Short Circuit Studies

Short Circuit Studies is critical for the safe, economical and efficient operation of all electrical power systems. In case of a short circuit, excessive electric current will flow through the system and could have disastrous consequences including personnel injury, damaged electrical equipment as well as costly downtime. Short circuit calculations provide currents and voltages on a power system during fault conditions. This information is required to design an adequate protective relaying system and to determine interrupting requirements for circuit breakers at each switching location. Fault conditions can be balanced or un-balanced shunt faults or series (open conductor) faults. Often information about contributions to a fault from rotating machines such synchronous machines, large motors would be required as a function of time to determine making and breaking requirements.

Recommendations and guidelines are given in the IEEE Violet Book (IEEE STD 551-2006) as well as North American ANSI C37.5, ANSI C37.010, ANSI C37.13 and International IEC-60909 guidelines. It also supports conventional short-circuit studies without reference to any particular standards.

The results of these studies are provided in tabular form giving details of making and breaking fault currents requirements at each breaker locations (bus locations). Further, plots of peak fault current, dc component of fault current, symmetrical RMS component of fault currents will be provided with respect to time. These calculations will be performed for all types of shunt faults such as 3 phase faults, single line to ground fault, line to line faults and double line to ground faults.

Apart from determining fault levels at various buses, short circuit studies are useful in determining post fault bus voltages in the entire system, post fault network currents in the entire system, and negative and zero sequence currents in various electrical network elements. These calculations generally provide most of the information needed for protection system design, protection setting calculations and relay coordination. These studies may be carried out for various operating scenarios of the plant with existing earthing system and the performance of the plant can be reviewed and remedies can be suggested.The results of these studies will be provided in tabular form and also on single line diagram of the plant. These calculations will be performed for all types of shunt faults.

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Overview of Power System Analysis

An electric power system is a network of electrical components used to generate, transmit and use electric power. This could be the elaborate network that supplies power to a region’s home and industry through an electrical grid transmission system from generating plants located faraway or even a captive power plant/microgrid which generates and consumes power within the same premises itself. The planning, design, and operation commercial and industrial power systems require indepth engineering studies to evaluate existing and proposed system performance, reliability, safety, and economics. Due to high risk of human life and costly equipments associated with any power systems, properly conceived and conducted power system studies and simulations are the only cost-effective way to prevent surprises and to optimize equipment selection. These power system studies identify and help avoid potential deficiencies in the system before it goes into operation. In the case of existing systems, the studies help locate the cause of equipment failure and misoperation, and determine corrective measures for improving system performance. 

In power engineering, an oneline diagram or single-line diagram (SLD) is a simplified notation for representing a three-phase power system. The one-line diagram has its largest application in power flow studies and in engineering design. Electrical elements such as circuit breakers, transformers, capacitors, bus bars, and conductors are shown by standardized schematic symbols. It is a form of block diagram graphically depicting the paths for power flow between entities of the system. Instead of representing each of three phases with a separate line or terminal, only one conductor is represented.  Elements on the diagram do not represent the physical size or location of the electrical equipment, but it is a common convention to organize the diagram with the same left-toright, top-to-bottom sequence as the switchgear or other apparatus represented. A typical SLD is illustrated below for reference:

Today, with the advent of digital computers, Power System Simulations are the basis of modern design of power systems for all industries. Typical studies most likely needed are load flow studies, ground mat design, harmonic measurements, cable ampacity studies, short-circuit studies, switching transient studies, coordination studies, and motor starting studies. The responsible engineer for system design must decide which studies are needed to ensure that the system will operate safely, economically, and efficiently over the expected life of the system. 

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