Patent GB-1253983-A (GB)
Title: D.c. generator apparatus
IPC: A61G5/00
Filed: 1968-11-25
Abstract: 1,253,983. Electric protective systems. LEAR SIEGLER Inc. 25 Nov., 1968 [4 Dec., 1967], No. 55793/68. Heading H2K. [Also in Divisions G2-G3] A D.C. generator whose field winding is automatically controlled to maintain the output voltage at a desired value (see Division G2- G3), is provided with protection control circuits to: (1) automatically disconnect the generator from load and equalizing bus-bars if reverse current flows through the generator; (2) prevent connection of the generator to the load bus-bar if a local ground fault exists, until this fault has been cleared; (3) de-energize the field winding of the generator and disconnect it from the load and equalizing bus-bars, in the event of a fault causing abnormal voltage or current conditions in the generator. In Fig. 4 an operational amplifier 96 compares the voltage (at input 98) of the generator 11 with the voltage (at input 100) of the load bus-bar L, prior to closure of a main contactor 19 controlled by a coil 21. When the two voltages are nearly equal the output of amplifier 96 actuates a control circuit comprising an operational amplifier 110 (connected to operate as a bi-stable step function device), Zener diode 119 and transistors 121, 125, 126, 127, which circuit energizes the coil 21, a coil 32 controlling a contactor (84) connecting the generator to the equalizing bus-bar, Fig. 2 (not shown) and a coil 131 of a start-fault relay (28), Fig. 1 (not shown) maintained energized by its hold-on contacts 133. After closure of contactors 19, (84) the amplifier 96 and control circuit respond to reverse current through the generator, which current flows through an interpole winding 12 causing the voltage at point 15 to become positive with respect to ground. Hence voltage at input 100 increases causing the amplifier output at 108 to exceed a reference voltage E R  and so reverse the control circuit operation to de-energize coils 21, 32. The start fault relay provides an input B-B to amplifier 110 to prevent a closing operation of the control circuit, this input being present in the event of a local ground fault prior to closure of contactors 19 (84). The Zener diode 119 is chosen so that with a fault on the load bus-bar L the control circuit operates to hold contactors 19, (84) closed. Zener diode 124 is chosen to ensure that if these contactors are opened by a fault on generator 11, they cannot be reclosed until its voltage is at least equal to that of the diode 124. In Fig. 5 an operational amplifier 136 is supplied with a signal indicative of the generator voltage (input 139) and a reference signal E R  (from 146). When the difference therebetween exceeds a predetermined value, circuit elements 148, 149 conduct to energize the coil 30 of a trip relay (26) which opens a switch (56) in the field-winding control circuit, Fig. 2 (not shown) to de-energize the winding 13. The switch 129 in Fig. 4 is also opened to de-energize coils 21, 32 and 131. The relay (26) must be manually reset. The amplifier 136 is arranged to respond to generator over-voltages not corrected by the automatic voltage control system. The latter also includes a current equalizing control circuit whose output A-A is also fed to the amplifier 136, so that if this control circuit does not function correctly elements 148, 149 conduct to energize the coil 30 as before. An integrating operational amplifier 161, which supplies the signal B-B to amplifier 110 of Fig. 4, also supplies the amplifier 136 to energize coil 30 under local ground fault conditions prior to closure of contactor 19. When the latter closes, a contact 168 opens to take amplifier 161 out of circuit. Thereafter such faults are sensed by the "out-of-balance" condition of current transformers 22, 23 which monitor the cable between contactor 19 and ground and which control the voltage across circuit members 175, 176 and hence the firing of an element 177 also controlling the coil 30.