While reviewing the various types of power quality disturbances I’ve previously covered as well as the phenomena PQ standards frequently reference, one of them stood out. Harmonics, sags and swells, transients, interruptions, unbalance, flicker, interharmonics and now even rapid voltage changes have received a good amount of press. But what about noise?
The often-cited IEEE Standard 1159, Recommended Practice for Electric Power Quality, contains few mentions of noise—in this case, electrical noise. Line 5.5 in Table 2 refers to noise being steady-state waveform distortion having a typical frequency spectrum classified as broadband and a typical magnitude of 0–1 percent of the nominal. Another reference up front in the definitions refers to a noise floor being an “artifact of the measuring system, including the transducers, input amplifiers, analog-to-digital converters, etc., which is approximately 5 percent of the nominal. This noise floor is not the noise described in Table 2 as wide spectrum noise that can be considered as a problem to equipment.”
Right away, we might see an issue ensuing if we are looking for noise in the 1 percent of nominal range that may cause trouble, but the measuring equipment typically has a noise floor of 5 percent of nominal, which would obscure and prevent any accurate measurement of the alleged noise.
The section of the standard covering waveform distortion allocates only one paragraph to discuss issues with noise, and little information is there to help the reader determine if the electrical system is having problems caused by noise. Noise is “unwanted electrical signals,” but so are the other types of PQ phenomena. The aforementioned spectral content is further classified as “typically lower than 200 kHz,” which encompasses just about all of the other phenomena.
The latest standards efforts on the 9 kilohertz (kHz) to 150 kHz region often refer to those as “supra-harmonics.” The waveform distortion produced by arcing is classified as transients, being a burst of higher frequency noise sort of like oscillatory transients, but random in frequency content. It goes on to state that “basically, noise consists of any unwanted distortion of the power signal that cannot be classified as harmonic distortion or transients.” So now we know what noise is not, if we could measure it when hidden in the noise floor.
The PQ chapter in NFPA 70B, Recommended Practice for Electrical Equipment Maintenance, goes a bit further in the discussion of noise. Noise is “undesirable electrical signals in an electrical or electronic circuit.” To further narrow down what noise is, the chapter states: “It can be random or continuous in nature … can occur at any frequency and amplitude … [and] can be introduced into a circuit from a multitude of sources … . The type and sources of noise are as diverse and numerous as the number of facilities that contain power systems.”
If it is seen equally on all phase conductors and the neutral, it is common mode noise versus traverse noise. If one is only measuring line-to-neutral voltages, noise would never appear on the measuring instrument because there would be no difference between the line and the neutral conductor voltages when the noise was equally there or not. It does seem to be quite elusive, which is perhaps why the standards coverage is the way it is.
Like the other PQ phenomena, noise is said to disturb electronic devices such as microcomputers and programmable controllers, or cause data corruption or unexplained equipment malfunction. It is not likely to cause breakers to trip and normally doesn’t destroy equipment like a nasty impulsive transient from a lightning strike could. But, if something unexplained happens, it may be that elusive noise.
A lot of electrical and electronic equipment, such as motors, transformers, rectified input switch mode power supplies, and improperly made electrical connections within equipment or on the electrical system, can produce noise. Given that it is made up of many random signals from many potential sources, it’s likely to be difficult to identify what it is and where it’s coming from due to its nature. The typical PQ tools are replaced by specialized instruments, such as spectrum analyzers, RFI/EMI recorders for radiated and conducted signals, and digital oscilloscopes, which not many of us have on hand.
Given the elusive nature of the offender that requires specialized equipment to try to see something that may be coming from a multitude of sources that may or may not be interacting with each other and the electrical environment, how can we get rid of it? Basically, we try to snuff it and not let it propagate by using high pass filters, isolation transformers equipped with multiple electrostatic shields, a low impedance equipotential ground plane (such as used in data centers), properly terminated shield cables that run between pieces of equipment, and sometimes even shielding and grounding the entire room where the noise is suspected of coming from or where the equipment that gets affected is located.
Maybe noise doesn’t get much attention in the standards because of its nature. If something generates undesirable signals that can’t be realistically measured and can’t be determined to have disturbed anything, is it noise?