Protection of EHV transmission lines with series compensation: BC Hydro´s lessons learned

The BC Hydro transmission network includes 33 wholly owned 500 kV circuits and three 500 kV interconnections with other utilities. Eleven of these lines include series compensation, and many were upgraded to provide single-pole tripping (SPT) capability to improve power transfer capability and stability margins. To reduce construction costs, the majority of these lines are not shielded, which increases the probability of low-grade, single-line-to-ground faults due to high tower grounding resistance. Over the years, BC Hydro has developed strict performance specifications for both sensitivity and speed of protection systems based upon fault location and fault type. We also have a well-developed process for model power system testing of protection systems to verify conformance to these performance specifications. In this paper, we discuss the lessons learned in applying protection systems and optimizing protection sensitivity and speed for these challenging applications. We also discuss a number of problems and challenges discovered in real-world experiences and during simulation, many of which are associated with discerning the difference between normal system unbalances and/or unbalances caused by operation of series capacitor protection and resistive ground faults up to 300 Omega. The paper includes a discussion of the impact of reclosing control and automatic reinsertion control of series capacitors on protection systems. The paper also includes a number of digital fault recorder (DFR) records for actual power system faults on series-compensated lines.