Explosion site 1: Tantalum capacitor
reverse connection Explosion site 2: Aluminum electrolytic capacitor
, super-voltage explosion site 3: Super capacitor short-circuit explosion site 4: High-voltage discharge component design notes: The cause of the capacitor explosion and the treatment method: once switched at 400V The room uses the PGJ1-5 reactive power compensation screen, which is equipped with 10 BCMJ shunt capacitors, each with a rated output of 16kVar, a rated voltage of 0.4kV, a rated current of 25A, and a temperature category of -25°C/45°C connection. The causes of these two accidents were carefully analyzed and thoroughly dealt with. Cause analysis and treatment 1. The reactive power compensation screen is installed in the 400V switch room with high ambient temperature. There are 8 switch cabinets in the room, and the area is only 30m2. The opposite is the SZ7-800kVA 35kV/0.4 transformer room, which has poor overall ventilation. In the hot summer, the indoor temperature of the switch is as high as 48°C. It can be seen that the high ambient temperature is one of the reasons that cause the capacitor to explode. The compensation screen should be moved to a single ventilated control room, and a wax sheet (temperature indicator sheet) should be affixed to the capacitor shell, which can indirectly monitor the temperature of the capacitor medium from the displayed temperature. 2, the voltage is extremely unstable we can see from the formula QCu003d2πfCV2: the reactive capacity of the capacitor is proportional to the square of the voltage. When the voltage drops, the reactive capacity of the capacitor will decrease in proportion to the square of the voltage, that is, the capacity of the capacitor cannot be fully utilized. When the operating voltage increases, the temperature rise of the capacitor will increase, and even the thermal balance of the capacitor will be destroyed and the capacitor will explode. Therefore, the national standard stipulates that the capacitor is allowed to operate at 1.1 times the rated voltage for a long time, but the operation time at 1.15 times the rated voltage within every 24h shall not exceed 30 minutes. The voltage of 400V is extremely unstable, and the voltage fluctuation range is 0.9Ue～1.15Ue (Ue is the rated voltage of 400V). The power consumption in the valley period is often about 450V, and the running time is up to 7h. This is the second reason for the explosion and burnout of our factory’s capacitors. . Because the SZ7-800kVA power transformer is an on-load tap-regulating transformer, to solve this problem, only an on-load tap-regulating controller needs to be installed, which can always control the voltage to the rated voltage. 3, the existence of harmonic current uses a high-power silicon controlled rectifier as the direct current power supply of the rotary kiln and runs in parallel with the compensation screen. As the thyristor device connected to the power grid operates, it objectively acts as a high-order harmonic generator, which will cause the waveform distortion of the circuit voltage and current. The presence of harmonic currents often causes abnormal noises in the capacitors, which may cause the capacitors to swell in severe cases, which is the third reason for the explosion of the capacitors. The main reasons for this situation are: (1) The high-order harmonic current is superimposed on the fundamental wave current, which increases the total current of the capacitor; (2) A certain high-order harmonic is caused between the inductance of the system and the capacitive reactance of the capacitor. Parallel resonance doubles the current flowing into the capacitor; (3) A local series resonance occurs inside the capacitor to a certain high-order harmonic, which causes an overload. Treatment measures In order to prevent these situations from happening, an air-core reactor can be connected in series in each phase of the compensation capacitor bank to limit the current. The combined reactance of the capacitor circuit becomes inductive reactance for higher harmonics. Among the higher harmonics, the 3rd harmonic is short-circuited by the delta connection of the transformer, so this is a measure for the 5th or higher harmonic. If the reactance of the series reactor is selected to make the 5th harmonic resonate, the 5th harmonic will be short-circuited. For the higher harmonics above the 5th harmonic, the capacitor circuit becomes inductive, so the waveform is improved, thereby fundamentally The above eliminates the possibility of resonance. The reactance of the anti-resonance series air-core reactor can be calculated: is XL》4%XC where: L——inductance of the series reactor, H; C——compensation capacitor capacity, F; The inductive reactance, Ω; XC-the capacitive reactance of the compensation capacitor, Ω. It can be seen that the reactance of the series reactor is about 4% or more of the capacitive reactance. Considering that the system frequency is low and the capacity of the capacitor is reduced in the event of an accident, the inductive reactance is actually 5% to 6%XC. After doing the above analysis and processing, the reactive power compensation screen has achieved the effect of safe and energy-saving operation.
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