Causes and Countermeasures of overheating of the h

2022-08-21
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Reasons and Countermeasures for overheating of primary electrical equipment

the overheating fault of primary electrical equipment is analyzed, and it is pointed out that poor contact and overload of contact surface are the main reasons for overheating of electrical equipment. The preventive measures proposed are: finding overheated defects or hidden dangers in time during spot inspection; Regularly repair and test the conductive contact surface according to the standard; Conduct thermal imager test on electrical equipment regularly; Handle as soon as possible in case of abnormality or excessive direct resistance. Key words: electrical equipment overheating

direct resistance exceeds the standard point inspection and periodic maintenance. The main functions of electrical equipment are used to transmit, distribute and convert electric energy. The realization of these functions is ultimately completed through the flow of current. Using the characteristic of current heating through conductors, many electrical products have served human beings, such as electric cookers, heaters, etc; However, if the inherent characteristic of resistance is not supervised and limited, it will endanger the safe operation of electrical equipment, lead to equipment damage or power failure, and even lead to major fire accidents

1 cause analysis

common metal conductors include copper, aluminum, tin, silver, steel, etc. Since any metal conductor has a certain resistance, its resistance is related to its own resistivity and average temperature coefficient, and has a corresponding melting point. For pure resistance equipment of electrical connector, according to R= ρ× L/s2h and q=i2 × R × t. It can calculate the resistance of the conductor and the heating amount when the current flows through the conductor; By formula q=i2 × R × T it can be seen that when the contact resistance of the electrical connector increases due to some factors, such as poor contact surface condition, serious oxidation, small contact pressure, reduced effective contact area, or when the current increases, its calorific value (temperature) will increase correspondingly, and the resistance will increase correspondingly due to thermal effect; The increase of resistance will increase the temperature. In such a vicious cycle, the temperature of the contact surface will rise above its melting point and melt, so that the joint temperature will melt beyond the melting point temperature; When there is a short circuit in the system, with the sharp increase of short-circuit current, the joint is most likely to melt or fuse due to overheating, which will expand into fire accidents and insulation damage accidents at the same time

case 1: shortly after the overhaul of a unit in a factory, the 6kV auxiliary section was short circuited between phases due to the damage of the cable insulation of a motor. The strong short-circuit current caused the current transformer connector with large contact resistance in the high-voltage distribution cabinet to melt and splash, resulting in the breakdown of phase to ground and phase to phase insulation. Then the Cainiao network with the largest business volume in the express industry caused the short circuit of the arithmetic mean value of 2 points in the high-voltage distribution cabinet, As a result, the incoming switch of 6kV auxiliary section fell off, and the bus was powered off and shut down. After the accident, the inspection found that the diameter of the screw hole on the contact surface of the current transformer terminal was expanded by 6mm due to overheating and melting through the short-circuit current, and the thickness of the terminal was reduced by 3mm. The oxygen index value of the contact surface in contact with the new national standard flame retardant class B1 must be oi ≥ 30%. The reason for the increase in resistance is that because the material of the conductive connecting plate in the distribution cabinet is copper and the terminal of the current transformer is aluminum, effective copper aluminum transition measures were not taken on the copper aluminum contact surface during installation, and electrochemical corrosion occurred on the contact surface due to the difference in chemical properties between the two metals, A gray corrosion layer is formed on the contact surface, causing the contact resistance of the contact surface to increase. After reprocessing the copper aluminum transition joint, the switchgear operates normally

case 2: a 380V incoming switch in a factory, due to the insufficient spring force of the contact spring, the contact pressure was insufficient, the contact resistance increased, overheated in operation, damaged the insulation, and short circuit and fire occurred, which led to the "fire pool" accident

case 3: for a 220kV on load voltage regulating transformer, the gas relay of the tap changer acted for many times, and it was found that the contact of the tap changer was seriously oxidized and the contact resistance exceeded the standard; The contact returns to normal after silver plating

Case 4: the high voltage bushing of a 500kV transformer in a factory was damaged in the partial discharge test because the terminal block bolts at the bottom of the bushing were not tightened during installation. During the minor repair of Datang Pandian #4 unit, it was found that a wiring board of phase B of the enclosed bus at the low voltage side of #4 high voltage station transformer should be fastened with four bolts, but only one bolt was fastened. Due to the contact pressure and effective contact ● the area of the exchange servo speed regulation system was reduced, and the contact surface had changed color. Fortunately, no accident was caused

case 5: a 220kV switch in a substation was found to be seriously overheated during maintenance. The reason was that the overtravel of the conductive rod was insufficient, and the effective contact area was reduced, resulting in the increase of contact resistance

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