JP2019215212A - Power cable insulation deterioration detection device and insulation deterioration inspection method - Google Patents

Abstract

To provide a power cable insulation deterioration inspection device that can highly accurately detect insulation deterioration of a power cable.SOLUTION: A power cable insulation deterioration detection device comprises: an AC power source that is electrically connected to a shield layer of a power cable, and grounded, and applies an AC voltage to the shield layer under a live wire; a control unit that causes an output frequency the AC power source applies to change in a frequency area shifting from an integral multiple of a commercial frequency to be supplied to the power cable; an electricity measurement unit that measures electricity flowing from the power cable to the ground via the AC power source; and a signal analysis unit that performs a frequency analysis of the electricity measured by the electricity measurement unit. A differential frequency between the output frequency and the integral multiple of the commercial frequency is equal to or more than 0.5[Hz].SELECTED DRAWING: Figure 6

Description

  The present invention relates to a device and a method for detecting insulation deterioration of a power cable.

  Conventionally, devices and methods for detecting insulation deterioration of a power cable have been proposed. For example, in Patent Document 1, an AC power source applies an AC voltage having a frequency of an integral multiple of the commercial frequency ± aHz (0 <a ≦ 10) to a shielding layer of a power cable to be measured under a live line. A current flowing through the AC power supply to the ground through a current measuring means, and diagnosing the degree of insulation deterioration of the power cable in a live state by a diagnostic means based on the current measured by the current measuring means. The technique of the insulation deterioration diagnosis method described above is disclosed.

  According to the technique of Patent Document 1, the superimposed voltage is an AC voltage, and a stray current generated via a sheath insulation resistance does not increase unlike a DC voltage, and only a deteriorated signal is amplified. .

Japanese Patent No. 3317391

  Here, depending on the current measurement line, noise other than the DC component current may be scattered. When such noise is present, it may be difficult to detect a deteriorated signal due to the influence of the noise. It is desired that the influence of noise can be reduced and the detection accuracy of insulation deterioration can be improved.

  An object of the present invention is to provide a power cable insulation deterioration detection device and an insulation deterioration detection method that can detect insulation deterioration of a power cable with high accuracy.

  The power cable insulation deterioration detection device of the present invention is electrically connected to a shielding layer of the power cable, and is grounded, and an AC power supply that applies an AC voltage to the shielding layer under a live line. A control unit that changes an output frequency applied by the AC power supply in a frequency range deviated from an integral multiple of a commercial frequency supplied to the power cable, and a current flowing from the power cable to the ground via the AC power supply. A current measuring unit for measuring; and a signal analyzing unit for performing frequency analysis of the current measured by the current measuring unit, wherein a difference frequency between the output frequency and an integral multiple of the commercial frequency is 0.5 [Hz]. It is characterized by the above.

  An insulation deterioration detection device for a power cable according to the present invention is electrically connected to a shield layer of the power cable, and is grounded, and an AC power supply that applies an AC voltage to the shield layer under a live line. A control unit that changes the output frequency applied by the AC power supply in a frequency range deviated from an integral multiple of the commercial frequency supplied to the power cable, and a current measurement unit that measures a current flowing from the power cable to the ground via the AC power supply. And a signal analysis unit that analyzes the frequency of the current measured by the current measurement unit.

  The difference frequency between the output frequency and the integral multiple of the commercial frequency is 0.5 [Hz] or more. ADVANTAGE OF THE INVENTION According to the insulation deterioration detection apparatus of the power cable which concerns on this invention, the influence of a stray current etc. can be suppressed because a lower limit of a difference frequency is set to 0.5 [Hz]. Therefore, the power cable insulation deterioration detecting device according to the present invention has an effect that the insulation deterioration of the power cable can be detected with high accuracy.

FIG. 1 is a schematic configuration diagram of a power cable insulation deterioration detection device according to the first embodiment. FIG. 2 is a flowchart of the power cable insulation deterioration detection method according to the first embodiment. FIG. 3 is a diagram illustrating a display example of a frequency analysis result according to the first embodiment. FIG. 4 is another diagram showing a display example of the frequency analysis result of the first embodiment. FIG. 5 is still another diagram illustrating a display example of a frequency analysis result according to the first embodiment. FIG. 6 is a diagram showing a list display of the frequency analysis results of the first embodiment. FIG. 7 is a diagram illustrating frequency characteristics of the deteriorated signal. FIG. 8 is a schematic configuration diagram of a power cable insulation deterioration detection device according to the second embodiment. FIG. 9 is a flowchart according to the power cable insulation deterioration detection method of the second embodiment. FIG. 10 is another flowchart according to the power cable insulation deterioration detection method of the second embodiment. FIG. 11 is a diagram illustrating an example of a band shift. FIG. 12 is a diagram illustrating another example of the band shift.

  Hereinafter, a power cable insulation deterioration detection device and insulation deterioration detection method according to an embodiment of the present invention will be described in detail with reference to the drawings. In addition, this invention is not limited by this embodiment. In addition, constituent elements in the following embodiments include those that can be easily assumed by those skilled in the art or those that are substantially the same.

[First Embodiment]
The first embodiment will be described with reference to FIGS. The present embodiment relates to a device and a method for detecting insulation deterioration of a power cable. FIG. 1 is a schematic configuration diagram of a power cable insulation deterioration detection device according to the first embodiment, FIG. 2 is a flowchart of a power cable insulation deterioration detection method according to the first embodiment, and FIG. FIG. 4 is a diagram showing a display example of the frequency analysis result of the first embodiment. FIG. 4 is another diagram showing a display example of the frequency analysis result of the first embodiment. FIG. 5 is a table of the frequency analysis result of the first embodiment. FIG. 6 is a diagram showing a list display of frequency analysis results of the first embodiment, and FIG. 7 is a diagram showing frequency characteristics of a deteriorated signal.

  As shown in FIG. 1, a power cable insulation deterioration detection device 1 (hereinafter, simply referred to as “cable deterioration detection device 1”) according to the present embodiment includes an AC power supply 2, a diagnosis device 3, a current measurement unit 4, It has a security circuit 5 and a frequency acquisition unit 29. A power cable 10 to be measured for deterioration detection is connected to a high-voltage bus 12 via a cable terminal 11. The power cable 10 is, for example, a crosslinked polyethylene insulated vinyl sheath cable (CV cable). The power cable 10 has a core wire 10a and a shielding layer 10b. An insulating layer (not shown) is provided between the core wire 10a and the shielding layer 10b. The shielding layer 10b is a layer that covers the insulating layer from the outside, and is made of, for example, a conductive metal such as copper.

  The AC power supply 2 applies an AC voltage to the shielding layer 10b under the live line. The AC power supply 2 includes an AC voltage generation circuit 21, a transformer 22, and a control unit 25. The AC voltage generation circuit 21 is a circuit that generates an AC voltage. The AC voltage generation circuit 21 outputs an AC voltage having a commanded output frequency f0. The transformer 22 boosts the AC voltage generated by the AC voltage generation circuit 21. The transformer 22 has a primary winding 23 and a secondary winding 24. The primary winding 23 is connected to the output side of the AC voltage generation circuit 21. One end of the secondary winding 24 is electrically connected to the shielding layer 10b via a connection line 26. The other end of the secondary winding 24 is grounded via a connection line 27. That is, the secondary winding 24 is interposed between the shielding layer 10b and the ground part 28. The connection lines 26 and 27 and the secondary winding 24 function as ground lines for grounding the shielding layer 10b. The connection line 27 may be connected to a capacitor ground.

  The control unit 25 controls the AC voltage generation circuit 21 based on an operation input to an operation unit 34 described later. More specifically, the control unit 25 instructs the AC voltage generation circuit 21 on the output frequency f0 of the AC voltage to be generated, based on the command signal sent from the operation unit 34.

  The current measuring section 4 is provided on the connection line 27. The current measuring unit 4 measures a current flowing through the connection line 27, in other words, a current flowing between the secondary winding 24 and the grounding unit 28. That is, the current measuring unit 4 measures a current flowing from the power cable 10 to the ground via the AC power supply 2. The security circuit 5 is connected to the secondary winding 24 and the current measuring unit 4 in parallel. The security circuit 5 protects the secondary winding 24 and the current measuring unit 4 from overcurrent and overvoltage.

  The diagnostic device 3 includes a signal analysis unit 31, a synchronization circuit 32, a display 33, an operation unit 34, a data recording medium 35, and a determination unit 36. The signal analysis unit 31 performs a frequency analysis of the current measured by the current measurement unit 4. The signal analyzing unit 31 is communicably connected to the current measuring unit 4 and acquires a measurement result by the current measuring unit 4. The signal analysis unit 31 performs signal analysis on the signal of the acquired measurement result. The signal analysis unit 31 of the present embodiment performs a signal analysis on the acquired signal by a fast Fourier transform (hereinafter, also referred to as “FFT”). By the FFT, a current component for each frequency is calculated. The synchronization circuit 32 is a circuit that synchronizes the output frequency f0 of the AC voltage generation circuit 21 with the detection signal acquired by the signal analysis unit 31.

  The display 33 is a display device that displays an image of a measurement result obtained by the current measurement unit 4. The display 33 displays an image related to the measurement result by the current measuring unit 4 on the screen based on the information acquired from the signal analyzing unit 31. The operation unit 34 is an operation input unit operated by the user, and is, for example, a switch, a dial, a button, a touch panel, or the like. The operation unit 34 is communicably connected to the control unit 25. A signal indicating an operation input to the operation unit 34 is transmitted to the control unit 25.

  The data recording medium 35 is a recording medium that can be read and written by a computer. The data recording medium 35 is for recording data relating to the measurement results obtained by the current measuring unit 4, and for example, records data of the analysis results obtained by the signal analyzing unit 31, and the like. The program executed by the signal analysis unit 31 may be recorded on the data recording medium 35. The data recording medium 35 may be removable from the diagnostic device 3.

  The determination unit 36 determines the insulation deterioration of the power cable 10 based on the result of the signal analysis performed by the signal analysis unit 31. The determination method by the determination unit 36 will be described later.

  The frequency acquisition unit 29 acquires a commercial frequency f1, which is a frequency of a commercial power supply supplied to the power cable 10. The frequency acquisition unit 29 is electrically connected to the core wire 10a of the power cable 10, for example, and detects the actual commercial frequency f1. The frequency acquisition unit 29 may acquire the actual commercial frequency f1 from an existing device. The frequency acquisition unit 29 outputs the acquired information on the commercial frequency f1 to the control unit 25. The frequency acquisition unit 29 of the present embodiment detects the commercial frequency f1 in response to a command from the control unit 25, and transmits a detection result to the control unit 25. The frequency acquisition unit 29 may repeatedly detect the commercial frequency f1 at predetermined time intervals, and transmit the latest value of the commercial frequency f1 in response to a request from the control unit 25. In the following description, the value of the actual commercial frequency f1 acquired by the frequency acquisition unit 29 is particularly referred to as “actual frequency fr”.

  The operation of the cable deterioration detection device 1 according to the present embodiment and the cable deterioration detection method will be described in detail. The deterioration detection operation by the cable deterioration detection device 1 is started, for example, by a user's instruction. The user performs an operation input to the operation unit 34 to instruct the start of the deterioration detection operation. In addition, the user performs an operation input to the operation unit 34 to specify a width, a resolution, a speed, and the like as sweep conditions for changing the output frequency f0 of the AC power supply 2. The width of the sweep condition is the width of the frequency domain that changes the output frequency f0. The resolution is a change width when changing the output frequency f0. The speed is the time from when the output frequency f0 is set to one value until it changes to the next value.

  With reference to FIG. 2, an operation of the cable deterioration detection device 1 of the present embodiment, that is, a cable deterioration detection method will be described. In step S1, the control unit 25 acquires the commercial frequency f1 from the frequency acquisition unit 29. That is, in step S1, the control unit 25 acquires the latest value of the actual frequency fr. The control unit 25 may cause the frequency acquisition unit 29 to detect the actual commercial frequency f1 at that time, and acquire the value of the detection result. When step S1 is executed, the process proceeds to step S2.

  In step S2, the control unit 25 determines the output frequency f0. The control unit 25 calculates the output frequency f0 based on the commercial frequency f1 acquired from the frequency acquisition unit 29 in step S1. In other words, the control unit 25 determines the output frequency f0 according to the actual frequency fr. When step S2 is performed, the process proceeds to step S3.

  In step S3, the control unit 25 causes the AC voltage generation circuit 21 to generate an AC voltage having an output frequency f0. The AC voltage generation circuit 21 applies an AC voltage having an output frequency f0 to the shielding layer 10b of the power cable 10. When step S3 is executed, the process proceeds to step S4.

  In step S4, the control unit 25 causes the current measuring unit 4 to measure the current. The current measuring unit 4 measures a current flowing from the power cable 10 to the grounding unit 28 via the AC power supply 2. When step S4 is executed, the process proceeds to step S5.

  In step S5, the signal analyzer 31 performs a frequency analysis. The signal analyzer 31 performs a frequency analysis of the current value measured in step S4. The result of the frequency analysis is displayed on the display 33. When step S5 is executed, the process proceeds to step S6.

  In step S6, the control unit 25 determines whether to end the measurement. The control unit 25 of the present embodiment detects the deterioration of the power cable 10 by sequentially applying AC voltages having different output frequencies f0 to the shielding layer 10b. If the application of the plurality of output frequencies f0 has not been completed, a negative determination is made in step S6, and the process proceeds to step S1. On the other hand, when the application of the plurality of output frequencies f0 is completed, an affirmative determination is made in step S6, and the present control flow ends.

  The output frequency f0 will be described in more detail. The control unit 25 of the present embodiment sets the output frequency f0 to a frequency shifted from an integral multiple of the actual frequency fr, in other words, a frequency near an integral multiple of the actual frequency fr. In this specification, a frequency that is an integral multiple of the actual frequency fr is referred to as an “N-fold frequency fn” (N = 0, ± 1, ± 2,...).

When an AC voltage having a frequency shifted from the N-fold frequency fn is applied, the voltage V applied to the insulating layer of the power cable 10 is approximated by the following equation (1). Here, ω1 = 2πfr, ω2 = 2πΔa, fr is the actual frequency fr, Δa is the frequency of the difference between the output frequency f0 and the N-fold frequency fn (hereinafter simply referred to as “difference frequency”), and V1 is the commercial power supply. The voltage V2 is the output voltage of the AC voltage generation circuit 21. In other words, the power cable 10 is in a state where an AC voltage V2 whose frequency is the difference frequency Δa is applied in addition to the voltage V1 of the commercial power supply. In other words, the same state as when the voltage V2 having the difference frequency Δa is superimposed on the power cable 10 is realized. Therefore, deterioration diagnosis of the power cable 10 can be performed based on the current component of the difference frequency Δa.
V = V1sinω1t−V2sinω2t (1)

  The signal analysis unit 31 starts analyzing the signal acquired from the current measurement unit 4 in synchronization with the start of the generation of the AC voltage by the AC voltage generation circuit 21. This synchronization is performed by the synchronization circuit 32. The synchronization circuit 32 acquires, for example, information on the operation state of the AC voltage generation circuit 21 from the control unit 25. When the operation state of the AC voltage generation circuit 21 is a state in which an AC voltage is being generated, the synchronization circuit 32 issues a command to the signal analysis unit 31 to execute analysis of a signal acquired from the current measurement unit 4. Do. The signal analysis unit 31 that has received the analysis execution instruction performs signal analysis on the signal acquired from the current measurement unit 4 while a predetermined period elapses. In the present embodiment, the predetermined period is, for example, 20 [seconds]. Therefore, the signal analysis unit 31 performs the FFT analysis on the signal acquired from the current measurement unit 4 within, for example, 20 seconds after receiving the analysis execution command.

  Further, the signal analysis unit 31 performs frequency analysis of an AC voltage applied to the power cable 10 by the AC power supply 2. The AC power supply 2 has a voltage measuring unit (not shown) that measures the AC voltage generated by the AC voltage generating circuit 21. The signal analysis unit 31 calculates a frequency characteristic of the voltage by FFT analysis from the measured voltage acquired from the voltage measurement unit. The frequency characteristic data of the current and the frequency characteristic data of the voltage calculated by the signal analyzer 31 are recorded on the data recording medium 35 together with the frequency data of the AC voltage.

  The display 33 displays a graph of the result of the signal analysis by the signal analysis unit 31 on a screen. FIG. 3 is a diagram illustrating an example of a graph displayed on the display 33. In the upper and lower parts of FIG. 3, the horizontal axis represents the frequency [Hz]. The vertical axis in the upper part shows voltage [V], and the vertical axis in the lower part shows current [A]. The upper part of FIG. 3 shows the frequency characteristics of the voltage applied (superimposed) to the power cable 10 by the AC voltage generation circuit 21. The lower part of FIG. 3 shows the frequency characteristics (FFT graph) of the current measured by the current measuring unit 4. FIG. 3 shows the frequency characteristics of the current measured when the value of the difference frequency Δa is Δ1.

  The user can determine the deterioration of the power cable 10 based on the frequency characteristics in the lower part of FIG. These frequency characteristics show current components of each frequency when the AC voltage is superimposed on the power cable 10 by the AC voltage generation circuit 21. It is considered that the magnitude of the current component at the difference frequency Δa (= Δ1) reflects the presence or absence of insulation deterioration such as a water tree in the power cable 10 and the degree of insulation deterioration. That is, when the value of the current component at the difference frequency Δa is large, there is a high possibility that the power cable 10 has deteriorated. Also, it is considered that the degree of insulation deterioration of the power cable 10 increases as the value of the current component at the difference frequency Δa increases. The user can determine the presence or absence of insulation deterioration of the power cable 10 based on, for example, a comparison between the value of the current component at the difference frequency Δa and the determination value.

  The difference between the value of the current component at the peripheral frequency and the value of the current component at the difference frequency Δa is likely to indicate the presence or absence of the power cable 10 and the degree of the deterioration. For this reason, the user may judge the deterioration of the power cable 10 by comparing the value of the current component at the peripheral frequency with the value of the current component at the difference frequency Δa.

  In the cable deterioration detection device 1 of the present embodiment, the control unit 25 switches the output frequency f0 of the AC voltage generation circuit 21 to a plurality of values. More specifically, the control unit 25 changes the output frequency f0 in a frequency region shifted from the N-fold frequency fn. This frequency region is a region near the N-fold frequency fn and not including the N-fold frequency fn, that is, a frequency region adjacent to the N-fold frequency fn. Here, the vicinity is, for example, an area defined within a predetermined frequency b [Hz] from the N-fold frequency fn. The predetermined frequency b may be, for example, 10 [Hz], 5 [Hz], 2 [Hz], or the like. Further, the size of the predetermined width c [Hz], which is the width of the nearby frequency region, is appropriately determined, for example, within 1 [Hz] or within 2 [Hz]. The frequency region in which the output frequency f0 is changed may be a region including a frequency shifted by 1 [Hz] from the N-fold frequency fn. This frequency region may be a region centered on a frequency shifted by 1 [Hz] from the N-fold frequency fn.

The cable deterioration detection device 1 of the present embodiment sequentially changes the output frequency f0 to a plurality of values and measures the current under the AC voltage of each output frequency f0. In the cable deterioration detecting device 1, the current measuring unit 4 measures the current flowing to the ground via the AC power supply 2 at each output frequency f0. The cable deterioration detection device 1 provides the user with the frequency characteristics obtained from the measurement results of the current measurement unit 4 for each value of the output frequency f0. The user determines the deterioration of the power cable 10 based on the frequency characteristics of the current value for each value of the output frequency f0. FIG. 4 shows the frequency characteristics of the current measured when the value of the difference frequency Δa is Δ2. The frequency Δ2 is a frequency higher than the frequency Δ1 in FIG. FIG. 5 shows the frequency characteristics of the current measured when the value of the difference frequency Δa is Δ3. The frequency Δ3 is a frequency higher than the frequency Δ2 in FIG. That is, the frequencies Δ1, Δ2, and Δ3 have the relationship of the following equation (2).
Δ1 <Δ2 <Δ3 (2)

  As can be seen from FIGS. 3 to 5, the peak in the frequency characteristic of the current changes according to the change in the output frequency f0 of the AC voltage generation circuit 21. More specifically, as shown in FIGS. 3 to 5, the values Δ1, Δ2, and Δ3 of the difference frequency Δa have peaks PK1, PK2, and PK3 of the maximum values of the frequency characteristics, respectively. When the positions of the peaks PK1, PK2, and PK3 of the frequency characteristic change following the change in the value of the difference frequency Δa, it can be determined with high reliability that the insulation of the power cable 10 is deteriorated. Also, the peak PK1 that exists when the difference frequency Δa is set to Δ1 (FIG. 3) does not appear when the difference frequencies Δa are set to Δ2 (FIG. 4) and Δ3 (FIG. 5). In this case, the correlation between the fact that the difference frequency Δa of the applied AC voltage is the frequency Δ1 and the appearance of the peak PK1 is high, and the peak PK1 is likely to indicate the deterioration of the power cable 10. Therefore, the cable deterioration detection device 1 of the present embodiment can provide a determination material that enables the insulation deterioration determination of the power cable 10 to be performed with high accuracy.

An example of detecting cable deterioration by the cable deterioration detection device 1 of the present embodiment will be described in more detail. The control unit 25 of the present embodiment sets the difference frequency Δa to, for example, ± 1.5 [Hz], ± 1.25 [Hz], ± 1.0 [Hz], ± 0.75 [Hz], ± 0. Change to 5 [Hz]. That is, the control unit 25 changes the output frequency f0 to five different frequencies at intervals of, for example, 0.25 [Hz]. FIG. 6 shows a graph of frequency characteristics acquired under the following conditions. The FFT analysis time described below is a measurement period of a current to be subjected to FFT analysis. That is, in the present embodiment, the current is measured by the current measuring unit 4 for one value of the difference frequency Δa for T1 seconds, and the FFT analysis is performed on the measurement data for T1 seconds. T1 of the FFT analysis time is, for example, several tens of seconds.
Integer N: 2 (N times frequency fn = 2 × fr [Hz])
FFT analysis time: T1 [second]

  The cable deterioration detection device 1 changes the difference frequency Δa from ± 1.5 [Hz] to ± 0.5 [Hz] every 0.25 [Hz] every T1 seconds. The control unit 25 sets a value obtained by adding the difference frequency Δa to twice the obtained actual frequency fr as the output frequency f0. The AC voltage generation circuit 21 generates the set output frequency f0. The cable deterioration detection device 1 measures a current by the current measuring unit 4 at each difference frequency Δa, and calculates a frequency characteristic from each measurement result by FFT analysis. The cable deterioration detecting device 1 sequentially displays the FFT graph on the display 33 every time the current measurement and the FFT analysis are completed. Thereby, the FFT graph is sequentially and additionally displayed on the display 33 from FIG. 6 (a) to FIG. 6 (e). In the present embodiment, since the FFT analysis time is T1 [second], the FFT graph is added on the display 33 every T1 seconds.

  6A to 6E, the horizontal axis indicates frequency [Hz], and the vertical axis indicates current [A]. A broken line is shown as a mark at the position of the difference frequency Δa on the horizontal axis. As shown in FIG. 6A, in the frequency characteristics when the difference frequency Δa is ± 1.5 [Hz], a peak PKa is recognized in the difference frequency Δa. Based on this frequency characteristic, it can be determined that insulation deterioration has occurred in the power cable 10. However, the peak PKa may be generated due to other causes such as noise. As shown in FIG. 6B, when the difference frequency Δa is switched to ± 1.25 [Hz], a peak PKb is recognized at a position of 1.25 [Hz]. Therefore, it can be said that the frequency characteristics in FIG. 6B also indicate that insulation deterioration has occurred in the power cable 10. As shown in FIG. 6C, the frequency characteristic when the difference frequency Δa is ± 1.0 [Hz] has a peak PKc at a position of 1.0 [Hz]. The peak PKc has a greater degree of protrusion than the peaks PKa and PKb. Therefore, it is considered that the peak PKc clearly indicates the existence of insulation deterioration in the power cable 10.

  Further, as shown in FIGS. 6D and 6E, even when the difference frequency Δa is set to ± 0.75 [Hz] and ± 0.5 [Hz], the peak characteristic PKd, PKe is present. The user can determine that insulation deterioration such as water tree has occurred in the power cable 10 to be detected based on these data.

  In the cable deterioration detection device 1 of the present embodiment, the lower limit of the difference frequency Δa is set to 0.5 [Hz]. This improves the detection accuracy, as described below with reference to FIG. FIG. 7 is a diagram illustrating frequency characteristics of the deteriorated signal. FIG. 7 shows current values actually measured by the cable deterioration detection device 1 for a power cable in which insulation deterioration has occurred. For example, the current value plotted against 1.0 [Hz] is a component value of 1.0 [Hz] in the current measured when the difference frequency Δa is 1.0 [Hz].

  As shown in FIG. 7, as the difference frequency Δa decreases, the magnitude of the current component at the difference frequency Δa decreases. Further, when the difference frequency Δa decreases, the influence of the DC current on the measurement circuit tends to be dominant in the current value measured by the current measurement unit 4. In other words, if the output frequency f0 is too close to the N-fold frequency fn, the stray current or the like easily affects the accuracy of the deterioration detection.

  In the cable deterioration detection device 1 of the present embodiment, the lower limit of the difference frequency Δa is set. In the present embodiment, the lower limit value of the difference frequency Δa is set to 0.5 [Hz]. As a result, it is possible to suppress the influence of the direct current and improve the accuracy of the deterioration detection.

  Further, in the cable deterioration detection device 1 of the present embodiment, the upper limit of the difference frequency Δa is set. In the present embodiment, the upper limit of the difference frequency Δa is set to 1.5 [Hz]. As a result, the deterioration can be detected while avoiding the influence of the commercial power supply (50 [Hz], 60 [Hz]).

  In the present embodiment, the allowable range Bx of the difference frequency Δa is determined based on the lower limit and the upper limit. The allowable range Bx is a frequency range with a lower limit of 0.5 [Hz] and an upper limit of 1.5 [Hz]. That is, the cable deterioration detection device 1 does not set the output frequency f0 in a range of less than 0.5 [Hz] around the N-fold frequency fn. Further, the cable deterioration detection device 1 does not set the output frequency f0 in a range farther than 1.5 [Hz] from the N-fold frequency fn. The cable deterioration detection device 1 of the present embodiment can improve the accuracy of deterioration detection by changing the difference frequency Δa in the allowable range Bx.

  The cable deterioration detection device 1 of the present embodiment can determine the insulation deterioration of the power cable 10 by the determination unit 36 in addition to providing the frequency analysis result of the measured current. The determination unit 36 detects insulation deterioration of the power cable 10 based on the frequency analysis result of the current value measured by the current measurement unit 4. For example, the determination unit 36 reads the value of the difference frequency Δa from the data recording medium 35 and data of the frequency characteristic corresponding to the difference frequency Δa. The determination unit 36 acquires the magnitude of the current component at the difference frequency Δa from the read frequency characteristic data. The cable deterioration detection device 1 of the present embodiment calculates and records the frequency characteristics of the current at a plurality of difference frequencies Δa. Therefore, for the power cable 10 to be detected, there are a plurality of combinations of the value of the difference frequency Δa and the magnitude of the current component at the difference frequency Δa.

  A basic method for determining whether or not the power cable 10 has deteriorated insulation depends on the result of comparison between the magnitude of the current component and the determination value. For example, the determination unit 36 may determine that the power cable 10 is insulation-degraded when the magnitude of the current component is equal to or greater than the determination value for at least one difference frequency Δa. By doing so, it is possible to prevent erroneous determination of overlooking insulation deterioration. In addition, the determination unit 36 may determine that the power cable 10 is insulation-degraded when the magnitude of the current component is equal to or greater than the determination value at a value of the difference frequency Δa equal to or greater than the predetermined number. This predetermined number may be a number that is a majority of the total number of changes in the difference frequency Δa. The determination unit 36 may determine that the insulation of the power cable 10 is deteriorated when the magnitude of the current component is equal to or greater than the determination value at all the changed values of the difference frequency Δa.

  The determination unit 36 may determine the presence or absence of insulation deterioration based on the amount of protrusion of the peak PKx (x = 1, 2, 3, a, b, c, d, e) at the difference frequency Δa. The protrusion amount may be a difference between the average value of the magnitude of the current component at a frequency near the difference frequency Δa and the peak value of the peak PKx. When the amount of protrusion is equal to or greater than the determination value regarding the amount of protrusion, it is determined that the insulation of the power cable 10 is deteriorated. Also in this determination, when the amount of protrusion is equal to or greater than the determination value at at least one difference frequency Δa, it may be determined that insulation has deteriorated. Alternatively, it may be determined that the insulation of the power cable 10 is deteriorated when the amount of protrusion is equal to or greater than the determination value at a value of the difference frequency Δa equal to or greater than the predetermined number.

  Further, the determination unit 36 may determine the insulation deterioration based on a frequency region where the noise level is low. For example, in the frequency characteristic shown in FIG. 6, in the frequency range from 0.5 [Hz] to 1.5 [Hz] in which the difference frequency Δa is changed, the magnitude of the current component on the low frequency side is smaller than that on the high frequency side. Smaller than the size of. That is, in this frequency region, the low-frequency noise level is lower than the high-frequency noise level. In the case of such frequency characteristics, the determination unit 36 may determine the insulation degradation based on the frequency characteristics obtained when the difference frequency Δa is a value on the low frequency side. As an example, for the power cable 10 having the frequency characteristics as shown in FIG. 6, based on the frequency characteristics when the difference frequency Δa is a value lower than the center frequency 1.0 [Hz]. Alternatively, the insulation deterioration may be determined.

  Further, the determination unit 36 may perform weighting according to the noise level with respect to the determination of insulation deterioration. For example, when the magnitude of the current component is equal to or larger than the determination value in the plurality of difference frequencies Δa, the weight for the difference frequency Δa in the low noise level region is increased, and the weight for the difference frequency Δa in the high noise level region is reduced. Alternatively, the insulation deterioration may be determined. As an example, when the current component is equal to or greater than the determination value at the difference frequency Δa in the low noise level region, one point is given to each of the portions, and the current component is determined at the difference frequency Δa in the high noise level region. In the case of the above, 0.5 point is added to each of the portions. When the current component is less than the determination value, 0 points are given to each of the locations. Based on the total points and the average points, the presence or absence of insulation deterioration of the power cable 10 and the degree of insulation deterioration are determined.

  The determination unit 36 determines insulation degradation based on a comparison between frequency characteristics when the AC power supply 2 is applying an AC voltage and frequency characteristics when the AC power supply 2 is not applying an AC voltage. Is also good. In this way, the influence of stationary noise can be reduced and the accuracy of determining insulation deterioration can be improved.

  As described above, the cable deterioration detection device 1 of the present embodiment includes the AC power supply 2, the control unit 25, the current measurement unit 4, and the signal analysis unit 31. The AC power supply 2 is electrically connected to the shielding layer 10b of the power cable 10 and is grounded. The AC power supply 2 applies an AC voltage to the shielding layer 10b under the live line.

  The control unit 25 changes the output frequency f0 applied by the AC power supply 2 in a frequency range shifted from an integral multiple of the commercial frequency f1. The current measuring unit 4 measures a current flowing from the power cable 10 to the ground via the AC power supply 2. The signal analysis unit 31 performs a frequency analysis of the current measured by the current measurement unit 4.

  The difference frequency Δa between the output frequency f0 and the integral multiple of the commercial frequency f1 is 0.5 [Hz] or more. Therefore, the cable deterioration detection device 1 of the present embodiment is less likely to be affected by a DC current such as a stray current and can improve the accuracy of deterioration detection.

  The difference frequency Δa in the present embodiment is equal to or less than 1.5 [Hz]. This makes it possible to reduce the influence of the commercial power supply and improve the accuracy of deterioration detection.

  The cable deterioration detection device 1 according to the present embodiment further includes a frequency acquisition unit 29 that acquires the actual frequency fr of the commercial power supplied to the power cable 10. The cable deterioration detection device 1 uses the actual frequency fr as the commercial frequency f1. Therefore, the cable deterioration detection device 1 of the present embodiment can accurately detect deterioration even when the actual frequency fr is deviated from the specified frequency.

  When changing the difference frequency Δa to a different value, the control unit 25 of the present embodiment acquires the latest value of the actual frequency fr from the frequency acquisition unit 29 and sets the output frequency f0. Therefore, the cable deterioration detection device 1 of the present embodiment can accurately detect deterioration even when the actual frequency fr fluctuates in a short term.

  In the present embodiment, the integer N of the N-fold frequency fn is N = 2, but the present invention is not limited to this, and the integer N may be another even number. In the present embodiment, the actual frequency fr is used as the commercial frequency f1, but the present invention is not limited to this. As the value of the commercial frequency f1, a prescribed frequency of 50 [Hz] or 60 [Hz] may be used.

  The configuration of the AC power supply 2 for boosting the AC voltage generated by the AC voltage generation circuit 21 is not limited to the transformer 22. The AC power supply 2 may boost the AC voltage by using, for example, a differential amplifier instead of the transformer 22. The sweep condition (width, resolution, speed) for changing the output frequency f0 of the AC power supply 2 may be specified by the cable deterioration detection device 1 instead of being input by the user.

[Modification of First Embodiment]
A modification of the first embodiment will be described. The integer N may be an odd number. When the integer N is an odd number, the voltage V applied to the insulating layer of the power cable 10 is approximated by the following equation (3). Here, α is a coefficient, ω1 = 2πfr, ω2 = 2πΔa.
V = (V1 + α · ω2) sin ω1t (3)

  That is, when the integer N is an odd number, the amplitude of the voltage applied to the power cable 10 changes according to the difference frequency Δa. Therefore, it is possible to detect the deterioration of the insulation of the power cable 10 based on the measurement results of the current before and after the AC power supply 2 applies the AC voltage. For example, the current is measured by the current measurement unit 4 before and during the application of the AC voltage by the AC power supply 2. The signal analysis unit 31 performs signal analysis on the measurement results before and after the application, and calculates frequency characteristics. The insulation deterioration of the power cable 10 is detected based on the comparison result between the frequency characteristics before the application and the frequency characteristics at the time of the application. In comparing the two frequency characteristics, for example, the magnitudes of the current components at the actual frequency fr and the N-fold frequency fn are compared.

The output frequency f0 of the AC power supply 2 may change in a frequency region deviated from an integral multiple of the actual frequency fr. The output frequency f0 is represented, for example, by the following equation (4). In the formula (4), M is an integer of 2 or more.
f0 = (fr / M) ± Δa (4)

The frequency f2 of the degraded signal obtained when such an AC voltage having the output frequency f0 is applied to the power cable 10 is represented by the following equation (5). That is, it is possible to detect the insulation deterioration of the power cable 10 based on the current component of the frequency f2 from the frequency characteristics obtained by performing the frequency analysis on the measured current.
f2 = | M · f0−f1 | (5)

[Second embodiment]
The second embodiment will be described with reference to FIGS. In the second embodiment, components having the same functions as those described in the first embodiment are denoted by the same reference numerals, and redundant description will be omitted. FIG. 8 is a schematic configuration diagram of a power cable insulation deterioration detection device according to the second embodiment, FIG. 9 is a flowchart of a power cable insulation deterioration detection method of the second embodiment, and FIG. 10 is a second embodiment. FIG. 11 is a diagram illustrating an example of a band shift, and FIG. 12 is a diagram illustrating another example of a band shift.

  The second embodiment differs from the first embodiment in, for example, having a function of estimating the cause of the inability of the cable deterioration detection device 1 to determine and a function of giving advice on a coping method. Further, the cable deterioration detection device 1 according to the second embodiment has a function of improving the determination accuracy by shifting the frequency band of the AC voltage applied to the shielding layer 10b.

  As shown in FIG. 8, the cable deterioration detection device 1 according to the second embodiment includes an AC power supply 2, a diagnostic device 3, a current measurement unit 4, a security circuit 5, a switching circuit 6, a DC power supply 7, and a frequency acquisition unit 29. Having. The AC power supply 2, the current measurement unit 4, the security circuit 5, and the frequency acquisition unit 29 are the same as the AC power supply 2, the current measurement unit 4, the security circuit 5, and the frequency acquisition unit 29 of the first embodiment.

  The switching circuit 6 is a circuit that switches the voltage applied to the shielding layer 10b to an AC voltage or a DC voltage. More specifically, the switching circuit 6 can electrically connect the shielding layer 10b to either the AC power supply 2 or the DC power supply 7. Further, the switching circuit 6 can shut off the shielding layer 10b from both the AC power supply 2 and the DC power supply 7. The switching circuit 6 has a first relay 61 and a second relay 62. The first relay 61 is interposed between one end of the secondary winding 24 of the AC power supply 2 and the connection line 26. The first relay 61 connects or disconnects the secondary winding 24 and the connection wire 26.

  The DC power supply 7 is a power supply that outputs a DC voltage, and is, for example, a battery. The negative electrode of the DC power supply 7 is grounded via the connection line 27. The second relay 62 is interposed between the positive electrode of the DC power supply 7 and the connection line 26. The second relay 62 connects or disconnects the DC power supply 7 and the connection line 26. The first relay 61 and the second relay 62 are opened and closed by the control unit 25, for example.

  The diagnostic device 3 according to the second embodiment includes a signal analysis unit 31, a display 33, an operation unit 34, a determination unit 36, and a digital filter 37. The signal analysis unit 31, the display 33, and the operation unit 34 according to the second embodiment are the same as the signal analysis unit 31, the display 33, and the operation unit 34 of the first embodiment. Note that the diagnostic device 3 may include a synchronization circuit 32 and a data recording medium 35 as in the diagnostic device 3 of the first embodiment.

  As described below, the determination unit 36 of the present embodiment executes the steps of estimating the cause of the determination failure, giving advice on the coping method to the user, and improving the determination accuracy by the band shift. The determination unit 36 executes the above steps based on a program stored in advance. The determination unit 36 is connected to the control unit 25 of the AC power supply 2 and can control the operation of the control unit 25.

  The determination unit 36 is connected to the display 33 and causes the display 33 to display various information. For example, the display 33 displays questions to the user and advice to the user. The display 33 of the second embodiment has a touch panel. The user inputs an answer to the question displayed on the display 33 on the touch panel. The determination unit 36 performs various determinations and advice based on the answer input to the touch panel.

  The digital filter 37 performs a filtering process on the signal measured by the current measuring unit 4. The digital filter 37 detects a degraded signal buried in noise by digital filter processing such as phase detection and a lock-in amplifier. The information processed by the digital filter 37 is displayed on the display 33, for example. The information processed by the digital filter 37 may be further analyzed by the signal analysis unit 31, and the analysis result may be displayed on the display 33. The determination unit 36 is connected to the digital filter 37. The determination unit 36 switches whether or not the digital filter 37 processes the signal detected by the current measurement unit 4.

  The insulation deterioration detection method according to the second embodiment will be described with reference to FIGS. The flowchart shown in FIG. 9 is executed in a state where the attachment of the cable deterioration detection device 1 to the power cable 10 to be determined is completed. The flowchart in FIG. 9 is started, for example, in response to a measurement start command from the user.

  In step S21, the determination unit 36 measures the sheath insulation resistance Rs of the shielding layer 10b. The sheath insulation resistance Rs is measured with the first relay 61 opened and the second relay 62 closed. In the following description, a state in which the first relay 61 is opened and the second relay 62 is closed, that is, a state in which the DC power supply 7 applies a DC voltage to the shielding layer 10b is referred to as a “first state”. . A state in which the first relay 61 is closed and the second relay 62 is open, that is, a state in which the AC power supply 2 applies an AC voltage to the shielding layer 10b is referred to as a “second state”.

  The determination unit 36 causes the control unit 25 to set the first relay 61 and the second relay 62 to the first state. The determining unit 36 calculates the sheath insulation resistance Rs based on the current value detected by the current measuring unit 4 in the first state. When the measurement of the sheath insulation resistance Rs is completed, the process proceeds to step S22.

  In step S22, the determination unit 36 determines whether the sheath insulation resistance Rs is less than 250 [kΩ]. In step S22, the value of the sheath insulation resistance Rs measured in step S21 is compared with the determination value of 250 [kΩ]. As a result of the determination in step S22, when it is determined that the sheath insulation resistance Rs is less than 250 [kΩ] (step S22-Y), the process proceeds to step S23, and when the determination is negative, the process proceeds to step S31.

  In step S23, the determination unit 36 determines that the deterioration determination is not possible, and proceeds to step S24.

  In step S24, the determination unit 36 inquires of the user whether the terminal has been cleaned. When the sheath insulation resistance Rs is lowered, one of the causes may be contamination of the terminal of the shielding layer 10b. The determination unit 36 displays a question, “Is the terminal cleaned?” On the display 33. The determination unit 36 proceeds to step S25 when the user inputs a reply that “the terminal has been cleaned”. On the other hand, when the user inputs an answer that “the terminal has not been cleaned”, the determination unit 36 proceeds to step S26 and advises that it is better to start the re-measurement after cleaning the terminal. Is displayed on the display 33. When step S26 is performed, the present flow ends.

  In step S25, the determination unit 36 asks the user whether the metal contact of the ground wire has been released. When the sheath insulation resistance Rs is reduced, one of the causes may be metal contact of the connection line 26. The determination unit 36 displays a question on the display 33, "Is the connection wire 26 separated from the metal?" The determination unit 36 proceeds to step S27 when the user inputs a reply that “the connection line 26 is separated from the metal”. On the other hand, when the user inputs an answer that “the connection wire 26 is not separated from the metal”, the determination unit 36 proceeds to step S28 and separates the connection line 26 from the metal before the start of the re-measurement. Is displayed on the display 33. When step S28 is performed, the present flow ends.

  In step S27, the determination unit 36 asks the user whether the weather is rain. When the sheath insulation resistance Rs has decreased, rainfall can be considered as one of the causes. The determination unit 36 displays a question “Is the weather raining?” On the display 33. When the user inputs an answer “rainy”, the determination unit 36 proceeds to step S30, and displays an advice to perform measurement on a different day on the display 33. If the terminal has been cleaned (step S24-Y) and the ground wire is separated from the metal (step S25-Y), it is highly likely that the cause of the decrease in the sheath insulation resistance Rs is rainfall. The determination unit 36 displays an advice on the display 33 to perform re-measurement when the weather is other than rainfall. When step S30 is executed, the present flow ends.

  The determination unit 36 proceeds to step S <b> 29 when the user inputs an answer “not rain”. In step S29, the determination unit 36 determines that the determination is impossible. The determination unit 36 displays on the display 33 that it is difficult to determine the insulation deterioration of the power cable 10 in the current state. When step S29 is performed, the present flow ends.

  In step S31, the determination unit 36 determines whether or not the AC superimposed current Isa is less than the determination value. The determination value is, for example, 10 [nA], but is not limited thereto. The determination unit 36 sets the first relay 61 and the second relay 62 in the second state, and measures the AC superimposed current Isa [A]. The determination unit 36 causes the AC voltage of the AC power supply 2 to be applied to the shielding layer 10b in the second state. The output frequency f0 at the time of measuring the AC superposition current Isa is a frequency value obtained by adding the difference frequency Δa to the actual frequency fr. The AC superposition current Isa is a frequency analysis result of the current value measured by the current measuring unit 4, and is a value of a current component corresponding to the difference frequency Δa. The determination unit 36 performs the determination in step S31 based on, for example, the value of the AC superimposed current Isa when the difference frequency Δa is set to 1.0 [Hz].

  The determining unit 36 changes the value of the difference frequency Δa in a predetermined frequency region (for example, 0.5 [Hz] −1.5 [Hz]) as shown in FIG. The AC superposition current Isa may be calculated from the current component values PKa, PKb, PKc, PKd, and Pke of the respective values of. In this case, the AC superimposed current Isa may be, for example, an average value or a maximum value of the current component values PKa, PKb, PKc, PKd, and Pke.

  The determination unit 36 compares the value of the AC superimposed current Isa with the determination value. As a result of the determination in step S31, when it is determined that the AC superimposed current Isa is less than the determination value (step S31-Y), the process proceeds to step S32, and when the determination is negative, the process proceeds to step S33.

  In step S32, the determination unit 36 performs a good determination. The determination unit 36 displays a result of the determination that the insulation state of the power cable 10 is good on the display 33. When step S32 is executed, the present flow ends.

  In step S33, the determination unit 36 makes a temporary deterioration determination, and proceeds to step S34.

  In step S34, the determination unit 36 determines whether the sheath insulation resistance Rs is less than 1 [MΩ]. The determination unit 36 performs the determination in step S34 based on the value of the sheath insulation resistance Rs measured with the first relay 61 and the second relay 62 in the first state. The determination value of the sheath insulation resistance Rs in Step S34 is a value larger than the determination value of the sheath insulation resistance Rs in Step S22. As a result of the determination in step S34, when it is determined that the sheath insulation resistance Rs is less than 1 [MΩ] (step S34-Y), the process proceeds to step S24, and when negative, the process proceeds to step S35.

  In step S35, the determination unit 36 determines whether or not the unbalanced charging current Iac is, for example, 100 [mA] or more. The unbalanced charging current Iac is a current value due to three-phase unbalance in the power cable 10. The determining unit 36 acquires the unbalanced charging current Iac. When the value of the unbalanced charging current Iac is large, the AC superimposed current Isa is affected, and it becomes difficult to determine the deterioration. If it is determined in step S35 that the unbalanced charging current Iac is equal to or greater than 100 [mA] (step S35-Y), the process proceeds to step S36, and if negative, the process proceeds to step S39.

  In step S36, the determination unit 36 inquires of the user whether the cable length is, for example, 1 [km] or more. The determination unit 36 displays a question on the display 33, "Is the length of the power cable 10 equal to or greater than 1 [km]?" When the user inputs a reply that “the length is 1 km or more”, the determination unit 36 proceeds to step S37, and inputs a reply that “the length is not 1 km or more”. If so, the process proceeds to step S38.

  In step S37, the determination unit 36 displays on the display 33 that the cause of the unbalanced charging current Iac may be an imbalance in capacitance due to a difference in the length of each phase. The determination unit 36 advises the user to check the imbalance of the capacitance in the power cable 10 by a power failure diagnosis or the like on the display 33. When step S37 is performed, the present flow ends.

  In step S38, the determination unit 36 displays on the display 33 that the cause of the unbalanced charging current Iac may be an imbalance in capacitance due to a factor such as breakage of the shielding copper tape. The determination unit 36 advises the user to check the cause of the imbalance of the capacitance in the power cable 10 in the power failure diagnosis or the like by displaying on the display 33. When Step S38 is performed, the present flow ends.

  In step S39, the determination unit 36 performs a band shift. When the unbalanced charging current Iac is less than the determination value (step S35-N), the cable deterioration detection device 1 of the present embodiment executes the band shift. The determination unit 36 shifts the frequency band of the AC voltage applied to the shielding layer 10b by the band shift. The band shift can reduce the influence of noise and improve the accuracy of determining insulation deterioration. The band shift will be described with reference to FIGS.

In step S41, the determination unit 36 compares the magnitude of the spectrum value for each band. The spectrum values compared in step S41 are the spectrum value TSP1 of the intermediate band shown in [Equation 1] and the spectrum value TSP0 of the low frequency band shown in [Equation 2].

  FIG. 11 shows an example of the intermediate band B1 and the low frequency band B0. The intermediate band B1 is a band currently set as a frequency region in which the difference frequency Δa is changed. The intermediate band B1 is, for example, a band of 0.9 [Hz] -1.1 [Hz]. The low frequency band B0 is a band on the lower frequency side than the intermediate band B1. The low frequency band B0 of the present embodiment is a band that is continuous with the intermediate band B1. The low frequency band B0 of the present embodiment is, for example, a band of 0.7 [Hz] -0.9 [Hz].

  The intermediate band spectral value TSP1 is the sum of the spectral values of the intermediate band B1. In other words, the spectrum value TSP1 of the intermediate band is the area of the region C1 in FIG. The spectrum value TSP0 of the low frequency band is the sum of the spectrum values of the low frequency band B0. In other words, the spectrum value TSP0 of the low frequency band is the area of the region C0 in FIG.

  In step S41, it is determined whether or not the spectrum value TSP0 of the low frequency band is less than half the spectrum value TSP1 of the intermediate band. As a result of the determination in step S41, if the affirmative determination is made that the spectrum value TSP0 of the low frequency band is less than half the spectral value TSP1 of the intermediate band (step S41-Y), the process proceeds to step S42, and a negative determination is made. If so, the process proceeds to step S43.

  In step S42, the determination unit 36 performs a band shift to the low frequency band B0. The determination unit 36 changes the difference frequency Δa of the AC voltage applied to the shielding layer 10b to a frequency value in the low frequency band B0. When the band is shifted, the AC superimposed current Isa at the changed difference frequency Δa is measured. The newly measured AC superimposed current Isa may be, for example, the value of the current component when the center value of the low frequency band B0 is set as the difference frequency Δa. Alternatively, the difference frequency Δa may be changed to a plurality of values in the low frequency band B0, and the AC superposition current Isa may be calculated from the value of each current component.

  Since the spectrum value TSP0 of the low frequency band is less than half of the spectrum value TSP1 of the intermediate band, it is considered that the noise level of the low frequency band B0 is lower than the noise level of the intermediate band B1. Therefore, the accuracy of determination of insulation deterioration can be improved by shifting the band to the low frequency band B0. When step S42 is executed, the process proceeds to step S46.

In step S43, the determination unit 36 compares the magnitude of the spectrum value for each band. The spectrum values compared in step S43 are the spectrum value TSP1 of the middle band and the spectrum value TSP2 of the high frequency band shown in [Equation 3].

  FIG. 12 shows an example of the intermediate band B1 and the high frequency band B2. The high frequency band B2 is a band on the higher frequency side than the intermediate band B1. The high frequency band B2 of the present embodiment is a band that is continuous with the intermediate band B1. The high frequency band B2 of the present embodiment is, for example, a band of 1.1 [Hz] to 1.3 [Hz].

  The high frequency band spectrum value TSP2 is the sum of the spectrum values of the high frequency band B2. In other words, the spectrum value TSP2 of the high frequency band is the area of the region C2 in FIG.

  In step S43, it is determined whether or not the spectrum value TSP2 of the high frequency band is less than half the spectrum value TSP1 of the intermediate band. As a result of the determination in step S43, when the affirmative determination is made that the spectrum value TSP2 of the high frequency band is less than half the spectral value TSP1 of the intermediate band (step S43-Y), the process proceeds to step S44, and a negative determination is made. If so, the process proceeds to step S45.

  In step S44, the determination unit 36 performs a band shift to the high frequency band B2. The determination unit 36 changes the difference frequency Δa of the AC voltage applied to the shielding layer 10b to a frequency value of the high frequency band B2. When the band is shifted, the AC superimposed current Isa at the new difference frequency Δa is measured. The newly measured AC superimposed current Isa may be, for example, the value of the current component when the center value of the high frequency band B2 is set as the difference frequency Δa. Alternatively, the difference frequency Δa may be changed to a plurality of values in the high frequency band B2, and the AC superposition current Isa may be calculated from the value of each current component.

  Since the spectrum value TSP2 of the high frequency band is less than half of the spectrum value TSP1 of the intermediate band, it is considered that the noise level of the high frequency band B2 is lower than the noise level of the intermediate band B1. Therefore, the accuracy of determining insulation deterioration can be improved by shifting the band to the high frequency band B2. When step S44 is performed, the process proceeds to step S46.

  In step S45, the determination unit 36 performs a digital filter process. The determination unit 36 causes the signal detected by the current measurement unit 4 to be processed by the digital filter 37, and causes the signal analysis unit 31 to analyze the processed signal. The determination unit 36 acquires the AC superposition current Isa from the analysis result of the signal analysis unit 31. When step S45 is performed, the process proceeds to step S46.

  In step S46, the determination unit 36 determines whether or not the AC superimposed current Isa is less than the determination value. The determination value is, for example, 10 [nA], but is not limited thereto. As a result of the determination in step S46, when it is determined that the superimposed AC current Isa is less than the determination value (step S46-Y), the process proceeds to step S47, and when the determination is negative, the process proceeds to step S48.

  In step S47, the determination unit 36 performs a good determination. The determination unit 36 displays a result of the determination that the insulation state of the power cable 10 is good on the display 33. When Step S47 is executed, the present flow ends.

  In step S48, the determination unit 36 performs a deterioration determination. The determining unit 36 determines that insulation deterioration has occurred in the power cable 10. The determination unit 36 displays, on the display 33, a determination result indicating that insulation degradation has occurred in the power cable 10. When Step S48 is executed, the present flow ends.

  The power cable insulation deterioration detection method according to the present embodiment includes a band changing step. The band changing step is a step of changing the frequency range of the AC voltage applied by the AC power supply 2 to the shielding layer 10b based on the spectrum distribution of the frequency analysis result (steps S42 and S44). According to the power cable insulation deterioration detection method of the present embodiment, the insulation deterioration of the power cable 10 can be detected with high accuracy by changing the frequency range of the AC voltage. The cable deterioration detection device 1 changes the frequency domain within the allowable range Bx. Therefore, the cable deterioration detection device 1 of the present embodiment can shift the frequency region to a low noise region while suppressing the influence of the stray current and the commercial power supply.

  As described above, the cable deterioration detection device 1 according to the present embodiment uses the diagnosis support logic (FIGS. 9 and 10) for determining and selecting a means for reducing the influence of noise from the measurement conditions, the measurement results, and the FFT analysis results. Have. The display 33 displays measurement results, advice for eliminating noise factors, FFT analysis results, digital filter processing results, and the like.

  Therefore, the cable deterioration detection device 1 of the present embodiment can advise a means for eliminating a noise factor due to the measurement environment from the measurement conditions and the measurement results. In addition, the cable deterioration detection device 1 of the present embodiment shifts the deteriorated signal to a frequency band less affected by noise based on the FFT analysis result (FIG. 10). Therefore, the cable deterioration detection device 1 of the present embodiment can reduce the influence of the measurement environment and noise regardless of the user's skill level, and can determine the insulation deterioration of the power cable 10 with high accuracy.

  It should be noted that the numerical values of the determination values in the above flowchart are merely examples, and may be different values. The cable deterioration detection device 1 may determine the presence or absence of insulation deterioration without performing a band shift. For example, when the spectrum value TSP1 of the intermediate band is smaller than a predetermined value, the comparison between the spectrum value TSP1 of the intermediate band and the other spectrum values TSP0 and TSP2 may not be performed. In this case, the presence or absence of insulation deterioration is determined based on the value of the AC superimposed current Isa in the intermediate band B1.

  The criteria for determining the band shift are not limited to the illustrated criteria. In the above example, the band shift is performed when the other spectrum values TSP0 and TSP2 are less than half the spectrum value TSP1 of the intermediate band. However, the coefficient by which the spectrum value TSP1 of the intermediate band is multiplied is not limited to １ ／.

  The contents disclosed in each of the above-described embodiments and modified examples can be appropriately combined and executed.

1 Cable deterioration detecting device (power cable insulation deterioration detecting device)
2 AC power supply 3 Diagnostic device 4 Current measuring unit 5 Security circuit 6 Switching circuit 7 DC power supply 10 Power cable 10a Core wire 10b Shielding layer 11 Cable terminal 12 High voltage bus 21 AC voltage generation circuit 22 Transformer 23 Primary winding 24 Secondary winding Line 25 control unit 26, 27 connection line 28 ground unit 29 frequency acquisition unit 31 signal analysis unit 32 synchronization circuit 33 display 34 operation unit 35 data recording medium 36 determination unit 37 digital filter 61 first relay 62 second relay Δa differential frequency B0 Low frequency band B1 Intermediate band B2 High frequency band Bx Allowable range f0 Output frequency f1 Commercial frequency fn N times frequency fr Actual frequency Iac Unbalanced charging current Isa AC superimposed current PK1, PK2, PKc, PKa, PKb, PKc, PKd, PKe Mountain Rs Sheath insulation resistance TSP0 Spectrum value of low frequency band TSP1 Spectrum value of middle band TSP2 Spectrum value of high frequency band V1 Voltage of commercial power supply V2 Output voltage

Claims (6)

An AC power supply that is electrically connected to the shield layer of the power cable, and is grounded, and applies an AC voltage to the shield layer under a live line;
A control unit that changes an output frequency applied by the AC power supply in a frequency range shifted from an integral multiple of a commercial frequency supplied to the power cable,
A current measurement unit that measures a current flowing from the power cable to the ground via the AC power supply,
A signal analysis unit that performs frequency analysis of the current measured by the current measurement unit,
With
A device for detecting insulation deterioration of a power cable, wherein a difference frequency between the output frequency and an integral multiple of the commercial frequency is 0.5 [Hz] or more. The power cable insulation deterioration detection device according to claim 1, wherein the difference frequency is equal to or less than 1.5 [Hz]. Further, a frequency acquisition unit for acquiring the actual frequency of the commercial power supplied to the power cable,
The power cable insulation deterioration detection device according to claim 1 or 2, wherein the actual frequency is used as the commercial frequency. The said control part acquires the latest value of the said actual frequency from the said frequency acquisition part, and sets the said output frequency, when changing the said difference frequency to a different value, The insulation deterioration of the power cable of Claim 3. Detection device. Applying an AC voltage from an AC power source under the live line to the shield layer of the power cable,
While changing the output frequency of the AC power supply in a frequency range shifted from an integral multiple of the commercial frequency, a current flowing from the power cable to the ground via the AC power supply is measured by a current measurement unit,
Detecting insulation deterioration of the power cable based on a frequency analysis result of the current value measured by the current measuring unit,
A method for detecting insulation deterioration of a power cable, wherein a difference frequency between the output frequency and an integral multiple of the commercial frequency is 0.5 [Hz] or more. The method according to claim 5, wherein an actual frequency of a commercial power supply supplied to the power cable is used as the commercial frequency.

Images