GB569028A - Improvements in or relating to fault indicating devices for electric supply systems - Google Patents

Abstract

569,028. Indicating faults. STANDARD TELEPHONES & CABLES, Ltd., TERRY, V. J., and KELLY, R. Oct. 26, 1943, No. 17610. [Glass 37] [Also in Group XXXVII] A voltage operated phase failure indicator for a three-phase electric power supply circuit comprises three primary transformer windings connected in star formation in shunt to the conductors of the supply circuit and three corresponding secondary transformer windings connected in a local circuit including a phase failure relay, means to generate in the secondary windings voltages at the third harmonic of the supply frequency, the connections being so arranged that when the supply circuit is operating normally a resultant third harmonic voltage for operating the relay is produced in the local circuit with substantially no voltage at the fundamental frequency. Three similar transformers A, B, C have their primary windings a, b, c connected in star formation to three-phase conductors on the line side of the main switch S by which the load L is connected. These transformers each have two secondary windings d, g e, h ; f, j. The windings d, e, f are similar and are connected in a series-aiding circuit to a single phase bridge rectifier D. The windings g, h, j are also alike and are connected in star formation to a three-phase bridge rectifier E comprising three pairs of rectifiers E1, E2, E3. Across the D.C. output terminals of this rectifier is the winding of a mains failure relay J shunted by a condenser K. The D.C. output terminals of the rectifier D are connected to a mains winding m of a phase failure relay G, which also has a bias winding n connected across the relay J in series with an adjustable resistance H. The magnetic cores of the transformers A, B, C are so designed that a large third harmonic of the supply frequency is generated in the windings d, e, f. When all the phases are operating normally, the voltages of supply frequency generated in the windings d, e, f are added vectorially producing zero resultant, so that the voltage of the fundamental frequency applied to the rectifier D is zero. Third harmonic voltages, however, combine to produce a resultant voltage, which is rectified by the rectifier D and maintains the relay G operated through the winding m. Should one phase be open-circuited, no voltage at the fundamental or harmonic frequency is applied to the rectifier D. The relay G accordingly releases and gives an indication of failure of the phase. The rectifier E produces a resultant rectified fundamental frequency voltage to maintain the relay J operated so long as at least two of the phases are functioning. When two phases have failed, the relay J releases and gives a corresponding indication. The current in the bias winding n is adjusted so that the relay G operates if the disconnection of the faulty phase is not complete, or if the supply voltage varies within commercial limits. If the common point of the primary windings a, b, c is connected to the neutral conductor of the supply or to earth, the relay J is maintained operated even if only one phase is working, and only releases on total failure of the supply. The switch S may be arranged to be opened by release of either or both of the relays J, G. On restoration of the supply to normal, both these relays are reset and can be arranged to close the switch S. The windings g, h, j may be connected-in delta instead of star formation, and the windings d, e, f may be connected in parallel aiding instead of in series aiding. A single three-phase transformer may replace the three transformers A, B, C.