Method for evaluating cable insulation, system for measuring cable insulation layer, and power cable

Abstract

To provide an evaluation method of a cable insulating layer capable of accurately evaluating electric characteristics of a power cable or an insulating material for DC in a state of simulating the power cable.SOLUTION: The evaluation method of the cable insulating layer 116 includes a voltage application step of applying a DC voltage to the conductor 112 and applying a DC voltage to the conductor 112 not via the current integrator 20 but via the shield ring 30, and an integrated value measurement step of measuring a current value flowing through the cable insulating layer 116 and obtaining a charge amount obtained by integrating the current value.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to evaluation of DC insulation, and more particularly to a method for evaluating a cable insulation layer using a power cable, a system for measuring a cable insulation layer, and a power cable evaluated as having predetermined characteristics by the evaluation method of the present invention. [Background technology]

[0002] Patent Document 1 discloses a technique for evaluating a DC insulator using a sample formed into a sheet shape using an insulating material. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. WO2017 / 150691 Summary of the Invention [Problem to be solved by the invention]

[0004] DC insulators are subject to a degradation phenomenon caused by the accumulation of electric charges specific to DC, known as space charge. When evaluating the electrical properties of DC insulators, it is essential to evaluate space charge. The PEA method (Pulse Electrostatic Absorption Method) is known as a method for evaluating space charge. Patent Document 1 also discloses a method for evaluating space charge using a current integration method, which is a different evaluation method from the PEA method.

[0005] However, conventional methods for evaluating the electrical properties of DC insulators have the problem that it is difficult to accurately evaluate the electrical properties of DC insulating materials in a power cable or a power cable simulation state.

[0006] Therefore, an object of the present invention is to provide a method for evaluating a cable insulation layer that can accurately evaluate the electrical characteristics of a DC insulating material in a power cable or a state simulating a power cable, a system for measuring a cable insulation layer, and a power cable that is evaluated by the evaluation method as having predetermined characteristics. [Means for solving the problem]

[0007] (1) A method for evaluating a cable insulation layer in a cable sample including a conductor and a cable insulation layer covering the conductor, the method comprising: a voltage application step of applying a DC voltage to the conductor via a current integrator, and also applying a DC voltage to the conductor via a shield ring without using the current integrator; and an integral value measurement step of measuring the value of the current flowing through the cable insulation layer when the DC voltage is applied to the conductor for a predetermined measurement time, and integrating the current value to determine the amount of charge obtained.

[0008] (2) In the evaluation method for a cable insulation layer described in (1), the integrating capacitor in the current integrator and the cable sample are connected in series, and the integrating capacitor and the shield ring are connected in parallel between a DC power source that supplies the DC voltage and the cable sample.

[0009] (3) In the evaluation method of a cable insulation layer described in (1) or (2), a test terminal is provided at the end of the cable sample, in which the cable conductor and cable insulation are exposed or covered with an insulating material, and the exposed conductor part of the test terminal and the shield ring are electrically insulated.

[0010] (4) In the method for evaluating a cable insulation layer according to any one of (1) to (3), the formula Q(0)=C cable ×V cable =I HV ×Δt (Q(0): initial charge accumulation amount, C cable : capacitance of the cable insulation layer, V cable : voltage applied to the cable insulation layer, I HVFor Δt calculated from the DC voltage (i.e., charging current), the predetermined measurement time starts after 1 × Δt seconds to 10 × Δt seconds have elapsed since the DC voltage was applied to the cable insulation layer, and the measurement ends after the predetermined application time has elapsed. After the charge in the integrating capacitor in the current integrator is discharged, the voltage applied to the conductor is reduced.

[0011] (5) In the evaluation method for a cable insulation layer according to any one of (1) to (4), the cable sample has an outer semiconductive layer outside the cable insulation layer and further has a cable shielding layer outside the outer semiconductive layer, and further comprises an electrostatic stress measurement step of arranging a pulse electrostatic stress measurement unit having an embedded piezoelectric sensor in a portion of the cable sample where the outer semiconductive layer is exposed, applying a pulse voltage to the cable shielding layer of the cable sample, and sensing elastic waves from the cable sample with the piezoelectric sensor to measure the pulse electrostatic stress.

[0012] (6) In the method for evaluating a cable insulating layer according to (5), the electrostatic stress measuring step and the integrated value measuring step are at least partially carried out simultaneously.

[0013] (7) In the method for evaluating a cable insulating layer according to (5), the integral value measuring step is performed before or after the electrostatic stress measuring step, or either one of them.

[0014] (8) The method for evaluating a cable insulation layer according to any one of (1) to (7), Formula Q(t)=C INT ×V INT (Q(t): amount of charge accumulated after t seconds, C INT : The capacitance of the integrating capacitor placed in the current integrator, V INT : Upper limit voltage applied to the current integrator) C obtained from INT is set so that the charge accumulation change amount ΔQ(t) in a predetermined measurement time is 5% or more and 50% or less with respect to Q(0).

[0015] (9) In the method for evaluating a cable insulation layer described in any one of (1) to (8), the cable sample includes at least a first cable and a second cable, and an intermediate joint is arranged between the first cable and the second cable.

[0016] (10) A power cable has a cable insulation layer in which the ratio Q(t) / Q(0) of the total charge Q(t) including the initial charge storage Q(0) after the predetermined measurement time to the initial charge storage Q(0), measured by the evaluation method for a cable insulation layer described in any one of (1) to (9), is 1 or more and 2.5 or less.

[0017] (11) A measurement system for a cable insulation layer included in a cable sample, the measurement system comprising a DC power supply, a current integrator, a shielding ring, a first wiring, a second wiring, and a third wiring, wherein the cable sample includes a conductor arranged in order from the center of the cable sample to the outside, a cable insulation layer, and a cable shielding layer, one end of the first wiring is connected to the DC power supply, the other end of the first wiring is connected to the conductor via the current integrator, one end of the second wiring is connected to the DC power supply, the other end of the second wiring is connected to the shielding ring without via the current integrator, and one end of the third wiring is connected to the cable shielding layer.

[0018] (12) In the measurement system for a cable insulation layer of (11), the cable sample includes an outer semiconductive layer between the cable insulation layer and the cable shielding layer, and is arranged so as to sandwich the outer semiconductive layer in the exposed portion of the cable sample, and further includes a pulse electrostatic stress measurement unit including a piezoelectric sensor and a resistance unit for applying a pulse voltage to the cable shielding layer. [Effects of the Invention]

[0019] According to the present invention, it is possible to provide a cable insulation layer evaluation method that can accurately evaluate the electrical characteristics of a DC insulating material in a power cable or a state simulating a power cable, a cable insulation layer measurement system, and a power cable that is evaluated by the evaluation method as having predetermined characteristics. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a diagram showing an outline of a circuit configuration included in a cable insulation layer measurement system according to a first embodiment of the present invention. [Figure 2A] FIG. 2A is a cross-sectional view of a cable sample taken along a plane perpendicular to the elongation direction. [Figure 2B] FIG. 2B is a side view of the cable sample. [Figure 3] FIG. 3 is a diagram showing a measurement system for measuring the electrical characteristics of a cable insulating layer using the measurement system of the first embodiment. [Figure 4] FIG. 4 is a diagram showing an overview of the measurement system of the first embodiment when viewed in the direction of arrow 312 in FIG. [Figure 5] FIG. 5 is a flowchart showing the flow of steps in the method for evaluating a cable insulating layer according to the first embodiment. [Figure 6] FIG. 6 is a diagram showing the relationship between the applied voltage and the measurement time. [Figure 7] FIG. 7 is a diagram showing the relationship between the capacitance of the current integrator and the change over time in the amount of accumulated charge. [Figure 8] FIG. 8 is a diagram showing a measurement system for measuring the electrical characteristics of a cable insulating layer using the measurement system of the second embodiment. [Figure 9] FIG. 9 is a diagram showing a measurement system for measuring the electrical characteristics of a cable insulating layer using the measurement system of the third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0021] (Embodiment 1) A cable insulation layer measurement system 1 according to a first embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a diagram showing an outline of the circuit configuration included in the cable insulation layer measurement system 1. The measurement system 1 is a system for measuring the electrical characteristics of an insulation layer included in a cable.

[0022] (Circuit configuration) 1, the measurement system 1 includes a DC power supply 10, a current integrator 20, and a shield ring 30. The current integrator 20 includes an integrating capacitor 22 and a switch 24.

[0023] In the measurement system 1, a DC voltage is applied to the cable sample 100 from a DC power supply 10, and the current flowing through the cable insulation layer included in the cable sample 100 is measured. The current flowing through the cable insulation layer can be determined from the current flowing through an integrating capacitor 22. The amount of charge can be determined by integrating the current flowing through the integrating capacitor 22. The current integrator 20 is also called a Q(t) meter. Before describing specific connection aspects in the circuit, the layer structure of the cable sample 100 will be described.

[0024] (Layer structure of cable sample) FIG. 2A is a cross-sectional view of the cable sample 100 taken along a plane perpendicular to the extension direction 311. The extension direction 311 is the direction in which the cable extends. The extension direction 311 is not shown in FIG. 2A, but is shown in FIG. 2B. FIG. 2B is a side view of the cable sample 100. Note that in FIG. 2B, for ease of understanding, the ends of the extension direction 311 of each layer included in the cable sample 100 are drawn at different positions.

[0025] (Layer structure of cable sample) 2A and 2B, the cable sample 100 includes, from the inside to the outside, a conductor 112, an inner semiconductive layer 114, a cable insulation layer 116, an outer semiconductive layer 118, a cable shielding layer 120, and a corrosion protection layer 122. The measurement system 1 makes it possible to measure the electrical properties of the cable insulation layer 116 in the form of a power cable.

[0026] 1, a description will be given of specific connections in the circuit included in the measurement system 1. The circuit configuration shown in FIG.

[0027] (First wiring) First wiring 41 is a wiring that connects DC power supply 10 and cable sample 100 via current integrator 20. One end 411 of first wiring 41 is connected to DC power supply 10, and the other end 412 of first wiring 41 is connected to cable sample 100 via integrating capacitor 22 of current integrator 20. More specifically, the other end 412 of first wiring 41 is connected to conductor 112 of cable sample 100. An arbitrary resistor may be inserted between cable sample 100 and current integrator 20, or between current integrator 20 and DC power supply 10, or either one of them.

[0028] (Second wiring) The second wiring 42 is a wiring that connects the DC power supply 10 and the shield ring 30. One end 421 of the second wiring 42 is connected to the DC power supply 10, and the other end 422 of the second wiring 42 is connected to the shield ring 30. The shield ring 30 plays a role as an "electric field mitigation" or "guard ring."

[0029] The shield ring 30 is disposed near the conductor 112 of the cable sample 100 with a gap or an insulator interposed therebetween. Therefore, in FIG. 1 , the shield ring 30 and the cable sample 100 are depicted as being connected by an imaginary fourth wiring 44 via a resistor 90.

[0030] By wiring first wiring 41 and second wiring 42 as described above, DC power supply 10, integrating capacitor 22 in current integrator 20, and cable sample 100 are connected in series. In addition, integrating capacitor 22 and shield ring 30 are connected in parallel between DC power supply 10 and cable sample 100.

[0031] (Third wiring) The third wiring 43 is a wiring that grounds the cable sample 100. One end 431 of the third wiring 43 is connected to the cable sample 100, and the other end 432 of the third wiring 43 is connected to the earth 98. More specifically, one end 431 of the third wiring 43 is connected to the cable shielding layer 120 of the cable sample 100.

[0032] In the measurement system 1, the current flowing in the thickness direction of the cable insulating layer 116 can be measured by the circuit described above.

[0033] (measurement system) The measurement system 1 will be described in more detail with reference to Fig. 3. Fig. 3 is a diagram showing more specifically each component, wiring, etc. when measuring the electrical characteristics of the cable insulating layer 116 using the measurement system 1. Fig. 3 shows a measurement system when measuring the electrical characteristics of the cable insulating layer 116 using the measurement system 1. The following explanation will mainly focus on matters that are different from those explained with reference to Fig. 1.

[0034] As shown in Figure 3, the cable sample 100 is arranged in a ring shape via the current-carrying / voltage-applying bar 200. For example, when only voltage is applied, the current-carrying / voltage-applying bar 200 only needs to have its exposed conductor portion located far enough away from ground. One end of the current-carrying / voltage-applying bar 200 is called the first bar end 201, and the other end of the current-carrying / voltage-applying bar 200 is called the second bar end 202.

[0035] (Cable sample classification) The layer exposed as the outermost layer of cable sample 100 varies depending on the position within measurement system 1. One end of cable sample 100 is called first end 101, and the other end is called second end 102. If cable sample 100 from first end 101 to second end 102 is divided by its exposed layers, they are, in order, exposed conductor portion 501, near-conductor insulating portion 502, test terminal portion 503, exposed shielding layer portion 504, exposed shielding layer portion 504, test terminal portion 503, near-conductor insulating portion 502, and exposed conductor portion 501.

[0036] (Exposed conductor) The conductor exposed portion 501 is a portion where the conductor 112 is exposed. In the conductor exposed portion 501, the conductor 112 is connected to the current-carrying / voltage-applying bar 200. In addition, the other end 412 of the first wiring 41 is connected to the conductor 112.

[0037] Specifically, the conductor 112 of the cable sample 100 is exposed near the first end 101 and the second end 102. At the first end 101, the conductor 112 is connected to the first bar end 201 of the energizing / voltage-applying bar 200. At the second end 102, the conductor 112 is connected to the second bar end 202 of the energizing / voltage-applying bar 200. The other end 412 of the first wiring 41 is connected to the conductor 112 of the exposed conductor portion 501 via the integrating capacitor 22 of the current integrator 20. In the example shown in FIG. 3 , the other end 412 of the first wiring 41 is connected to the conductor 112 at the exposed conductor portion 501 on the second end 102 side, not on the first end 101 side. However, this is merely an example, and the other end 412 of the first wiring 41 may be connected to the conductor 112 of the cable sample 100 at any desired location.

[0038] The vicinity of the portion of the first bar end 201 of the energizing / energizing bar 200 where the first end 101 is connected, and the vicinity of the portion of the second bar end 202 where the second end 102 is connected, are covered with an insulating material 210 such as tape.

[0039] (Insulation area near conductor) Conductor-proximate insulating portion 502 is a portion that is insulated by providing an insulating material such as insulating tape (not shown) on the outside of exposed conductor 112, or by providing a gap and insulating it with air. Conductor-proximate insulating portion 502 is located at a position following exposed conductor portion 501.

[0040] A shield ring 30 is disposed in the conductor-proximal insulating portion 502. The shield ring 30 is disposed so as to annularly surround the cable sample 100 in the conductor-proximal insulating portion 502, with an insulating material such as insulating tape or an air gap between the shield ring 30 and the cable sample 100.

[0041] 3, two shield rings 30 are arranged. The shield ring 30 arranged on the first end 101 side is called the first shield ring 301, and the shield ring 30 arranged on the second end 102 side is called the second shield ring 302.

[0042] Corresponding to the arrangement of two shield rings 30, the other end 422 of the second wiring 42 branches into two. The other end 422 of one branch is called a first other end 423. The other end 422 of the other branch is called a second other end 424.

[0043] The first other end 423 is connected to the first shield ring 301. The second other end 424 is connected to the second end 102.

[0044] (Test terminal) The test terminal portion 503 is the portion where the test terminal 130 is provided. The test terminal portion 503 is arranged next to the conductor-proximal insulating portion 502. The test terminal 130 may be used in the cable sample 100 with the cable insulation layer 116 exposed, or an insulating reinforcement layer may be formed thereon. The test terminal 130 is located near the end of the cable sample 100. In the test terminal portion 503, the insulation and insulating reinforcement layer are exposed. The conductor exposed portion 501 and the shield ring 30 are electrically insulated.

[0045] (Exposed shielding layer) The exposed shielding layer portion 504 is a portion where the cable shielding layer 120 is exposed. The exposed shielding layer portion 504 is arranged next to the test terminal portion 503. One end 431 of the third wiring 43 is connected to the cable shielding layer 120 at the exposed shielding layer portion 504. The cable shielding layer 120 is connected to the earth 98 via the third wiring 43. The shielding layer may be configured as an externally attached metal material.

[0046] The connections of the wiring will be described with reference to the cross-sectional configuration of the cable sample 100. Fig. 4 is a diagram showing an overview of the measurement system 1 as viewed in the direction of arrow 310 in Fig. 3. As shown in Fig. 4, the conductor 112 of the first end 101 and the conductor 112 of the second end 102 are connected via a current-carrying / voltage-applying bar 200. The other end 412 of the first wiring 41 is connected to the conductor 112 of the first end 101.

[0047] The other end 422 of the second wiring 42 is connected to the second shield ring 302. In the example shown in Fig. 4, the second wiring 42 is connected to the first shield ring 301 via the second shield ring 302. Note that the second wiring 42 can also be connected to the first shield ring 301 without going through the second shield ring 302, as shown in Fig. 3.

[0048] Furthermore, one end 431 of the third wiring 43 is connected to the cable shielding layer 120 .

[0049] The role of the shield ring 30 is to alleviate electric field concentration in the high-voltage section of the test terminal and to alleviate leakage current on the surface of the cable. The shield ring 30 is positioned so that it does not come into contact with the conductor 112 of the cable sample 100. To prevent contact between the shield ring 30 and the conductor 112, for example, a gap 220 can be formed between the shield ring 30 and the conductor 112, providing insulation by an air layer. Alternatively, an insulator such as insulating tape can be placed around the conductor 112, and the gap 220 can be filled with the insulator.

[0050] In measurement system 1 of this embodiment, current integrator 20 is disconnected from shield ring 30 and connected to cable sample 100. In other words, current integrator 20 and shield ring 30 are connected to different wirings. The current passing through integrating capacitor 22 of current integrator 20 passes only through conductor 112 of cable sample 100 without passing through shield ring 30.

[0051] Therefore, in the measurement system 1 of this embodiment, it is possible to prevent the current integrator 20 from detecting leakage current from the shield ring 30 or the test terminal 130. As a result, the measurement system 1 of this embodiment can accurately evaluate the electrical characteristics of the DC insulator in the form of a power cable. In other words, the measurement system 1 of this embodiment can accurately evaluate the electrical characteristics of the cable insulating layer 116.

[0052] (Cable insulation layer evaluation method) A method for evaluating a cable insulation layer according to an embodiment of the present invention will now be described. The method for evaluating a cable insulation layer according to an embodiment of the present invention is a method for evaluating the electrical properties of the cable insulation layer 116 in the state of a power cable in a cable sample 100 including a conductor 112 and a cable insulation layer 116 covering the conductor 112. Fig. 5 is a flow chart showing an overview of the method for evaluating a cable insulation layer according to this embodiment.

[0053] (S1) S1 is a voltage application step. In S1, a DC voltage is applied to all conductors 112 (cable conductors) of cable sample 100 through current integrator 20. A DC voltage is applied to shield ring 30 without passing through current integrator 20.

[0054] (S2) S2 is a step of determining whether a predetermined time has elapsed since the application of the DC voltage. In the method for evaluating a cable insulation layer of this embodiment, a DC voltage is applied to the conductor 112, and then the potential difference and current value are determined (measured) after a predetermined time has elapsed, and the amount of charge is determined by integrating them. This will be explained with reference to FIG. 6.

[0055] Fig. 6 is a diagram showing the relationship between the applied voltage and the measurement time, etc. Graph 601 in Fig. 6 is a diagram showing the relationship between time and the applied voltage to the cable sample 100. The horizontal axis of graph 601 represents time, and the vertical axis represents the applied voltage to the cable sample 100.

[0056] Graph 602 in Fig. 6 shows the measurement time of the accumulated charge amount. The measurement time of the accumulated charge amount is the time from the timing when measurement of the accumulated charge amount starts to the timing when measurement of the accumulated charge amount ends. The horizontal axis of graph 602 indicates time, and the vertical axis indicates the measurement OFF and measurement ON states. OFF on the vertical axis of graph 602 means measurement OFF, and ON means measurement ON.

[0057] Graph 603 in Fig. 6 shows the relationship between time and amount of charge accumulated Q in current integrator 20. The horizontal axis of graph 603 represents time, and the vertical axis represents amount of charge accumulated Q in integrating capacitor 22 of current integrator 20, scaled relative to initial amount of charge accumulated Q(0). Note that the position of time zero on the horizontal axis and the scale of the horizontal axis are the same in graphs 601, 602, and 603.

[0058] In the method for evaluating the cable insulating layer 116 of this embodiment, the cable insulating layer 116 is evaluated using a cable sample 100. Therefore, the applied voltage and the amount of accumulated charge are larger than when determining the amount of charge stored in an insulator in sheet form. For example, when the thickness of the cable insulating layer 116 is 6 mm or more and 30 mm or less, the applied voltage may be approximately 800 kV.

[0059] As the applied voltage increases, the initial charge accumulation amount Q(0) increases. Therefore, if the initial charge accumulation amount Q(0) is included in the measurement time, the charge accumulation amount Q(t) will reach the allowable capacity of the integrating capacitor 22 of the current integrator 20 before the desired measurement time arrives. Therefore, measurement of the charge accumulation amount is started after the applied voltage reaches a predetermined voltage.

[0060] Furthermore, after the applied voltage reaches a predetermined voltage, a time lag occurs before integrating capacitor 22 of current integrator 20 is charged. Therefore, measurement of the accumulated charge amount is started after the applied voltage reaches the predetermined voltage and after integrating capacitor 22 is charged.

[0061] The following description will be given with reference to graphs 601 to 603. In graph 601, t1 indicates the timing when the applied voltage reaches a predetermined voltage. In graph 602, t2 indicates the timing when measurement of the accumulated charge amount starts. Δt indicates the time difference between timing t1 and timing t2. Δt is preferably set so that a sufficient time is ensured after the applied voltage reaches the predetermined voltage. In other words, it is preferable to start measurement of the accumulated charge amount after the integrating capacitor 22 is sufficiently charged.

[0062] Therefore, in S2, it is determined whether a predetermined time has elapsed after the DC voltage is applied. Then, only if the predetermined time has elapsed, proceed to the integral value measurement step in S3. By providing S2 before S3, it is possible to apply a predetermined voltage to the cable sample 100 and start measurement after it has stabilized sufficiently. This avoids measuring the initial amount of charge accumulation. As a result, it is possible to take a longer measurement time.

[0063] The predetermined time that should elapse after the DC voltage is applied will now be described. The predetermined time can be calculated, for example, by the following formula.

[0064] Formula Q(0)=C cable ×V cable =I HV ×Δt (Q(0): initial charge accumulation amount, C cable : capacitance of the cable insulation layer, V cable : voltage applied to the cable insulation layer, I HV :charging current) The Δt calculated from the above formula can be used as a reference for determining the predetermined time that should elapse after the application of the DC voltage, which may mean, for example, that measurement of the amount of accumulated charge should start after 1×Δt seconds or more and 10×Δt seconds or less have elapsed since the application of the DC voltage.

[0065] (S3) S3 is an integral value measurement step. In S3, an integral value of the current flowing through the cable insulating layer 116 is measured when a DC voltage is applied to the conductor 112 for a predetermined measurement time. The predetermined measurement time may be, for example, 3000 seconds. The predetermined measurement time is not particularly limited.

[0066] In the integral value measurement step, the electrical characteristics of the cable insulating layer 116 can be evaluated, for example, as follows. For example, the initial charge accumulation amount Q(0) and the charge accumulation amount Q(t) after a predetermined time t seconds have elapsed are calculated from the capacitance of the cable insulating layer 116 measured using a capacitance measuring device and the applied voltage. This calculation can be performed using the formula Q=CV (Q: charge accumulation amount, C: capacitance, V: applied voltage). Next, the accumulated charge increase rate Q(t) / Q(0) is calculated. Using this accumulated charge increase rate as an index, the electrical characteristics of the cable insulating layer 116 can be evaluated.

[0067] The ratio Q(t) / Q(0) of the total charge amount Q(t) after a predetermined measurement time to the initial charge accumulation amount Q(0), measured by the above-mentioned evaluation method, is preferably, for example, 1 or more and 2.5 or less.

[0068] It is preferable that a power cable or the like has a cable insulation layer 116 in which the ratio Q(t) / Q(0) according to the above-mentioned evaluation method is, for example, between 1 and 2.5. Furthermore, it is preferable that not only power cables but also power cable systems having intermediate joints and terminal joints, which will be described later, have cable insulation layers 116 within the above-mentioned numerical range.

[0069] The measurement can be terminated after a predetermined measurement time has elapsed. Graph 602 indicates the timing of measurement termination as t3. The time from timing t2 to timing t3 is the predetermined measurement time. After the measurement is terminated, the charge in integrating capacitor 22 is discharged. Graph 603 indicates the timing of charge discharge as t4. Timing t4 occurs before timing t3. After the charge in integrating capacitor 22 is discharged, the voltage applied to conductor 112 is dropped. Graph 601 indicates the timing of dropping the voltage applied to conductor 112 as t5. Timing t5 occurs after timing t3. In other words, the order is discharge t4, measurement termination t3, and voltage drop t5. Dropping the voltage applied to conductor 112 completes the flow of the cable insulation layer evaluation method.

[0070] (Capacitance of the integrating capacitor) The capacitance of the integrating capacitor 22 of the current integrator 20 will now be described. As mentioned above, when evaluating the electrical properties of the cable insulation layer 116 using the cable sample 100, the capacitance is larger than when using a sheet sample. The capacitance of the cable sample 100 is, for example, 10 times or more the capacitance of the sheet sample. Therefore, when evaluating the electrical properties of the cable insulation layer 116 using the cable sample 100, it is preferable to increase the capacitance of the integrating capacitor 22 of the current integrator 20. However, if the capacitance of the integrating capacitor 22 is too large, the measurement sensitivity may be reduced. Therefore, it is necessary to optimize the capacitance of the current integrator 20 to match the capacitance of the cable sample 100.

[0071] Specifically, if the capacitance of the integrating capacitor 22 is too large compared to the capacitance of the cable sample 100, the capacitance measurement sensitivity will be reduced. In particular, to detect minute differences in capacitance due to deterioration of the cable insulation layer 116, it is necessary to maintain the measurement sensitivity. On the other hand, if the capacitance of the integrating capacitor is too small compared to the capacitance of the cable sample 100, the amount of accumulated charge will reach the capacity limit of the integrating capacitor 22 soon after the start of measurement. As a result, the required measurement time cannot be secured. Therefore, it is preferable to determine the capacitance of the integrating capacitor 22 in terms of its ratio to the capacitance of the cable sample 100.

[0072] The capacitance of the integrating capacitor disposed in the current integrator 20 can be determined based on, for example, the following equation: Formula Q(t)=C INT ×V INT (Q(t): amount of charge accumulated after t seconds, C INT : The capacitance of the integrating capacitor placed in the current integrator, V INT : Upper limit voltage applied to the current integrator) C calculated from the above formula INT is set so that the charge accumulation change amount ΔQ(t) in a predetermined measurement time is 5% or more and 50% or less with respect to Q(0).

[0073] This allows the capacitance of integrating capacitor 22 arranged in current integrator 20 to be determined from the perspective of what is the desirable ratio of the amount of charge accumulated after t seconds Q(t) to the amount of initial charge accumulated Q(0). In other words, the capacitance of integrating capacitor 22 can be determined from the perspective of what level of sensitivity is required when observing the accumulation of space charge relative to the capacitance of cable sample 100.

[0074] The amount of change in charge accumulation ΔQ(t) relative to the initial amount of charge accumulation Q(0) will be described with reference to Fig. 7. Fig. 7 is a diagram showing the relationship between the capacitance of current integrator 20 and the change in the amount of charge accumulation over time. In the graph shown in Fig. 7, the horizontal axis represents time, and the vertical axis represents the amount of charge accumulation Q in integrating capacitor 22 of current integrator 20.

[0075] As shown in FIG. 7, the amount of charge accumulation at time 0 is defined as the initial amount of charge accumulation Q(0). In the example shown in FIG. 7, the measurement time is 3000 seconds. C1 to C4 shown in FIG. 7 have different capacitances of integrating capacitor 22 of current integrator 20. The capacitances increase in the order from C1 to C4. The ratio of the amount of charge accumulation change ΔQ(t) to Q(0) at 3000 seconds is 100% for C1, 50% for C2, 10% for C3, and 5% for C4. When the capacitance of integrating capacitor 22 is small, the ratio of the amount of charge accumulation change ΔQ(t) to Q(0) is large, and when the capacitance of integrating capacitor 22 is large, the ratio of the amount of charge accumulation change ΔQ(t) to Q(0) is small.

[0076] Therefore, from the viewpoint of maintaining the necessary measurement sensitivity while avoiding reaching the capacity limit, the capacitance of the integrating capacitor 22 is determined so that the charge accumulation change amount ΔQ(t) relative to Q(0) is 5% to 50%. In the example shown in FIG. 7, C2 to C4 are within this range. In other words, the capacitance of the integrating capacitor 22 from C2 to C4 is the preferable capacitance value of the integrating capacitor 22. If the capacitance value of the integrating capacitor 22 is smaller than the value at C2, there is a high risk that the amount of accumulated charge will reach the capacity limit during measurement. On the other hand, if the capacitance value of the integrating capacitor 22 is larger than the value at C4, there is a high risk that the measurement sensitivity will be reduced and minute differences in electrostatic capacitance will not be detected.

[0077] The capacitance of integrating capacitor 22 of current integrator 20 can also be adjusted depending on the measurement time. Generally, when the measurement time is short, the change in the amount of accumulated charge is small, and when the measurement time is long, the change in the amount of accumulated charge becomes large. Therefore, the capacitance of integrating capacitor 22 at which the amount of charge accumulation change ΔQ(t) relative to Q(0) is between 5% and 50% changes depending on the measurement time.

[0078] (Embodiment 2) A measurement system 1 for a cable insulation layer and an evaluation method for a cable insulation layer according to a second embodiment of the present invention will be described with reference to FIG. 8. FIG. 8 is a diagram showing a measurement system for measuring the electrical properties of a cable insulation layer using the measurement system according to the second embodiment. The following description will focus on differences between the second embodiment and the first embodiment. Unlike the measurement system 1 according to the first embodiment, the measurement system 1 according to the second embodiment includes a pulse electrostatic stress measurement unit 250. The pulse electrostatic stress measurement unit 250 is a component that enables measurement of the pulse electrostatic stress of the cable sample 100. The measurement system 1 according to the second embodiment can measure the pulse electrostatic stress (PEA) in addition to the charge accumulation amount Q(t) according to the first embodiment. The charge accumulation amount Q(t) was measured by placing a current integrator 20 between the conductor 112 of the cable sample 100, which serves as a terminal high-voltage unit, and the DC power supply 10, and measuring the charge accumulation amount of an integrating capacitor 22 in the current integrator 20. In contrast, pulse electrostatic stress is measured by applying a pulse voltage from the cable shielding layer 120 side rather than from the terminal high-voltage section side of the cable sample 100, i.e., by injecting a pulse, and then sensing the elastic waves caused by the accumulated charge with a pressure sensor such as a piezoelectric sensor to measure the pulse electrostatic stress.

[0079] (Cable sample classification) 8, in the measurement system 1 of the second embodiment, a pulse electrostatic stress measurement unit 250 is provided in a portion of the cable sample 100 that is sandwiched between two anticorrosion layers 122. Before describing the pulse electrostatic stress measurement unit 250, the classification of the cable sample 100 from the viewpoint of the layer that is exposed as the outermost layer will be described.

[0080] The second embodiment differs from the first embodiment in the arrangement of the exposed shielding layer portion 504 and other components in the cable sample 100. In the second embodiment, when the cable sample 100 from the first end 101 to the second end 102 is divided into exposed layers, the layers are, in order, the exposed conductor portion 501, the conductor-proximal insulating portion 502, the test terminal portion 503, the exposed shielding layer portion 504, the exposed outer semiconductive layer portion 506, the exposed shielding layer portion 504, the test terminal portion 503, the conductor-proximal insulating portion 502, and the exposed conductor portion 501. The exposed outer semiconductive layer portion 506 refers to the portion where the outer semiconductive layer 118 is exposed. The anticorrosion layer 122 does not have to be attached during measurement.

[0081] (Pulse electrostatic stress measurement unit) The pulse electrostatic stress measurement section 250 includes a pulse electrostatic stress measurement device 260 and a resistance section 270 .

[0082] (Pulse electrostatic stress measurement device) The pulse electrostatic stress measurement device 260 is a device that detects charge oscillations that occur inside the cable sample 100. The pulse electrostatic stress measurement device 260 is disposed in the outer semiconductive layer exposed portion 506. The pulse electrostatic stress measurement device 260 includes a piezoelectric sensor 262. The piezoelectric sensor 262 detects charge oscillations that occur inside the cable sample 100 that is located on the pulse electrostatic stress measurement device 260.

[0083] (Resistance part) The pulse voltage 272 is applied to the cable sample 100 via a resistor 270. The resistor 270 is provided on each of the two exposed portions 504 of the shielding layer that sandwich the exposed portion 506 of the outer semiconducting layer. Each resistor 270 is connected to the exposed portion 504 of the shielding layer.

[0084] Further, each resistor portion 270 is connected to the ground 98. In the first embodiment, in order to measure the amount of accumulated charge Q(t), a shielding layer exposed portion 504 is provided following the test terminal portion 503, and the cable shielding layer 120 and the ground 98 are connected at the shielding layer exposed portion 504 via the third wiring 43. In contrast to this, in the second embodiment, the cable shielding layer 120 and the ground 98 are connected at the shielding layer exposed portions 504 located on both sides of the outer semiconductive layer exposed portion 506 via the resistor portion 270.

[0085] In the cable insulation layer evaluation method of the second embodiment, pulse electrostatic stress can be measured in the measurement system 1 shown in FIG. 8 as follows. In the pulse electrostatic stress measurement, a high pulse voltage 272 is applied to the cable shielding layer 120 of the cable sample 100 via a resistor 270 from the exposed shielding layer portions 504 that sandwich the exposed outer semiconducting layer portion 506 from both sides. The application of this pulse voltage 272 generates elastic waves due to charge accumulation in the cable sample 100, causing the cable sample 100 to vibrate. In other words, the application of a high-voltage pulse causes the charges inside the cable sample 100 to vibrate. This vibration is detected by a pulse electrostatic stress measurement device 260, particularly by a piezoelectric sensor 262 of the pulse electrostatic stress measurement device 260, allowing the amount of charge accumulation and the charge distribution to be measured.

[0086] As described above, the cable insulation layer evaluation method of this embodiment can be expressed as follows: The outer semiconductive layer exposed portion 506, which is the portion of the cable sample 100 where the outer semiconductive layer 118 is exposed, is placed in a pulse electrostatic stress measurement device 260 in which a piezoelectric sensor 262 is embedded. A pulse voltage is then applied to the cable shielding layer 120 of the cable sample 100. Elastic waves from the cable sample 100 are sensed by the piezoelectric sensor 262 to measure the pulse electrostatic stress. The process described above is called an electrostatic stress measurement process.

[0087] (Electrostatic stress measurement process) In the evaluation method for a cable insulating layer of this embodiment, various timings are possible for performing the electrostatic stress measurement step in relation to the integral measurement step described in embodiment 1. For example, the electrostatic stress measurement step and the integral measurement step can be performed at least partially simultaneously. Also, the integral measurement step can be performed before or after the electrostatic stress measurement step.

[0088] The measurement of the accumulated charge Q(t) and the pulse electrostatic stress can be performed on the same cable sample 100 using the same measurement system. In other words, the space charge can be measured by two methods on the same cable sample 100 using the same measurement system. This is because the pulse electrostatic stress can be measured on the exposed shielding layer portion 504 side, and the measurement of the accumulated charge Q(t) can be performed on the exposed conductor portion 501 side on the high-voltage side, for example. In other words, the part for measuring the pulse electrostatic stress and the part for measuring the accumulated charge Q(t) can be arranged separately in the measurement system 1. This allows both the measurement of the pulse electrostatic stress and the measurement of the accumulated charge Q(t) to be performed using the same measurement system. Measuring the accumulated charge Q(t) and the pulse electrostatic stress on the same cable sample 100 reduces the variation in measurement results due to differences in the cable sample 100. Furthermore, by simultaneously measuring the accumulated charge Q(t) and the pulse electrostatic stress, data on the accumulated charge Q(t) and the pulse electrostatic stress can be obtained for the same cable sample 100 immediately after application of a DC voltage and a pulse voltage.

[0089] (Embodiment 3) A cable insulation layer measurement system 1 and a cable insulation layer evaluation method according to a third embodiment of the present invention will be described with reference to Fig. 9. Fig. 9 is a diagram showing an outline of a measurement system in the cable insulation layer measurement system 1 according to the third embodiment. The following description will focus on differences between the third embodiment and the first embodiment.

[0090] The measurement system 1 of the third embodiment differs from the measurement system 1 of the first embodiment in that the cable sample 100 includes a first cable 1001 and a second cable 1002. The measurement system 1 of the third embodiment also includes an intermediate connection part 350. The intermediate connection part 350 is a part that connects the first cable 1001 and the second cable 1002.

[0091] The cable insulation layer measurement system 1 and the cable insulation layer evaluation method of the third embodiment are capable of measuring a cable sample 100 in which a plurality of cables are connected. Fig. 9 illustrates a case in which two cables are connected at an intermediate joint 350. One of the two cables constituting the cable sample 100 is referred to as a first cable 1001, and the other cable is referred to as a second cable 1002.

[0092] One end of the first cable 1001 is referred to as a third end 103, and the other end of the first cable 1001 is referred to as a fourth end 104. Furthermore, one end of the second cable 1002 is referred to as a fifth end 105, and the other end of the second cable 1002 is referred to as a sixth end 106.

[0093] The third end 103 of the first cable 1001 is connected to the first bar end 201 of the energizing / voltage-applying bar 200, and the fifth end 105 of the second cable 1002 is connected to the second bar end 202 of the energizing / voltage-applying bar 200. The third end 103 corresponds to the first end 101 in embodiment 1, and the fifth end 105 corresponds to the second end 102 in embodiment 1.

[0094] The fourth end 104 of the first cable 1001 and the sixth end 106 of the second cable 1002 are connected via an intermediate connection part 350 .

[0095] (Cable sample classification) 9, in the measurement system 1 of the third embodiment, an intermediate joint 350 is provided in a portion of the cable sample 100 that is sandwiched between the portions where the two anticorrosion layers 122 are exposed. In both the first cable 1001 and the second cable 1002, the portion connected to the intermediate joint 350 is an exposed shielding layer portion 504. That is, the vicinity of the fourth end 104 and the vicinity of the sixth end 106 are exposed shielding layer portions 504.

[0096] (Intermediate connection part) Intermediate joint 350 is a portion that connects two cables. The cables are connected by inserting them into intermediate joint 350 with the conductors, insulating layer, and outer semiconductive layer exposed in stages. In intermediate joint 350, at least a portion of corresponding layers of first cable 1001 and second cable 1002, such as conductor 112 of first cable 1001 and conductor 112 of second cable 1002, and cable insulating layer 116 of first cable 1001 and cable insulating layer 116 of second cable 1002, are connected by connecting members provided inside intermediate joint 350.

[0097] The measurement system 1 and the method for evaluating a cable insulation layer of the third embodiment can measure the electrical characteristics of a cable sample 100 in which two cables are previously connected at an intermediate joint 350. Furthermore, the measurement system 1 and the method for evaluating a cable insulation layer of the third embodiment can measure the electrical characteristics of a first cable 1001 and a second cable 1002, for example, and then connect the first cable 1001 and the second cable 1002 at the intermediate joint 350, and measure the electrical characteristics of the connected cable sample 100. This makes it possible to compare the electrical characteristics of each cable with the overall electrical characteristics after connection.

[0098] The above describes a case where two cables are connected. However, the number of intermediate connectors 350 as joints is not limited to one. In other words, the number of cables included in the cable sample 100 is not limited to two.

[0099] 9, the cable sample 100 has four exposed shielding layer portions 504. The third wiring, which is a wiring connected to the earth 98, can be connected to any of the four exposed shielding layer portions 504. In other words, the exposed shielding layer portions 504 to which the third wiring is connected are not limited to the example shown in FIG.

[0100] The measurement system 1 and the method for evaluating a cable insulation layer according to the embodiment of the present invention facilitate sample investigation of the product by using the cable itself that will actually be delivered as a product as a sample for development testing or type testing.

[0101] Although the present invention has been described above as an embodiment, it is not limited to the above-described embodiment, and various changes, modifications, and combinations are possible.

[0102] According to the embodiment described above, the following effects are achieved.

[0103] The method for evaluating a cable insulation layer 116 in this embodiment is a method for evaluating a cable insulation layer 116 in a cable sample 100 including a conductor 112 and a cable insulation layer 116 covering the conductor 112, and includes a voltage application process in which a DC voltage is applied to the conductor 112 via a current integrator 20, and a DC voltage is applied to the conductor 112 via a shield ring 30 without via the current integrator 20, and an integral value measurement process in which the integral value of the current flowing through the cable insulation layer 116 when the DC voltage is applied to the conductor 112 for a predetermined measurement time is measured.

[0104] This makes it possible to accurately evaluate the electrical characteristics of a DC insulating material in a power cable or in a state where the power cable is simulated.

[0105] In addition, in the evaluation method for the cable insulation layer 116 according to this embodiment, the integrating capacitor 22 in the current integrator 20 and the cable sample 100 are connected in series, and the integrating capacitor 22 and the shield ring 30 are connected in parallel between the DC power supply 10 that supplies a DC voltage and the cable sample 100.

[0106] This makes it possible to accurately measure the current flowing through the cable sample while taking shielding measures and suppressing the influence of leakage current from shield rings, etc.

[0107] In addition, in the evaluation method for the cable insulation layer 116 according to this embodiment, a test terminal 130 covered with an insulating material is provided at the end of the cable sample 100, and the test terminal 130 and the shield ring 30 are electrically insulated from each other.

[0108] This allows for more reliable shielding measures.

[0109] In the method for evaluating the cable insulating layer 116 according to this embodiment, the formula Q(0)=C cable ×V cable =I HV ×Δt (Q(0): initial charge accumulation amount, C cable : capacitance of the cable insulating layer 116, V cable : voltage applied to the cable insulating layer 116, I HV Regarding Δt obtained from the DC voltage (= Δt / √Hz), the predetermined measurement time starts after 1×Δt seconds to 10×Δt seconds have elapsed since the DC voltage was applied to the cable insulating layer 116, and ends after the predetermined application time has elapsed. After the measurement is completed and the charge in the integrating capacitor 22 is discharged, the voltage applied to the conductor 112 is reduced.

[0110] This prevents the capacitance generated by the current flowing through the cable sample from reaching the allowable capacitance of the capacitor of the current integrator, making it impossible to measure the electrical characteristics.

[0111] Furthermore, in the evaluation method for the cable insulating layer 116 according to this embodiment, the cable sample 100 has an outer semiconductive layer 118 on the outside of the cable insulating layer 116, and further has a cable shielding layer 120 on the outside of the outer semiconductive layer 118, and further includes an electrostatic stress measurement step of arranging the exposed portion of the outer semiconductive layer 118 of the cable sample 100 in a pulse electrostatic stress measurement unit 250 having a piezoelectric sensor 262 embedded therein, applying a pulse voltage 272 to the cable shielding layer 120 of the cable sample 100, and sensing elastic waves from the cable sample 100 with the piezoelectric sensor 262 to measure the pulse electrostatic stress.

[0112] This allows measurement of pulsed electrostatic stress (PEA) in addition to charge accumulation Q(t) on the same sample.

[0113] In the method for evaluating the cable insulating layer 116 according to this embodiment, the electrostatic stress measuring step and the integrated value measuring step are at least partially carried out simultaneously.

[0114] This makes it possible to shorten the time required to measure the charge accumulation amount Q(t) and the pulsed electrostatic stress (PEA).

[0115] In the evaluation method for the cable insulating layer 116 according to this embodiment, the integral value measurement step is performed after the electrostatic stress measurement step.

[0116] This makes it easy to measure the pulse electrostatic stress immediately after applying a DC voltage and measuring the charge accumulation amount Q(t).

[0117] In addition, in the evaluation method of the cable insulating layer 116 according to this embodiment, Formula Q(t)=C INT ×V INT (Q(t): amount of charge accumulated after t seconds, C INT : The capacitance of the integrating capacitor placed in the current integrator, V INT : Upper limit voltage applied to the current integrator) C obtained from INTis set so that the charge accumulation change amount ΔQ(t) in a predetermined measurement time is 5% or more and 50% or less with respect to Q(0).

[0118] This prevents the measurement sensitivity from becoming dull, while also preventing the capacitance generated by the current flowing through the cable sample from reaching the allowable capacitor capacity of the current integrator, making it impossible to measure the electrical characteristics.

[0119] In addition, in the measurement system for the cable insulation layer 116 and the evaluation method for the cable insulation layer 116 according to this embodiment, the cable sample 100 includes at least a first cable 1001 and a second cable 1002, and an intermediate connection portion 350 is arranged between the first cable 1001 and the second cable 1002.

[0120] This facilitates the measurement of the electrical properties of the power cable system.

[0121] Furthermore, the power cable of this embodiment has a cable insulating layer 116 in which the ratio Q(t) / Q(0) of the total charge amount Q(t) including the initial charge accumulation amount Q(0) after a predetermined measurement time to the measurement system (0) of the cable insulating layer 116 of this embodiment, as measured by the evaluation method of the cable insulating layer 116 of this embodiment, is 1 or more and 2.5 or less.

[0122] This makes it possible to obtain a DC power cable with excellent electrical characteristics.

[0123] The measurement system 1 for a cable insulation layer 116 in this embodiment is a measurement system 1 for a cable insulation layer 116 included in a cable sample 100, which comprises a DC power source 10, a current integrator 20, a shield ring 30, a first wiring 41, a second wiring 42, and a third wiring 43.The cable sample 100 includes a conductor 112 arranged in order from the center of the cable sample 100 to the outside, a cable insulation layer 116, and a cable shielding layer 120, and one end 411 of the first wiring 41 is connected to the DC power source 10, the other end 412 of the first wiring 41 is connected to the conductor 112 via the current integrator 20, one end 421 of the second wiring 42 is connected to the DC power source 10, the other end 422 of the second wiring 42 is connected to the shield ring 30 without via the current integrator 20, and one end 431 of the third wiring 43 is connected to the cable shielding layer 120.

[0124] This makes it possible to accurately evaluate the electrical characteristics of a DC insulating material in a power cable or in a state where the power cable is simulated.

[0125] Furthermore, in the measurement system 1 for the cable insulating layer 116 according to this embodiment, the cable sample 100 includes an outer semiconductive layer 118 between the cable insulating layer 116 and the cable shielding layer 120, and is arranged to sandwich the exposed portion of the outer semiconductive layer 118 in the cable sample 100, and further includes a pulse electrostatic stress measurement unit 250 including a piezoelectric sensor 262, and a resistance unit 270 that applies a pulse voltage 272 to the cable shielding layer 120.

[0126] This allows measurement of pulsed electrostatic stress (PEA) in addition to charge accumulation Q(t) on the same sample. [Explanation of symbols]

[0127] 1. Measurement System 10 DC power supply 20 Current integrator 22 Integration capacitor 30 Shield Ring 41 First Wiring 42 Second Wiring 43 Third Wire 98 Earth 100 cable samples 112 Conductor 114 Internal semiconducting layer 116 Cable insulation layer 118 Outer semiconducting layer 120 Cable shielding layer 122 Anticorrosion Layer 130 Test terminal

Claims

1. A method for evaluating a cable insulation layer in a cable sample including a conductor and a cable insulation layer covering the conductor, comprising: a voltage applying step of applying a DC voltage to the conductor via a current integrator and also applying a DC voltage to the conductor via a shield ring without via the current integrator; an integral value measurement step of measuring the value of the current flowing through the cable insulation layer when a DC voltage is applied to the conductor for a predetermined measurement time, and integrating the current value to determine the amount of charge obtained.

2. an integrating capacitor in the current integrator and the cable sample are connected in series; 2. The method for evaluating a cable insulation layer according to claim 1, wherein the integrating capacitor and the shield ring are connected in parallel between a DC power source that supplies the DC voltage and the cable sample.

3. a test terminal is provided at an end of the cable sample, in which the cable conductor and the cable insulation are exposed or covered with an insulating material; 3. The method for evaluating a cable insulation layer according to claim 1, wherein the exposed conductor portion of the test terminal and the shield ring are electrically insulated from each other.

4. Equation Q(0) = C ｃａｂｌｅ ×V ｃａｂｌｅ =I ＨＶ ×Δt (Q(0): initial charge accumulation amount, C ｃａｂｌｅ : capacitance of the cable insulation layer, V ｃａｂｌｅ : voltage applied to the cable insulation layer, I ＨＶ :Charging current) Regarding Δt, 3. The method for evaluating a cable insulation layer according to claim 1, wherein the predetermined measurement time starts after a lapse of 1 × Δt seconds or more and 10 × Δt seconds or less after application of a DC voltage to the cable insulation layer, the measurement ends after the lapse of the predetermined application time, and after discharging an electric charge in an integrating capacitor in the current integrator, the voltage applied to the conductor is reduced.

5. The cable sample has an outer semiconductive layer on the outside of the cable insulation layer, and further has a cable shielding layer on the outside of the outer semiconductive layer, a pulse electrostatic stress measuring unit having an embedded piezoelectric sensor disposed in a portion of the cable sample where the outer semiconductive layer is exposed; applying a pulse voltage to the cable shielding layer of the cable sample; 3. The method for evaluating a cable insulation layer according to claim 1, further comprising an electrostatic stress measurement step of measuring a pulse electrostatic stress by sensing elastic waves from the cable sample with the piezoelectric sensor.

6. The method for evaluating a cable insulating layer according to claim 5 , wherein the electrostatic stress measuring step and the integrated value measuring step are at least partially performed simultaneously.

7. The method for evaluating a cable insulating layer according to claim 5 , wherein the integral value measuring step is performed before or after the electrostatic stress measuring step, or both of them.

8. Equation Q(t) = C ＩＮＴ ×V ＩＮＴ (Q(t): amount of charge accumulated after t seconds, C ＩＮＴ : the capacitance of the integrating capacitor placed in the current integrator, V ＩＮＴ : Upper limit voltage applied to the current integrator) C obtained from ＩＮＴ 3. The method for evaluating a cable insulation layer according to claim 1, wherein the change in charge accumulation ΔQ(t) over a predetermined measurement time is set to be 5% or more and 50% or less of Q(0).

9. the cable sample includes at least a first cable and a second cable; 3. The method for evaluating a cable insulation layer according to claim 1, wherein an intermediate joint is disposed between the first cable and the second cable.

10. 3. A power cable comprising a cable insulation layer, in which the ratio Q(t) / Q(0) of the total charge Q(t) including the initial charge storage Q(0) after the predetermined measurement time to the initial charge storage Q(0) is 1 or more and 2.5 or less, as measured by the evaluation method for a cable insulation layer according to claim 1 or 2.

11. A measurement system for a cable insulation layer included in a cable sample, comprising a DC power supply, a current integrator, a shield ring, a first wire, a second wire, and a third wire, The cable sample includes a conductor, a cable insulating layer, and a cable shielding layer, which are arranged in this order from the center of the cable sample toward the outside, one end of the first wiring is connected to the DC power supply, and the other end of the first wiring is connected to the conductor via the current integrator; one end of the second wiring is connected to the DC power supply, and the other end of the second wiring is connected to the shield ring without passing through the current integrator; A system for measuring a cable insulation layer, wherein one end of the third wiring is connected to the cable shielding layer.

12. The cable sample includes an outer semiconductive layer between the cable insulation layer and the cable shielding layer; a pulse electrostatic stress measurement unit including a piezoelectric sensor, the pulse electrostatic stress measurement unit being disposed so as to sandwich the exposed portion of the outer semiconductive layer of the cable sample; The cable insulation layer measurement system of claim 11, further comprising a resistor for applying a pulse voltage to the cable shielding layer.

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