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Different kinds of disturbances
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2 Different kinds of disturbances
It is possible to define some characteristic parameters in order to assess the quality of power delivered from the supply:
• Frequency;
• Amplitude;
• Waveform;
• Symmetry of the voltage system.
These characteristics may vary during the normal operation of the electrical system because of load changes, disturbances introduced by user apparatus and fault occurrence. As a consequence, such characteristics may be time variant at each point where energy is delivered and for a given instant they may not be equal at different points in the network. In most of the cases, statistical evaluation of such phenomena may represent an important means to collect information about power quality.
2.1 Frequency variations
Large generators switch-off or important load commutation may lead to transient variations of the frequency, which are quickly compensated through the primary regulation of the generators. Then the power exchanged among interconnected grids is balanced by the generation station, which has to perform the secondary regulation. The primary regulation achieves a null average value for the power exchanges among interconnected grids due to frequency variations. Grid frequency affects the behaviour of motors (speed variations), the performance of some electronic devices where it is used for synchronisation purposes, the losses in magnetic materials and the usefulness of filters to suppress harmonics. Frequency variations are defined in terms of percentage deviation from the nominal frequency.
2.2 Voltage amplitude variations
The grid voltage continuously changes because of the commutation of the electrical devices connected to the grid. The voltage variation may be slow or quick depending on whether an overall load progressive change or a step change for a large load is happening. The grid impedance deeply affects the amount of voltage variations as a consequence of load change: the higher the impedance, the larger the variation.
2.3 Voltage fluctuations
A set of quick voltage variations is referred to as voltage fluctuation. The limit between slow and quick variations is not so definite and can range from a few seconds to one minute. Slow variations are assessed through the average value calculated over contiguous intervals of ten minutes.
Rapid variations may be single or repetitive and their amplitude usually does not exceed 6–8% of the nominal voltage. Usually electrical apparatus are able to work even in the presence of this kind of disturbance (in most of the cases corrected by voltage regulation) unless initial voltage is too low. These kinds of variations are caused by variable loads such as welding machines, arc furnaces and mills. Rapid variations of over 10% amplitude irrespective of the duration, are considered voltage dips.
2.4 Flicker
The term flicker is referred to as a systematic or casual variation of the voltage amplitude ranging from 0.9 to 1.1 p.u.. Sometimes the terms flicker and voltage fluctuations are interchangeably used. Notwithstanding, the term flicker is strictly related to the impression 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.
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