Abridgment of excitation systems their influence on short circuits and maximum power

Abstract

Since 1920, when the general subject of excitation systems was reviewed at the White Sulphur Springs Convention, two important problems regarding these systems have arisen: One relates to the required excitation characteristics during system disturbances, and the other to the characteristics which are necessary in order to increase the maximum power above the steady slate or static limit — i. e., in order to operate the synchronous machines under the condition of dynamic stability. With respect to the former problem, the advantages and disadvantages of quick response excitation are considered. Such excitation tends, of course, to hold up the voltage during system disturbances, and is thus advantageous. However, it also increases the short-circuit current which circuit breakers must interrupt. The general trend in installing such systems is therefore in the direction of requiring larger circuit breakers. Such an excitation system is justified in many cases, and, indeed, it is essential in some. The extent to which the quickness of response and the maximum value of the excitation voltage are carried, is a question which, at present, should be settled by the conditions of the particular case. As to increasing power limits, results are given which are very promising with respect not only to long distance transmission, but also to power systems which have approached the power limit as determined by the condition of present normal operation. A new regulator, unique in its operating characteristic, has been developed which makes it possible to sustain stable operation under the condition of dynamic stability, thus increasing the maximum power by taking advantage of a heretofore unexploited range of operation of synchronous machines. Comparative test results are given for different types of regulators. The new regulator alone showed extraordinary gain, giving an increase of maximum power from a steady state (steady-field excitation) value of 110 kw., to a maximum of 415 kw., on a system comprising a synchronous generator supplying power directly to a synchronous motor. This shows the extent of improvement obtainable in the machines themselves. With an artificial 500-mile straightaway transmission line between the machines, a maximum power (received at the motor) equal to 90 per cent of the “infinite bus” value was obtained. The infinite bus value was 61 kw., 55 kw. was obtained, 44 kw. being the steady state power limit. The excitation system, as controlled by the new regulator, provides a component of excitation voltage which is at all instants equal to the i r drop in the field circuit during the necessary small oscillation under dynamic stability. The i r drop is therefore compensated, the characteristic of the regulator being to introduce the effect of negative resistance. And with zero effective field resistance, the maximum power corresponds to the condition of constant flux linkages — the power under that condition is greatly increased above the constant field current value. This condition is approached by the new regulator.