This paper studies protection and control methods for a large DC grid based solely on Full Bridge MMC (Modular Multilevel) converters. An initial theoretical study concludes that DC CB energy dissipation will depend on inductance and square of fault current but not on protection operating time. It is proposed to use differential protection because of robust selectivity and also since it operates well with small series inductors. The analysis of DC CB dissipated energy leads to new protection logic design that delays tripping signals until local current reduces to low values. Low speed DC Circuit Breakers with very small inductors are adequate. The design of controllers for MMC converters should be coordinated with DC grid protection and the study derives values for current controller references. The fault recovery time is found to depend on current reference settings at MMC terminals, and optimal values are derived. The conclusions are confirmed using EMTP simulation on a 400kV, 4-MMC DC grid considering two topologies: with 5 overhead lines and with 5 DC cables.
|Number of pages||10|
|Journal||IEEE Transactions on Power Delivery|
|Early online date||10 Jan 2018|
|Publication status||Published - 31 Aug 2018|
- AC-DC power conversion
- DC power systems
- DC power transmission
- HVDC converters
- HVDC transmission
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- Aberdeen HVDC Research Centre
- Centre for Energy Transition
- Engineering, Engineering - Chair in Engineering