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Voltage sag: the most significant of all Power Quality issues

Modern industry is becoming more automated and the sensitivity of processes to power quality events is increasing.

It is generally recognized that quality is an important aspect of the electricity service. Not only low prices are important, also high-quality matters to customers. Price and quality are often complementary aspects; together they define the value that customers derive from consuming electricity.

Companies are more and more sensitive to Power Quality issues because they can cause troubles and damages to equipment, up to interrupting the production cycle.The most significant of all Power Quality phenomena is the voltage sag: more than 60% of poor Power Quality costs are consequence of Voltage SAGs. SAG cost is normally lower than a Voltage Supply Interruption one, but the first is by far more frequent. Troubles increase with sophisticated and electronic appliances.


Temporary reduction of the Voltage RMS below a specific threshold at an electrical supply line point.

Voltage SAGs are generally caused by faults in the public network or in the installations of network users, in few cases by transient overloads due to the gearing up of large motors or the switching on of large loads. Voltage SAGs are unpredictable and random.

SAG starts when Voltage goes lower than 90% of nominal value and ends when voltages resume above 90%. Voltage SAG duration is considered within 10ms up to 1min.

The great deal of Voltage SAG has a duration lower than 1 second and a residual voltage higher than 40% of the rated value.

The voltage SAG propagates from the higher voltage levels to the lower ones, the load is often connected to a voltage level lower than the point of failure. Faults in the network cause deep voltage SAGs if they occur near loads. The incidence of voltage SAGs is much greater in the case of an aerial MV network than with underground cables.


The more modern the equipment is and the more electronics is required, the more serious are the problems caused through voltage SAG. With the increasing number of regenerative energy sources, energy sags, fluctuations and frequency deviations also increase.

Example of costs due to voltage SAGs:

  • Costs for unproductive personnel due to the sudden termination of the production cycle.
  • Costs for raw materials and production lost.
  • Costs for damages and/or malfunctions of machineries (repairs to them, temporary hire of new ones).
  • Penalties caused by contractual shortcomings.
  • Sanctions for damage to the environment. Increase in general insurance costs.


Many businesses require immunity to voltage SAGs and voltage conditioning rather than battery back-up power, provided by UPS system. In those cases where back-up power is unnecessary, a sag compensator provides superior protection and additional power quality functions, such as protecting against SAGs, over/under voltage, voltage fluctuations.

Moreover protecting a whole plant by UPS, which can guarantee SAGs immunity, may be very costly, due to battery and maintenance costs.

The right solution is the sag compensator, that permits SAGs correction up to -50% for 1 min.

It’s an economical solution: no maintenance and operation costs: no battery energy storage required and efficiency >98% at nominal power.


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The 3 main causes of low Power Quality

Power Quality is gaining increasing attention in the electric power industry. The consumer of electrical energy requires electric power with a certain quality, but loads can have a negative impact on the electrical system and are thus also subject to an assessment in terms of quality. Power quality is therefore intrinsically linked to the interaction between the electrical system and loads and must take into account both the voltage quality and power quality.

Possible consequences of low power quality that affect business costs are:

  • Power failures (Release switches, fuses blowing).
  • Breakdowns or malfunctions of machines.
  • Overheating of machines (transformers, motors, etc.) leading to reduced useful life.
  • Damage to sensitive equipment (computers, production line control systems, etc.).
  • Electronic communication interference.
  • Increased distribution system losses.
  • The need to oversize systems to cope with additional electric stress, resulting in higher installation and operational costs.
  • Luminosity flickering

Interruption of production due to these impacts of low power quality entails high costs due to production loss and the associated waste. For the Industrial sector, the estimated costs due to poor power quality represent 4% of turnover (Source: Studio Leonardo Energy). The impact of production interruptions is greatest in companies with continuous production.

Among the main causes of poor power quality in low voltage are:

  • Voltage sags, because equipment operates less efficiently and can stop.
  • Harmonic pollution, which causes additional stress on the networks and systems, causing them to operate less efficiently.
  • Excessive reactive power, because it charges useless power to the system.

The solutions vary for each cause. The voltage sags can be eliminated with a sag compensator, which ensures a voltage output at a nominal value. Reduced productivity, loss of data, loss of security, and machine breakdowns are only some of the problems caused by an unstable power supply that can be solved with a voltage sag compensator.

Harmonic pollution is caused by large amounts of non-linear consumption (from inverters, soft starters, rectifiers, power electronics, non-filament lighting, presses, etc.). Such devices deform the electrical current causing disturbances and problems to the system. Harmonic pollution is solved by active filters that are capable of eliminating the current harmonics in the system by measuring and injecting the same current, but in the opposite phase.

Excessive reactive power is regulated by a power factor correction system, which not only avoids any penalties due to excessive reactive energy, but reduces the “unnecessary” electrical current that flows into the lines and power components, yielding substantial benefits, such as reducing voltage drops along the lines and leakages due to the Joule effect.

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Harmonics: active or passive filter?

Loads operated by electronic devices are increasingly adopted in more and more industrial and commercial applications such as: Variable Speed Drives, rectifiers, UPS systems, DC power supplies, welding machines, computers, TV, energy efficient lamps, photocopiers.

These loads generate waveform distortions that become threat for network components.

In particular voltage and current harmonics, where the latter is the more dangerous, generate a series of problems, like:

  • Cable overheating
  • Undue tripping of circuit breakers
  • Blowing of fuses
  • Capacitor overloading and network resonance
  • Neutral cable overload
  • Transformer premature ageing
  • Electronic appliances disturbance.

These effects are seldom related to harmonic current and voltage distortion, while effective remedy for harmonics are passive and active filters. Active filters achieve much higher levels of efficiency in harmonics cleaning the first does not:


  • it can compensate many harmonic currents simultaneously (ICAR FA40 up to the 50°)
  • It does not saturate or overload
  • It is uninfluenced by the network conditions (capacitance/Inductances)
  • it hasn’t capacitance
  • it is extremely flexible, it can target few harmonic currents
  • It can compensate power factor, even at high speed
  • it can compensate unbalanced loads
  • it attenuates voltage flicker
  • it can be set in parallel with other filters, even at later stages
  • it automatically and semalessly adjustes to the load


  • it can compensate only one or very few harmonic current
  • it is extremely difficult to compensate few harmonics at the same time.
  • it can saturate or be overloaded
  • it is influenced by the network as for the existing capacitance and inductance
  • it is a capacitive load, hence improve power factor.
  • it is fix and not adjustable
  • it can be ON/OFF, or in steps.
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