ABSTRACT:
A sliding-mode grid voltage observer (SMGVO) is proposed
and experimentally verified in this paper for voltage-sensorless operation
under an unbalanced network. Fundamental positive sequence component (FPSC) and
fundamental negative sequence component (FNSC) are inherently separated in the observer
without employing any additional filters. Due to embedded filtering effect,
high frequency chattering and harmonic ripples can be well suppressed.
Additionally, DC component can be completely rejected. As a result, DC offset
would not cause fundamental frequency oscillations in magnitude and frequency
of the estimated FPSC and FNSC. Owing to the predictive ability of SMGVO,
one-step delay can be directly compensated using state variables in the
observer. By combining estimation and prediction into one stage, the designed
SMGVO turns out to be a compact solution for finite control set-model
predictive power control (FCS-MPPC) without voltage sensors. Theoretical proof
is derived to verify that FPSC and FNSC can be accurately estimated and
separated. Experimental results obtained from a two-level PWM rectifier confirm
the effectiveness of the whole control system.
KEYWORDS:
1.
Predictive
power control
2.
Unbalanced
grid
3.
Voltage
observer
4.
Voltage
sensorless
SOFTWARE: MATLAB/SIMULINK
BLOCK DIAGRAM:
Fig.
1. Control diagram of SMGVO based FCS-MPPC.
Fig.
2. Startup responses with 50% voltage dip in phase A. (a) Actual grid voltages,
currents and estimated voltages; (b) comparison between estimated voltages from
SMGVO and calculated voltages from DSOGI.
Fig.
3. Operation from balanced condition to unbalanced condition. (a) Actual grid
voltages, currents and estimated voltages; (b) comparison with usogi p and
usogi n calculated from actual voltage by DSOGI.
Fig.
4. Dynamic responses when Pref steps from 600 W to 1000 W.
Fig.
5. Steady state responses with 1.5 V DC component and 50% AC voltage dip in
phase A.
Fig.
6. Average switching frequency fsw when Pref = 1 kW.
Fig.
7. Spectrum analysis of (a) grid voltage, (b) ^up and (b) ^un under unbalanced
and distorted grid conditions.
Fig.
8. Estimated grid frequency with sudden frequency step change of +10 Hz under
unbalanced and distorted grid conditions.
CONCLUSION:
A
SMGVO is designed and experimentally verified in this paper. It has the
following properties: 1) inherent separation of FPSC and FNSC without utilizing
any filters; 2) no high frequency chattering; 3) satisfactory DC component
rejection; 4) comparable performance with DSOGI based sequence separation using
measured voltage; 5) predictive ability to compensate one-step delay in
predictive control. FCS-MPPC is implemented based on SMGVO and tested on a
two-level PWM rectifier to verify the effectiveness of the control system.
Experimental results show that FPSC and FNSC can be accurately estimated and
separated. The dynamic performance of SMGVO during voltage sag is similar to that
of DSOGI. The implemented voltage sensorless FCSMPPC presents fast dynamic
responses which can track power reference quickly. Direct start without initial
knowledge of grid voltage is possible due to fast converging rate of SMGVO and high
regulation bandwidth of FCS-MPPC.
REFERENCES:
[1]
Z. Zhang, H. Xu, M. Xue, Z. Chen, T. Sun, R. Kennel, and C. M. Hackl,
“Predictive control with novel virtual-flux estimation for backto- back power
converters,” IEEE Trans. Ind. Electron., vol. 62, no. 5, pp. 2823–2834, May
2015.
[2]
A. M. Razali, M. A. Rahman, G. George, and N. A. Rahim, “Analysis and design of
new switching lookup table for virtual flux direct power control of
grid-connected three-phase PWM AC - DC converter,” IEEE Trans. Ind. Appl., vol.
51, no. 2, pp. 1189–1200, March 2015.
[3]
J. Kukkola and M. Hinkkanen, “State observer for grid-voltage sensorless control
of a converter equipped with an LCL filter: Direct discretetime design,” IEEE
Trans. Ind. Appl., vol. 52, no. 4, pp. 3133–3145, July 2016.
[4]
H.-S. Song, I.-W. Joo, and K. Nam, “Source voltage sensorless estimation scheme
for PWM rectifiers under unbalanced conditions,” IEEE Trans. Ind. Electron.,
vol. 50, no. 6, pp. 1238–1245, Dec 2003.
[5]
K. H. Ahmed, A. M. Massoud, S. J. Finney, and B. W. Williams, “Sensorless
current control of three-phase inverter-based distributed generation,” IEEE
Trans. Power Del., vol. 24, no. 2, pp. 919–929, April 2009.
