Arc Interruption Theory

Arc Interruption Theory

 

The insulating material used in a circuit breaker serves two critical functions:

1.  Providing ample insulation between the contacts when the circuit breaker opens.
2.  Extinguishing the arc that forms between the contacts when the circuit breaker opens.

To elaborate on the second point, let's consider a scenario where a fault or short circuit occurs in the system. In response, the relay sends signals to the circuit breaker to open its contacts and prevent the fault from continuing. However, when the circuit breaker opens, an arc forms between the contacts. This arc needs to be safely extinguished to prevent further damage or hazards. The insulating material and specific techniques employed within the circuit breaker are designed to interrupt and extinguish this arc effectively.

Methods of Arc Interruption

There are two methods by which interruption is done.

1.     High resistance method,
2.     Low resistance method or current zero interruption method.

The high interruption method involves significantly increasing the electrical resistance to force the current to zero, thereby preventing the re-ignition of the arc. However, it is crucial to carefully manage the rate of resistance change to avoid generating harmful induced voltages in the system. Various techniques, such as arc lengthening or cooling, can be employed to elevate arc resistance.

Proper precautions must be taken to ensure that the rate of resistance increase or decrease remains within acceptable limits to mitigate the risk of inducing harmful voltages in the system. Increasing arc resistance through methods like arc lengthening or cooling can effectively suppress the possibility of arc re-ignition and enhance the interruption process in circuit breakers.

Limitations of High Resistance Method

Arc discharge exhibits a resistive nature, causing most of the energy to be absorbed by the circuit breaker itself. Therefore, careful attention must be paid to the manufacturing of circuit breakers, ensuring adequate mechanical strength and durability. This method is commonly applied in DC power circuit breakers, as well as in low and medium AC power circuit breakers.

In contrast, the low resistance method is specific to AC circuits, facilitated by the natural zero of current present in the alternating current waveform. The arc is extinguished at this natural zero point, and its re-ignition is prevented by the rapid buildup of dielectric strength within the contact space.

There are two theories which explain the phenomenon of arc extinction:

1.     Energy balance theory,
2.     Voltage race theory.

Before going in details about these theories, we should know the following terms.

Restriking Voltage

Restriking Voltage refers to the voltage appearing across the breaking contact at the moment of arc extinction.

Recovery Voltage

Recovery Voltage is the voltage present across the breaker contact after the transient oscillations have ceased and the arc has been completely extinguished across all poles.

Active Recovery Voltage

Active Recovery Voltage represents the instantaneous recovery voltage precisely at the moment of arc extinction.

Arc Voltage

Arc Voltage is the voltage observed across the contact during the arcing period when the current flows as an arc. Generally, it remains low except at the point where it rapidly rises to a peak value as the current reaches zero.

Energy Balance Theory

When the contacts of a circuit breaker are on the verge of opening, the restriking voltage is zero, resulting in zero heat generation. Similarly, when the contacts are fully open, there is infinite resistance, leading again to no heat production. Consequently, the maximum heat generated lies between these two scenarios and can be approximated. This theory is based on the premise that the rate of heat generation between the contacts of the circuit breaker is lower than the rate at which heat dissipates between the contacts. Therefore, if it's possible to rapidly remove the generated heat by cooling, lengthening, and splitting the arc, the arc can be extinguished.

Voltage Race Theory

The arc forms due to the ionization of the gap between the contacts of the circuit breaker. Consequently, the resistance at the initial stage is very small, i.e., when the contacts are closed. As the contacts separate, the resistance begins to increase. If we remove ions at the initial stage by either recombining them into neutral molecules or inserting insulation at a rate faster than the rate of ionization, the arc can be interrupted. The ionization at zero current depends on the voltage, known as the restriking voltage.

Let us define an expression for restriking voltage. For loss-less or ideal system we have,

Here, v = restriking voltage.

V = value of voltage at the instant of interruption.

L and C are series inductor and shunt capacitance up to fault point.

Thus from above equation we can see that lower the value of product of L and C, higher the value of restriking voltage.

The variation of v versus time is plotted below:

Now, let's examine a practical scenario where there are finite losses in the system. As depicted in the figure below, in such cases, the restriking voltage is damped out due to the presence of some finite resistance. Here, it is assumed that the current lags behind the voltage by an angle (measured in degrees) of 90. However, in practical situations, this angle may vary depending on the time in the cycle at which the fault occurs.

Considering the effect of arc voltage, if the arc voltage is included in the system, there is an increase in the restriking voltage. However, this effect is offset by another effect of the arc voltage, which opposes the current flow and alters the phase of the current. Consequently, the current is not at its peak value when the voltage passes through zero.

Rate of Rise of Restriking Voltage (RRRV)

It is defined as the ratio of the peak value of restriking voltage to the time taken to reach the peak value. It is one of the most important parameters because if the rate at which the dielectric strength developed between the contacts is greater than the RRRV, then the arc will be extinguished.

FAQs

1. What is arc interruption theory?

   - Arc interruption theory explains the methods and mechanisms used to extinguish electrical arcs that occur during the opening of circuit breakers or switches.

 

2. Why is arc interruption important in electrical systems?

   - Arc interruption is crucial in electrical systems to safely and efficiently interrupt the flow of current during faults or switching operations, preventing damage to equipment and ensuring system reliability.

 

3. How do circuit breakers interrupt arcs?

   - Circuit breakers interrupt arcs using various techniques such as high resistance, low resistance, and natural current zero methods, along with techniques to cool, lengthen, or split the arc to facilitate extinction.

 

4. What are the factors influencing arc interruption?

   - Factors influencing arc interruption include the magnitude of the fault current, the type of circuit breaker technology used, the characteristics of the arc, and the design of the interrupting chamber.

 

5. What is restriking voltage?

   - Restriking voltage is the voltage that appears across the breaking contacts at the instant of arc extinction, which can lead to re-ignition of the arc if not properly managed.

 

6. How does resistance affect arc interruption?

   - High resistance methods increase the electrical resistance between the contacts to force the current to zero and restrict arc re-ignition, while low resistance methods utilize the natural zero of the AC waveform to extinguish the arc.

 

7. What is the role of arc voltage in interruption theory?

   - Arc voltage affects interruption by opposing the current flow and altering the phase relationship between current and voltage, influencing the effectiveness of interruption techniques.

 

8. How does the rate of rise of restriking voltage impact arc interruption?

   - The rate of rise of restriking voltage (RRRV) is a critical parameter that determines the ability of the circuit breaker to extinguish the arc. If the dielectric strength develops faster than the RRRV, the arc can be successfully interrupted.

 

9. What are some practical considerations in arc interruption?

   - Practical considerations include the design of circuit breakers, the selection of interruption methods based on system requirements, and the management of factors such as ionization and heat generation during interruption.

 

10. How does arc interruption theory contribute to electrical safety?

    - By understanding arc interruption theory, engineers can design and operate electrical systems with effective protection mechanisms, minimizing the risk of electrical faults, and ensuring the safety of personnel and equipment.