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Appendix A



This appendix discusses some of the technical concepts involved with prime power operations. Although knowledge of these concepts is not critical, it is useful for planners and commanders who employ engineer prime power units.


Prime power is electrical power that is continuously produced. It is not necessarily uninterrupted power. Power is produced by generators. Generators are machines and as such are subject to mechanical or electrical failure. They require periodic maintenance and service to avoid breakdown. To obtain a source of continuous or prime power, multiple generators are installed in parallel. This arrangement allows the performance of maintenance on one or more generators while the others produce power. Simply having a backup generator that can be installed in the event of a generator failure does not constitute a prime power plant. The same multigenerator principle is used in the production of commercial power and can be used with some models of TACGENS.

Prime power plants can produce power at either 50 or 60 Hz. When operating at 60 Hz, the frequency common to most US systems, the generator output voltage is 4,160 volts, 3 phases. At 50 Hz, the output voltage is 3,800 volts, 3 phase. These output voltages are in the range called medium voltage.

Figure A-1 depicts a typical prime power plant. It shows that a generator may be isolated from the power bus by opening the air switch between the bus and the generator. With the air switch open, maintenance can safely be performed on the isolated generator while other generators continue to produce power. A four-generator plant has an installed output capacity of 2.25 MW, based on continuous operation of three generators. The peak capacity of the plant is 3 MW. This may be attained for limited periods of time.


Primary Distribution

Primary distribution networks carry medium-voltage power from the power plant to the transformers or mobile substations. Primary systems are constructed with extra-heavy-duty, multiconductor, shielded power cable that is suitable for ground-laid or buried applications. These networks can be laid out in radial or loop patterns.

The radial layout has the advantage of being quicker and more economical to install. The loop layout is more reliable. Figures A-2 and A-3 depict stand-alone prime power plants powering loads over radial and loop primary distribution networks. These two figures show that a radial layout can be upgraded to a loop layout to increase reliability.


The medium-voltage power distributed on the primary system is stepped down to user-voltage power by transformers. In the simplest terms, transformers are electromagnetic devices that use mutual inductance to transfer energy from one circuit to another. Most transformers are more than 95 percent efficient. As a result, very little energy is lost in the transformation process. The power put into a transformer approximately equals the power coming out. In the case of stepdown distribution transformers, the high-voltage, low-current power going into a transformer approximately equals the low-voltage, high-current power coming out. When a transformer reduces voltage, it increases the current proportionally.

Primary distribution voltage (medium voltage) is stepped down to user voltage by distribution transformers or mobile substations. A primary distribution system may incorporate either or both of these items. The system can incorporate distribution transformers and switch gear that are organic to the prime power team or commercially obtained or both. Use of distribution transformers is advantageous when the electrical load consists of several small power requirements dispersed over a relatively wide area. Figure A-4 shows a typical primary distribution feeder using distribution transformers. Use of distribution transformers allows power to be distributed at a higher voltage on smaller conductors and helps to reduce voltage drop and line loss.

Mobile substations are simply large, trailer-mounted transformers with self-contained switching and protective devices. Use of mobile substations is advantageous when providing power to larger loads concentrated in a smaller area. Mobile substations are well suited for powering industrial areas and large facilities. Multiple mobile substations can be employed in parallel to increase capacity. Figure A-5 depicts a typical application of mobile substations.

Secondary Distribution Networks

Once the voltage is stepped down to user voltage at the transformer, the secondary distribution network carries the power from the transformer to the user. Secondary distribution systems are constructed with multiconductor cable when possible. Figure A-6 depicts a typical, simple secondary distribution network.


Consideration of power systems characteristics is important in determining power use for specific applications. Some power system characteristics can be altered to suit user needs.

Output Voltage

Output voltage is the measure of the voltage at the output terminals of the power system. Large output voltage alterations can be made by using transformers. Small output voltage changes can be made by adjusting generator controls on TACGENS and prime power generators. Devices such as voltage regulators can be used to make small voltage adjustments to commercial power.

Single-Phase or Three-Phase Power

Most alternating current (AC) power is generated as three-phase power. Single-phase power can easily be obtained from a three-phase source. Three-phase power is provided at three separate output terminals that share a common neutral terminal. The voltage difference between phases is the result of each being 120 degrees out of phase with the other two. For many applications where higher voltage single-phase power is required, two-phase power can be used to provide power at approximately the needed single-phase voltage. For example, a 120/208-volt system can provide single-phase 120-volt power. It can also provide 208-volt three-phase and/or 208-volt two-phase power.

Three-phase power systems should be designed so that each phase carries approximately the same amount of load as the other two. This concept is called load balancing. Badly unbalanced loads will result in frequent tripping of protective devices and may damage equipment.

Output Capacity

Output capacity is the amount of power a system can deliver. It is usually measured either in kilovoltamperes (kVA) or in kW. Output capacity is limited not only by the size of the generation equipment but also by the rated capacity of the distribution system. Electrical conductors and devices such as transformers, breakers, and switches are designed and manufactured with specific limitations on current and voltage. When user power demand exceeds output capacity, the system is said to be overloaded and one of two things may occur. Either protective devices such as fuses, breakers, or relays are blown or tripped or else the system is damaged. The damage can occur in the form of melted conductors, burned connections, or blown transformers. Output capacity may be increased by upgrading distribution systems and by employing additional or larger generators.


Reliability is the measure of a power system's ability to fulfill all user demands without failure for a long period of time. Systems that are susceptible to outages, either scheduled or unscheduled, or that cannot provide all the power a user needs are not very reliable. Reliability can be improved by the employing standby and load-sharing generators. It can also be improved by using redundant distribution systems and enhanced by maintaining existing distribution systems and generation equipment.


The ability to rapidly relocate a power system may be critical to certain operations. TACGENS are the most portable systems available. Since commercial power is tied to fixed facilities, it is the least portable. Prime power systems are portable but require more effort and time to move and install than TACGENS. Prime power plant installation may be feasible if the plant remains in operation (stationary) for 30 days or longer.


AC power frequency is given in cycles per second, or Hz. The most common worldwide systems are 50 and 60 Hz. The accepted US standard is 60 Hz. Most countries establish one or the other as a national standard. They build their commercial power systems accordingly. In a few countries, both systems may be encountered. Appendix C lists the commercial power-grid frequencies and user voltages for various countries.

Some equipment is sensitive to AC frequency and will not operate properly when powered by a source with a different frequency than the equipment is designed for. Units should ensure frequency compatibility for this equipment to avoid damaging it. Most transformers designed for 50-Hz operation can be used for 60-Hz application. Most 60-Hz transformers cannot be used for 50-Hz application unless they are significantly derated.

Prime power generation equipment can operate at 60 Hz or 50 Hz. Most TACGENS operate at 60 Hz. Some specialized TACGENS operate at 400 Hz. 400-Hz frequency power is used extensively for aircraft systems, missile and avionics systems, signal systems, and some shipboard systems. Frequency alterations are possible with the use of frequency converters.

Line Loss and Voltage Drop

Electrical conductors have some resistance. The amount of resistance depends on the type of metal, cross-sectional area and length, and temperature of the conductor. Copper is less resistive than aluminum. Conductors with larger cross sections and shorter lengths are less resistive than those with smaller cross sections and longer lengths. Conductors are less resistive at lower temperatures than at higher temperatures.

When electrical current flows through a resistive material, some of the energy is converted to heat, causing a drop in voltage. The energy converted to heat is called line loss and the drop in voltage is called voltage drop. Distribution systems must be designed to safely carry the required amount of current while maintaining output voltage within the operating parameters of the devices being powered.

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