Improved control strategy of current-source-converter for HVDC system application

Pulse width modulated current-source converter (PWM-CSC) has great prospects in high voltage direct current transmission system (HVDC) due to its attractive features, such as flexible control characteristics, ability to avoid commutation failure, and lower cost. However, the valve voltage of the PWM-CSC is the jump value between the filter capacitors line-voltage and zero voltage due to its bypass operation, resulting in the high peak voltage withstand by the converter for a rated HVDC system and high voltage ripper ratio. In order to solve these issues, an improved modulation method called specific carrier frequency of SPWM was proposed. After adopting the improved modulation method, the number of the reverse blocking integrated gate commutated thyristor (RB-IGCT) in series can reduce by 31.5% under the unity power factor operation and the value of the dc choke can reduce by about 78.6% compared to traditional modulation method, improving the practicability of engineering application. Moreover, the active power and reactive power operating range are derived under the different modulation methods and the relationship of the power factor, modulation in q-axis and DC current are studied. Finally, the effectiveness of the improved modulation method and the comparisons of power operating range are verified in PSCAD/EMTDC.