Delta-Alpha Limitation

In addition to the limit on the value of α, a limit is also imposed on the variation rate (Δα) of α as shown. On the rectifier side, is limited between two adjustable values.

The typical values are:

• Note that in the "equiangular" synchronization system, Da is the difference between the current reference and the previous firing reference.
• In an “equidistant” synchronization system, Δα is the difference between the reference “a” and value of “a” reflecting the position of the oscillator phase (See section 11.1.5).
• At the rectifier and the inverter, Δα is within a value range where only the vertical coordinates are adjustable.
• The typical values for the ordinates (X-axis) are: [Da1, Da2, Da3, Da4] = [90°, 5°, -10° and -5°] (rectifier); [Da1, Da2, Da3] = [6°, 1°, -6°] (inverter).

Limitation of Δα

The values marked are for illustration only, they can be adapted to the dynamic characteristic of a particular network.

The purpose of imposing limits on Δα is to:

• reduce the risk of commutation failure particularly when the value of Δα is high;
• improve the stability of the control system in case of faults or other large disturbances.

The side effect of limiting the variation rate of Δα is to slow down the response of the control system.