A microprocessor based firing scheme for three-phase converters working under a variable frequency supply

In a weak power system like the one in India, power supply faces a lot of voltage swells and sags. Apart from this, the supply frequency also deviates up to ±5% of its rated value in accordance with the variations in the load. This can cause malfunctioning of digitally controlled converters as it is required that timings of the control signals fed to such converters should be accurate. In this paper, a generalised microprocessor based firing scheme for converters working under a three-phase variable frequency supply is presented. The scheme is built using the Intel processor 8086 along with a timer 8253 and a peripheral interface 8255. The firing scheme uses the concept of hardware interrupt to generate the firing signals so that the CPU time of the processor can be used for other useful purposes. The truncation error involved in the calculation of delay corresponding to the delay angle of the converter is also minimised by incorporating an algorithm for correction. This scheme allows the firing angle of the converter to be varied as and when required. The firing scheme enables the converter to be operated over a wide range of frequencies so that the converter could be employed in the rotor side of a static Kramer drive. The firing scheme responds to any variation in the supply frequency within one cycle duration. In general, this robust firing scheme can be used for any three-phase converter such as an AC chopper for switching PF correction capacitors or a rectifier in a HVDC transmission system