Comparison of DC/DC Converters in DCM for Reducing Low-Frequency Input Current Ripple of Single-Phase Two-Stage Inverters

ABSTRACT

Single-phase two-stage inverters generally use an intermediate capacitor to buffer the power imbalance between DC input and AC output. However, the resultant low-frequency voltage ripple on this intermediate capacitor may produce low frequency ripple at the source side, especially when the front-end dc/dc converter operates in continuous conduction mode (CCM). Some common solutions to reducing this ripple are feed forward control and power decoupling circuits. Alternatively, this paper analyzes a two-stage inverter where the front-end is a dc/dc converter operating in discontinuous conduction mode (DCM). In general dc/dc converters operating in DCM have inherent natural capability to reduce this low-frequency input current ripple, without needing a sophisticated control or complex circuitry as compared with its CCM operation. Analysis with simulation verification is reported to demonstrate such capability.

KEYWORDS

1.      Dc/ac

2.      Low-frequency ripple

3.      Single-phase

4.      Two stage

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1. A simplified power-stage diagram of a single-phase two-stage inverter.

EXPECTED SIMULATION RESULTS

          

(a)   CCM operation: _vin = 3:3V

(b) DCM operation: _vin = 0:88V

Fig. 2. DCM boost front-end converter has lower voltage ripple than CCM.

Fig. 3. DCM buck-boost front-end converter does not contain low-frequency

ripple but only high-frequency ripple.

                                                                         

Fig. 4. SEPIC front-end converter operating in DCM+CCM contains negligible

low-frequency ripple but only high-frequency ripple.

Fig. 5. High-gain front-end converter operating in DCM does contains

significant low-frequency ripple.

CONCLUSION

This paper analyzes basic and several higher-order front-end dc/dc converters for single-phase two-stage inverter design. Through inspecting the instantaneous average input current of those converters in discontinuous conduction mode (DCM), it has confirmed that buck-boost converter and buck-boost derived converters such as ZETA are free of low-frequency (mainly double ac line frequency) input current ripple due to the lack of direct connection between input and output during switching actions. For boost converter based converters such as SEPIC and C´ uk converters, their input currents contain lower low-frequency content thanks to the cascaded design. For boost converter based high voltage gain converters, its input current may not necessarily reduce the low-frequency content effectively. It depends on how the high-gain sub circuit is constructed and interacts with the input inductor. Further research is necessary to identify suitable converter topologies which have both smooth input current and low frequency content.

REFERENCES

 [1] K. Fukushima, I. Norigoe, M. Shoyama, T. Ninomiya, Y. Harada, and K. Tsukakoshi, “Input Current-Ripple Consideration for the Pulse-link DC-AC Converter for Fuel Cells by Small Series LC Circuit,” in 2009 Twenty-Fourth Annual IEEE Applied Power Electronics Conference and Exposition, Feb 2009, pp. 447–451.

[2] L. Jianguo, H. Wenbin, Y. Kai, L. Xiaoyu, W. Fuyun, and W. Junji, “Research on input current ripple reduction of two-stage single-phase PV grid inverter,” in 2014 16th European Conference on Power Electronics and Applications, Aug 2014, pp. 1–8.

[3] B. Ge, Y. Liu, H. Abu-Rub, R. S. Balog, F. Z. Peng, S. McConnell, and X. Li, “Current Ripple Damping Control to Minimize Impedance Network for Single-Phase Quasi-Z Source Inverter System,” IEEE Transactions on Industrial Informatics, vol. 12, no. 3, pp. 1043–1054,

June 2016.

[4] Y. Zhou, H. Li, and H. Li, “A Single-Phase PV Quasi-Z-Source Inverter With Reduced Capacitance Using Modified Modulation and Double- Frequency Ripple Suppression Control,” IEEE Transactions on Power Electronics, vol. 31, no. 3, pp. 2166–2173, March 2016.

[5] D. B. W. Abeywardana, B. Hredzak, and V. G. Agelidis, “An Input Current Feedback Method to Mitigate the DC-Side Low-Frequency Ripple Current in a Single-Phase Boost Inverter,” IEEE Transactions on Power Electronics, vol. 31, no. 6, pp. 4594–4603, June 2016.

Improved Particle Swarm Optimization For Photovoltaic System Connected To The Grid With Low Voltage Ride Through Capability

ABSTRACT:

Grid connected photovoltaic (PV) system encounters different types of abnormalities during grid faults; the grid side inverter is subjected to three serious problems which are excessive DC link voltage, high AC currents and loss of grid-voltage synchronization. This high DC link voltage may damage the inverter. Also, the voltage sags will force the PV system to be disconnected from the grid according to grid code. This paper presents a novel control strategy of the two-stage three-phase PV system to improve the Low-Voltage Ride-Through (LVRT) capability according to the grid connection requirement. The non-linear control technique using Improved Particle Swarm Optimization (IPSO) of a PV system connected to the grid through an isolated high frequency DCeDC full bridge converter and a three-phase three level neutral point clamped DC-AC converter (3LNPC2) with output power control under severe faults of grid voltage. The paper, also discusses the transient behavior and the performance limit for LVRT by using a DC-Chopper circuit. The model has been implemented in MATLAB/SIMULINK. The proposed control succeeded to track MPP, achieved LVRT requirements and improving the quality of DC link voltage. The paper show

KEYWORDS:

1.      Particle swarm optimization

2.      Maximum power point tracking

3.      PV system

4.      High frequency isolated converter

5.      Low voltage ride through

6.      Grid

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Block diagram of the PV system connected to the grid.

 EXPECTED SIMULATION RESULTS:

Fig. 2. PV module characteristics (a) Current-voltage characteristics (b) power-voltage characteristics.

Fig. 3. Behavior of PV array under normal condition using IPSO.

Fig. 4. DC-link voltage under normal condition using IPSO.

Fig. 5. Behavior of PV array under normal condition using IC.

Fig. 6. DC-link voltage under normal condition using IC.

Fig. 7. Behavior of grid connected inverter system under normal operation.

Fig. 8. The grid voltage fault.

Fig. 9. Behavior of PV array under fault condition.

Fig. 10. DC-link voltage under fault condition.

Fig. 11. Behavior of grid connected inverter system under fault condition.

Fig. 12. Behavior of PV array with LVRT capability.

Fig. 13. DC-link voltage during a grid fault with LVRT capability.

Fig. 14. Behavior of grid connected inverter system with LVRT capability.

CONCLUSION:

Based on the existing grid requirements, this paper discussed the potential of a two-stage three-phase grid-connected PV system operating in grid fault condition. The power control method proposed in this paper is effective when the system is under grid fault operation mode. It can be concluded that the future three-phase grid-connected PV systems are ready to be more active and more “smart” in the regulation of power grid.

Non-linear robust control technique using IPSO control is implemented for MPPT of 100.7 kW PV system connected to the grid. Complete control of both active and reactive powers is implemented using Matlab/Simulink with complete simulation under severe faults of grid voltage. The results show superior behavior of the IPSO; it has a faster dynamic response and better steady-state performance than the traditional algorithm; IC method, thus improving the efficiency of the photovoltaic power generation system. The use of full bridge single phase inverter with a high frequency transformer which combines the advantages of 60 Hz technology and transformer- less inverter technology, achieved MPPT requirements with IPSO. Also, this system overcomes the drawbacks of DC-chopper parameters design.

Two loops of control for the utility-connected 3LNPC2 are implemented which improve the performance of inverter and reduces the harmonics in output voltage. This control, also, increases the power injected to the grid and consequently increases the total efficiency of the system. The results show that the DC chopper circuit is capable of reducing the DC-link voltage below threshold values during the fault and protect it from failure or damage. The IPSO is capable of tracking MPP with LVRT capability included.

An anti-wind up conditioned strategy is used in order to improve the quality on the DC link voltage during and after the grid fault. It succeeds to stop accumulation of the integral part during fault, which helps system to follow up pre-faults values rapidly after clearing the fault. Finally, simulated results have demonstrated the feasibility of the IPSO algorithm and capability of MPPT in grid-connected PV systems with LVRT enhancement.

REFERENCES:

[1] Ramdan B.A. Koad, Ahmed. F. Zobaa, Comparison between the conventional methods and PSO based MPPT algorithm for photovoltaic systems, Int. J. Electr. Electron. Sci. Eng. 8 (2014) 619e624.

[2] Ali Reza Reisi, Mohammad Hassan Moradi, Shahriar Jamas, Classification and comparison of maximum power point tracking techniques for photovoltaic system: a review, Renew. Sustain. Energy Rev. 19 (2013) 433e443.

[3] N.H. Saad, A.A. Sattar, A.M. Mansour, Artificial neural controller for maximum power point tracking of photovoltaic system, in: MEPCON’2006 Conference, II, El-MINIA, Egypt, 2006, pp. 562e567.

[4] Raal Mandour I. Elamvazuthi, Optimization of maximum power point tracking (MPPT) of photovoltaic system using artificial intelligence (AI) algorithms, J. Emerg. Trends Comput. Information Sci. 4 (2013) 662e669.

[5] Saeedeh Ahmadi, Shirzad Abdi, Maximum power point tracking of photovoltaic systems using PSO algorithm under partially shaded conditions, in: The 2nd Cired Regional Conference, Tehran, Iran, 14, 2014, pp. 1e7.