Load-induced harmonics and interharmonics have since muddied the waters, and generators are still trying to produce a pure sine wave at 60Hz. But it doesn’t always look that way to the loads, and problems can result when it isn’t pure.
The sine wave is a common phenomenon in science that has significance in many areas. The velocity of a pendulum follows a sine wave, reaching the maximum velocity at the bottom of the arc and slowing to zero at the top, before reversing its direction and repeating the pattern in the other direction.
As the magnetic field rotates around inside the generator, the strength of the field and the induced electrical field follows the same sinusoidal pattern. Frequency is measured as the inverse of the period, which is the time it takes to repeat the pattern.
Normally, we measure the period from the positive-going zero crossing (Point A in the figure) to the next positive-going zero-crossing (Point B). Taking the reciprocal yields the frequency. In the North American grid, it is 60Hz that makes the electric clocks keep proper time.
The fundamental power frequency isn’t only 60Hz. It is 50Hz in Europe and other parts of the world, 20Hz for electric railroad, and 400Hz in some aircraft and shipboard applications. But the principle is the same. The prime mover (hydro, steam, diesel motor) is used to turn the generator at the required speed. When the loading on a generator changes abruptly, it will cause the frequency to change: more load, slower frequency; less load, higher frequency.
If one generator experiences this more than one paralleled on the same grid, then the two generators would be out of synch, causing undesirable power flows. This is compensated for by adjusting the speed of the prime mover that is driving the “disturbed” generator to get the frequencies back in synch. The good news is that on the North American grid, the deviation is rarely beyond 0.05Hz from the nominal 60Hz.
But it can be, because there are different types of equipment that rely on that clean sine wave for timing and control information. If the area around the zero crossings is distorted, from either high harmonic content or transients, then there can be more than two zero crossing transitions in each cycle. If the timing circuit in the equipment is counting zero crossings, then the equipment time can be off from real time.
This was apparent for a number of years in some of the earlier digital clock radios, where complaints of timing running twice as fast were heard.
On a process control system, this can also wreak havoc and damage the quality of the product being produced. Random transients causing such are probably the more difficult case to track down, as it is almost impossible to detect without a power quality monitor in place.
The ultimate in distorted sine waves is probably the voltage output of a pulse-width-modulated (PWM) adjustable-speed drive. The width of each pulse is dependent on the load requirements, and the time between pulses is based on the switching frequency of the drive.
The current waveform would be similar to the sine wave, as the current doesn’t change instantaneously when the voltage does. It acts sort of like a low pass filter, and the fundamental power frequency signal becomes dominant.
However, the drive can change this fundamental power frequency depending on the load, ranging from 20 to 70Hz or wider. All of those fast transients that make up the PWM voltage waveform can create another set of problems, from degradation in the windings of the motor, to electric and magnetic fields being coupled into other wiring in the immediate area. Fields of 10 volts aren’t uncommon anymore in a room full of large drives.
Though the pure sine wave for voltage may be a thing of the past, there is still equipment out there that relies on having a reliable and stable fundamental power frequency.
Knowing what your equipment susceptibilities are, even if the disturbing phenomena is as infrequent as frequency problems, can go a long way toward keeping your facility running smoothly. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.