Figure 4-36. - Three-phase oil switch recloser.
In high-voltage circuits or in circuits carrying large currents, a current transformer is connected into the line and the secondary leads from this transformer supply the current to the trip coil of the oil switch (fig. 4-37). Since there is a fixed ratio between primary and secondary currents of the current transformer, the coil can be adjusted to trip at any predetermined value of current in the line.
The use of the current transformer on such circuits serves the dual purpose of providing a small current for operating the tripping coil and insulating the coil from the high voltage of the line.
Figure 4-37. - Current transformer used to supply current to the trip coil of an oil switch.
An oil circuit recloser is a type of oil switch designed to automatically interrupt and reclose an alternating-current circuit. It can be made to repeat this cycle several times. Reclosers are designed for use on single-phase circuits or on three-phase circuits.
A recloser opens the circuit in case of fault as would a fuse or circuit breaker. The recloser, however, recloses the circuit after a predetermined time (for hydraulically controlled reclosers about 2 seconds). If the fault persists, the recloser operates a predetermined number of times (1 to 4) and "locks out," after which it must be manually reset before it can be closed again. If, however, the fault was of a temporary nature and cleared before lockout, the recloser would reset itself and be ready for another full sequence of operations.
Temporary faults arise from wires swinging together when improperly sagged, from tree branches falling into the line, from lightning surges causing temporary flashover of line insulators, and from animals on the conductors short circuiting the insulators.
A recloser is unlike a fuse link because it distinguishes a temporary from a permanent fault. A fuse link interrupts temporary and permanent faults alike. Reclosers give temporary faults repeated chances, usually four, to clear or be cleared by a subordinate device, like a fuse or sectionalizer. If the fault is not cleared after four operations, the recloser recognizes it as a permanent fault and operates to lock out and leave the line open.
A recloser can be magnetically operated by a solenoid connected in series with the line. Minimum trip current is usually twice the normal load current rating of the recloser coil. The operations are performed by a hydraulic mechanism and a mechanical linkage system. When the fault current reaches twice the normal line current, the increased magnetic field pulls the plunger down into the coil. As the plunger moves downward, the lower end trips the contact assembly to open the contacts and break the circuit. As soon as the contacts are open, there is no more current in the coil to hold them open, so a spring closes the mechanism and reenergizes the line.
Protection to the lineman is most important when a transmission or distribution line or a portion of a line is removed from service to be worked on using de- energized procedures. Precautions must be taken to be sure the line is de-energized before the work is startedContinue Reading