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Created Date : Sun, 26/02/2012 - 21:36
Project Description :


customers are, e.g., generator stations, distribution networks, large industrial customers (who could be generating or consuming electricity at different times, based on the electricity price at that moment), and other transmission networks.    For a distribution network, the customers are currently mainly end-users that only consume electricity, but also the transmission network and smaller generator stations are customers.   Note that all customers are equal, even though some may be producing energy while others are consuming it.   The aim of the network company is only to transport the energy, or in economic terms: to enable transactions between customers.


                The technical aim of the power network becomes one of allowing the transport of electrical energy between the different customers, guaranteeing an acceptable voltage and allowing the currents taken by the customers.


                With an ideal network each customer should perceive the electricity supply as an ideal voltage source with a zero impedance.   Whatever the current is, the voltage should be constant.   As always, reality is not ideal.    Power quality concerns this deviation between reality and ideal.


                Note that this same model also becomes attractive when considering the integration of renewable or other environmentally-friendly sources of energy into the power system.   The power network is no longer the boundary condition that limits e.g. the amount of wind power that can be produced at a certain location.   Instead the power network’s task becomes to enable the transport of the amount of wind power that is produced.   It will be clear to the reader that the final solution should be found in co-operation between the customer and the network operator considering various technical and economic constrains.


                There are many aspects to the limitations set by the network on the market.   A much discussed one is the limited ability of the network to transport energy.   Note that lack of generation capacity is not a deficiency of the network but a deficiency of the market.


                In this modern way of looking at power systems, the utility no longer buys and sells energy, but instead sells transport capacity and access to the network.


                This paper will give a short overview of power quality with emphasis on the two issues that currently receive most attention:  harmonic distortion and voltage dips.   But first another attempt will be made at defining the term ‘power quality’.





2.             Definitions


                There is a lot of confusion on the meaning of the term ‘power quality’, not in the least because ‘power’ is used as a synonym  for ‘electricity’ in American English whereas it is also the energy transport per unit of time.   Different authors use different definitions.   A consistent set of definitions is given as follows:


  • Voltage qualityis concerned with deviations of the voltage from the ideal.   The ideal voltage is a single frequency sine wave of constant amplitude and frequency.
  • Current qualityis the complementary term to voltage quality:  it is concerned with the deviation of the current from the ideal.   The ideal current is again a single-frequency sine wave of constant amplitude and frequency, with the additional requirement that the current sine wave is in phase with the voltage sine wave.
  • Power qualityis the combination of voltage quality and current quality.
  • Quality of supplyis a combination of voltage quality and the non-technical aspects of the interaction from the power network to its customers.
  • Quality of consumptionis the complementary term to quality of supply.


                Note that not all these terms are equally commonly used, especially current quality and quality of consumption are used more frequently.   Also note that other sources give other, often conflicting, definitions.    All definitions given above apply to the interface between the network (company) and the customer.   This may be for example a domestic customer and the public low voltage distribution network, an individual plant and the industrial medium-voltage distribution network, a power station and a transmission network, or a transmission network and a distribution network.    The term power quality is certainly not restricted to the interaction between the power grid and end-user equipment.


                The term electromagnetic compatibility  (EMC)  has in this context a more restricted meaning:  it applies only to the interaction between equipment and its electromagnetic environment (e.g. the power system).   Strictly speaking it would thus only apply to low-voltage networks but the terminology is also being applied to higher voltage levels.   Note that in the international (IEC) standards power quality is treated as a subset of EMC.


                Power quality disturbances (i.e. deviations of voltage and/or current from the ideal) come in two types, based on the way a characteristic of voltage or current is measured:

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