Toshiya Imai

Development of high gradient zinc oxide nonlinear resistors and their application to surge arresters

Zinc oxide nonlinear resistors with double the conventional reference voltage gradient, while improving their protective characteristics and raising discharging capability, were developed and applied to tank-type surge arresters. On the basis of high-performance elements, which provide good protective characteristics, a simulation was performed on the relation between the composition of the raw material acid the current-voltage (I-V) characteristics, consequently achieving a higher reference voltage gradient and an improved I-V nonlinearity. Moreover, the discharging capability was raised by improving the thermal distribution in the element when a current is applied and reducing their pores. This, when applied to surge arresters, permitted the number of elements, with the same diameter as conventional elements, to be halved. As a result, the number of parts, size and weight of the surge arrester were reduced and its structure was simplified. A series of tank-type surge arresters using these elements for 66-550 kV power systems was developed, tested in various ways without problems, and applied in the field.

Computation of Arrester Thermal Stability

Full-scale thermal stability type tests on substation class metal oxide arresters are not practical. As a result, standards such as IEC 60099-4 specify the use of a reduced-scale model and define test methods which are intended to assure that the reduced-scale model is conservative relative to the full-scale arrester. The authors succeeded in computing the thermal stability of full-scale arresters based on the operating duty arrester type test specified in IEC60099-4 and ANSI in a few hours using transient nonlinear finite element software customized for the purpose. Based on these computations, the cooling mechanism of a polymer arrester and the effect of arrester structure on thermal stability could be evaluated. The authors are continuing this approach with the expectation of achieving more accurate prediction of full-scale arrester stability computationally than can be achieved using reduced-scale models. The present paper contributes to the understanding of appropriate reduced-scale models, their relationship to the thermal stability of full-scale arresters, and how to define the thermal stability of a full-scale arrester.

Effect of Externally Gapped Line Arrester Placement on Insulation Coordination of a Twin-Circuit 220 kV Line

Application of externally gapped lightning arresters (EGLAs) to a twin circuit 220 kV transmission line was simulated using Alternative Transients Program/Electromagnetic Transients Program. The lightning current to the tower top as well as to phase conductors was estimated using the electrogeometric model. Increasing the number of phases on which EGLAs were installed in one circuit reduces the likelihood of back flashover. Installing EGLAs on the three phases of one circuit has two advantages, viz., 1) eliminating double circuit outages and 2) preventing lightning-induced back flashovers for strikes to the tower top ≤ 200 kA. However, EGLAs on one circuit cannot improve lightning performance related to the shielding failure when the unprotected circuit is hit. EGLA duty varied little with the number of phases on which EGLAs were installed, which indicates that the required EGLA rating is unlikely to depend on the number of phases on which EGLAs are installed.