JP2004354093A - Deterioration diagnostic method of power cable - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a deterioration diagnostic method of power cable, capable of performing a precise deterioration diagnosis of a power cable by deriving and removing a third harmonic component in a loss current which appears by the influence of a third harmonic voltage included in an AC voltage applied to the power cable. <P>SOLUTION: This deterioration diagnostic method of the power cable comprises applying the AC voltage to the power cable to extract the loss current from the current carried in an insulator, and performing the deterioration of diagnosis of the power cable by use of the third harmonic component included in the loss current. In this method, paying attention, as the AC voltage to be applied to the power cable, to a first voltage V1 having a main frequency f and a superposed voltage V3 in which a second voltage V2 having a frequency 3f three times the frequency f is superposed on the first voltage V1, a third harmonic component I<SB>3n</SB>included in the loss current, which appears by the influence of the third harmonic voltage V<SB>3n</SB>included in the first voltage V1, and a third harmonic component I<SB>3m0</SB>included in the loss current is corrected, whereby the deterioration diagnosis of the power cable is performed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for diagnosing deterioration of a power cable.
[0002]
[Prior art]
At present, the main form of deterioration of cross-linked polyethylene insulated cables (hereinafter referred to as cables), which is the most widespread power cable, is water tree deterioration. The water tree is a deteriorated portion in the insulator which is generated by the action of moisture and electric field existing in the cable insulator, and increases with the passage of time, thereby deteriorating the insulation performance of the cable. Various degradation diagnosis techniques have been studied with the aim of efficiently diagnosing the state of water tree degradation.
[0003]
As one of the prior arts, an AC voltage (test voltage) is applied to a cable, a loss current having the same phase as the AC voltage is extracted from a current flowing through an insulator, and a harmonic component included in the loss current is extracted. There is a method of diagnosing cable deterioration by using the method. Harmonic components in the loss current used as a degradation signal in this method appear due to the effect of the nonlinear electrical conduction characteristics of the water tree. Can be diagnosed. More specifically, an AC voltage is applied to the cable and a lossless standard capacitor connected in parallel with the cable, and the current flowing through the standard capacitor is used to apply a current flowing through the cable to the cable, leading the applied voltage by 90 ° with respect to the applied voltage. The loss current is extracted by removing the component (capacitive current), and the amplitude (magnitude) and the superimposed phase (order with respect to the fundamental wave component) of the harmonic component, which determine the generation state of the third harmonic component therein, are determined. The degree of deterioration of the cable is evaluated by comparing the value of (phase difference) with data (for example, see Patent Documents 1 and 2).
[0004]
[Patent Document 1]
JP-A-7-151815 (paragraph 0003, paragraph 0030 to paragraph 0035 and FIG. 2 of the detailed description of the invention)
[Patent Document 2]
JP 2002-196030 A (paragraph 0003, paragraph 0005 and FIG. 2 of the detailed description of the invention)
[0005]
[Problems to be solved by the invention]
In the conventional degradation diagnosis technology, if noise having the same frequency as the third harmonic component in the loss current used as the degradation signal is present, the noise signal cannot be distinguished from a true degradation signal reflecting the state of water tree degradation. , It is not possible to perform highly accurate deterioration diagnosis.
[0006]
The main source of this noise is the third harmonic voltage contained in the AC voltage applied to the cable. If a voltage that does not include the third harmonic voltage at all can be used as the AC voltage, the third harmonic component contained in the loss current can be directly evaluated as a water tree deterioration signal, and accurate deterioration diagnosis can be performed. It is possible to do.
[0007]
However, the AC voltage usually contains a harmonic voltage having a frequency that is an integral multiple of the fundamental component in addition to the fundamental voltage, regardless of the level difference, and is called an AC voltage that does not include any harmonic voltage. It is practically impossible to make things happen.
[0008]
The device for generating an AC voltage is roughly divided into a transformer, a variable reactor, and a power supply for supplying power thereto. As a power source used at the diagnostic site, if there is a switchboard equipment permanently installed at the site, bring commercial power from there, and if there is no such equipment, bring a mobile power generator to the site, A power supply from the power supply will be used, but such a power supply originally contains a harmonic component. Both transformers and variable reactors are power devices that use the magnetic properties of the internal core, and the magnetic properties of the core generally have nonlinear characteristics. Harmonic voltage is generated.
[0009]
Conventional degradation diagnosis technology uses filters that reduce harmonic components in the power supply to reduce the effect of harmonic voltages in the AC voltage on measurement results, or as cores used in transformers and variable reactors. Attention has been paid to maximally suppress harmonic voltages generated in an AC voltage by using a magnetic material having a margin in which a non-linear characteristic hardly appears. However, such measures cannot reduce the harmonic voltage in the AC voltage, but do not completely eliminate the harmonic voltage. The harmonic component in the AC voltage changes according to the load connected to the transformer output, that is, the capacitance of the cable to be diagnosed, and the diagnostic accuracy varies depending on the capacitance of the cable. This can lead to undesirable situations.
[0010]
The present invention has been made to solve the above problems, and an object of the present invention is to provide a third harmonic included in a loss current appearing due to an influence of a third harmonic voltage originally included in an AC voltage applied to a power cable. By accurately evaluating the wave component (noise that is not a degraded signal) and correcting the influence, the true third harmonic component (degraded signal) emitted from the water tree is extracted, and high-precision power An object of the present invention is to provide a power cable deterioration diagnosis method capable of performing cable deterioration diagnosis.
[0011]
[Means for Solving the Problems]
According to the present invention, an AC voltage is applied to a power cable, a loss current having the same phase as the AC voltage is extracted from a current flowing through an insulator, and a third harmonic component included in the loss current is used for the power cable. In the method for diagnosing deterioration of a power cable, the first voltage V1 having a main frequency f is applied as an AC voltage applied to the power cable, and the first voltage V1 has a frequency three times the frequency f. Paying attention to a superimposed voltage V3 in which a second voltage V2 having 3f is superimposed, the third harmonic voltage V included in the first voltage V1_3nHarmonic component I contained in the loss current appearing due to the influence of_3nFrom the third harmonic component I contained in the loss current extracted when the first voltage V1 is applied._3m0This is a method for performing a power cable deterioration diagnosis by correcting.
[0012]
Then, the third harmonic voltage V included in the first voltage V1_3nHarmonic component I contained in the loss current appearing due to the influence of_3nAs a specific means for deriving the third harmonic component I included in the loss current extracted when the first voltage V1 is applied._3m0And the third harmonic voltage V included in the first voltage V1_3nAnd the third harmonic component I included in the loss current extracted when the superimposed voltage V3 is applied._3m1And the third harmonic voltage V included in the superimposed voltage V3_3m1And the third harmonic voltage V_3m1And the third harmonic voltage V_3nAs a result, the third harmonic voltage V which is the second voltage V2 superimposed on the first voltage V1_3s1And the third harmonic component I_3m1And the third harmonic component I_3m0The third harmonic voltage V_3s1Harmonic component I contained in the loss current appearing due to the influence of_3s1And the third harmonic voltage V_3s1And the third harmonic component I_3s1The third harmonic voltage V included in the first voltage V1_3nHarmonic component I contained in the loss current appearing due to the influence of_3nIs derived.
[0013]
As a result, the third harmonic voltage V included in the first voltage V1 (AC voltage) applied to the power cable_3nThird harmonic component I in the loss current generated by the_3n(Noise that is not a degraded signal) can be quantitatively evaluated. Therefore, the third harmonic component I included in the loss current extracted when the first voltage V1 is applied._3m0To correct the third harmonic component I_3nHarmonic component I, which is a true degradation signal generated from a water tree generated in the insulation of the power cable by removing the influence of the_3rCan be extracted, and a highly accurate power cable deterioration diagnosis can be performed.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing an example of a measuring circuit for implementing the present invention. First, as a first step, a signal having a main frequency f, for example, a commercial frequency of 50 Hz is generated by the waveform generator 3 and the signal is input to the primary side of the transformer 7 via the power amplifier 5 and the reactor 6. As a result, the transformer 7 outputs a first voltage V1 having a main frequency of 50 Hz as an AC voltage (test voltage).
[0015]
Next, the first voltage V1 is applied to the power cable 1 to be diagnosed and a lossless standard capacitor 9 connected in parallel to the power cable 1 to be diagnosed, and the current transformer 11 inserted between the power cable 1 and the transformer 7 is applied. The current flowing through the insulator of the power cable 1 and the current flowing through the standard capacitor 9 are input to the loss current measurement bridge 13, and the current flowing through the insulator of the power cable 1 is applied to the loss current measurement bridge 13. A component (capacitive current) having a phase leading by 90 ° with respect to the first voltage V1 is removed, and a loss current having the same phase as the first voltage V1 is extracted.
[0016]
Next, the waveform of the loss current output from the loss current measurement bridge 13 was fetched as discrete numerical data by the digital oscilloscope 15 and subjected to FFT analysis by the waveform analysis computer 17 to apply the first voltage V1. Third harmonic component I included in the loss current that is sometimes extracted_3m0, The amplitude (magnitude) and the superimposed phase (the phase difference with respect to the fundamental wave component) are measured. At the same time, the oscilloscope 15 captures the voltage waveform as discrete numerical data, and the waveform analysis computer 17 uses the third harmonic voltage V1 included in the first voltage V1._3nThe amplitude and the superimposed phase are measured. Third harmonic voltage V_3nFIG. 2 (a) shows a vector diagram of the third harmonic component I._3m0Is shown in FIG. 2 (b). Third harmonic component I extracted here_3m02B is a third harmonic component I which is a true degradation signal generated from a water tree generated in the insulator of the power cable 1 as is clear from FIG._3rIn addition to the third harmonic voltage V included in the first voltage V1._3nHarmonic component I contained in the loss current appearing due to the influence of_3nIs also included.
[0017]
Next, as a second step, the waveform generator 3 generates a signal obtained by adding a signal having a frequency 3f, that is, a signal having a frequency of 150 Hz to a signal having a frequency f, that is, a commercial frequency of 50 Hz. Outputs a superimposed voltage V3 in which the second voltage V2 having a frequency of 150 Hz is intentionally superimposed on the first voltage V1, and outputs the superimposed voltage V3 in parallel with the power cable 1 and a lossless connection. The third harmonic component I contained in the loss current applied to the standard capacitor 9 and extracted in the same manner as described above_3m1, The superimposed phase, and the third harmonic voltage V included in the superimposed voltage V3_3m1Are measured by the waveform analysis computer 17.
[0018]
Third harmonic voltage V_3m1FIG. 3 (a) shows a vector diagram of the third harmonic component I._3m1Is shown in FIG. 3 (b). Third harmonic component I extracted here_3m13B is a third harmonic component I which is a true deterioration signal generated from a water tree generated in the insulator of the power cable 1 as is clear from FIG._3r, The third harmonic voltage V included in the first voltage V1_3nHarmonic component I contained in the loss current appearing due to the influence of_3nAnd a third harmonic voltage V, which is a second voltage V2 superimposed on the first voltage V1._3s1Harmonic component I contained in the loss current appearing due to the influence of_3s1Is also included.
[0019]
Third harmonic voltage V included in first voltage V1 obtained in the first step_3nAnd the third harmonic voltage V included in the superimposed voltage V3 obtained in the second step._3m1Is used, the third harmonic voltage V2, which is the second voltage V2 intentionally superimposed on the first voltage V1_3s1Can be specified. More specifically, the third harmonic voltage V_3m1From the third harmonic voltage V_3nIs added to the third harmonic voltage V2, which is the second voltage V2 intentionally superimposed, by taking the amplitude and the superimposed phase into account and subtracting the vector by the waveform analysis computer 17._3s1Can be derived (see FIG. 3A).
[0020]
On the other hand, the third harmonic component I included in the loss current extracted when the first voltage V1 obtained in the first step is applied._3m0And the third harmonic component I included in the loss current extracted when the superimposed voltage V3 obtained in the second step is applied._3m1Is used, the third harmonic voltage V2 which is the second voltage V2 intentionally superimposed on the first voltage V1_3s1Harmonic component I contained in the loss current appearing due to the influence of_3s1Can be specified. Also in this case, the third harmonic component I_3m1From the third harmonic component I_3m0Is added to the third harmonic voltage V2, which is the second voltage V2 intentionally superimposed, by taking the amplitude and the superimposed phase into account and subtracting the vector by the waveform analysis computer 17._3s1Harmonic component I contained in the loss current appearing due to the influence of_3s1Can be derived (see FIG. 3B).
[0021]
Thus, the third harmonic voltage V, which is the second voltage V2 intentionally superimposed on the first voltage V1,_3s1And the third harmonic voltage V_3s1Harmonic component I contained in the loss current appearing due to the influence of_3s1And the third harmonic voltage V_3s1And the third harmonic component I_3s1The third harmonic voltage V originally contained in the first voltage V1 is determined by examining and evaluating the correspondence between_3nHarmonic component I contained in the loss current appearing due to the influence of_3nCan be derived by analysis by the waveform analysis computer 17.
[0022]
Then, as is clear from FIG. 2B, the third harmonic component I included in the loss current extracted when the first voltage V1 is applied._3m0(Synthesis of the degradation signal due to the water tree and the noise due to the power supply) from the third harmonic voltage V included in the first voltage V1_3nHarmonic component I contained in the loss current appearing due to the influence of_3nIs added to the amplitude and the superimposed phase, and the vector is subtracted by the waveform analysis computer 17 to obtain the third harmonic component I included in the loss current extracted when the first voltage V1 is applied._3m0To correct the third harmonic component I_3nThe third harmonic component I, which is a true deterioration signal emitted from the water tree generated in the insulator of the power cable 1 by removing the influence of the_3rCan be reliably extracted, and highly accurate power cable deterioration diagnosis can be performed.
[0023]
Further, a second voltage V2 (third harmonic voltage V3) superimposed on the first voltage V1._3s1) Are two or more, so that for each of the superimposed phases, the superimposed second voltage V2 (third harmonic voltage V_3s1) And the third harmonic component I included in the loss current that appears due to this effect._3s1It can be evaluated by examining the corresponding relationship with. By evaluating these plural correspondences, the third harmonic voltage V included in the first voltage V1 is obtained._3nHarmonic component I contained in the loss current appearing due to the influence of_3nIs reduced, and the third harmonic component I included in the loss current extracted when the first voltage V1 is applied is reduced._3m0Can be improved.
[0024]
In the above embodiment, when an AC voltage is applied to the power cable 1, the first voltage V1 is first applied to the third harmonic component I included in the loss current._3m0And then the third harmonic voltage V2, which is the second voltage V2, to the first voltage V1._3s1Is applied, and the third harmonic component I included in the loss current is applied._3m1The measurement when the first voltage V1 is applied and the measurement when the superimposed voltage V3 is applied are independent measurement actions, and one measurement action affects the other measurement action. There is no. Therefore, the order of applying the AC voltage may be reversed, and the measurement may be performed by applying the superimposed voltage V3 first and then applying the first voltage V1.
[0025]
【Example】
The power cable 1 to be diagnosed has a voltage class of 22 kV and a conductor size of 100 mm.²A water tree deteriorated cable having an insulator thickness of 6 mm and a length of 50 m was used. Also, the third harmonic voltage V included in the first voltage V1 of the AC voltage to be applied_3nIn order to confirm the effect by changing the situation of (1), a capacitor 19 of 900 nF is connected to the transformer 7 in parallel with the standard capacitor 9 between the transformer 7 and the standard capacitor 9 as a load of the transformer 7 (see FIG. 1). And, if not, the third harmonic voltage V included in the first voltage V1._3nWere realized, and the loss current of the power cable 1 was measured. The basic frequency f of the AC voltage was set to 50 Hz.
[0026]
The measurement circuit shown in FIG. 1 can generate and control an AC voltage (test voltage) by inputting the signal of the waveform generator 3 to the primary side of the transformer 7 via the power amplifier 5 and the reactor 6. It is configured to be able to. When the waveform generator 3 generates a signal of 50 Hz corresponding to the frequency f, the third harmonic voltage V of only 150 Hz is included in the voltage of 50 Hz as the main component._3nCan be obtained, and when a signal is generated by adding 150 Hz which is a frequency 3f that is three times the frequency of 50 Hz to 50 Hz, a second voltage of 150 Hz is added to the first voltage V1. A superimposed voltage V3 obtained by superimposing V2 can be obtained. Since the waveform generator 3 can arbitrarily set the superposition phase of the 50 Hz signal and the 150 Hz signal, the superposition phase of the first voltage V1 and the second voltage V2 can be arbitrarily set as the AC voltage. . The test conditions of this embodiment are as follows: an effective value of 18 kV as a 50 Hz component of the first voltage V1, an effective value of about 200 V (150 Hz) as the second voltage V2, and a superposition phase of the first voltage V1 and the second voltage V2. , Six values were set.
[0027]
In addition, when the 900 nF capacitor 19 is not connected as a load of the transformer 7 (condition (1)) and when it is connected (condition (2)), the capacitor 19 included in the AC voltage (first voltage V1) is used. 3 harmonic voltage V_3nAnd the third harmonic component I contained in the loss current extracted when the first voltage V1 in the prior art is applied._3m0(No noise correction) and the third harmonic voltage V included in the first voltage V1 in the present invention._3nHarmonic component I contained in the loss current appearing due to the influence of_3nThird harmonic component (deteriorated signal) I that has been corrected for and removed the effect of (noise)_3r4 (with noise correction) is shown in FIG. The third harmonic component I shown here_3m0, I_3rThe value of the amplitude and the superimposed phase of the fundamental wave of the AC voltage (first voltage V1) is expressed by V = V_nsin ｛n (ωt + θ_vn)｝, N = 1, 2, 3,..., Where θ_v1= 0, the loss current is expressed as I = I_nsin ｛n (ωt + θ_n), N = 1, 2, 3,... Are the amplitudes and superimposed phases for n = 3 obtained as a result of defining.
[0028]
FIG. 5A shows the third harmonic voltage contained in the AC voltage in the measurement under the condition (1), and FIG. 5B shows the third harmonic voltage contained in the loss current extracted when the AC voltage is applied. FIG. 6A is a vector diagram showing a harmonic component. FIG. 6A shows the third harmonic voltage included in the AC voltage in the measurement under the condition (2), and FIG. 6B shows the third harmonic voltage when the AC voltage is applied. FIG. 7 is a vector diagram showing a third harmonic component included in a loss current to be performed.
[0029]
A third harmonic voltage V included in a superimposed voltage V3 obtained by superimposing a second voltage V2 having a frequency 3f (150 Hz) three times the frequency f (50 Hz) on the first voltage V1._3m1Or V_{3m 6}(A circle in FIGS. 5A and 6A) indicates the third harmonic voltage V included in the first voltage V1 before the second voltage V2 is superimposed._3n(Circles in FIGS. 5A and 6A) are obtained by drawing a circle on a plane. This corresponds to a second voltage V2 (third harmonic voltage V3) superimposed on the first voltage V1._3s1Or V_{3s 6}This is because the amplitude (magnitude) is fixed and only the superposition phase is changed. Note that V_3s1Or V_{3s 6}Is the third harmonic voltage V_3m1Or V_{3m 6}From the third harmonic voltage V_3nOf the second voltage V2 (third harmonic voltage V_3s1Or V_{3s 6}).
[0030]
On the other hand, the third harmonic component I contained in the loss current extracted when the superimposed voltage V3 is applied is also included in the third harmonic component I contained in the loss current._3m1Or I_{3m 6}(Indicated by Δ in FIGS. 5B and 6B) are included in the loss current extracted when the first voltage V1 is applied before the second voltage V2 is superimposed. Third harmonic component I_3m0(FIG. 5 (b) and FIG. 6 (b)) are obtained by drawing a circle on a plane. Note that I_3s1Or I_{3s 6}Is the third harmonic component I_3m1Or I_{3m 6}From the third harmonic component I_3m0Is the third harmonic voltage V derived by vector subtraction_3s1Or V_{3s 6}Harmonic component I contained in the loss current appearing due to the influence of_3s1Or I_{3s 6}Is a vector.
[0031]
As is clear from those shown in FIGS. 5A and 5B and FIGS. 6A and 6B, the second voltage V2 (the third harmonic voltage V_3s1Or V_{3s 6}) And the third harmonic component I included in the loss current that appears due to this effect._3s1Or I_{3s 6}Are independently in a one-to-one correspondence. Therefore, by evaluating the correspondence between them, the third harmonic voltage V originally contained in the first voltage V1 is obtained._3nHarmonic component I contained in the loss current appearing due to the influence of_3n(Noise) can be derived, and using the result, the third harmonic component included in the loss current extracted when the first voltage V1 is applied I_3m0Is corrected to eliminate the influence of noise, so that the third harmonic component I, which is a true degraded signal emitted from the water tree, is corrected._3r(X marks in FIGS. 5B and 6B) can be obtained.
[0032]
Condition (1) is that the first harmonic voltage V1 has an amplitude of 34.0 V and a third harmonic voltage V of 0.13% of the basic voltage (50 Hz)._3nThe condition (2) is that the third harmonic voltage V has an amplitude of 142.4 V in the first voltage V1 and is 0.56% of the basic voltage (50 Hz)._3nIs included. Third harmonic voltage V included in first voltage V1_3nIs the third harmonic component I contained in the loss current_3m0If the condition (2) is affected, the third harmonic component I in the loss current is larger in the condition (2) than in the condition (1)._3m0Of the third harmonic voltage V included in the first voltage V1_3nA large error due to the influence of appears.
[0033]
As shown in FIG. 4, under the condition (1), the third harmonic component I included in the loss current in the prior art (without noise correction) and the present invention (with noise correction)_3m0, I_3rAlthough the difference between the amplitude and the superimposed phase is small, it can be confirmed under the condition (2) that the difference between the conventional technology (without noise correction) and the present invention (with noise correction) is large. In the technology for diagnosing deterioration of the power cable 1 using the third harmonic component in the loss current, it is determined that the larger the amplitude and the smaller the absolute value of the superimposed phase, the more the cable is deteriorated. From the viewpoint of the result of the present embodiment, from the viewpoint, although the same power cable 1 is measured under the condition (1) and the condition (2), in the prior art, the condition between the conditions (1) and (2) is not satisfied. A large difference between the amplitude and the value of the superimposed phase is obtained. In the condition (2), the amplitude is small and the absolute value of the superimposed phase is included in the AC voltage (first voltage V1). Third harmonic voltage V_3nHas an effect, and there is a risk of underestimating the deterioration. On the other hand, in the present invention, the values of the amplitude and the superimposed phase are almost the same between the conditions (1) and (2), and it can be seen that the noise correction is correctly performed and only the true deteriorated signal is derived. .
[0034]
【The invention's effect】
As described above, according to the power cable deterioration diagnosis method of the present invention, as the AC voltage applied to the power cable, the first voltage V1 having the main frequency f, and the first voltage V1 having the frequency f Paying attention to a superimposed voltage V3 obtained by superimposing a second voltage V2 having a frequency 3f that is three times as large as the third harmonic voltage V3 included in the first voltage V1._3nHarmonic component I contained in the loss current appearing due to the influence of_3nFrom the third harmonic component I contained in the loss current extracted when the first voltage V1 is applied._3m0To correct the power cable for deterioration diagnosis, the third harmonic voltage V included in the first voltage V1 applied to the power cable is corrected._3nThird harmonic component I in the loss current generated by the_3n(Noise that is not a degraded signal) can be quantitatively evaluated. Therefore, the third harmonic component I included in the loss current extracted when the first voltage V1 is applied._3m0To correct the third harmonic component I_3nHarmonic component I, which is a true degradation signal generated from a water tree generated in the insulation of the power cable by removing the influence of the_3rCan be extracted, and a highly accurate power cable deterioration diagnosis can be performed.
[0035]
In addition, a second voltage (third harmonic voltage V) superimposed on the first voltage V1 to carry out the present invention._3s1Or V_{3s 6}) May have a small capacity of about 1% of the first voltage V1 and does not require a capacity change of a device such as a transformer constituting a device for generating an AC voltage (test voltage). The equipment used conventionally can be diverted as it is. Further, the method according to the present invention can correct the influence of the harmonic voltage generated by the test apparatus regardless of the level of the harmonic voltage. It is not necessary to take measures such as adding an iron core or using an iron core having a sufficient magnetic characteristic as a transformer or the like, and the cost of the test apparatus can be reduced.
[0036]
Further, a second voltage V2 (third harmonic voltage V3) superimposed on the first voltage V1._3s1Or V_{3s 6}) Are two or more, so that the second voltage V2 (the third harmonic voltage V_3s1Or V_{3s 6}) And the third harmonic component I included in the loss current that appears due to this effect._3s1Or I_{3s 6}And the third harmonic voltage V included in the first voltage V1._3nHarmonic component I contained in the loss current appearing due to the influence of_3nIs reduced, and the third harmonic component I included in the loss current extracted when the first voltage V1 is applied is reduced._3m0This is preferable because the accuracy of correcting is improved.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating an example of a measurement circuit for implementing the present invention.
FIG. 2A shows a third harmonic voltage V included in a first voltage V1 applied to a power cable._3n(B) shows the third harmonic component I contained in the loss current extracted when the first voltage V1 is applied._3m0FIG.
FIG. 3A shows a third harmonic voltage V included in a superimposed voltage V3 applied to a power cable._3m1(B) shows the third harmonic component I included in the loss current extracted when the superimposed voltage V3 is applied._3m1FIG.
FIG. 4 shows a third harmonic voltage V included in an AC voltage (first voltage V1) under conditions (1) and (2)._3nOf the third harmonic component I contained in the loss current in the prior art._3m0And the third harmonic component (deteriorated signal) I contained in the loss current in the present invention._3rIt is a figure showing the derivation result of.
5A is a diagram illustrating a third harmonic voltage included in an AC voltage in the measurement under the condition (1), and FIG. 5B is a diagram illustrating a third harmonic voltage included in a loss current extracted when the AC voltage is applied. FIG. 4 is a vector diagram showing three harmonic components.
FIG. 6 (a) shows the third harmonic voltage included in the AC voltage in the measurement under the condition (2), and FIG. 6 (b) shows the third harmonic voltage included in the loss current extracted when the AC voltage is applied. FIG. 4 is a vector diagram showing three harmonic components.
[Explanation of symbols]
1 Power cable
3 Waveform generator
5 Power amplifier
6 reactor
7 Transformer
9 Standard capacitors
11 Current transformer
13 Loss current measurement bridge
15 Digital oscilloscope
17 Waveform analysis computer
19 Capacitor

Claims (3)

An AC voltage is applied to the power cable, a loss current having the same phase as the AC voltage is extracted from a current flowing through the insulator, and a deterioration diagnosis of the power cable is performed using a third harmonic component included in the loss current. In the method of diagnosing deterioration of a power cable to be performed, as an AC voltage applied to the power cable, a first voltage V1 having a main frequency f and a first voltage V1 having a frequency 3f which is three times the frequency f are used as the first voltage V1. Focusing on the superimposed voltage V3 on which the second voltage V2 is superimposed, the third harmonic component I _3n included in the loss current caused by the influence of the third harmonic voltage V _3n included in the first voltage V1 is _calculated . derived, thereby characterized in that the deterioration diagnosis of the power cable by correcting the third harmonic component I _3M0 contained in loss current is extracted when applying the first voltage V1 power Deterioration diagnosis method of Buru. A third harmonic component I _3m0 included in the loss current extracted when the first voltage V1 is applied, a third harmonic voltage V _3n included in the first voltage V1, and the superimposed voltage V3. _{Is measured when} the third harmonic component I _3m1 included in the loss current extracted when the voltage is applied and the third harmonic voltage V _3m1 included in the superimposed voltage V3, and the third harmonic voltage V _3m1 and the by third harmonic voltage _{V 3n,} we derive a third harmonic voltage _{V 3S1} is a second voltage superimposed V2 to the first voltage V1, the third harmonic component _{I 3m1} and third harmonic The component I _{3m0 derives} a third harmonic component I _3s1 included in the loss current appearing _under the influence of the third harmonic voltage V _3s1, and obtains the third harmonic voltage V _3s1 and the third harmonic component I _3s1. Check the correspondence with 2. The deterioration of the power cable according to claim 1, wherein a third harmonic component _I3n included in a loss current appearing under the influence of a third harmonic voltage _V3n included in the first voltage V1 is derived. Diagnostic method. 3. The method for diagnosing deterioration of a power cable according to claim 1, wherein the superimposed phases of the second voltage V2 superimposed on the first voltage V1 are two or more types.

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