Three level back-to-back HVDC based on H-bridge converter

This paper introduces modeling and control of three level back-to-back high voltage direct current (HVDC) utilizing H-bridge voltage source converters (VSCs) as a result of its attractive features over conventional two-level converters. With contribution of pulse width modulation (PWM), instantaneous real and reactive powers can be controlled independently. This remedies power system phenomena such as power quality, voltage and first swing stability. By injecting or curtailing real and reactive powers to power grid, HVDC has superfluous capability to damp oscillations caused by short circuit faults or abrupt change in power system. Digital current control method in a synchronous rotating reference frame (d-q) controls direct and quadrature currents (Id, Iq) to obtain independent control of real and reactive powers. This is achieved by means of PI compensators. The design procedures using transfer functions attained by applying Laplace transformation to small signal equations, which obtained by linearizing averaged equations, are also studied. Practical PI compensators are designed with proper loop gain characteristics of voltage and current loops. Simulation results show the ability to control real and reactive powers independently in four quadrants while predicating constant DC voltage.