Voltage sags disrupt control processes in Programable Logic Controllers (PLCs), variable frequency drives (VFDs) and relays. This leads to outages in manufacturing lines and direct costs as a result of lost supplies and damage to equipment. Indirect costs such as delayed deliveries and potential safety hazards (due to sudden stalling of equipment) may also result from sags.

A SAG compensator detects the sag in less than 3 milliseconds and is capable of supplying 60% of the nominal voltage (a dip of 138V) during 1 minute.

This ensures a stable operating voltage within the installation, despite the voltage sag on the supplying grid. This prevents ‘brownouts’ (safety shutting down due to low voltage) and therefore allows the plant to operate without noticing the sag.

Application of a voltage SAG compensator

Of all Power Quality phenomena, voltage sags are responsible for the most economic damage. SAG compensators are not emergency power generators, but prevent disruption caused by voltage sags. This makes them the most cost-effective solution for plants in countries with a strong electricity grid (few outages) but where sags still occasionally occur.

Costs of voltage sags:

  • Loss of production
  • Loss of supplies
  • Personnel costs
  • Damage to equipment
  • Safety risks (due to sudden stalling of equipment)

Causes of voltage sags

Voltage sags are caused by large inrush currents (from drives, for instance). However most voltage sags are induced by short circuits in the public grid. These can be caused by excavation work, gusts of wind during storms that cause conductors to touch,or by improper switching at distribution stations. This means that the possibility of a voltage sag occuring is always present.

Free energy?

A SAG compensator does not create energy, nor does it store energy. A SAG compensator uses the grid connection to temporarily increase current consumption despite the sag. This current is subsequently used to temporarily boost the voltage to it’s nominal value by means of a converter which protects the equipment in the installation.


Uninteruptable power supplies (UPS) based on batteries or UPSs based on flywheels with an emergency generator (DRUPS) can also protect against voltage sags. However these are more often very expensive solutions, whilst complete loss of voltage does not occur often enough to justify the costs.

SAG Compensator UPS (batterijen) DRUPS (Flywheel+ EG)
+ No increased harmonic voltages - Additional fire risks - Higher harmonic pollution as a result of increased impedance
+ No mechanical parts, no batteries - Short life span of the batteries - Relatively large amount of maintenance due to constantly moving mechanical parts
+ Up to 4000kVA in a compact formfactor - Large volume needed - Needs a lot of room: supply of fresh air, removal of combustion gasses and optionally storage for fuel.
- Not suitable for isolated operation +/- Short duration emergency power solution + Long duration emergency power solution
++ Cost-effective solution for grids with few outages but dips still occur - Expensive - Constant energy usage to keep the flywheel rotating

Combinations to arrange the optimal solution are possible .


Sag compensators are often applied in the following industries


Non critical processes but where outages should be minimized.

Voeding & Drank
Food industry

High speed systems such as packaging lines and photo-diagnostics

Chemical industry

Prevent rejected batches and improve consistency in continuous production


Prevent outages in telecommunicationsystems and errors during data processing

Maunfacturing industry

Prevent outages on fully automated processes

Semiconductor industry

Prevent outages on sensitive equipement

HyTEPS is an expert in cost-effective solutions for voltage sags


UPS systems are an expensive solution for a problem that very seldom occurs: total loss of voltage. SAG compensators offer protection to the most common kind of outages - Voltage sags - which make these the most cost-effective solution. No supplies are lost, and production is maintained. HyTEPS has over 14 years of experience in the effects of voltage dips and how to mitigate them.

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