Voltage sags – or dips which are the same thing – are brief reductions in voltage, typically lasting from a cycle to a second or so, or tens of milliseconds to hundreds of milliseconds. Voltage swells are brief increases in voltage over the same time range. Longer periods of low or high voltage are referred to as “undervoltage” or “overvoltage”.
Voltage sags are caused by abrupt increases in loads such as short circuits or faults, motors starting, or electric heaters turning on, or they are caused by abrupt increases in source impedance, typically caused by a loose connection.
Voltage swells are almost always caused by an abrupt reduction in load on a circuit with a poor or damaged voltage regulator, although they can also be caused by a damaged or loose neutral connection.
Voltage sags are the most common power disturbance. At a typical industrial site, it is not unusual to see several sags per year at the service entrance, and far more at equipment terminals.
Voltage sags can arrive from the utility; however, in most cases, the majority of sags are generated inside a building. For example, in residential wiring, the most common cause of voltage sags is the starting current drawn by refrigerator and air conditioning motors.
Sags do not generally disturb incandescent or fluorescent lighting. motors, or heaters. However, some electronic equipment lacks sufficient internal energy storage and, therefore, cannot ride through sags in the supply voltage. Equipment may be able to ride through very brief, deep sags, or it may be able to ride through longer but shallower sags.
“Well-designed” equipment should be able to tolerate any power event that lies in the shaded area. Note that the curve includes sags, swells, and transient overvoltages. The semiconductor industry developed a more recent specification (SEMI F47) for tools used in the semiconductor industry in an effort to achieve better ride through of equipment for commonly occurring voltage dips and therefore improving the overall process performance. It is basically the same as the ITI Curve but specifies an improved ride through requirement down to 50% retained voltage for the first 200 msec. Many short voltage dips are covered by this additional requirement. IEC 61000-4-11 and IEC 61000-4-34 provide similar voltage dip immunity standards.
Many utilities have benchmarked performance of the supply system for voltage dips but it has not been the general practice to specify any required performance levels for the system. Performance is often specified using the SARFI index that provides a count of all events with magnitudes and durations outside of some specifications. For instance, SARFI-70 would provide a count of all voltage dips with a retained voltage less than 70% (regardless of duration). SARFI-ITIC would provide a count of all voltage dips that exceeded the ride through specifications of the ITI Curve.
The table below provides a summary of voltage dip performance levels from a few major benchmarking efforts. Note that these are average performance levels and it would not be reasonable to develop limits based on an average expected performance (although these are the correct values to use when evaluating the economics of investments in ride through solutions).
The voltage dip performance can vary dramatically for different kinds of systems (rural vs urban, overhead vs underground). It may be important to include some of these important factors in the specification of the power quality grades.
It will also be important to specify the performance for momentary interruptions. These events can be a particular problem for customers and are not included in most assessments of reliability.
A previous CEA Technologies report prepared by Electrotek Concepts recommended that the SARFI indices be calculated for the following magnitude and duration categories:
The reasons for these categories were explained as follows:
As a final note, remember that voltage sags are voltages, and therefore always occur between two conductors – there is no such thing as a “sag on phase A” — it must be a sag between phase A and phase B, or a sag between phase A and Neutral.
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