News Details

 1. Types and Judgment of Cable Faults

Whether high-voltage or low-voltage cables, during construction, installation, and operation, faults often occur due to short circuits, overload operation, insulation aging, or external damage. Cable faults are classified into three types: grounding, short circuit, and open circuit. The main fault types for three-core cables include: one or two cores in contact; short circuit between two phase cores; complete short circuit of three phase cores; one phase core open circuit or multiple phase open circuits. For direct short circuit or open circuit faults, a multimeter can be used for direct measurement and judgment. For non-direct short circuit and grounding faults, use a megohmmeter to measure the insulation resistance between cores or between cores and ground, and determine the fault type based on the resistance value. Dew point meters, moisture meters, and SF6 gas detectors are widely applicable in industries such as electric power, chemical, aviation, metallurgy, and medicine. Hubei Yitiancheng Electric specializes in the production and sales of various gas detection equipment such as dew point meters, moisture meters, and SF6 gas detectors.

  2. Methods for Locating Cable Fault Points

   1. Sound Detection Method: The so-called sound detection method is to locate the fault based on the discharge sound of the faulty cable. This method is effective for flashover discharge between the high-voltage cable core and the insulation layer. The equipment used in this method is a DC withstand voltage tester. The circuit connection is shown in Figure 1, where SYB is the high-voltage test transformer, C is the high-voltage capacitor, ZL is the high-voltage rectifier silicon stack, R is the 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 to the faulty cable core, and at the fault location, the cable core discharges to the insulation layer producing a "buzzing, buzzing" spark discharge sound. Then, when the ambient noise is low, use audio amplification devices such as hearing aids or medical stethoscopes to locate the fault. During the search, place the pickup close to the ground and move slowly along the cable route. When the "buzzing, buzzing" discharge sound is loud, that location is the fault point. Safety must be emphasized when using this method, and dedicated personnel should be assigned to monitor both the test equipment end and the cable end.

   2. Bridge Method: The bridge method uses a double bridge to measure the DC resistance value of the cable core, then accurately measures the actual length of the cable, and calculates the fault point based on the proportional relationship between cable length and resistance. This method has a judgment error generally not exceeding 3m for faults where the cable cores are directly short-circuited or the contact resistance at the short-circuit point is less than 1Ω. For faults where the contact resistance at the fault point is greater than 1Ω, a method of applying high voltage to burn through can be used to reduce the resistance to below 1Ω, and then measure using this method.

In the measurement circuit, first measure the resistance R1 between cores a and b, then R1 = 2RX + R, where R is the resistance of one phase from phase a or b to the fault point, and R is the contact resistance at the short-circuit point. Then measure the DC resistance R2 between cores a' and b' at the other end of the cable, then R2 = 2R(L-X) + R, where R(L-X) is the resistance of one phase from phase a' and b' to the fault point. After measuring R1 and R2, short-circuit b' and C' according to the circuit shown in Figure 3, and measure the DC resistance between cores b and c. Half of this resistance value is the resistance of each phase core, represented by RL. RL = RX + R(L-X), from which the contact resistance at the fault point can be obtained: R = R1 + R2 - 2RL. Therefore, the resistance values of the cores on both sides of the fault point can be expressed as: RX = (R1 - R)/2, R(L-X) = (R2 - R)/2. After determining the three values RX, R(L-X), and RL, the distance X or (L-X) from the fault point to the cable end can be calculated using the proportional formula: X = (RX/RL)L, (L-X) = (R(L-X)/RL)L, where L is the total length of the cable. When using the bridge method, measurement accuracy should be ensured. The bridge connection wires should be as short as possible, with sufficient cross-section, and connections to the cable cores should be made by crimping or soldering. All decimal places should be retained during calculations.

   3. Capacitance Current Measurement Method: During cable operation, there is capacitance between cores and between cores and ground. This capacitance is uniformly distributed, and the capacitance value is linearly proportional to the cable length. The capacitance current measurement method is based on this principle and is very accurate for determining open circuit faults in cable cores. The measurement circuit is shown in Figure 4, using a 1-2kVA single-phase voltage regulator, a 0-30V, 0.5-class AC voltmeter, and a 0-100mA, 0.5-class AC milliammeter.

    Measurement steps:

  (1) First, measure the capacitance current of each core at the head end of the cable (keeping the applied voltage equal) to obtain the values of Ia, Ib, and Ic.

  (2) Then, at the end of the cable, measure the capacitance current of each phase core to obtain the values of Ia', Ib', and Ic', to compare the ratio of intact cores to broken cores, and preliminarily determine the approximate distance to the break point.

  (3) According to the capacitance calculation formula C = 1/(2πfU), when voltage U and frequency f are constant, C is proportional to I; since the frequency f of power frequency voltage is constant, as long as the applied voltage is kept constant during measurement, the ratio of capacitance currents is the ratio of capacitances. Assuming the total cable length is L and the distance to the core break point is x, then Ia/Ic = L/x, so x = (Ic/Ia)L. During the measurement, as long as the voltage is kept constant, the ammeter reading is accurate, and the total cable length is measured precisely, the measurement error is relatively small.

   4. Zero Potential Method: The zero potential method, also known as the potential comparison method, is suitable for ground faults in relatively short cable cores. This method is simple and accurate, requiring no precision instruments or complex calculations. The connection is shown in Figure 5. The measurement principle is as follows: Connect the faulty cable core in parallel with a comparison wire of equal length. When voltage E is applied at both ends, it is equivalent to connecting a power source across two parallel uniform resistance wires. At this time, the potential difference between any point on one resistance wire and the corresponding point on the other resistance wire must be zero. Conversely, two points with zero potential difference must be corresponding points. Since the negative terminal of the microvoltmeter is grounded and is at the same potential as the cable fault point, when the positive terminal of the microvoltmeter moves on the comparison wire and the reading is zero, that point is at the same potential as the fault point, i.e., the corresponding point of the fault point. In Figure 5, K is a single-phase knife switch, E is a 6V battery or four No. 1 dry cells, and G is a DC microvoltmeter. The measurement steps are as follows:

  (1) First, connect battery E to cores b and c, then lay a comparison wire S on the ground with a length equal to the faulty cable. This wire should be bare copper or bare aluminum wire with equal cross-section and no intermediate joints.

  (2) Connect the negative terminal of the microvoltmeter to ground, and the positive terminal to a long flexible wire, the other end of which should be able to make full contact when sliding on the laid comparison wire.

  (3) Close the knife switch K, and slide the free end of the flexible wire on the comparison wire. When the microvoltmeter indicates zero, that position is the location of the cable fault point.