Comparative evaluation of converter-based compensation schemes for VSC systems to achieve full-range active power transfer in very weak grids

Voltage source converter (VSC) is the expected core technology that supports power system de-carbonization by allowing renewable energy development. However, with the increasing penetration of renewables and continuing decommissioning of thermal generators, challenges of integrating VSC-based systems into weak and even very weak ac grids become apparent. Therefore, this paper presents theoretical analysis describing the relationship between active power transfer range and weak grid factors of the generic VSC-grid system, and aims to identify the most effective way to allow the VSC to exchange the rated active power in both directions (+/- 1 pu) with the weak grid. Thus, converter-based compensation is presented to extend the operation boundary and avoid voltage collapsing. Nevertheless, the effects of reactive current provision and series voltage compensations should be recognized; therefore, operational characteristics of two arrangements, namely, shunt and series VSC-based compensation schemes, are comparatively evaluated. In extremely weak grid cases, shunt compensation converter cannot ensure a full active power transfer range of the targeted VSC due to the inherent voltage limitation, whilst series compensation converter can assist the targeted VSC to achieve full-range active power transfer. Effectiveness and performance of the presented compensation methods during power reversal and ac fault are demonstrated with a typical extremely weak grid, and system boundaries with different schemes are given.