Electric power conversion systems – Current conversion – Including automatic or integral protection means
Patent
2000-06-07
2000-12-19
Riley, Shawn
Electric power conversion systems
Current conversion
Including automatic or integral protection means
H02H 7125
Patent
active
061634719
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method for damping an oscillation in a current in the direct current link in a plant for transmission of high voltage direct current, which oscillation has a frequency equal to the fundamental frequency of an alternating current network to which the plant is coupled, and to a device for carrying out the method.
BACKGROUND ART
HVDC plants comprise converters which on their alternating current sides are coupled to alternating current networks via converter transformers, and on their direct current sides are coupled to each other via a direct current link. The alternating current networks each have a fundamental frequency, the nominal value of which is usually equal to 50 or 60 Hz.
During certain circumstances, oscillations of a frequency equal to the fundamental frequency can be initiated in the direct current transferred in the direct current link. In particular, the phenomena might occur when the plant exhibits on one hand a resonance on the alternating current side of the converter at a frequency equal to twice the fundamental frequency, and on the other hand a resonance on the direct current side at a frequency equal to the fundamental frequency, such as a combination of low impedance at the fundamental frequency as seen from the converter to the direct current link and a high impedance at twice the fundamental frequency as seen from the converter to the alternating current network. This phenomena might give rise to direct currents through the secondary windings of the converter transformer with an amplitude sufficient to bring the magnetic circuit of the transformer into saturation. The oscillations might build up comparatively slowly, during periods of the order of minutes and even hours.
The normal current control of the converter is not always able to sufficiently suppress this type of oscillation, which has as consequence that it will be sustained until the protective system of the plant is activated and trips the plant.
In the European Patent EP 0 671 067 a method and an arrangement for damping oscillations at or near a natural resonance frequency in a power transmission system are described. An oscillator is adapted to generate reference signals of the form sin .omega..sub.r t and cos .omega..sub.r t where .omega..sub.r is a pre-set frequency, selected with knowledge of a natural resonance frequency of the power transmission system, a frequency which can be equal to the fundamental frequency or to another frequency, for example a sub-synchronous frequency.
A measured value of a quantity sensed in the power transmission system, exemplified as the direct current in an HVDC transmission svstem, is multiplied with the reference signals. The products obtained are of the form X sin .PHI. and X cos .PHI., respectively, where X is the amplitude of the measured value, and .PHI. the phase position thereof in relation to a reference phase position. Each product is treated in a separate calculating circuit to the effect to add to the phase position .PHI. an adjustable phase angle displacement .alpha., is multiplied with the reference signals cos .omega..sub.r t and sin .omega..sub.r t , respectively, and thereafter added. The sum is multiplied with an amplification constant K. The result is a control signal of the form K cos (.omega..sub.r t+a+.PHI.), which control signal is supplied to the power transmission system, in the case this is an HVDC plant, for example in such a way that the ordered control angle is generated in direct dependence on the control signal. The control system operates as a sampled system and the phase angle displacement .alpha. is chosen for best damping with respect to delays in the sampling.
The products X sin .PHI. and X cos .PHI. are supplied to a diagnostic and determination circuit, wherein each of them is compared to a predetermined limiting value. If anyone of the limit values is exceeded, the control signal is supplied to the power transmission system in dependence on a decision circuit comprising delay circuits with pre-se
REFERENCES:
patent: 4042871 (1977-08-01), Grubbs et al.
patent: 4797833 (1989-01-01), El-Amawy et al.
ABB AB
Riley Shawn
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