Power System Studies

April 8, 2002
Designing a Safe and Reliable Electrical System - Part 3
There are several different types of power systems studies that can be performed on an electrical system. The proper study choice depends upon the areas of concern identified on the one-line diagram or the problems the system has already experienced.Following is a description of several power systems studies that can help facilities managers develop and maintain a good electrical distribution system:• Short Circuit Study. A short circuit study calculates the available short circuit currents at each protective device. Even the best-designed electrical distribution systems will occasionally experience short circuits. Over-current protective devices, such as fuses and circuit breakers, should safely isolate the fault current at desired locations with minimal equipment damage and minimal disruption to the facility. Other components of the distribution system, such as transformers, cables, and disconnect switches, must be able to withstand the mechanical and thermal stresses produced by the fault current flowing through them. Results of a short circuit study will provide the necessary electrical equipment rating for each protective device.In a newly designed system, a short circuit study ensures that the equipment is properly rated. However, if this new system is being added to an existing electrical system, the study must include the entire system. The existing system is often overlooked in the short circuit study of a new expansion. A comprehensive study of the complete system ensures that all equipment is properly rated and that the existing electrical equipment is capable of withstanding the changing electrical needs brought on by the new system. Remember that this design issue must be addressed for the comprehensive system and not separately for each individual segment. • Protective Device Coordination Study. A protective device coordination study determines the necessary characteristics, ratings, and settings of all the over-current protective devices within the electrical distribution system. This study ensures that the minimum unfaulted load is interrupted when the protective devices isolate a fault or overload anywhere in the distribution system. It also ensures that the devices and settings are selected to provide satisfactory protection against overloads on equipment and interrupt short circuits as rapidly as possible.The coordination study evaluates current transformer ratios, protective relay characteristics and settings, fuse ratings, low-voltage circuit breaker ratings, characteristics, and trip settings. The study provides the information needed to design a system with the optimum level of protection and selectivity in coordination of devices. When coordinating a new system into an existing electrical system, the protective device study ensures that the two systems are coordinated.• Load Flow Study. The load flow study is an analysis of system capability to supply energy to the connected load under steady state conditions. It demonstrates the distribution of power and voltage levels needed throughout the system for selected operating scenarios, and may include normal and emergency operating modes, present and future circuit arrangements, and alternative designs and equipment components.Study results allow the power system engineer to identify overloaded transformers and cables, select proper transformer tap settings, determine the need for power factor correction capacitors, and identify suitable areas for additional loads.

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