Improved Phase Control With Two Degrees of Freedom for Current Source Converter Using in HVDC Systems

The actively commutated current source converter (CSC) is a competitive conversion method for HVDC transmission systems. However, conventional CSCs have the defects of high DC fluctuations and high losses when utilizing pulse width modulation (PWM). Conventional phase control is an attractive solution. Yet, its degree of freedom is single, making the active and reactive power of the converter coupled, which simultaneously deteriorates the control flexibility of the system. To solve these problems, an improved phase control (IPC) method, which introduces an extra compensation angle, is proposed for the actively commutated CSC employed in an HVDC transmission system. Meanwhile, a control method based on IPC is proposed, which possesses a dual degree of freedom, such as the trigger angle and compensation angle, resulting in independent control of the active power and reactive power. In this paper, the operating principle of IPC is analyzed in detail, with the mathematical model of the system structured. Furthermore, A control strategy is designed to realize the decoupling of active power and reactive power, where the corresponding operating power range is further investigated. Added to that, simulation and experimental results confirm the effectiveness of the introduced IPC and the control strategy. Finally, the economic comparison of IPC-CSC with other modulation methods is analyzed to verify the advantages in HVDC scenarios.