Causes, Characteristics and Limiting Methods of Capacitor Switching Inrush Current

I. Causes of Capacitor Switching Inrush Current

The core cause of capacitor switching inrush current is that a capacitor is an energy storage component, and the voltage across its terminals cannot change abruptly. At the moment of switching on, it is equivalent to a "capacitive load being suddenly connected to the circuit" with extremely low loop impedance, resulting in an instantaneous large current impact. The specific conditions are detailed as follows:

 

1. First Switching (Uncharged State): At the moment of the first switching operation of the capacitor, its plates are in an uncharged state, and the initial voltage across the terminals is 0. After switching on at this time, the system voltage is instantly applied to both ends of the capacitor. Since the capacitor voltage cannot change abruptly, an instantaneous charging current is generated in the circuit. This current is only limited by the total loop impedance (including line inductance, transformer leakage inductance, and the capacitor's own equivalent series resistance ESR). Because the equivalent loop impedance is extremely small (close to a short-circuit state) at the moment of switching on, a large inrush current is generated. The maximum inrush current usually occurs at the moment of switching on (related to the phase of the system voltage; when the system voltage is near the peak value during switching on, the inrush current amplitude is relatively larger).

 

2. Re-switching Without Sufficient Discharge (Charged Switching): If the capacitor is re-switched on without sufficient discharge after being cut off and taken out of operation, the amplitude of the switching inrush current can reach twice that of the first switching (uncharged state). The core reason is that after the capacitor is cut off, residual charges remain on the plates (if not discharged, the residual voltage is close to the rated phase voltage of the system). When re-switching on, if the system voltage and the residual voltage of the capacitor are exactly in an opposite phase state of "equal magnitude and opposite direction", the instantaneous potential difference across the capacitor terminals is twice the rated phase voltage, leading to a sharp increase in charging current and thus a larger switching inrush current. Therefore, to avoid large inrush current damage to equipment caused by charged switching, the capacitor must be fully discharged (usually discharged to below the safe voltage through a discharge resistor) before being re-switched on after being cut off and taken out of operation.

 

II. Related Characteristics of Capacitor Switching Inrush Current

The amplitude multiple of the capacitor switching inrush current is mainly related to two factors: first, the capacity of the connected capacitor (the larger the capacity, the larger the inrush current amplitude usually); second, the short-circuit capacity of the installation site (the larger the system short-circuit capacity, the smaller the equivalent loop inductance, the larger the inrush current amplitude and the higher the oscillation frequency). If the capacitor is installed in a circuit with a large short-circuit capacity, the loop inductance is small and the impedance is low, so the inrush current not only has a large amplitude but also a significantly higher oscillation frequency.

 

Measured data show that in a conventional power system, the amplitude of the capacitor switching inrush current is usually 5~15 times the rated current of the capacitor (the specific value is affected by loop parameters; for example, the configuration of a series reactor can significantly reduce the inrush current amplitude); the oscillation frequency of the inrush current is generally 250~400Hz (belonging to high-frequency inrush current); at the same time, an instantaneous overvoltage is accompanied during the switching process, and its amplitude is about 2~3 times the system phase voltage (this overvoltage is a transient overvoltage with an extremely short duration, but it may still impact the capacitor insulation).

Supplementary Professional Explanation: In practical engineering, to suppress the switching inrush current, a small reactor (inrush current suppression reactor) is usually connected in series in the capacitor loop, which can limit the inrush current amplitude to 2~5 times the rated current, and at the same time reduce the transient overvoltage amplitude to protect the capacitor and loop equipment.

 

III. Methods to Limit Capacitor Switching Inrush Current

The operating current of the capacitor is not only related to the power supply fundamental voltage but also positively related to the voltage frequency (according to the capacitive reactance formula X_C = 1/2πfC): when the power supply voltage waveform is distorted and high-order harmonic voltage is applied to the capacitor, since the harmonic frequency is much higher than the fundamental frequency, the capacitive reactance of the capacitor will be significantly reduced, leading to a substantial increase in the harmonic current passing through the capacitor; at the same time, the capacitor current is also positively related to the capacitance. The larger the capacitance, the smaller the capacitive reactance, and the larger the passing current (including fundamental current and harmonic current), which further aggravates the voltage waveform distortion and forms a vicious circle.

 

1.To effectively limit the capacitor switching inrush current and suppress the impact of high-order harmonics in the power grid on the capacitor, connecting a reactor in series in the capacitor loop is the most commonly used and effective method in engineering. Its core principle is: Jinneng electric Iron-Core harmonic filter reactors increase the total inductive reactance of the circuit, form series resonance with the capacitive reactance of the capacitor (for specific harmonics), which can not only reduce the discharge current when the capacitor is switched on but also suppress the intrusion of harmonic current. If the parameters of the series reactor are properly selected, the switching inrush current can be stably limited within the allowable range of the equipment.

When the series reactor is only used to limit the switching inrush current, its capacity (reactance rate) should be selected as a small value, usually 0.2%~1% of the rated capacity of the capacitor (reactance rate 0.2%~1%). Engineering operation and test data show that when a reactor with a series reactance rate of 0.2% is connected, the capacitor switching inrush current can be significantly suppressed to meet the operation requirements of conventional equipment; if harmonic suppression (such as suppressing 3rd and 5th harmonics) needs to be considered at the same time, the reactance rate can be appropriately increased (such as 3%, 5%).

 

2.In addition to the series reactor, the inrush current-free capacitor switching switch (such as thyristor switch) is also a commonly used inrush current suppression device. This type of switch is mainly composed of a bidirectional thyristor, trigger circuit, absorption circuit, protection circuit, and intelligent heat sink. Relying on the independently patented zero-voltage switching-on and zero-current switching-off technology, it can realize inrush current-free and impact-free switching of capacitors, with fast response speed and an average response time of less than 10ms. It can effectively compensate for the reactive power demand of impact loads, and can well replace the traditional contactor switching device, avoiding the arc impact and inrush current problem during contactor switching.