3a Short Circuit Study Opening Section.pdf

Short Circuit Analysis Study

Short Circuit Analysis Study Page 1

A Short Circuit Analysis Study is typically comprised of three parts – Findings and Recommendations, Short Circuit Data and Short Circuit Calculations. The Findings and Recommendations are the culmination of the engineer’s observations and assessment of present conditions. These findings are generally related to equipment being exposed to conditions where available fault currents exceed the equipment’s ratings.

This section on Short Circuit Data is used to report the studied system’s actual system configuration (generally by one-line diagrams) and a detailed listing of the component pieces comprising the system (e.g., cables, transformer, motors, breakers, etc.). The Short Circuit Calculation section contains the computer-generated analysis of the system(s). Multiple scenarios are often generated to analyze different operating conditions and/or possible alternate configurations for future considerations.

A Short Circuit Study forms the foundation for the overall power system analysis. To conduct a short circuit study the electrical system(s) is modeled capturing the system’s overall configurations. System components such as transformers, switchgear, panelboards, cable data, motors, etc. are input into the software in a manner to interconnect the individual components as they occur in the real world. The completed model is used to simulate 3-phase faults at each piece of equipment. In this section the resultant fault currents are compared with the associated equipment’s withstand rating (ability to withstand the mechanical forces produced by fault currents). An Equipment Duty Report is generated by the Easy Power that tabulates a listing of equipment and their corresponding exposure to fault currents and as a percent the equipment’s rating related to available fault current.

The fault currents are required as input data for both the coordination and arc flash studies.

Findings and Recommendations

Throughout the campus there are multiple incidences where the available fault currents exceed the equipment’s withstand rating. These conditions range from values just under the equipment’s ratings, designated as warning to that of danger when the available fault currents exceed the equipment’s rating.

In most incidences these locations are panelboards. The overall panelboard’s rating is established by the lowest rated device. This could be a single branch breaker, the main breaker or the panel’s bus bracing. If one or more breakers are the reason for placing the panel in the danger or warning condition, it may be possible to just replace these breakers with properly sized breakers.

At the end of this section is a condensed tabulation of equipment by system found to be in Violation or noted as Warning.

Short Circuit Data

Before an electrical system can be analyzed and Short Circuit Calculations performed, system data must be collected and modeled to represent the studied system. Collecting system data is generally an exercise of reviewing existing documents, particularly existing one line diagrams, and the collection of site data.

Collection of site data typically requires a site investigation to record equipment manufacturer data and assesses actual field conditions. Collected data is generally a mixture of electrical and non-electrical (wire gauge, length, Model numbers) properties and their interconnection within the electrical system. An

Short Circuit Analysis Study Page 2 electrical system’s physical components include such items as listed below and their physical properties as indicated:

Utility Data – System Nominal Voltage, Available Fault Current and X/R ratio and/or Symmetrical Component Values.

Cables – Voltage and Type Insulation, Length (in feet), Wire gauge, Copper or Aluminum, Type (single conductor, multiple conductor, in conduit (steel or aluminum), etc.).

Cable or Bus Duct –Voltage, Ampacities, Length, and related impedance data.

Transformers – Primary and Secondary Voltage, kVA rating, Percent Impedance, Winding configuration.

Switchgear and Protective Devices – Voltage Rating, Ampacities, AIC ratings.

Generators – Base kVA, Voltage Rating, X’’dv Sub transient reactance.

Motors – Base Voltage, Horsepower, Type (induction, synchronous, etc.), RPM.

Current Limiting Reactors – Voltage, Ampacities, kVA rating, winding configuration and related impedance data.

While physical properties are familiar and readily identifiable to an individual, they are not suitable for a digital computer to perform short circuit calculations. This data has to be converted into its electrical properties – impedance (resistive and reactance values). The Short Circuit Program contains sub programs to convert the individual electrical components to their equivalent electrical properties. Some of these conversions are direct mathematical conversions yielding precise impedances. Other conversions use empirical data from the software’s equipment library to establish typical impedance values. This is a standard practice in the industry and is necessitated because manufacturer impedance data is not published or available.

The one line diagram is used both to establish interconnections and arrangement of the electrical system’s components forming the system and for presentation purposes. The collected system’s components are input into the electrical analysis software to create a one line diagram establishing their relationship with respect to each other while displaying their physical properties (cable, motor, etc.). The resultant one line diagram is a visual tool for presentation and for verification of data input accuracy. The software in the background translates the one line into an impedance diagram for matrix computational purposes.

A system bus is established in the impedance diagram for every interconnection of two or more system components. A bus is also referred to as a node and is a point in the electrical system model for the termination of system components. A system component has two or more termination points with the exception of voltage sources such as utilities, generators and motors. For example, a cable connecting a switchgear breaker to a motor is terminated on both ends of the bus. The same cable without the motor is still terminated on both ends to the same buses.

Short Circuit Calculation and Theory

Short Circuit Calculations are used to simulate fault conditions and the resulting maximum current flow that the system devices and interconnections would be subject to in the event of a system fault. The

Short Circuit Analysis Study Page 3 computer short circuit program calculates fault currents for each bus within the system. The resulting calculations are used to evaluate adequacy of installed equipment’s bracing; interrupting capabilities of switches, breakers and fuses; and to establish protective device coordination settings.

Short Circuit calculations use the impedance diagram established from the one line diagram to perform digital matrix calculations to generate system fault currents. Fault currents are determined for each bus by computer simulation of a fault at each bus. Depending on the study’s objectives, different fault types can be simulated such as three phase faults (three phases effectively bolted together), phase to ground, etc.

The fault calculations were performed using Easy Power Version 8.0.2 software by ESA. This program is compliant with standard established by ANSI/IEEE Standards C37.13.1 “IEEE Standard for Definite Purpose Switching Devices for Use in Metal-Enclosed Low-Voltage”, C37.01 “IEEE application guide for AC high-voltage circuit breakers rated on a symmetrical current basis”, and C37.5 “IEEE Guide for Calculation of Fault Currents for Application of AC High-Voltage Circuit Breakers Rated on a Total Current Basis”, NEMA, and NEC.

The program offers four different fault types for short circuit analysis. Three phase is generally used to determine the highest available currents for equipment duty comparison and establishing relay settings.

Line to Ground is generally used to determine ground fault values for setting ground relays. Double Line to Ground and Line to Line are generally used for specialized relaying applications or system trouble shooting. This study included Three Phase and Line to Ground.

Short circuit calculations are based on ANSI Standards. The ANSI Standard method used a separate R network for the interrupting duty (2-5 cycles) network to determine a conservative Z/R ratio. This ratio is then used as the Thevenin equivalent fault point X/R ratio for determining the appropriate breaker contact parting time multipliers and NACD ratios. NACD (No AC Decrement) ratios are calculated with consideration of generator Local and Remote contributions as outlined in ANSI Std. C37.010 and Interpretation of New American National Standards For Power Circuit Breaker Applications by Walter C.

Huening Jr. IEEE Oct 1969. Current and voltage calculations are based on a complex (R+jX) network reduction. Both the momentary (1/2 Cycle) and the 30 cycle calculations use a complex network reduction for all voltages, current, and X/R ratio calculations.

ULow Voltage Analysis Low voltage power circuit breakers are applied in accordance with ANSI Std.

C37.13. Current manufactured breakers are rated on a symmetrical interrupting basis. Pre-1957 low breakers had an average asymmetrical rating and were required to be at least 1.25 times the ½ cycle symmetrical short circuit duty, For momentary duty (1/2 cycle) faults, the positive sequence impedance is assumed equal to the negative sequence impedance. X/R ratios are derived from the complex network.

UInterrupting Analysis The values calculated are to be compared with the interrupting duties for high voltage (above 1 kV) circuit breakers. Duty faults are modeled using multipliers to modify rotating machine sub-transient impedances (positive sequence) as outlined in ANSI Std C37.010. Negative sequence impedances are modeled using the rotating machine subs transient impedances with no multipliers. A separate “R”

Short Circuit Analysis Study Page 4

(resistance) network is formed for calculation of the fault point X/R ratio. The X/R ratio used for the calculation of the interrupting duty multipliers is then found from the relationship Z/R. This method fully complies with the ANSI Standard.

For 30 cycle faults, all motor contributions have decayed to zero, and a modified generator impedance of

1.5 X”dv is used. This provides conservative results which are typically higher than most dynamic studies indicate.

Symmetrical Fault currents are AC currents without any DC component. Low Voltage equipment bracing and interrupting duties are evaluated based on symmetrical current.

Asymmetrical Fault currents have both an AC and DC component. The asymmetrical value is the maximum possible asymmetrical value in any phase. Asymmetrical currents are based on the following equation:

Iasym = I sym [ 1 + 2e{-4πτ / (X/R)}]1/2 τ = 0.49 – 0.1 e{-(X/R)/3}

For interrupting duty currents the asymmetrical value is based on the fault point X/R ratio, the No AC Decay (NACD) ratio, the contact parting time of the breaker, and whether the breaker is rated on a Total or Symmetrical basis. The NACD ratio consists of two factors dependent upon whether generation is Local or Remote. The basis of whether a generation is local or remote is based on ANSI standards. A utility source is almost always Remote because a system fault has no impact on the utility fault contribution.

ULow Voltage (LV) DutiesU are based on momentary or ½ cycle faults. Per ANSI terminology, the term “interrupting” as used for low voltage equipment applies to momentary or ½ cycle faults. This is because low voltage equipment typically interrupts within ½ cycle of fault inception. The interrupting currents are calculated by adjusting the symmetrical current if the equipment test X/R is less than the fault point X/R ratio. The following equation is used to determine the currents:

Iadjsym = I sym [ 1 + e{-2πτ / (X/Rsys)}]/ [ 1 + e{-2πτ / (X/Rtest)}]

X/Rtest = TAN[ARCCOS(Tested PF)]

High Voltage (HV) Breaker Duties are based on adjusted symmetrical currents. The interrupting currents are calculated by adjusting the symmetrical currents with a multiplying factor. For breakers the multiplying factor is based on the fault point X/R ratio, the No AC Decay (NACD) ratio, and the contact parting time of the breaker, and whether the breaker is rated on a Total or Symmetrical basis. The NACD ratio consists of two factors that are dependent upon whether generation is Local or Remote. These calculations are based on ANSI standards and are too detailed for inclusion in this report.

Short Circuit Analysis Study Page 5

Fuse Duties (HV & LV) are based on adjusted symmetrical currents. Fuse multiplying factors are based on the fault point X/R ratio and the fuse test X/R ratio. The interrupting currents are calculated by adjusting the symmetrical current if the equipment test X/R is less than the fault point X/R ratio. The following equation is used to determine the currents:

Iadjsym = I sym [ 1 + 2e{-4πτ / (X/Rsys)}]1/2/ [ 1 + 2e{-4πτ / (X/Rtest)}]1/2

Standard test X/R ratios are 5, 8, 12, and 15 for distribution and power fuses.