Patent GB-1282455-A (GB)
Title: Rectifier controlled apparatus for an a.c. generator excitation system
Inventor: YOKOKAWA SUMIO
IPC: H02P9/38
Filed: 1969-07-14
Abstract: 1282455 Controlling thyristors FUJI DENKI SEIZO KK 14 July 1969 [12 July 1968 15 July 1968 (2)] 35238/69 Heading H2F [Also in Division G3] An excitation system for an A.C. generator 1 comprises a rectifier bridge 2 which supplies the generator field winding la from an A.C. source VE obtained by connecting in series the outputs of voltage and current transformers 8, 9 energized by the generator output, the bridge 2 comprising diodes 2a and thyristors 2b which are supplied with firing signals from a control unit 3 dependent upon the deviation of the generator output from a predetermined value and upon a phase reference signal relatively free from harmonics. In a comparator 5, the output of a voltage transformer 6 sensing the generator voltage is compensated by the output of a current transformer 7 responsive to the generator current, the compensated voltage so produced being compared with a reference voltage to derive an error signal for application to the unit 3 via an amplifier 4. This arrangement provides a voltage control which is stabilized for the reactive power of the generator. To derive the phase reference signal for the unit 3, voltage and current transformers 10 and 11 are connected to the outputs of transformers 6, 7 and have their outputs connected in series to provide a three-phase supply V S  which, compared to the supply V E  for the bridge 2, is relatively free from harmonics. The phase voltages U, V, W of the supply V S  are applied to the primary windings P3, P2, P1 of a transformer TR (Fig. 4) whose secondary windings S1, S2, S3 supply a circuit consisting of diodes, switching transistors and resistors, the whole arrangement constituting a synchronizing signal generator 31 having square wave outputs u, v, w which are respectively applied to the three pulse generators for the thyristors 2b. These generators, one of which is shown in Fig. 5, and the generator 31 constitute the control unit 3. The output voltages of windings S1, S2, S3 cause each of the diodes D1, D2, D3 to be conductive for a period of 120 degrees during which a corresponding one of the transistors T1, T2, T3 is switched off, so producing a negative voltage at a corresponding terminal u, v, w. For the remaining 240 degrees the corresponding transistor T1, T2, T3 is switched on to produce a positive voltage at the corresponding terminal u, v, w, Fig. 6(c) (not shown). The square wave output is applied to terminal u of the pulse generator 32 of Fig. 5, where it controls transistors T4, T5, T6 and an RC circuit to form a sawtooth voltage which is applied to the base of a transistor T7. This base also receives signals from terminals C, D respectively corresponding to the error signal from amplifier 4 and a bias voltage which determines the quiescent firing angle of the thyristors 2b for the constant load condition of the generator. The transistor T7 is turned on or off according as the combined signal from terminals C, D is less or greater than the sawtooth voltage, Fig. 6 (d) (not shown), and the switching of transistor T7 controls that of transistor T8 through an RC circuit, the latter transistor in turn controlling the switching of transistors T9, T10 to produce at output terminal Pu a square wave voltage, Fig. 6(h) (not shown), to fire the thyristor. Transformers 8, 9 have response characteristics chosen to produce a supply voltage V E  substantially in proportion to the field current required for the load condition of the generator. Transformer 9 may be of the gap-provided type or may comprise an ordinary current transformer in parallel with a reactor, Fig. 2 (not shown). Instead of voltage control, the generator may be controlled to maintain a desired reactive power or power factor. Tne invention may be applied to control the excitation of the stationary field winding of the A.C. exciter of a brushless A.C. generator, Fig. 7 (not shown).