Patent GB-622726-A (GB)
Title: Improvements in and relating to electric transformers
IPC: H01F38/32
Filed: 1944-10-18
Abstract: 622,726. Current, voltage and power measurements; supply meters. BRITISH THOMSONHOUSTON CO., Ltd. Oct. 18, 1944, No. 20165. Convention date, Oct. 18, 1943. [Class 37] [Also in Group XXXV] An A.C. transforming system for measuring, indicating or controlling purposes comprises a potential or current transformer, a measuring or indicating instrument or a controlling device constituting the burden of the secondary winding of the transformer, and amplifying means, the input of which is derived from this secondary winding or from another secondary winding of the same transformer or of an auxiliary transformer energized by the same source as the first transformer, and the output of which is supplied to the burden to reduce the load on the transformer. The burden may be a voltmeter, ammeter, watts meter, watt-hour meter, or a relay. As shown in Fig. 1, a current-carrying conductor 11 forms the primary winding of a current transformer 12 and passes through the annular core 14, which carries a secondary winding 15. Current from this is supplied to a wattmeter 13, which constitutes the burden. The conductor 11 also forms the primary winding of an auxiliary current transformer 17 having a core 18 and a secondary winding 19 which is in series with the winding 15 and the burden 13. An amplifier 24 has its input terminals 33 connected across the winding 19 and its output terminals 34 across the winding 15. An additional current is thereby supplied to the winding 19 and burden 13 which corrects for the exciting current error in the output current of the system. In a modification, Fig. 3, the transformer 12 has a tertiary winding 16, and the transformer 17 has a tertiary winding 20. The amplifier 24 is a balanced amplifier with triodes 25, 26, grid bias battery 29 in the grid neutral line 31, and plate battery 30 in the plate neutral line 32. A phase-angle adjustment impedor 21 is connected across the winding 20. The output of the amplifier is fed into the coil 16 and by induction, through the coil 15, into the circuit of the coil 19 and burden 13. The amplifier grids are excited from the coil 20. In another modification, a single current transformer is employed, the amplifier input being derived from a tertiary winding and the amplifier output being applied via a transformer to the main circuit which includes the burden and the secondary winding of the instrument transformer. In the apparatus shown in Fig. 5, the electronic amplifier is replaced by a synchronous motor - generator system. The voltage across the coil 20 is rectified by a rectifier 36 and delivered to the field winding 37 of an A.C. generator 38 driven by a synchronous motor 39. This has the same number of poles as the generator 38 and has its field winding 40 supplied from a D.C. source and its armature fed from an A.C. source synchronous. with the current in the conductor 11. The phase angle is adjusted by the angular position of the generator shaft in relation to the motor shaft, fine adjustment being effected by an impedor 55. Fig. 6 shows the application to a potential transformer system. A main potential transformer 42, having primary and secondary windings 43, 44, is connected across the mains 50. An auxiliary transformer 45 has primary and secondary windings 46, 47 with the same turn ratio as that of the windings 43, 44. Across the secondary winding 44 of the main transformer is connected a burden 48, such as the potential coil of a wattmeter, in series with a transformer 35. The coil 47 is connected in series opposition with the burden 48 through the grid-filament circuit of the amplifier system 24. The output circuit of the amplifier is connected in series with the instrument circuit through the transformer 35 so that a corrective voltage is delivered to the output circuit of the potential transformer proportional to the error caused by the impedance drop of the main potential transformer 42. The transformer 45 may be omitted, the winding 47 being a tertiary winding on the transformer 42. In a modification, Fig. 8, a resistor 56 is connected in the output circuit of the potential transformer 45 and the voltage across its terminals is impressed on the control terminals of the amplifier 24. The output terminals of the amplifier are connected in parallel with the burden terminals in correct polarity, so that a portion of the burden current is supplied from the amplifier, reducing the current to be fed from the potential transformer 45. The voltage across the resistor 56 is thereby reduced and the amplifier acts as a degeneratively coupled amplifier and carries nearly all the load, so that the impedance drop error of the output voltage of the transformer 45 becomes negligible.