Coordination control of hybrid AC/DC Microgrid

 ABSTRACT:

The hybrid AC/DC microgrid is considered to be the more and more popular in power systems as increasing DC loads. In this study, it is presented that a hybrid AC/DC microgrid is modelled with some renewable energy sources (e.g. solar energy, wind energy), typical storage facilities (e.g. batteries), and AC, DC load, and also the power could be transformed smoothly between the AC and DC sub-grids by the bidirectional AC/DC converter. Meanwhile, coordination control strategies are proposed for power balance under various operations. In grid-connected mode, the U–Q (DC bus voltage and reactive) or PQ method is adopted for the bidirectional AC/DC converter according to the amount of exchange power between AC and DC system in order to improve the DG utilisation efficiency, protecting the converter and maintain the stable operation of the system. In islanded mode, V/F control is applied to stabilising the entire system voltage and frequency, achieving the power balance between the AC and DC systems. Finally, these control strategies are verified by simulation with the results showing that the control scheme would maintain stable operation of the hybrid AC/DC microgrid.

SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:

Fig. 1 Compact hybrid AC/DC microgrid system

 EXPECTED SIMULATION RESULTS:

Fig. 2 AC bus voltage and current of A phase in grid-connected mode

Fig. 3 SOC of the battery in grid-connected mode

Fig. 4 Power of wind, DC side power flowed into AC side and the output

of battery in grid-connected mode

Fig. 5 PV output power versus 50*solar irradiation in islanded mode

Fig. 6 DC bus voltage with the influence of solar irradiance variation

and pulse load

Fig. 7 SOC of the battery in islanded mode

Fig. 8 AC bus voltage and current of A phase in islanded mode

Fig. 9  Power of wind, DC side power flowed into AC side and the output

of battery in islanded mode

 CONCLUSION:

In this paper, the coordination control strategies are proposed for the hybrid AC/DC microgrid, operating in grid-connected mode and islanded mode. The control strategies are verified with Matlab/ Simulink under various operations and load conditions. The simulation results show that the control strategies of the hybrid AC/DC microgrid system are efficient. In grid-connected mode, both the bidirectional AC/DC converter and the batteries can keep the DC bus voltage stable, and ensure the converter smoothly operates in U–Q or PQ methods under the various solar irradiation conditions. In islanded mode, the AC bus voltage and frequency are provided by bidirectional AC/DC converter, the battery is to maintain DC bus stability and system power balance under pulse load and various solar irradiation conditions.

REFERENCES:

[1] Unamuno, E., Barrena, J.: ‘Hybrid ac/dc microgrids – part I: review and classification of topologies’, Renew. Sust. Energy Rev., 2015, 52, pp. 1251– 1259

[2] Khederzadeh, M., Sadeghi, M.: ‘Virtual active power filter: a notable feature for hybrid ac/dc microgrids’, IET Gener. Transm. Distrib., 2016, 10, (14), pp. 3539–3546

[3] Salomonsson, D., Soder, L., Sannino, A.: ‘An adaptive control system for a dc microgrid for data centers’, IEEE Trans. Ind. Appl., 2008, 44, (6), pp. 1910– 1917

[4] Anand, S., Fernandes, B.G., Guerrero, J.M.: ‘Distributed control to ensure proportional load sharing and improve voltage regulation in low-voltage dc microgrids’, IEEE Trans. Power Electron., 2013, 28, (4), pp. 1900–1913

[5] Wu, W., Wang, H., Liu, Y., et al.: ‘A dual buck-boost AC/DC converter for DC nanogrid with three terminal outputs’, IEEE Trans. Ind. Electron., 2017, 64, (1), pp. 295–299

A Single-Phase Buck-Boost Matrix Converter with Only Six Switches and Without Commutation Problem

ABSTRACT:

 In this paper, a single-phase buck-boost matrix converter is proposed which can both buck and boost the input voltage with step-changed frequency. It consists of only six unidirectional current flowing bidirectional voltage blocking switches, two input and output filter capacitors, and one inductor. It has following advantages over the existing single-phase matrix converters: 1) it can both buck and boost input voltage solving the limited voltage transfer ratio (only boost or buck) problem; 2) it also has enhanced reliability as it is immune from shoot-through problem of voltage source when all switches are turned-on simultaneously, and therefore, it has no need of PWM dead times and RC snubbers or dedicated soft-commutation strategies to solve the commutation problem; 3) it can also use high speed power MOSFETs as their body diodes never conduct, which eliminate their poor reverse recovery problem. The operation principle of the proposed converter is given, and switching strategies are developed to obtain various multiples and submultiples of input frequency. To verify its performance, a laboratory prototype is fabricated and experiments are performed to produce step-down and step-up voltage with three different frequencies of 120, 60 and 30 Hz.

KEYWORDS:

1.      Buck-boost operation

2.      Commutation problem

3.      Single-phase matrix converter

4.      Step-changed frequency

5.      Z-source

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1. Circuit topology of the proposed single-phase buck-boost MC.

 EXPERIMENTAL RESULTS:

Fig. 2. Experimental results of the proposed ac-ac converter under non-inverting buck-boost mode operations for  and   . (a) Boost operation when,  . (b) Buck operation when ,  . (c) Components stresses. (d) Zoom-in waveforms of (c).

Fig. 3. Experimental results of the proposed ac-ac converter under inverting buck-boost mode operations for   and  . (a) Boost operation when . (b) Buck operation when . (c) Components stresses. (d) Zoom-in waveforms

of (c).

Fig. 4. Experimental results of the proposed ac-ac converter under buck-boost mode operations for  and step-down frequency operation when . (a) Boost operation when . (b) Buck operation when . (c) Switch voltage and inductor current stresses (d) Zoom-in waveforms of (c).

Fig. 5. Experimental results of the proposed ac-ac converter under buck-boost mode operations for  and   step-up frequency operation when . (a) Boost operation when. (b) Buck operation when . (c) Switch voltage and inductor current stresses (d) Zoom-in waveforms of (c).

Fig. 6. Experimental waveforms of input voltage , output voltage , input current 

, and output current during 60 Hz inverting mode operation with inductive load    when 

 Fig. 7. Experimental waveforms of input voltage , output voltage , input current , and output current during 30 Hz mode operation with inductive load when 

Fig. 8. Efficiency of the proposed single-phase buck-boost matrix converter.

CONCLUSION:

In this paper, a single-phase buck-boost MC is proposed which consists of one inductor, two filter capacitors, and six unidirectional current conducting bidirectional voltage blocking switches. It can step-changed the output frequency with both voltage buck and boost operation, therefore, solves the limited gain (only buck or boost) ability of the existing single-phase MCs. The proposed single-phase MC is more reliable than the existing MCs as it can turn on all switches simultaneously without current overshoot problem caused by short-circuit of voltage source. Therefore, it does not have commutation problem and eliminates the need for PWM dead times and lossy RC snubbers or dedicated soft-commutation strategies, which is a significant advantage.

A detailed analysis of the proposed topology and switching strategies are given for buck-boost operation with step-down, same and step-up frequency. A scaled down laboratory prototype of the proposed MC with output voltage of 70 Vrms was fabricated based on TMS320F28335 DPS-kit to generate the control signals, and experimental results under buck and boost modes were given for output frequencies of 30 Hz (step-down frequency), 60 Hz (same frequency) and 120 Hz (step-up frequency). The proposed MC can be used in applications which require voltage regulation along with frequency variation such as to control the speed of a fan or a pump, to drive induction motor, for induction heating, and to implement a high boost AC-DC MC based on Cockcroft-Walton voltage multiplier, etc.

REFERENCES:

[1] B. H. Kwon, B. D. Min, and J. H. Kim, “Novel topologies of AC choppers,” IEE Proc. Electr. Power Appl., vol. 143, no. 4, pp. 323–330, Jul. 1996.

[2] X. P. Fang, Z. M. Qian, and F. Z. Peng, “Single-phase Z-source PWM ac–ac converters,” IEEE Power Electron. Lett., vol. 3, no. 4, pp. 121–124, Dec. 2005.

[3] T. B. Lazzarin, R. L. Andersen, and I. Barbi, “A switched-capacitor three-phase ac-ac converter,” IEEE Trans. Ind. Electron., vol. 62, no. 2, pp. 735–745, Feb. 2015.

[4] D.-C. Lee, and Y.-S. Kim, “Control of single-phase-to-three-phase ac/dc/ac PWM converters for induction motor drives,” IEEE Trans. Ind. Electron., vol. 54, no. 2, pp. 797– 804, Apr. 2007.

[5] J. E. C. d. Santos, C. B. Jacobina, N. Rocha, and E. R. C. d. Silve, “Six-phase machine drive system with reversible parallel ac-dc-ac converters,” IEEE Trans. Ind. Electron., vol. 58, no. 5, pp. 2049– 2053, May. 2011.