The Art of Protecting Electrical Systems
From 1965 through 1970, Consulting-Specifying Engineer’s predecessor, Actual Specifying Engineer, ran a series of articles on overcurrent protection.
From 1965 through 1970, Consulting-Specifying Engineer’s predecessor, Actual Specifying Engineer, ran a series of articles on overcurrent protection. Due to the immense popularity of the 31 installments, the authors, George Farrell, PE, and Frank Valvoda, PE, reprised the series in an updated version beginning in the Feb. 1989 issue of CSE . Over the years since the last installment ran in the late ’90s, CSE has received many requests to rerun this series. Addenda describing new information that has been added will appear at the end of each article. “The Art of Protecting Electrical Systems” has made the transition from 20th to 21st century—and from print to the web.
“The overcurrent protective system is the very heart of the electrical distribution system. Faulty or inadequate overcurrent protection can bring about the loss of the entire facility served by the system—or, worse, it can cause needless death or injury to personnel.”
In this article, some of the concepts underlying the study of electrical systems are outlined: linearity, superimposition, the Thevenin equivalent circuit, the sinusoidal forcing function, vector representation, the single-phase equivalent circuit, symmetrical components and the per-unit method. Understanding these concepts is necessary for comprehending the material that will following in the series.
In the previous article, the single-phase equivalent of a typical power distribution system was discussed. It was said to consist of a voltage source with a load, plus series impedance, i.e., resistance and reactance connecting the source and the load. Capacitance is not usually a significant factor in the typical distribution system, except in long transmission lines and when introduced to improve power factor. In consequence, even with intentionally introduced capacitance, almost every system has a net inductive balance, that is, a resistance-inductance (R-L) circuit with current lagging the voltage.
Engineers designing protection for electrical systems must consider the many changes that take place when a short circuit occurs. Protective equipment must be able to withstand the effects of short circuits, minimize damage and restore service as quickly as possible.
When a short circuit occurs, generators—whether utility or on-site—do not stop generating instantly, even if the drivers are immediately shut down. Because the only thing limiting the initial current a generator can deliver to its shorted terminals is its internal impedance, initial current flow may be 20 or more times the rated load current.
This sixth part in our series discusses the need for short-circuit calculations and the current-limiting effect of some overcurrent protective devices. Whenever a short circuit occurs, every component in the system carrying the fault current must safely withstand the heating and magnetic stresses caused by the current. In addition, the protective device interrupting the fault current must do so safely and reliably.
Part Seven of this ongoing series discusses the importance of overcurrent protection theorems used in short-circuit calculations and component ratings.
Preceding articles explored a number of fundamentals involved in ensuring protection of electrical systems, emphasizing the need for short-circuit calculations and ratings. This article presents general definitions and comments on electrical-component short-circuit ratings. It points out the need to determine the maximum short-circuit currents as accurately and completely as possible and introduces ways to calculate short-circuit currents.
This installment of our ongoing series introduces short-circuit calculations, starting with the fundamentals and proceeding on through available computer programs. In previous articles, we discussed the importance of overcurrent protection, presented fundamental theorems used in short-circuit calculations and examined component short-circuit ratings. This article begins the study of calculation methods.
Part nine of this ongoing series continues the discussion of short-circuit calculations and details the use of two manual per-unit steps: diagramming the system and finding the Thevenin Equivalent. Examples given here detail only the first and third steps; the other two will be covered in subsequent articles in our series.
Part 10 continues the series with a discussion of calculating the effects of fault current by assigning resistance and reactance values to an electrical system. In Part 9 of this series, we introduced methods of diagramming electrical systems and reducing a diagram to its Thevenin Equivalent. Here, we continue the study by considering a simple system, assigning impedance values to its components and determining the fault current at representative locations.
Part 11 continues the discussion of short-circuit calculations. In Part 10, calculations were presented for systems without rotating machinery. Here, the authors introduce rotating-machine impedance as a factor in the process.
The Art of Protecting Electrical Systems, Part 12: Approximating short-circuit calculations for conductors
The ohmic and per-unit methods, covered in previous articles, are the most accurate and flexible means of fault calculation. However, they are also the most time-consuming. Large or complex radial systems, closed-loop systems and networks require analysis by the per-unit method. However, for many straight-forward radial systems, short-cut methods provide a quick, safe approach to determining fault currents for equipment application. Often, these estimates are valid even for final calculations—depending on the project’s size and the fault current available from the utility. This article describes a short-cut method for calculating the decrease in fault current caused by circuit conductors.
The Art of Protecting Electrical Systems, Part 13: Calculating short-circuit current at secondary of transformers
In the last part (Part 12) of this series, we presented a point-to-point method of calculating short-circuit currents. Known as the LICE method, this shortcut is based on circuit parameters of conductor length, initial fault current, reciprocal of the conductor’s impedance (C value), and operating voltage. Part 12 presented the C-values for wire and cable of various sizes and constructions. This article presents available fault currents at the secondaries of three-phase transformers having secondary voltages of 120/208 to 600 V. Short-circuit contribution to the transformers’ primary terminals are included, allowing rapid determination of available root-mean-square (rms) symmetrical fault current for systems with available primary fault energy from 50,000 kVA through unlimited kVA.
The Art of Protecting Electrical Systems, Part 14: Single-phase short circuit calculations, a step-by-step guide
Previous articles in this series have presented several methods for calculating available short-circuit current in three-phase systems with three phases shorted. While the fundamental principles are the same for single-phase short circuits, whether the fault is line-to-line or line-to-neutral, a slightly different approach is required to compensate for the different voltages and the effect of single-phase impedance.
Previous articles in this series have covered per-unit calculation of fault currents for both single-phase and three-phase systems. We have also presented several short-cut methods of estimating fault current using point-to-point calculations and transformer short-circuit tables. This installment in the series introduces tables that indicate the fault current available at the end of various feeders, and which calculate fault current at the ends of feeders of various size and length. The first set of tables has calculations for three-phase transformers from 112.5 kVA through 1,500 kVA with secondary voltages of 208, 240, and 480.
In the past few years the art of protecting electrical systems has undergone a dramatic change. The ready availability of personal computers with greatly expanded memory and storage has brought forth a myriad of software programs. Software is so user friendly and easy to learn that engineers and designers do not need to be computer experts to master their operation quickly.
A wide variety of computer programs has become available in recent years for enhancing the art of protecting electrical systems. Although long available for mainframe computers, many programs now are available for more readily accessible microcomputers. These programs cover the design of power distribution systems. This series examines one program as an example of the genre. The program facilitates the complete design of an electrical system from loading, sizing of feeders, and transformers, and voltage drop and load flow to short circuit calculations.
Part 18 of our continuing series on protecting electrical systems begins a series of articles on protection of electrical systems components, starting with conductors. Designing reliable electrical systems requires consideration of many interrelated details. It is not enough to depend only on protective devices, which, by definition, function after the fact. Engineers need to seek means to prevent equipment and component failures, and they need to understand how equipment selection and application affect system reliability.
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