Multi-terminal Direct-Current Grids: Modeling, Analysis, and Control

Chapter 6Protection of MTDC Grids

6.1 Introduction

Protection of a VSC DC grid in the event of a fault in the DC cable network is a research problem till date. It was mentioned in Section 2.4.2.4 that regardless of the control strategy, conventional VSCs cannot limit or interrupt the current that flows following a DC-side fault. DC circuit breakers are yet to be commercially available for high power ratings. Therefore, the only way to clear a DC-side fault is to open the circuit breakers on the AC side. For a point-to-point VSC HVDC link, this is not a major problem as the loss-of-infeed is not unacceptably high even if the entire link (e.g., 500 MW) is taken out of service by opening the AC breakers at both ends. However, for a DC grid with a capacity in the range of tens of GWs, opening all the AC circuit breakers to clear a DC-side fault would result in unacceptably large loss-of-infeed. Thus, one cannot rely on the AC circuit breakers to clear a fault within the DC grid.

There are essentially two ways of clearing DC grid faults without causing a large loss-of-infeed. One option is to use DC breakers (as and when they are commercially available) to isolate only the faulty cable while continuing to operate the rest of the DC grid as usual. Another option is to use converter topologies which are capable of interrupting the DC fault current. Such topologies are usually less efficient and a proper balance between fault-blocking capability and efficiency is required.

Detection ...