Analysis and Control of Modular Multi-terminal DC Power Flow Controller with Fault Current Limiting Function

In order to overcome the problems of power flow control and fault current limiting in multi-terminal high voltage direct current(MTDC)grids,this paper proposes a modular multi-terminal DC power flow controller(MM-DCPFC)with fault current limiting function.The topology structure,operation principle,and equivalent circuit of MM-DCPFC are introduced,and such a structure has the advantages of modularity and scalability.The power balance mechanism is studied and a hierarchical power balance control strategy is proposed.The results show that MM-DCPFC can achieve internal power exchange,which avoids the use of external power supply.The fault characteristics of MM-DCPFC are analyzed,fault current limiting and self-protection methods are proposed,and the factors affecting the current limiting capability are studied.The simulation models are established in PLECS,and the simulation results verify the effectiveness of MM-DCPFC in power flow control,fault current limiting,and scalability.In addition,a prototype is developed to validate the function and control method of MM-DCPFC.

Evaluation of Feasible Interlinking Converters in a Bipolar Hybrid Microgrid

A bipolar hybrid microgrid is a new topology which benefits from the advantages of both alternating current(AC)and direct current(DC)microgrids.Interlinking AC/DC converter is the key of this topology which has the following characteristics:being able to provide two equal pole voltages in DC side;complying with the standards of current quality at AC side;being able to control active and reactive power independently in AC side,and transmitting bidirectional power.In this paper,two categories of power converters including single-stage and two-stage converters are proposed for this topology.A new cost-effective control strategy is added to the control of general grid-connected converter for each interlinking converter,and the control of autonomous DC-link pole voltage for both candidates is achieved.Detailed simulations based on the designed control strategies are conducted to validate the function of control strategies under the operation conditions of different DC sides.The performances of two selected interlinking converters with balanced and unbalanced DC loads are analyzed.Suggested power quality of microgrid and total harmonic distortion(THD)analysis are demonstrated in grid-tied and islanded modes.Eventually,semiconductor power loss simulations based on a closed-loop thermal network simulation are conducted.Thereby,the mutual effects of power loss and initial junction temperature are investigated.