Patent GB-919002-A (GB)
Title: Automatic paralleling of alternating current generators
IPC: H02J3/42
Filed: 1961-02-06
Abstract: 919,002. Synchronizing. WESTINGHOUSE ELECTRIC CORPORATION. Feb. 6, 1961 [Feb. 11, 1960], No. 4365/61. Class 38 (4). Sensing means provide an output voltage to effect paralleling of an A.C. generator to an energized line or generator, when the phase difference between them is within predetermined limits, the output voltage being suppressed by a second sensing means when the frequency difference between the generators and the line, or second generator, exceeds a predetermined limit. An A.C. generator 1 may be paralleled to a line 4 energized by a generator 2 by means of a contactor 6 having a closing coil 7. Voltage regulators, not shown, ensure that the respective generator voltages are sufficiently close to permit paralleling. The secondary winding of a sensing transformer 9 whose primary is connected to corresponding phase voltages of both generators, produces an output voltage having a maximum value when the phase voltages are 180 degrees apart and a minimum value when they are in phase. This output voltage, whose frequency is equal to the frequency difference of the two generators, is rectified by half-wave rectifier 10 and filtered by capacitor 11 and potentiometer 12, which applies a predetermined part of the signal voltage to the base of a transistor 13. When the signal voltage is above the minimum base voltage, the transistor conducts, but when it falls below this value at a point when the alternators are in phase to within a predetermined limit, the transistor is " cut off " and an output voltage appears at a terminal 18 which either directly or through current amplifying means, energizes the closing coil 7 of the contactor 6. The predetermined limit is set by means of a potentiometer 12 which changes the point at which the voltage falls below the minimum base voltage. A tapping 20 at the mid-point of the primary of transformer 9 is connected to a capacitor 22 and a potentiometer 23 through a half-wave rectifier 21 and produces a rectified and filtered voltage at the base of a transistor 24 which has a maximum value when the generator voltages are in phase, and a minimum value when they are 180 degrees apart. The R.C. circuit is arranged to modify the output voltage so that at a low difference frequency it falls below the minimum base voltage for the transistor 24 to conduct, while at one above the desired limit, the output voltage does not fall below it. An output voltage thus appears at the transistor collector when the difference frequency of the generator is within a predetermined paralleling limit. This output voltage controls a bi-stable circuit 28 of two transistors 29, 30, and including a semi-conductor switch device 41 which may be a PNPN switch. The circuit 28 is arranged to be initially in a condition where the transistor 30 conducts and hence provides a shunt path to each from the output terminal 18 so that any output from the phase sensing circuit will be prevented from energizing the contactor closing coil 7. The semi-conductor switch 41 operates so that it is non-conductive until a voltage across it rises to a breakdown point after which it becomes highly conductive until the current through it is reduced to a very low value when it again becomes non-conductive. The voltage supplied to the base of the transistor 30 from the phase-sensitive circuit through conductor 38 is. shunted to earth through the semi-conductor switch 41 so that transistor 30 remains conductive until required to be otherwise. With a manual switch 19 closed, when the slip frequency is below the limit permissible for paralleling, the output voltage from the transistor 24 will cause the bi-stable circuit 28 condition to be reversed so that the transistor 30 is now "cut-off" and thus interrupts the shunt path from the terminal 18, thus allowing the output voltage of the phase-sensing circuit to effect paralleling of the generators at the appropriate moment when both the frequency and phase relationship of the two generator voltages are within the limits set on the potentiometers 23 and 12, respectively. When the contactor 6 has paralleled the generators, the phase difference passes through zero and then increases so that the transistor 13 again becomes conductive. At the moment of zero phase shift, the phase sensitive output voltage will drop so that the current through the semiconductor switch 41 drops below the critical value and becomes non-conductive, so enabling the transistor 30 to again become conducting; the bi-stable circuit 28 thus reverting to its original condition. Due to the two sensing circuits possessing separate adjusting potentiometers the output voltage effecting the closure of the closing coil 7 may be set to appear a predetermined number of degrees prior to zero phase shift and remain for an equal number of degrees after this point, thus ensuring that the contacts have adequate time to respond and various different circuit and contactor characteristics may be accommodated. Specification 861,656 is referred to.