A nonlinear optimal control method for AC/AC converter-based solid state transformers

The article treats the problem of nonlinear optimal control of solid state transformers (AC/AC converters) consisting of a three-phase AC/DC converter and a three-phase DC/AC inverter. It is proven that the dynamic model of the AC/AC converter-based solid state transformer is differentially flat. To apply the proposed nonlinear optimal control method the state–space model of the solid state transformer undergoes approximate linearisation with the use of first-order Taylor series expansion and through the computation of the associated Jacobian matrices. The linearisation takes place at each sampling instance around a temporary operating point which is defined by the present value of the system's state vector and by the last sampled value of the control inputs vector. For the approximately linearised model of the system an H-infinity (optimal) feedback controller is designed. To compute the feedback gains of this controller an algebraic Riccati equation is solved repetitively at each time-step of the control algorithm. The global stability properties of the control scheme are proven through Lyapunov analysis.