Cable-Capacitance Discharge Time With and Without the Application of Grounding Device

This paper provides inherent cable capacitance, its stored energy, its maximum transient voltage, and time to discharge such a voltage and energy to a safe level with and without the application of a grounding device. Maintenance procedures require that appropriate grounding devices be applied to electrical equipment upon its deenergizing before performing inspection or maintenance for safety reasons. The authors have not come across published literature describing how to determine the discharge time of an inherent cable-capacitance charged voltage. Shore power supply to a berthing ship is called cold ironing; it helps to minimize air pollution. In the cold-ironing power system design, medium-voltage heavy plug and receptacle assemblies are required to make and break shore power connections to the ship power system as the ship arrives and leaves the port after the cold-ironing operation. Therefore, there was a need to know the discharge time of the cable capacitance without the application of the grounding device for safety reasons. A recommendation for a future research project to develop a recording/measuring device for the cable-capacitance voltage discharge time to validate the calculated discharge time presented in this paper is included. For safe and efficient operation of the cold-ironing operation, implementation of a grounding switch key interlocked with an associated main disconnect switch and power plugs (for discharging quickly cable-capacitance voltage and charged energy to ground) shall be included.

I. Introduction

The shore-side power system infrastructure requires a medium-voltage power system to provide large amounts of power of up to 10–20 MW to bigger ships. The exact voltage rating of such a power supply system lies with the ship's owners, who make provisions inside the ships to receive shore power. At present, there are two common voltages, namely, 11 and 6.6 kV, at 60 Hz, three-phase, and three-wire, at the ships for shore power connections. This paper provides a discussion of inherent cable capacitance, charged transient voltage, and energy and their discharging time profile with and without the application of a grounding device. This discussion is based upon an actual cold-ironing project at a specific port. To comply with new air-pollution laws, many large ports with large ships all around the world are facing the challenge of implementing cold-ironing infrastructure, and the design of such a power system must implement all components known to a power system engineer for safety reasons.