Q19Power Quality and FACTS

Definition and Principle of Voltage Sag

A voltage sag (or dip) is a short-duration reduction in RMS voltage magnitude to between 0.1 pu and 0.9 pu of nominal voltage, lasting from half a cycle to one minute, as per IEEE 1159 classification. Voltage sags are among the most frequent and economically damaging power quality disturbances faced by industrial and commercial customers.

Causes of Voltage Sag

The most common cause of voltage sags is short-circuit faults (line-to-ground, line-to-line, or three-phase) occurring anywhere in the transmission or distribution network. During a fault, a large fault current flows through the system impedance, causing a temporary voltage drop that is felt by all customers electrically close to the fault, until the protection system clears the fault. The magnitude and duration of the sag experienced by a particular customer depend on the fault type, the fault location, the system impedance between the fault and the customer, and the fault-clearing time of the protective devices. Other causes include starting of large induction motors (which draw high inrush starting current), energization of large transformers, and sudden connection of heavy loads.

Characterizing Sag Performance

Voltage sag performance at a given location is characterized using two parameters: retained voltage magnitude (as a percentage of nominal) and duration (in cycles or milliseconds). Equipment sensitivity to sags is commonly represented using tolerance curves such as the CBEMA curve or its successor the ITIC (Information Technology Industry Council) curve, which plot the magnitude and duration combinations that equipment can withstand without malfunction. If the sag magnitude-duration point falls below the ITIC curve's lower boundary, the connected equipment is likely to trip or malfunction. Utilities and industries use sag statistics (SARFI indices - System Average RMS Variation Frequency Index) to quantify how often a given bus experiences sags below specified thresholds over a period.

Impact of Voltage Sags

Even brief sags of only a few cycles can cause adjustable-speed drives to trip on under-voltage protection, contactors to drop out, PLCs to reset, and computer/data-processing equipment to lose data or restart, leading to costly production interruptions in continuous-process industries such as semiconductor manufacturing, paper mills, and textile plants, where a single sag can cause losses far exceeding the cost of mitigation equipment.

End-User Level Solutions

Since utilities cannot economically eliminate all faults causing sags, mitigation is often applied at the end-user level, close to the sensitive load. A Dynamic Voltage Restorer (DVR) is one of the most effective solutions: it is installed in series with the incoming supply to the sensitive load and injects a compensating voltage during a sag so that the load sees a constant, undisturbed voltage. Uninterruptible Power Supplies (UPS), either online or line-interactive, use stored energy (batteries) to supply the load during a sag or interruption, completely decoupling the load from supply disturbances.

Ferroresonant (constant-voltage) transformers exploit magnetic saturation to maintain a relatively constant output voltage despite input voltage variation, offering a low-cost passive solution for smaller loads. Motor-generator (M-G) sets use flywheel energy storage coupled to a motor-generator combination to ride through short interruptions and sags. Additionally, improving the inherent ride-through capability of end-use equipment itself, for example by adding hold-up capacitors in DC power supplies or programming adjustable-speed drives to ride through momentary disturbances, reduces sensitivity without requiring a separate compensating device. The selection among these options depends on load criticality, sag frequency and severity at the site, and the economic cost of downtime versus the mitigation equipment investment.

• DVR: series voltage injection, corrects sag/swell in real time
• UPS: battery-backed decoupling from supply disturbances
• Ferroresonant transformer: passive, low-cost voltage regulation
• Motor-generator set: flywheel-based ride-through
• Improved equipment ride-through capability (hold-up capacitors, control settings)

Additional Consideration: Point-on-Wave and Phase-Angle Jump

Beyond magnitude and duration, a voltage sag caused by an unbalanced fault also typically introduces a phase-angle jump, i.e., a sudden shift in the phase angle of the voltage waveform at the instant the fault occurs, in addition to the change in magnitude. Sensitive equipment such as adjustable-speed drives that use phase-locked loops for synchronization can mis-operate due to this phase jump even if the magnitude drop itself is within tolerance. Point-on-wave characteristics, i.e., the instant within the cycle at which the sag begins, also affect the severity of impact on rectifier-front-end equipment, since a sag beginning near a voltage zero-crossing can cause a different transient response than one beginning near the peak. A complete voltage sag characterization for end-user mitigation studies must therefore consider magnitude, duration, phase-angle jump, and point-on-wave, not magnitude and duration alone.

Economic Perspective on Mitigation Selection

The choice of an appropriate end-user mitigation solution is ultimately an economic decision that weighs the capital and maintenance cost of the mitigation device against the expected cost of downtime avoided. For a small number of critical control circuits, a simple constant-voltage transformer or a small UPS may be adequate and inexpensive. For an entire sensitive production line drawing hundreds of kilowatts, a custom power device such as a DVR sized to the specific sag depth and duration profile recorded at the site (through power quality monitoring over several months) offers the most cost-effective solution, since its rating only needs to cover the missing voltage component rather than the full load power. Facilities with very high sensitivity to any interruption, such as data centers, typically deploy a combination of UPS for ride-through of very short events together with standby generation for sustained interruptions.

Standards and Industry Practice

Utilities and industries commonly reference IEEE 1159 for disturbance classification, IEEE 1346 for compatibility assessment between the equipment sensitivity (via ITIC/CBEMA curves) and the site's sag performance, and IEC 61000-4-11 for equipment immunity test procedures against short voltage interruptions and sags. Site power quality audits typically begin with a monitoring campaign using a power quality analyzer over a representative period (often three to twelve months) to statistically characterize the expected sag magnitude-duration distribution at the site, which then forms the technical basis for selecting and sizing the most appropriate end-user level mitigation equipment, whether a DVR, UPS, ferroresonant transformer, or improved equipment ride-through specification.

Utility-side measures also contribute to reducing sag exposure even though the questions here focus on end-user solutions: faster fault clearing through improved relay coordination and current-limiting fuses reduces sag duration, while installing fault current limiters at strategic network points reduces the severity of voltage drop seen by nearby customers during a fault. Nonetheless, since utilities cannot guarantee zero sags at every customer connection point, the end-user level solutions described above (DVR, UPS, ferroresonant transformers, motor-generator sets, and improved equipment ride-through) remain the primary practical means by which a facility protects its own sensitive processes from the residual sag exposure that utility-side measures cannot fully eliminate.