A Nonlinear Controller for Neutral Point Piloted (T-Type) Multilevel Inverter-Based Three-Phase Four-Wire DSTATCOM

Table 5

Comparison between NPP-based DSTATCOM with two-level capacitor midpoint and four-leg topologies.

Active filter topology	NPP (three-level)	Capacitor midpoint (two-level)	Four-leg APF (two-level)
Number of switching devices	12	6	8
Number of capacitors	2	2	1
DC voltage sensor requirement	Two	Two	One
DC-side voltage ( = line-to-line voltage)
Need of coupling transformer	Not necessary	Necessary	Necessary
Control over neutral current	Indirect	Indirect	Direct (using 4th leg)
Main advantage	Reduced dc voltage requirement. high voltage capability, improved performance	Reduced number of switching devices	Better controllability than capacitor midpoint
Main disadvantage	Capacitor unbalance problem due to voltage difference across two capacitors	Capacitor unbalance problem due to voltage difference across two capacitors	Additional control circuit for 4^th leg
Application and topology selection	Suitable for high voltage, medium to high-power Applications	Suitable for low-power applications	Suitable for low-power applications