JP2014190758A - Deterioration diagnostic method for power cable - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method capable of easily performing deterioration diagnosis of a power cable in a hot-line state without having to attach equipment required for diagnosis in service interruption in advance in a preliminary step for performing the deterioration diagnosis of the power cable.SOLUTION: In a deterioration diagnostic method for a power cable, following successive steps are performed: a step of installing a high frequency current measuring part capable of detecting a high frequency pulse current having a frequency at least 10,000 or more times as much as the commercial frequency in a shield ground line under a situation where a diagnosis target power cable is in a hot-line state; a step of installing a filter part for cutting off a commercial frequency component contained in an electric signal detected by the high frequency current measuring part, and a waveform detection part for detecting the signal waveform of the electric signal outputted via the filter part; a step of recognizing a time difference t of a pair of pulse signals discretely repeated contained in the signal waveform detected by the waveform detection part; and a step of specifying a point advanced from one end of the diagnosis target power cable toward the other end thereof by a length corresponding to a value obtained a predetermined calculation using the time difference t as a deterioration point.

Description

  The present invention relates to a method for diagnosing insulation deterioration of a power cable, and more particularly to a method for diagnosing live lines.

  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. 4 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. 4, 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. 4, 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.

  Up to now, a so-called DC high voltage leakage current method has been known as a method for diagnosing power cable deterioration with high accuracy (see, for example, Patent Document 1 below). However, in this method, since it is necessary to apply a high DC voltage to the cable, there is a problem that the cable to be diagnosed must be in a power failure state, which inconveniences consumers. Moreover, in order to reduce the burden on the customer as much as possible, it is necessary to reduce the power failure time as much as possible, so the cable diagnosis must be performed with a limited diagnosis time. For this reason, there exists a subject that it is difficult to diagnose all the power cables 10 drawn in by the customer by one operation | work, for example.

  On the other hand, Patent Document 2 below discloses a method for performing insulation deterioration diagnosis of a high-voltage cable under a live line. According to this method, since the insulation deterioration diagnosis of the high-voltage cable can be performed in a live line state, it is not necessary to force the customer to stop power during the diagnosis work.

JP 2005-195503 A JP 2007-33365 A

  The method described in Patent Document 2 is a method of measuring a DC component due to a tree rectification effect from a leakage current flowing in a shield ground line. In the case of this method, the live line diagnostic device is inserted between the load side shield and the ground. During normal operation, only the power supply shield is grounded. At the time of hot line diagnosis, it is necessary to attach the diagnostic device and disconnect the power supply shield from the ground so that the leakage current passes through the hot line diagnostic device.

  Therefore, in this method, in order to perform live line diagnosis, a grounding wire should be connected to both ends of the cable shield in advance, and a switchgear for disconnecting the power-side shield should be connected between the load-side shield and the ground. Needs to be provided with a terminal to which a live line diagnostic device can be attached in advance. That is, since it is necessary to remove the grounding of the neutral point during the preliminary work necessary for performing live line diagnosis, a power failure work is necessary during this period. In other words, once the installation of the diagnostic device is completed, the cable deterioration diagnosis work under the live line can be performed, but a power outage of about several hours is required at the preparation stage, so it is still necessary for the consumer. It will be inconvenient.

  In view of such problems, the present invention eliminates the need to attach equipment necessary for diagnosis under a power outage in advance before performing power cable deterioration diagnosis, and easily performs power cable deterioration diagnosis in a live state. The object is to provide a method that can be performed.

  The present invention relates to a method for diagnosing insulation deterioration of a power cable in which a conductor, an insulator, a shield layer, and a cable sheath are coaxially formed in this order from the inside, wherein the shield layer is one end of the power cable to be diagnosed. The following steps are executed under the condition that the ground is connected through the shield ground line while the other end is in an open state and the diagnosis target power cable is in a live state.

Each step is as follows.
(A) installing a high-frequency current measuring unit capable of detecting a high-frequency pulse current at least 10,000 times the commercial frequency on the shield ground line;
(B) A filter unit that cuts off the commercial frequency component included in the electrical signal detected by the high-frequency current measurement unit, and a waveform detection unit that detects a signal waveform of the electrical signal output through the filter unit Installing the steps;
(C) identifying a time difference t between a pair of discretely repeated pulse signals included in the signal waveform detected by the waveform detector;
(D) Specify a portion advanced from the one end of the diagnosis target power cable to the other end as a degradation portion by a length corresponding to a value obtained by performing a predetermined calculation using the time difference t. Step.

  In general, the shielded ground wire of the power cable is installed in the field with one side grounded. However, there are also sites where the shield ground wire is grounded at both ends when the induced voltage at the shield open end is high due to a long cable length or the like. When performing insulation deterioration diagnosis of a power cable using the method of the present invention at such a site, for example, one ground wire may be disconnected.

  Here, as the high-frequency current measuring unit, for example, a clamp-type CT (instrument transformer) can be used. In this case, it can be installed simply by fitting this CT into the shield ground wire. A high current does not flow through the shield ground line, and the operation of installing such a high-frequency current measuring unit on the shield ground line can be performed safely and easily even in a live line state.

  In addition, the operation of connecting the high-frequency current measurement unit and the filter unit with a signal line and the operation of connecting the filter unit and the waveform detection unit with a signal line can be performed by, for example, a normal plug connection operation, which is extremely easy. There is no safety problem even in a live state. For example, it may be performed before the work of installing the high-frequency current measuring unit on the shield ground wire.

  Strictly speaking, the signal line may be connected before the CT is installed on the shield ground line. Although this is small, current flows through the grounding wire, so if CT is installed first, the secondary circuit of CT is opened, and there is a possibility of damage to the measuring instrument when CT is broken down or connected. Because.

  Here, a general high-pass filter or a band-pass filter can be employed as the filter unit.

  In addition, as the waveform detection unit, for example, a device that displays a signal waveform such as an oscilloscope can be employed.

  When insulation deterioration has occurred in the power cable to be diagnosed, partial discharge occurs in the insulator in a live line state, that is, in a state where voltage is applied. The current pulse derived from the partial discharge propagates to one end shielded via the shield layer and the other open end on the opposite side. The current pulse reaching the other end is reflected by the other end and propagates to the one end. A current pulse (hereinafter referred to as “direct pulse”) directly from the deteriorated portion toward the one end and a current pulse reflected from the other end and directed toward the one end (hereinafter referred to as “reflection pulse”). , And detected by a high-frequency current measuring unit installed on each shield ground line with a time difference. The detected signal appears as a pulse signal in the waveform detection unit after the commercial frequency component is cut off in the filter unit.

  Therefore, the waveform detection unit detects the pulse signal derived from the direct pulse and the pulse signal derived from the reflected pulse with a time difference. This time difference t can be recognized by checking the waveform detector. This time difference t changes depending on which of the portions between the one end and the other end of the diagnostic target power cable is located. That is, the length from one end of the diagnosis target power cable to the deteriorated portion can be expressed as a function of the time difference t. Therefore, by calculating based on this time difference t, the length from one end of the power cable to be diagnosed to the deteriorated portion can be obtained, so that the portion advanced from one end to the other end by that length is the deteriorated portion. Can be specified as

  Here, when the length L from the one end to the other end of the power cable to be diagnosed and the speed v of the high-frequency pulse current propagating through the shield layer are known in advance, these values and the time difference t On the basis of the above, it is possible to specify a point advanced from the one end of the diagnosis target power cable to the other end by the value x calculated by x = L−v · t / 2 as the deterioration point. The basis for this calculation will be described in more detail later in the section “Detailed Description of the Invention”.

  On the other hand, when the length L from the one end to the other end of the power cable to be diagnosed and the speed v of the high-frequency pulse current propagating through the shield layer are not known in advance, the following steps are further added. Then, it is characterized by specifying a deterioration location.

Specifically, the following steps are performed.
(E) installing an electrode on a surface in the vicinity of the one end of the cable sheath;
(E2) A test pulse having a frequency higher than the commercial frequency is injected from the electrode, and after the test pulse is injected, a signal waveform derived from a reflected wave of the test pulse is detected by the waveform detection unit. Measuring the pulse propagation time t ₀ for inspection required for.

In the step (d), x / L = 1− based on the length L from the one end to the other end of the power cable to be diagnosed, the inspection pulse propagation time t ₀ , and the time difference t. The part advanced from the one end of the diagnosis target power cable to the other end by the ratio of t / t ₀ is specified as the deteriorated part.

  When a high-frequency inspection pulse is injected from an electrode attached to the surface in the vicinity of the one end of the cable sheath, an induced voltage is generated in the shield layer by the inspection pulse. The high-frequency pulse current derived from this voltage is detected by the high-frequency current measuring unit directly via the shield ground line, and flows toward the other end via the shield layer, and after being reflected at the other end, It is divided into those detected by the high-frequency current measuring unit through the ground wire. Both signals appear as pulse signals in the waveform detection unit after the commercial frequency component is cut off in the filter unit.

Therefore, by identifying these time differences t ₀ in the waveform detection unit, when the propagation speed of the high-frequency current pulse is v, the total length L from one end of the power cable to be diagnosed to the other end is expressed as L = v · t it can be recognized as a _0/2. Therefore, x / L, which is the ratio of the distance x from one end to the deteriorated portion with respect to the total length L of the cable, can be calculated as 1−t / t ₀ . Thereby, a deterioration location is specified. The basis for this calculation will be described later in more detail in the “Detailed Description of the Invention” section.

  Further, when the diagnosis target power cable has three phases and these shield layers are collectively grounded by the shield ground wire, the diagnosis target power cables for three phases are in a live state. In addition to the steps (a) to (d), the following steps are further executed.

Each step is as follows.
(E) installing an electrode on the surface in the vicinity of the one end of the cable sheath of each of the three phases;
(F) a step of inputting a voltage signal induced in each of the three-phase electrodes to a signal processing unit to perform phase discrimination and waveform processing;
(G) The voltage signal that has undergone waveform processing in the signal processing unit is taken into the waveform detection unit, and the phase of the voltage signal corresponding to the variation in the polarity of the pair of discretely repeated pulse signals is shown. Qualifying the power cable to be diagnosed as a degraded power cable.

  Then, in the step (d), the portion advanced from the one end to the other end of the deteriorated power cable certified in the step (g) is specified as the deterioration portion.

  When the shield layer of the power cable for diagnosis of three phases is collectively grounded, even if the pulse signal derived from the direct pulse and the pulse signal derived from the reflected pulse are detected with a time difference in the waveform detection unit, It is impossible to determine which phase of the electric power pulse is derived from the partial discharge from the power cable.

  Here, the current pulse derived from the partial discharge generated in the insulator in the power cable shows a polarity synchronized with the polarity of the voltage of the cable. This is because the pulse transmitted through the shield layer is reflected in the forward direction at the open end and invertedly reflected at the ground end. That is, in the case of the above configuration, since the reflection end of the pulse is open, the direct pulse and the reflection pulse have the same polarity. Therefore, a positive pulse generated at the rising time of the voltage has a positive reflected pulse, and similarly, a negative pulse generated at the falling time has a negative reflected pulse.

  The direct pulse and the reflected pulse are synchronized with the period of voltage fluctuation during the rising period from the time when the cable voltage reaches zero crossing until the peak value of the positive voltage is reached and from the time when the peak voltage reaches the peak value of the negative voltage. Occur. Therefore, by comparing the polarity fluctuation of the voltage signal of each power cable and the polarity fluctuation of the pulse signal of the direct pulse and the reflected pulse, and confirming the phase of the synchronized power cable, the power cable of this phase It can be judged that it has deteriorated.

  Here, in order to take each cable voltage into the waveform detector, an electrode is provided on the surface of the cable sheath. Since the cable sheath is an insulator, a high current does not flow, and the operation of installing the electrode on the surface of the cable sheath can be easily and safely performed even in a live line state. Since an insulator is formed inside the cable sheath and a conductor to which a voltage is applied is formed inside the cable sheath, a voltage applied to the conductor is induced in the electrode. Therefore, the phase difference of the voltage signal applied to the conductor of each cable can be recognized by taking out the voltage signal induced in the electrode attached to each phase.

  Here, in step (f), the extracted voltage signal is once input to the signal processing unit, and after phase discrimination and waveform processing are performed, the processed signal is input to the waveform detection unit. While the period of the voltage signal is on the order of milliseconds, the period of the pair of pulse signals to be confirmed for the cable deterioration certification is on the order of microseconds. Therefore, if these pulse signals are simultaneously displayed on the waveform detection unit on the same scale as the voltage signal, it becomes difficult to confirm the correspondence between the polarity fluctuation of the voltage signal and the polarity fluctuation of the pulse signal. Therefore, the signal processing unit processes the voltage waveform in accordance with the order of the period of the pulse signal.

  Further, the signal processing unit determines which phase each cable is based on the phase difference of the input three-phase voltage signals. For example, a general phase discrimination circuit can be used.

  Under such a configuration, the waveform detection unit compares the fluctuation of the voltage signal of each phase with the appearance timing of the pair of pulse signals and the polarity fluctuation thereof, and has a deteriorated power cable with a phase synchronized with the voltage fluctuation. Can be identified.

  As a more specific method, the voltage phase of each phase may be obtained from the voltage signal, and a signal corresponding to the phase and phase (phase angle) to be measured may be output from the signal processing unit. At this time, the phase and phase to be measured can be freely adjusted by the measurer, and a signal corresponding to the phase and phase (phase angle) to be measured is used as an external trigger of the waveform detection unit (eg oscilloscope), The target phase and waveform at the time of phase can be measured.

  In addition, since the operation | work which connects each 3 phase electrode and a signal processing part with a signal line, and the operation | work which connects a signal processing part and a waveform detection part with a signal line can be performed by the normal connection work, for example, it is very easy. There is no safety problem even if it is in a live state.

In the case where a three-phase power cable is provided and the shield layers for the three phases are collectively grounded by the shield ground wire, the degradation point can be specifically specified in the same manner. That is, when the length L from one end of the power cable to be diagnosed to the other end and the value of the velocity v of the high-frequency pulse current propagating through the shield layer are known in advance, x = L−v based on the time difference t. -The part advanced from the said one end of the said diagnostic object electric power cable to the said other end only by the value x calculated in t / 2 is specified as a degradation location. If the length L from one end of the power cable to be diagnosed to the other end and the value of the velocity v of the high-frequency pulse current propagating through the shield layer are not known, an inspection pulse is injected from the electrode in advance. , The inspection pulse propagation time t ₀ required from the injection to the detection of the signal waveform derived from the reflected wave of the inspection pulse is measured, and x / L = 1−t based on this t ₀ and the time difference t. The part advanced from the one end of the diagnosis target power cable to the other end by the ratio of / t ₀ is specified as the deterioration part.

  According to the power cable deterioration diagnosis method of the present invention, it is not necessary to attach a facility necessary for diagnosis under a power outage in advance before performing the power cable deterioration diagnosis, and the power cable deterioration diagnosis can be easily performed in a live state. Can be performed.

It is a block diagram which shows typically the power cable degradation diagnostic method of this invention. It is a figure which shows typically the cross section of the power cable of the state which attached the electrode. It is a figure which shows typically an example of the screen of a waveform detection part. It is a figure which shows typically the cross section of an electric power cable.

[First Embodiment]
A first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram schematically showing a method for diagnosing deterioration of power cable insulation according to the present invention.

(Description of preparation work)
In FIG. 1, it is assumed that the diagnosis target power cable has three phases (10A, 10B, 10C). The cross-sectional structure of each cable is the same as that shown in FIG. In the following description, A, B, and C are added to the end of the reference when describing each phase while noting that each reference is omitted. That is, “conductor 11A” refers to a conductor included in power cable 10A, and “conductor 11” simply refers to all conductors of power cables 10A, 10B, and 10C.

  In FIG. 1, for convenience of explanation, the shield layer 15 (15A, 15B, 15C) is intentionally illustrated separately from the cable.

  Each shield layer 15 is grounded via a common shield ground line 40 at one end 41 (41A, 41B, 41C). The other end 43 (43A, 43B, 43C) is in an open state.

  In the present embodiment, the power cable 10 to be diagnosed has a three-phase configuration (10A, 10B, 10C) as described above, and the shield layer 15 of each cable is collectively grounded at one end and is open at the other end. This will be explained assuming such a site.

  The operator brings the high-frequency current measurement unit 2, the filter unit 3, the signal processing unit 4, the waveform detection unit 5, and the electrodes 6 (6A, 6B, 6C), and installs these devices at the site. All the diagnosis target power cables 10 are performed under a live line condition.

  The high-frequency current measurement unit 2 can employ, for example, a clamp-type CT (instrument transformer). The worker performs an operation of attaching the high-frequency current measuring unit 2 to the shield ground wire 40. Since a high current does not flow through the shield ground line 40, this operation can be easily and safely performed even in a live line state. The high-frequency current measuring unit 2 may be configured to be able to detect a high-frequency current pulse of, for example, about several MHz. That is, it is a configuration capable of detecting a high-frequency pulse current at least 10,000 times the commercial frequency.

  The filter unit 3 can employ, for example, a general high-pass filter or a band-pass filter. The filter unit 3 only needs to have a function of cutting off a signal of a commercial frequency component (for example, 60 Hz) out of at least an input signal and outputting a higher frequency signal.

  Further, as the waveform detection unit 5, for example, a device that displays a signal waveform such as an oscilloscope can be employed.

  The worker connects the high-frequency current measuring unit 2, the filter unit 3, and the waveform detecting unit 5 via signal lines, respectively, so that the commercial frequency component of the electrical signal detected by the high-frequency current measuring unit 2 can be obtained. The waveform detector 5 can check the signal waveform of the high frequency component from which the above signal is cut off.

  Further, the worker attaches the electrodes 6 (6A, 6B, 6C) to the surface in the vicinity of the one end 41 among the surfaces of the cable sheath 17 of each power cable 10 (see also FIG. 2). As a method for attaching the electrode 6, for example, a method of attaching a copper foil tape with adhesive, a method of winding a metal foil around the power cable 10 with a tape, or the like can be employed. The installation location of the electrode 6 is a location in the vicinity of the end (one end) of the power cable 10 on the side where the shield layer 15 is installed via the shield ground wire 40. More specifically, the time required for the high-frequency current pulse to propagate a distance twice as long as the electrode 6 and the other end 43 is ignored. The electrode 6 is installed in the vicinity of the one end 41 as much as possible.

  In addition, since the cable sheath 17 is an insulator, a high current does not flow. For this reason, the operation | work which attaches the electrode 6 to the surface of this cable sheath 17 can be performed safely and easily, even if the electric power cable 10 is a hot wire state.

  When the electrode 6 is attached to the cable sheath 17 of each phase, the insulator 13 is formed inside the cable sheath 17, and the conductor 11 to which a voltage is applied is formed inside thereof. The voltage applied to 11 is induced. Therefore, the phase difference of the voltage signal applied to the conductor of each cable 10 can be recognized by taking out the voltage signal induced in the electrode 6 attached to each phase.

  Then, the worker connects the electrode 6 (6A, 6B, 6C) attached to each phase and the signal processing unit 4 with a signal line.

  The signal processing unit 4 has a function (phase discrimination function) for discriminating the phase of each cable 10 (10A, 10B, 10C) based on the phase difference when the voltage signal from each electrode 6 is input. . In addition, for the purpose of making the waveform of the voltage signal (sine waveform) supplied from each electrode 6 easier to see visually, the cycle is extended or shortened, or the waveform itself is changed to a triangular wave or a rectangular wave. Function (waveform processing function). To realize the phase discrimination function, for example, a general phase discrimination device that specifies the phase based on the time difference passing through the zero cross can be used. The waveform processing function can use a general image processing circuit or the like. Some phase discrimination devices have the above-described waveform processing function. In this case, the signal processing unit 4 can be configured by one phase discrimination device.

  The signal processing unit 4 has a function of outputting a signal corresponding to the phase and phase to be measured designated by the measurer to the waveform detection unit 5. This signal becomes an external trigger of the waveform detector 5 (for example, an oscilloscope).

  Then, the worker connects the signal processing unit 4 and the waveform detection unit 5 with a signal line. Thereby, in the waveform detection part 5, the aspect of the voltage fluctuation of each cable 10 (10A, 10B, 10C) in the state to which the signal processing was performed in the signal processing part 4 can be confirmed.

  Thereby, in the waveform detection part 5, the signal from which the commercial frequency component was cut among the electric signals detected by the high frequency current measurement part 2 and the signal indicating the voltage fluctuation of each cable 10 (10A, 10B, 10C). Can be confirmed at the same time. As will be described later, the period of the electric signal detected by the high-frequency current measuring unit 2 is on the order of μ seconds, whereas the period of the voltage signal supplied from each electrode 6 is on the order of m seconds. Therefore, the signal processing unit 4 performs waveform processing of the voltage signal supplied from each electrode 6 for the purpose of simultaneously displaying the waveforms on the waveform detection unit 5 so that the synchronization of these polarity fluctuations can be seen.

(Description of detection work)
When the above-described preparation work is completed, the worker checks the screen of the waveform detection unit 5. FIG. 3 is an example schematically showing a screen of the waveform detector 5 when the cable 10A is deteriorated. Hereinafter, a description will be given with reference to FIGS. 1 and 3.

  When insulation degradation has occurred at the degradation location 30 of the power cable 10A, partial discharge occurs in the insulator 13A in a live line state, that is, in a state where a voltage is applied. This partial discharge propagates through the shield layer 15A to the one end 41A on the shield ground line 40 side and to the other end 43A opened on the opposite side, and then reflects at the other end 43A. On the other hand, a reflected pulse 32 traveling toward the end 41A is formed.

  The direct pulse 31 directly reaches the one end 41 </ b> A from the deteriorated portion 30 and then flows to the ground via the shield ground line 40. On the other hand, the reflected pulse 32 reaches the other end 43A from the deteriorated portion, is reflected by the other end 43A, reaches the one end 41A, and then flows to the ground through the shield ground line 40. That is, there is a time difference t between the timing when the direct pulse 31 flows through the shield ground line 40 and the timing when the reflected pulse 32 flows through the shield ground line 40.

  When a high-frequency current flows through the shield ground line 40, the high-frequency current measuring unit 2 detects the current signal. The detected signal is sent to the waveform detection unit 5 after the commercial frequency component is cut off by the filter unit 3.

  The current pulse derived from the partial discharge generated in the insulator 13A becomes a high-frequency current of about several MHz, for example. For this reason, as described above, the high-frequency current measuring unit 2 needs to have a function of detecting such a high-frequency current.

  As described above, there is a time difference t between the timing at which the high-frequency current measuring unit 2 detects the current signal derived from the direct pulse 31 and the timing at which the current signal derived from the reflected pulse 32 is detected. Therefore, a pair of pulse signals having a time difference t is displayed on the waveform detector 5 (signals 51d and 51r, signals 52d and 52r in FIG. 3). In addition, compared with the current signal derived from the direct pulse 31, the level of the current signal derived from the reflected pulse 32 is attenuated because the propagation distance is long.

  In addition, the voltage detector 5 simultaneously displays voltage fluctuation modes of the cables 10 (10A, 10B, 10C) (waveforms 61, 62, 63). The operator compares the polarity of the pair of pulse signals 51 and 52 with the variation of the polarity of the voltage waveform from such a screen of the waveform detector 5. A pair of high-frequency signal pulses (51, 52,...) Derived from the partial discharge has a rising period from the time of zero crossing of the cable voltage of the cable where deterioration has occurred until the peak value of the positive voltage is reached, and from the time of zero crossing to the negative voltage. It occurs in synchronization with the period of voltage fluctuation during the falling period until the peak value is reached.

  That is, the worker confirms the screen of FIG. 3, and while the pair of pulse signals 51 (51d, 51r) shows positive polarity, the voltage value tends to increase within a positive range, While the pulse signal 52 (52d, 52r) shows negative polarity, a phase showing a voltage fluctuation curve in which the voltage value tends to fall within a negative range may be specified. In the case of FIG. 3, the curve 61 corresponds to this. Then, the phase discrimination function of the signal processing unit 4 specifies that the voltage fluctuation like the curve 61 is the phase of the power cable 10A.

  From the above, it can be determined that the degraded power cable 10 of the three phases is the power cable 10A.

  Next, a method for identifying which part of the power cable 10A has deteriorated will be described.

  First, the case where the length L from one end 41A to the other end 43A of the power cable 10A and the propagation speed v of the high-frequency pulse current propagating through the shield layer 15 are known in advance will be described. Hereinafter, the distance from the one end 41A of the power cable 10A to the deteriorated portion 30 is assumed to be x.

A direct pulse (direct wave) 31 is detected by the high-frequency current measuring unit 2 after proceeding by x from the deteriorated portion 30. In other words, the traveling distance Z _d of the direct pulse 31 is represented by the following equation (1).
(Equation 1)
Z _d = x

On the other hand, the reflected pulse (reflected wave) 32 travels from the degraded portion 30 to the other end 43A, and then is reflected and detected by the high-frequency current measuring unit 2, and therefore the travel distance _Zr is _expressed by the following equation 2. Is done.
(Equation 2)
_Zr = 2 (L-x) + x = 2L-x

  In reality, the distance d from the one end 41A to the high-frequency current measuring unit 2 via the shield ground line 40 is also added, but the cable length L is generally more than a few tens m to more than 100 m. On the other hand, since the distance d is an order of about several tens of cm to 30 cm at most, it can be ignored in the calculation.

The difference between the traveling distance _Zd of the direct pulse 31 and the traveling distance _Zr of the reflected pulse 32 coincides with the product of the time difference t and the pulse propagation velocity v as shown in the following equation 3.
(Equation 3)
Z _r -Z _d = v · t

Substituting Equations 1 and 2 into Equation 3 and solving for x yields Equation 4 below.
(Equation 4)
x = L−v · t / 2

  Therefore, the worker reads the time difference t between the high-frequency signal pulse 51d derived from the direct pulse 31 and the high-frequency signal pulse 51r derived from the reflected pulse 32 from the screen of the waveform detection unit 5 shown in FIG. The value of x can be obtained by substituting the values of L and v and the values of L and v known in advance. Since the value of x corresponds to the distance from the one end 41A of the power cable 10A to the deteriorated portion 30, the position of the deteriorated portion 30 is specified.

  Next, a case where the length L from one end 41A to the other end 43A of the power cable 10A and the propagation speed v of the high-frequency pulse current propagating through the shield layer are not known will be described.

  The operator injects a high-frequency inspection pulse current from the electrode 6A. At this time, an induced voltage is generated in the shield layer 15A by this inspection pulse. The high-frequency pulse current derived from this voltage is detected by the high-frequency current measurement unit 2 directly through the shield ground line 40 (hereinafter referred to as “direct inspection pulse”), and the other through the shield layer 15A. After flowing toward the end 43A and being reflected at the other end 43A, it is divided into those detected by the high-frequency current measuring unit 2 via the shield ground line 40 (hereinafter referred to as “reflection inspection pulse”).

Therefore, the waveform detector 5 detects a pair of pulse signals having a time difference t ₀ . When this pulse signal is detected, it is input in advance with a high current amount so as not to be mixed with the pair of pulse signals (51, 52,...) Derived from the partial discharge, or discretely repeatedly over a predetermined time. It is also possible to input an inspection pulse current.

The difference in the moving distance between the direct inspection pulse and the reflection inspection pulse corresponds to twice the distance from one end 41A of the power cable to the other end 43A, that is, 2L. This difference in distance, appears as a time difference t _0. That is, the following formula 5 is established.
(Equation 5)
2L = v · t ₀

Solving this for v yields:
(Equation 6)
v = 2L / t ₀

Substituting Equation 6 into Equation 4 yields Equation 7 below.
(Equation 7)
x = L− (t / 2) · (2L / t ₀ ) = (1−t / t ₀ ) · L

By transforming Equation 7, the following Equation 8 is obtained.
(Equation 8)
_{x / L = 1- (t /} t 0)

Therefore, the worker reads the time difference t between the high-frequency signal pulse 51d derived from the direct pulse (direct wave) 31 and the high-frequency signal pulse 51r derived from the reflected pulse (reflected wave) 32 from the screen of the waveform detection unit 5, and this The value of x / L can be known by substituting the value of t and the value of the time difference t ₀ of the test pulse measured in advance into Equation 8. Since the value of x / L corresponds to the ratio of the distance x from the one end 41A to the deteriorated portion 30 with respect to the total length L of the power cable 10A, the position of the deteriorated portion 30 is specified.

In the above-described embodiment, a high-frequency inspection pulse current is injected from the electrode 6A attached to the power cable 10A. This operation is for recognizing the relationship between the length L of the power cable 10 and the pulse propagation velocity v by measuring the time difference t ₀ . Therefore, when there is almost no difference in the length and material of the power cable 10 (10A, 10B, 10C) of each phase, the phase to which the pulse current for inspection is applied may be any phase. In the field where the power cables 10 (10A, 10B, 10C) having a three-phase structure are attached, there is usually almost no difference in the length and material of each cable.

  According to the above-described method, the worker performs all the work after arriving at the site where the deterioration diagnosis is performed, before preparing for the deterioration diagnosis, actually performing the diagnosis and withdrawing. It can be performed in a live line state. Thereby, the deterioration diagnosis of a power cable can be performed without forcing a power failure to a consumer at all.

  In addition, the equipment that the operator needs to bring to the site includes a high-frequency current measuring unit 2 composed of a clamp-type CT (instrument transformer), a filter unit composed of a high-pass filter, etc. A signal processing unit 4 composed of a discriminating device, a waveform detection unit 5 composed of an oscilloscope, and an electrode 6 composed of a foil electrode, etc., which can be easily brought by an operator with one hand It is. Therefore, the work can be easily performed by a single worker.

  In addition, according to this method, all work including preparation work, diagnosis work and withdrawal work can be completed in about 30 minutes after arrival at the site. As a result, it is possible to perform deterioration diagnosis of many on-site power cables in one day.

  Further, according to this method, measurement can be performed by attaching the above-described equipment only to the one end 41 side of the power cable 10. For this reason, even if the length of the power cable 10 is very long, the deterioration diagnosis work can be completed in a short time.

  Furthermore, according to this method, the position of the deteriorated portion 30 of the deteriorated power cable can be specified. Thereby, for example, when the degraded portion 30 is an end portion of the power cable 10, it can be known that it can be handled by repairing only the end portion at a later date. On the other hand, when the power cable 10 is in the substantially central portion, it can be known that the power cable 10 itself needs to be replaced at a later date. Thereby, after the end of the deterioration diagnosis work, it is possible to explain to the customer the appropriate treatment content to be performed thereafter.

[Second Embodiment]
Only a different part from 1st Embodiment is demonstrated about 2nd Embodiment of this invention.

  In the first embodiment, it is assumed that the power cable 10 has a three-phase structure (10A, 10B, 10C), and deterioration diagnosis is performed at a site where the shield layers 15 of all phase cables are collectively grounded. It was. On the other hand, when it is a single phase structure or when the shield layer 15 is grounded for each phase, the diagnosis can be performed by a simpler method.

  That is, in the first embodiment, since the three-phase cables 10 (10A, 10B, 10C) are all grounded via the common shield ground line 40, the high-frequency current measuring unit 2 detects the high-frequency pulse current. However, it has not been possible to identify which phase of the detected pulse current is propagated from the shield layer 15 of the power cable 10. For this reason, in the first embodiment, the waveform of the voltage induced on the electrode 6 (6A, 6B, 6C) (the waveform subjected to signal processing based on the waveform) and the high-frequency current pulse are simultaneously displayed on the waveform detector 5. Thus, by comparing the variation modes of the polarity, the worker was able to know which phase of the cable had deteriorated for the first time.

  On the other hand, when the single-phase structure is used, or when the shield layer 15 is grounded for each phase, the deteriorated cable is detected when the high-frequency current measuring unit 2 detects the high-frequency pulse current. Can be specified. Therefore, it is not necessary to perform signal processing on the voltage signal induced in the electrode 6 (6A, 6B, 6C) by the signal processing unit 4. That is, the signal processing unit 4 is not necessary in this embodiment.

  Further, when the length L from one end 41 to the other end 43 of the power cable 10 and the propagation speed v of the high-frequency pulse current propagating through the shield layer 15 are known, the pulse current for inspection is transmitted from the electrode 6 to the cable. Therefore, it is not necessary to install the electrode 6.

2: High-frequency current measurement unit
3: Filter section
4: Signal processor
5: Waveform detector
6 (6A, 6B, 6C): Electrode 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 drop 30: Degraded portion 31: Direct pulse derived from partial discharge 32: Reflected pulse derived from partial discharge 40: Shield ground wire 41 (41A, 41B, 41C): One end of shield layer (shield ground wire side)
43 (43A, 43B, 43C): The other end (open end side) of the shield layer
51, 52: a pair of pulse signals 51d, 52d: a pulse signal derived from a direct pulse 51r, 52r: a pulse signal derived from a reflected pulse 61, 62, 63: a signal indicating a voltage fluctuation mode of the power cable

Claims (5)

A method for diagnosing insulation deterioration of a power cable in which a conductor, an insulator, a shield layer, and a cable sheath are coaxially formed in this order from the inside,
The shield layer is grounded through a shield ground wire at one end of the power cable to be diagnosed, and is in an open state at the other end.
Under the situation where the power cable to be diagnosed is in a live state,
(A) installing a high-frequency current measuring unit capable of detecting a high-frequency pulse current at least 10,000 times the commercial frequency on the shield ground line;
(B) A filter unit that cuts off the commercial frequency component included in the electrical signal detected by the high-frequency current measurement unit, and a waveform detection unit that detects a signal waveform of the electrical signal output through the filter unit The steps of installing
(C) certifying a time difference t between a pair of discretely repeated pulse signals included in the signal waveform detected by the waveform detector;
(D) Specify a portion advanced from the one end of the diagnosis target power cable to the other end as a degradation portion by a length corresponding to a value obtained by performing a predetermined calculation using the time difference t. A diagnostic method comprising performing steps. In the one end of the diagnosis target power cable for three phases, when the shield layer for three phases is collectively grounded by the shield ground wire,
Under the situation where the diagnosis target power cable for three phases is in a live state,
(E) installing an electrode on the surface in the vicinity of the one end of the cable sheath of each of the three phases;
(F) inputting a voltage signal induced in each of the three-phase electrodes to a signal processing unit to perform phase discrimination and waveform processing;
(G) The voltage signal that has undergone waveform processing in the signal processing unit is taken into the waveform detection unit, and the phase of the voltage signal corresponding to the variation in the polarity of the pair of discretely repeated pulse signals is shown. Certifying the power cable to be diagnosed as a degraded power cable,
In the step (d), a step of specifying a portion advanced from the one end to the other end of the deteriorated power cable identified in the step (g) is specified as a deterioration portion. Item 2. The diagnostic method according to Item 1.   In the step (d), based on the length L from the one end to the other end of the power cable to be diagnosed, the velocity v of the high-frequency pulse current propagating through the shield layer, and the time difference t, x = 2. The step of specifying, as a deterioration point, a portion advanced from the one end of the diagnosis target power cable to the other end by a value x calculated by Lv · t / 2. Or the diagnostic method of 2. (E) installing an electrode on the surface in the vicinity of the one end of the cable sheath;
(E2) A test pulse having a frequency higher than the commercial frequency is injected from the electrode, and after the test pulse is injected, a signal waveform derived from a reflected wave of the test pulse is detected by the waveform detection unit. A step of measuring an inspection pulse propagation time t ₀ required for
In the step (d), x / L = 1−t / based on the length L from the one end to the other end of the power cable to be diagnosed, the inspection pulse propagation time t ₀ , and the time difference t. The diagnosis method according to claim 1, wherein a step of specifying a portion advanced from the one end of the diagnosis target power cable to the other end as a deterioration portion by a ratio of t ₀ is performed. (E2) A test pulse having a frequency higher than the commercial frequency is injected from the electrode, and after the test pulse is injected, a signal waveform derived from a reflected wave of the test pulse is detected by the waveform detection unit. A step of measuring an inspection pulse propagation time t ₀ required for
In the step (d), the length L from the one end to the other end of the deteriorated power cable certified in the step (g), the inspection pulse propagation time t ₀ , and the time difference t are set. 3. The step of identifying, as a deteriorated portion, a portion advanced from the one end of the deteriorated power cable to the other end by a ratio of x / L = 1−t / t ₀ is performed. The diagnostic method according to.

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