Cause of cable failure
In order to accurately and timely determine the cable fault, before the cable fault is detected, the cause of the cable fault must be analyzed first, and the appropriate and appropriate cable fault detection method should be selected to eliminate the cable fault in time.
The most direct cause of a cable fault is a breakdown of insulation and breakdown. There are many ways to reduce the amount of guided-sensitive insulation. According to the actual operating experience, the following situations can be summed up:
1, external damage. From the operation analysis of recent years, especially in the high-speed economic development of Haipu East, a considerable number of cable faults are caused by mechanical damage. For example, when the cable is laid and installed, the construction is not standardized and it is easy to cause mechanical damage. It is also very easy to damage the cable during operation by carrying out the civil works on the buried cable. l Sometimes if the damage is not serious, it will take several months or even years to cause complete breakdown of the damaged part to form a fault, and sometimes severe damage may lead to road failure, directly affecting the safe production of electric power units.
2, insulation damp. This situation is also very common, usually occurs in the cable joints in the direct burial or exhaust pipe. For example: The unqualified cable joints and joints in humid climate conditions will cause the joints to enter water or mix in water vapor. The water branches will form under the effect of the electric field for a long time, which will gradually damage the dielectric strength of the cables and cause failures.
3, chemical corrosion. Cables directly buried in areas with acid and alkali effects often cause cable armoring, lead sheathing, or outer sheathing corrosion. The protective layer is subject to long-term chemical or electrolytic corrosion, resulting in failure of the protective layer and reduced insulation. Cable failure.
4, long-term overload operation. Overload operation, due to the thermal effect of the current, the load current will inevitably lead to heating of the conductor when passing through the cable. At the same time, the skin effect of the charge and the eddy current loss and insulation loss of the steel crucible will also generate additional heat, thus increasing the temperature of the cable. During long-term overload operation, an excessively high temperature will accelerate the aging of the insulation so that the insulation is broken down. Especially in the hot summer season, the temperature rise of the cable often leads to the first breakdown of the weak cable insulation. Therefore, in the summer, the cable fault is particularly high.
5, cable connector failure. Cable joints are the weakest link in the cable route. Faults of cable joints caused by direct personnel faults (bad construction) occur frequently. In the process of making cable joints, if the joints of the joints are not tightly connected and the heating is insufficient, the insulation of the cable heads will be reduced and the accident may be caused.
6, the environment and temperature. The external environment and heat source of the cable can also cause the cable temperature to be too high, insulation breakdown, and even explosion.
7. Other reasons such as normal aging of cables or natural disasters.
Cable fault detection method
A comprehensive analysis of the causes of cable faults, then we must select the appropriate cable fault detection methods. The method of cable fault detection is various and it can be summarized as follows:
1, zero potential method
Zero potential method is the potential comparison method, which is one of the commonly used methods for cable fault detection. It is suitable for cable core ground faults of relatively long lengths. This method is simple, accurate and does not require precision instruments and complicated calculations. The principle of measurement is as follows: the faulty core wire is connected in parallel with the equal length of the comparative conductor. When the voltage VE is applied across the terminals b and c, it is equivalent to connecting the power supply at both ends of the two parallel uniform resistance wires. At this time, a resistance wire is used. The potential difference between any point on the resistance wire and the corresponding point on the other resistance wire must be zero. Otherwise, the two points where the potential difference is zero must be the corresponding point. Because the negative terminal of the microvoltmeter is grounded, and the point of failure of the cable is equalized, therefore, when the positive terminal of the microvoltmeter moves on the comparative lead to the point where the indicated value is zero and the fault point is equal, that is, the corresponding point of the fault point. S is a single-phase switch, E is a 6E battery or 4 AA batteries, G is a DC microvoltmeter, and the measurement procedure is as follows:
1) Connect the batteries E to the cores of phases b and c first, and then lay a comparative wire S equal to the length of the faulty cable on the ground. The bare wire or bare aluminum wire should have the same cross-section. There can be no intermediate connector.
2) Connect the negative pole of the microvoltmeter to ground and the positive pole to a long soft wire. The other end of the wire requires full contact when sliding on the comparative wire.
3) Close the knife switch S and slide the end of the soft wire on the comparative wire. When the microvoltmeter indicates zero, the position is the position of the cable fault point.
2. Bridge method
The bridge method uses the double-arm bridge to measure the DC resistance of the cable core, and then accurately measures the actual length of the cable. According to the proportional relationship between the length of the cable and the resistance, the fault point is calculated and cable fault detection is achieved. This method determines that the error is generally not more than 3m for the faults with the contact resistance between direct and path points between cable cores being less than 1Ω. For faults where the contact resistance at the fault point is greater than 1Ω, the resistance can be reduced to 1Ω by increasing the voltage through burning. Below, measure again according to this method. When measuring the circuit, first measure the resistance R1 between the cores a and b, R1=2RX+R where RX is a phase resistance value of phase a or phase b to the point of failure, which is only the contact resistance of the contacts. Then the bridge is moved to the other end of the cable, and the DC resistance value R2 between the cores a1 and b1 is measured. Then R2 = 2R(LX)R, R(LX) is the phase a1 or b1 core to the fault point A phase resistance value. After measuring R1 and R2, and then b1 and c1 road, measure the DC resistance between the two phases of the b, c core, then the 1/2 of the organization for the resistance of each phase of the core, represented by RL, RL =RX R(LX), from which the contact resistance value of the fault point can be found: R=R1 R2-2RL table, therefore, the resistance value of the core on both sides of the fault point can be expressed by the following formula: RX=(R1-R) /2, R(LX) = (R2-R)/2. After the three values ​​of RX, R(LX), and RL are determined, the distance X from the fault point to the end of the cable or (LX) can be obtained by a proportional formula: X=(RX/RL)L, (LX)=(R (LX)/RL)L, where L is the total length of the cable. When the bridge method is adopted, the accuracy of measurement shall be ensured. The bridge connection line shall be as far as possible, and the wire diameter shall be large enough. The connection with the cable core shall be crimped or welded. The decimal places shall be retained during the calculation.
3, capacitance current measurement
During the operation of the cable, capacitance exists between the core and the core. The capacitance is evenly distributed. The capacitance is linearly proportional to the length of the cable. Capacitance current measurement is based on this principle to detect cable faults. The determination of the breakage of the cable core is very accurate. The measuring circuit uses a 1-2kVA single-phase voltage regulator 2S, 1 to 100mA, 0.5 mA milliampere meter. Measurement steps:
1) First, measure the value of the capacitance current of each phase of the core wire (the voltage should be kept equal) Ia, Ib, Ic at the head end of the cable.
2) At the end of the cable, the value of the capacitance currents Ia1, Ib2, and Ic3 of the cores of each phase is measured to check the ratio of the capacitances of the perfect cores to the disconnected cores, and an approximation point of the disconnection distance can be preliminarily determined.
3) According to the capacitance calculation formula C=I/(2Î fU), when positive voltage U and frequency f are constant, C is proportional to I. Since the f-frequency of the power-frequency voltage does not change, the applied voltage must remain constant during the measurement, and the ratio of the capacitance current is the ratio of the capacitance. Let the total length of the cable be L, and the distance of the broken line of the core is X, then Ia/Ic=L/X, X=(IC/Ia)L. During the measurement process, as long as the voltage is kept constant, the current meter reading is accurate, and the total length of the cable is measured accurately, and the measurement error is relatively small.
4, sound measurement
The so-called sound measurement method is based on the sound of the fault cable discharge to find, to achieve the cable fault detection method, the method for the high-voltage cable core insulation layer flashover discharge is more effective. The equipment used in this method is a DC voltage tester. TB is a high voltage test transformer, C is a high voltage capacitor, VE is a high voltage rectifier silicon stack, R is a current limiting resistor, Q is the discharge ball gap, and L is the cable core. When the capacitor C is charged to a certain voltage value, the ball gap discharges the cable fault core wire. At the fault, the cable core wire is discharged to the insulating layer to generate a “zig and zi†spark discharge sound. If it is a buried cable, you must first determine and mark the direction of the cable. When the noise is at a minimum, search by means of audio amplification devices such as deafness hearing aids or medical stethoscopes. When looking up, place the pickup close to the ground and slowly move along the direction of the cable. When you hear the sound of the “zigand and sizzle†discharge, it is the point of failure. Use this method must pay attention to safety, in the end of the test equipment and the end of the cable should be set up to monitor.
Through the cable fault detection, the cable fault is accurately determined, followed by the selection of appropriate maintenance methods and means to repair and eliminate the cable fault.
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