The Not-So-Dumb Grid

With all this talk about the coming of the smart grid, it seems like a good idea to review what currently delivers the electrons to your door. The majority of electricity is generated and delivered in the form of alternating current (AC) rather than direct current (DC), thanks to Westinghouse winning out over Edison in the War of the Currents in the late 1880s. Most electric power plants or generators produce in the range of 12 to 15 kilovolts (kV) AC. The generation of electricity usually involves either a rotating electromagnetic generator at the proper speed required for the fundamental frequency or a synthesized waveform from a DC source using power electronics. The former needs a prime mover, such as water in a hydro plant or steam in a coal/gas/nuclear facility, to turn the generator. Alternative power sources, such as storage batteries, wind and photovoltaic, have a power converter that takes the DC source and makes it into an AC sine wave.

Beside the voltage, the fundamental power frequency also must be tightly controlled. In North America, it is 60 hertz (Hz), but it is 50 Hz in Europe and other parts of the world, 16 or 20 Hz for electric railroad, as well as 400 Hz in some aircraft and shipboard applications. One of the challenges in regulating the frequency is that, when the loading on a generator changes abruptly, it will tend to cause the frequency to change: more load, slower frequency; less load, higher frequency. If one generator experiences this more than another one that is “paralleled” on the same grid, the two generators are out of sync with each other, causing undesirable power flows. Adjusting the speed of the prime mover or power converter that is driving the “disturbed” generator to get the frequencies back in sync compensates for this. On the current North American grid, the deviation is rarely beyond 0.05 Hz from the nominal 60 Hz, as shown in Figure 1.

To provide the necessary power for the industrial, commercial and residential loads at those medium-voltage levels, it would require tremendously high currents and, therefore, very large diameter cables. Instead, the voltage is stepped up at the transmission substations from 69 kV to 765 kV and sent along its way through the smaller wires installed on the tall towers. This results in much lower current levels for the same power delivery. Lower current levels also means lower losses in the wires, since the voltage drop in the wires is proportional to the current. Hence, more power gets to the loads, rather than being lost along the way.

To protect the transmission system, very fast-acting breakers (which usually operate in less than six cycles with one recloser attempt about a half-second later) protect against the unlikely occurrence of faults on transmission lines. They do occur, though, from contact with airplanes, hot air balloons, unmaintained vegetation, and lightning, which can provide paths for phase-to-phase or phase-to-ground shorts, resulting in dramatic arcs that usually are short in duration due to the not-so-dumb system protection (see Figure 2).

At distribution stations, the high-voltage transmission levels are reduced back down to medium-voltage levels, generally between 5 kV and 35 kV. A distribution substation is fed by one or more sets of transmission lines and then distributes that power out to between three and 10 distribution lines. These are the voltage levels found on the utility poles (often single wood poles) found running along residential and industrial streets. Faults are much more common on these systems, though some power quality phenomena on the distribution system originates in facilities and is carried over the wires to other customers. Being closer to the ground (and to humans), distribution systems can have problems created by motor vehicles hitting the poles, incidental contact with vegetation from the wind, animals (the tail to the paws of a squirrel can make a good short circuit), falling tree limbs from ice and wind storms, and lightning as the sources of transients, sags and interruptions. System protection schemes at the distribution level tend to have breakers that operate in six to 10 cycles, along with localized fuse schemes that operate in a cycle but only take out a small number of customers. Whereas the breakers will go through an automatic recloser sequence (unless it locks out after several failed attempts to get rid of the fault), fuses require manual intervention to get the circuits back operating, which can take hours.

The system-protection schemes are actually quite complex, as there are current and frequency levels, timing settings, and carrier signals that are sent out to try to minimize which breakers in an interconnected scheme will operate to quickly clear the fault but keep the disruption in service (where a sag or interruption depending on where the fault is relative to the user and the substation). A fault on one circuit can be seen on hundreds of circuits with millions of customers. A recent event in a Latin American country had more than 400 events occur within 50 milliseconds across 40 substations, originating from a single fault, though most customers experienced nothing more than a barely noticeable blink of the lights.

The not-so-smart electrons eventually make it all the way from the generator to where they are needed to do work. Most industrial facilities would be fed by three-phase power that is reduced by a facility transformer to 480V, though larger industrial facilities may have their own substations and are fed the medium voltage levels directly. Residential service is usually provided with a single-phase conductor into an autotransformer (utility pole-mounted transformers) that is center tap grounded to produce two 120V circuits. Time will tell how much smarter this elaborate scheme will get and at what cost to us all.

BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.

About the Author

Richard P. Bingham

Power Quality Columnist
Richard P. Bingham, a contributing editor for power quality, can be reached at 732.287.3680.

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