Three-phase Four-wire Active Power Filter Detection Technology

The problem of power quality is gaining more and more concern due to the widespread of power electronic based loads. It is because these nonlinear loads may seriously pollute power lines with their high level of harmonic currents and poor power factor. Reactive power and harmonic currents are two problems that threat the system power. Reactive power requires more generator size and transmission/distribution losses, while harmonic currents could harm consumer side, cause disturbance and affect stability of the system.Shunt active power filters have been considered as an effective solution for these problems as they can compensate simultaneously these harmonics and reactive power. When designing a shunt active power filters (SAPF), it is crucial to generate reference currents for determining actual compensating current injections to the point of common coupling. Traditionally, we use instantaneous reactive power theory (ie. p-q theory), proposed by Akage at al. in 1983, to compute SAPF reference compensating currents.In p-q theory, voltages and currents in a three-phase three wire system are transformed into two-phase current/voltage components on orthogonal-coordinates, and then the instantaneous real and imaginary powers can be calculated without any time delay from the two-phase components. Though the instantaneous reactive power theory has a conceptual limitation, such as only complete for three-phase systems without zero-sequence currents and voltage, it inspired the development of many other p-q theory-based methods for realizing the SAPF.Instantaneous power in 0-α-βtransformation discussed by Ferrero and Superti-Furga, "Nabae power" named by Akagi and Nabae are such methods that consider the zero sequence components. It can easily be seen that the zero sequence currents and voltage can contribute to the real power, when it comes to the influence on the imaginary power, there is on brief definition. Nabae believes 0-sequence axis have the same situation withαandβaxis, and instantaneous active/reactive power, respectively, by the scalar/vector product of the voltage and the current space vectors in 0~α-βcoordinate. While Ferrero and Superti-Furga insist 0 axis independent of the other two, the 0-sequence component only related with active power. Moreover, the upper two methods cannot be derived from the single-phase power systems as a special case. In 1999, Hyosung Kim proposes a power compensation algorithm in a three-phase system by using a so-called p-q-r theory. It transforms the system voltages and currents from the a-b-c coordinates to the p-q-r coordinates that are rotating 0-α-βcoordinates. Since the axis rotates along with the voltage space vector, there is only one voltage component e_p, the other two components e_q and e_r are equal to zero. An instantaneous active power p and two instantaneous reactive powers q_q and q_r are defined by a scalar product and a vector product, respectively. In this method both instantaneous active and reactive powers can be defined in the zero-sequence circuit in three-phase four-wire systems. Although the calculation is more complex than others, it can be derived from the single-phase power systems as a special case.Also, cross vector theory has the same character and the calculation is easy. It defines instantaneous active/reactive power, respectively, by the scalar/vector product of the voltage and the current space vectors in a-b-c coordinates in a three-phase four-wire system. There are one instantaneous real power and three instantaneous imaginary powers that follow power conservation and agree well with the general understanding of power. Traditionally, when a zero-sequence voltage exists, the neutral-line current could not be eliminated completely by compensating the instantaneous imaginary power. This article proposes a novel cross vector theory that can compensates for three reactive power and eliminates the neutral current in unbalanced three-phase four-wire systems whether with or without zero-sequence components in the currents and voltages.