Standards Development

Scope

This International Standard outlines procedures for calculating short-circuit currents that may occur on a marine or offshore AC electrical installation.The calculation methods are for use on unmeshed three-phase alternating current systems,a)      operating at 50 Hz or 60 Hz;b)      having any system voltage specified in IEC 60092-201;c)      having one or more different voltage levels between 1kV and 15kV specified in IEC 60092-503;d)      comprising generators, motors (both synchronous and asynchronous), transformers, reactor coils, cables, and converter units;e)      having their neutral point connected to the ship’s hull through an impedance (designed to limit the short-circuit current flowing to the ship’s hull);f)       having their neutral point isolated from the ship’s hull;g)      having their neutral point directly connected to the ship’s hull.The calculation procedures are for–        a three-phase symmetrical short-circuit condition i.e. three- phase conductors shorted together;–        or shorted to the ship’s hull and for which the short-circuit occurs on all three poles simultaneously–        or a line-to-neutral or a line-to-earth short-circuit condition for systems with directly earthed neutral.The calculating formulae and methods described produce sufficiently accurate results to calculate the short-circuit current during the first 100 ms of a fault condition. For time periods beyond 100 ms the controlling effects of the system voltage regulators may be predominant.The object of this standard is to present formulae for calculating short-circuit currents of the active components of an electrical system and to indicate how such formulae may be simplified when calculating the short-circuit current at various locations on the electrical system. The calculations give estimates of the prospective short-circuit current when the system’s active components produce their maximum contribution.The calculating formulae are developed from basic electrical engineering theories relevant to the system components. To simplify the calculation methods yet retain an acceptable level of accuracy in the results, suitable assumptions are outlined and their effects on the calculation results indicated.This document specifies calculation methods, which can be used to find maximum and minimum short-circuit currents for the fault situations specified.NOTE The generator short circuit decrement curves can be lower than the steady state current between 100 ms and 1 s. Therefore this behaviour should be considered when setting the protection devices. Similar effects may apply for static converters.This standard provides a calculation procedure. It gives guidance on the interpretation and application of simplifying formulae and the corresponding results when applied to the selection of switchgear. The standard is not intended to provide any other information than the calculation of short-circuit current which may flow in the network.In order to understand the methods, and consequences of the results, it is assumed that the person responsible for initiating short-circuit current calculations, is thoroughly familiar with valid electrical engineering fundamentals.