How to avoid production losses in the event of voltage sags on a distribution system

A predictable event with unpredictable effects

This article is intended to complement the publication Problems with power quality at electrical installations, which addresses power quality issues that can impact a facility’s power supply, in particular electrical equipment downtime caused by system voltage sags.

When voltage sags occur on a distribution system, the residual voltage, in one or more phases, can vary from 10% to 90% of its nominal value for less than one minute. In comparison, residual voltage under normal conditions is at least 40% for less than one second and most often for about one-tenth of a second. Voltage sags can significantly impact certain industrial activities, resulting in production losses or downtime for certain types of critical processes and facilities (pharmaceutical, semi-conductor manufacturing, hospitals). In other cases, even if there are fewer financial losses, having to restart systems can also cause significant inconvenience.

The equipment sensitivity threshold is highly variable and fluctuates depending on the severity and duration of voltage sags; 120V microprocessors and automated controls are generally more sensitive than electromechanical equipment. However, the latter are definitely more vulnerable when coupled with a variable frequency drive (VFD) or an automated control, currently a wide-spread trend in industrial processes.

Voltage sags usually result from short-circuits on the power supply system. They can be caused by a variety of events, for example equipment failures or contact between two conductive elements, or damage incurred during unusual weather events. Generally, public distribution systems experience three times more voltage sags than complete power outages, and the sags are often more frequent in rural areas. Compared with transmission systems, distribution systems are more exposed to voltage sags because of the amount of equipment required and their proximity to vegetation.

Acceptability criteria for voltage sags

Some organizations have proposed acceptability curves for voltage sags based on duration, such as the ITIC (CBEMA) curve, applied below to single-phase 120V / 60Hz computer equipment, and the SEMI F47 curve issued for semi-conductor manufacturers.

Although this type of curve is used to “qualify” a series of events, it cannot be used to get a precise analysis of potential impacts on critical processes. For example, imported electronic equipment that operates on a slightly different assigned voltage (e.g., 110V) may react differently because it’s more sensitive. Also, a voltage sag, even if it doesn’t cause a service interruption, may prevent a read-write operation and thus have unexpected effects on a process, even in the absence of a reboot.

Mitigation adapted to current and future facility needs

Voltage sag solutions are often difficult to implement directly on the distributor’s system and require consideration of the system’s features and mitigation opportunities. The costs associated with such a review can increase rapidly, since the mitigation affects a large number of consumers, and the majority of them therefore run the risk of not being able to pay for the implementation of a global solution. Furthermore, the mitigation solution selected may be inconvenient for some clients (e.g., a decrease in the short-circuit level by isolating sensitive loads).

Voltage sag mitigation solutions generally use the residual power on the system at the time of the event (dynamic voltage restorer DVR) or stored power (RUPS flywheel, batteries, capacitors, etc.). Less equipment is needed when using the system’s residual power, but its performance depends on the status of the system at the time of the disturbance. Energy storage offers greater flexibility and by avoiding a power interruption, sometimes allows the load to be transferred to generating units, but the costs of maintaining the stored energy may be high. Impacts on the short-circuit (motor performance) and on other power quality phenomena are also aspects to be considered in the mitigation analysis.

The mitigation solution can be deployed at numerous locations on the system, i.e., close to the problem equipment only, across an entire sector or at the facility intake for full coverage. A technical-financial analysis of current and future issues is required to identify the best strategy. The analysis should specifically consider the cost of production delays and losses, environmental impacts, safety risks and impacts on business reputation, as well as the benefits and issues of the mitigation solution, specifically:

• Integration challenges (restricted space, prolonged outages, connection complexities, etc.)
• Component lifespan
• Initial costs and the possibility of postponing certain investments
• Correction of additional problems
• Maintenance procedures and related interruptions

Solutions adapted to each facility

Voltage sag mitigation analysis for a given facility requires:

• An historical log of the defects, including financial losses.
• A solid understanding of the process, critical elements and upcoming changes.
• A review of the ability to implement, operate and maintain mitigation components.

Close co-operation with the power distributor is essential to correctly identify the origin of the disturbances and understand the upcoming changes that may modify the frequency and severity of voltage sags on the system. With over 150 power disturbances studies to its credit, BBA has leading-edge expertise and demonstrated ability to work effectively with a variety of stakeholders. We can propose innovative solutions that satisfy the technical and financial needs of electricity consumers. Contact us to learn more!