Capacitor compensation should pay attention to the technical problems - Database & Sql Blog Articles

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1. Prevent inrush currents. When the capacitor is put into operation, in general, with a large inrush current, the calculation formula for the capacitor inrush current in the IEC publication 831 capacitor is Is=In×√2S/Q.
Is---Inrush current when the capacitor is put in (A)
In--- is the rated current of the capacitor (A)
S--- is the short-circuit power (MVA) at the installation capacitor
Q---for capacitor capacity (Mvar)
In the low-voltage capacitor circuit, the following methods can be used: (1) is a series reactor; (2) is to increase the capacity of the switching capacitor; (3) is a contactor that is switched by a dedicated capacitor.
2. Prevent the effects of system harmonics. Since the capacitor loop is an LC circuit, it is easy to generate resonance for some harmonics, causing harmonic amplification, increasing current and increasing voltage. For this purpose, a series reactor with a certain inductance can be used to avoid resonance. For example, the percentage of the reactor is K. When the 5th harmonic in the power grid is higher, and the 3rd harmonic is not too high, K should be 4.5%. If the third harmonic is higher, K should adopt 12%. When the harmonics in the power grid are not high, K should adopt 0.5%.
3. Prevent self-excitation. The capacitor is used to compensate the reactive power of the motor in situ. The capacitor is directly connected in parallel with the motor. After the power is cut off, the motor continues to operate under the inertia. At this time, the discharge current of the capacitor becomes the excitation current. If the capacity of the compensation capacitor is too large, the magnetic field of the motor can be self-excited to generate a voltage, and the motor is operated in a power generation state. Therefore, the compensation capacity is less than the no-load capacity of the motor, and it is generally not necessary to take 0.9 times.
QC=0.9×3UI0
In the formula above:
Qc---To compensate capacitor capacity U--- for system voltage I0--- for motor no-load current