Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Patent
2000-01-07
2000-09-05
Nguyen, Matthew
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
363 34, 363 51, H02J 336, H02M 540
Patent
active
061152692
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a pair of first and second control equipment for a first and a second active filter, respectively, in a bipolar converter station for conversion between alternating current and high-voltage direct current, the filters being intended for reduction of harmonics in a dc link connected to the converter station, and to a method for reduction of harmonics in a dc link, connected to a bipolar converter station, by means of a first and a second active filter.
Each one of the pieces of control equipment comprises frequency-selecting means which influence the frequency content of an influencing quantity, generated by the respective active filter, to essentially lie within one of a first and a second of at least two predetermined, mutually non-overlapping frequency bands.
BACKGROUND ART
A converter for conversion between alternating current and high-voltage direct current, connected between an ac link and a dc link, generates, through its mode of operation, harmonics in the current in the ac network and in the voltage of the dc link. The currents associated with the latter voltage harmonics, and in particular the so-called ground-mode current, constitute a source of disturbance for telecommunication equipment located in the vicinity of the dc link. To eliminate these disturbances to the greatest possible extent, passive shunt filters are often used in the dc link, tuned to frequencies which are integer multiples of a product of the pulse number of the converter and the system frequency in the ac network, as well as designed as high-pass filters. These filters are generally not capable of completely extinguishing harmonics in the dc link. The reasons therefor are, for example, drift in the system frequency and in component values in the filters, but also the fact that the filter impedance at resonance frequency is not always negligible compared with the impedance in the other parts of the system. Further, during commutations and because of phase asymmetries in the ac network, harmonics of other frequencies than those mentioned above are normally also generated.
The passive filters are therefore nowadays often supplemented with active filters, which supply to the dc link an influencing quantity in the form of a current or a voltage for the purpose of reducing the amplitude of the harmonics which are generated by the phenomena described above. By measuring the remaining harmonic content in the direct current, the supplied current or voltage may thus be given such a form that, in principle, it fully eliminates the harmonic content. Such a filter comprises a power amplifier which is controlled by control equipment which is supplied with a sensed value of the harmonics, usually an actual current value formed in dependence on the harmonic current in the dc link. The control equipment forms, in dependence on a difference of a reference value for the harmonics and their sensed value, a control signal which is supplied to the power amplifier. In dependence on the control signal, the power amplifier generates the influencing quantity and supplies this to the dc link as one of a series quantity or a shunt quantity. Such filter circuits are exemplified in L. Gyugyi and E. C. Strycula: Active AC Power Filters, IAS 76 Annual, Paper 19-C. The document describes filter circuits for ac networks, but circuits which, in principle, are similar may also be applied to a dc link. In practice, the influencing quantity is usually supplied as a shunt quantity in that the power amplifier is connected in parallel with the disturbance source, in this case the converter.
The control signal is formed by a controller, which usually has a proportional or a proportional/integrating characteristic, which controller is supplied with the above-mentioned difference of the reference value for the harmonics and their sensed value.
An embodiment of such a controller is described in the published international patent application WO 93/09585. The controller forms the control signal in the form of a pulse tra
REFERENCES:
patent: 4835411 (1989-05-01), Takeda
patent: 5296764 (1994-03-01), Asplund et al.
patent: 5666275 (1997-09-01), Inokuchi et al.
patent: 5701241 (1997-12-01), Dofnas et al.
Gunnarsson Stefan
Nordstrom Boris
ABB AB
Nguyen Matthew
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