Grid Voltages Estimation for Three-Phase PWM Rectifiers Control Without AC Voltage Sensors

 ABSTRACT

This paper proposes a new AC voltage sensorless control scheme for three-phase pulse-width modulation rectifier. A new startup process to ensure a smooth starting of the system is also proposed. The sensorless control scheme uses an adaptive neural (AN) estimator inserted in voltage-oriented control to eliminate the grid voltage sensors. The developed AN estimator combines an adaptive neural network in series with an adaptive neural filter. The AN estimator structure leads to simple, accurate and fast grid voltages estimation, and makes it ideal for low cost digital signal processor implementation. Lyapunov based stability and parameters tuning of the AN estimator are performed. Simulation and experimental tests are carried out to verify the feasibility and effectiveness of the AN estimator. Obtained results show that; the proposed AN estimator presented faster convergence and better accuracy than the second order generalized integrator based estimator; the new startup procedure avoided the over-current and reduced the settling time; the AN estimator presented high performances even under distorted and unbalanced grid voltages.

KEYWORDS

1.      AC voltage sensorless control

2.      Adaptive neural (AN) estimator

3.      Grid voltages estimation

4.      Neural networks (NNs)

5.      Pulse-width modulation (PWM) rectifier

6.       Startup process

7.      Voltage-oriented control (VOC)

SOFTWARE:  MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Overall structure of the developed AC voltage sensorless control.

EXPECTED SIMULATION RESULTS

Fig. 2. Steady-state performances of the AN estimator in diode rectifier operation mode (experiment): (a) computed input voltages vαn and vβn, (b) actual AC-line currents iα and iβ, (c) actual grid voltage eα, estimated grid voltage eα,est and estimation error and (d) actual grid voltage eβ, estimated grid voltage eβ,est and estimation error.

Fig. 3. Steady-state performances of the PLL in diode rectifier operation mode (experiment): (a) computed dq components (ed, eq) with actual grid  voltages and computed dq components (ed,est, eq,est) with estimated grid  voltages and (b) computed angles θ and θest with actual and estimated grid voltages, respectively.

Fig. 4. Performances of the AN estimator at startup (experiment): (a) input voltages vαn and vβn, (b) actual AC-line currents iα and iβ, (c) reference and  measured DC-link voltages (Vdc ref, Vdc), (d) actual grid voltage eα, estimated  grid voltage eα,est and estimation error and (d) actual grid voltage eβ, estimated grid voltage eβ,est and estimation error.

Fig. 5. Performances of the PLL at startup (experiment): (a) computed dq components (ed, eq) with actual grid voltages and computed dq components (ed,est, eq,est) with estimated grid voltages and (b) computed angles θ and θest with actual and estimated grid voltages, respectively.

Fig. 6. Transient performances of the AN estimator under Vdc ref step change (experiment): (a) actual grid voltage eα, estimated grid voltage eα,est and estimation error, (b) actual grid voltage eβ, estimated grid voltage eβ,est and estimation error, (c) actual AC-line currents iα and iβ and (d) reference and measured DC-link voltages.

Fig. 7. Transient performances of the PLL under Vdc ref step change (experiment): (a) computed dq components (ed, eq) with actual grid voltages and computed dq components (ed,est, eq,est) with estimated grid voltages and (b) computed angles θ and θest with actual and estimated grid voltages, respectively.

Fig. 8. Transient performances of the AN estimator under load resistance variation (experiment): (a) actual grid voltage eα, estimated grid voltage eα,est and estimation error, (b) actual grid voltage eβ, estimated grid voltage eβ,est and estimation error and (c) actual AC-line currents.

Fig. 9. Transient performances of the PLL under load resistance variation (experiment): (a) computed dq components (ed, eq) with actual grid voltages and computed dq components (ed,est, eq,est) with estimated grid voltages and (b) computed angles θ and θest with actual and estimated grid voltages, respectively.

Fig. 10. Transient performances of the AN estimator under symmetric grid voltages sag (experiment): (a) actual grid voltage eα, estimated grid voltage eα,est and estimation error and (b) actual grid voltage eβ, estimated grid voltage eβ,est and estimation error and (c) actual AC-line currents.

Fig. 11. Transient performances of the PLL under symmetric grid voltages sag (experiment): (a) computed dq components (ed, eq) with actual grid voltages and computed dq components (ed,est, eq,est) with estimated grid voltages and (b) computed angles θ and θest with actual and estimated grid voltages, respectively.

Fig. 12. Transient performances of the AN estimator under grid voltages unbalance (experiment): (a) actual grid voltage eα, estimated grid voltage eα,est and estimation error and (b) actual grid voltage eβ, estimated grid voltage eβ,est and estimation error and (c) actual AC-line currents.

Fig. 13. Transient performances of the PLL under grid voltages unbalance (experiment): computed dq components (ed, eq) with actual grid voltages and computed dq components (ed,est, eq,est) with estimated grid voltages and (b) computed angles θ and θest with actual and estimated grid voltages, respectively.

Fig. 14. Transient performances of the AN estimator under distorted grid voltages (simulation): (a) actual grid voltage eα and estimated grid voltage eα,est, (b) actual grid voltage eβ and estimated grid voltage eβ,est and (c) actual AC-line currents.

Fig. 15. Transient performances of the PLL under distorted grid voltages (simulation): computed dq components (ed, eq) with actual grid voltages and computed dq components (ed,est, eq,est) with estimated grid voltages and (b) computed angles θ and θest with actual and estimated grid voltages respectively.

CONCLUSION

In this work, a new AN estimator for eliminating the grid voltage sensors in VOC of three-phase PWM rectifier has been proposed. The developed AN estimator combines estimation capability of the ANN and filtering property of the ANF. Lyapunov’s theory based stability analysis has been exploited for optimal tuning of the AN estimator. Hence, simple, accurate and fast grid voltages estimation has been obtained. To avoid current overshoot and to reduce the settling time at the startup, a new startup process has been proposed to initialize the VOC. The effectiveness of the proposed procedure has been experimentally demonstrated. A comparison between the proposed AN estimator and the recently developed SOGI based estimator has been conducted. This comparison has clearly indicated faster convergence and better accuracy of the proposed estimator. Finally, robustness of the AN estimator regarding to step change in DC-link voltage reference, load resistance variation and non-ideal grid voltages conditions (symmetrical sag, unbalance, distortion) has been investigated through simulation and experimental tests. The obtained results have demonstrated high performances of the proposed AN estimator within the analyzed working conditions.

REFERENCES

[1] R. Teodorescu, M. Liserre, and P. Rodriguez, Grid converters for photovoltaic and wind power systems, John Wiley & Sons, 2011.

[2] A.-R. Haitham, M. Malinowski, and K. Al-Haddad, Power electronics for renewable energy systems, transportation and industrial applications, John Wiley & Sons, 2014.

[3] T. Friedli, M. Hartmann, and J. W. Kolar, “The essence of three-phase PFC rectifier systems–Parte II,” IEEE Trans. Power Electron., vol. 29, no. 2, pp. 543–560, Feb. 2014.

[4] M. B. Ketzer and C. B. Jacobina, “Sensorless control technique for PWM rectifiers with voltage disturbance rejection and adaptive power factor,” IEEE Trans. Ind. Electron., vol. 62, no. 2, pp. 1140–1151, Feb. 2015.

[5] A. Bechouche, H. Seddiki, D. Ould Abdeslam, and K. Mesbah, “Adaptive AC filter parameters identification for voltage-oriented control of three-phase voltage-source rectifiers”, Int. J. Modell. Identification Control, vol. 24, no. 4, pp. 319–331, 2015.

Direct Power Control of Pulse Width Modulated Rectifiers without DC Voltage Oscillations under Unbalanced Grid Conditions

ABSTRACT:

Direct power control with space vector modulation (DPC-SVM) features simple structure, fast dynamic performance and little tuning work. However, conventional DPC-SVM can not achieve accurate power control under unbalanced grid conditions. A modified DPC-SVM is thus proposed for accurate power control under both ideal and unbalanced grid conditions. Though power control accuracy is improved when compared with conventional DPCSVM, it still suffers highly distorted grid current and DC voltage oscillations with an unbalanced network. Therefore, a power compensation method is subsequently derived aiming at the following targets: eliminating DC voltage oscillations, achieving sinusoidal grid current and obtaining unity power factor. To that end, average grid-side reactive power and oscillations in converter-side active power are controlled as zero by simply adding a compensation to original power reference. Additionally, the proposed method does not require extraction of positive sequence or negative sequence component of grid voltage. Compared with conventional DPC-SVM in ideal grid, only additional compensation of power reference is required. As a result, control performance can be significantly improved without substantial increase of complexity. The superiority of the proposed method over the prior DPC-SVM is validated by both simulation and experimental results obtained on a two-level PWM voltage source rectifier.

KEYWORDS:

1.      Predictive power control

2.      Power compensation

3.      Unbalanced grid

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Control diagram of the proposed DPC-SVM.

EXPECTED SIMULATION RESULTS:

Fig. 2. Simulation results of Uabc, Pin, Pref , Qin, Qref and Iabc for (a) the MDPC-SVM and (b) CDPC-SVM.

Fig. 3. Simulation results of Uabc, Pin, Pref , Pout, Udc and Iabc for (a) MDPC-SVM-PC and (b) MDPC-SVM

Fig. 4. Simulation results of MDPC-SVM-PC when 50% voltage dip in phase A is suddenly applied.

Fig. 5. Simulation results of MDPC-SVM-PC when both R and L in the controller are (a) 50% and (b) 150% of their actual value.

Fig. 6. Simulated results of MDPC-SVM-PC under one phase grounding fault.

CONCLUSION:

In existing literature, most studies on DPC-SVM were carried out under balanced grid voltage conditions. Under unbalanced grid voltage conditions, the steady-state performance of DPC-SVM are seriously deteriorated by exhibiting highly distorted current and oscillations in the DC-link voltage. To cope with these problems, this paper proposes a novel DPC-SVM method, which is able to work effectively under both balanced and unbalanced grid conditions. An appropriate power compensation is derived, which only requires the grid/converter voltages and their delayed values. By adding this power compensation to the original power references without modifying the internal control structure, constant DC-link voltage and sinusoidal grid currents are achieved simultaneously without affecting the average value of gridside active power and reactive power. The proposed DPC-SVM is compared to conventional DPC-SVM and its effectiveness is confirmed by the presented simulation and experimental results.

Due to additional calculation of power compensation, complexity of the proposed DPC-SVM is higher than conventional power control schemes. However, twice grid voltage frequency oscillations can be completely eliminated in theory by the proposed method under unbalanced grid conditions, which is beneficial to the lifetime and maintenance of capacitors. Although using a larger capacitor can also reduce DC voltage ripples, it may increase hardware cost and volume of the system. In this sense, the proposed method is more suitable for the application where a high quality DC voltage is required under unbalanced grid conditions.

REFERENCES:

[1] Z. Zhang, H. Fang, F. Gao, J. Rodríguez, and R. Kennel, “Multiplevector model predictive power control for grid-tied wind turbine system with enhanced steady-state control performance,” IEEE Trans. Ind. Electron., vol. 64, DOI 10.1109/TIE.2017.2682000, no. 8, pp. 6287– 6298, Aug. 2017.

[2] A. Koran, T. LaBella, and J. S. Lai, “High efficiency photovoltaic source simulator with fast response time for solar power conditioning systems evaluation,” IEEE Trans. Power Electron., vol. 29, DOI 10.1109/TPEL.2013.2262297, no. 3, pp. 1285–1297, Mar. 2014.

[3] A. Camacho, M. Castilla, J. Miret, A. Borrell, and L. G. de Vicuña, “Active and reactive power strategies with peak current limitation for distributed generation inverters during unbalanced grid faults,” IEEE Trans. Ind. Electron., vol. 62, DOI 10.1109/TIE.2014.2347266, no. 3, pp. 1515–1525, Mar. 2015.

[4] W. Jiang, Y. Wang, J. Wang, L. Wang, and H. Huang, “Maximizing instantaneous active power capability for pwm rectifier under unbalanced grid voltage dips considering the limitation of phase current,” IEEE Trans. Ind. Electron., vol. 63, DOI 10.1109/TIE.2016.2577544, no. 10, pp. 5998–6009, Oct. 2016.

[5] H. Yang, Y. Zhang, J. Liang, J. Gao, P. Walker, and N. Zhang, “Sliding mode observer based voltage-sensorless model predictive power control of pwm rectifier under unbalanced grid condition,” IEEE Trans. Ind. Electron., vol. PP, DOI 10.1109/TIE.2017.2774730, no. 99, pp. 1–1, 2017.

A New PWM and Commutation Scheme for One Phase Loss Operation of Three- Phase Isolated Buck Matrix-Type Rectifier

ABSTRACT:

In this paper, a new PWM scheme and commutation method is presented for one phase loss operation of three-phase isolated buck matrix-type rectifier. With the proposed PWM scheme, the maximum allowable voltage gain for one phase loss operation can be achieved, which permits the continuous operation of the converter to deliver 2/3 of rated power and regulate the output voltage with maximum output voltage drop less than 5% of nominal output voltage. In addition, with the proposed commutation method, a safe transition from one phase loss operation to normal operation and vice versa can occur with minimum commutation steps (two-step) under zero voltage switching (ZVS) condition. The performance of the proposed PWM scheme and commutation schemes with one phase loss operation is evaluated and verified by simulations and experiments on a 5kW prototype.

KEYWORDS:

1.      PWM

2.      Commutation

3.      Matrix converter

4.      Three phase

5.      One phase loss

6.      Isolated

7.      Buck rectifier

8.      ZVS

9.      MOSFET

10.  High frequency

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1. ZVS three-phase PWM rectifier.

EXPECTED SIMULATION RESULTS:

Fig. 2. Simulated waveforms for 2/3PO_max, vLL = 480V and ma = 0.75 when “phase C” is shorted at t1 and recovered at t2: (a) input phase voltages, (b) input phase currents, (c) transformer secondary voltage, (d) output of bridge rectifier, (e) output voltage and battery set point, (f) output inductor current.

CONCLUSION:

In this paper, operation of the three-phase isolated Buck matrix-type rectifier under one phase loss condition is described and a new PWM scheme and commutation method for the one phase loss operation is proposed. With the proposed switching scheme and commutation method, two step commutation with ZVS (here either using ZVS or zero voltage turn-ON) can be realized for one phase loss operation and also for the transition from normal operation to one phase loss operation and from one phase loss operation to normal operation. Operation and performance of the converter with the proposed PWM and commutation method are verified with simulation and experimental results. Based on the experimental results obtained from a 5 Kw prototype, it is shown that the converter is able to deliver 2/3 of maximum output power to the load and regulate the output voltage with maximum voltage drop less than 5% of nominal output voltage. Current stress of the converter and input current THD and spectrum analysis are also provided in the experimental results with one phase loss operation. The relatively large THD (around 40%) is one of the drawbacks for this converter when operating under one phase loss condition.

REFERENCES:

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[2] K. Inagaki, T. Furuhashi, A. Ishiguro, M. Ishida, and S. Okuma, “A New PWM Control Method for ac to dc Converters with High- Frequency Transformer Isolation”, IEEE Trans. Industry Applications, Vol. 29, No. 3, pp. 486-492, May/Jun., 1993.

[3] V. Vlatković and D. Borojević “Digital-Signal-Processor-Based Control of Three- Phase Space Vector Modulated Converters”, IEEE Trans. Industrial Electronics, vol. 41, no. 3, pp. 326-336, Jun., 1994.

[4] V. Vlatković and D. Borojević, and F. C. Lee, “A Zero-Voltage Switched, Three-phase Isolated PWM Buck Rectifier”, IEEE Trans. Power Electronics, vol. 10, No. 2, pp. 148-157, Mar., 1995.

[5] R. García-Gil, J. M. Espí, E. J. Dede, and E. Sanchis-Kilders, “A Bidirectional and Isolated Three-Phase Rectifier With Soft-Switching Operation,” IEEE Trans. Industrial Electronics, vol. 52, no. 3, pp. 765-773, Jun, 2005.