GB833255A - Improvements relating to electric supply systems - Google Patents

Abstract

833,255. Automatic voltage control systems. GENERAL ELECTRIC CO. Oct. 26, 1956 [ Oct. 28, 1955], No. 32723/56. Class 38 (4). In an A.C. supply system in which the output voltage of three parallel connected transformers is automatically controlled by tap changing, closer voltage regulation at the expense of some circulating current is obtained by adjusting the transformers one tap at a time in sequence. In one embodiment, Fig. 1, the load circuit 20 is connected to the supply 16 by transformers 10, 11, 12 having tapped secondaries and tap changing motors 30, 31, 32 which are under the general control of a voltage-responsive relay 22 connected to the load circuit 20 by a transformer 26 via a line drop load current compensator consisting of a current transformer 28 and impedances 23, 24. The relay 22 controls a master " raise contactor 85 and a master " lower " contactor 86. Each tap change motor, 30, 31, 32 additionally operates banks of cam-actuated contacts. Cams 57, 67, 77 open their contacts between each tap change and interrupt the master contactor energizing circuit to prevent over-run. The associated cams, e.g. 53, 63, 73, provide an interlock to prevent energization of the master contactors if the transformers get more than one tap out of step. The remaining cams, e.g. 50, 60, 70, control subsidiary " raise " and " lower " contactors 110, 111, 112, 113 to decide the sequence of tap-changing operations. If all three transformers are in the same tap position, then the first adjustment is made on transformer 12 which moves one tap. If the correction is insufficient, transformer 11 moves one tap, then transformer 10, and thence back to transformer 12 and so on. If adjustment in the reverse sense is required, the transformer which last operated is adjusted one tap in the new direction, followed by the others in the sequence 10, 11, 12, until all three are again level, when transformer 12 again takes the lead in the new direction. In a second embodiment, Fig. 2 (not shown) each transformer is controlled by its own voltage sensitive relay and line-drop compensator as in Specification 589,853, but the voltage relays of the two " follower " transformers are set to respond to progressively lower voltages than the " leader," and an additional coil carrying circulating current is added to bias each line drop compensator. Should the output voltage fall, the " leader " moves one tap, and the resulting circulating current biases its relay to render it inoperative and biases the relay of the next transformer to the operating point so that it would respond to any further fall in voltage. This in turn biases the third relay to the operating point. Specification 587,881 also is referred to.