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A perfect flicker compensation is not possible, but an attenuation of this phenomena can be achieved through:

• an increase of the short circuit power;

• a reduction of the reactive power flux;

• a limitation of the motor starting currents. 2.5 Voltage dips – short interruptions Voltage dips are bi-dimensional electromagnetic distortions which are characterised by the amplitude and duration. Voltage dip means that energy is not properly provided to loads during this event and this could result in different consequences depending on the kind of load. According to International Electrotechnical Commission (IEC) standards, voltage dips are referred to as a sudden reduction of voltage affecting a point of the distribution network below 90% of the reference voltage. This reduction has to be recovered within 60 s. Whenever the voltage falls down to zero the event is classified as a short interruption.

The duration of a voltage dip is the interval between the instant when the voltage falls below the threshold value and the instant when the voltage rises again above the threshold. The depth of a voltage dip is the difference between the reference and the residual voltage.

The starting of large loads and faults on the network are the main causes of voltage dips. Dips caused by starting currents are less deep and longer (up to a few seconds) than the ones caused by faults on the grid (less than one second).

When large loads are switched on, the starting current could be much higher than the steady-state current. Since the feeders and the cable of a distribution system are designed for steady-state operation, the high current value is responsible for a considerable voltage drop. 2.6 Waveform variation Harmonics If an electric quantity is distorted and periodical it can be split into three terms: the mean value calculated over one period of the considered signal, the fundamental component having the same frequency of the considered signal and the sum of the harmonic components. The amplitude of the harmonics decreases with the frequency. The representation of such amplitudes is referred to as spectrum.

As regards symmetrical waveforms (perfectly matching of the positive and negative half-waves), the even harmonics are nihil. This type of harmonics were common when half-wave rectifiers were used.

Power suppliers provide a 50 Hz sinusoidal voltage, but the current drawn by a load is not always sinusoidal. The current is not sinusoidal anymore when the load impedance varies during one period T(the load voltage/current characteristic is not linear). Such type of loads is referred to as non-linear loads. For example, the magnetising current of a transformer is deformed by a third-order harmonic because of the non-linear magnetisation curve of the machine. Rectifiers (battery chargers, welding machines, etc.), inverters, electronic starters, adjustable speed drives, discharge lamps are other examples of non-linear loads. A distorted current causes distorting voltage drops so that the resulting voltage supplying a circuit will not be sinusoidal anymore. The voltage provided is the transformer voltage minus the voltage drop across the feeder. Thus, the voltage distortion depends on the distance from the transformer and on the line impedance. In short, the voltage distortion affecting the grid at a certain location depends on the value of the short-circuit current of that point. Also, once the grid voltage is distorted a linear load absorbs a distorted current. The presence of such harmonics on the grid is responsible for detrimental effects. Moreover, at higher frequencies, iron losses (hysteresis losses and eddy current losses) as well as the losses in the cables increase. Finally, electronic equipments may experience failures due to the presence of harmonics.

Another aspect which should not be neglected is the resonance issue related to the presence of harmonics in electrical networks. In fact, in this case the amplitude of a specific harmonic may rise up to several times that of normal operation. Consequently this high-value current may seriously damage capacitors and equipments connected to the grid.

In order to prevent this kind of event, the resonance frequency of the grid at a certain point has to be known and, additionally, the insertion of well-fitted anti-resonance coils may be considered to damp the oscillatory phenomena. 2.7 Interharmonics Interharmonics are particular harmonics whose frequency is not an integer multiple of the fundamental frequency. The analysis of such interharmonics has attracted increasing interest over the last few years since the massive use of power electronic equipments has caused an increment in their amplitude. They can be observed where there is at least a part not pulsating synchronously with the fundamental power system frequency. There are many loads introducing voltage or current interharmonics such as arc furnaces, welding machines and cycloconverters. 2.8 Unbalance A three-phase system is symmetrical and balanced when voltages and currents have the same amplitude in each phase and 120. phase shifted. To assess the degree of unbalance of a three-phase system it should be split into a positive sequence component, a negative sequence component and a zero sequence component.

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