The lag angle reference (a) is a firing system input. It is generated by the regulation system and the AC voltages of the switching bus. The synchronization system generates firing pulses for each thyristor. The generic synchronization system used in a 6-pulse converter is of the “equiangular” type. It can be used with a 12-pulse converter for which the user also has the option of using an “equidistant” synchronization system.
Equiangular synchronization mode
The firing pulses are synchronized with the zero crossings of the switching voltages.
Equidistant synchronization mode
The firing pulses are synchronized with an oscillator whose frequency is 12 times that of the power grid. The oscillator phase is controlled by the regulation system (current, voltage, etc.) which lags or leads the firing, if necessary. The oscillator synchronizes itself slowly at the zero crossings of the switching voltages, but quickly at the previous firing instant. The oscillator gain, set by the user, allows users to determine the speed at which the oscillator synchronizes itself with the zero crossings. A gain of 1 (minimum allowed) means a full synchronization with the zero crossings1. As the gain increases, the oscillator synchronizes itself more slowly with the zero crossings (1000 is the maximum limit imposed by the model). Hence, the system is not greatly affected by transients and unbalances of the AC switching voltages. A gain of 32 (default value) is a good compromise between the synchronization speed and the immunity to the harmonics on the switching voltages. In transient state or during AC faults, when the switching voltages are absent or severely distorted, the oscillator gain is automatically increased to cancel any zero-crossing synchronization.
Note that a valve will be fired (pulse generated) only when the voltage at its terminals has reached a minimum threshold, adjustable by the user.
As it is the case for the equiangular synchronization system:
The General Diagram of a 12-Pules Converter Bridge