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"Power quality" conjures up mysterious terms like sags, swells, transients, and harmonics. Let's explain what these terms mean and then how to measure them with a PowerSight Power Quality Monitor.
The concept of Power Quality aims to define the conditions that allows electrical systems to operate as intended, and what limits would adversely affect their performance or life-span.
In electrical parlance "Power" has a specific meaning; it describes the flow of energy in a circuit and is the product of voltage and current (and power factor in AC systems). For the most part “Power Quality” refers to the voltage conditions with respect to time. Most power quality measurements are made in the voltage domain. However, the issue of harmonics – discussed later – often gets included with power quality investigations and harmonics measurements are made in both the voltage and current domains.
Sags - this a low voltage condition. Voltage sags are often momentary and last from a few cycles to minutes, or in the unusual case of a "brownout" last for hours. Sometimes referred to as a "dip". Most utility power companies follow the definitions that are established by technical committees such as the IEEE or IEC who define sags more rigorously; i.e.: when the RMS voltage is below the nominal voltage by 10 to 90% for 0.5 cycle to 1 minute. If the voltage dips below 90% for more than one minute many technical committees describe this situation specifically as an undervoltage or sometimes as a brownout.
Here are two examples; a short-term sag of a few cycles presented in the sinusoidal domain, and a longer 5 second sag presented in the RMS domain.
Effects of voltage sags.
All electric machines and appliances must have an adequate voltage source. Without the proper operating voltage being supplied (when the voltage falls below a certain limit usually specified by the manufacturer) the machine or appliance will mis-operate, or shut down and turn off, or become damaged.
Swells - this the opposite to a low voltage condition, it's when voltage is suddenly increased. Voltage swells are often momentary and usually last just a few cycles to minutes. Similar to sags, technical committees define swells as when the RMS voltage exceeds the nominal voltage by 10 to 80% for 0.5 cycle to 1 minute.
Effects of voltage sags.
When the voltage rises above a certain limit, usually specified by the manufacturer, the machine or appliance can malfunction, shut down and turn off, or (usually) become damaged.
Transients (Impulses) - these are very short duration little bursts of energy on the power line that show up as a brief, fast-rising voltage excursions on the sine wave. You also may see the terms: "spike", "impulses" or "surge" being used to describe these phenomena. They typically last for a few microseconds to several milliseconds and are caused by a variety of loads turning on or off (specifically, most electrical devices will either require an inrush of energy to charge them when power is first applied, and conversely will discharge some stored energy when the device is turned off). Transients can also be caused by faults on the power system, or in the extreme, by lightning. Here is an example of a voltage transient: