Improvements in or relating to relays for the differential protection of power transformers

摘要

480,394. Protective cut-out arrangements. COMPAGNIE POUR LA FABRICATION DES COMPTEURS ET MATERIEL D'USINES A GAZ. Dec. 7, 1936, No. 33580. Convention date, Dec. 7, 1935. [Class 38 (v)] In order to prevent the operation of differential protective systems for two- or threewinding transformers by the momentary unbalance when switched in, and by throughfaults, the active couple of the protective relay is arranged to be little influenced by the unidirectional component of the primary current during transience, and, further, depends on the frequency and changes in sign for a frequency comprised between the normal frequency and twice this frequency, whilst the opposing couple is an increasing function of the main currents which traverse the windings of the transformer. The restraining element of the relay comprises a core 8 of irregularhexagon shape having poles 9 .. 11 co-operating with an armature 7 of star shape. The poles have windings 12 .. 14 connected respectively to be energized by the currents in the windings of the transformer, mutual interference between the windings of the element being reduced by suitably dimensioning the core. The operating element comprises a core 4 carrying twoparallel-connected fixed coils 5, 6 arranged in series with a movable coil 2 on a frame 1 in the air-gap of the core 4. The armature 7 and frame 1 are mounted on an axle 3, which also carries a damping plate 21 and a movable contact 19 co-operating with a fixed contact 18. Coils 2 and 5, 6 are connected across the secondary of a transformer 23 energized by the unbalance current, the primary of the transformer having three tappings in order to give three different percentage values of operation. The D.C. component of unbalance is byepassed by an inductance 24 of low ohmic resistance. A condenser 25 is shunted across the winding 5, 6 and is of such value that the driving couple tends to close the contacts with current at the supply-frequency but at a frequency lying between the supply-frequency and twice that frequency, it tends to open the contacts. The condenser and winding 2 are protected against transient over-voltage by neon tubes 26, 27. In the case of a two-winding transformer, Fig. 4 (not shown), one coil of the restraining element may be left unconnected or, alternatively, two coils may be connected in parallel and fed from one winding of the transformer, the other winding of which feeds the third coil. Electromagnetic switches.-The frame 1 is of moulded artificial resin (phenol formaldehyde resin).