The technology behind the world’s electrical grids is likely to change dramatically in the future with the spread of renewable energy sources such as wind and solar power. The controls handling grid functions can get to be complicated in such distributed energy environments. But it is easier to understand these trends when one has a good grasp of traditional grid functions.
To begin, two or more dc generators can be connected in parallel and run simultaneously, forming a grid that will power common loads. For proper operation, the voltages should be the same and polarities connected correctly to avoid a short circuit. The generators could be connected in series so the voltages would be additive, but this is not generally done.
Two or more ac generators can also connect and run in parallel, or one ac generator can hook into a grid. The process is more complex. For ac cogeneration, there are five requirements. The source that is contributing energy to the larger network must match its line voltage, frequency, phase sequence, phase angle and waveform. The waveform depends upon the nature of the sources. A rotary generator normally outputs a sine wave. Sine is a trigonometric function and it is equal to the ratio of the opposite over hypotenuse sides of a right triangle where the phase angle θ is located at the intersection of the X and Y axes. The opposite side is the amplitude of the signal, the adjacent side is equal to elapsed time and the hypotenuse is the vector. As the generator rotor turns the output in volts varies in accordance with the sine function.
Line voltage and frequency depend upon speed of the source, generally measured in revolutions-per-minute. Phase sequence depends upon the electrical connections as terminated at the generator or at the grid. Phase angle will change as the speed is adjusted. The task of the operator or the generator is to reduce to zero the phase angle between generator and grid.
Each time two ac sources are paralleled, voltage, frequency and phase angle must be re-established.
Synchronization may be achieved either manually or automatically. The first step is to start the generator and after warm up and stabilization, to increase the speed until generator and grid voltages are substantially equal.
If the generator is out of phase with the grid by a slight amount, it will pull in and synchronize itself to the stiffer source when the electrical connection between them is completed. The stiffer source will not be affected to an appreciable degree. If the phase difference is too great, there will be excessive cross current flow and damage to the smaller machine unless fusing is in place to interrupt the circuit.
Years ago, three light bulbs served as a phase indicator when connected between generator and grid terminals. As the generator was made to close in on the grid, the light bulbs would quickly flicker and then more slowly blink in time with the beat frequency between the two power sources. When they became in phase, the bulbs would go out. Then, the electrical connection could be completed by switching on the three-pole breaker and from that point the two sources would remain in phase or more precisely the smaller generator would remain synchronized with the unmoving grid.
We are talking here in terms of rotating generators, but the same priniple applies when switching in solar arrays, wind turbines, or other distributed sources. These sources must synchronize with the grid frequency.
The solar option is becoming a realistic alternative. This solution lends itself to decentralization. But nevertheless, long, high-capacity electrical transmission lines will be around for many years.
Aerial lines require lots of land because of the wide right-of-way for tall towers needed for high-voltage transmission. Land is becoming a scarce resource. Running transmission lines underground, at one time prohibitively expensive, is now more viable because of technological innovations such as affordable horizontal boring and pre-softened rights of way along major roadways.
Given that the wave of future for high-voltage electrical power lines is underground, we have to understand that this trend implies a switch back from ac to dc transmission. Aerial lines are separated from each other and from the ground by distance. These setbacks are greatly reduced in an underground line, the reduction made possible by relatively thin layers of insulation that have a high dielectric constant.
Accordingly, a high-voltage underground power line of any appreciable length becomes in effect a giant capacitor. Because capacitive reactance is frequency dependent, line-to-line and line-to-ground impedances are prohibitively low and loss is high in ac lines. The effect of capacitance in a dc line is momentary, lasting milliseconds and arising only when power switches on or off. Another issue is the skin effect. Due to self-inductance, ac current is repelled away from the center of a large conductor, confined to the region near its surface. For this reason, a large portion of the conductor’s cross section is not usable.
Traditionally, the great advantage in ac has been that inexpensive step-up and step-down transformers could easily, and with little loss, increase and reduce the voltage level as needed for transmission or for the end-user.
To change the voltage in a dc line, huge 20-acre rectifier and inverter stations were needed. Additionally, dc circuit breakers were highly problematic. All this is changing. At one time, state-of-the-art dc high-voltage rectification used mercury arc valves. These are cold-cathode gas-filled tubes characterized by a cathode that consists of a pool of mercury and is thus self-restoring. Such low-tech devices were replaced in the 1970s by semiconductors such as diodes and thyristors, which could be designed for high-voltage applications.
Line-commutated current-source converters employing thyristors have been successfully used in high-power projects over 1,000 MW. Forced-commutated voltage-source converter gate-turn-off thyristors or insulated-gate bipolar transistors in 350-MW installations have been used in many areas. It seems certain that similar innovations will enable dc to re-emerge as a dominant technology as Thomas Edison once envisioned.
Finally, a word about China’s electrical grid. Its State Grid Corporation of China is the largest electric utility in the world. In 2011, China passed the U.S. to become the world’s largest consumer of electricity. China has massive coal reserves and hydro-electric resources, but therein lies a problem. Carbon emissions are out of control, and the result is a relentless blanket of smog assaulting urban populations. Most people consider hydro to be a perfectly green energy source, but the fact is that the huge flooded areas upstream of every hydro dam have become permanently deforested, eliminating the most effective CO2 mitigation. As we all know, the nuclear option is problematic.
The location of China’s grid unfortunately does not correspond with the country’s greatest need, lying in the rapidly developing industrial areas of the east and south. Abundant coalfields are in the northeast and the hydropower potential lies in the southwest. This mismatch implies a need for many large transmission lines. (Wind, solar and nuclear power are expanding rapidly, but coal could account for as much as 75% of the total in 2020.)
Distribution is complicated by the fact that there is not a single national grid. Of the six regional grids, five are managed by the State Grid Corporation. The South Grid is managed by the much smaller South China State Grid Corporation.
Coal shortages are a problem particularly in northern and high-altitude areas where deliveries are delayed by intense winter weather. At present China is committing an enormous amount of capital (300 billion yuan = 47.2 billion USD) in electrical distribution infrastructure. It has been suggested that increasing the distribution capability will result in further air quality degradation, so the road ahead is uncertain at best.