Comparative Analysis of Conventional VSC and MMC on the Basis of Output Power Quality and Control Performance

Abstract

The conventional two-level voltage source converter (VSC) was first commercialized by ABB in 1997 in an HVDC project. Being cost effective, compact, having simpler design structure and stiff control, it attracted the vendors to be utilized in high voltage high power applications. But it offered certain limitations one of which includes; the requirement of bulky and expensive filters to mitigate the low frequency harmonics present in the output voltage waveform. Although the use of high switching frequency pushes the low order frequency spectrum to higher order frequencies which helps with reduced filter costs and size but corresponds another cumbersome issue of increased converter switching losses up to 1.7%. Thus there is always a conflicting design compromise between converter losses and need for filtration. Another troublesome issue with conventional VSC is that it requires the chains of series connected IGBTs to make converter arm valves. Being low to medium voltage rated devices typically ranging from (1.7k-6.5kV), these devices does not hold the capability to withstand the high voltage operating ratings. Therefore these switching devices require sophisticated gate drives to provide dynamic and static voltage balancing to ensure the simultaneous switching of all the IGBTs in the same converter arm. This cumbersome issue corresponds a water bed situation which limits the overall working efficiency of 2-level VSC. This work is mainly aimed to propose a compact design solution for all the foregoing problems, by using 21-level Modular multilevel converter (MMC) that being multilevel does away with massive and expensive filter requirements and being modular eliminates the need for fussy series connected IGBTs when utilized in high voltage high power applications like HVDC. The converter switches commutate at lower switching frequency thus converter losses are greatly reduced. The design structure involves the use of 100kV converter station tied to a 220kV and 50Hz Grid station connected with two 50MW parallel loads through two ix successive transmission lines of 10km. The modeling and design of grid tied conventional VSC and MMC is presented. Converter control is implemented in direct quadrature(dq0) frame using vector control technique, where active and reactive power are efficiently and independently controlled at point of common coupling (PCC). The Proportional (P) and Integral (I) gains of the PI compensator are optimized by using Modulus optimum tuning criteria. Furthermore, both conventional VSC and MMC are analyzed and compared on the basis of mathematical model of their AC side dynamics, control complexity and stability conditions, output voltage quality, switching losses, total harmonic distortion (THD), AC harmonic spectrum analysis, filter requirement, costs and design structure.