An Alternate Grid-splitting Scheme and Efficient Algorithm for Voltage Source Converter Based Multiterminal DC Grid Protection and Restoration Control

Despite of the numerous advantages in realizing the super grid known as multi-terminal dc (MTDC) grid, core design requirement is the lack of a fast mature protection strategy (least tripping time) to quickly detect the dc fault, react to it and restore grid back to its normal operation within a few milliseconds. To bridge this gap, this paper investigates cost-effective novel protection scheme by installing minimum number of fast reacting grid splitting devices along with the other easily available alternate protection devices like high voltage AC circuit breakers (ACCBs), high speed DC switches (HSSs), fault current limiting inductors, with real time fault detection and control. Individual protection of each faulty zone is done by isolating the faulty part, resuming operation of the healthy grid zones. Rapidly increasing fault current is quickly reduced to sufficiently lower interruptible levels so that the no-load HSSs of faulty zone quickly and selectively open to isolate the faulty lines. Fault feeding from healthy zones of the grid into the faulted zone is eliminated and also the delay in current decaying behavior of ACCBs used for the dc fault current interruption is compensated improving their performance. This greatly assists in reducing the overall grid outage time or total fault processing time. The strategy is extensible, used to protect the smaller and un-transposed sections of a high impact MTDC grid by splitting it into a number of smaller faulty and healthy zones upon dc fault detection. Fault detectors with control continuously monitor the entire network to identify the faults correctly, quickly and differentiate between various fault types. The effectiveness of the proposed scheme was tested using MATLAB simulations.