Method for determining the condition of a power cable in a power supply network using broadband powerline communication signals

Abstract

Method for determining a condition of a power cable in a power supply network, in particular in a low-voltage or medium-voltage network, wherein the power supply network has at least two broadband powerline modems and at least one power cable coupled between the broadband powerline modems for transmitting a data signal, comprising the following method steps: S1) Determining a signal-to-noise ratio per transmission frequency of the data signal at the broadband powerline modem for one time step, S2) Determining a slope of the signal-to-noise ratio over the transmission frequencies for the time step, S3) Repeating steps S1) and S2) for several time steps within a predetermined period to obtain several gradients, S4) interpolating the plurality of gradients for the predetermined period and determining a gradient coefficient, wherein the gradient coefficient indicates the change in the gradients from step S2) over the plurality of time steps, S5) Providing a cable length and the cable type of the power cable and normalizing the slope coefficient over the cable length, S6) Determining the state of the power cable depending on a predetermined limit value, wherein the state is specified as a function of the time steps by means of the slope coefficient and an output value.

Description

[0001] The invention relates to a method for determining a state of a power cable in a power supply network, in particular in a low-voltage or medium-voltage network, wherein the power supply network has at least two broadband power-line modems and at least one power cable coupled between the broadband power-line modems for transmitting a data signal.

[0002] Traditionally, the condition of low-voltage power cables is not usually determined, as using conventional methods, such as dissipation factor measurement, would entail significant effort and expense for grid operators, resulting in supply disruptions to connected customers. Low voltage refers to alternating voltages up to 1000 V and direct voltages up to 1500 V. Low-voltage networks are defined as sections of the power grid that distribute electrical energy to the majority of electrical end users or low-voltage devices. Low-voltage networks are preferably operated with a single-phase voltage of 230 V or a three-phase voltage of 400 V.

[0003] At the medium-voltage level, conventional methods are occasionally used to record the cable condition. However, this method can only provide information about the condition for individual cable sections at the current time.

[0004] DE 10 2020 003 143 A1 describes a device for diagnosing an electrical line in order to predict failure, wear, and / or aging of the line. The electrical line is intended to connect two electrical units. The electrical line comprises at least two sections. The device is inserted between these two sections. For this purpose, the device comprises a first and a second interface for contacting a respective end of one of the two sections of the electrical line. A detection device with a digital signal processor is provided at at least one of the second interfaces.

[0005] WO 2022 / 002442 A1 describes a system for industrial automation, comprising a field device with a functional unit, in particular an actuator unit, sensor unit and / or control unit, which is designed to provide a function according to received payload data, and a communication unit for receiving a payload signal containing the payload data via a cable, wherein the communication unit is designed to provide a parameter set based on the payload signal and to carry out signal processing of the payload signal using the parameter set in order to obtain the payload data from the payload signal, wherein the system further comprises a diagnostic device which is designed to determine a cable wear condition of the cable according to an indicator variable based on the parameter set.

[0006] DE 10 2022 103 006 A1 describes a system for determining a state of a power supply network and / or a network component in the power supply network using powerline communication, preferably broadband powerline. This system comprises a plurality of powerline nodes configured to transmit data via powerline signals over the power supply network, to register with one of several powerline cells, and to determine information about a state of adjacent parts of the power supply network.

[0007] EP 2 608 417 B1 describes a method for monitoring the status of a supply network, wherein the supply network is designed for transmitting electrical energy and for data transmission using a PLC (Power-Line Communication) system.

[0008] EP 3 538 905 B1 describes a method for monitoring the condition of an electrical line, in which data communication is carried out via an electrical line, parameters for the data communication being determined, and a condition of the electrical line being estimated from the parameters.

[0009] The devices or methods known from the prior art do not yet allow the aforementioned disadvantages to be overcome and do not provide a way to continuously determine the technical condition of a power cable of a low-voltage or medium-voltage network during operation using Britband powerline communication.

[0010] Based on this, the object of the invention is to provide a method with which the technical condition of a power cable of a low-voltage or medium-voltage network can be determined simply and cost-effectively and without interrupting the supply.

[0011] This object is achieved by the subject matter of patent claim 1. Preferred developments can be found in the subclaims.

[0012] According to the invention, a method is provided for determining the condition of a power cable in a power supply network, in particular in a low-voltage or medium-voltage network, wherein the power supply network has at least two broadband powerline modems and at least one power cable coupled between the broadband powerline modems for transmitting a data signal, comprising the following method steps: S1) Determining a signal-to-noise ratio per transmission frequency of the data signal at the broadband powerline modem for one time step, S2) Determining a slope of the signal-to-noise ratio over the transmission frequencies for the time step, S3) Repeating steps S1) and S2) for several time steps within a predetermined period to obtain several gradients, S4) interpolating the plurality of gradients for the predetermined period and determining a gradient coefficient, wherein the gradient coefficient indicates the change in the gradients from step S2) over the plurality of time steps, S5) Providing a cable length and the cable type of the power cable and normalizing the slope coefficient over the cable length, S6) Determining the state of the power cable depending on a predetermined limit value, wherein the state is specified as a function of the time steps by means of the slope coefficient and an output value.

[0013] A key aspect of the invention is therefore that the technical condition of the power cable can be monitored using broadband power line modems (BPL modems). Broadband over Power Cable (BPL) is a method of power cable (PLC) communication that enables digital data transmission over public power distribution lines. BPL uses higher frequencies, a wider frequency range, and different technologies than other forms of power line communication to enable high-speed communication over longer distances. BPL uses frequencies that are part of the radio frequency spectrum allocated for wireless communication services. BPL modems are usually already installed in the power grid, so no modifications are required; the necessary data is continuously retrieved, thus saving costs.

[0014] It has been shown that the quality of data transmission correlates with the technical condition of power cables. Therefore, the signal-to-noise ratio (SNR) is measured across the entire transmission frequency range, particularly from 2 to 28 MHz. The change in SNR across the transmission frequencies can be plotted as a gradient in a graph. This gradient, in conjunction with a baseline value and predetermined or individually defined thresholds, can be used to determine the technical condition.

[0015] Since the noise power at each BPL modem is unknown and can vary significantly, it is essential to the invention that trends are analyzed. Therefore, the slope of the SNR is determined across the transmission frequencies for several different time steps. Seasonal effects and impulses can preferably be filtered out using low-pass filtering, such as a moving average. The change in the slope over these time steps, i.e., the change in state, can then also be specified as a slope. The change in state is then determined as the slope or change in the slopes of the SNR over the several time steps. This second slope or change in state is described using a slope coefficient. This slope coefficient is determined over the cable length l Kabel standardized. The cable type includes, in particular, paper, PVC, or XLPE insulation.

[0016] Using an initial value and the standardized gradient coefficient, a function can be created that indicates the condition of the power cable as a function of time. This is preferably a linear function of the form f(x) = mx+n. Where f is the technical condition, x is the time, m is the gradient coefficient, and n is the initial value. The initial value is understood to be a predetermined and defined value that represents a new cable in perfect technical condition without wear. The initial value is preferably determined in advance according to steps S1) and S2) for a starting time of an initial power cable. These initial values differ, in particular, depending on the insulation of the power cable.

[0017] The slope coefficient is interpolated over the entire operating period t in years. The technical condition of the power cable is preferably described by the following linear function: State(t)=Initial value+mCablelCable⋅12Mta⋅t.

[0018] Preferably, the initial values for paper insulation or PVC insulation include the following values: Output value paper = −7.88⋅10−3dBMHz⋅m, Output valuePVC=−7.99⋅10−3dBMHz⋅m.

[0019] The condition of the power cable can then be determined using predetermined or predefined threshold values. Exceeding the threshold value indicates a critical technical condition. These threshold values can be freely selected depending on the network operator's risk affinity.

[0020] Preferably, the predetermined limit values depend on the material properties of the insulation of the power cable. In particular, the limit values for paper insulation and PVC insulation include the following values: Limit Paper>−4.94⋅10−3dBMHz⋅m, Limit value PVC<−9.58⋅10−3dBMHz⋅m.

[0021] By monitoring long-term SNR trends, the cable condition can be continuously monitored and a "live monitoring system" can be established. This provides added value for network operators, allowing them to monitor the condition of their cable network during ongoing operations without additional effort and plan renewal measures more effectively than before.

[0022] According to a preferred development of the invention, the slope of the signal-to-noise ratios over the transmission frequencies and / or the change in the slope coefficients over the time steps is determined using linear regression. Linear regression (LR for short) is a special case of regression analysis, i.e., a statistical method that attempts to explain an observed dependent variable using one or more independent variables. Linear regression assumes a linear model (LM for short). Therefore, only relationships are considered in which the dependent variable is a linear combination of the regression coefficients (but not necessarily of the independent variables). It has been shown that the power cable exhibits linear aging; therefore, linear regression is suitable for interpolating the values over a longer period of time and determining the slope.

[0023] According to a preferred embodiment of the invention, the predetermined period covers at least three years. The longer the SNR trend tracked across the transmission frequency, the better the significance of the analysis. It has been shown that meaningful results can be achieved with a minimum period of three years.

[0024] According to a preferred development of the invention, the method comprises the following further method steps: S3a) Repeating steps S1) and S2) for several points in time included in the time step, and S3b) Filtering the data from step S3b) by determining a median for a defined period and low-pass filtering.

[0025] The data is filtered using medians and low-pass filtering to minimize the influence of noise. To filter out individual noise pulses that occur in the SNR curve, the median of the determined slope coefficients is first determined for each time step for a defined period. This ensures that the median is an "information vector" and not a "noise vector." The time step can, in particular, cover a month. Within this month, several measurements were taken at different times, for example, 30 days. The multiple time steps, i.e., the months, add up to the predetermined period, for example, one, two, or three years.

[0026] According to a preferred development of the invention, the method comprises the following further method steps: S3c) comparing the slope of a first time step and the slope of a second time step following the first time step and determining a deviation, S3d) if the deviation is greater than a predetermined threshold, then defining the second time step as a new start time for the predetermined period.

[0027] In this way, topology changes can be detected and taken into account. Topology changes are detected by comparing the gradients of successive time steps. If the deviation is too large, for example, greater than 0.39 dBMHz A topology change has occurred. If a topology change has been detected, the next steps must be performed separately.

[0028] According to a preferred embodiment of the invention, the condition is classified into the following categories: "action required" or "no action required" or "monitor cable." The condition can be provided to an evaluation unit or warning unit so that further action can be initiated in the event of a poor condition.

[0029] According to a preferred development of the invention, the method comprises the following further method steps: S4a) Carrying out steps S1) to S4) for an incoming data signal from a broadband powerline modem, and S4b) Determining an overall slope coefficient, wherein the overall slope coefficient results from the mean value of the slope coefficient of the incoming data signals.

[0030] In this way, both the forward and return connections of a cable route, or the incoming data signals at the broadband powerline modems, can be described and taken into account when determining the slope coefficient. The average of the two slope coefficients of the forward and return connections is calculated and used as the overall slope coefficient m. Kabel the cable route.

[0031] To determine this mean value, the method according to a preferred development of the invention comprises the following further method steps: S4b') if one of the slope coefficients of one of the incoming data signals is negative and the power cable is covered with paper insulation, then overwriting the respective slope coefficient with the value zero to determine the mean value of the slope coefficients, or S4b'') if one of the slope coefficients of one of the incoming data signals is positive and the power cable is sheathed with PVC insulation, then overwriting the respective slope coefficient with the value zero to determine the average value of the slope coefficients.

[0032] To determine the mean value of the slope coefficients of the forward and return connections, it is assessed whether the slope coefficients are positive or negative. Since an improvement in the condition of cables is unrealistic and would only indicate an excessive influence of noise, two rules are defined: For paper-insulated cables, the slope coefficient must not be negative. If this is the case, it is set to zero.

[0033] For PVC-insulated cables, the gradient coefficient must not be positive. If this is the case, it is set to zero. The average of the two gradient coefficients for the forward and return connections is then calculated and used as the total gradient coefficient m. Kabel the cable route is assumed.

[0034] The invention will be explained in more detail below using a preferred embodiment with reference to the drawings.

[0035] The drawings show Fig. 1 schematically shows a method for determining a condition of a power cable according to a preferred embodiment of the invention, and Fig. 2 schematically shows a graphical representation for determining the change in state for a period of time.

[0036] Out of Fig. Figure 1 schematically shows a method for determining the condition of a power cable in a power supply network, in particular in a low-voltage network or medium-voltage network. The power supply network comprises at least two broadband powerline modems and at least one power cable coupled to the broadband powerline modems for transmitting a data signal.

[0037] In a first step S1, an SNR is recorded for each transmission frequency of the BPL modem's frequency spectrum for one time step. This SNR is plotted against the transmission frequency, and a slope is determined from it (S2).

[0038] The aforementioned steps are repeated for several time steps of a predetermined period S3. To filter the data, the SNR is recorded repeatedly for each time step at several different points in time S3a, after which the median is determined and filtered using low-pass filtering S3b. A slope is therefore determined for several points in time and from this the median is calculated, which is used as the slope for the time step for the subsequent steps. The slopes of the time steps, which each represent the median of the slopes from several points in time, are then interpolated for the predetermined period. The change in the slope over time is then specified using a slope coefficient S4. In parallel, the topology check takes place. Topology changes can be detected by comparing the slopes of two consecutive time steps S3c.If the deviation exceeds a predetermined threshold, a topology change has occurred. In this case, the subsequent steps must be performed separately (S3d).

[0039] The aforementioned steps are performed for both the forward and return connections, i.e., for both the incoming data signals on two interconnected broadband powerline modems (S4a). The slope coefficient of the forward path and the slope coefficient of the return path are averaged (S4b), thus providing an overall slope coefficient for the subsequent process steps.

[0040] When determining the mean value, two rules are established: either S4b' if one of the slope coefficients of the incoming data signal or the outgoing data signal is negative and the power cable is sheathed with paper insulation, then the respective slope coefficient is overwritten with the value zero, or S4b'' if one of the slope coefficients of the incoming data signal or the outgoing data signal is positive and the power cable is sheathed with PVC insulation, then the respective slope coefficient is overwritten with the value zero. In this way, incorrect values that indicate an unrealistic condition improvement are not taken into account when calculating the mean value.

[0041] The provided slope coefficient is then standardized over the cable length S5 and combined with an output value to form a function of the condition of the power cable S6. It has been shown that the condition of the power cable influences the transfer function and thus the SNR. For paper-insulated cables, a deterioration in condition results in a lower slope of the attenuation constant over the frequency range under consideration. This in turn means that the transfer function also has a lower slope and, as a result, the SNR in the upper and lower frequency ranges converges. For PVC-insulated cables, the opposite behavior is observed, with the slope of the attenuation constant increasing as the condition deteriorates over the frequency range under consideration. As the condition of PVC-insulated cables deteriorates, the SNR shows an increasingly larger difference between the upper and lower frequency ranges.

[0042] If the damping constant dα(f)df for trend analysis at several points in time and then plotted over time, the diagram shows Fig. 2. It is evident that the aging or deterioration of the condition decreases linearly. Therefore, a linear regression is performed on the values to interpolate the values over a longer period and determine a slope. The slope is defined as the slope coefficient m. Kabel specified and the cable length l Kabel standardized so that, together with an initial value representing a new cable in perfect technical condition without wear, the condition of the power cable can be determined using the following formula: State(t)=Initial value+mCablelCable⋅12Mta⋅t.

[0043] The invention underlying this patent application was created in a project funded by the BMBF under the funding code 03SF0568A (“Sensors in the Network 2.0”). List of reference symbols S1 Acquisition of a signal-to-noise ratio per transmission frequency for one time step S2 Determine a slope of the signal-to-noise ratio over the transmission frequencies for the time step S3 Repeat steps S1) and S2) for several time steps S3a Repeating steps S1) and S2) for several points in time included in the time step S3b Filtering the data S3c Comparing the slope of a first time step and the slope of a second time step following the first time step and determining a deviation S3d Defining the second time step as a new start time for the predetermined period S4 Interpolating the multiple slopes for the predetermined period and determining a slope coefficient S4a Performing steps S1) to S4) for each of the incoming data signals S4b Determining an overall slope coefficient S4b', S4b'' Overwrite the respective slope coefficient with the value zero S5 Providing a cable length of the power cable and normalizing the slope coefficient S6 Determining the condition of the power cable depending on a predetermined limit value

Claims

[1] Method for determining a condition of a power cable in a power supply network, in particular in a low-voltage or a medium-voltage network, wherein the power supply network has at least two broadband powerline modems and at least one power cable coupled between the broadband powerline modems for transmitting a data signal, comprising the following method steps: S1) Determining a signal-to-noise ratio per transmission frequency of the data signal at the broadband powerline modem for one time step, S2) Determining a slope of the signal-to-noise ratio over the transmission frequencies for the time step, S3) Repeating steps S1) and S2) for several time steps within a predetermined period to obtain several gradients, S4) interpolating the plurality of gradients for the predetermined period and determining a gradient coefficient, wherein the gradient coefficient indicates the change in the gradients from step S2) over the plurality of time steps, S5) Providing a cable length and the cable type of the power cable and normalizing the gradient coefficient over the cable length, S6) Determining the state of the power cable depending on a predetermined limit value, wherein the state is specified as a function of the time steps by means of the gradient coefficient and an output value. [2] The method according to claim 1, wherein the output value was determined in advance after steps S1) and S2) for a starting time of an output power cable. [3] Method according to claim 1 or 2, wherein the slope of the signal-to-noise ratios over the transmission frequencies and / or the change in the slope coefficients over the time steps is determined by means of linear regression. [4] Method according to one of the preceding claims, wherein the predetermined limit value is dependent on material properties of an insulation of the power cable. [5] Method according to one of the preceding claims, wherein the predetermined period comprises at least three years. [6] Method according to one of the preceding claims, with the following further method steps: S3a) Repeating steps S1) and S2) for several points in time included in the time step, and S3b) Filtering the data from step S3b) by determining a median for a defined period and low-pass filtering. [7] Method according to one of the preceding claims, with the following further method steps: S3c) comparing the gradient of a first time step and the gradient of a second time step following the first time step and determining a deviation, S3d) if the deviation is greater than a predetermined threshold, then defining the second time step as a new start time for the predetermined period. [8] Method according to one of the preceding claims, wherein the condition is classified into the following categories: "need for action" or "no need for action" or "watch cable". [9] Method according to one of the preceding claims, with the following further method steps: S4a) Carrying out steps S1) to S4) for an incoming data signal from a broadband powerline modem, and S4b) Determining a total slope coefficient, wherein the total slope coefficient results from the mean value of the slope coefficient of the incoming data signals. [10] Method according to claim 9, wherein the power cable is covered with a paper insulation or with a PVC insulation, with the following further method steps: S4b') if one of the slope coefficients of one of the incoming data signals is negative and the power cable is covered with paper insulation, then overwriting the respective slope coefficient with the value zero to determine the mean value of the slope coefficients, or S4b'') if one of the slope coefficients of one of the incoming data signals is positive and the power cable is sheathed with PVC insulation, then overwriting the respective slope coefficient with the value zero to determine the average value of the slope coefficients.

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