What is Short Circuit Analysis, and Why is it Done?

Published by Carelabs (Carelabz), Website: carelabz.com


Image: Carelabz

A Short circuit analysis is used to determine the magnitude of short circuit current, the system is capable of producing, and compares that magnitude with the interrupting rating of the overcurrent protective devices (OCPD). Since the interrupting ratings are based by the standards, the methods used in conducting a short circuit analysis must conform to the procedures which the standard making organizations specify for this purpose. The American National Standards Institute (ANSI) publishes both the standards for equipment and the application guides, which describes the calculation methods. 

Short-Circuit Currents are currents that introduce large amounts of destructive energy in the forms of heat and magnetic force into a power system. A short circuit is sometimes called a fault. It is a specific kind of current that introduces a large amount of energy into a power system. It can be in the form of heat or in the form of magnetic force. Basically, it is a low-resistance path of energy that skips part of a circuit and causes the bypassed part of the circuit to stop working. The reliability and safety of electric power distribution systems depend on accurate and thorough knowledge of short-circuit fault currents that can be present, and on the ability of protective devices to satisfactorily interrupt these currents. Knowledge of the computational methods of power system analysis is essential to engineers responsible for planning, design, operation, and troubleshooting of distribution systems. 

Short circuit currents impose the most serious general hazard to power distribution system components and are the prime concerns in developing and applying protection systems. Fortunately, short circuit currents are relatively easy to calculate. The application of three or four fundamental concepts of circuit analysis will derive the basic nature of short circuit currents. These concepts will be stated and utilized in a step-by step development. 

The three phase bolted short circuit currents are the basic reference quantities in a system study. In all cases, knowledge of the three phase bolted fault value is wanted and needs to be singled out for independent treatment. This will set the pattern to be used in other cases. 

A device that interrupts short circuit current, is a device connected into an electric circuit to provide protection against excessive damage when a short circuit occurs. It provides this protection by automatically interrupting the large value of current flow, so the device should be rated to interrupt and stop the flow of fault current without damage to the overcurrent protection device. The OCPD will also provide automatic interruption of overload currents. 

Short-circuit calculations are required for the application and coordination of protective relays and the rating of equipment. All fault types can be simulated. Carelab’s short-circuit study provides a detailed report identifying breaker ratings, breaker fault duties, discussions, and recommendations for any deficiencies found 

Risks Associated With Short Circuit Currents 

The building/facility may not be properly protected against short-circuit currents. These currents can damage or deteriorate equipment. Improperly protected short-circuit currents can injure or kill maintenance personnel. Recently, new initiatives have been taken to require facilities to properly identify these dangerous points within the power distribution of the facility. 

Why Is A Short Circuit Dangerous? 

A short circuit current can be very large. If unusually high currents exceed the capability of protective devices (fuses, circuit breakers, etc.) it can result in large, rapid releases of energy in the form of heat, intense magnetic fields, and even potentially as explosions known as an arc blast. The heat can damage or destroy wiring insulation and electrical components. An arc blast produces a shock wave that may carry vaporized or molten metal, and can be fatal to unprotected people who are close by. 

Fault current calculations are necessary to properly select the type, interrupting rating, and tripping characteristics of power and lighting system circuit breakers and fuses. Results of the fault current calculations are also used to determine the required short-circuit ratings of power distribution system components including bus transfer switches, variable speed drives, switchboards, load centres, and panel boards. In calculating the maximum fault current, it is necessary to determine the total contribution from all generators that may be paralleled and the motor contribution from induction and synchronous motors. 

Short Circuit Analysis is performed to determine the currents that flow in a power system under fault conditions. If the short circuit capacity of the system exceeds the capacity of the protective device, a dangerous situation exists. Since growth of a power system often results in increased available short-circuit current, the momentary and interrupting rating of new and existing equipment on the system must be checked to ensure the equipment can withstand the short-circuit energy (see Device Evaluation). Fault contributions for utility sources, motors and generators are taken into consideration.  

A Short Circuit Analysis will help to ensure that personnel and equipment are protected by establishing proper interrupting ratings of protective devices (circuit breaker and fuses). If an electrical fault exceeds the interrupting rating of the protective device, the consequences can be devastating.  It can be a serious threat to human life and is capable of causing injury, extensive equipment damage, and costly downtime. 

On large systems, short circuit analysis is required to determine both the switchgear ratings and the relay settings.  No substation equipment can be installed with knowledge of the complete short circuit values for the entire power distribution system.  The short circuit calculations must be maintained and periodically updated to protect the equipment and CC the lives. It is not safe to assume that new equipment is properly rated. 

The results of a Short Circuit Analysis are also used to selectively coordinate electrical protective devices. 

What is Short Circuit Analysis? 

Short circuit analysis essentially consists of determining the steady state solution of a linear network with balanced three phase excitation. Such an analysis provides currents and voltages in a power system during the faulted condition. This information is needed to determine the required interrupting capacity of the circuit breakers and to design proper relaying system. To get enough information, different types of faults are simulated at different locations and the study is repeated. Normally in the short circuit analysis, all the shunt parameters like loads, lime charging admittances are neglected* Then the linear network that has to be solved comprises of  

  • Transmission network  
  • Generator system and  
  • Fault. By properly combining the representations of these components we can solve the short circuit problem

Carelabs allows you to perform a per unit calculation on any system you are working with. We automatically converts the entire system (panel boards, transformers, generators, motorized items and cables) into a unique impedance unit from which you can obtain the rating of the short circuit current at any given point. The process is simple, efficient and will save you both money and time. 

Carelabs provides short-circuit calculations for single and multiple faults, together with a number of reporting options. As short-circuit calculations are needed for a variety of purposes, the short-circuit calculation in Carelabs supports different representations and calculation methods based on a range of international standards, as well as the superposition method (also known as the Complete Method).

What Are Bolted, Arcing and Ground Faults?

A bolted fault typically results from a manufacturing or assembly error that results in two conductors of different voltages being “bolted” together, or a source of power being directly connected (bolted) to ground. Since the connectors are solidly bolted there in no arc created and the high current quickly trips a protective device limiting the damage. 

An arc fault is one in which the short circuit creates an arc. An arc is a flow of electricity between two conductors that are not in contact. The resulting intense heat can result in a fire, significant damage to the equipment, and possibly an arc flash or arc blast resulting in serious injuries. 

A ground fault is when electricity finds an unintended, low resistance, path to ground. When that path goes through a human body the resulting heat can cause serious burns, and the electrical shock can disrupt the functioning of the human heart (fibrillation). 

What Are Symmetrical and Asymmetrical Currents?

A polyphase system may experience either a symmetrical or an asymmetrical fault. A symmetrical fault current is one that affects all phases equally. If just some of the phases are affected, or the phases are affected unequally, then the fault current is asymmetrical. 

Symmetrical faults are relatively simple to analyse, however they account for very few actual faults. Only about 5% of faults are symmetrical. Asymmetrical faults are more difficult to analyse, but they are the more common type of fault. 

What Are Protective Devices for Short Circuit Analysis?

Protective devices are designed to detect a fault condition and shut off the electric current before there is significant damage. There are a number of different types of protective devices, the two most common are: 

Fuses and Circuit Breakers

Fuses and circuit breakers are used to protect an electrical circuit from an over-current situation, usually resulting from a short circuit, by cutting off the power supply. Fuses can only be used once. Circuit breakers may be reset and used multiple times. 

Ground Fault Interrupter (GFI)

This is a device that detects when the current flow in the energized conductor does not equal the return current in the neutral conductor. The GFI protects people by quickly cutting off the current flow preventing injuries resulting from shock.  Ground Fault Interrupters are typically used in homes for bathroom, kitchen, and outdoor electrical sockets. The GFI will typically be built into the electrical socket. 

A GFI does not provide over-current protection, and the circuit that includes a GFI will also include a fuse or circuit breaker. 

In addition to fuses, circuit breakers, and GFIs, there are electrical protection devices that: 

  • detect changes in current or voltage levels 
  • monitor the ratio of voltage to current 
  • provide over-voltage protection 
  • provide under-voltage protection 
  • detect reverse-current flow 
  • detect phase reversal 
When are Short Circuit Analysis Needed? 

The first short-circuit analysis should be performed when a power system is originally designed, though this should not be the only time. These studies need to occur with any facility expansion or with the addition of any new electrical equipment such as circuit breakers or new transformers and cables. Without any new additions or changes, short circuit studies still need to occur on a regular basis of at least every 5-6 years. 

How Is Short Circuit Current Calculated? 

Short-circuit calculations are required to correctly apply equipment in accordance with NEC, and ANSI standards. Depending on the size and utility connection, the amount of detail required to perform these calculations can vary greatly. Carelabs short-circuit analysis will include calculations performed in accordance with the latest ANSI standards. 

Switches, fuses, and breakers that need to interrupt or close into a fault are of special concern. Cables and buswork also have short-circuit withstand limitations, and a thorough study will examine non-interrupting equipment, as well as switches and breakers. Standards such as ANSI C37.010 and C37.13 outline the recognized calculation methods for these equipment-rating analyses. 

These short circuit studies are performed using power system software as per IEEE standards. For larger systems, these short circuit calculations to be performed for both switch gear ratings and relay settings. Knowledge of the computational methods of power system analysis is essential to engineers responsible for planning, design, operation, and troubleshooting of distribution systems. A short-circuit study is an analysis of an electrical system that determines the magnitude of the currents that flow during an electrical fault. Comparing these calculated values against the equipment ratings is the first step to ensuring that the power system is safely protected. Once the expected short-circuit currents are known, a protection coordination study is performed to determine the optimum characteristics, ratings and settings of the power system protective devices. 

NEC 110 requires that a short circuit analysis be done for all electrical equipment and panels. The two most common standards for short circuit current calculations are the ANSI/IEEE C37.010-1979 standard and the International Electro-technical Commission (IEC) 60909 standard. 

The ANSI C37.010 standard was intended to be used for power circuit breaker selection, but it does provide the information needed for NEC 110 required labelling. The IEC 60909-3:2009 standard is more generic. It is intended to provide general guidelines for short-circuit analysis of any asymmetrical short circuit in a three-phase 50 Hz or 60Hz A.C. electrical system. 

Either the ANSI or the IEC short circuit calculation method can be used. They have been compared and found to produce similar results. The ANSI method is commonly used in short circuit current calculation software.  

Our short circuit analysis service: 

  • Is done with support of IEC 60909 (including 2016 edition), IEEE 141/ANSI C37, VDE 0102/0103, G74 and IEC 61363 norms and methods 
  • Is calculation of short-circuit currents in DC grids according to IEC 61660 and ANSI/IEEE 946 
  • We do complete superposition method, including dynamic voltage support of generators connected via power electronics 
  • Multiple fault analysis of any kind of fault incl. single-phase interruption, inter-circuit faults, fault sweep along lines, etc. 
Diakoptic Model For The Short Circuit Analysis (Do We Use This?)

In the short circuit analysis, it is customary to neglect the loads and other shunt parameters to the ground. Under this condition, impedance representation for the transmission network with ground as reference does not exist. However, connection to the ground is established at the generator buses, representing the generator as a constant voltage source behind appropriate reactant. Hence let us consider the combined transmission-generator network and while tearing the network, let us ensure that each sub-network has at least one generator. In practice this should pose no difficulty since large power system 84 networks normally consist of different areas having generations in each area.  

Neplan

Short circuit analysis is performed so that existing and new equipment ratings were sufficient to with stand the available short circuit current. This short circuit analysis can be done either through hand calculations or through known software like NEPLAN. 

Using NEPLAN we can perform short circuit studies on electrical systems in a quick time and effective manner in four steps. 

  • Data Collection and SLD Preparation 
  • Short circuit calculations 
  • Relay Coordination Studies 
  • Load flow Analysis  
Why chose Carelabs for Short Circuit Analysis? 

At Carelabs we differ from competitors in our size and structure and this allows us to be more responsive to change. It also allows us to provide personalized and superior services to you. We follow NFPA-70E and IEEE 1584 guidelines in order to guarantee that we always meet the highest industry standards.  

Benefits of Short Circuit Analysis 

Conducting a short circuit analysis has the following benefits: 

  • Helps avoid unplanned outages and downtime
  • Is critical for avoiding interruptions of essential services
  • Reduces the risk of equipment damage and fires
  • Increases safety and protects people from injuries
  • Determines the level and type of protective devices that are needed
  • Provides the information needed for NEC and NFPA required labels 
  • Keeps you in compliance with NEC requirements 
  • Reduces the risk a facility could face and help avoid catastrophic losses
  • Increases the safety and reliability of the power system and related equipment

Source: https://carelabz.com/what-short-circuit-analysis-done-why/

Published by PQTBlog

Electrical Engineer

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