Estimation of stored energy for TWIN source HVDC transmission line

•For Negative Neutral Beam Injectors (NNBI) utilizing RF negative ion source, grid breakdown is a routine phenomenon during system conditioning and operation phases. During grid breakdown event, there are chances that electrostatic energy stored in the high voltage transmission line is dumped in the grids, which may possibly damage them.•NNBI system with long high voltage transmission line, major contribution for the stored energy emanates from the inter-conductor capacitance of transmission line. The inter-conductor capacitance and thus stored energy of high voltage transmission line could be reduced by proper designing of it.•It is equally difficult and challenging to estimate the inter-conductor capacitance of the transmission line with non-generic cross sections that forms initial step for designing high voltage transmission line. Estimation of inter-conductor capacitance for non-generic cross section (vertical configuration) of transmission line is done.•Stored energy for 1m long prototype of 1:1 scale is estimated experimentally and verified with energy estimated from inter-conductor capacitance.•The stored energy for 120m long high voltage transmission line during grid breakdown is estimated.