Patent GB-1256617-A (GB)
IPC: H02M7/757
Filed: 1969-05-07
Abstract: 1,256,617. Static inverters. ALLMANNA SVENSKA ELEKTRISKA A.B. 7 May, 1969 [8 May, 1968], No. 23339/69. Heading H2F. An inverter connecting a D.C. network with an A.C. network has a control system which controls the delay angle (Fig. 5) of the inverter rectifiers with respect to a certain operating magnitude (e.g. the D.C. input or output power) and in addition ensures a necessary commutation margin by pre-calculating continuously at every moment the remaining time integral of the commutation voltage U K t until the zero passage of U K . Ignition must thus be initiated when the predicted total voltage time integral U K t<SP>*</SP> is equal to the voltage time integral calculated to be necessary for commutation at the actual direct current value J (area K 2 J) plus a desired margin (area M). Control circuit (Figs. 1 and 2).-A control pulse generator 6 for valves 1, 2 is operated via an OR gate 5 either from a discriminator 12 in dependence upon a desired operating magnitude (direct current input) or from a discriminator 13 for securing a desired commutation margin. An amplifier 9 receives as inputs a desired direct current value as pre-set on a potentiometer 10 and an actual direct current value as sensed by a transductor 11. The amplified difference value is fed to a summator 7 together with a sawtooth signal U 8  (Fig. 4, not shown) from an integrator 8 and a direct voltage level U 14  from a unit 14 which level corresponds to the actual period time T divided by the inverter pulse number (2 in present case). When U 8  equals U 14  the input voltage to discriminator 12 is zero and a pulse is sent to the control pulse generator 6. Equidistant ignition pulses are obtained provided the output of amplifier 9 is zero. If the set value current is exceeded the resulting output from 9 causes a delay of the next ignition. In Fig. 5 commutation between valves starts at the delay angle α. Commutation occurs during an angle u in which the commutation voltage/time integral is proportional to load current J and the inductance of transformer winding 3. The area M corresponds to a necessary minimum margin of the commutation voltage time integral. As long as the control angle α is so small that an intermediate area B remains #0, the said margin is granted. Thus it is arranged that discriminator 13 initiates an ignition pulse from generator 6 at the latest when B equals zero only if the discriminator 12 has not earlier initiated an ignition pulse. Commutation margin control.-The discriminator 13 is controlled by the circuit, Fig. 2, which circuit has three inputs-one from a transformer 16 which senses the commutation voltage U K,  one from the transductor 11 which senses the load current, and one from a potentiometer 25 which pre-sets the margin M. At every instant the remaining voltage/time integral up to the zero passage of U K  is calculated continuously i.e. the areas K 2 J, B and M, from the measured actual value of U K  and the predicted remaining time t*. Since no special curve shape of U K  is pre-supposed a practical prediction is obtained by calculating the product of U K  and t* and correcting the result by a term An for each phase or each valve. The product of U K  and t<SP>*</SP> is obtained by a unit 17 having inputs representing U K  and t*. The value t* is formed in a summing device 22 as the difference between the calculated half period from an integrator unit 21 and the time measured by a unit 19 from the last zero passage of the actual commutation phase. The units 19 and 21 are operated by discriminators 18, 20 respectively, indicating the zero passages of commutation voltage. The output of 17 is fed with a negative sign to a summator 24 which also receives the correction term An witha positive sign from a unit 23. From units 26, 27 a magnitude is fed corresponding to the precalculated voltage time integral. Unit 26 gives the product K 2 J while 27 takes account of change in current during the commutation period and gives an output K 3 (dJ/dt). The output of 26 is thus K 2 [J+K 3 (dJ/dt)]. From the summator 24 is thus obtained the margin B and if ignition is not initiated by discriminator 12 before the magnitude from 24 passes zero the discriminator 13 sends a pulse to pulse generator 6.  The correction (An) member 23 (detailed Fig. 3, not shown) stores the actual value of K 1 U K t<SP>*</SP> from unit 17 at the ignition instant. From this stored magnitude is subtracted the voltage time integral U K t* from the ignition instant to zero passage. The difference is then stored to be used as a correction term the next time the same valve or a valve in the same phase is ignited.