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

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1. Prevent inrush currents. When a capacitor is energized, it often experiences a large inrush current. According to the IEC 831 standard, the formula for calculating this inrush current is: Is = In × √2S/Q. - Is: Inrush current when the capacitor is turned on (in Amperes) - In: Rated current of the capacitor (in Amperes) - S: Short-circuit power (in MVA) at the installation point - Q: Capacitor's reactive power (in Mvar) To reduce this inrush current in low-voltage circuits, you can use one or more of the following methods: - Install a series reactor to limit the current. - Increase the switching capacity of the capacitor. - Use a dedicated contactor designed for capacitors. 2. Prevent harmonic effects. Since a capacitor forms an LC circuit, it may resonate with certain harmonics in the grid, leading to harmonic amplification and increased voltage and current. To avoid this, a series reactor with a specific inductance value is typically used. The reactor’s percentage (K) depends on the harmonic content: - If the 5th harmonic is high and the 3rd is not, K should be around 4.5%. - If the 3rd harmonic is dominant, K should be set to 12%. - If harmonics are minimal, a 0.5% reactor is sufficient. 3. Prevent self-excitation. When a capacitor is used to compensate for motor reactive power, it is connected directly in parallel. After the power is cut off, the motor may continue running due to inertia. At this point, the capacitor discharges and acts as an excitation source. If the compensation capacity is too large, it can cause the motor’s magnetic field to self-excite, potentially turning the motor into a generator. To prevent this, the compensation capacity should be less than the motor’s no-load capacity. It is generally recommended to keep the compensation at about 0.9 times the no-load reactive power. The formula for the required compensation is: QC = 0.9 × 3UI₀ - QC: Required capacitor reactive power - U: System voltage - I₀: Motor no-load current These measures ensure safe and efficient operation of capacitors in various electrical systems.