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.
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
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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.
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 its 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.
|DRUPS ( Flywheel + NSA)
|+ 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
|– Constant energy usage to keep the flywheel rotating
Sag compensators are often applied in the following industries
Non critical processes but where outages should be minimized.
High-speed systems such as packing lines and photodiagnostics
Prevent rejected batches and improve consistency in continuous production
Prevent failures in telecommunications systems and errors in data processing
Prevent outages on fully automated processes
Prevent failure of sensitive machinery