During World War II it became necessary due to increasing demand for power to use single-circuit tie lines between systems to transfer firm blocks of power. Single-pole reclosing breakers have been used successfully in this application. Ground-fault neutralizers are useful in this application, but provide improvement only in the case of single line-to-ground faults, whereas single-pole reclosing breakers improve the stability limits of a single tie line for all types of faults, except three-phase. In this case the operation is that of a gang-operated reclosing breaker.
The advantage of single-pole operation lies in the fact that power can be transferred over the unfaulted phase(s) during the period when the breakers are open to clear the fault. Since most line interruptions do not permanently ground a phase conductor, successful reclosure is obtained in the majority of cases and thus restores the system to its original condition without at any time reducing the power limits to as low a value as would be the case if all three conductors were disconnected.
If single-pole reclosing is to be used, the transformer banks at both ends of the tie line should be grounded solidly or through low values of impedance in order that power may be transferred during the breaker operating period. A comparison of three-pole and single-pole reclosure has been made for the system shown in the insert of Fig.73. The result of stability calculations for the two types of reclosure are plotted in Fig. 73 in terms of the permissible transmitted loads and reclosure times. The curves show the advantages of single-pole reclosure which can be used (1) to transmit greater power, (2) to provide greater deionizing time, (3) to permit the use of slower-speed breakers for fault clearing or reclosure, or a combination of these three. The power-transferring ability of a systemfor a sustained one-phase open condition varies with the zero-sequence impedance of the circuit between the limits of (1) infinite impedance which obtains with the ungrounded system, and (2) zero impedance, a theoretical condition which is rarely approached even for solidly grounded systems. The practical case for grounded systems lies between these two extremes.
The single-pole reclosing breaker is somewhat more expensive than the three-pole breaker because of the three separate mechanisms and the more complicated relay system required.
Electrical Transmission and Distribution Reference Book (Westinghouse Electric Corporation)