Abstract :
In order to operate two power plants satisfactorily in parallel, the transmission line which ties them together must have sufficient synchronizing power, as well as sufficient carrying capacity. The “synchronizing power” of a line depends upon its resistance and reactance, the bus voltages maintained at its ends, and the maximum kilovolt-amperes it must transmit. The ability of different lines to provide satisfactory parallel operation cannot be measured by any standard which does not take account of all of these factors. Limiting values for “synchronizing power” of lines under various operating conditions are given. The division of load between two plants in parallel is regulated by steam control; the division of wattless current associated with the load depends upon the voltages generated and may be in proportion to the division of real load, when the difference in voltage at the two busses will vary with the load transmitted, or may be arbitrary so that regardless of the load transmitted, the voltages on the two busses will be maintained constant. The latter plan generally gives better operating conditions on the system as a whole, but creates demands for wattless currents at either or both plants in excess of their normal capacity, and in so doing, involves an additional cost. The excess wattless crosscurrent so created can be materially reduced by varying the voltage with changes in load transmitted through the use of taps on the line transformers, by inserting additional reactance in the line at light loads, or, when the stations are tied together by several parallel lines, by cutting out one or more lines as the load decreases. The design of a transmission line involves a consideration of load to be transmitted, voltage, reactance, resistance, losses, and charging current of the line, and of wattless generating capacity at the receiving end of the line. The wattless generating capacity at the end of the line determines how man- kilowatts will be transmitted for each ampere of line current, by fixing the power factor of the load transmitted and the voltage at the receiving end of the line. When a line is to be designed for paralleling two plants, it must have sufficient “synchronizing power” to hold the two plants together.
