About power quality

Power quality is a generic term for disturbance-free electricity supply. Deviation from voltage sinusoidal waveform (50 Hz), which may result in equipment being disturbed or damaged, is to be regarded as a power quality disturbance. This means that the more sensitive equipment you connect to the grid, the higher should your demands on power quality and disturbance-free environment be.

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Voltage dips (sags)

By dips we normally mean voltage drops that are deeper than 10 percent of the nominal voltage level and have a duration longer than 20 ms (one period). Compared with transients dips are slower disturbances; therefore, they have an impact on voltage rms.

Sources of disturbance

Common sources of disturbance that cause voltage dips are thunder, earth fault, short circuit, motor starting, pumps, mills, and welding machines.

Consequences

When a voltage dip occurs, it results in an energy deficit which disturbs electronics, control systems, computers, robots and other equipment. Other consequences are disruptions of drives, blinking of lightbulbs, disturbances in thermal processes/foundries, machine breakdowns, shutdowns etc. It has also been shown that voltage dips can cause surges in electronic devices that may be damaged by this.

Costs

Total costs for voltage dips with short breaks (less than 3 minutes) for Swedish electricity customers are estimated at 1,400 million SEK per year (UPN, 2006). Hardest hit are the process and manufacturing industries, where different types of downtime cause major economic losses.

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Swells (Voltage Increases)

By voltage swells we mean transient voltage rises above 3 percent of nominal voltage and with a duration longer than 20 ms (one period).

Sources of disturbance

Connection of capacitor banks, disconnection of reactors, error compensation, poor grounding, etc.

Consequences

Recurring voltage swells lead to exhaustion of insulation material, which ultimately can lead to insulation failure and subsequent failure of devices. Occasional high voltage swells can cause more direct errors, such as disruptive discharge and other insulation errors.

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Transients

Transients are fast, positive or negative voltage peaks which have a duration of less than 20 ms (one period). In other words, transients are a faster voltage change than, for example, voltage dips.

Sources of disturbance

Transients are caused by lightning, connecting/disconnecting of capacitor banks, switching in the grid, etc.

Consequences

Equipment breakdowns, disruption in electronics, control systems, computers, disruption of drives etc. Transients that break through the zero-crossing can cause disturbance to sync devices that trigger on the zero-crossing.

Costs

Transients are estimated to cost Swedish electricity customers 460 million SEK per year (UPN, 2006) (it is possible that this cost even includes costs for swells).

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Harmonics

Harmonics are voltage and currents with a frequency different from the fundamental (50 Hz). When harmonics occur in the grid, the voltage and current waveforms and the original sine wave get distorted. When we talk about harmonics in these contexts, we usually refer to harmonic harmonics, i.e. harmonics that are integer multiples of the fundamental. The most common measurement parameter of harmonics is THD (Total Harmonic Distortion). It is a measure of the overall harmonic content.

Disturbance source

All nonlinear loads produce harmonics. Examples are computers, compact fluorescent lamps, switching power supplies, frequency converters, electric arc furnaces, etc. If you measure high levels it can be an indicator of faulty harmonic filters.

Consequences

Impaired efficiency in engines, increased energy losses, overheating of motors, overload of transformers and other equipment, resonance resulting in over-current or over-voltage, disturbance with electronics and control systems, currents in neutral mm.

Costs

For Swedish electricity customers, the total annual cost for harmonic disturbances are estimated at 345 million SEK (UPN, 2006). Hardest hit are the manufacturing and process industries.

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Unbalance

Unbalance means that the voltage in the three phases is not equal. It arises from uneven load between the three phases of a 3-phase system. Non-uniform load is caused by uneven distribution of one-phase and two-phase loads; the phases are loaded unevenly. Unbalance is calculated as the ratio between the negative and positive phase sequence component.

Disturbance sources

The problem of unbalance has increased in recent years as the major household appliances such as washing machines and dishwashers are increasingly one-phase instead of 3-phase. Other sources of unbalance could be trains, arc furnaces or twisted transmission lines.

Consequences

Currents in the 0-conductors, warming, efficiency of 3-phase motors decreases, voltage drives that are fed with asymmetrical voltages can give rise to harmonics.

Costs

Unbalance causes breakdowns, accelerated wear on equipment, as well as increased energy losses etc. Cost estimates for this parameter are missing.

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Flicker

Flicker is a measure of the fluctuations (repeated variations) in voltage. Flicker makes light bulbs flash or pulsate. These fluctuations arise by frequent connection and disconnection of loads, often in combination with a weak grid.

Disturbance source

Disturbance sources causing flicker are welders, arc furnaces, heat pumps, induction cooktops, car scrap plants, rolling mills, etc.

Consequences

At high levels flicker is perceived as psychologically irritating. The wide deployment of heat pumps have had a major impact on the number of affected subscribers. Most commonly, however, it is the subscriber himself who causes the disturbances.

Costs

Private housholds are most commonly affected, especially in weak networks. The direct economic impact is difficult to calculate as flicker causes reduced productivity.

RVC – Rapid Voltage Change

Rapid voltage change is a parameter that is used when looking at voltage changes that are greater than 3 percent and less than 10 percent. This has been added as a supplement to flicker and voltage dips where its original purpose has been to capture individual (not cyclical) yet common disorders that cause flashing of lights.

Disturbance sources

Switchings, starting engines, welding, induction cookers, etc.

Consequences

Electrically, the consequences of RVCs are usually small. However, like flicker they are perceived as irritating as these cause lights to flash.

Costs

Private housholds are most commonly affected, especially in weak networks. The direct economic impact is difficult to calculate as flicker causes reduced productivity.

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Frequency deviations

One type of disturbance that has received increasingly more attention in recent years are frequency deviations. With wind and thermal power plants we have got a more distributed generation and hence equipment with sensitive frequency protection. If a large load is disconnected, or a larger power plant abruptly stops, a momentary overplus or deficit occurs in the energy production system which in turn disturbs the frequency. Although frequency disturbance itself is relatively rare, the consequence is very extensive once it happens because the disturbance affects the whole grid. On December 1, 2005, we could clearly see how a disturbance in the northern part of Sweden had nationwide effects with major implications for both Sweden and Norway.

Disturbance sources

Frequency disturbances occur with abrupt disconnection of major power generation, such as when large hydro or nuclear power plants suddenly stop.

Consequences

Motors and generators with frequency protection might be disconnected. It is important that the frequency caps for the equipment are properly set on the basis of the frequency deviations in the network and the requirements of the equipment itself. Especially thermal power plants have shown to be vulnerable to this type of disturbance.

Costs

Cost estimates for this parameter are missing.