mpression of instability of the visual sensation produced by a light whose intensity and spectral distribution are time variant. The amplitude of the voltage variations is usually less than 10% and the behaviour of the electrical apparatus is not affected. Notwithstanding, these small disturbances can result in lightning variations which may affect the human eye. This sensitivity is strictly dependent on the frequency of the phenomena reaching its peak value around 7-10 Hz. In this range, even a 0.3% variation of the rms voltage feeding an incandescent lamp may be perceived.
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 ...
