What is automatic reclosing and what are the types of reclosing?

A widely used and effective anti-accident measure on transmission and power supply lines. When a line fault occurs, after the relay protection trips the circuit breaker, the automatic reclosing device recloses the circuit breaker after a short time interval. In most cases, line faults (such as lightning strikes, wind damage, etc.) are temporary. After the circuit breaker trips, the insulation performance of the line (insulators and air gaps) can be restored, and reclosing can be successful, thus improving the reliability of the power supply system. In a few cases, the fault is permanent. After the automatic reclosing device operates, the relay protection trips again to identify the cause and eliminate it before power is restored. Generally, the faster the reclosing after a line fault trips, the better the effect. The minimum allowable interval for reclosing is 0.15 to 0.5 seconds. The higher the rated voltage of the line, the longer the insulation deionization time. The success rate of automatic reclosing varies depending on the line structure, voltage level, meteorological conditions, and the main fault type. According to statistics from the Chinese power sector, it generally reaches 60% to 90%. Another widely used accident prevention measure in the power sector is automatic backup power transfer, which typically takes 0.2 to 0.5 seconds. It requires minimal investment while providing significant economic benefits in maintaining normal power supply.

Automatic reclosing devices are automatic devices that automatically reconnect circuit breakers that have tripped due to faults as needed. Power system operating experience shows that the vast majority of faults on overhead lines are "transient," with permanent faults generally accounting for less than 10%. Therefore, after a short-circuit fault is cleared by relay protection, the arc will automatically extinguish, and in most cases, the insulation at the short-circuit point can automatically recover. Therefore, automatically reclosing circuit breakers not only improves the safety and reliability of power supply and reduces power outage losses, but also improves the transient level of the power system, increases the transmission capacity of high-voltage lines, and can correct false trips caused by circuit breakers or relay protection devices. Therefore, automatic reclosing should be used for overhead lines.

(1) Significantly improving the reliability of power supply and reducing the number of line outages, especially for single-circuit lines with a single power source;

(2) In high-voltage transmission lines, reclosing can also improve the stability of parallel operation of the power system;

(3) In the design and construction of power grids, in some cases, considering the role of reclosing, the construction of double-circuit lines can be postponed to save investment;

(4) It can also correct erroneous tripping caused by circuit breaker malfunctions or relay protection errors.

The economic benefits of reclosing should be measured by the economic losses caused by power outages without reclosing. Because the investment in reclosing devices is very low and their operation is reliable, they have been widely used in power systems. However, everything has two sides. When reclosing is used in conjunction with a permanent fault, it also brings some adverse effects, such as:

(1) subjecting the power system to another fault impact;

making the operating conditions of the circuit breaker more severe, as it must interrupt the short-circuit current twice in a very short time. This situation must be considered for oil circuit breakers, because during the first trip, the oil's insulation strength is reduced due to the arcing effect. The second trip after reclosing occurs under these unfavorable conditions of reduced insulation. Therefore, after adopting reclosing, the breaking capacity of oil circuit breakers will also decrease to varying degrees (generally to about 80%). Thus, in power systems with large short-circuit capacities, these adverse conditions often limit the use of reclosing.

(1) According to the type of reclosing action, it can be divided into mechanical and electrical types.

(2) According to the way the reclosing acts on the circuit breaker, it can be divided into three-phase, single-phase combined reclosing, and other types.

(3) According to the number of operations, it can be divided into single-phase and double-phase (multiple-phase) reclosing.

According to the operating conditions, it can be divided into single-side power supply reclosing and double-side power supply reclosing. Double-side power supply reclosing can be further divided into reclosing under no-voltage verification, reclosing under synchronous verification, and non-synchronous reclosing.

ZW32-24 outdoor vacuum pole-mounted circuit breaker (hereinafter referred to as the circuit breaker) is a control and protection device used in a 24kV, 50Hz three-phase AC medium-voltage distribution network. It is primarily used for switching and interrupting load currents, overload currents, and short-circuit currents in power transmission lines. It is suitable for use in substations and industrial and mining enterprise distribution systems for protection and control purposes.

This circuit breaker features a pole-mounted, dry-type structure with reliable insulation, long electrical life, and maintenance-free operation. It is particularly suitable for applications requiring frequent operations. It is also ideal for installation in rural power grids with limited maintenance and repair capabilities. The circuit breaker can also function as a sectionalizer in the power grid, and when equipped with a controller, it can achieve distribution network automation.