Christoph Haederli - Les thèses en ligne de l'INP - Institut National ...

NP Control with Carrier based PWM 101
modulation depth and load angle cos(ϕ) to determine the transition points to switch over from one
scheme to another, which is illustrated in Figure 67. DC CM injection (with limitation to maximum
NP current CM values as indicated in TABLE 33) is very effective over a very large range of
operation, including reactive power at low modulation depth. However, it has very poor
performance for pure reactive power at high modulation depth, which is pointed out by the zoom
in Figure 67. In this region, clearly 6 th harmonic injection performs better.
(a)
(b)
Figure 67, Maximum obtainable NP currents, a) DC CM injection, b) 6 th harmonic injection
To determine the transition point, the two graphs have been combined into one (Figure 68 b).
The area improved by the 6 th harmonic injection is rather small. However, this is the area where
STATCOM’s are operating: high modulation index and very low cos(ϕ). The performance
improvement is very significant in this case. This even holds true for pure reactive power and
highest physically possible modulation index.
(a)
(b)
Figure 68, maximum obtainable NP currents, a) optimal CM injection based on true real time
NP current function, b) constrained DC and 6 th harmonic injection