Safety problems of the most thermal power vessel g

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Safety problems of capacitor bank used in power compensator

capacitor (bank) is a common equipment used to compensate inductive reactive power and improve electric power factor. This paper expounds the safety problems of using capacitor banks in three aspects

1. Prevent resonance overvoltage

the capacitor bank is often connected to the bus thread of the substation for reactive power compensation. When the bus is connected with silicon rectifier and other harmonic sources, 4) multiple force sensors and deformation sensors can be configured for standby, it is possible to divide into three areas: fatigue source (the core source of fatigue cracks); The fatigue crack expansion area (the shell shape with concentric arc is left in the expansion process) generates harmonic overvoltage, because the circuit at this time is equivalent to the R-L-C series circuit, and its natural frequency fo=1/2 л LC, if the frequency Fo of a certain harmonic in the electric voltage is equal to or close to fo, resonance will occur under the action of this harmonic voltage. At this time, the Nth harmonic voltage at both ends of the capacitor bank:

because the capacitance is relatively large, fo is not high, which is likely to be the main higher harmonic with the 5th and 7th harmonics. Therefore, high overvoltage may occur in electricity. The capacitor bank may be damaged due to overvoltage, and the electricity cannot work normally

the measure to limit harmonic overvoltage is to string a reactor L in the capacitor circuit, as shown in the attached figure, so as to eliminate the possibility of resonance under the possible harmonic conditions. The inductive reactance value of reactor can be calculated according to the following formula:

where k - reliability coefficient, generally 1.25~1.4

xc -- power frequency capacitive reactance of compensation capacitor bank

n -- the lowest harmonic that may be generated

for example, in order to limit the 5th harmonic,

2 Prevent capacitor explosion

because the power loss and calorific value of the capacitor are directly proportional to the square of the voltage, such as high electric voltage, high environmental humidity and difficult heat dissipation, under the action of high temperature and high electric field strength for a long time, the insulation accelerates aging, which will lead to the breakdown of capacitor components. After the breakdown, not only the breakdown phase current increases, but also other capacitors in parallel discharge to the breakdown capacitor, causing the capacitor to generate intense heat, so that the insulating oil decomposes and produces a large amount of gas, resulting in the explosion of the box shell and porcelain conduit

in addition, harmonic overvoltage and switching overvoltage can directly cause capacitor explosion

the measures to prevent capacitor explosion are as follows:

(1) configure fuses to protect capacitors. For a single capacitor, the rated current of the fuse can be selected as 1.5~2.5 times of the rated current of the capacitor; For capacitor banks (generally not more than 4), the rated current of fuse can be selected as 1.3~1.8 times of the rated current of this group of capacitors

(2) strengthen patrol inspection. The high-voltage capacitor is equipped with an ammeter for monitoring. If the three-phase current is found to be seriously unbalanced or more than 1.3 times the rated current, it should be out of operation, find out the cause and deal with it properly. During the inspection, if the capacitor is found to have discharge sound, serious oil leakage, rusted shell, bulge or serious heating, it should be out of operation in time for maintenance and treatment. The actual performance of the experimental machine is usually measured according to the most basic physical characteristics

(3) reasonable operation. Before the capacitor bank is reclosed every time, it must be discharged for 3min. It is forbidden to switch on the capacitor with charge to avoid damage to the capacitor by operating overvoltage. In order to ensure the normal discharge of the capacitor, check whether the discharge device of the capacitor is in good condition every month. When high-power silicon rectifier equipment and high-capacity electric machine suddenly throw off the load or repeatedly impact the load, the voltage of the capacitor will rise. At this time, the capacitor should also be out of operation. Cuilixin, chairman of Shandong innovation group: build the "small workshop" into a domestic large-scale aluminum alloy material production base to avoid resonance overvoltage

3. Prevent electric shock when detecting capacitors

some workers believe that capacitors are equipped with discharge devices, so manual discharge is not required during maintenance. In fact, this idea is wrong. Although the charge stored in the capacitor is discharged by the discharge device, there may still be residual charge (especially in the capacitor), so no matter what kind of discharge device is installed, it must be repaired after manual discharge

the method of artificial discharge is: first fix the grounding terminal of the grounding wire, and then discharge the capacitor several times with the grounding rod until there is no discharge spark and discharge sound. If it is to repair the internal fault of the capacitor, although there may be residual charge at the fault part through manual discharge, at this time, wear insulating gloves and short circuit the two poles of the fault capacitor for discharge before maintenance

(yuan Shiqi)

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