JPH0236192B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD The present invention relates to a method for monitoring cable insulation under live power cables, which can monitor cable insulation defects under live power cables. FIG. 1 is a diagram showing a conventional method for monitoring the insulation of high-voltage power cables under live lines. Various methods have been known to monitor the insulation deterioration status of high-voltage power cables under live wires, but among them, there is a method that does not require the preparation of a power source to be applied to the cable for measurement. A known method is shown in FIG. In FIG. 1, reference numeral 1 indicates a monitored cable that is transmitting high-voltage power. 2
At one end of the cable, the potential at the lower end of the cable is taken out by a grounding wire 3. At the other end 2' of the cable, that end of the cable sheath is not grounded. Reference numeral 4 denotes a current transformer that is magnetically coupled to the grounding wire 3 and detects the grounding current Io flowing through the grounding wire 3. This situation is the so-called
Although it is similar to ZCT, the purpose of ZCT is to detect the sum of the conductor current for three phases of the cable, so the three-phase conductor of the cable passes through the center of the magnetic core, and the detection sensitivity is about 100 mA. It's the smallest and not very expensive. However, in No. 4, the aim is to detect a ground current Io of 10 mA or less.
Reference numeral 5 denotes an amplifier for amplifying the secondary current of 4 and supplying the amplified current to the ammeter 6. FIG. 2 is an equivalent circuit of the circuit shown in FIG.
R ₁ , C ₁ , R ₂ , C ₂ , R ₃ , and C ₃ indicate the insulation resistance and capacitance of each phase of the cable. Since it is assumed that C ₁ =C ₂ =C ₃ , I〓C ₁ +IC ₂ +IC ₃ =0, and the ground current Io to be detected is I〓o=I _R1 +I _R2 +I〓 _R3 . That is, Io is determined by calibrating the amplified value of the ammeter 6, and when a current value exceeding a certain limit of Io is observed, the insulation resistance of the cable is determined to be defective. This conventional monitoring method has the following drawbacks. In other words, if the three-phase insulation deteriorates to the same degree, it is impossible to detect a defect. No matter how much the insulation resistance value of each phase decreases, if there is no unbalance between them, that is, the absolute value of the current of each phase |I _R1 |= |I _R2 |=
If ｜I _R3 ｜, no matter how large the value becomes
I _R1 +I _R2 +I〓 _R3 = 0, making it impossible to detect a defective state. Only when there is a large difference in the insulation resistance values of each phase
Io can be detected. Io detection sensitivity increased to 10m
As A, the defective insulation resistance value that can be detected in a 3KV power cable is |I _R2 |= |I _R3 |=0 |I _R1 |=|Io
Under the ideal unbalanced condition of |

[Formula] is impractical if it cannot be detected unless the insulation resistance value falls to such a low value. It is susceptible to the effects of capacitance unbalance and other induction, and there is a high possibility of misperception of performance. If the detection sensitivity can be increased to 1mA, the capacitance of a cable with a corresponding charging current is 0.00167μF at 50Hz, 3.3KV/√3, which corresponds to a length of approximately 3m for a 150mm ² cable. Capacitance imbalances of this magnitude are usually quite possible. Also, transformer coils and amplifiers tend to pick up external noise. An object of the present invention is to provide a method for monitoring cable insulation under live power cables that can easily monitor insulation defects in power cables with good detection sensitivity without requiring a separate power supply for measurement. That's true. The present invention will be described in detail below with reference to the drawings. FIG. 3 is a diagram illustrating a method for monitoring cable insulation under a live wire according to the present invention. 1 is a monitored cable that is transmitting high-voltage power. At one end of the cable 2, the potential at the other end of the cable is taken out by the ground wire shown at 3. In addition, the other terminal 2' of the cable
In this case, the terminal on that side of the cable cable is not grounded. 7 is a switch inserted in the middle of the connection of the grounding wire 3 to the ground, and 8 and 9 are capacitance and safety arresters connected in parallel to the switch 7. Furthermore, the DC voltmeter 11 is connected to the multiplier resistor 1
0 and is connected to measure the terminal voltage of the capacitance 8. FIG. 4 is an equivalent circuit diagram of FIG. 3. R ₁ , C ₁ ,
R ₂ , C ₂ , R ₃ , and C ₃ indicate the insulation resistance and capacitance of each phase of the cable. K ₁ , K ₂ , and K ₃ indicate rectifying elements that are assumed to exist in each phase insulator, and although such elements are not found in good insulators,
As the cable insulator deteriorates, a phenomenon occurs that is effectively equivalent to having a rectifying element inside the insulator, and the polarity of the AC voltage applied to the conductor becomes positive (or negative). There is a difference in the current flowing through the insulator when the current becomes negative (positive), and as a result, I _R1 , I _R2 , I _R3
The DC component currents of each phase shown by are shown to the outside in the direction of flowing toward the ground with the weaker side at a positive potential. DC current Io that is the sum of the three phases = I _R1 +
I _R2 + I _R3 charges the capacitance Co inserted in the middle of the grounding wire with a time constant determined by approximately Co x Ro.
The temporal change in the terminal voltage is measured by a DC voltmeter with R _M as the multiplier resistance. When charging of Co finishes, the current flowing through R _M becomes equal to Io, but in reality Co also has a finite internal parallel resistance circuit, and the insulation resistance of the cable's anti-corrosion layer is assumed to be a fairly low value. exist, so their parallel resistance is
Shown as Ro. In reality, it is Ro that suppresses the terminal voltage of Co, where _Ro≪RM . This is because I _R1 , I _R2 , and I _R3 are considered to be the output currents of a constant current generator having internal resistance (R ₁ , R ₂ , R ₃ ) higher than Ro. If you want to read the terminal voltage of Co without being influenced by the magnitude of the cable's anti-corrosion layer insulation resistance value, you need to prepare a somewhat low Ro in parallel with Co from the beginning, or set the value of R _M itself to be low from the beginning. You may adopt something. Now, with reference to FIGS. 3 and 4, a method for specifically monitoring cable insulation under live wire conditions will be described below. First, a switch 7, a capacitor 8, a safety arrester 9, a multiplier resistor 10, and a DC voltmeter 11 are connected to the middle of the grounding wire 3 of the monitored cable 1 to complete the circuit shown in FIG. As for the specific method of this connection, it is possible to always have these things fixed and wired, or it is possible to connect them each time a measurement is made. 8 or less as a rotating measurement set, etc.
Any method of dividing the fixed part and the rotating part can be considered. Further, in the case of constant connection, measurement may be performed by switching between a large number of cables, or these switching operations may be performed automatically. When it is confirmed that the circuit is completed, switch 7 is opened. (Alternatively, a measuring circuit is inserted in the middle of the grounding wire.) The DC component current Io, which has been flowing directly to the ground, flows to charge the capacitance 8. The terminal voltage gradually increases with a time constant determined by a circuit constant, and its value is measured by a DC voltmeter 11. In this case, the polarity of the terminal voltage is positive on the side connected to the ground, and negative on the side connected to the earth. If there are pinholes in the cable corrosion protection layer and a voltage that clearly exceeds the voltage generated by a local battery that can occur between the cable layer and the ground is observed by the above operation, this cable has poor insulation. An alert will be issued if there is. For this purpose, an observer may read the voltmeter indication and take appropriate measures, or an alarm may be automatically issued if a preset voltage value is exceeded. This can be easily done by converting the DC voltmeter 11 into a meter relay. The limit voltage value at which an alarm should be issued is, for example, 1.0V, with the cable end at a positive potential and the ground side at a negative potential. The metals that make up cable cables and shields are aluminum, lead, and copper, in order of their base potential. On the other hand, underground metal bodies (ground electrodes) that can cause local batteries to occur in areas with defects in the corrosion protection layer include magnesium, aluminum, zinc, iron, copper, etc., in terms of base metals.
Among these combinations, excluding those in which the insulation side is at a negative potential and the ground side is at a positive potential, and excluding combinations with special sacrificial electrodes used for galvanic corrosion protection, the most likely combination to be encountered is copper. The theoretical maximum potential difference is 0.96V with respect to iron. In reality, such a voltage is not generated with this combination, so it is sufficient to adopt the easy-to-understand 1.0V as the limit voltage value for issuing an alarm, and by this, it is possible to observe the local battery voltage and determine whether the insulation is defective. First of all, the possibility of misidentification can be eliminated. FIG. 5 is a diagram showing observed values actually obtained by the method of the present invention. Cable: 3KV3×150mm ² CV cable approximately 50m long Capacitance value 40μF Multiplier resistance 2MΩ Observation voltage 2.8V, 3 and a half minutes after charging started, the charging curve is shown in Figure 5 Insulation resistance measured using a megger after removal Red phase 350MΩ White phase 25MΩ Corrosion protection layer 100MΩ or more Black phase 40MΩ Same as above Insulator Visually inspected and found countless water trees in each phase As shown in the observation example above, cables with insulation deterioration to this extent can be separately applied to the cable under live wires according to the present invention. The industrial value obtained is extremely high because it is possible to detect insulation defects by obtaining a high observation voltage that avoids misunderstandings caused by measuring local battery voltage in the case of poor insulation of the corrosion protection layer without preparing a power source for measurement. The monitoring method of the present invention has the following effects. (1) Even if three phases deteriorate to the same degree, it can be detected as defective. What is involved in the measurement is only the direct current generated by rectification within the insulator, so there is no need to consider it as a vector sum of phase currents with phase differences, as is the case when measuring AC ground current. Deterioration can be detected simply by measuring the terminal voltage at which the capacitance is charged by the added current. Therefore, it does not matter at all whether the deterioration states of each phase are equal or unequal. (2) Good detection sensitivity. As seen in the actual measurement example, insulation resistance defects in a high region can be detected, which is equivalent to increasing the ground current detection sensitivity to 0.1 mA, which is actually impossible, using the conventional method. (3) It is less susceptible to the effects of capacitance unbalance and other induction, and there is less possibility of misidentification. Even if the capacitance is unbalanced and the total AC charging current is not zero, the DC voltage to be detected is also at a high level of 1V or more, so there will be no misidentification. This is because the low impedance of the capacitance connected between the capacitance and the ground reduces the generation of capacitance potential due to unbalanced alternating ground currents and induced noise currents. Since the alarm transmission voltage is set to a voltage higher than the local battery generated voltage in the pinhole portion of the corrosion protection layer, there is no possibility of misidentification from this point of view. (4) It must be easy to implement. The parts are simple and few.
It is easy to implement because it is cheap and lightweight.

[Brief explanation of drawings]

Fig. 1 is a diagram showing a conventional method for monitoring the insulation of high-voltage power cables under live lines, Fig. 2 is an equivalent circuit diagram of the circuit shown in Fig. 1, and Fig. 3 is a diagram showing the method for monitoring cable insulation under live lines according to the present invention. The explanatory diagram, FIG. 4, is an equivalent circuit diagram of the circuit shown in FIG. 3. FIG. 5 is a diagram showing observed values obtained by the method of the present invention. 1: Monitored cable, 2, 2': Cable terminal,
3: Ground wire, 4: Current transformer, 5: Amplifier, 6:
Ammeter, 7: Switch, 8: Capacitance, 9: Security arrester, 10: Multiplier resistance, 11: DC voltmeter.

Claims (1)

[Claims] 1. A capacitor is inserted in the middle of a thin ground wire of a high-voltage power cable under a live wire, and the voltage charged by the capacitor is charged by a direct current generated in an insulator. The cable is characterized in that when it is detected that the polarity is positive with the negative side being positive and the value has reached a specified value or more, an alarm is generated indicating that the cable has poor insulation. Cable insulation monitoring method under live wires. 2. A cable insulation monitoring method under a live wire according to claim 1, characterized in that the specified voltage value at which the alarm should be generated is 1V.