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

Application and Verification 161
TABLE 70, PERFORMANCE OF THE INDIVIDUAL NP CONTROL SCHEMES AT ACTIVE POWER OPERATION
AND LOW MODULATION DEPTH (m = 0.1)
Unconstrained DC
CM injection
- m = 0.1
CH1 (yellow):
CH2 (cyan):
U out1
U out2
CH3 (magenta): U NP
CH4 (green):
MATH (red):
Time/div:
I out2
U out1-2
100ms
Modified and
virtual vectors
- m = 0.1
CH1 (yellow):
CH2 (cyan):
U out1
U out2
CH3 (magenta): U NP
CH4 (green):
MATH (red):
Time/div:
I out2
U out1-2
100ms
The operating point of very high modulation depth is even more critical than the operation at
low modulation depth. At high modulation depth, typically load currents are higher and distortion
my produce significant NP bias currents that need to be compensated. This is especially true for
line connected applications, where exceptional line conditions may be very demanding. TABLE 71
shows experimental results for very high modulation depth and different NP control schemes.
Unconstrained DC CM injection is capable of controlling the NP voltage up to maximum
sinusoidal line to line voltage. However, even without line distortion or any other source of NP bias
current, there is a low frequency ripple apparent on the NP voltage, indicating that the controller is
operating at its limits and cannot compensate the full NP current. The transient has a duration of
~80ms corresponding with an average dv/dt of 250V/s. The corresponding NP current is 1A or
8.5% of 11.5A peak phase current. With the modified and virtual vector scheme, the NP voltage is
significantly more stable and the transient has a duration of 50ms (400V/s). This corresponds with
1.6A NP current or 14% of the peak phase current. The difference is significant, but it may not
justify the use of the more complicated scheme, introducing distortion in the line to line voltage
(nicely visible in TABLE 71 b, red waveform during transient). However, the unconstrained DC CM
injection does not work anymore for maximum over modulation (NP voltage diverging in
experimental verification), whereas the modified and virtual vector scheme keeps its properties also
in this operating mode. TABLE 71 I shows maximum trapezoidal operation still yielding a NP
voltage dv/dt of 250V/s or 1A NP current (8% of peak phase current). A hysteresis controller as
introduced in the previous paragraph can make best use of that control power and the good
performance regarding switching losses and output harmonics of the other modulators.