Whether you have a question or just want to browse, take a few moments to read over our collection of commonly asked technical questions and help tips. You might find what you’re looking for without even contacting our support staff. These can help you get started and provide on-going reference.
Harmonics are typically defined as "frequencies that are integer multiples of the fundamental frequency".
For 60 Hz power systems, these means that the 2nd harmonic is 120 Hz, the third harmonic is 180 Hz, the fourth is 240 Hz, ..., the nth harmonic is n*60. Frequencies are found that are actually in between these harmonic frequencies are called interharmonics (such as 185 Hz), but are generally much less common than harmonic frequencies themselves. Frequencies below the fundamental frequencies are called subharmonics (such as 9 Hz), and often contribute to the phenomena of light flicker.
Variations in the RMS value are often used to trigger capturing of PQ data. The most common type of RMS variation is the sag, (or dip in European lingo). Some studies show over 60% of the PQ disturbances are sags, which is when the RMS value goes below 90% of the nominal value. On a typical office or residential outlet, that would be dropping from 120 Vrms to 108 Vrms. If the voltage goes down below 10% of nominal, we call that an interruption. Conversely, if it increases above 110% of nominal, that is a swell.
If you have taken a distribution panel off for whatever reason, it is a good time to use one of the most important power quality tools, the screwdriver. Be sure to wear the proper safety equipment and to follow all necessary safety precautions. In most facilities, the current flows during only part of the day. Today, this current is often contains heat-generating harmonic currents. The heating/cooling/heating/cooling cycle and resulting expansion and contraction of the wires can cause the connections to loosen over time. This loosening increases the impedance of the connection, which further increases the heating effects. Tightening loose connections with the screwdriver can help reduce voltage drops and minimize the fire potential.
Swells are increases in the voltage, typically above 110% of the nominal. Though must less common than sags, swells can cause catastrophic failures in equipment if the voltage exceeds the safe input level of the equipment for too long. Swells can be caused when a large load is suddenly turned off (opposite of the cause of sags). The voltage will increase for 30-60 cycles, until the automatic tap changers can bring the voltage back into normal regulation Limits.
Sags are often caused by sudden, large increases in current, which causes a proportional voltage drop in the wiring, leaving less voltage remaining for the loads. If it is a fault on the electric distribution system, such as a phase-to-ground short circuit caused by lightning, animals, tree branches, or accidents, then the direction of the sag is called upstream or source side, or towards the generating source. If a load starts up, such as a large HP motor, then the direction of the sag is said to be downstream or load side. If the remaining voltage during the sag is too low for the equipment to operate properly, the process can be interrupted or corrupted. Though equipment is usually not damaged during such, the product being produced often has to be scrapped, and there may be a significant restart time to get the operation running smooth again.
Some changes or modulations of the voltage that aren’t large enough to be consider sags may not seem to effect equipment operation. However, these anomalies can result in quality variations in extrusion and textile processes and in flickering lights that can cause human discomfort. In the case of flicker, the frequency of the modulation is critical as to whether it will be noticeable to the particular susceptibilities of the human eye and brain. For example, it would only take about 0.3 volts of modulation at 9 Hz on a 120 V system for most people to notice the flicker in a 60 W light bulb. However, at 1 Hz, it would take nearly 10 times the modulation to be noticeable.