A novel time domain protection technique for multi-terminal HVDC networks utilising travelling wave energy

Fault vulnerability and protection issues are major challenge in realising multi-terminal HVDC transmission system, also termed HVDC grids. This paper presents a novel time domain and transient based protection technique for application to HVDC grids. The technique utilises the energy of the forward and backward travelling waves produced by a fault to distinguish between internal and external faults. For an internal fault, the calculated forward or backward travelling wave energy for a pre-set time duration following the occurrence of a fault must exceed a predetermined setting otherwise the fault is external. This characteristic is largely due to the DC inductor located at the cable ends, as per HVDC breakers or fault current limiters, which provides attenuation for the high frequency transients resulting from an external fault. The ratio between the forward travelling wave energy and the backward travelling wave energy provides directional comparison. For a forward directional fault with respect to a local relay, this ratio must be less than unity whereas the ratio is greater than unity for a reverse directional fault. The simulation results presented based on full scale Modular Multilevel Converter Based HVDC grid shows the suitability of the proposed technique. An advantage of this technique is that it is non-unit based and as such no communication delay is incurred. Furthermore, it is simple as it does not require complex mathematical/DSP technique; and as such can be easily implemented at each independent relay since it will require minimal hardware resources hence reduces cost.