Patent GB-773759-A (GB)
Title: Improvements in and relating to circuit arrangements for regulating transformers
IPC: H01H9/00
Filed: 1955-08-24
Abstract: 773,759. Transformers. BRITISH THOMSON-HOUSTON CO., Ltd. Aug. 24,1955 [Aug. 24, 1954], No. 24353/55. Class 38 (2). A transformer tap-changing arrangement comprises a main transformer having tapped and untapped portions in its secondary winding, a balancing transformer and an autotransformer, the three transformers being so interconnected by means of two reversing switches that a large number of voltage steps are available at the output of the arrangement with only a small number of tappings on the main transformer secondary winding. A main transformer 1 has a secondary winding in three parts 2, 3, 4, the parts 2, 3 having no tappings thereon, the part 4 having three tappings 5, 6, 7, these being connected to contactors 8, 9, 10 and by means of contactors 29 ... 34 to a reactor 35, a centretap 36 of which together with the tapping 5 are connected over a reversing switch 39 to the primary winding 20 of a balancing transformer 15. The ends 12, 13 of the secondary winding 14 of this transformer are connected to the contactors 8, 10 connected together and 9 respectively, whilst a centre tap 16 of the winding 14 and the tapping 7 are connected by way of a further reversing switch 19 to an autotransformer 25 connected between the parts 2, 3 of the secondary winding of the main transformer 1. In operation the contactors 8, 33, 34 are first closed. The voltage between the tapping 5 and the mid-point 36 of the reactor is thus a maximum and is applied to the primary winding 20 with the reversing switch 39 in such a position that the voltage induced between the terminals 12, 16 of the secondary winding 14 aids the voltage between the tappings 5, 7. These added voltages are then applied to the autotransformer 25 with the reversing switch 19 in such a position that the added voltages are in opposition with the voltages of the parts 2, 3 of the secondary winding of the transformer 11. The ratios of the transformers are such that, with contactors 8, 33, 34 closed the voltages in the output circuits between terminals 42, 44 and 43, 44 are a minimum. In the next step contactor 32 is closed and contactor 34 is opened. Thus if the voltage between tappings 6, 7 is represented by u the voltage now applied to the primary winding 20 will be decreased by u/2. Having regard to the ratio of the transformer 15 the voltage applied to the autotransformer 25 will be decreased by u/8, thus the voltage in each output circuit will increase by u/16. In succeeding steps the contactors 29-34 are operated such that the voltages in the output circuits will increase by increments of u/16. When the position is reached where contactors 29, 30 are closed the reversing switch 39 is changed over and the contactors then operated in the reverse order. The voltage induced in the secondary winding 14 between terminals 12, 16 is now in opposition with the voltage in the winding 4, then as the contactors 29-34 are operated the output voltages will further increase in steps of u/16. Further operation of the contactors 29-34, the contactors 8-10 and the reversing switch 39 result in a point being reached in which the voltage applied to the autotransformer 25 is zero. At this point, in which the output voltages are equal to those in the windings 2, 3, the reversing switch 19 is changed over. The voltages applied to the autotransformer 25 now aid the voltages in the windings 2, 3 and operation of the contactors will result in further increases of u/16 of the voltages in the output circuits until the voltage in each of these circuits is about double that in each winding 2, 3. The element 45 connected to the terminal 44 may be used in any suitable protective device such as a current-responsive unbalance relay. The autotransformer may be arranged to provide more than two outputs.