Bruce L. Graves A Defining a Power System A power system is an assembly of generators, transformers, power lines, fuses, circuit breakers, protective devices, cables, and associated apparatus used to generate and deliver electrical energy to the intended load. A power system s reliability is influenced by a number of factors, some of which are environmental, weather, human error, equipment quality, maintenance, and equipment application. The purpose of a power system is to generate and deliver electrical energy safely, reliably, efficiently, and economically to the intended load. Each of these factors (safety, efficiency, reliability, and economy) plays a major role in the design, modification, or ex- 1997 PhotoDisc, Inc. ccording to the IEEE Brown Book (ANSI/IEEE Standard 399-1990), The planning, design and operation of a power system requires continual and comprehensive analyses to evaluate current system performance and to establish the effectiveness of alternative plans for system expansion. Bruce L. Graves is with World Wide Technologies, Inc. of Greenville, South Carolina. This article appeared in its original form at the 2000 IEEE IAS Pulp and Paper Industry Technical Conference. 14 IEEE Industry Applications Magazine March/April 2001 1077-2618/01/$10.00 2001 IEEE
pansion of a power system. If any of these factors is left unattended, it can jeopardize the future profitability of the facility. Here s how: Safety: If ignored, can cause injury, loss of life, property damage, and destruction. Reliability: If ignored, can cause needless interruption of power, resulting in costly manufacturing outages. Efficiency: If ignored, can cause excessive equipment and energy costs, reducing the overall profitability of the manufacturing operation. Economy: If ignored, economy or false economy can influence the initial installation costs and long-term maintenance and repair costs, either of which reduce the profitability of the facility. Performance Problems Factors that keep a power system from doing its job can be divided into two categories: equipment or component failures and equipment or component operation (intentional or unintentional). Equipment or component that can fail include cable insulation, bus insulation, transformers, circuit breakers, and motors. Some of these failures may be related to the original system design and equipment or device failure. Equipment, component, or erratic operation are represented by actions such as circuit breaker trips, fuses blowing, switch operation, and protective relay operation. Some of those operations are correctly doing their job, and others are operating on conditions that should not cause power system interruptions. Any factor that prevents a power system from doing its job reduces the productivity of the power system and ultimately the facility that it serves. Improving the Productivity Implementing the results and recommendations of power system studies will improve the productivity of a power system. Chapter 2 of ANSI/IEEE 399-1990 states the following: The planning, design, and operation of industrial power systems require engineering studies to evaluate existing and proposed system performance, reliability, safety, and economics. Studies, properly conceived and conducted, are a cost-effective way to prevent surprises and to optimize equipment selection. In the design stage, the studies identify and avoid potential deficiencies in the system before it goes into operation. In existing systems, the studies locate the cause of equipment failure and misoperation and determine corrective measures for improving system performance. The most common system studies done on power systems includes: load flow, short-circuit, stability, motor starting, harmonic analysis, switching transient, reliability, cable ampacity, IEEE Industry Applications Magazine March/April 2001 15
ground mat, and coordination studies. The three studies addressed in this article are short-circuit, coordination, and harmonic studies. Short-Circuit Studies A short-circuit study is the analysis of a power system that determines the magnitude of the currents that flow during a fault. These magnitudes are determined at various points in time after the fault inception. They are then compared to the ratings of the electrical components in the power system. The comparison determines the suitability of the equipment for use in the analyzed power system. Components The list below outlines the various components that comprise a short-circuit study. With the availability of computer software that can complete the calculations, some of these components may actually be performed by the software: Define the study scope (what are the boundary points of the study scope?). Gather data on the power system (including that of the utility serving the facility) and the associated equipment (either in the field for existing power systems or from manufacturers data sheets for new systems). Categorize and tabulate the data. Create a power system one-line diagram (or update an existing one). Convert the raw data into usable data. Create or construct needed impedance diagrams. Calculate the fault current flows throughout the power system, especially at major points of interest (usually related to equipment or device ratings). Read and compare the results of the calculations to the ratings of the associated equipment (this is commonly called a protective device evaluation). Table I. Typical Electrical Equipment Fault Current Ratings Type of Equipment Fuses Molded case circuit breakers Air power circuit breakers Typical Fault Current Ratings in kilo amps 10 ka, 50 ka, 100 ka, etc. 14 ka, 25 ka, 35 ka and up 35 ka, 50 ka, 65 ka and up Switchgear bus bracing 35 ka, 50 ka, 65 ka, 100 ka, etc. Motor control center bus bracing Medium voltage switchgear 35 ka, 50 ka, 65 ka, 100 ka, etc. Momentary and interrupting ratings of 58 and 28 ka Tabulate the results of the protective device evaluation, which should include recommendations for corrective actions where they may be required. Results The result of a short-circuit study is a compilation of data (usually from a computer analysis) indicating the available fault current at all major components of a power system. These results are then compared to the ratings of the equipment (existing or proposed). The comparison is commonly called a protective device evaluation. The protective device evaluation is made to determine where and if the electrical components are exposed to current magnitudes greater than the ratings of the equipment. This portion of the study should also provide recommendations on corrective actions. Why a Short-Circuit Study? The specification and purchase of new electrical power equipment is dependent on selecting the equipment with the proper ratings for the application. The selection of the equipment is partially based on the magnitude of the fault currents that can flow through the equipment. The equipment selected must be designed and built so that it can successfully handle (carry and interrupt) the fault currents that flow during a short circuit. A shortcircuit study is needed to determine the magnitude of these fault currents. It is imperative that the ratings of the selected equipment equal or exceed the magnitude of the calculated values of the short-circuit currents that can flow during a fault. After the initial design work is completed successfully, many power systems experience changes that result in the need for new short-circuit studies. Table I lists a few of the various types of equipment and their ratings. This table is not intended to be complete. It is suggested that the reader refer to the appropriate manufacturers data sheets and the appropriate equipment standard (IEEE, ANSI, NEMA, or UL) for a complete listing of ratings. How Often Is a Short-Circuit Study Performed? Short-circuit studies should be performed as a part of the initial design of an electrical power system and at any time a significant modification is anticipated. The original or base study must be completed so that the electrical power system components can be properly selected. After a power system design has been completed and placed into operation, the usual expectation is that the loads will grow with the expansion and growth of the facility. The expansion and growth will usually involve the addition of new motors and loads, necessitating the installation of new motor control centers, circuit breakers, switchgear, switchboards, transformers, and associated electrical system components. 16 IEEE Industry Applications Magazine March/April 2001
As motor loads increase, their fault contribution to the existing system increases, exposing the existing power equipment to higher available fault current magnitudes. These additions can reach a point where the fault currents equal or exceed the ratings of the installed electrical equipment. Adding load to a power system must not expose the existing equipment to fault current magnitudes in excess of the equipment ratings. Continued evaluation of the short-circuit currents associated with the modifications must be completed to ensure that the fault current magnitudes are not allowed to increase to a point above the equipment ratings. If modifications to a facility are constant, then short-circuit studies must be completed regularly in anticipation of the modifications. The frequency of the studies should follow the frequency of the modifications to the facility. Some facility owners have adopted a regular schedule of updating their studies every five years, when little or no change is made to a facility. Planned regular updates will catch changes made by the utility as well as the plant. Some facility owners prepare five-year plans for their facility s operation and growth. Short-circuit studies should be used as a tool in assisting in the preparation of these plans. They can be used to assist in evaluating proposed changes and additions to facilities and determine how the facility can best spend capital for new electrical equipment. Depending on the magnitude of the proposed changes, the costs of the new electrical equipment can impact the costs of the project significantly. Short-circuit tables and curves are found in application literature. These tables and curves are often used as a quick means of determining shortcircuit currents. They are based on assumptions unknown to the user and do not cover the entire power system being evaluated. Some of these may match equipment ratings to transformer ratings. A word of caution is in order: tables and curves do not replace exact engineering calculations. Coordination Studies A coordination study is the process of determining the optimum characteristics, ratings, and settings of the power system protective devices. The optimum settings are focused on providing systematic interruptions to the selected power system segments during fault conditions. Engineers in process-related industries whose processes can cause equipment damage and/or human injury in case of an unplanned shutdown will design the protective system on the operating philosophy consistent with the safety of the process. Design and coordination of these systems requires special considerations that are beyond the scope of this article. Those industries engineers whose processes can afford limited outages generally design and coordinate their electrical system in a manner so that electrical equipment damage is minimized and the power system outage is limited to the smallest selected portion of the power system. Components The list below outlines the various components that make up a coordination study. Computer software may be able to perform many of these tasks. Define the study scope (what are the boundary points of the system included in the study scope?). Gather data on the power system (including that of the utility serving the facility) and the associated equipment (either in the field for existing power systems or from manufacturers data sheets for new systems). Categorize the data. Create a power system one-line diagram (or revise an existing one). Review the protective device characteristics, ratings and settings. Define the desired results (level of selectivity). Select preliminary device characteristic, rating or setting. Prepare the final settings and/or time current curves. Tabulate the results and recommendations. Several of the above items are similar to those of a short-circuit study. One usually gathers field data for both a short-circuit and a coordination study at the same time. The results of a coordination study are recommendations on the proper protective device characteristic, rating, or setting selection for all of the system protective devices. These recommendations should provide, when implemented, a power system whose protection devices will provide the level of system protection and selectivity planned. Why a Coordination Study? A coordination study and the implementation of the resulting recommendations will minimize device erratic operation and (i.e., nuisance power outages) electrical equipment damage and provide improved power system reliability. A coordination study is needed: When the initial power system is designed, When new loads are added to the power system, When existing equipment is replaced with equipment whose ratings are higher, When a fault on the periphery of the system shuts down a major portion of the system, When the utility or source short-circuit current changes, and When new relays are installed to replace or upgrade the existing relays. Properly coordinated power system protective devices will result in a power system that is highly reliable and minimizes equipment damage during IEEE Industry Applications Magazine March/April 2001 17
faults. It will also afford operation with the least exposure to outages. In many cases, coordination studies reveal areas where exact coordination between two devices is not possible. In these cases, engineering judgment is used to select the most desirable configuration usually one that will minimize equipment damage. Harmonic Analyses A harmonic analysis evaluates the steady-state effects of nonsinusoidal voltages and currents on the power system and its components. Some of the sources of these wave shape disturbances are: DC rectifiers, adjustable-speed drives, arc furnaces, welding machines, static power converters of all kinds, and transformer saturation. Why Harmonic Analysis? Electrical equipment used to generate, distribute, and utilize electrical energy is designed for use on power systems that supply pure sine waves. When a voltage or current wave shape is distorted, it causes abnormal operating conditions in the system and equipment. Various types of equipment are affected differently by these wave shape distortions. Some examples are: Voltage distortion can cause additional heating in induction and synchronous motors and generators. Voltage distortion with high peak values can weaken insulation in cables, windings, and capacitors. Voltage wave shape distortion can also cause malfunction of electronic devices that use wave shape for timing. Harmonic currents in motor windings can cause higher noise emissions. Harmonic currents cause additional heating in transformers. Harmonic currents flowing through cables cause higher heating over and above the heating expected from rms currents. Harmonic currents flowing through switchgear can increase heating and losses in the switchgear and circuit breakers. Resonant current flows can cause capacitor failures and/or fuse failures in the capacitor or other electrical equipment. Protective relays and circuit breakers can trip falsely due to harmonic currents. When Is a Harmonic Analysis Needed? A harmonic analysis is usually needed when any of the following conditions exists: During the design phase of a facility that consists of a large harmonic-generating load (arc furnaces, rectifier lines, a large concentration of variable-speed drives), When a utility limits its distortion to the system voltages and currents, Plant expansions that include large amounts of harmonic generating loads, Installation of power-factor-correction capacitors on power systems that contain large harmonic generating loads, and History of capacitor fuse failures. What Completes a Harmonic Analysis? The same data needed to complete the short-circuit study is needed for this work. In addition to that data, a good understanding of the power system serving the facility is needed by the engineer performing the analysis. The engineer must know if there are capacitors on the utility system that may affect the results of the study. Additionally, he or she must also know: 1) the utility harmonic voltage spectrum at the point of common coupling, 2) data on all system capacitors, 3) specific system configurations and operating characteristics and procedures on all converter circuits to be studied, and 4) any utility limitations on distortion limit. Results The result or solution to a harmonic problem is to shift the harmonic resonant point to some other frequency not generated by the equipment in the system. Through the course of the evaluation, the system power factor may also be adjusted to an improved point. The use of filters may be required to shift the resonant point. The analysis may reveal that some system components require derating due to the heating caused by harmonic currents. The net result of the harmonic analysis, once the recommendations of the analysis are implemented, is a power system whose components are not exposed to voltages or currents that exceed their capability or the mill or utility requirements. It is important that these studies, along with others, be given the attention needed at the appropriate times throughout the life of a facility. References [1] IEEE Recommended Practice for Power Systems Analysis, IEEE Std. 399, 1990. [2] IEEE Recommended Practice for Electric Power Distribution for Industrial Plants, IEEE Std. 141, 1993. [3] D. L. Beeman, Ed, Industrial Power Systems Handbook. New York: McGraw-Hill, 1955. [4] J.L. Blackburn, Protective Relaying Principles and Applications. New York: Marcel Dekker, 1987. [5] IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems, IEEE Std. 242, 1986. [6] IEEE Recommended Practices for Harmonic Control in Electric Power Systems, IEEE Std. 519, 1992. [7] C.K. Duffey, ESA s Practical Guide To Power System Harmonics, ESA seminar notes. [8] R.H. McFadden, Power-System Analysis: What It Can Do for Industrial Plants, IEEE Trans. Ind. Applicat., Mar./Apr. 1971. 18 IEEE Industry Applications Magazine March/April 2001