A Highly Reliable Single-Phase High-Frequency Isolated Double Step-Down AC-AC Converter with Both Non-Inverting and Inverting Operations

IEEE Transactions on Industry Applications, 2016

ABSTRACT: In this paper, the switching cell concept is extended to isolated ac-ac converters, and a highly reliable double step-down ac-ac converter is proposed with high-frequency transformer (HFT) isolation. By using the switching cell structure and coupled inductors, the proposed converter has no commutation problem as it is immune from both short-circuit and open-circuit problem, even when all the switches are turned-on or turned-off, simultaneously. Therefore, it does not require PWM dead-times along with bulky and lossy RC snubbers or voltage sensing circuitry to implement soft-commutation strategies; resulting in high reliability and high quality output waveforms. The HFT in the proposed converter provides electrical isolation and safety which is required in applications such as dynamic voltage restorer (DVR) and solid state transformer (SST), etc., without the need for external bulky line frequency transformer. Moreover, all the passive components experience twice the switching frequency, therefore, their size can be reduced. The proposed converter is very suitable for application as DVR, to compensate both voltage sags and swells, owing to its ability to provide both inverting and non-inverting outputs. A detailed theoretical analysis and operation of the proposed ac-ac converter are provided, and its applications as DVR and SST are also discussed. Experimental results with scaled-down prototype are also provided to verify its performance.

KEYWORDS:

1.      AC-AC converter

2.      Commutation problem

3.      Double-step down

4.      High frequency transformer (HFT)

5.      Non-inverting and inverting operation.

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1. Circuit topology of the proposed HFT isolated double step-down ac-ac converter.

 EXPECTED EXPERIMENTAL RESULTS:

Fig. 2. Input voltage, output voltage and current waveforms for: (a) Non-inverting mode operation. (b) Inverting mode operation.

Fig. 3. Capacitor voltage waveforms for: (a) Non-inverting mode operation. (b) Inverting mode operation.

Fig. 4. Voltage waveforms of: (a) Input voltage, and switches voltage stresses. (b) Zoom in waveforms of (a).

Fig.5. Experimental results of switch voltage and winding current stresses: (a) Winding current and switches voltage stresses. (b) Zoom in waveforms of (a).

Fig. 6. Experimental results of switch voltage and winding current stresses: (a) Transformer secondary winding current and bidirectional switch voltage stresses. (b) Zoom in waveforms of (a).

CONCLUSION:

This paper proposed a very robust high frequency transformer isolated double step-down ac-ac converter with both non-inverting and inverting operations. The proposed ac-ac converter uses the SC structure with CLs at primary side, which make it immune from both short-circuit and open-circuit problems, even when all switches are turned-on or turned-off, simultaneously. Therefore, the proposed converter is highly reliable as it has no commutation problem. Moreover, it does not need any PWM dead time along with RC snubers or soft-commutation strategies by sensing voltage polarity. The proposed converter provides the electrical isolation and safety with HFT, thus, it eliminates the need of external bulky line frequency transformer, in the applications such as DVR and SST, etc. The size of all the passive components in the proposed converter can be reduced, owing to fact that they experience twice the switching frequency. It has both non-inverting and inverting operations which are utilized to compensate both voltage sags and swells in its application as DVR. The operating principle and circuit analysis of the proposed converter is explained, and then, the structures of DVR and SST are developed based on the proposed converter. A scaled-down prototype is also fabricated in laboratory and experimental results are given to validate its advantages.

REFERENCES:

[1]         C. Liu, B. Wu, N. R. Zargari, D. Xu and J. Wang, “A novel three-phase three-leg ac-ac converter using nine IGBTs,” IEEE Trans. Power Electron., vol. 24, no. 5, pp. 1151–1160, May. 2009.

[2]         C. B. Jacobina, I. S. d. Freitas, E. R. C. d. Silva, A. M. N. Lima, and R. L. d. A. Riberio, “Reduced switch count dc-link ac-ac five-leg converter,” IEEE Trans. Power Electron., vol. 21, no. 5, pp. 1301–1310, Sep. 2006.

[3]         P. Alemi, Y.-C. Jeung, and D.-C. Lee, “Dc-link capacitance minimization in T-type three-level ac/dc/ac PWM converters,” IEEE Trans. Ind. Electron., vol. 62, no. 3, pp. 1382– 1391, Mar. 2015.

[4]         J. W. Kolar, T. Friedli, J. Rodriguez, P. W. Wheeler, “Review of three-phase PWM ac-ac converter topologies,” IEEE Trans. Ind. Electron., vol. 58, no. 11, pp. 4088– 5006, Nov. 2011.

[5]         A. Ecklebe, A. Lindemann, and S. Schulz, “Bidirectional switch commutation for a matrix converter supplying a series resonant load,” IEEE Trans. Power Electron., vol. 24, no. 5, pp. 1173– 1181, May. 2009.

Improved Fault Ride Through Capability in DFIG Based Wind Turbines Using Dynamic Voltage Restorer With Combined Feed-Forward and Feed-Back Control

IEEE ACCESS, volume 5, date of current version October 25, 2017.

ABSTRACT: This paper investigates the fault ride through (FRT) capability improvement of a doubly fed induction generator (DFIG)-based wind turbine using a dynamic voltage restorer (DVR). Series compensation of terminal voltage during fault conditions using DVR is carried out by injecting voltage at the point of common coupling to the grid voltage to maintain constant DFIG stator voltage. However, the control of the DVR is crucial in order to improve the FRT capability in the DFIG-based wind turbines. The combined feed-forward and feedback (CFFFB)-based voltage control of the DVR verifies good transient and steadystate responses. The improvement in performance of the DVR using CFFFB control compared with the conventional feed-forward control is observed in terms of voltage sag mitigation capability, active and reactive power support without tripping, dc-link voltage balancing, and fault current control. The advantage of utilizing this combined control is verified through MATLAB/Simulink-based simulation results using a 1.5-MW grid connected DFIG-based wind turbine. The results showgood transient and steady-state response and good reactive power support during both balanced and unbalanced fault conditions.

 KEYWORDS:

1.      Doubly-fed induction generator (DFIG)

2.      Dynamic voltage restorer (DVR)

3.      Fault Ride-Through (FRT)

4.      Low Voltage Ride Through (LVRT)

5.      Combined feed forward feedback control

6.      MPPT

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Schematic Diagram of DVR with DFIG.

   

EXPECTED SIMULATION RESULTS:

Fig 2. DVR using CFFFB control: (a) supply voltage with 35 % balanced sag in pu, (b) load voltage in pu, and (c) DVR injection voltage in Volts.

Fig. 3. (a) Active Power of DFIG with CFFFB control DVR with 35 % balanced sag in pu. (b) Reactive Power of DFIG with CFFFB control DVR with 35 % balanced sag in pu. (c) Rotor speed control of DFIG with CFFFB controlled DVR with 35 % balanced sag in pu. (d) DC-link voltage with CFFFB controlled DVR with 35 % balanced sag in pu. (e) Stator current (GSC current) of DFIG with CFFFB controlled DVR with 35 % balanced sag in pu. (f) Rotor current (RSC current) of DFIG with CFFFB controlled DVR with 35 % balanced sag in pu.

Fig. 4 DVR using CFFFB control: (a) supply voltage with 35 % unbalanced sag of Phase A (in red) in pu, (b) load voltage in pu, and (c) DVR injection voltage in Volts.

Fig. 5. (a) Active Power of DFIG with CFFFB control DVR with 35 % unbalanced sag in pu. (b) Reactive Power of DFIG with CFFFB control DVR with 35 % unbalanced sag in pu. (c) Rotor speed control of DFIG with CFFFB controlled DVR with 35 % unbalanced sag in pu. (d) DC-link voltage with CFFFB controlled DVR with 35 % unbalanced sag in pu. (e) Stator current (GSC current) of DFIG with CFFFB controlled DVR with 35 % unbalanced sag in pu. (f) Rotor current (RSC current) of DFIG with CFFFB controlled DVR with 35 % unbalanced sag in pu.

Fig. 6. DVR using CFFFB control: (a) supply voltage with short circuit three phase to ground fault in pu, (b) load voltage in pu, and (c) DVR injection voltage in Volts.

Fig. 7 (a) Active Power of DFIG with CFFFB control DVR with short circuit three phase to ground fault in pu. (b) Reactive Power of DFIG with CFFFB control DVR with short circuit three phase to ground fault in pu (c) Rotor speed control of DFIG with CFFFB controlled DVR with short circuit three phase to ground fault in pu. (d) DC-link voltage with CFFFB controlled DVR with short circuit three phase to ground fault in pu. (e) Stator current (GSC current) of DFIG with CFFFB controlled DVR with short circuit three phase to ground fault in pu. (f) Rotor current (RSC current) of DFIG with CFFFB controlled DVR with short circuit three phase to ground fault in pu.

Fig. 8 Harmonic spectrum of DVR Load voltage with Feed Forward control shows THDD5.24 %.

Fig. 9 Harmonic spectrum of DVR Load voltage with CFFFB control shows THDD4.47 %.

 CONCLUSION:

This paper investigates the performance of DVR with combined Feed-Forward and Feed-Back control for the FRT capability improvement in DFIG based wind turbines. Series compensation scheme using DVR proves to be very effective with good reactive power compensation scheme, voltage control and power flow control. The performance comparison suggests that the operation of DVR is a good suit for improving FRT capability in DFIG based variable speed wind turbines as per grid code standards. The investigated combined Feed Forward and Feed Back (CFFFB) control has many advantages like simplicity with limited controller complexity. The controller is used to investigate the improvement in performance of FRT capability operation in DFIG wind turbine while modifying the voltage control of a DVR. The DVR proves to deliver very good transient voltage control, fault current control and reactive power support. The controller contributes in better harmonic compensation compared to conventional control as per IEEE 519 standards. The simulation results performed using a 1.5 MW DFIG based wind turbine connected to electrical grid show better performance of DVR with the combined Feed-Forward and Feed-Back control for improving the FRT capability of DFIG based wind turbines.

REFERENCES:

[1]         R. A. J. Amalorpavaraj, K. Palanisamy, S. Umashankar, and A. D. Thirumoorthy, ``Power quality improvement of grid connected wind farms through voltage restoration using dynamic voltage restorer,'' Int. J. Renew. Energy Res., vol. 6, no. 1, pp. 53_60, Mar. 2016.

[2]         R. A. J. Amalorpavaraj, P. Kaliannan, and U. Subramaniam, ``Improved fault ride through capability of DFIG based wind turbines using synchronous reference frame control based dynamic voltage restorer,'' ISA Trans., vol. 70, no. 1, pp. 465_474, Jul. 2017.

[3]         J. Morren and S. W. H. D. Haan, ``Ridethrough of wind turbines with doubly-fed induction generator during a voltage dip,'' IEEE Trans. Energy Convers., vol. 20, no. 2, pp. 435_441, Jun. 2005.

[4]         L. Holdsworth, X. G. Wu, J. B. Ekanayake, and N. Jenkins, ``Comparison of _xed speed and doubly-fed induction wind turbines during power system disturbances,'' IEE Proc.-Gen. Transmiss. Distrib., vol. 150, no. 3, pp. 343_352, May 2003.

[5]         A. D. Hansen and G. Michalke, ``Fault ride-through capability of DFIG wind turbines,'' Renew. Energy, vol. 32, no. 9, pp. 1594_1610, Jul. 2007.

Transformerless Z-Source Four-Leg PV Inverter with Leakage Current Reduction

Transformerless Z-Source Four-Leg PV Inverter with Leakage Current Reduction

ABSTRACT:

Due to the lack of electrical isolation, the leakage current is one of the most important issues for transformerless PV systems. In this paper, a new modulation strategy is proposed to reduce the leakage current for Z-Source four-leg transformerless PV inverter. Firstly, the common mode loop model is presented. And then the common mode voltage behavior and the effect of factors on the leakage current are discussed. A new modulation strategy is proposed to achieve the step-up function and constant common mode voltage. Therefore, the leakage current can be suppressed effectively. Finally, the proposed strategy is digitally implemented and tested. The simulation results verify the effectiveness of the proposed solution.

KEYWORDS:

1.      Transformerless photovoltaic system

2.      Z source inverter

3.      Modulation

4.      Leakage current.

 SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1. Z-source four-leg inverter for transformerless PV systems

EXPECTED SIMULATION RESULTS:

                 (a)Common mode voltage VCM        

(b) Parasitic capacitance voltage VPV

                     

                                                                                                      (c) Leakage current ICM  

                                                                                                                      

   (d) Spectrum analysis of ICM

                                 

 (e) Grid current 

 (f) Spectrum analysis of grid current

                                                     Fig.2 Simulation results of conventional modulation strategy

(a) Common mode voltage VCM  

 

                      (b) Parasitic capacitance voltage VPV                            

 (c) Leakage current ICM      

                                                                                                                                                  (d) Spectrum analysis of ICM

           (e) Grid current    

                                                                                               (f) Spectrum analysis of grid current

Fig.3 Simulation results of proposed modulation strategy

                                                       

          (a) Conventional modulation strategy. 

   (b) Proposed modulation strategy

Fig. 4 Simulation results of d from 0.3 to 0.1

Fig.5 Simulation results of duty cycle and leakage current (RMS)

CONCLUSION:

This paper has presented the analysis and simulation verification of a new modulation strategy to reduce the leakage current of Z-source four-leg inverter for transformerless PV systems. Our finding indicates that the conventional method fails to eliminate the leakage current. Meanwhile, the leakage current will be higher as the shoot-through duty cycle increases. As for the proposed method, the effect of shoot-through duty cycle variation on the leakage current is small, and the leakage current can be effectively reduced. On the other hand, there is one drawback that the number of switching for the proposed solution is slightly more than that of the traditional one during a carrier cycle. However, compared with the conventional solution, both the leakage current and the THD of grid current can be reduced effectively with the proposed solution. Moreover, the four-leg solution can enable the zero sequence current to circulate, avoiding the dc bias in the load output currents in case of unbalanced loads. Aside from that, the power losses of semiconductor devices can be reduced significantly. Therefore, the proposed solution is attractive for transformerless PV systems.

REFERENCES:

[1]         X. Guo, Y. Yang, and T. Zhu, “ESI: A novel three-phase inverter with leakage current attenuation for transformerless PV systems," IEEE Trans. Ind. Electron., vol. 65, no. 4, pp. 2967-2974, Apr.2018.

[2]         H. Xiao, L. Zhang, and Y. Li, “An improved zero-current-switching single-phase transformerless PV H6 inverter with switching loss-free,” IEEE Trans. Ind. Electron., vol. 64, no. 10, pp. 7896-7905, Oct. 2017.

[3]         L. Zhang, K. Sun, Y. Li, X. Lu, and J. Zhao, "A distributed power control of series connected module-integrated inverters for PV grid-tied applications,” IEEE Trans. Power Electron., vol. 33, no. 9, pp. 7698-7707, Sept. 2018.

[4]         W. Li, Y. Gu, H. Luo, W. Cui, X. He, and C. Xia, “Topology review and derivation methodology of single-phase transformerless photovoltaic inverters for leakage current suppression,” IEEE Trans. Ind. Electron., vol. 62, no. 7, pp. 4537–4551, Jul. 2015.

[5]         Yam Siwakoti, and Frede Blaabjerg, "Common-ground-type transformerless inverters for single-phase solar photovoltaic systems,” IEEE Trans. Ind. Electron., vol. 65, no. 3, pp. 2100–2111 Mar. 2018.