GB733272A - Control means for static current converters - Google Patents

Abstract

733,272. Converting. ALLMANNA SVENSKA ELEKTRISKA AKTIEBOLAGET. June 3, 1953 [June 4, 1952], No. 15303/53. Class 38 (2). [Also in Group XXXVI] In a static current converter, during commutation, the time integral of the voltage determining the commutation is proportional to the load current of the converter at the instant of initiation of the commutation. It is stated that this is achieved if commutation is initiated at the instant when the function traverses zero. In this function, E is the peak value of the commutating voltage, x and y are the electrical angles between the initiation and termination respectively and the zero passage of the voltage ; LK is the inductance of the commutating circuit, and I is the load D.C. Fig. 1 shows an example of a circuit in which, when the commutation voltage, e.g. in a polyphase circuit the voltage between two phases, is introduced at 1, the voltage on the grid of valve 10 traverses zero at the same time as the value of the above-mentioned function. The commutation voltage is applied across transformer '24, and voltage derived from the commutation voltage through a 90 degree phase change circuit comprising capacitor 2, resistor 3 and transformer winding 4, is applied to transformer 25. These two voltages are applied to apparatus 6 which may be of the kind described in Specification 733,254, [Group XXXVI], whereby a voltage proportional to the square root of the sum of their squares, i.e. proportional to E, is developed across resistor 8, thereby supplying the term E cos y. The winding 12 of transformer 25 supplies the term E cos x and apparatus 9 which may be of the kind shown in Fig. 4 derives a voltage proportional to the load current and supplies the term #LKI. The phase change network may be an artificial line comprising condensers and inductors, loaded by a resistor, Fig. 2 (not shown). Transformer winding 12 may be replaced by two reversely connected windings to supply two valves controlling opposite phases of a polyphase converter. In a further modification, the square root of the sum of the squares of the two 90 degrees phase-spaced voltages is derived in a circuit comprising two transductors the magnetizing windings of which are energized respectively by the two voltages. The main windings of the transductors are fed with H.F. current displaced by 90 degrees, the values of which are proportional to the currents in the magnetizing windings and therefore to the applied voltages, Fig. 3 (riot shown). The required result is given by the voltage drop across resistor 8 from the rectified algebraic sum of the H.F. currents. The circuit of Fig. 4 furnishes a voltage across resistor 31, 32, with low resistance inductor 35 in parallel with the latter, proportional to the load current derivative I when this is constant. During increase of current, the voltage increases with increased volt drop across inductor 35. Also capacitor 37 is charged through valve 40 from winding 45 of transformer 36 and a voltage is developed across resistor 39 through capacitor 40. When the current ceases to increase, the capacitor 37 and resistor 39 add to the voltage proportional to the current, a voltage which is at first equal to that which was developed across inductor 35 and then declines to zero. Similarly when the current falls, the voltage at first remains constant by the addition of a voltage which is developed across capacitor 41 through valve 44, and then declines to a value corresponding to the instantaneous value of the current. Valves 33, 34 prevent a reverse current from flowing through resistors 31, 32 so that the voltage drop across them is never less than that corresponding to 1. Arrangements are described in which the value of y varies with I/E, Fig. 2 (not shown), and in which by connecting series connected batteries, valves and resistors across resistor 31, Fig. 8 (not shown), the voltage drop across it is maintained and the commutation instant is not retarded. To prevent undue retardation of commutation when the peak value of the voltage suddenly rises, the rise is arranged to induce a voltage component in opposition to the other voltages in the circuit, Fig. 9 (not shown). Specifications 537,930, 585,765 and 682,743 also are referred to.