The Active Power Filter And Its Application In The Control Of Electrical Railway Power Quality

With the development of power system and traction power system,the requirement for power quality is increasingly high by the people.Most of the traction loads in electrical railway are phased rectifier loads,with the features of low power factor and high harmonic contents and big negative sequence currents.The influences of those loads are bigger on the common grid.Measures must be taken to improve the power quality.As one of the effective methods to improve the power quality,active power filter can better adapt to the changes in the system.It has the theoretical significance and practical value to research the active power filter and apply it to the electrical railway for improving the power quality.Considering the vector space,this paper establishes the coordinate transformation system in the three-dimensional space.The equivalent transformation between two different coordinate systems is deduced through strict mathematical derivation.The transform coefficient is just the one that used in the instantaneous reactive power theory(IRPT).The new coordinate transform system provides a theoretical basis for the coordinate transform used in IRPT.From the point of rotate vector in circuit theory,this paper explains the detected principle of i_p-i_q method.It can be inferred that the DC parts of instantaneous active and reactive power reflect the information of fundamental current.This method resolves the problems of direct-inferred tediousness and distinguishing order currents under distortion voltage.The reasons of bringing errors to the fundamental active currents detection are analyzed in the paper.On the basis of the improvement capacitor voltage control scheme in the DC side and a new fundamental active current detection method,the phase locked loop is omitted and the power frequency and arbitrary angle are provided to detect the fundamental active current under distortion voltage.It can realize the integrative currents compensation without phase locked loop.From the angle of probability theory,the thesis discusses the criterion for DC voltage reference value.It concludes that the direct-current side reference voltage has to be bigger than three times of system voltage peak value is merely a sufficient condition for APF to work normally.It has the guiding significance for the device selection. The fundamental current separation method based on the orthogonal character of sin and cosine functions is improved.It resolves the problem of detecting the fundamental active current in single phase circuit under distortion voltage without phase locked loop.A new direct detection method based on IRPT is proposed in single-phase circuit for harmonic currents detection.There is no need to construct three- phase system by time-delay link.The delay error is reduced for harmonic currents detection.The symmetrical decomposition algorithm based on instantaneous value in time domain is derived from the symmetrical component method based on the phasor in frequency domain.It is verified and provides the basis for the positive sequence fundamental current detection of three-phase asymmetrical loads.This paper proposes a new scheme using active power filter combined with symmetrical decomposition algorithm to compensate the harmonic,reactive power and negative sequence currents in electrical railway.Making the three-phase currents be positive sequence fundamental active currents is the compensation target in the scheme.Two single-phase active power filter devices with common detection circuit are installed in the low-voltage side.The purpose of detection circuit is to get the directive compensation currents for three-phase windings.The directive compensation currents for each power arm are distributed according to the three-phase and two-phase relationship in traction transformer.The single-phase active power filters generate the compensation currents respectively.The simulation system is built with actual surveys by matlab.It's verified that this method can realize the compensation for harmonic and reactive power even negative order currents and improve the traction power quality.