This zero current crossing method is only applicable to alternating current circuit breaker. We all know that an alternating current naturally passes through zero at regular intervals (every half-cycle of the AC waveform). This method safely interrupts the alternating current at the natural zero-crossing of its waveform.
Working Principle of a CB based on Zero Current Crossing
The alternating current follows a sinusoidal waveform, which crosses zero twice per cycle. When a circuit breaker opens at a non-zero current, the energy stored in the inductance and capacitance of the system can cause high-voltage transients, leading to arc re-ignition.
In other words, during current interruption at a non-zero crossing point in the waveform, there is a sudden drop in current, which can cause high-voltage transients. The Zero Current Method ensures that the circuit breaker operates only at the zero-current crossing, where the stored energy in the system is at its lowest.
By interrupting the current at the zero-crossing, the arc formed between the breaker contacts is easily extinguished, preventing excessive overvoltage transients. This method avoids current chopping by allowing the current to naturally reach zero at the zero-crossing point of the waveform. Afterward, the dielectric strength between the separated breaker contacts is rapidly increased to prevent the reestablishment of current after zero-crossing.
Steps in the Zero Current Method
After the fixed and moving contacts separate, the arc is naturally extinguished near the instant when the current crosses zero. At that moment, due to the high voltage stress between the contacts, a significant number of electrons and ions remain in the gap. As mentioned earlier, the arc is not extinguished exactly at the zero-crossing point but rather near it. Consequently, there is a sudden drop in current when the arc is extinguished. This abrupt change in current causes a voltage surge between the contacts, known as restriking voltage. Due to this restriking voltage, the arc can reignite, allowing the current to continue flowing. If such a breakdown occurs, the arc will persist for another half-cycle, and the process will repeat. Therefore, to ensure the final extinguishing of the arc, it is essential to remove the residual ions and electrons as soon as the current crosses zero.
The removal of electrons and ions occurs in two ways:
(1) through the recombination of electrons and ions and
(2) by sweeping them away from the contact gap.
Ideally, the current should be interrupted exactly at the zero-crossing point. After zero crossing, a rapid increase in dielectric strength is required to withstand the voltage between the contacts. Therefore, during on-load opening, the circuit breaker contacts first establish an arc, which is then extinguished at a suitable subsequent zero-crossing point. The recombination of electrons and ions is accelerated by cooling the contact gap and increasing the pressure surrounding the arc.
SF₆ Circuit Breakers: The Industry Standard
Sulfur hexafluoride (SF₆) gas can very efficiently quench arcs due to its high dielectric strength and thermal conductivity. Also, SF6 gas ions deionized very fast and absorb electrons from the arcing medium. In puffer-type breakers, compressed SF₆ blasts the arc, cooling and deionizing it. However, SF₆ is a potent greenhouse gas, driving research into alternatives like CO₂ or synthetic gases.
Vacuum Circuit Breakers: Eco-Friendly Innovators
A truly perfect vacuum doesn’t exist; it’s always just near perfect. In these near-perfect vacuums, electrical arcs can vaporize the contact material. This happens because the boiling point of the metal drops significantly under low pressure. However, when the current reaches zero, the arc loses its energy source, causing the metal vapor to instantly condense back into a solid. After this point, there’s often no remaining vapor in the contact gap to sustain another arc in the next cycle. Since the contacts are already far enough apart to prevent restriking in the initial cycles, the current is successfully interrupted. These breakers are compact, durable, and well-suited for medium-voltage applications. As a result, they’re becoming increasingly popular as a sustainable alternative.
More Close Look in The Current Zero Method
Due to the accelerated cooling method applied during contact separation, the heat energy dissipated from the arc exceeds the energy it receives from the system. As a result, the current through the arc drops to zero slightly before its natural zero crossing. Because of the inductances (L) present in the power network, the current cannot vanish instantly. Instead, it is diverted through the parallel capacitive path (C) associated with the circuit breaker. Notably, there is no physical capacitor connected across the circuit breaker; rather, this is due to the natural capacitive behavior inherent in all electrical circuits. The reactive circuit associated with the circuit breaker increases the voltage across the separated contacts. At the moment of current interruption, an oscillatory circuit is formed by the inductance and capacitance of the system, and the frequency of this oscillation is given by $$ f = \frac{1}{2\pi\sqrt{LC}} $$
The voltage appearing across the contacts has this frequency. The voltage across the contact gap increases along with the dielectric strength. The dielectric strength increases due to the quenching medium of the circuit breaker. If the rate of rise of dielectric strength between the gap is slower than the rate of rise of restriking voltage, the reignition of the arc occurs, and the arc cannot be extinguished until the next passage of current through zero. If the rate of rise of dielectric strength of the medium increases at least at the same rate as the voltage build-up due to current interruption, the restriking of the arc does not take place again, and the circuit breaking process gets completed. The voltage appearing across the open contacts, also known as restriking voltage, settles down after a few superimposed oscillations have died away. In an AC circuit breaker, the main feature is its ability to establish dielectric strength between open contacts faster than the rate of rise of the restriking voltage.