The actual firing is accomplished for each valve by comparing the alpha order input (AO) to the value of the ramp. The interpolated firing time is calculated and passed to the Thyristor component routine so that the system may be interpolated to this point. The Thyristor includes logic so that if the valve is pulsed, but its voltage is still less than the forward voltage drop, the firing will be delayed until the voltage is exactly equal to the forward voltage drop. The firing pulse is maintained across each valve for 120°.
When the default internal firing controls are used, the user merely supplies the alpha order input (AO) for firing. The user may also, externally generate firing pulses and input them directly to each of the thyristors.
The measured firing angle output (AM) and the measured extinction angle output (GM) are calculated for both modes (inverter/rectifier) of operation.
The blocking logic is controlled by the KB external input signal and is effective whether the user has supplied the pulse, or if the internal firing controls are used. The following summarizes the input values needed for blocking control:
KB = 0: Blocks all firing pulses
KB = 1: Deblock (normal firing)
KB = -1 to -6: Block a single valve indicated by the number
KB = -7: Block and form by-pass pair
The only complex function is the formation of a bypass pair (KB = -7). In this case, all pulses are blocked except for two valves connected to the same leg (1 and 4 for example), and which remain continuously pulsed. The pair of valves chosen are those connected to the phase whose zero crossing (for a measurement) is to be observed next.