Series capacitors for transmission circuits

This paper presents the results of an analytical and experimental investigation of the use of series capacitors to increase the permissible loadings of long high-voltage a-c transmission lines. Because of limitations imposed by synchronous stability and reactive kilovolt-ampere requirements, conventional lines, when extended to the higher voltages and greater lengths, cannot be loaded to values of power sufficiently high to develop maximum over-all economy. It is shown that, when suitably applied, series capacitors will make possible the desired optimum loadings and will very materially reduce transmission costs. The theoretical possibilities of using series capacitors to compensate the excessive inductive reactance of long lines have received consideration for many years. Practical series capacitors require protective equipment, but shunting of the capacitors in the earlier schemes resulted in a decrease instead of an increase in the transient-stability limits. The authors propose series capacitors of the limited-voltage type in combination with auxiliary equipment which not only protects the insulation but quickly restores the capacitors to the circuit after the faulted conductors are isolated, thus preventing a decrease in system power limit at the time of need. The application requirements of necessary apparatus are also briefly outlined. Results of analytical studies and miniature-system tests in connection with a typical application of series capacitors to one of the major lines of a typical transmission system are included. This work deals with relative transmission costs, transient stability, spontaneous hunting, and the sub-synchronous operation in the induction starting of machines. The conclusion is reached that the proposed series-capacitor scheme appears practicable for long transmission lines.