METHOD AND ELECTRICAL CIRCUIT ARRANGEMENT FOR BRANCH-SELECTIVE DETERMINATION OF RESISTIVE AND CAPACITIVE, ISOLATION-DETERMINING QUANTITIES IN A BRANCHED UNGROUNDED POWER SUPPLY SYSTEM

Description

[0001] The invention relates to a method and an electrical circuit arrangement for the branch-selective determination of resistive and capacitive, insulation-determining quantities (insulation resistance and leakage capacitance) in a branched ungrounded power supply system with insulation monitoring.

[0002] According to DIN VDE 0100-410, unearthed power supply systems (power grids) must be equipped with an insulation monitoring device (IMD) in accordance with DIN EN 61557-8.

[0003] The insulation monitoring device is located at a central location near the feed-in point in the main system of the power supply system and, in accordance with the standard, records the insulation resistance (resistive or ohmic component of an insulation impedance) and, depending on the device configuration, also the leakage capacitance (capacitive component of the insulation impedance) of the unearthed electrical system against earth potential (earth).

[0004] For this purpose, the insulation monitoring device has a measuring path between at least one active conductor of the power system and ground, via which a measuring voltage is generated using a measuring voltage source and fed into the power system via a coupling resistor (active measuring method). The (measurement) current driven by the measuring voltage in the measuring path and adjusted according to the insulation fault is measured as a voltage drop (measured voltage) at a measuring resistor and evaluated in an IMD evaluation electronics.

[0005] If ohmic insulation faults occur, the insulation monitoring device triggers an alarm as soon as the insulation resistance falls below an adjustable response value adapted to the power supply system.

[0006] From the combination of the insulation-determining quantities of insulation resistance and leakage capacitance, the specialist can draw important conclusions regarding the insulation condition and the protective effect of the unearthed power supply system.

[0007] In order to meet the normative requirement for rapid elimination of the insulation fault and to localize the insulation fault in a branched power supply system consisting of the main system and (line) branches (line outgoing lines), insulation fault location systems (IFLS - Insulation Fault Location System) according to DIN EN 61557-9 are used.

[0008] State-of-the-art fault-finding solutions are known that aim to locate line branches with low ohmic insulation levels, for example, the search for the line branch with the largest common-mode fault current component. For this purpose, a test current generator is connected to the ungrounded power system, which injects a line-driven or locally generated test current in the mA range between at least one active conductor and ground potential.

[0009] In contrast to the measuring current supplied by the insulation monitoring device during insulation resistance measurement, the current supplied during insulation fault location is referred to as the test current. However, as advantageously evident from the present invention, the measuring current of the insulation monitoring device can be used as a test current for insulation fault location and, according to the invention, for the branch-selective determination of the resistive and capacitive insulation-determining variables.

[0010] The insulation fault location device excites the power system with a test current, preferably a rectangular signal, with the fundamental frequency of the rectangular excitation in the single-digit or sub-hertz range. The test current generator can be located within an insulation monitoring device and is usually mains-driven. To ensure that the excitation can be superimposed and measured without interference, the fundamental frequency of the test current must be adapted to the low-pass characteristics determined by the resistive and capacitive variables and the resulting system frequency of the power system.

[0011] The line branch(es) to be monitored in the power grid are each monitored by a sensor (CT - Current Transformer), consisting of a (toroidal) current transformer with a secondary winding. The line branch routed through the toroidal core of the current transformer acts as a primary winding with (only) one turn. If an insulation fault occurs in this line branch, a (common-mode) branch current flows through the sensor's current transformer, which is transformed to the secondary side and is available for evaluation as a secondary current.

[0012] A solution according to the above-mentioned method with supply of a test current by a test current generator is offered by Bender GmbH & Co. KG and is in Fig. 1 shown schematically.

[0013] This method can be used to locate faulty line branches, but it does not provide direct information about the size of the insulation resistance and leakage capacitance.

[0014] Another solution is described in published patent application EP 1 589 345 A1. The setting of an alternating test current generated in a separate test current generator as a branch current with a maximum permissible test current amplitude and a minimum test pulse time is carried out in conjunction with the recording and evaluation of the temporal progression of network disturbances in a residual current transformer by observing a current-time surface. Since the AC measuring current transformers, in conjunction with the necessary burden resistance, act as a high-pass filter, slow systems with a high leakage capacitance and / or a high insulation resistance can no longer be accurately evaluated using this technique, as the high-pass filter no longer allows the system response to pass through.

[0015] An insulation fault location with adaptive adjustment of the test pulse period in a test current generator is described in DE 10 2017 209 663 B3. For branch-selective determination of a partial insulation resistance in a line branch to be monitored, information about the total insulation resistance is required. The determination of a capacitive element is not performed.

[0016] In the patent specification DE 10 2017 217 473 B3, a total insulation resistance and a total system leakage capacitance of the power supply system must first be continuously determined during operation using an extended insulation monitoring system with a test current generator.

[0017] The published patent application DE 10 2013 218 836 A1 describes an insulation fault location system with branch-selective feed-in and a selective insulation monitoring system for a branched IT system consisting of several subsystems. For insulation fault location, a differential current measuring device, a test signal generator, and a blocking device are arranged in each subsystem.

[0018] Furthermore, the published patent application DE 10 2015 216 915 A1 relates to a method and an insulation fault location system for an IT power supply system with a test current generator for feeding in a test current, with a test current sensor for detecting a test current component in a branch of the IT power supply system and with an evaluation device for evaluating the detected test current component.

[0019] In the methods and corresponding circuit arrangements known from the state of the art for insulation monitoring, including insulation fault location, either a determination of capacitive quantities is not carried out, or, if a capacitive quantity is determined in addition to the (ohmic) insulation resistance, the calculations require the inclusion of a total insulation resistance and a total system leakage capacitance of the power supply system.

[0020] In addition, the devices mentioned require a separate, configurable test current generator for insulation fault location in addition to the measuring voltage source required for insulation resistance measurement, and are therefore quite complex in terms of circuitry.

[0021] The present invention is therefore based on the object of proposing a method and an electrical circuit arrangement for a branched ungrounded power supply system, which enable a branch-selective determination of resistive and capacitive, insulation-determining variables of the power supply system, wherein the circuitry and manufacturing expenditure should be as low as possible.

[0022] This object is achieved by a method for the branch-selective determination of resistive and capacitive, insulation-determining variables in a branched ungrounded power supply system with insulation monitoring according to claim 1 and by an electrical circuit arrangement implementing this method according to claim 4.

[0023] Basic elements of the claimed method and the implementing electrical circuit arrangement are an insulation monitoring device according to the invention installed in the branched ungrounded power supply system and (in each case) a sensor according to the invention arranged in the line branch(es) to be monitored.

[0024] The insulation monitoring device is connected between at least one active conductor and earth and has a measuring path with a measuring voltage source, a coupling resistor and a measuring resistor with IMD evaluation electronics.

[0025] The (respective) sensor is arranged in the line branch(es) to be monitored in the ungrounded power system and has a measuring current transformer with a secondary winding for measuring a branch current using a secondary current established in the secondary winding. The measuring current transformer is preferably designed to be universally sensitive and thus suitable for detecting both alternating currents and direct currents. A (pure) AC measuring current transformer can also be used, particularly for cost reasons, provided its high-pass cutoff frequency is lower than the system frequency.

[0026] The active conductors of the line branch to be monitored, which are led through the measuring current transformer, form a primary winding with usually (only) one turn through which a real branch current flows as the primary current.

[0027] After switching on a measuring voltage generated by the measuring voltage source, a start signal is first sent from the insulation monitoring device to the sensor via a communication connection.

[0028] The measurement voltage is superimposed on the power supply system and has a periodic waveform, preferably composed of bipolar, rectangular measurement pulses because this pulse shape is easy to generate and provides broadband excitation. Other pulse shapes, such as triangular or sinusoidal, are also possible.

[0029] A voltage drop across the measuring resistor as a result of the (measuring) current driven by the measuring voltage is measured and recorded as the measured voltage.

[0030] At the same time, a real branch current, also driven by the measuring voltage, is measured in the sensor by means of the measuring current transformer with toroidal core and the secondary winding via a real secondary current that occurs in the secondary winding.

[0031] The real secondary current is transformed into a time-discrete real output signal by means of a transmission path in the sensor, which is recorded in a CT evaluation electronics of the sensor.

[0032] After a predeterminable number of periods of the measuring voltage, the measuring voltage is switched off.

[0033] A measuring cycle can comprise one or more periods of the measuring voltage, whereby in the case of several periods the periods of the measured voltage curves are superimposed and optimized with a view to a signal-to-noise ratio.

[0034] The IMD evaluation electronics then calculates a branch voltage from the measured voltage, the measured voltage, the coupling resistance, and the measuring resistance. This voltage is then transmitted from the insulation monitoring device to the sensor of the line branch to be monitored via the communication link. In the (normal) case of multiple line branches to be monitored, each with an installed sensor, all relevant sensors receive the same branch voltage curve simultaneously ("broadcasting").

[0035] The CT evaluation electronics then calculate a simulated branch current from the branch voltage, the time derivative of the branch voltage, an estimated insulation resistance of the line branch to be monitored, and an estimated leakage capacitance of the line branch to be monitored. The calculations are based on the laws applicable to linear networks (nodal point rule, mesh rule), and, in particular, the current-voltage relationships applicable to capacitive elements.

[0036] Estimates based on experience with comparable power systems are used as starting values ​​for the insulation resistance and leakage capacitance used in the calculation of the simulated branch current. These starting values ​​can be narrowed down to obtain a quick approximate solution from the ongoing measurement of a total insulation resistance or total leakage capacitance, provided the insulation monitoring device can provide these values ​​via the communication link.

[0037] The calculated simulated branch current is fed through the sensor by means of the measuring current transformer via an auxiliary winding and a current source controlled by a digital-to-analog converter (D / A converter), which functions as an additional primary current—in addition to the real branch current flowing through the active conductors. In addition to the toroidal core and the secondary winding, the measuring current transformer thus comprises an additional winding, namely the auxiliary winding for feeding the simulated branch current, as a further element essential to the invention.

[0038] The auxiliary winding does not have to be an integral physical component of the measuring current transformer; instead, a winding that can be separated from the measuring current transformer, for example, in the form of a conductor loop routed through the toroidal core, can be used. This has the advantage that installed conventional, state-of-the-art measuring current transformers can remain in the power system and can be easily retrofitted.

[0039] The simulated branch current fed in via the auxiliary winding is measured by means of a simulated secondary current occurring in the secondary winding of the measuring current transformer, transformed into a time-discrete simulated output signal by means of the transmission path and recorded in the CT evaluation electronics.

[0040] Thus, the real branch current and the simulated branch current follow the same transmission path from the detection in the toroidal core to the recording of the discrete-time real output signal and the discrete-time simulated output signal in the CT evaluation electronics. This path consists of the measurement in the measuring current transformer with toroidal core and secondary winding and the transformation in the transmission path. As an essential element of the invention, it can be considered a black box.

[0041] This offers the advantage that the precise transmission characteristics of this black box, which are often difficult to determine and vary depending on time, temperature, environmental conditions, and interference, do not need to be known. This significantly simplifies the process because the discrete-time output signals can be directly compared and evaluated.

[0042] After switching off the supply of the simulated branch current, the time-discrete output signals are evaluated in the CT evaluation electronics by calculating the error sum of squares between the real output signal and the simulated output signal.

[0043] If the sum of squares of the errors is sufficiently small, i.e. falls below a predefined limit, the insulation resistance and the leakage capacitance are transmitted as calculation results to the insulation monitoring device via the communication connection and can be displayed or further processed.

[0044] Estimated values ​​for insulation resistance and leakage capacitance are continuously recalculated in the CT evaluation electronics.

[0045] In each measurement cycle, an iteration step is carried out with the recalculation of the estimated values ​​for the insulation resistance and the leakage capacitance with the aim of minimizing the error sum of squares and thus approximating the estimate of the insulation resistance and the leakage capacitance to the actual values.

[0046] By switching on the measuring voltage, a new measuring cycle can then be started with the newly calculated insulation resistance and the newly calculated leakage capacitance.

[0047] The method according to the invention thus advantageously allows the branch-selective measurement of the insulation resistance and the leakage capacitance when using and modifying according to the invention from the insulation resistance measurement and insulation fault location of known and proven system components (insulation monitoring device and sensor) and at the same time dispensing with a separate test current generator, since the test current driven by the measuring voltage in the standardized insulation monitoring device is used as a real branch current for fault location to generate the test current.

[0048] In a further advantageous embodiment, the transformation of both the real and the simulated secondary current on the transmission path is carried out by tapping the voltage of the respective secondary current at a burden resistor, low-pass filtering by means of a low-pass filter with the effect of a coarse mains frequency filtering, variable amplification by means of an amplifier with variable gain and analog-to-digital conversion in an analog-to-digital converter (AD converter).

[0049] Because the system is considered a black box, there are only two boundary conditions that must be met regarding the properties of the transmission path within the black box. The AD converter's control level must be optimal, and the superimposed measurement signal must be distinguishable from the system-specific input signal. The first condition can be achieved by adjusting the load impedance and the variable amplifier settings. The latter condition can be influenced primarily by the cutoff frequency of the high-pass filter consisting of the secondary winding and the load resistor, and the cutoff frequency of the low-pass filter, and is directly related to the settling time of the power system being monitored.

[0050] By arranging the auxiliary winding as an additional winding on the measuring current transformer belonging to the sensor in conjunction with the iterative simulation of the real current curve, the associated determination of the line branch variables insulation resistance and leakage capacitance takes place in an advantageous manner according to the invention.

[0051] This approach, which deliberately avoids a complex (mathematical) description of the current measurement transmission path, allows for the use of lower-quality standard system components (amplifiers, magnetic core materials). Linearity, temperature stability, and saturation behavior play only a minor role in the measurement method according to the invention and at this point in the measurement chain, thus enabling a cost-effective solution to the task at hand.

[0052] Further advantages are that no calibration of the measuring system is required and that the method works with significantly lower levels of the measuring current, which can prevent the disturbance of other sensitive system components due to excessive common-mode currents. Furthermore, it is not necessary to wait for steady-state system states for the rectangular excitation periods and the associated evaluation to run through, and therefore in particular the identification of a faulty line branch with the normative requirement for rapid elimination of the insulation fault is fulfilled.

[0053] Another advantage is that the insulation monitoring device can measure the total insulation impedance of the power system while continuously identifying the faulty line branch. This is often not the case with conventional insulation fault location systems using a locating current generator.

[0054] Further advantageous design features will become apparent from the following description and the drawings, which illustrate a preferred embodiment of the invention using examples. They show: Fig. 1: the functional principle of insulation fault location according to the state of the art, Fig. 2: an equivalent circuit diagram of a branched unearthed power supply system with insulation monitoring device, Fig. 3: schematically shows a structure of the electrical circuit arrangement according to the invention, Fig. 4: a block diagram of a sensor according to the invention, Fig. 5: a flow chart of the method according to the invention and Fig. 6: Time signals and their evaluation.

[0055] Fig. 1 shows an example of an insulation fault location device 12 from Bender GmbH & Co. KG [EDS309x_D00012_xx_M_XXDE.pdf (www.bender.de), page 21, Abb. 4 .1] the functional principle of a state-of-the-art insulation fault location for a branched, unearthed power supply system 2 with a main system 3 and a (line) branch 4.

[0056] The insulation fault location device 12 comprises a test current generator 14, a state-of-the-art sensor ZCT and an insulation fault location device 16.

[0057] The test current generator 14 feeds a line-driven test current IL into the power supply system 2, which is measured as a real branch current i ca in the line branch 4 affected by the insulation fault R fN by a sensor ZCT. According to the prior art, the sensor ZCT is designed as a toroidal current transformer 20 with a toroidal core 22 and a secondary winding 24, which is connected to the insulation fault locator 16.

[0058] In Fig. 2 An equivalent circuit diagram of the branched ungrounded power supply system 2 with an insulation monitoring device IMD is given.

[0059] The insulation monitoring device IMD is represented by a measuring path 42 which runs between an active conductor L1, N and earth PE and comprises a coupling resistor R a , a measuring voltage source 44 for feeding in a measuring voltage ug as (measuring) system excitation and a measuring resistor R m with IMD evaluation electronics 46 for tapping and evaluating the measured voltage um.

[0060] For the (measurement signal) behavior, it is assumed that the higher frequency mains voltage (50 / 60 Hz or higher) in relation to the measurement voltage ug , whose frequency is a few Hz or in the sub-Hertz range, is decoupled from the measuring system by suitable filters.

[0061] The measuring voltage ug is specified, and the measured voltage um is known by measurement. Furthermore, the measuring resistance R m and the coupling resistance R a are known.

[0062] The respective branch current i 1 , i 2 ,..., in is to be calculated for the assumed combination of the insulation-determining quantities insulation resistance R f,n and leakage capacitance C e,n . The respective branch current i 1 , i 2 ,..., in corresponds to the real branch current ia flowing in the selected line branch to be monitored ( Fig. 3 ).

[0063] First, the branch voltage uc is calculated according to Equation 1.1. Assuming negligible cable impedances, this voltage is identical for all line branches 1, 2, ..., n. u c = u g − u m 1 + R a R m

[0064] The total current ig corresponds to the (measurement) current driven by the measurement voltage ug and corresponds to the sum of the branch currents i 1 , i 2 ,..., in according to Eq.1.2. i g = i 1 + i 2 + … + i n

[0065] In the time domain, the respective branch current (here, for example, the branch current in ) is calculated from the branch voltage uc with the assumed values ​​R f,n and C e,n according to Eq. 1.3. i n = u c R f , n + C e , n ⋅ du c dt

[0066] Transferring Eq. 1.3 to the discrete-time domain, we arrive at Eq. 1.4, where fs is the sampling frequency and k is the sampling index. i n k = u c k R f , n + f s ⋅ C e , n ⋅ u c k − u c k − 1

[0067] Fig. 3 shows schematically a structure of the electrical circuit arrangement 10 according to the invention, consisting of the insulation monitoring device IMD and the sensors CT according to the invention.

[0068] The insulation monitoring device IMD includes (see Fig. 2 ) the measuring path 42 connected between the active conductor L1, N and earth PE with the coupling resistor R a , the measuring voltage source 44 and the measuring resistor R m with IMD evaluation electronics 46.

[0069] In the line branches 4, the sensors CT measure the respective actual branch currents i 1 , i 2 ,..., in , where the respective branch current i 1 , i 2 ,..., in - in Fig. 3 For example, the branch current in - is referred to as the actually measured, real branch current ia.

[0070] A preferably bidirectional communication connection 70, which can be implemented, for example, as an RS485 interface, runs between the insulation monitoring device (IMD) and the CT sensors. The requirements for the communication connection 70 are that trigger signals for calibrating the measuring systems can be transmitted in real time (latency < 10 ms).

[0071] In addition and alternating with the injection and measurement of the real branch current ia, the above-described calculation of the branch current in based on the estimated quantities insulation resistance R f and leakage capacitance C e according to Eqs. 1.3 and 1.4 serves to conduct and measure a corresponding current flow as a simulated branch current ib through the respective sensor CT.

[0072] For this purpose, the measuring current transformer 20 of the ZCT sensor known from the prior art is Fig.1 by an auxiliary winding 26 with (only) one turn. The auxiliary winding 26 is embodied here, for example, as a conductor loop and can supplement an already installed ZCT sensor known from the prior art as a separate component. Alternatively, the auxiliary winding 26 can also be an integral physical component of the measuring current transformer 20.

[0073] The calculated and with a current source 28 ( Fig. 4 ) into the auxiliary winding 26 corresponds to the current characteristic of the real branch current in =ia if the combination of insulation resistance R f and leakage capacitance C e is calculated with sufficient accuracy.

[0074] Both current waveforms, that of the real branch current ia and that of the simulated branch current ib , are recorded one after the other via the secondary winding 24 of the measuring current transformer 20 of the sensor CT as real secondary current i ca and as simulated secondary current i cb and are compared with each other.

[0075] Fig. 4 shows a block diagram of the sensor CT according to the invention.

[0076] The sensor CT comprises, as elements known from the prior art, an AC measuring current transformer 20 with the toroidal core 22 and secondary winding 24. The secondary winding 24 is preferably designed with a number of turns in the hundreds range, e.g. 600.

[0077] As the primary winding, the line branch 4 carrying the real branch current ia is led - usually once - through the toroidal core 22.

[0078] In addition to the state-of-the-art sensor ZCT from Fig. 1 The measuring current transformer 20 of the sensor CT according to the invention has Fig. 4 the auxiliary winding 26.

[0079] The auxiliary winding 26 of the measuring current transformer 20 represents a key, new element through which the simulated branch current ib is fed. The auxiliary winding 26 is fed by the current source 28, which in turn is controlled by a digital-to-analog converter 29 as a µC peripheral. Alternatively, control via pulse width modulation (PWM) with subsequent low-pass filtering is possible.

[0080] The sensor CT further comprises a CT evaluation electronics 50, which includes a microprocessor for carrying out the calculations.

[0081] Furthermore, according to the invention, the sensor CT comprises a transmission path 30 having a load resistor 32, a low-pass filter 34, a variable-gain amplifier 36, and an analog-to-digital converter 38. The transmission path 30 with the transfer function G(s) transforms the continuous-time real secondary current i ca =i ca (t) and the continuous-time simulated secondary current i cb =i cb (t) into the corresponding discrete-time real output signal ya (k) and the simulated output signal yb (k).

[0082] The transmission system, consisting of the measuring current transformer 20 and the transmission link 30 - with the real / simulated branch current ia (t) / ib (t) as successively applied input signal up to the corresponding real / simulated output signal ya (k) / yb (k) - is assumed to be a black box 60 with the transfer function G'(s).

[0083] The transfer function G'(s) of this entire transmission system (measuring current transformer 20 and transmission link 30) does not need to be known, which significantly simplifies the problem-solving process. The properties of the black box 60 are computationally irrelevant. The only relevant factor is that no measurable change in the properties of the transmission system occurs during a measurement cycle, i.e., the sequential recording of the branch currents ia (t) and ib (t) with a duration of < 1 minute.

[0084] In Fig. 5 A flowchart of the method according to the invention is shown. The left side reflects the sequence of process steps in the insulation monitoring device (IMD), while the right side shows the sequence in the CT sensor.

[0085] The insulation monitoring device IMD assumes the role of master, while the sensors CT act as slaves. The insulation monitoring device IMD starts by switching on (activating) the measuring voltage ug (t), which is preferably formed by a bipolar periodic square-wave function. Directly connected to this, a start signal St is distributed to the sensors CT as a trigger pulse. While the sensors CT then measure the respective real branch current ia (t) in the form of a real secondary current i ca (t), transform it into a real output signal ya (k), and record it, the insulation monitoring device IMD records the measured voltage um (t) (after AD conversion) in a time-discrete manner.

[0086] After a fixed, predeterminable number of periods (measurement cycle), the measuring voltage ug (t) is switched off (deactivated) and both the recording of the measured voltage um (t) in the insulation monitoring device IMD and the real output signal ya (k) in the sensor CT are stopped.

[0087] If the measuring cycle comprises several periods, the measurement curves of the measured voltage um (t) and the real output signal ya (k) are optimized with regard to the signal-to-noise ratio by digital signal processing filter algorithms such as averaging.

[0088] This is followed by the calculation of the branch voltage uc (t) in the insulation monitoring device IMD, which is then transmitted to the sensors CT, preferably as an array with fixed dimensions.

[0089] In the CT evaluation electronics 50 of the CT sensors, the simulated branch current ib (t) is then calculated using the estimated quantities of insulation resistance R f and leakage capacitance C e as well as the branch voltage uc (t) and is impressed on the auxiliary winding 26 by means of the digital-to-analog converter 29 and the current source 28.

[0090] Meanwhile, the measured value of the simulated branch current ib (t) is recorded as the simulated secondary current i cb (t) and transformed into the simulated output signal yb (k) via the transmission path 30. This signal curve is also optimized using digital signal processing.

[0091] This is followed by a calculation of the error sum of squares according to ∑ k = 0 n y b k − y a k 2 → Min between the real output signal ya (k) and the simulated output signal yb (k)) and an optimization of the simulation variables insulation resistance R f and leakage capacitance C e in the CT evaluation electronics 50 by means of an iterative approximation method before the measurement cycle starts again.

[0092] Only when the sum of squares of the errors reaches a sufficiently minimal value, the corresponding pair of values ​​insulation resistance R f and leakage capacitance C e is output as a result value in the currently running measuring cycle and transmitted to the insulation monitoring device IMD.

[0093] In a further step, the estimated values ​​of the insulation resistance and the leakage capacitance are recalculated in order to further minimize the error sum of squares by purposefully changing the estimated values ​​in the continuously continuing sequence of measuring cycles within the following measuring cycle with the iteration step to be carried out therein.

[0094] The procedure is illustrated by the Fig.6 displayed time signals and their evaluation.

[0095] Shown is the time-limited application of the bipolar, rectangular measuring voltage ug (t), the sequential feed-in of the real branch current ia (t) and the simulated branch current ib (t), a corresponding input voltage u adc (t) of the analog-digital converter 38 with obvious deformation by the transmission system (black box 60) as well as the output signals ya (k), yb (k) derived from the recording of the branch currents ia (t), ib (t) - simplified representation as a continuous time course - of the transmission link 30 in comparison.

[0096] Comparing the real output signal ya (k) with the simulated output signal yb (k) shows that by minimizing the sum of squares, a close agreement between the two output signals ya (k) and yb (k) can be achieved. This means that the insulation resistance R f and leakage capacitance C e determined iteratively also correspond to the values ​​actually prevailing in the ungrounded power system.

Claims

1. A method for determining resistive and capacitive, insulation-determining variables (Rf, Ce) in a selected branch of a branched ungrounded power supply system (2) using insulation monitoring, the method comprising the following steps: switching on a periodic measuring voltage (ug(t)) by means of an insulation monitoring device (IMD) which is connected between at least one active conductor (L1) and ground (PE) and has a measuring path (42) having a measuring-voltage source (44), having a coupling resistor (Ra) and having at least one measuring resistor (Rm) having IMD evaluation electronics (46), sending a start signal (St) via a communication connection (70) from the insulation monitoring device (IMD) to a sensor (CT), which is disposed in a line branch (4) to be monitored of the ungrounded power supply system (2), measuring and recording a measured voltage (um(t)) dropping at the measuring resistor (Rm), measuring a real branch current (ia(t)), which is driven by the measuring voltage (ug(t)), in the sensor (CT) via a real secondary current (ica(t)), which is set in a secondary winding (24), by means of a measuring current transformer (20) having a toroid (22) and the secondary winding (24), transforming the real secondary current (ica(t)) into a time-discrete real output signal (ya(k)) by means of a transmission path (30) in the sensor (CT), recording the real output signal (ya(k)) in CT evaluation electronics (50) of the sensor (CT), switching off the measuring voltage (ug(t)) in the insulation monitoring device (IMD) in accordance with a pre-determinable period number, computing a branch voltage (uc(t)) from the measuring voltage (ug(t)), the measured voltage (um(t)), the coupling resistor (Ra) and the measuring resistor (Rm) in the IMD evaluation electronics (46), transmitting the branch voltage (uc(t)) from the insulation monitoring device (IMD) to the sensor (CT) of the line branch (4) to be monitored by means of the communication connection (70), computing a simulated branch current (ib(t)) from the branch voltage (uc(t)), the time derivative (d(uc(t)) / dt) of the branch voltage (uc(t)), an insulation resistance (Rf) of the line branch (4) to be monitored and a leakage capacitance (Ce) of the line branch (4) to be monitored in the CT evaluation electronics (50), respective estimated values being used as the initial value for the insulation resistance (Rf) and the leakage capacitance (Ce), supplying the simulated branch current (ib(t)) via the sensor (CT) by means of the measuring current transformer (20) via an auxiliary winding (26) and a power source (28) controlled by a DA converter (29), measuring the simulated branch current (ib(t)) by means of a simulated secondary current (icb(t)) set in the secondary winding (24) of the measuring current transformer (20), transforming the simulated secondary current (icb(t)) into a time-discrete simulated output signal (yb(k)) by means of the transmission path (30), and recording the simulated output signal (yb(k)) in the CT evaluation electronics (50), switching off the supply of the simulated branch current (ib(t)), computing the error square sum between the real output signal (ya(k)) and the simulated output signal (yb(k)) in the CT evaluation electronics (50), transmitting the insulation resistance (Rf) and the leakage capacitance (Ce) as the computation result to the insulation monitoring device (IMD) via the communication connection (70) should the error square sum be sufficiently small, newly computing estimated values for the insulation resistor (Rf) and for the leakage capacitance (Ce) in the CT evaluation electronics (50), starting a new measuring cycle when switching on the measuring voltage (ug(t)) and using the newly computed insulation resistance (Rf) and the newly computed leakage capacitance (Ce).

2. The method according to claim 1, characterized in that the transformation on the transmission path (30) takes place by tapping the voltage of the corresponding secondary current (ica(t), icb(t)) at a load resistor (32), low-pass filters (34), variable amplification (36) and analog-to-digital conversion (38).

3. The method according to claim 1 or 2, characterized in that an AC / DC-sensitive measuring current transformer (20) is used for measuring the branch current (ia(t), ib(t)).

4. The method according to any one of the claims 1 to 3, characterized in that the periodic measuring voltage (ug(t)) is comprised of bipolar square measuring pulses.

5. The method according to any one of the claims 1 to 4, characterized in that a winding separable from the measuring current transformer (20) is used as the auxiliary winding (26) of the measuring current transformer (20).

6. An electric circuit arrangement (10) for determining resistive and capacitive, insulation-determining variables (Rf, Ce) in a selected branch of a branched ungrounded power supply system (2) using insulation monitoring, the electric circuit arrangement (10) consisting of an insulation monitoring device (IMD), which is connected between at least one active conductor (L1) and ground (PE) and has a measuring path (42) having a measuring voltage source (44), having a coupling resistor (Ra) and having a measuring resistor (Rm) having IMD evaluation electronics (46) and of a sensor (CT), which is disposed in a line branch (4) to be monitored of the ungrounded power supply system (2) and has a measuring current transformer (20) having a toroid (22) and a secondary winding (24) in order to measure a branch current (ia(t), ib(t)) by means of a secondary current (ica(t), icb(t)), which is set in the secondary winding (24), the insulation monitoring device (IMD) is configured for switching on a periodic measuring voltage (ug(t)) generated by the measuring voltage source (44) and for switching off the measuring voltage (ug(t)) after a pre-determinable period number and for measuring and recording a measured voltage (um(t)) dropping at the measuring resistor (Rm), characterized in that the IMD evaluation electronics (46) are configured for computing a branch voltage (uc(t)) from the measuring voltage (ug(t)), the measured voltage (um(t)), the coupling resistor (Ra) and the measuring resistor (Rm), in that the sensor (CT) comprises a transmission path (30) for transforming the corresponding secondary current (ica(t), icb(t)) into a time-discrete output signal (ya(k), yb(k)), an auxiliary winding (26) of the measuring current transformer (20) and a power source (28) controlled by a DA converter (29) in order to supply a simulated branch current (ib(t)) and CT evaluation electronics (50), the CT evaluation electronics (50) being configured for recording a real output signal (ya(k)) and a simulated output signal (yb(k)), for computing the simulated branch current (ib(t)) from the branch voltage (uc(t)), the time derivative (d(uc(t)) / dt) of the branch voltage (uc(t)), an insulation resistance (Rf) of the line branch (4) to be monitored and from a leakage capacitance (Ce) of the line branch (4) to be monitored, for computing the error square sum between the real output signal (ya(k)) and the simulated output signal (yb(k)) in the CT evaluation electronics (50), for transmitting the insulation resistance (Rf) and the leakage capacitance (Ce) to the insulation monitoring device (IMD) should the error square sum be sufficiently small, for newly computing estimated values for the insulation resistance (Rf) and for the leakage capacitance (Ce), and in that a communication connection (70) is disposed between the insulation monitoring device (IMD) and the sensor (CT) in order to send a start signal, for transmitting the branch voltage (uc(t)) and for transmitting the insulation resistance (Rf) and the leakage capacitance (Ce).

7. The electric circuit arrangement (10) according to claim 6, characterized in that the transmission path (30) has a load resistor (32), a low pass filter (34), an amplifier having variable amplification (36) and an AD converter (38).

8. The electric circuit arrangement (10) according to claim 6 or 7, characterized in that the measuring current transformer (20) is configured to be AC / DC sensitive for measuring the branch current (ia(t), ib(t)).

9. The electric circuit arrangement (10) according to any one of the claims 6 to 8, characterized in that the periodic measuring voltage (ug(t)) generated by the measuring voltage source (44) is composed of bipolar square measuring pulses.

10. The electric circuit arrangement (10) according to any one of the claims 6 to 8, characterized in that the auxiliary winding (26) of the measuring current transformer (20) is configured as a winding separable from the measuring current transformer (20).