In a dead tank type circuit breaker, the switching device (consisting of interrupters) is situated within a metallic vessel filled with insulating medium and supported by suitable insulators. This vessel is at ground potential. In contrast, in a live tank circuit breaker, the switching device is positioned on insulated bushings at the system potential. Live tank circuit breakers are more economical and require less mounting space.
As mentioned previously, there are primarily three types of circuit breakers used in high voltage switchgear systems: air blast circuit breakers, SF6 circuit breakers, and oil circuit breakers. Vacuum circuit breakers are rarely used in comparison.
Air Blast Circuit Breaker
In this design, a blast of high-pressure compressed air is employed to extinguish the arc between two separating contacts precisely when the ionization of the arc column is minimal, typically occurring at current zero crossings.
Oil Circuit Breaker
This classification further divides into bulk oil circuit breaker (BOCB) and minimum oil circuit breaker (MOCB). In BOCB, the interrupting unit is housed within an oil tank at earth potential. Here, oil serves as both the insulating and interrupting medium. In contrast, MOCB minimizes the need for insulating oil by placing the interrupting units in an insulated chamber at live potential on an insulator column.
SF6 Circuit Breaker
SF6 gas is widely utilized as an arc quenching medium in high voltage applications due to its highly electronegative nature and excellent dielectric and arc quenching properties. These characteristics enable the design of high voltage circuit breakers with smaller dimensions and shorter contact gaps. Moreover, its superior insulating ability facilitates the construction of indoor type switchgear for high voltage systems.
Vacuum Circuit Breaker
In a vacuum, no additional ionization occurs between two separated current-carrying contacts after the current reaches zero. The initial arc caused by it will extinguish as soon as the next zero crossing is reached. Since there is no provision for further ionization once the current passes its first zero, arc quenching is completed. Although the arc quenching process is very rapid in vacuum circuit breakers (VCBs), they are not considered a suitable solution for high voltage switchgear because VCBs designed for very high voltage levels are not economical.
Essential Features of High Voltage Circuit Breaker
High voltage circuit breakers, used in high voltage switchgear, must possess essential features to ensure safe and reliable operation. These breakers must be capable of safely operating under the following conditions:
1. Terminal faults.
2. Short line faults.
3. Magnetizing current of transformers or reactors.
4. Energizing long transmission lines.
5. Charging capacitor banks.
6. Switching out-of-phase sequences.
Terminal Fault
Typically, the load connected to the power system is inductive, which can lead to high restriking voltage when a short circuit current is interrupted by a circuit breaker. This voltage comprises two parts:
- Transient recovery voltage with high-frequency oscillation immediately after the arc extinction.
- After the high-frequency oscillation dies down, power frequency recovery voltage appears across the circuit breaker contacts.
Transient Recovery Voltage
Immediately after the arc extinction, transient recovery voltage appears across the circuit breaker (CB) contacts, characterized by high frequency. This transient recovery voltage gradually approaches the open circuit voltage.
The frequency of oscillation is determined by the circuit parameters L and C, where L represents inductance and C represents capacitance. The presence of resistance in the power circuit helps dampen this transient voltage. The transient recovery voltage consists of multiple frequencies rather than a single frequency, reflecting the complexity of the power network.
Power Frequency Recovery Voltage
After the transient recovery voltage dissipates, the open circuit voltage emerges across the circuit breaker (CB) contacts. In a three-phase system, the power frequency recovery voltage varies among different phases, with the highest voltage observed in the first phase. If the network neutral is not earthed, the voltage across the first pole to be cleared is 1.5U, where U represents the phase voltage. In an earthed neutral system, it will be 1.3U. Damping resistors can be utilized to limit the magnitude and rate of rise of the transient recovery voltage.
The dielectric recovery of the arc quenching medium and the rate of rise of the transient recovery voltage significantly impact the performance of circuit breakers used in high voltage switchgear systems. In an air blast circuit breaker (ABCB), the de-ionization of ionized air occurs slowly, resulting in a prolonged recovery of dielectric strength. Therefore, using a low-value breaker resistor can slow down the rate of rise of the recovery voltage. Conversely, SF6 circuit breakers are less sensitive to the initial recovery voltage due to the high arc voltage in SF6, resulting in a faster recovery of dielectric strength compared to air. The lower arc voltage in SF6 circuit breakers makes them more sensitive to the initial recovery voltage.
In oil circuit breakers (OCBs), pressurized hydrogen gas generated during the recombination of oil due to arc temperature provides rapid recovery of dielectric strength immediately after current zero. Hence, OCBs are more sensitive to the rate of rise of recovery voltage and the initial transient recovery voltage.
Short Line Fault
A short line fault in a transmission network is defined as a short circuit fault occurring within a distance of 5 km from the line length. During such faults, a double frequency is imposed on the circuit breaker, and both the source and line side transient recovery voltages start from their instantaneous values prior to interruption. On the supply side, the voltage oscillates at the supply frequency and eventually approaches the open circuit voltage. On the line side, after interruption, trapped charges initiate traveling waves through the transmission line. Since there is no driving voltage on the driving side, the voltage ultimately diminishes to zero due to line losses.
0 Comments