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.

Performance Enhancement of Actively Controlled Hybrid DC Microgrid Incorporating Pulsed Load

ABSTRACT:

In this paper, a new energy control scheme is proposed for actively controlled hybrid dc microgrid to reduce the adverse impact of pulsed power loads. The proposed energy control is an adaptive current-voltage control (ACVC) scheme based on the moving average measurement technique and an adaptive proportional compensator. Unlike conventional energy control methods, the proposed ACVC approach has the advantage of controlling both voltage and current of the system while keeping the output current of the power converter at a relatively constant value. For this study, a laboratory scale hybrid dc microgrid is developed to evaluate the performance of the ACVC strategy and to compare its performance with the other conventional energy control methods. Using experimental test results, it is shown that the proposed strategy highly improves the dynamic performance of the hybrid dc microgrid. Although the ACVC technique causes slightly more bus voltage variation, it effectively eliminates the high current and power pulsation of the power converters. The experimental test results for different pulse duty ratios demonstrated a significant improvement achieved by the developed ACVC scheme in enhancing the system efficiency, reducing the ac grid voltage drop and the frequency fluctuations.

KEYWORDS:

1.      Hybrid dc microgrid

2.      Energy control system

3.      Pulse load

4.      Supercapacitor

5.      Active hybrid power source

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Schematic diagram of the hybrid dc microgrid under study

EXPECTED SIMULATION RESULTS:

Fig. 2: Experimental test results of ACVC and CACC technique during constant pulse load operation.

 Fig. 3: Experimental test results of CACC method and ACVC technique when pulse load frequency changes from 0.1-Hz to 0.2-Hz and its duty ratio increased from 20% to 40%.

Fig. 4: Variation of the normalized average dc bus voltage and the kv in the proposed ACVC technique when pulse load frequency changes from 0.1-Hz to 0.2-Hz and its duty ratio increased from 20% to 40%.

Fig. 5: Experimental test results of CACC method and ACVC technique when pulse load changed from 2-kW to 3-kW.

Fig. 6: Hybrid DC microgrid performance comparison when ACVC, LBVC and IPC methods are utilized.

CONCLUSION:

              In this paper, a new energy control scheme was developed to reduce the adverse impact of pulsed power loads. The proposed energy control was an adaptive current-voltage control (ACVC) scheme based on the moving average current and voltage measurement and a proportional voltage compensator. The performance of the developed ACVC technique was experimentally evaluated and it was compared to the other common energy control methods.

             The test results showed that the ACVC scheme has a similar performance with the continuous average current control (CACC) method during a constant pulsed power load operation. However, the transient response of the ACVC technique during pulse load variation was effectively improved and it prevented any steady state voltage error or dangerous over voltage.

               Also, the performance of the developed ACVC technique was compared with the limit-based voltage control (LBVC) and instantaneous power control (IPC) methods for different pulse rates and duty ratios. The comparative analysis showed that although the maximum dc bus voltage variation in the case of ACVC scheme was higher than the IPC and LBVC methods, the proposed ACVC technique required smaller power capacity of the converter and energy resources. Moreover, the developed ACVC method effectively eliminated the power pulsation of the slack bus generator and frequency fluctuation of the interconnected AC grid while the ac bus voltage drop was well reduced. Additionally, the efficiency analysis for different pulse duty ratios showed that the developed ACVC method considerably improved the efficiency of the system since the maximum current of the converter was reduced and the converter was operating at a relatively constant value.

REFERENCES:

[1] M. E. Baran and N. R. Mahajan, “DC Distribution for industrial systems: opportunities and challenges,” IEEE Trans. on industrial applications, vol. 39, no. 6, pp. 1596-1601, November/December 2003.

[2] M. Farhadi, A. Mohamed and O. Mohammed, "Connectivity and Bidirectional Energy Transfer in DC Microgrid Featuring Different  Voltage Characteristics," Green Technologies Conference, 2013 IEEE, vol., no., pp.244-249, 4-5 April 2013.

[3] D. Salomonsson, L.Soder, A. Sannino, "An Adaptive Control System for a Dc Microgrid for Data Centers," Industry Applications Conference, 2007. 42nd IAS Annual Meeting. Conference Record of the 2007 IEEE, vol., no., pp.2414,2421, 23-27 Sept. 2007.

[4] M. Falahi, B K.L. utler-Purry and M. Ehsani, "Reactive Power Coordination of Shipboard Power Systems in Presence of Pulsed Loads," Power Systems, IEEE Transactions on, vol.28, no.4, pp.3675-3682, Nov. 2013.

[5] M. Farhadi, and O. Mohammed, "Realtime operation and harmonic analysis of isolated and non-isolated hybrid DC microgrid," Industry Applications Society Annual Meeting, 2013 IEEE , vol., no., pp.1,6, 6-11 Oct. 2013.

A New Control Strategy for a Multi-Bus MV Microgrid Under Unbalanced Conditions

ABSTRACT:

This paper proposes a new control strategy for the islanded operation of a multi-bus medium voltage (MV) microgrid. The microgrid consists of several dispatchable electronically-coupled  distributed generation (DG) units. Each DG unit supplies a local load which can be unbalanced due to the inclusion of singlephase  loads. The proposed control strategy of each DG comprises a proportional resonance (PR) controller with an adjustable resonance frequency, a droop control strategy, and a negative-sequence impedance controller (NSIC). The PR and droop controllers are, respectively, used to regulate the load voltage and share the average power components among the DG units. The NSIC is used to effectively compensate the negative-sequence currents of the unbalanced loads and to improve the performance of the overall microgrid system.Moreover, the NSIC minimizes the negative-sequence currents in the MV lines and thus, improving the power quality of the microgrid. The performance of the proposed control strategy is verified by using digital time-domain simulation studies in the PSCAD/EMTDC software environment.

KEYWORDS:

1.      Distributed generation

2.       Medium voltage (MV)

3.      Microgrid

4.       Negative-sequence current

5.       Power sharing

6.       Unbalance load

7.       Voltage control

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1. MV multi-bus microgrid consisting of two DG units.

 EXPECTED SIMULATION RESULTS:

             

Fig. 2 Unbalanced load changes in feeder F₁ (a) instantaneous real, and (b)

reactive power components.

Fig. 3. Amplitude of (a) positive- and (b) negative-sequence currents of the

feeders.

Fig. 4. Instantaneous voltages at DG terminals during unbalanced load

changes in feeder F₁, (a) DG₁and (b) DG₂ .

Fig.5. Frequency of islanded microgrid during unbalanced load changes.

Fig. 6. (a) Negative-sequence output impedance of each DG, and (b) amplitude

of negative-sequence current of DG units.

Fig. 7. Dynamic response of DG units to unbalanced load changes in feeder

F₁: (a) real power, and (b) reactive power components of DG units.

Fig. 8. Unbalanced load changes in feeders F₃ and F₂ (a, b) instantaneous

real and reactive power of feeders.

Fig. 9. Amplitude of (a) positive and (b) negative-sequence currents of the

feeders.

Fig. 10. (a) Negative-sequence output impedance, and (b) amplitude of negative-

sequence current for each DG.

CONCLUSION:

This paper presents a new control strategy for amulti-bus MV microgrid consisting of the dispatchable electronically-coupled DG units and unbalanced loads. The negative-sequence current of a local load is completely compensated by its dedicated DG. However, the negative-sequence current of the nonlocal loads is shared among the adjacent DGs. The proposed control strategy is composed of a PR controller with non-fixed resonance frequency, a droop control, and a negative-sequence impedance controller (NSIC). The PR and droop controllers are, respectively, used to regulate the load voltage and to share the average power among the DG units. The NSIC is used to improve the performance of the microgrid system when the unbalanced loads are present. Moreover, the NSIC minimizes the negative- sequence currents in the MV lines, and thus, improving the power quality of the microgrid. The performance of the proposed control strategy is investigated by using digital time-domain simulation studies in the PSCAD/EMTDC software environment. The simulation results conclude that the proposed strategy:

• robustly regulates voltage and frequency of the microgrid;

• is able to share the average power among the DGs;

• effectively compensates the negative-sequence currents of local loads; and

• shares the negative-sequence current of the nonlocal loads such that the power quality of the overall microgrid is not degraded.

REFERENCES:

[1] N. Hatziargyriou, H. Asano, R. Iravani, and C. Marnay, “Microgrids,” IEEE Power Energy Mag., vol. 5, pp. 78–94, Jul.–Aug. 2007.

[2] A. G. Madureira and J. A. P. Lopes, “Coordinated voltage support in distribution networks with distributed generation and microgrids,” IET Renew. Power Gener., vol. 3, pp. 439–454, Sep. 2009.

[3] IEEE Recommended Practice for Monitoring Electric Power Quality, IEEE Std. 1159, 2009.

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

[5] R. Lasseter, “Microgrids,” in Proc. IEEE Power Eng. Soc. Winter Meeting, 2002, pp. 305–308.