The Spanish Grid Code and the grid codes from other countries require some quantities, such as active and reactive currents and powers, must be controlled in order to avoid unexpected disconnections of the wind farms submitted to voltage dips. These grid codes implicitly propose the traditional well-known formulations, included in the IEEE Standard 1459-2010, for measuring active and reactive powers and currents. For balanced voltage dips, these formulations are adequate to verify grid code requirements, although the different values of the active and reactive phase currents may difficult the verification process. However, for unbalanced voltage dips, traditional formulations include components which are a result of the imbalances and, thus, mistakes in the magnitude and duration of the active and reactive quantities may be presented.
Fundamental positive-sequence active and reactive formulations, also included in the IEEE Standard 1459-2010, are a more adequate alternative than the traditional theory for verifying the accomplishment of the grid code requirements. Several reasons justify the use of the fundamental positive-sequence quantities: (a) active and reactive currents have only one component so much for balanced as unbalanced voltage dips and, thus, the verification process of the grid code requirements is simplified; (b) positive-sequence active and reactive powers do not contain negative-sequence components caused by the voltage unbalances and, thus, these quantities exactly quantify active and reactive phenomena effects, respectively; (c) positive-sequence active and reactive powers and currents can be decomposed into two components, due to the loads and caused by the unbalances.
This decomposition established by the Unified Theory has been expressed in section 2. It shows how imbalances of supplies and loads originate additional positive-sequence powers and currents, which either can increase or decrease total values of these quantities and, therefore, the accomplishment of the grid code requirements can be better explained and new wind-generator support procedures can be proposed by applying the Unified Theory.
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