Unified Cost Function Model Predictive Control for a three-stage Smart Transformer

Abstract

The massive integration of power electronics-based renewable energy sources has profoundly changed the electrical grid. In this scenario, the smart transformer, which is a solid-state transformer with advanced control and communication features, has been proposed as one of the solutions to offer new grid services, while mitigating electrical grid issues, including voltage/frequency disturbance, harmonics, voltage instability, and to pave the way towards dc grids. The commonly proposed topology for ST is the three-stage ac-dc-dc-ac converter, due to the availability of the dc link at both medium-and low-voltage sides. The control design usually relies on the well-known techniques of pole/zero placement and each conversion stage is considered separately. This paper proposes a unified predictive control of the three stages of the ST that allows to control all the variables with a single cost function. Simulation results show the effectiveness of the proposed solution in guaranteeing excellent current tracking performance and good disturbance rejection.

Model Predictive Control for Solid State Transformers: Advances and Trends

“…This table indicates the work reference, SST application (according to the respective authors), SST conversion stages, used power converters, type of MPC (FCS-MPC, CCS-MPC or both), if MPC is used in all stages, and the used physical controllers. These works will be described next, according to the following order: first, MPC strategies in AC/AC SSTs without [24][25][26][27] and with [28][29][30] internal DC-links are addressed; then, the use of MPC in AC/DC SSTs with internal DC-links [31,32] are described; finally, the existing MPC strategies in a flexible single-stage SST topology [33][34][35] are also discussed.…”

“…However, this topology corresponds to a three-stage SST architecture, as illustrated in Figure 3. Three-stage SST architecture (adopted in [28][29][30]).…”

“…More recently, in [30], another MPC strategy has also been proposed for the same SST. This work represents an extended version of [29].…”

“…This work represents an extended version of [29]. Even though the same SST topology is used in [29,30], there are some significant differences between these two works. First, conversely to [29], in [30], an MPC scheme is also proposed for the third stage of the SST, which is formed by a 2-level DC/AC converter.…”

“…Even though the same SST topology is used in [29,30], there are some significant differences between these two works. First, conversely to [29], in [30], an MPC scheme is also proposed for the third stage of the SST, which is formed by a 2-level DC/AC converter. Secondly, instead of independent control actions being taken for the SST converters (using a cost function for each of them), a single cost function is defined for the whole system.…”