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Gapless Metal (Zinc) Oxide Surge Arresters for a.c. systems contribute to the insulation co-ordination based on the suppression of lightning surges and switching surges. These gapless metal oxide surge arresters using ZnO elements are effective to HVDC systems. This paper describes basic characteristics of ZnO (zinc oxide) elements for d.c. systems and applications of gapless surge arresters to HVDC 125kV frequency converters, HVDC 250kV, upgrade HVDC 500kV converter stations, and HVDC 500kV cables of Japan through the experience of developments and applications of gapless metal oxide surge arresters.

This paper presents a new concept for back-to-back (BTB) dc systems. The main feature of the proposed BTB is referred to as “massively parallel.” This massively parallel BTB system consists of a number of small or mediumcapacity inverter units that are connected in parallel. The concept of transmission power control using the massively parallel BTB is investigated using MATLAB for a case where a fault occurs. In addition, a method for modeling the proposed BTB in EMTP is investigated. The work presented in this paper shows that the proposed BTB system is very promising for large power system applications because the losses of the system are very small compared to conventional BTB using VSC (voltage source converter). The massively parallel BTB can keep operating even if an inverter unit fails, thus improving overall power transmission capability. © 2001 Scripta Technica, Electr Eng Jpn, 138(3): 34–42, 2002