Control of VSC based multi-terminal DC system for support to critical loads on grid side witnessing loss of generation

Often in a networked AC system, substation witnessing loss of generation fails to supply the loads, which are sensitive and critical. Creation of dedicated infrastructure for support is quite costly. The paper proposes to augment a Multi-Terminal Medium Voltage DC Sub-Transmission System (MT-MVDCS) between the AC grid and AC distribution system, to enhance unlimited expansion, selection of power input, availability, stability and reliability at low cost. The control algorithm of the proposed system is discussed for control power flow, decoupled power control to input only real power from the mains, supply desired reactive power and cater to the critical load connected to the grid side witnessing the loss of generation. The indirect current control is utilized to control the Voltage Source Converters (VSC) under grid tied conditions, whereas, the VSCs are PWM controlled when supplying power to passive loads. The controller in later case regulates the AC side voltage by tight control on modulation index, and evenly compensates voltage drop on DC bus through injection of third harmonic component. Simulation based results demonstrate that proposed method provides effective control of VSC both in current controlled mode and voltage controlled mode for the aforesaid objectives. The presented results show that seamless power is provided to the critical loads connected to the grid with loss/failure of generation, without any transients in voltage.