DISTRIBUTED POWER GENERATION BASIC AND TUTORIALS

Distributed generation (DG) refers to small (a few watts up to 1 MW) power plants at or near the loads, operating in a stand-alone mode or connected to a grid at the distribution or subtransmission level, and geographically scattered throughout the service area. Typically they harness unconventional energy resources such as insolation, wind, biomass, tides and waves, and geothermal.

Small plants powered by site-specific conventional energy resources such as low-head and small hydro and natural gas are also included in this general group. Interest in DG has been growing steadily since the dramatic oil embargo of 1973.

In addition to the obvious advantages realized by the development of renewable energy sources, DG is ideally suited to power small remote loads, located far from the grid. An entire family of small power sources has been developed and employed for space, underwater, and biomedical applications.

Another niche for these systems is in energizing remote rural areas of developing countries. It is estimated that there are more than one million remote villages in the world with no grid connection and minimally sustained by locally available energy sources. Integrated renewable energy systems (IRES), a special subset of DG, are ideally suited for these situations.

General Features
DG will have one or more of the following features:
• Small size
• Intermittent input resource
• Stand-alone or interface at the distribution or subtransmission level
• Extremely site-specific inputs
• Located near the loads
• Remoteness from conventional grid supply
• Availability of energy storage and reconversion for later use

Potential and Future
Globally, the potential for DG is vast. Even extremely site-specific resources such as tides, geothermal, and small hydro are available in significant quantities. Assessments of the future for various DG technologies vary, depending on the enthusiasm of the estimator.

However, in almost all cases, the limitations are economic rather than technical. Concerns over the unrestricted use of depletable energy resources and the ensuing environmental problems such as the greenhouse effect and global warming are providing the impetus necessary for the
continued development of technologies for DG.

Motivation
Among the powerful motivations for the entry of DG are:
• Less capital investment and less capital at risk in the case of smaller installations
• Easier to site smaller plants under the ever-increasing restrictions
• Likely to result in improved reliability and availability
• Location near load centers decreases delivery costs and lowers transmission and distribution losses
• In terms of the cost of power delivered,

DG is becoming competitive with large central-station plants, especially with the advent of open access and competition in the electric utility industry

DG Technologies
Many technologies have been proposed and employed for DG. Power ratings of DG systems vary from milliwatts to megawatts, depending on the application. A listing of the technologies is given below.

•Photovoltaics (PV)
•Wind-electric conversion systems
• Mini and micro hydro
• Geothermal plants
• Tidal and wave energy conversion
•Fuel cells
•Solar-thermal-electric conversion
• Biomass utilization
•Thermoelectrics
•Thermionics
• Small cogeneration plants powered by natural gas and supplying electrical and thermal energies

The technology involved in the last item above is mature and very similar to that of conventional thermal power plants and therefore will not be considered in this section.

BRIEF HISTORY OF THE ELECTRIC POWER SYSTEM – BASIC INFORMATION

Over the past century, the electric power industry continues to shape and contribute to the welfare, progress, and technological advances of the human race. The growth of electric energy consumption in the world has been nothing but phenomenal.

In the United States, for example, electric energy sales have grown to well over 400 times in the period between the turn of the century and the early 1970s. This growth rate was 50 times as much as the growth rate in all other energy forms used during the same period. It is estimated that the installed kW capacity per capita in the U.S. is close to 3 kW.

Edison Electric Illuminating Company of New York inaugurated the Pearl Street Station in 1881. The station had a capacity of four 250-hp boilers supplying steam to six engine-dynamo sets. Edison’s system used a 110-V dc underground distribution network with copper conductors insulated with a jute wrapping.

In 1882, the first water wheel-driven generator was installed in Appleton, Wisconsin. The low voltage of the circuits limited the service area of a central station, and consequently, central stations proliferated throughout metropolitan areas.

The invention of the transformer, then known as the “inductorium,” made ac systems possible. The first practical ac distribution system in the U.S. was installed by W. Stanley at Great Barrington, Massachusetts, in 1866 for Westinghouse, which acquired the American rights to the transformer from its British inventors Gaulard and Gibbs.

Early ac distribution utilized 1000-V overhead lines. The Nikola Tesla invention of the induction motor in 1888 helped replace dc motors and hastened the advance in use of ac systems. The first American single-phase ac system was installed in Oregon in 1889. Southern California Edison Company established the first three phase 2.3 kV system in 1893.

By 1895, Philadelphia had about twenty electric companies with distribution systems operating at 100 V and 500-V two-wire dc and 220-V three-wire dc, single-phase, two-phase, and three-phase ac, with frequencies of 60, 66, 125, and 133 cycles per second, and feeders at 1000-1200 V and 2000- 2400 V.

The subsequent consolidation of electric companies enabled the realization of economies of scale in generating facilities, the introduction of equipment standardization, and the utilization of the load diversity between areas. Generating unit sizes of up to 1300 MW are in service, an era that was started by the 1973 Cumberland Station of the Tennessee Valley Authority.

Underground distribution at voltages up to 5 kV was made possible by the development of rubber-base insulated cables and paper-insulated, leadcovered cables in the early 1900s. Since then, higher distribution voltages have been necessitated by load growth that would otherwise overload low-voltage circuits and by the requirement to transmit large blocks of power over great distances. Common distribution voltages presently are in 5-, 15-, 25-, 35-, and 69-kV voltage classes.

The growth in size of power plants and in the higher voltage equipment was accompanied by interconnections of the generating facilities. These interconnections decreased the probability of service interruptions, made the utilization of the most economical units possible, and decreased the total reserve capacity required to meet equipment-forced outages.

This was accompanied by use of sophisticated analysis tools such as the network analyzer. Central control of the interconnected systems was introduced for reasons of economy and safety. The advent of the load dispatcher heralded the dawn of power systems engineering, an exciting area that strives to provide the best system to meet the load requirements reliably, safely, and economically, utilizing state of-the-art computer facilities.

Extra higher voltage (EHV) has become dominant in electric power transmission over great distances. By 1896, an 11-kv three-phase line was transmitting 10 MW from Niagara Falls to Buffalo over a distance of 20 miles. Today, transmission voltages of 230 kV, 287 kV, 345 kV, 500 kV, 735 kV, and 765 kV are commonplace, with the first 1100-kV line already energized in the early 1990s.

The trend is motivated by economy of scale due to the higher transmission capacities possible, more efficient use of right-of-way, lower transmission losses, and reduced environmental impact.

In 1954, the Swedish State Power Board energized the 60-mile, 100-kV dc submarine cable utilizing U. Lamm’s Mercury Arc valves at the sending and receiving ends of the world’s first high-voltage direct current (HVDC) link connecting the Baltic island of Gotland and the Swedish mainland. Currently, numerous installations with voltages up to 800-kV dc are in operation around the world.

In North America, the majority of electricity generation is produced by investor-owned utilities with a certain portion done by federally and provincially (in Canada) owned entities. In the United States, the Federal Energy Regulatory Commission (FERC) regulates the wholesale pricing of electricity and terms and conditions of service.

The North American transmission system is interconnected into a large power grid known as the North American Power Systems Interconnection. The grid is divided into several pools. The pools consist of several neighboring utilities which operate jointly to schedule generation in a cost-effective manner.

The electric power industry is undergoing fundamental changes since the deregulation of the telecommunication, gas, and other industries. The generation business is rapidly becoming market-driven. The power industry was, until the last decade, characterized by larger, vertically integrated entities.

The advent of open transmission access has resulted in wholesale and retail markets. Utilities may be divided into power generation, transmission, and retail segments. Generating companies (GENCO) sell directly to an independent system operator (ISO). The ISO is responsible for the operation of the grid and matching demand and generation dealing with transmission companies as well (TRANSCO).

This scenario is not the only possibility, as the power industry continues to evolve to create a more competitive environment for electricity markets to promote greater efficiency. The industry now faces new challenges and problems associated with the interaction of power system entities in their efforts to make crucial technical decisions while striving to achieve the highest level of human welfare.

THE TRANSMISSION AND DISTRIBUTION SYSTEM BASIC AND TUTORIALS

Like any other industry, the electric power system may be thought of as consisting of three main divisions:

1. manufacture, production or generation, cogeneration,
2. delivery or transmission and distribution,
3. consumption.

The figure below show a typical transmission and distribution system in both pictorial and block diagram forms. Although geographical difficulties, demand variances, and other reasons may make for minor differences in some transmission and distribution systems, the voltages chosen here are pretty typical.

This is what happens to electricity between the generator and a home, office, store, or factory. There are many definitions of transmission lines, distribution circuits, and substations specifying distinctions between them.

However, none of these definitions is universally applicable. To give some idea of where one ends and the other begins: Transmission may be compared to bulk delivery of a commodity from factory to regional depots; subtransmission from the depot to central area warehouses; primary distribution from area warehouse to local wholesale vendors; secondary distribution from the vendors to local stores; services from store to consumer.

In the pictorial rendition, note that the generator produces 20,000 volts. This, however, is raised to 138,000 volts for the long transmission journey. This power is conducted over 138,000-volt (138 kV) transmission lines to switching stations located in the important load area served.

These steel tower or wood frame lines, which constitute the backbone of the transmission system, span fields and rivers in direct cross country routes. When the power reaches the switching stations, it is stepped down to 69,000 volts (69 kV) for transmission in smaller quantities to the substations in the local load areas. (In some cases it might be stepped down to 13,800 volts [13.8 kV] for direct distribution to local areas.)

Transmission circuits of such voltages usually consist of open wires on wood or steel poles and structures in outlying zones (along highways, for example) where this type of construction is practicable.

Other transmission-line installations can provide an interchange of power between two or more utility companies to their mutual advantage. Sometimes, in more densely populated areas, portions of these transmission lines may consist of high-voltage underground systems operating at 69,000, 138,000, 220,000, 345,000, 500,000, and 750,000 volts.

At the substations, the incoming power is lowered in voltage for distribution over the local area. Each substation feeds its local load area by means of primary distribution feeders, some operating at 2400 volts and others at 4160 volts and 13,800 volts or higher.

Ordinarily, primary feeders are one to five miles in length; in rural sections where demands for electricity are relatively light and scattered, they are sometimes as long as 10 or 12 miles. These circuits are usually carried on poles; but in the more densely built-up sections, underground conduits convey the cables, or the cable may be buried directly in the ground.

Distribution transformers connect to the primary distribution lines. These transformers step down the primary voltage from 2400 volts, 4160 volts, or 13,800 volts, as the case may be, to approximately 120 volts or 240 volts for distribution over secondary mains to the consumer’s service.

The lines which carry the energy at utilization voltage from the transformer to consumer’s services are called secondary distribution mains and may be found overhead or underground. In the case of transformers supplying large amounts of electrical energy to individual consumers, no secondary mains are required.

Such consumers are railroads, large stores, and factories. The service wires or cables are connected directly to these transformers. Transformers may also serve a number of consumers and secondary mains; they are located in practically every street in the area served by utility companies.

Services and meters link the distribution system and the consumer’s wiring. Energy is tapped from the secondary mains at the nearest location and carried by the service wires to the consumer’s building.

As it passes on to operate the lights, motors, and various appliances supplied by the house wiring, it is measured by a highly accurate device known as the watt-hour meter. The watt-hour meter represents the cash register of the utility company.