Integrating Hybrid Power Source Into an Islanded MV Microgrid Using CHB Multilevel Inverter Under Unbalanced and Nonlinear Load Conditions

ABSTRACT:

This paper presents a control strategy for an islanded medium voltage microgrid to coordinate hybrid power source (HPS) units and to control interfaced multilevel inverters under unbalanced and nonlinear load conditions. The proposed HPS systems are connected to the loads through a cascaded H-bridge (CHB) multilevel inverter. The CHB multilevel inverters increase the output voltage level and enhance power quality. The HPS employs fuel cell (FC) and photovoltaic sources as the main and supercapacitors as the complementary power sources. Fast transient response, high performance, high power density, and low FC fuel consumption are the main advantages of the proposed HPS system. The proposed control strategy consists of a power management unit for the HPS system and a voltage controller for the CHB multilevel inverter. Each distributed generation unit employs a multiproportional resonant controller to regulate the buses voltages even when the loads are unbalanced and/or nonlinear. Digital time-domain simulation studies are carried out in the PSCAD/EMTDC environment to verify the performance of the overall proposed control system.

KEYWORDS:

1.      Cascaded H-bridge (CHB) multilevel inverter

2.      Fuel cell (FC)

3.       Hybrid power source (HPS)

4.       Multiproportional resonant (multi-PR)

5.       Photovoltaic (PV)

6.       Supercapacitor (SC)

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Single-line diagram of MV microgrid consisting of two DG units.

Fig. 2. Proposed structure of the hybrid FC/PV/SC power source.

EXPECTED SIMULATION RESULTS:

Fig. 3. Microgrid response to unbalanced and nonlinear load changes in feeder

F₁ . (a) and (b) Instantaneous real and reactive powers of feeders.

Fig. 4. Microgrid response to the unbalanced and nonlinear load changes applied

to feeder F₁ ; positive-sequence, negative-sequence, and harmonic components

of loads currents at (a) feeder F₁ and (b) feeder F₂ .

Fig. 5. Dynamic response of DG units to unbalanced and nonlinear load

changes applied to feeder F₁ . (a) and (b) Real and reactive power components

of DG units.

Fig. 6. Microgrid response to the unbalanced and nonlinear load changes applied

to feeder F₁ ; positive-sequence, negative-sequence, and harmonic currents

of (a) DG₁ and (b) DG₂ .

Fig. 7. (a) Instantaneous current waveforms, (b) five-level-inverter output

voltage, and (c) voltage waveforms of each phase of DG₁ ’s CHB inverter due

to the nonlinear load connection to feeder F₁ .

Fig. 8. (a) Instantaneous current waveforms, (b) five-level-inverter output

voltage, and (c) voltage waveforms of each phase of DG₁ ’s CHB inverter due

to the single-phase load disconnection from feeder F₁ .

Fig. 9. (a) Voltage THD and (b) VUF at DG₁ ’s terminal.

Fig. 10. Voltages of dc links for DG₁ ’s units.

Fig. 11. Dynamic response of DG₁ to load changes; currents of FC stacks

and PV units for each HPS. (a) Phase a, (b) phase b, and (c) phase c.

Fig. 12. Dynamic response of DG₁ to load changes; average current of SC

module of each HPS. (a) Phase a, (b) phase b, and (c) phase c.

CONCLUSION:

This paper presents an effective control strategy for an islanded microgrid including the HPS and CHB multilevel inverter under unbalanced and nonlinear load conditions. The proposed strategy includes power management of the hybrid FC/PV/SC power source and a voltage control strategy for the CHB multilevel inverter. The main features of the proposed HPS include high performance, high power density, and fast transient response. Furthermore, a multi-PR controller is presented to regulate the voltage of the CHB multilevel inverter in the presence of unbalanced and nonlinear loads. The performance of the proposed control strategy is investigated using PSCAD/EMTDC software. The results show that the proposed strategy:

1) regulates the voltage of the microgrid under unbalanced and nonlinear load conditions,

2) reduces THD and improves power quality by using CHB multilevel inverters,

3) enhances the dynamic response of the microgrid under fast transient conditions,

4) accurately balances the dc-link voltage of multilevel inverter modules, and

5) effectively manages the powers among the power sources in the HPS system.

REFERENCES:

[1] H.Zhou,T. Bhattacharya,D.Tran,T. S. T. Siew, and A. M. Khambadkone, “Composite energy storage system involving battery and ultracapacitor with dynamic energymanagement in microgrid applications,” IEEE Trans. Power Electron., vol. 26, no. 3, pp. 923–930, Mar. 2011.

[2] W. S. Liu, J. F. Chen, T. J. Liang, and R. L. Lin, “Multicascoded sources for a high-efficiency fuel-cell hybrid power system in high-voltage application,” IEEE Trans. Power Electron., vol. 26, no. 3, pp. 931–942, Mar. 2011.

[3] A. Ghazanfari, M. Hamzeh, and H. Mokhtari, “A control method for integrating hybrid  power source into an islanded microgrid through CHB multilevel inverter,” in Proc. IEEE Power Electron., Drive Syst. Technol. Conf., Feb. 2013, pp. 495–500.

[4] IEEE Recommended Practice for Electric Power Distribution for Industrial Plants. ANSI/IEEE Standard 141, 1993.

[5] IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power System. IEEE Standard 519, 1992.