①Three-phase alternating current
In power transmission, the AC power output by the power station is high-voltage three-phase power. After the electricity is distributed to cities or towns, it needs to be transformed into low-voltage three-phase or single-phase alternating current. Three-phase electricity is more efficient than single-phase electricity, so it is widely used in industrial and commercial users who use high-power motors and have greater demand for electricity. Most residential communities use single-phase power distribution, because electricity demand is relatively small. Figure 1a and Figure 1b show the waveforms of single-phase electricity and three-phase electricity, respectively. Single-phase electricity returns to the zero position three times in each cycle and changes polarity twice, so its energy becomes zero three times in each cycle. Each phase of the three-phase electricity will also return to the zero position and change polarity, but the three phases will not reach the zero position at the same time, so the energy of the three-phase system will never become zero.
Because three-phase electricity is more efficient, a three-phase motor can be made smaller (smaller core) than a single-phase motor of the same horsepower. Under certain conditions, the wires in a three-phase system can be thinner.
Figure 1b shows the output waveform of a three-phase generator, with an interval of 120° for each phase. The three-phase generator contains three independent coil windings in structure, which separates the three outputs by 120°. The single-phase generator has an output winding. In practical applications, power companies use high-voltage three-phase power generation systems, and then transformers convert the power into single-phase power distribution. Single-phase generators are usually used in independent houses or small commercial occasions that cannot be connected to the grid.
The three-phase transformer is used to increase the voltage to facilitate long-distance transmission, and then lower the voltage for power distribution. The coil windings and line configurations of these transformers vary according to different applications. Figure 2 shows two common three-phase configurations, namely “delta” (△-shaped) and “Y”-shaped configurations. The windings of each phase are respectively identified as A, B and C, and the space layout shall ensure that the phases are separated by 120°. The electrician connects the output of the secondary side to a three-phase or single-phase load.
The phasor diagram (see Figure 2) can show the angular displacement and amplitude of the waveform instead of the pure waveform. The arrows in the figure represent the angles between the amplitudes of voltage and current and each phase. This method is often used as a shorthand method to express important waveform information.
Figure 3 shows a transformer with a “Y”-shaped secondary winding connected to a three-phase 240V load.
②Advantages of AC
A significant advantage of alternating current is that it can be transformed into a high voltage or a low voltage by a transformer. When the voltage in the line increases, the current decreases in the same proportion. Smaller currents can reduce line losses, allowing the use of thinner transmission lines and reducing the construction and maintenance costs of the grid. We should understand that the same electric energy can be transmitted by high current and low voltage, or by low current and high voltage. E.g:
If E=1000V, I=1.0A. P=E×I=1000W
E=1.0V, I=1000A, P=E×I=1000W
Historically, most of the transmission and distribution of high-voltage electrical energy used three-phase alternating current. In the 21st century, high-voltage direct current (HVDC) long-distance transmission has been demonstrated and verified, and it has a higher cost performance in some applications. The phase synchronization between some AC power grids is very complicated and prone to failure, while HVDC is very suitable for such occasions. In Edison’s era, high-voltage direct current transmission was not feasible, because the technology to convert low-voltage direct current to high-voltage direct current was not mature and cost-effective. Therefore, AC power has become the first choice for power transmission and distribution and continues to this day. However, with the advent of DC-DC converters and high-efficiency DC-AC inverters, the feasibility of high-voltage direct current long-distance transmission has been improved.