Some of the concepts in electrical engineering are more easily visualized by using physical phenomena. Power quality phenomena is not different, since it is just a specialized application of the same principles, particularly Ohm’s and Kirchoff’s Laws. Ohm’s Law is that the voltage (or potential to do work) is equal to the current (or pressure or flow rate) multiplied by the impedance (or resistance against that pressure). Kirchoff’s Law is that the sum of the potential around a closed loop will equal zero, and the sum of all of the flows at a node or junction point will net out to zero also.
One of those tall water storage tanks that look like the Jolly Green Giant’s golf ball on a tee provides a good analogy. The water in the tank has potential to do work due to its stored kinetic energy. If the valve is closed, there is still energy stored in the tank but nothing is flowing. The same is true in a circuit where there is voltage, but the switch is open, which means no current flows. Once the valve in the pipe is fully opened, the rate of water flow will depend on the size of the pipe. A narrow pipe has more resistance to the water than a wide one and will impede its flow. However, if more potential is applied by having a taller water tank, more water can flow through the same size pipe. Likewise, if one lowers the resistance of the wire, more current will flow, and vice versa.
When the pipes come to a tee connection, all of the water that enters from the supply pipe will end up going into one of the two other pipes, just like Kirchoff’s law for current. If both pipes are the same diameter, the same flow will occur in both (assuming equal “loads” downstream). If one is larger than the other, the one with less resistance will have more water flow. But the sum of the two flows will equal the supply-side flow.
For the power quality equivalents:
Sag—When someone flushes the toilet while someone else is taking a shower, the person in the shower will most likely have reduced water flow until the toilet tank refills. This is because some of the original flow to the shower is now diverted to the toilet. When a large horsepower motor starts up on the same line as a PLC, there is a large increase in current that, when multiplied by the resistance of the wires, causes less potential to be available for the load, since the supply potential doesn’t change quickly to compensate for such.
Swell—When a large amount of water is flowing in a pipe and then the valve is shut off very abruptly, the phenomena called a water hammer is likely to occur. This can be indicated by a banging sound in the pipes as the plumbing system tries to reestablish equilibrium. Likewise, if a large electrical load suddenly turns off, there will be an increase in the voltage for the equipment on the same circuit. To reestablish equilibrium on the electrical distribution system, the utility has automatic tap changers that drop the voltage back down and bring an end to the swell.
Sustained undervoltage (brown out)—During the summer, when everyone has lawn sprinklers running or is filling swimming pools, the decrease in water flow may last for a much longer time than just the toilet flush. Or when a water main ruptures, the water takes the path of least resistance and escapes into the street, leaving much less water in the pipes for those downstream. Likewise, when it is above 90°F and everyone’s air conditioner is running, the distribution voltage can drop down for a sustained period of time.
Harmonics—The easiest example for harmonics is to compare with one in the music world, which are called “overtones.” When different types of instruments produce the same note, such as “middle A,” which has a fundamental frequency of 440 Hz, other frequencies are also produced at much lower volume, depending on the type of instrument. A trumpet will have a different set of overtone frequencies than a clarinet. The human voice, however, is rarely capable of producing overtones, only the fundamental. Whereas overtones or harmonics are desired in music, they are not so in electromagnetic equipment where they produce no useful work but result in extra heat that requires derating the operating point and shortens the life of the equipment.
Power factor capacitor transient—If one were to connect a balloon to a straw’s end, initially, the airflow would go into the balloon and fill it until it required more pressure than the person blowing it up could provide in one breath (assuming it didn’t pop). As the person paused to take his or her next breath, the air would flow back out of the balloon into the straw. When a PF capacitor on a distribution circuit is energized, it diverts a significant amount of the pressure to fill it up or charge the voltage on the capacitor. When charged, the inductance of the wire results in a kick-back, or an increase in voltage on the system, which goes back and forth in decaying amounts until the system reaches equilibrium.
Other power quality phenomena, such as flicker, impulsive transients and frequency changes, could also be visualized with physical analogies but might require more imagination. Give it a try, and see what you can come up with when trying to explain what your power quality monitor recorded to your clients.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.