PQ CASE STUDY: HARMONICS REDUCTION USING A CONTINOUSLY REACTIVE POWER COMPENSATION IN HVDC LINKS
ABSTRACT:
According to rectifying or inverting operation of HVDC converters, reactive power is absorbed from the bus in which the converter is connected. In either case of operation reactive power compensation in AC side of converters is quite necessary. In addition to reactive power compensation, due to nonlinear behavior of power electronics converters, considerable characteristic and uncharacteristic harmonics are produced in both sides of links and often they are filtered by passive and active filters.
There has been a great growth in application of AC/DC links and therefore the harmonic reduction and reactive power compensation method should be improved but this increases the complexity in hardware and control strategies and also increases the total cost of links. In this paper the new switching patterns of capacitance for achieving continuously controlled compensation and reduction of harmonics produced by HVDC converters are described. This method has a simpler structure and easier switching control strategies compared to active filter configurations.
INTRODUCTION:
The rapid development of power generated by increased demand for electric energy initially in industrialized countries and subsequently in developing countries led to different technical problems in the systems such as stability limitation and voltage problems.
However breaking advances in semiconductor technology then enabled the manufacture of powerful thyristors and later other elements such as the gate turn off thyristors and insulted gate bipolar transistors. High voltage DC transmission (HVDC) technology which is being considered as an alternative to long distance AC transmission is based on this development [1].
Harmonics problem generated by nonlinear loads and thyristor converters becomes increasingly serious as
they are widely used in industrial applications and transmission and/or distribution systems. Since the
HVDC converters are large power converters, they have become important harmonic sources in power
systems and without proper compensation the quality of power in system is deteriorate.
So far, the shunt passive filters due to their low cost and high efficiency have hitherto been used to reduce harmonics in power systems. However shunt passive filters have many problems to discourage their applications. As shown in fig. 1, the filtering performance of passive filters is influenced by the ratio of equivalent impedance of AC source link side and passive power filter impedance.
Since the source impedance is not accurately known and varies with the system configuration, strongly
influences characteristic of shunt passive filter. Furthermore the passive filter may fall in series resonance or in parallel resonance with source impedance.
Shunt active filters using PWM inverters have been developed as the solution of preceding problems in passive shunt filters [3]. In the beginning, shunt active filters were proposed to suppress the harmonics generated by large rated thyristor converters used in HVDC transmission systems.
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