JP6322681B2 - Measurement method of dielectric loss tangent of power cable - Google Patents

Description

  The present invention relates to a method for measuring a dielectric loss tangent of a power cable, and more particularly to a measurement method using a live wire.

  Tree degradation is one of the main insulation degradation phenomena that determines the withstand voltage life characteristics of rubber / plastic insulation cables (hereinafter referred to as “power cables”) such as CV cables.

  FIG. 5 schematically shows a cross section of the power cable. The power cable 10 is formed such that the conductor 11, the insulator 13, the shield layer 15, and the cable sheath 17 are located concentrically from the inside.

  The conductor 11 is an assembly of strands mainly composed of copper or aluminum. The insulator 13 is provided on the outer periphery of the conductor 11 for the purpose of forming electrical insulation, and is mainly composed of plastic synthetic rubber such as crosslinked polyethylene. The shield layer 15 is provided on the outer periphery of the insulator 13 in order to prevent the electric field from the conductor 11 from being radiated to the outside of the cable from the viewpoint of security and improvement in cable performance. It is configured. The cable sheath 17 is applied for the purpose of protecting the insulator 13 from damage, intrusion of moisture and organic substances, and is mainly composed of vinyl or neoprene rubber.

  Although not shown in FIG. 5, adhesion between the conductor 11 and the insulator 13 is improved between the conductor 11 and the insulator 13 to prevent generation of voids (bubbles) and to reduce electric field concentration. An internal semiconductive layer may be applied for purposes. In addition, an external semiconductive layer may be provided between the insulator 13 and the shield layer 15 for the purpose of preventing generation of voids between the insulator 13 and the shield layer 15 and alleviating electric field concentration.

  In the power cable 10 constructed in the configuration as shown in FIG. 5, a void 21 may be generated inside for some reason as time elapses. If the state in which the voltage is applied in such a state continues, the void 21 having a lower dielectric breakdown strength than the insulator 13 is discharged first (partial discharge), and this discharge is repeated, whereby the insulator 13 is repeated. A hollow with a sharp tip (projection shape) is formed inside. Once such a shape is formed in the insulator 13, the electric field concentrates at the location and dendritic breakdown occurs. Once this breakdown occurs, an air layer is formed at this location, so that the discharge further proceeds.

  In addition to the void 21, the power cable 10 may generate foreign matter 23 and moisture 25 inside. The moisture 25 existing in the insulator 13 further develops a tree-like defect by a synergistic action with a local high electric field applied to the foreign matter 23, the void 21, or the projection.

  Such a tree phenomenon decreases the insulation performance of the power cable 10 with its growth, and eventually causes an insulation breakdown accident during operation. For this reason, in the insulation deterioration diagnosis of the power cable 10, it is required to detect the tree deterioration nondestructively and with high reliability.

  When the tree progresses in the power cable 10, the electric field applied to the power cable 10 changes with time, and this changes to a resistance component. As a result, the change in the electric flux density is delayed, and the phase of the current is delayed by δ. As a result, the phase difference between current and voltage is not 90 °. This δ is called a dielectric loss angle. A method for diagnosing deterioration of the power cable 10 by measuring the dielectric loss tangent (tan δ) that is the tangent of δ is known.

  FIG. 6 shows a Schering bridge circuit which is a general technique for measuring the dielectric loss tangent (see, for example, Patent Document 1). The values of the variable capacitor C4 and the variable resistor R3 are determined so that the current flowing through the galvanometer G becomes substantially zero. Based on this value, the value of the dielectric loss tangent of the power cable 10 can be measured.

  However, in order to measure the dielectric loss tangent using the Schering bridge, a high voltage power supply HV is required. That is, it is necessary to prepare a large power supply device for measurement. Moreover, since the dielectric loss tangent cannot be measured while the power cable 10 is in a live line state, a power failure is forced.

  Against this background, a technique for measuring the dielectric loss tangent of a power cable in a live line state has been proposed (see, for example, Patent Documents 2 and 3).

JP-A-6-3391 JP-A-7-98347 JP 7-218564 A

  In the method described in Patent Document 2, it is necessary to connect a standard capacitor in advance to the power cable that is the object to be measured, and there is a concern that the work for actually measuring the dielectric loss tangent becomes complicated. Moreover, since the method described in Patent Document 3 is configured to apply a voltage from a current transformer, there is a problem that the accuracy of the measured dielectric loss tangent depends on the characteristics of the current transformer.

  An object of the present invention is to propose a new method for measuring the dielectric loss tangent of a power cable in a live line state.

The present invention is a method for measuring a dielectric loss tangent of a live cable in which a power flow of a commercial frequency is generated,
Obtaining a leakage current signal derived from the current flowing through the ground line of the power cable (a);
A first delay voltage signal whose phase is variably changed with respect to a first reference voltage signal whose frequency is a predetermined multiple of the commercial frequency, and a phase delayed by a predetermined angle from the first delay voltage signal. Generating a two-delayed voltage signal (b);
The product of the leakage current signal and the first delay voltage signal and the product of the leakage current signal and the second delay voltage signal are respectively calculated, and the first delay voltage at the time when the calculation result establishes a predetermined relationship. Identifying a phase difference between a signal and the first reference voltage signal (c);
And (d) calculating the dielectric loss tangent based on the phase difference specified in the step (c).

  As an example, step (a) can employ a method of measuring leakage current by attaching a current probe to the ground line of the power cable. The second delay signal may be 90 degrees out of phase with the first delay voltage signal.

  According to this method, the voltage phase of the power cable is specified based on the leakage current of the power cable. That is, when measuring the dielectric loss tangent, there is no need to acquire a voltage signal from the live power cable. For this reason, for example, a measuring instrument incorporating an oscillator that generates a voltage of several volts to several tens of volts, a signal delaying process, and a calculation unit that performs various calculations, and a leakage current of a power cable is measured. The dielectric loss tangent of the target power cable can be measured in a live state only by bringing the current acquisition unit to the site. Therefore, the number of steps involved in measuring the dielectric loss tangent can be greatly reduced. In addition, there is also an effect that it is not necessary to bring a large device for the measurement.

The step (c)
While adjusting the phase of the first reference voltage signal, the first operation value that is the product of the leakage current signal and the first reference voltage signal, and the product of the leakage current signal and the first delay voltage signal A second calculated value and a third calculated value that is the product of the first calculated value and the second calculated value are calculated, and the absolute value of the maximum value and the minimum value of the third calculated value are equal to each other. It may be a step of identifying the phase of the first reference voltage signal at the time.

The step (d)
A third delayed voltage signal having a phase difference specified in the step (c) with respect to a second reference voltage signal having a frequency that is the commercial frequency, and a phase delayed by a predetermined angle from the third delayed voltage signal Generating a fourth delayed voltage signal that is present (d1);
Calculating a product of the leakage current signal and the third delay voltage signal and a product of the leakage current signal and the fourth delay voltage signal, and calculating the dielectric loss tangent based on each calculation result (d2) It can have.

  Here, the fourth delay voltage signal may be 90 degrees behind the third delay voltage signal.

In addition, the step (d2)
A fourth calculated value that is a product of the leakage current signal and the second reference voltage signal, and a fifth calculated value that is a product of the leakage current signal and the second delayed voltage signal are calculated, respectively. It may be a step including calculating the dielectric loss tangent based on a ratio of a direct current component of four arithmetic values and a direct current component of the fifth arithmetic value.

  The first reference voltage signal may be a signal having a frequency twice the commercial frequency.

  According to the method of the present invention, it is possible to measure the dielectric loss tangent of a power cable in a live line state without using a large apparatus.

It is a block diagram which shows typically the power cable dielectric loss tangent measuring method of this invention. It is the block diagram which displayed functionally the structure of the dielectric loss tangent measuring apparatus. The waveforms of the current signal I ₁ , the first delay voltage signal V _1a , the first calculated value m ₁ , the second calculated value m ₂ , and the third calculated value m _{3 when} the phase φ is set at a certain angle are shown. It is a graph. Absolute value | m _{3 of} current signal I ₁ , first delay voltage signal V _1a , first calculated value m ₁ , second calculated value m ₂ , and third calculated value m ₃ at the time when phase φ is set to an angle. It is a graph which shows each waveform of |. It is a figure which shows typically the cross section of an electric power cable. 1 is a diagram showing a Schering bridge circuit which is a general method for measuring a dielectric loss tangent.

  FIG. 1 is a block diagram schematically showing a method for measuring a dielectric loss tangent of a power cable according to the present invention. In the method according to the present invention, it is possible to measure the dielectric loss tangent of the power cable 10 only by bringing the dielectric loss tangent measuring device 1 and the current measuring unit 2 to the site where the power cable 10 to be measured exists. is there.

  The current measuring unit 2 can be configured by a current probe, for example. The current measuring unit 2 is attached to a ground line 30 provided in the power cable 10. Thereby, in the live power cable 10, a current signal derived from the leakage current flowing through the ground wire 30 is measured by the current measuring unit 2. The current signal measured by the current measuring unit 2 is input to the dielectric loss tangent measuring apparatus 1.

  The dielectric loss tangent measuring apparatus 1 includes an oscillation unit 41, a filter unit 42, a calculation unit 43, and a display unit 44. The oscillating unit 41 generates an alternating voltage having a predetermined frequency. The filter unit 42 is composed of, for example, a band-pass filter, and removes a high frequency component from the current signal measured by the current measuring unit 2 and outputs it. The calculation unit 43 is a processing unit that performs predetermined calculation processing, which will be described later, based on a plurality of input electrical signals, and is configured by, for example, a microcomputer or software. Note that the circuit may be configured by hardware. The display unit 44 is a means for displaying the calculation result of the calculation unit 43. In the present embodiment, the value of the dielectric loss tangent of the power cable 10 is displayed on the display unit 44. Note that other information such as waveforms of various signals may be displayed on the display unit 44. Further, whether or not the dielectric loss tangent measuring apparatus 1 includes the filter unit 42 is arbitrary.

  FIG. 2 is a block diagram functionally showing the structure of the dielectric loss tangent measuring apparatus 1. Hereinafter, a method for measuring the dielectric loss tangent of the power cable 10 will be described with reference to FIG.

(Step S1)
The current measuring unit 2 is attached to the ground line 30 provided in the power cable 10 to acquire a current signal based on the leakage current. From this signal, the high frequency component is removed by the filter unit 42, the signal is substantially converted into a commercial frequency component signal, and is input to the arithmetic unit 43. This step S1 corresponds to the step (a). A current signal input to the calculation unit 43 is referred to as I ₁ .

(Step S2)
A voltage signal V ₁ (corresponding to “first reference voltage signal”) twice the commercial frequency is output from the oscillator 41 and input to the calculator 43.

(Step S3)
The calculating unit 43, the voltage signal V _1a of the phase-shifted with respect to the voltage signal V _1, to produce a V _1b. Here, the voltage signal V _1a is a signal generated by changing the phase of the voltage signal V ₁ as needed, and corresponds to the first delayed voltage signal. The voltage signal V _1b is a signal generated by delaying the phase by a predetermined angle with respect to the voltage signal V _1a , and corresponds to the second delayed voltage signal. Here, it is assumed that the voltage signal V _1b is a signal whose phase is delayed by 90 degrees with respect to the voltage signal V _1a .

  This step S3 corresponds to the step (b).

(Step S4)
The calculating unit 43, calculates a first calculation value m ₁ is the product of the voltage signal V _1a and the current signal I _1, the second calculation value m ₂ is the product of the voltage signal V _1b and the current signal I ₁ respectively. Further, the calculation unit 43 calculates a third calculation value m ₃ that is the product of the first calculation value m ₁ and the second calculation value m ₂ .

Then, the calculation unit 43 searches for and specifies the phase of the voltage signal V _1a so that the absolute values of the maximum value and the minimum value of the third calculation value m ₃ are equal to each other. Hereinafter, the phase identified in step S4 is denoted as φ. That is, the processes in steps S3 to S4 are repeatedly executed until the absolute values of the maximum value and the minimum value of the third calculated value m ₃ are equal to each other. Here, the case where the absolute values are equal includes the case where they are substantially equal. For example, if the difference is within ± 0.01%, it can be determined that they are substantially equal, and ± 0 More preferably, the difference is within 001%. Further, the value of the third calculated value m ₃ may be appropriately displayed on the display unit 44, and the phase of the voltage signal V _1a may be specified when this value becomes almost zero.

Incidentally, in the case of digitally calculation is performed in the arithmetic unit 43, compares the absolute value to each other of the third maximum value and the minimum value of the calculated value m ₃ compared with those predetermined factor numbers in the vicinity of the phase to be identified It is good also as what to do. In this case, since the value of the difference range determined to be substantially equal can be increased, the apparent difference value can be increased to simplify the comparison process.

  This step S4 corresponds to the step (c).

(Step S5)
A commercial frequency voltage signal V ₂ (corresponding to the “second reference voltage signal”) is output from the oscillator 41 and input to the calculator 43.

(Step S6)
The calculating unit 43, the voltage signal V _2a of the phase-shifted, to generate V _2b outputs the voltage signal V _2. Here, the voltage signal V _2a is a signal shifted from the voltage signal V ₂ by the phase φ specified in step S4, and corresponds to the third delayed voltage signal. The voltage signal V _2b is a signal generated by delaying the phase by a predetermined angle with respect to the voltage signal V _2a , and corresponds to the fourth delayed voltage signal. Here, it is assumed that the voltage signal V _2b is a signal whose phase is delayed by 90 degrees with respect to the voltage signal V _2a .

  This step S6 corresponds to the step (d1).

(Step S7)
The calculating unit 43, calculates a fourth operation value m _4, which is the product of the voltage signal V _2a and the current signal I _1, the fifth operation value m ₅ is the product of the voltage signal V _2b and the current signal I ₁ respectively. The arithmetic unit 43 extracts the respective calculated value m _4, m ₅ of the offset component (DC component), calculating the ratio therebetween. Based on this ratio, the dielectric loss tangent of the power cable 10 is calculated.

In this step S7, an arbitrary method can be adopted as a method for extracting the offset component (DC component) of each of the calculated values m ₄ and m ₅ . As an example, by calculating the time average of the calculated value m _4, m _5, it is possible to extract the offset component of each calculated value m _4, m _5.

  This step S7 corresponds to the step (d2). Steps S6 to S7 correspond to step (d).

  Hereinafter, the fact that the dielectric loss tangent of the power cable 10 can be calculated by the above method will be described.

The current signal I ₁ based on the leakage current flowing through the power cable 10 is expressed by the following equation (1), where δ is the dielectric loss angle of the power cable 10. A voltage signal (first reference voltage signal) V ₁ output from the oscillating unit 41 that is twice the commercial frequency is expressed by the following equation (2).

The voltage signal V _1a (first delay voltage signal) is generated by changing the phase of the voltage signal V ₁ in the calculation unit 43. When the phase difference between the voltage signal V ₁ and the voltage signal V _1a is φ, the voltage signal V _1a is defined by the following equation (3). The voltage signal V _1b (first delay voltage signal) is a signal delayed in phase by 90 degrees with respect to the first delay voltage signal V _1a , and is defined by the following equation (4).

As described above, in the calculation unit 43, the first calculation value m ₁ that is the product of the voltage signal V _1a and the current signal I ₁ and the second calculation value m ₂ that is the product of the voltage signal V _1b and the current signal I ₁ are obtained. Each is calculated. From the above formulas (1), (3), and (4), the calculated values m ₁ and m ₂ are expressed by the following formulas.

As described above, the calculation unit 43 calculates the third calculation value m ₃ that is the product of the first calculation value m ₁ and the second calculation value m ₂ . From the above formulas (5) and (6), the calculated value m ₃ is expressed by the following formula.

The calculation unit 43 sequentially performs the above calculation while changing φ, and searches for the value of the phase difference φ such that the absolute values of the minimum value and the maximum value of the third calculation value m ₃ are equal.

FIG. 3 shows current signal I ₁ , first delay voltage signal V _1a , first calculated value m ₁ , second calculated value m ₂ , and third calculated value m ₃ when the phase φ is set at a certain angle. These waveforms are displayed on the same graph. Note that the value on the vertical axis of the graph is changed as appropriate in order to make the change in each value easier to see.

FIG. 4 shows the absolute values of the current signal I ₁ , the first delay voltage signal V _1a , the first calculated value m ₁ , the second calculated value m ₂ , and the third calculated value m ₃ when the phase φ is set at a certain angle. Each waveform of the value | m ₃ | is displayed on the same graph. Incidentally, at the time of FIG. 4, the absolute value of the minimum value of the third operation value m _3, the maximum value of the third operation value m ₃ matches. The calculation unit 43 performs processing for specifying the phase φ that realizes such a relationship.

Such a calculation method for specifying the phase φ can be realized by an arbitrary method. As an example, the phase φ is specified when the difference between the absolute value | m _3min | of the third operation value and the maximum value | m _3max | of the third operation value is within a predetermined threshold. Can do. Mathematical analysis of equation (7) shows that | m _3min | = | m _3max | holds when φ = 2δ is satisfied. That is, the phase of the power supply voltage V can be specified by executing step S4 described above.

Next, in step S < _b > 6, the calculation unit 43 generates a voltage signal V _2a (third delay voltage signal) having a commercial frequency and synchronized with the power supply voltage V generated in the power cable 10. Further, the voltage signal V _2b (fourth delay voltage signal) generated in the arithmetic unit 43 is a signal whose phase is delayed by 90 degrees with respect to the third delay voltage signal V _2a . As described above, in the arithmetic unit 43, the fourth operation value m _4, which is the product of the voltage signal V _2a and the current signal I _1, the fifth operation value m ₅ is the product of the voltage signal V _2b and the current signal I ₁ Each is calculated. The calculated values m ₄ and m ₅ are expressed by the following equations. In the following formula, K is a constant term.

From the above formulas (8) and (9), the calculation unit 43 calculates the ratio of the offset (DC component) of the calculation values m ₄ and m ₅ to calculate tan δ.

[Another embodiment]
Hereinafter, another embodiment will be described.

  <1> In step S4, the dielectric loss angle δ of the power cable 10 is specified. Therefore, tan δ may be calculated based on the specified value of δ without performing Step S5 and subsequent steps. In addition, when calculating the dielectric loss angle δ of the power cable 10 through steps S5 to S7, the calculation unit 43 can be configured by an analog electronic circuit including an amplifier, a phase shifter, a multiplier, and a divider. is there.

<2> In the embodiment described above, the first reference voltage signal V ₁ is a signal having a frequency twice the commercial frequency, but may be a signal having an integral multiple such as three times.

  <3> In the embodiment described above, the configuration for calculating the dielectric loss tangent of the power cable 10 in the apparatus 1 has been described. However, the value of the dielectric loss angle (δ) may be calculated instead of the tangent.

<4> In step S4, it is possible to approximately calculate the phase φ such that the absolute values of the maximum value and the minimum value of the third calculated value m ₃ are equal to each other. As an example, a difference between the absolute value of the maximum value and the minimum value of the third calculation value m ₃ is calculated while discretely changing the value of φ, and the difference value is 0 by linearly approximating these difference values. Can be specified. The absolute value of the maximum value of the third operation value m _3, the difference between the absolute value of the minimum value of the third operation value m _3, since a linear relationship and the phase phi, the phi identified by the above approximation approaches The phase of the voltage signal V _1a can also be specified by the value.

1: Dissipation factor system
2: Current measurement unit 10 (10A, 10B, 10C): Power cable 11: Conductor 13: Insulator 15 (15A, 15B, 15C): Shield layer 17: Cable sheath 21: Void 23: Foreign object 25: Water droplet 30: Ground Line 41: Oscillating unit 42: Filter unit 43: Calculation unit 44: Display unit

Claims (7)

A method for measuring the dielectric loss tangent of a live power cable in which a power flow of a commercial frequency is generated,
Obtaining a leakage current signal derived from the current flowing through the ground line of the power cable (a);
A first delay voltage signal whose phase is variably changed with respect to a first reference voltage signal whose frequency is a predetermined multiple of the commercial frequency, and a phase delayed by a predetermined angle from the first delay voltage signal. Generating a two-delayed voltage signal (b);
The product of the leakage current signal and the first delay voltage signal and the product of the leakage current signal and the second delay voltage signal are respectively calculated, and the first delay voltage at the time when the calculation result establishes a predetermined relationship. Identifying a phase difference between a signal and the first reference voltage signal (c);
And (d) calculating the dielectric loss tangent based on the phase difference specified in the step (c). The step (c)
While adjusting the phase of the first delayed voltage signal, a first calculation value which is the product of the said leakage current signal and the first delayed voltage signal, the product of the said leakage current signal and the second delayed voltage signal A second calculated value and a third calculated value that is the product of the first calculated value and the second calculated value are calculated, and the absolute value of the maximum value and the minimum value of the third calculated value are equal to each other. The method of claim 1, comprising identifying a phase of the first reference voltage signal at a point in time.   The method according to claim 1, wherein the second delay voltage signal is 90 degrees out of phase with the first delay voltage signal. The step (d)
A third delayed voltage signal having a phase difference specified in the step (c) with respect to a second reference voltage signal having a frequency that is the commercial frequency, and a phase delayed by a predetermined angle from the third delayed voltage signal Generating a fourth delayed voltage signal that is present (d1);
Calculating a product of the leakage current signal and the third delay voltage signal and a product of the leakage current signal and the fourth delay voltage signal, and calculating the dielectric loss tangent based on each calculation result (d2) The method according to any one of claims 1 to 3, characterized by comprising: The step (d2)
A fourth operation value that is a product of the leakage current signal and the third delay voltage signal and a fifth operation value that is a product of the leakage current signal and the fourth delay voltage signal are respectively calculated, 5. The method according to claim 4, further comprising calculating the dielectric loss tangent based on a ratio of a direct current component of four arithmetic values and a direct current component of the fifth arithmetic value.   6. The method according to claim 4, wherein the fourth delay voltage signal is 90 degrees out of phase with the third delay voltage signal. The method according to claim 1, wherein the first reference voltage signal is a signal having a frequency twice as high as the commercial frequency.

Images