Verification of Lightning Protection by Current Injection in a LNG Plant

Due to its lightning prone location, the construction of a new Liquefied Natural Gas (LNG) plant in East-Malaysia required specific attention in terms of lightning protection of the electronic control systems. In line with the earthing and bonding approach of IEC 61000-5-2 [2], all cables entering the control building are led through special cable entry windows, which allow exchange of induced common mode currents between the cable armouring or shields and the local grounding system. In order to verify the correct implementation of this approach, the quality of the cable entry window was assessed by measurement To this end, currents were induced in a selected set of multi-core (10x2C and 20x2 C) steel wire armoured cables, first by a Common Mode (CM) current injection clamp (less than 1 A at 20 kHz), and subsequently, by a direct capacitor discharge (damped sine wave with a frequency of about 22 kHz and an amplitude up to 1.4 kA) on process modules. The frequency was selected to ensure that the current distribution would be dominated by induction, as will be the case for realistic lightning discharges. The linearity required for extrapolation to an actual lightning stroke has briefly been investigated. As a result of the exercise, safe operation of control equipment inside the control building is to be expected under lightning conditions. However, where direct discharges occur in the vicinity of field equipment, non-linear effects are expected due to local sparking and non-linear behaviour of steel Parallel Earthing Conductors (PEC) structures [3], resulting in surge voltages in excess of industrial surge immunity levels. For these locations, very strict bonding measures and proper selection of locations are required to protect the equipment.