POLITECHNIKA WARSZAWSKA

4. ANN based Current Controllers (CC)
If the sampling signal in the three-phase system is shifted 120° in each S&H block
(Fig.4.5b), only one of the inverter legs will change its state during the sample period
1/f s . This guarantees only adjacent and zero voltage vector selection, and
consequently a better quality of current waveform (lower RMS, J) at this same
sampling frequency f s (Fig.4.5c) [71]. It is noted that the DM-CC can also be applied
in the space vector based controllers working in either stationary or rotating
coordinates [75,79,81]. The main advantages of DM-CC are extremely simple and
tuning-free hardware implementation, and good dynamics.
2) Optimal discrete modulation algorithm
For the RDCL converters an optimal algorithm selects the voltage vector which
minimizes the RMS current error for each resonant pulse [80,93,106].
U s
Synch.
RDCL
i sc
+
- -
i s
T
e
Calculation
of voltage
referen
vector
Voltage
vector
Load
model
Three-phase
Load
Fig. 4.6. Optimal (mode) discrete modulation controller for RDCL
As shown in [106] this is equivalent to selecting the nearest available voltage vector
commands u sc (T). So, instead of the PWM algorithm (Fig.4.4a) only the voltage
vector selector is required (Fig.4.6). However, errors and subharmonics typical of the
discrete modulation are obtained.
4.2.3 Linear Current Controllers
The linear controllers operate with conventional voltage type PWM modulators [21-
36]. In contrast to the nonlinear controllers (see Section 4.2.2), linear controller
schemes have clearly separated current error compensation and voltage modulation
parts. This concept allows us to exploit the advantages of open loop modulators
(sinusoidal PWM, space vector modulator, optimal PWM) which are: constant
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