Band-Pass Filter

The voltages used for synchronization are filtered by band-pass filters with an adjustable tuning frequency. The transfer function of one band-pass filter is:

EQ 01

EQ 02

The General Diagram of the Generic Converter Control System

At fo frequency, H(s) has a gain of 1 and its phase is zero. As a result, the band-pass filter does not affect the fundamental component of the voltage (neither in amplitude nor in-phase), while it reduces other harmonic components.
The B parameter (proportional to the pass-band with a typical value of 45 Hz) is used to control the sensitivity and the speed of the filter. A small bandwidth improves harmonic filtering and gives less distorted wave-form at the filter output. However, the smaller the bandwidth, the slower the filter is in following the amplitude and phase fluctuations of the input signals.

The three-phase voltages on the primary side of the transformer are thus filtered by an adaptive band-pass filter. The six secondary (Y and Δ side) line voltages (phase to phase) calculated from the primary voltages are used to determine the zero-crossing times. For each voltage, there are two zero crossings in one cycle, one with a positive slope and another with a negative slope for a total of 12 zero crossings in each cycle.

The period T is updated 12 times per cycle. The measured period T is limited within an adjustable interval defined by the Freq max and Freq min parameters.

Low AC Voltage Detection

If the voltage is lower than a specified threshold during a specified time interval, the Low AC voltage detection function provides a low voltage flag. In the equiangular synchronization mode, this flag is on and the calculated period T is not updated. It is rather maintained at its past value in prevision of an eventual AC fault. Also, DC fault detection is inhibited. A falling edge delay (from True to False) is applied to the flag to ensure that it remains present for a minimum period of time. Finally, low voltage detection is used in the DC fault protection described further ahead.

Alpha Angle Regulation and Limiting System

The function of the regulation system is to regulate the direct current or voltage by adjusting firing angle α, and to limit the inverter extinction angle to a minimum value.