Patent GB-778419-A (GB)
Title: Improvements in or relating to electric generator systems
IPC: H02P9/32
Filed: 1954-12-15
Abstract: 778,419. Automatic voltage control systems. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. Dec. 15, 1954 [Dec. 24, 1953], No. 36297/54. Class 38 (4). In order to prevent the field excitation of a non-salient pole alternator which is connected in parallel with other alternators through an external reactance from being reduced below the limit of stability by an automatic voltage regulator when the load p.f. is loading, the voltage at the far end of the main system is computed from the terminal voltage and a voltage proportional to the external reactance voltage drop, and the open circuit E.M.F. is computed from the terminal voltage and a voltage proportional to the synchronous reactance drop in the machine. If the angle between the two computed vectors approaches 90 degrees then further reduction of the excitation is prevented. Basic voltage regulator.-The alternator 10 is excited by generator 20 having buck and boost fields 26, 28 energized via rectifiers 84, 82 from opposed self-saturating magnetic amplifiers 48, 46. The amplifiers have control windings 114, 116, 118, 120 energized via rectifier 122 and transformer 124 in accordance with the terminal voltage of the alternator, and bias windings 90, 92, 94, 96 which aid and oppose respectively the control windings on the two amplifiers so as to give the required differential control. A set of reference windings 102, 104, 106, 108 determine the operating point of the amplifiers. Excitation limit circuit.-The terminal voltage Et of the alternator is measured between lines 14, 18 by a potential transformer 132. The current in line 16 is measured by current transformer 140. The current in this line is (at unity power factor) 90 degrees out of phase with the voltage between lines 14, 18 and can therefore be used to give a measure of reactive voltage components. A resistor 131 connected across the C.T. 140 is adjusted so that the voltage across it represents the voltage drop jixe due to the external reactance assumed to exist between the alternator and the main system. This voltage jixe, Fig. 2, is added to the terminal voltage Et from P.T. 132 so that across the winding 182 there is a voltage Es representing the voltage at the main system. Fig. 2 is drawn for a leading p.f. system load. An auxiliary current transformer 142 is connected in series with C.T. 140, and is adjusted so that its output voltage would represent the approximate internal voltage drop of the alternator jixd due to " synchronous reactance." Since the resistor 131 is also included in series with C.T. 142 the output in fact represents the sum of both vectors jixe, jixd (Fig. 2). The remaining vector Ed then represents the opencircuit E.M.F. of the alternator for the particular excitation. It is stated that if the angle # between Es and Ed becomes greater than 90 degrees, then a non-salient pole alternator will fall out of step. Hence the limiting value occurs when cos # (jixd + jixe) = Es. The quantity Es is applied by transformer 182 to a push-pull pair of self-saturating magnetic amplifiers 150, 152 the outputs of the load windings 162, 164, 166, 168 appearing via rectifiers,188, 190 across load resistors 192, 194. D.C. control windings 204, 206, 208, 210 are also energized in accordance with Es via rectifier 214. The control windings assist the self-saturation of the load windings on amplifier 150 and oppose it on amplier 152 A.C. control windings 216, 218, 220, 222 are energized from C.T. 142 in accordance with jixd + jixe. Negative feedback windings 224, 226, 228, 230 are connected across the output, and bias windings 196, 198, 200, 202 determine the operating point of the amplifiers. Apart from the effect of the D.C: control windings, the amplifier output will inherently be proportional to the A.C. control voltage jixd + jixe times the cosine of the angle # between the control voltage and the supply voltage,Es. However, taking into consideration also the D.C. control windings energized by Es the final output is E s  - cos # (jixd + jixe). This, the required control voltage, is amplified by a positive feedback magnetic amplifier 40 and applied, via rectifier 272 in parallel with the normal voltage control signal to the control windings of the magnet amplifiers 46, 48. The amplifier 40 is arranged to provide a standing output across resistor 234 which is reduced by the control voltage and this reduction, by shunting the normal voltage control winding causes an increase in alternator excitation.