Back‐to‐Back Asynchronous Interconnection

Abstract

IntroductionAs the rating and acceptability of high power self-commutating switches improve, the boundaries between the HVDC and FACTS technologies are gradually becoming blurred. HVDC has already started using the new devices (thus improving its control flexibility) and FACTS controllers are increasing their power range (which may finally result in the control of the total power transfer of an asynchronous interconnection). Therefore, modern back-to-back (BTB) conversion could be considered as part of the HVDC and FACTS technologies.Bidirectional BTB asynchronous interconnections with ratings of up to 1500 MW are already being used for the interconnection of networks of different frequencies, like the recent Brazil-Argentina scheme [1], and for trading reserves [2] or shifting peak energy loading times between networks with different time zones, such as between Finland and Russia [3].The distinguishing feature of FACTS with respect to traditional HVDC has been its greater control flexibility. However, several fully flexible PWM-based HVDC voltage source conversion schemes in the 300 MW region are already operating successfully and the power rating capability of the PWM-VSC technology is on the increase.Although the large power BTB interconnections under consideration are still based on conventional thyristor conversion, the self-commutating multilevel technology is likely to be a viable alternative in the near future. However, under conventional control the self-commutating multilevel BTB interconnection lacks independence of reactive power controllability between the two ends of the link, an important shortcoming.The MLCR concept described in Chapter 4 and used in Chapter 8 for the series-connected double converter groups, is used here for the firing control of the parallel-connected BTB converter configuration to provide four-quadrant controllability. The parallel MLCR configuration has a simpler structure and is the preferred option for the large BTB asynchronous application, which will normally consist of groups of two paralleled bridges connected in series.