Fusion of Neural Networks, Fuzzy Systems and Genetic Algorithms: Industrial Applications Fusion of Neural Networks, Fuzzy Systems and Genetic Algorithms: Industrial Applications
by Lakhmi C. Jain; N.M. Martin
CRC Press, CRC Press LLC
ISBN: 0849398045   Pub Date: 11/01/98
  

Previous Table of Contents Next


The simulations results obtained for the XDFC adjustable speed drive considered are shown in Figure 12. The converter is operating at 70 Hz output frequency. The output line voltages and line currents show this. The converter’s low commutation frequency can be appreciated by looking at the output line voltage. This waveform has a reduced number of pulses, implying low commutation frequency on the converter. The load’s line currents have a distortion lower than the 6% limit imposed, which is why they clearly resemble sinusoidal waveforms. The input current control action is shown in the figure. This control produces the series of chops in each current pulse border. Regarding the input displacement factor, the phase voltage and line current are nearly in phase.

7. Evaluation

This section presents an evaluation of the proposed XDFC working in an ASD. In order to compare the proposed control algorithm, the Synchronous DFC (SDFC) presented in [13] is used as reference. That converter also achieved a unity ac-ac voltage gain, and kept the converter’s commutation frequency below 550 Hz. Thus, it also belongs to the high voltage gain and low commutation frequency/losses trend previously described in Section 1.

The SDFC uses the fictitious link concept to model and control the converter [6]. Under this approach, the converter’s transfer function H is split into a rectifying transfer function HR, and an inverting transfer function HI. Therefore, its operation can be described as a fictitious rectifier producing a fictitious dc link, which is then inverted at the desired output frequency and amplitude by a fictitious inverter. Matrix H is redefined as the dyadic product of column vectors HI and HR as follows:

where,

H = DFC transfer function, 3×3 matrix;
HR = Fictitious rectifier transfer function, 3×1 column vector;
HI = Fictitious inverter transfer function, 3×1 column vector.

With this decomposition of matrix H, the converter’s input-output relations (9) can be rewritten in the following way:

where,

Vdc = dc link voltage produced by the fictitious rectifier;
Idc = dc link current drawn by the fictitious inverter.

Transfer functions HR and HI elements can take values from {-1,0,1}, being forced to add to zero in order to comply with Kirchhoff’s voltage law. These transfer functions can then take seven possible combinations or Electric States, which are illustrated in Table VI.

Table VI Transfer Function States for 3-Phase Converters

H S1 S2 S3 S4 S5 S6 S7

H1   1   0 -1 -1   0   1 0
H2   0   1   1   0 -1 -1 0
H3 -1 -1   0   1   1   0 0

The SDFC models the fictitious rectifier as a diode bridge, and the fictitious inverter as a space vector modulated VSI. Consequently, its waveforms resemble the conventional drives. The SDFC is controlled by a predictive current loop which selects the next inverter state (thus, DFC state) by predicting the current trend produced by all the converter states, and chooses the one that satisfies the control objectives of this technique. These are to keep the load’s line currents space vector within the reference currents space vector Vr. Figure 13 shows the input and output waveforms of the SDFC operating at 70 Hz in an ASD with the parameters shown in Table V.


Figure 13  Input phase voltage Vr, input line current Ir, output line voltage Vab, and output line current Ia of the SDFC operating at 70 Hz.

The evaluation considers the ac-ac voltage gain (Gv), the input power factor (pf), the input current total harmonic distortion (THD), and the commutation frequency of both converters operating in an ASD. These results are shown in Figure 14. In order to assess the impact of the operation of multiple converters connected to a common feeder, the total input current waveforms and corresponding THD as a function of the number of converters connected was also determined. These results are important as converters are usually imbedded in environments with multiple nonlinear (converters) and linear loads. So the interaction between them should be considered.

Regarding the converters’ ac-ac voltage gain Gv, both DFCs achieve a unity voltage gain. The converters’ input power factor shows that the presented XDFC converter is superior to the SDFC. The input current distortion THD is also reduced in the XDFC when compared to the SDFC. The presented DFC offers better results; it has considerably reduced the input current distortion compared to the SDFC. This reduction is a consequence of the input current control introduced as second control objective of the XDFC. Finally, the converters’ commutation frequency is also shown.


Figure 14  Evaluation performed between the XDFC and SDFC. Figures show voltage gain Gv, input power factor pf, input current THD and commutation frequency.


Previous Table of Contents Next

Copyright © CRC Press LLC