring device or a test voltage of an insulation fault location device.

5. The method according to any one of the claims 1 to 4, characterized in that the residual current (Iz) is registered by means of a current sensor configured as a measuring-current transformer.

6. An electric circuit arrangement (10) for determining a branch insulation resistance (Rz) and a branch leakage capacitance (Cz) of a line branch to be monitored (4) in a branched, ungrounded power supply system (2) having active conductors (L1, L2), the circuit arrangement consisting of a controllable measuring voltage source (12) for generating a measuring voltage (Um), a coupling circuit (14) for feeding the measuring voltage (Um) in each of the active conductors (L1, L2) at a central location of the power supply system (2), a current sensor (20) for registering a residual current (Iz) caused by the measuring voltage (Um) in the line branch (4) to be monitored, characterized in that the controllable measuring voltage source (12) is configured for alternatingly forming the measuring voltage (Um) having a first frequency composition (F1) and having a second frequency composition (F2) over a generator period (P), the coupling circuit (14) is designed to be switchable by means of a switch unit (16) and has a first coupling resistance (Ra1) for supplying the first frequency composition (F1) and a second coupling resistance (Ra2) for supplying the second frequency composition (F2), the current sensor (20) has a data storage (26), in which the value of the first coupling resistance (Ra1) and the value of the second coupling resistance (Ra2) and the spectral composition of the first frequency composition (F1) and the spectral composition of the second frequency composition (F2) are stored, the current sensor (20) has a computing unit (22), which is configured to execute a frequency analysis of the residual current (Iz) over two subsequent generator periods (P) in order to identify which frequency composition (F1, F2) is presently fed, to determine the presently switched coupling resistance (Ra1, Ra2) from the assignment of the identified frequency composition (F1, F2) to the corresponding coupling resistance (Ra1, Ra2), and to compute the branch insulation resistance (Rz) and the branch leakage capacitance (Cz) from the measured residual current (Iz), the measuring voltage (Um), the frequencies (ω1, ω2) contained in the frequency composition (F1, F2) and the coupling resistance (R1, R2).

7. The electric circuit arrangement (10) according to claim 6, characterized in that the controllable measuring voltage source (12) is configured such that the first frequency composition (F1) and the second frequency composition (F2) each contain at least two frequencies, the first frequency composition (F1) differing from the second frequency composition (F2) in at least one frequency.

8. The electric circuit arrangement (10) according to claim 6 or 7, characterized in that the controllable measuring voltage source is configured such that the frequencies contained in the frequency compositions (F1, F2) are chosen such that their periods are contained as integral multiples in the generator period (P).

9. The electric circuit arrangement (10) according to claim 6 to 8, characterized in that the controllable measuring voltage source (12) is a measuring-pulse voltage source of an insulation monitoring device or a test voltage source of an insulation-fault location device.

10. The electric circuit arrangement according to any one of the claims 6 to 9, characterized in that the current sensor (20) is designed as a measuring current transformer (24).