Patent GB-437218-A (GB)
Title: Improvements in and relating to electric converting systems
IPC: H02M5/00
Filed: 1934-08-24
Abstract: 437,218. Frequency changing. BRITISH THOMSON - HOUSTON CO., Ltd., Crown House, Aldwych, London. Aug. 24, 1934, No. 24435. Convention date, Aug. 25, 1933. [Class 38 (ii)] A frequency changer for interconnecting A.C. networks 10, 11 comprises groups of anodes 17, 18 connected to the secondary windings 13, 14 of a transformer 12 and supplying the primary windings of a transformer 15, the groups of anodes being provided with " rectifier " control from grid transformers 24-29 and " inverter " control from grid transformer 30-35, these transformers being supplied from the A.C. network 10 through phase shifting transformers 36, 37. The grid transformers are of the self saturating type to give peaky-wave potentials. The transformers 24-29 are provided with additional saturating windings B, C and D. The windings D are supplied with unidirectional impulses at the frequency of the network 11 from an arrangement 39, 40 and effect the low frequency control. These impulses render the transformers associated with groups of anodes 17, 18 alternately ineffective. Under unity power factor conditions the discharge devices 43, 44 of the arrangement 39, 40 are alone conductive so that the current impulses supplied to the saturating windings D are slightly less than a half cycle of the L.F. supply 11 in duration. When the power factor is less than unity the duration of the current impulse is increased by altering the phase-shifting circuit 53 which allows the discharge devices 42, 45 to conduct a controlled amount. Since the grid transformers 24-29 associated with each group of anodes are rendered ineffective for more than a half cycle, during the additional part inverter action takes place and there is a reversal of energy flow. Automatic voltage regulation is effected by means of the saturating windings B which are in series in the connections from the starpoints of the windings 13, 14 to the ends of the primary of transformer 15. The moment of commutation when operating on low power factor is varied by means of saturation windings G on the transformers 30-35 connected in series in the load circuits so as to ensure a sufficient commutation angle. To avoid large circulating currents due to the variation in the moment of commutation,the groups of transformers 24-26, and 27-29 are provided with cross-connected saturating windings C so that when current is flowing in one group of valves the rectifier excitation of the other group is rendered ineffective. Where the frequency of circuit 11 is not a submultiple of the frequency of the circuit 10, difficulty arises due to the half cycles of the L.F. supply not being formed of equal number of half cycles of the L.F. supply. If, e.g. the negative half cycles of L.F. supply always have more than the positive half cycles, a unidirection component will result which tends to cause saturation of the transformer 15. To avoid this the transformer 51 has two windings 57, 58 connected respectively in series with the two halves of the primary of the transformer 15. The alternating component is choked by means of a winding 59 short-circuited through a resistance 60. If unbalance between the half cycles occurs a unidirectional component of flux will be produced in the transformer core causing phase displacement of voltage in opposite directions in the windings 49, 50 so that the low frequency swamping impulse supplied to one of the windings D is shortened and that supplied to the other is lengthened to cause restoration of balance.