Application, principle and instructions of phase-advancing capacitor

1.Switching On and Off Capacitors

When switching capacitors on or off, abnormal voltages may occur, potentially damaging the capacitors and shortening their lifespan. Therefore, the following precautions should be observed:

Simultaneous Disconnection 

When turning off the mechanical switch connected to a capacitor, always ensure the power supply is also cut off. Leaving the capacitor energized can cause adverse effects on both the capacitor and the power supply side.

Residual Voltage Delay

If a capacitor is disconnected while in use and not fully discharged before being switched on again, abnormal high voltage may result. Therefore, avoid re-energizing within 5 minutes after disconnection (within 3 minutes for low-voltage systems). For automatic power factor regulators (APFRs), automatic reconnection should be avoided within 1 minute for low-voltage systems. If reconnection within a short time is necessary, discharge coils must be installed and connection sequence must be considered. The switching frequency should be limited to no more than 300 times per day. If exceeded, a 0.1% current-limiting coil should be installed. 

Arcing During Switching

Disconnecting a capacitor can produce electric arcs between switch contacts. Continuous arcing can lead to abnormal voltage. Use switches with good insulation and arc suppression, s

Energizing (Switching On)

When energizing the first or only set of capacitors in a circuit, a charging current is generated.   

If other capacitors are already energized in the circuit, inrush current can reach 200–300 time s the rated current.   

After installing a series reactor

Using a 6% series reactor, inrush current can be limited to approximately 5 times the rated current.

Formula Parameters

1.  Im: Inrush current during capacitor energization (A)
2. Ic: Rated current of the capacitor (A)
3. Ss: Short-circuit capacity of the circuit (kVA)
4. Qc: Capacitor capacity (kVAR)
5. V: Line voltage (V)
6. Xl: Line impedance between capacitor banks (Ω)
7. Xc1: Reactance of already-energized capacitor (Ω)
8. Xc2: Reactance of capacitor to be energized (Ω)
9. Xc: Capacitor reactance (Ω)
10. Xsr: Series reactor inductive reactance (Ω) 
    

Disconnection (Switching Off)

Use a switch that can quickly disconnect the capacitor from the power source to prevent re-ignition (restriking). Otherwise, an overvoltage several times the rated voltage may be applied to the capacitor, leading to damage.

Discharging

Before re-energizing or servicing, the residual voltage of a disconnected capacitor must be reduced to below 50V, to avoid overvoltage damage due to residual charge.

Diagnosis in Capacitor Circuits

1. High-Voltage Circuits: Use a harmonic analyzer for measurement. Contact us for assistance if needed.
2. Low-Voltage Circuits: Use a clamp-type ammeter to measure capacitor circuit current or a voltmeter to measure the voltage across the series reactor. Compare values with rated specs to determine harmonic presence.
   

1. Voltage Stress: Harmonics may raise terminal voltage, exceeding acceptable limits and degrading insulation.
2. Current Stress: Harmonics may increase current, exceeding allowable levels, causing thermal damage and insulation failure.
   

Current Suppression (Series Reactors)

If capacitors are not frequently switched and circuit harmonics are within acceptable levels, avoid inserting series reactors. However, due to increasing harmonic problems from industrial semiconductor equipment, installing harmonic filters for both harmonic suppression and power factor correction should be considered.

For capacitors operating under harmonic stress or controlled by automatic power factor regulators, inserting series reactors with proper inductance is essential to:

1. Reduce harmonics
2. Limit inrush currents during capacitor energization

Note: Adding a series reactor reduces the total impedance of the circuit, increasing current and causing a voltage rise at the capacitor terminals. The rated voltage of capacitors must be selected to withstand this rise.

1. Suppress inrush current at the moment of capacitor energization.
2. Prevent arc restriking during disconnection.
3. Suppress harmonic current amplification.
   

1. Carefully evaluate reactor capacity to avoid harmonic resonance or amplification.
2. Capacitor terminal voltage will rise after inserting a reactor—ensure rated specifications are adjusted accordingly.
3. With increasing harmonic sources in the network, install harmonic filters to prevent degradation of power quality.
   
   

1. Residual charge inside capacitors before energization.
2. Inrush current during switching.
3. Switching frequency.
4. Segment capacity ratio (e.g., 1:1:1... for cyclic control).