Voltage sags are brief reductions in the voltage on ac power systems. (The American “sag” and the British “dip” have exactly the same meaning, and may be used interchangeably.) How brief? Between 1/2 cycle and a few seconds. Disturbances that last less than 1/2 cycle are commonly called “low frequency transients”; voltage reductions that last longer than a few seconds are commonly called “undervoltage.”
Power systems have non-zero impedances, so every increase in current causes a corresponding reduction in voltage. Usually, these reductions are small enough that the voltage remains within normal tolerances. But when there is a large increase in current, or when the system impedance is high, the voltage can drop significantly. So conceptually, there are two sources of voltage sags:
As a practical matter, most voltage sags are caused by increases in current.
It is convenient to think of the power system as a tree, with your sensitive load connected to one of the twigs. Any voltage sag on the trunk of the tree, or on a branch leading out to your twig, will cause a voltage sag at your load. But a short circuit out on a distant branch can cause the trunk voltage to diminish, so even faults in a distant part of the tree can cause a sag at your load.
Voltage sags have two main characteristics: depth and duration. Do a scatter plot of depth vs. duration, and you get a CBEMA or ITIC graph.
Most voltage sags originate within your facility. The three most common causes of facility-sourced voltage sags are:
Experts can identify the specific source of a voltage sag with an advanced power quality monitor, such as those found at PQMonitoring.com.
Voltage sags can also originate on your utility’s electric power system. The most common types of utility-sourced voltage sags are:
It is important to understand the source of the voltage sags before trying to eliminate them, because the wrong solution can actually make the problem worse. For example, if you install a ferro-resonant transformer as a voltage regulator, or a battery-operated UPS (a reasonable and common approach), but inadvertently install it upstream from the motor that is causing your voltage sags, the voltage sags will get worse, not better. In most cases, the correct solution is to adjust the equipment so that it is less sensitive to voltage sags.
Sag sensitivity – five ways equipment fails during voltage sags
CBEMA curve – voltage sag depth and duration at world-wide semiconductor plants
Sag immunity – inexpensive, simple ways to increase sag immunity
Semiconductor sag standards – industry standards, SEMI F47, F42
Power Standards Lab
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