New Control Strategy for Three-Phase Grid-Connected LCL Inverters without a Phase-Locked Loop

ABSTRACT:

The three-phase synchronous reference frame phase-locked loop (SRF-PLL) is widely used for synchronization applications in power systems. In this paper, a new control strategy for three-phase grid-connected LCL inverters without a PLL is presented. According to the new strategy, a current reference can be generated by using the instantaneous power control scheme and the proposed positive-sequence voltage detector. Through theoretical analysis, it is indicated that a high-quality grid current can be produced by introducing the new control strategy. In addition, a kind of independent control for reactive power can be achieved under unbalanced and distorted grid conditions. Finally, the excellent performance of the proposed control strategy is validated by means of simulation and experimental results.

KEYWORDS:

1.      Control strategy

2.       Grid-connected inverters

3.       Instantaneous power control scheme

4.       LCL filter

5.       Positive-sequence voltage detector

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Block diagram of the control system with LCL filter.

 EXPECTED SIMULATION RESULTS:

Fig. 2. Simulation results of the proposed control system. (a) Generated current reference signals. (b) A-phase grid voltage and three-phase current. (c) Actual active and reactive powers.

Fig. 3. Experimental results of the positive-sequence voltage detector under actual grid operating conditions. (a) Utility voltage and the detected positive-sequence signals. (b) Harmonic spectrum of the utility voltage. (c) Harmonic spectrum of the detected positive-sequence signals.

Fig. 4. Experimental results of a step in the reactive power reference. (a) A-phase grid voltage and three-phase current. (b) A-phase grid voltage and A-phase current.

 CONCLUSION:

A new control structure for three-phase grid-connected voltage source inverters (VSI) with an LCL-filter is proposed. By using the instantaneous power control scheme and the proposed positive-sequence voltage detector, the current reference can be indirectly generated, which avoids the complex PLL. The effectiveness of the proposed system for three-phase grid-connected VSIs is demonstrated via simulation results, which show a significant improvement in both the steady state and transient behavior. The same behavior is experimentally verified. The fast dynamic response to a reference step is not affected by the inclusion of additional control loops. Good performance is guaranteed even under unbalanced and distorted grid voltages.

REFERENCES:

[1] X. Wang, J. M. Guerrero, F. Blaabjerg, and Z. Chen, “A review of power electronics based microgrids,” Journal of Power Electronics, Vol. 12, No. 1, pp. 181-192, Jan. 2012.

[2] S. Peng, A. Luo, Y. Chen, and Z. Lv, “Dual-loop power control for single-phase grid-connected converters with LCL filters,” Journal of Power Electronics, Vol. 11, No. 4, pp. 456-463, July. 2011.

[3] F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus, “Overview of control and grid synchronization for distributed power generation systems,” IEEE Trans. Ind. Electron., Vol. 53, No. 5, pp. 1398-1409, Oct. 2006.

[4] R. Inzunza, T. Sumiya, Y. Fujii, and E. Ikawa, “Parallel connection of grid-connected LCL inverters for MW-scaled photovoltaic systems,” in Proc. IEEE IPEC, pp. 1988-1993, 2010.

[5] T. Noguchi, H. Tomiki, S. Kondo, and I. Takahashi, “Direct power control of PWM converter without power-source voltage sensors,” IEEE Trans. Ind. Appl., Vol. 34, No. 3, pp. 473-479, Mar./ Jun. 1998.

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.

Distributed Generation System Control Strategies in Microgrid Operation

ABSTRACT:

Control strategies of distributed generation (DG) are investigated for different combination of DG and storage units in a microgrid. This paper develops a detailed photovoltaic (PV) array model with maximum power point tracking (MPPT) control, and presents real and reactive power (PQ) control and droop control for DG system for microgrid operation. In grid-connected mode, PQ control is developed by controlling the active and reactive power output of DGs in accordance with assigned references. In islanded mode, DGs are controlled by droop control. Droop control implements power reallocation between DGs based on predefined droop characteristics whenever load changes or the microgrid is connected/disconnected to the grid, while the microgrid voltage and frequency is maintained at appropriate levels. This paper presents results from a test microgrid system consisting of a voltage source converter (VSC) interfacing with a DG, a PV array with MPPT, and changeable loads. The control strategies are tested via two scenarios: the first one is to switch between grid-connected mode and islanded mode and the second one is to change loads in islanded mode. Through voltage, frequency, and power characteristics in the simulation under such two scenarios, the proposed control strategies can be demonstrated to work properly and effectively.

KEYWORDS:

1.      Distributed generation

2.       PV

3.       Microgrid

4.       Droop control

5.       PQ control

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Schematic of the microgrid.

CONTROL SYSTEM:

Fig. 2. Schematic of the PQ control.

 Fig. 3. Schematic of the droop control.

EXPECTED SIMULATION RESULTS:

Fig. 4. PQ control under grid-connected mode.

Fig. 5. Droop control for switching modes.

Fig. 6. Droop control for varying load.

CONCLUSION:

In this paper a detailed PV model with MPPT, and PQ and droop controllers is developed for inverter interfaced DGs. The use of PQ control ensures that DGs can generate certain power in accordance with real and reactive power references. Droop controller is developed to ensure the quick dynamic frequency response and proper power sharing between DGs when a forced isolation occurs or load changes. Compared to pure V/f control and master-slave control, the proposed control strategies which have the ability to operate without any online signal communication between DGs make the system operation cost-effective and fast respond to load changes. The simulation results obtained shows that the proposed controller is effective in performing real and reactive power tracking, voltage control and power sharing during both grid-connected mode and islanded mode. To fully represent the complexity of the microgrid, future work will include the development of hierarchical controllers for a microgrid consisting of several DGs and energy storage system. The function of primary controller is to assign optimal power reference to each DG to match load balances and the secondary controllers are designed to control local voltage and frequency.

REFERENCES:

Barsali, S., Ceraolo M., Pelacchi, P., and Poli, D. (2002). Control techniques of dispersed generators to improve the continuity of electricity supply. IEEE Conf., Power Engineering Society, vol.2, pp.789-794.

Cai, N., and Mitra J. (2010). A decentralized control architecture for a microgrid with power electronic interfaces. IEEE conf., North American Power Symposium, pp. 1-8.

Chen, X., Wang, Y.H., and Wang, Y.C. (2013). A novel seamless transferring control method for microgrid based on master-slave configuration. IEEE Conf., ECCE Asia, pp. 351-357.

Cho, C. H., Jeon, J.H., Kim, J.Y., Kwon, S., Park, K., and Kim, S. (2011). Active synchronizing control a microgrid. IEEE Trans., Power Electron., vol. 26, no. 12, pp. 3707-3719

Choi, J.W. and Sul, S.K. (1998). Fast current controller in three-phase AC/DC boost converter using d-q axis crosscoupling. IEEE Trans., Power Electron., vol.13, no.1, pp. 179-185.