Gain scheduling scheme assisting the control strategy for three-level NPC VSC-HVDC transmission system

In this paper a gain scheduling control scheme is presented for a three-level three-phase voltage source converter for high-voltage direct-current transmission system (VSC-HVDC), based on two neutral point clamped (NPC) converters in Back-to-Back configuration. The proposed model in dq0 is developed in detail and it has the advantage over other solutions of facilitating the mathematical analysis. Then a direct power control for the entire VSC-HVDC system, based on the dq0 model, is proposed where the control tasks are assign as follow: one NPC-VSC controls the active power flow, the other one regulates the dc voltage level, both converters have the capability to control the flow of active power. Additionally, a closed loop to minimize the voltage imbalance is defined and placed at each NPC-VSC in order to mitigate the imbalance at both system sides in a independently way. After that, a gain scheduling scheme is presented which has the ability to well assign new compensator tuning parameters considering system states as scheduling variables, solving an important neutral point clamped topology weakness. Finally, in order to test the control performance, a detailed computer simulation is implemented in PSCAD/EMTDC software.