Automatic Reconnection

the rapid automatic return to operation of high-voltage electrical transmission lines and equipment after their automatic disconnection. This is one of the most effective emergency prevention techniques. It increases the reliability of electricity supply to users and reestablishes a normal set of conditions for operation of electrical systems. In many cases, after rapid disconnection of a sector of the electrical system in which a short circuit has occurred as a result of a short-term breakdown of insulation or a flashover of the air gap, no further short circuits occur during the subsequent reestablishment of voltage.

Automatic reconnection is accomplished by means of automatic devices acting upon high-voltage breakers after their automatic emergency disconnection by protective relays. Many of these automatic devices provide automatic reconnection with self-actuated breaker disconnection—for example, during strong ground tremors resulting from nearby explosions, earthquakes, and so on. The effectiveness of automatic reconnection increases in proportion to the speed with which it occurs after an emergency disconnection—that is, the shortness of the time of interruption of the supply to users. This time is dependent upon the duration of the automatic reconnection cycle. In electrical systems single automatic reconnection is used with a single cycle, double reconnection in case of an unsuccessful first cycle, and triple reconnection with three successive cycles. An automatic reconnection cycle—the time from the instant at which the command for disconnection is originated to the closing of the circuit at the main contacts of the breaker—consists of the disconnection and reconnection time of the breaker and the time for operation of the automatic reconnection device. The duration of the current-less interval, during which the user does not receive any electrical energy, is selected so that it will permit the reestablishment of the insulation (deionization of the medium) at the point of the short circuit, so that the breaker linkage will be ready for reconnection after the disconnection, and so that the breaker, at the instant of closing of its main contacts, will reestablish the ability to break the impaired circuit in case of unsuccessful automatic reconnection. The deionization time is dependent upon the medium, climatic conditions, and other factors. The time for reestablishment of the breaker’s disconnection capability is determined by its construction and the number of automatic reconnection cycles preceding the given cycle. The usual duration of the first cycle does not exceed 0.5–1.5 sec; of the second, 10–15 sec; and of the third, 60–120 sec.

Single-cycle automatic reconnection is the most prevalent, providing up to 86 percent successful connections on overhead high-tension lines (110 kV and higher) and up to 55 percent on cable lines (3–10 kV). Double-cycle automatic reconnection provides up to 15 percent successful connections in the second cycle. The third cycle raises the number of successful connections by only 3–5 percent. On high-tension electrical transmission lines (from 110 to 550 kV) single-phase automatic reconnection is used; in these cases the breakers must have separate linkage for each phase.

The use of automatic reconnection is economically desirable, since the cost and use of automatic reconnection devices is incomparably less than the damage caused by a break in the transmission of electric power.