Equipment within an end-user facility may have different sensitivity to voltage sags. Equipment sensitivity to voltage sags is very dependent on the specific load type, control settings, and applications. Consequently, it is often difficult to identify which characteristics of a given voltage sag are most likely to cause equipment to misoperate.
The most commonly used characteristics are the duration and magnitude of the sag. Other less commonly used characteristics include phase shift and unbalance, missing voltage, three-phase voltage unbalance during the sag event, and the point-in-the-wave at which the sag initiates and terminates.
Generally, equipment sensitivity to voltage sags can be divided into three categories:
■ Equipment sensitive to only the magnitude of a voltage sag. This group includes devices such as undervoltage relays, process controls, motor drive controls,6 and many types of automated machines (e.g., semiconductor manufacturing equipment).
Devices in this group are sensitive to the minimum (or maximum) voltage magnitude experienced during a sag (or swell). The duration of the disturbance is usually of secondary importance for these devices.
■ Equipment sensitive to both the magnitude and duration of a voltage sag. This group includes virtually all equipment that uses electronic power supplies. Such equipment misoperates or fails when the power supply output voltage drops below specified values.
Thus, the important characteristic for this type of equipment is the duration that the rms voltage is below a specified threshold at which the equipment trips.
■ Equipment sensitive to characteristics other than magnitude and duration. Some devices are affected by other sag characteristics such as the phase unbalance during the sag event, the point-in-thewave at which the sag is initiated, or any transient oscillations occurring during the disturbance.
These characteristics are more subtle than magnitude and duration, and their impacts are much more difficult to generalize. As a result, the rms variation performance indices defined here are focused on the more common magnitude and duration characteristics.
For end users with sensitive processes, the voltage sag ride-through capability is usually the most important characteristic to consider. These loads can generally be impacted by very short duration events, and virtually all voltage sag conditions last at least 4 or 5 cycles (unless the fault is cleared by a current-limiting fuse).
Thus, one of the most common methods to quantify equipment susceptibility to voltage sags is using a magnitude-duration plot as shown in Fig. 3.6. It shows the voltage sag magnitude that will cause equipment to misoperate as a function of the sag duration.
Figure 3.6 Typical equipment voltage sag ride-through capability curves.
The curve labeled CBEMArepresents typical equipment sensitivity characteristics. The curve was developed by the CBEMA and was adopted in IEEE 446 (Orange Book). Since the association reorganized in 1994 and was subsequently renamed the Information Technology Industry Council (ITI), the CBEMA curve was also updated and renamed the ITI curve.
Typical loads will likely trip off when the voltage is below the CBEMA, or ITI, curve. The curve labeled ASD represents an example ASD voltage sag ride through capability for a device that is very sensitive to voltage sags. It trips for sags below 0.9 pu that last for only 4 cycles.
The contactor curve represents typical contactor sag ride-through characteristics. It trips for voltage sags below 0.5 pu that last for more than 1 cycle. The area of vulnerability for motor contactors shown in Fig. 3.5 indicates that faults within this area will cause the end-user voltage to drop below 0.5 pu.
Figure 3.5 Illustration of an area of vulnerability.
Motor contactors having a minimum voltage sag ride-through capability of 0.5 pu would have tripped out when a fault causing a voltage sag with duration of more than 1 cycle occurs within the area of vulnerability. However, faults outside this area will not cause the voltage to drop below 0.5 pu.
The same discussion applies to the area of vulnerability for ASD loads. The less sensitive the equipment, the smaller the area of vulnerability will be (and the fewer times sags will cause the equipment to misoperate).
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