Small-signal modeling and multivariable PI control design of VSC-HVDC transmission link

•A small-signal model of VSC-HVDC transmission system is obtained for control design.•Multivariable LQ optimal PI control is employed for inner-loop control of the system.•The outer-loop SISO PI controllers are used for outer-loop control and designed using sequential method.•The performance of the proposed control is compared with the conventional decouple-based PI control.•The superiority of the proposed MIMO PI controller over the conventional one is studied through different performance and robustness measures, and tested through simulations.

This paper employs a multivariable PI (proportional-integral) controller for inner-loop control and SISO (single-input single-output) PI controllers for outer-loop control for a VSC-HVDC (voltage source converter-based high voltage direct current) transmission system. To design the control system, first, a state-space model of VSC-HVDC transmission link is obtained in dq-coordinate frame. Then, using linearization technique the small-signal model is obtained. Next, a multivariable optimal PI controller is designed for the (inner-loop) linearized model using LQR (linear quadratic regulator) approach. Finally, the outer-loop SISO PI controller parameters are obtained using sequential design method (i.e., assuming that the inner-loop is must faster than the outer-loop). The advantage of using the MIMO (multi-input multi-output) PI controller over the conventional decouple-based PI controller for inner-loop control is studied through different performance and robustness measures, and also verified through simulations in presence of several disturbances and parameter variations.