Third harmonic auxiliary impulse commutation inverter with means for precharging commutation capacitor

Claim 1

Means for precharging the commutation capacitor of a third harmonic auxiliary impulse commutated inverter in an electric power system comprising a pair of d-c terminals, means including an impedance that has appreciable electrical inductance for connecting said d-c terminals to a source of direct voltage the average magnitude of which normally does not exceed a predetermined amount, three a-c terminals adapted to be connected respectively to different phases of a 3-phase a-c inductive load circuit, at least three pairs of alternately conducting main controllable electric valves arranged in a 3-phase, double-way bridge configuration for interconnecting said d-c and a-c terminals, a commutation circuit including at least two auxiliary controllable electric valves interconnected in series aiding fashion between said d-c terminals and connected via a commutation capacitor to said load circuit, bistable means coupled to said capacitor for indicating whether the electrical potential on one side of the capacitor is measurably positive or negative with respect to the other side, and control means having a third harmonic commutation mode of operation in which periodic firing signals are cyclically produced, in a predetermined sequence and in synchronism with variable frequency alternating voltages developed at said a-c terminals, for causing said auxiliary valves to turn on alternately, at six times the fundamental frequency of said alternating voltages, and for turning on each of said main valves once per cycle, said control means also having a capacitor "ring-up" mode of operation which is achieved by the improvement comprising cyclically operative additional means coupled to said bistable means: a. for initially generating a first pair of concurrent firing signals to turn on a first one of said auxiliary valves and a preselected complementary one of said main valves if said potential is relatively positive, or a second pair of concurrent firing signals to turn on the second auxiliary valve and another preselected complementary one of said main valves if said potential is negative, whereupon in either case a pulse of direct current flows from said source through a path comprising said impedance, said load circuit, and said capacitor to incrementally charge the capacitor with reverse polarity until current in said path oscillates to zero and the conducting valves consequently turn off; and b. thereafter, in delayed response to the start of the preceding cycle of operation, for repeatedly generating said second pair of firing signals if said potential is negative and said first pair of firing signals if said potential is positive; c. said ring-up mode of operation continuing for a sufficient number of cycles to enable said capacitor to charge to a voltage magnitude many times higher than said predetermined amount.