Ground Rules by John Cadick: Power System Studies (Part 1 of 3)

Source: Cadick Corporation
By John Cadick, P.E., Cadick Corporation

A power system study is an engineering process that evaluates some aspect or aspects of your power system operation. There are eleven types of studies defined in IEEE standard Std 399. They are:

  1. Short circuit analysis
  2. Protective device coordination
  3. Load flow
  4. Cable ampacity
  5. Reliability
  6. Stability
  7. Motor starting
  8. Transient switching
  9. Ground Mat Analysis
  10. Harmonic analysis (Power Quality)
  11. DC supply systems
Each of these studies is intended to provide information that will allow you to operate your system more economically, safely, or reliably. In the next three Ground Rules, I'll briefly discuss each of these studies by describing its fundamental approach and benefits. My intent is to remind of the value of good engineering in power system applications. As we move further down the de-regulation path, economic pressures are likely to cause us to cut back on some expensive procedures. However, good engineering practice demands that the power system (whether utility, industrial or commercial) be well designed and operated. These eleven studies are stepping-stones to that meeting that demand.

The first three studies, short circuit analysis, protective device coordination, and load flow, are often performed together because they use much of the same information and the same system model.

Short Circuit Analysis
This study mathematically determines the amount of current that will flow for short circuits placed at various locations throughout the system. To perform such a study, the system must first be modeled on the computer. Although details are beyond our scope here, the process usually involves creating an impedance matrix that represents all of the system impedances throughout the system. By using clever mathematical manipulation, faults can be placed one at a time and the current values calculated.

The short circuit analysis results are used for at least three major purposes.

  1. The current flows can be compared to the interrupting ratings of the circuit breakers, fuses, and other protective devices throughout the system. Any location that has more current than its device can safely interrupt will require attention.
  2. The current values can be used in the performance of a protective device coordination study.
  3. Proper selection of flame retardant clothing can be made only when the fault duties are known for the point(s) of exposure. While rules-of-thumb are less expensive to use, they will probably give excessively conservative results and cost more money downstream.
Note that any facility that is regulated by the National Electrical Code is probably required to have an up-to-date short circuit analysis. Why? From article 110-9:
    "Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment. Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted."
And from Article 110-10:
    "The overcurrent protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit-protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors, or between any circuit conductor and the grounding conductor or enclosing metal raceway. Listed products applied in accordance with their listing shall be considered to meet the requirements of this section."
While there may be methods to "estimate" the short circuit values needed to comply with these articles, only by using a short circuit analysis can the owner of the system be certain that all protective devices are properly sized.

Protective Device Coordination
A coordination study is a graphical study that compares the tripping characteristic curves of the protective devices and the thermal or mechanical damage characteristics of the line equipment. It is usually done on computer using special software. A coordination study and a short circuit study are actually just opposite sides of the same coin.

A coordination study is performed for at least three reasons:

  1. To make certain that the closes upstream device to the short circuit is the one that operates. For example a fault on a feeder should trip the feeder breaker, not the main breaker.
  2. To make certain that the protective devices operate quickly enough to prevent collateral damage to uninvolved equipment. For example, a transformer feeding a short circuit should not be damaged by the short circuit. Protection should operate before that happens.
  3. Wherever possible, the next upstream device should operate if the first device does not. For example if the feeder breaker does not interrupt the short circuit, the main breaker should operate, usually after some time delay.
Note that Article 110-10 of the NEC can best be accommodated by the performance of a coordination study in conjunction with a short circuit analysis.

Load Flow
This study is performed by modeling the system and calculating currents, voltages, and phase angles for various load conditions. It is performed on a computer and is actually somewhat more mathematically complex that a short-circuit analysis. The system is usually modeled for several different load conditions so that overloaded or under loaded equipment can be pinpointed. It is particularly useful when new installations are being considered. A good load flow may show ways that the existing system can be used without having to purchase new equipment.

All of these three studies should be performed for even the smallest of power systems. Also, they should be periodically updated to allow for internal and external system changes.

A registered professional engineer, John Cadick has specialized for three decades in electrical engineering, training, and management. In 1986 he created Cadick Professional Services (forerunner to the present-day Cadick Corporation), a consulting firm in Garland, Texas. His firm specializes in electrical engineering and training, working extensively in the areas of power system design and engineering studies, condition based maintenance programs, and electrical safety. Prior to the creation of Cadick Corporation, John held a number of technical and managerial positions with electric utilities, electrical testing firms, and consulting firms. Mr. Cadick is a widely published author of numerous articles and technical papers. He is the author of the Electrical Safety Handbook as well as Cables and Wiring. His expertise in electrical engineering as well as electrical maintenance and testing coupled with his extensive experience in the electrical power industry makes Mr. Cadick a highly respected and sought after consultant in the industry. (Back to top)