25.2Electrical power grids

The term “grid” refers to the conductors and equipment interconnecting power sources to power loads in a wide-spread electrical system. Generating stations (i.e. “power plants”) convert various forms of energy such as fossil fuel, solar, wind, elevated water, and nuclear into electrical power; which is then sent through step-up transformers to raise the voltage and reduce current11; conveyed long distances over “transmission lines” at voltages ranging in the hundreds of kilovolts; received by “substations” which serve as interconnecting hubs for generating stations and loads, using step-down transformers to reduce transmission line voltage and increase current for distribution to local loads; conveyed to industrial, commercial, and residential points of use over “distribution lines” at tens of kilovolts (or less); and then finally stepped down in voltage once more at points of use by transformers located at power customer sites. A myriad of circuit breakers, disconnect switches, fuses, and other devices serve to disconnect and re-route power as needed within the grid. Sensing instruments monitor the flow of power throughout the grid, for regulatory (control), billing (metering), and protection (shutdown) purposes.

The innovation of alternating current (AC) is what made large-scale electrical power grids possible. DC power – at least when implemented with 19th century technology – is prohibitively expensive12 to transport over long distances due to the inability to easily transform voltage and current levels. AC by contrast allows the use of electromagnetic transformers, allowing voltage to be stepped up and current stepped down for economical transmission (i.e. small-gauge conductors suspended by long insulators), and then allowing voltage to be stepped down (for safety) with a proportional increase in current for driving heavy loads at points of use.

11The sole purpose of transforming voltage and current levels in a power grid is to minimize power losses due to the

electrical resistance of the conductors. Recall from basic DC electrical theory that the amount of power dissipated by a current-carrying resistance is P = I2R. This means doubling the current through a resistive conductor will increase that conductor’s power dissipation four-fold, all other factors being equal. Metal wire is expensive, especially when thousands of miles of it must be run to form a power grid. In the interest of reducing this expense, transformers are used to maintain long-distance power line voltages high and currents low, permitting the use of smaller-gauge conductors to carry that current.

12Thomas Edison’s original DC-based power grid was limited in radius to the size of a city, because all components operated at one voltage level (about 110 VDC). Large copper busbars served as distribution lines from coal-fired generating stations to points throughout the city, the sheer mass of these copper bars necessitating their installation in underground trenches rather than as overhead lines. Voltage losses from the generating station to points at the furthest reaches of the DC grid were significant, meaning customers at the “end of the line” had to tolerate dimmer lamps than customers located nearer the generating station.

The following illustration shows a simplified schematic diagram of a polyphase AC power system dating from the year 189513, showcasing an example of a Westinghouse power transmission and distribution system designed to generate power at Niagara Falls and send it some 60 miles distant:

This system was typical of its era, with one centralized generating station providing electrical power to all portions of the city. Electricity at the generators was stepped up in voltage to 10 kV to facilitate long-distance transmission over economically thin wires, then stepped back down through a series of transformers to voltages appropriate for direct use. Certain applications such as traction motors for electric streetcars required DC power rather than AC power (as e cient speed control for AC motors did not exist at that time), and given the lack of modern solid-state rectifier (diode) circuits the only practical solution was to employ motor-generator machines called rotary converters to convert AC into DC.

Modern electric power grids link dozens of large electric generating stations together with hundreds of cities to provide electricity across much larger geographic spans than the Westinghouse system of 1895. Modern power electronics has all but replaced rotary converters and made conversion between AC and DC relatively easy and e cient, allowing DC to now play a role in high-voltage power transmission. Some electrical power plants located in remote regions output DC along twoconductor transmission lines, which is later converted to AC and synchronized with AC generating stations on the grid.

13The source for this historical illustration is Cassier’s Magazine, which was an engineering periodical published in the late 1800’s and early 1900’s out of London, England. The Smithsonian Institute maintains online archives of Cassier’s spanning many years, and it is a treasure-trove for those interested in the history of mechanical, electrical, chemical, and civil engineering.

The following photograph shows a computer monitor plotting a trend over time of power sourced to and drawn from one electric utility company’s service region, in this particular case it is the one operated by Puget Sound Energy in northwest Washington state:

Lines of di ering color reveal power flow to and from this grid. The black trend represents total load on the system from all consumers: industrial, commercial, and residential. Each of the other colored trends represent di erent sources of power to the grid. Red represents thermal-based power plants, split between natural gas and coal energy sources. Yellow represents wind turbine power generating stations, with zero output between the times of 1:00 AM and 8:00 AM on the graph (i.e. negligible wind blowing during that timespan). Blue represents hydroelectric generators. Finally, green represents power purchased from other electric utilities (e.g. Bonneville Power Administration and Seattle City Light).

These trend graph reveals wind power to be a relatively small percentage of the total power generated during the timespan shown, with coal and hydroelectric sources remaining fairly constant over that time and the balance made up by power purchased from other utilities.

With no large-scale means to store excess electrical energy, the stability of an electrical grid depends on generating stations’ ability to alter their power output to meet a continuously changing demand14. Some types of generating stations lend themselves better to rapid changes in power

14Other options may exist for some grids. For example, large-scale industrial customers may be requested to curtail their power consumption at certain times in order to o set a deficit in supply. An example of this might be an aluminum smelter (which uses hundreds of megawatts of electricity to reduce alumina powder to molten aluminum metal) operating as a sheddable load while the same grid employs a nuclear fission power plant as one of its sources. If the nuclear generator’s reactor happens to “scram” (shut down for any reason), that reactor’s power output will