GB556014A - Improvements in and relating to protective systems for alternating current circuits and apparatus therefor - Google Patents

Abstract

556,014. Protective cut-out arrangements. BRITISH THOMSON-HOUSTON CO., Ltd. (General Electric Co.). March 23, 1942, No. 3857. [Class 38 (v)] A distance-protective system having the same reach for all types of faults, as in Specification 554,667, differs from the system of the latter Specification in that the connections of the voltage windings only are changed, by means responsive to negative phase sequence components of voltage operating on line-to-line to make the system response on such faults the same as for the other faults. The underlying theory is given and is adapted from that set out in the said Specification. Impedance relays 16 A .. C of the balanced-beam type are energized voltage windings 30 connected to the secondary windings 28 of a voltage transformer 26 so as normally to be energized by the voltage - to - earth of the respective lines, the windings being also connected, but normally ineffectively, to the delta voltages in series with impedances 37 which absorb half the delta voltage. Each beam is also acted upon by a line current winding 32 and by a winding 33, the latter being energized in series by a portion of the residual current adjustable by an autotransformer tapping 36. This arrangement is shown to give the same reach for three-phase, and single- and double-line-to-earth faults. Since a line-to-line fault produces negative phase sequence components only, the voltage-coil connections are changed to delta voltages by means of a relay 39 the winding 41 which is energised from the secondary 28 over a negative phase sequence mesh 42, 43, and opens contacts 40 to render the delta voltages effective. Since earth faults also may produce three negative components as well as zero components, relay 39 is rendered ineffective, if the latter components are present, by a residual-voltage relay 44 connected in the neutral lead as shown, or energized from an open-delta tertiary winding (not shown) on transformer 26. Tripping is also subject to a directional relay having current windings 23 and voltage windings 25 arranged as in Fig. 8 of Specification 508,005. The Specification refers also to discrimination between the different types of fault by means of power-directional relays responsive to positive, negative, or zero components.