Patent GB-399479-A (GB)
Title: Improvements in or relating to electrical power-transmission systems
IPC: H02H9/00
Filed: 1932-01-25
Abstract: 399,479. Supply systems. WESTING- HOUSE ELECTRIC & MANUFACTURING CO., East Pittsburg, U.S.A.-(Assignees of Bergvall, R. C. ; 321, South Avenue, Wilkinsburg, Pennsylvania, U.S.A.) Jan. 25, 1932, No. 2236. Convention date, Jan. 24, 1931. [Class 38 (iv).] To improve the stability of a transmission between synchronous machines where the transmission line includes a section which may be removed on the occurrence of a fault therein, means, operated automatically before the fault is cleared, insert a series resistance in the transmission line and thus quickly increase the effective load upon the generator. Preferably each generator is associated, on its neutral side, with a resistance which is normally short-circuited by a reclosing circuit-breaker, but the resistances may be interposed between the generator and the fault, for example in series with the neutrals of the high voltage windings of the transformers, or in the lines themselves. The resistances associated with the respective generators may be put out of operation in sequence on restoration of stable conditions. Where the receiving stations comprise a rotary converter or motor generator this may be unloaded under fault conditions by a circuit-breaker which inserts a series inductance in its generator circuit. The system, shown in Fig. 1, comprises water-wheel generators 17, 18, 19 at the sending end and synchronous machines 4, 5 at the receiver end, all provided with quick-response excitation systems which prevent material reduction of the useful internal flux during a first transient of reactive fault currents. The system also comprises pairs of sectionalizing, quick acting, trip-free circuit-breakers 7, 8 and 9, 10 for example, each having a selectively-operating, quick-acting, fault-responsive relay system 15 capable of clearing a fault on a 60 cycle system with 24 cycles, for example, and the circuitbreakers of each pair may operate in sequence. Resistances 21 Y-connected in the neutral of each generator armature circuit are normally short-circuited by quick-acting circuit-breakers 22 tripped by any relay apparatus responsive to a fault anywhere on the system or in any predetermined portion of the system, for example a low-volt relay, over-current relay or residual-current relay. As shown, a tripping coil 23 is energized by a residual-current relay 24, responsive to ground current in the system, and by low-volt relays 25 when the voltage of any phase falls. The automatic reclosing means shown comprises a back contact 30 which energizes a timing switch 31 operating a relay 33 through a back contact 35. A relay 42 short circuits the relay 33 when the back contact 41 closes as the circuit-breaker is closing, the stored energy in the relay coil 33 preventing interruption of the circuit of the closing coil 39 until the circuit-breaker has closed. Any suitable reclosing means may be employed. The Specification discusses the appropriate value of series resistance with the aid of power-angle diagrams, the optimum value being equal to the reactance from the internal generator voltage to the fault, economical values being somewhat smaller. The resistance is inserted as soon as possible after a fault but not after 9 cycles on a 60 cycle system, and may remain for two or three seconds or more, the quick-response excitation system co-operating meanwhile in increasing the stability of the system. Synchronous motors such as rotary converters or motor-generators may be caused to drop their load by the provision of a circuitbreaker, such as 22, arranged to remove a short circuit upon a reactance in the circuit of the generator element of such a machine.