GB542804A - Improvements in or relating to electric protective arrangements - Google Patents

Abstract

542,804. Protective cut-out systems. REYROLLE & CO., Ltd., A., LACKEY; C. H., and SCARRATT, G. G. May 17, 1940, No. 8843. [Class 38 (v)] A section which includes a power-transformer with a neutral-earth connection, such as a highvoltage feeder A<SP>1</SP> ... A<SP>3</SP> connected directly to a low-voltage line D<SP>1</SP> ... D<SP>3</SP> over a transformer having star-connected low-voltage windings, is protected as a unit by balancing voltages dependent on the phase-currents on opposite sides and by balancing the phase-currents against the neutral-earth current in a local circuit on the star-connected side. At the remote end, star-connected, five-ampere currenttransformers F', F<SP>3</SP> in the phases are connected, as shown, to a readily-saturable summation transformer having a twelve-turn primary G and one-turn and three-turn parts G' connected between F', F<SP>2</SP> and F<SP>2</SP>, F', respectively. The delta-connected 2.89 amp current transformers H<SP>1</SP> ...H<SP>3</SP> are connected to twenty-turn primaries K<SP>1</SP> ... K<SP>3</SP> of a further saturable summation transformer, the neutral-earth connection transformer H<SP>4</SP> being connected to a further twentyturn winding K<SP>4</SP>. A further primary J is tapped to provide a one-turn and a three-turn part connected, respectively, across H<SP>3</SP>, H<SP>2</SP>. The secondaries M, N are connected for voltagebalance over a pilot and trip circuit P<SP>1</SP>, P<SP>2</SP> containing relay transformers Q which are tuned to the fundamental frequency so as to byepass the higher harmonics constituting the greater part of the unbalance in the saturated summation transformers. The trip relays R also have restraining windings R<SP>1</SP> energized by a further secondary of the associated summation transformer. With an internal fault on the low-voltage side, the resulting current in winding K<SP>4</SP> is compared with that in the winding G' due to the compensating current in two of the high-voltage lines, and as the ampere-turns are unequal, unbalance produces tripping. With an internal fault on the high-voltage side, the high-voltage summation transformer only is energized and produces tripping. With an external earth fault on the low-voltage side, the effects produced in winding K<SP>4</SP> and the appropriate winding K', K<SP>2</SP> or K<SP>3</SP> balance out, also similar parts of the windings G', J are affected and their effects balance out, so that tripping does not occur. Similarly, phase faults produce balance or unbalance according as to whether they are external or internal. The neutral-earth current may be injected into either summation transformer or into a separate summation transformer, the secondary of which energizes a primary winding on the phase-fault summation transformer. Other arrangements of windings of the power transformer are also referred to. In order to avoid the possibility of an internal double line-to-earth fault producing balance in the system shown, the summation transformers may each be divided into two separate transformers dealing with earth and phase-faults, respectively, the two pilot circuits having a common return lead. The decrement of magnetizing surges due to switching-in the power transformer is increased by the transformers in the protective equipment, and the relay may have an inertia device giving it a short inverse time characteristic to ensure stability.