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.

Active Power Management of MultiHybrid Fuel Cell/Supercapacitor Power Conversion System in a Medium Voltage Microgrid

ABSTRACT:

This paper proposes a hierarchical active power management strategy for a medium voltage (MV) islanded microgrid including a multihybrid power conversion system (MHPCS). To guarantee excellent power management, a modular power conversion system is realized by parallel connection of small MHPCS units. The hybrid system includes fuel cells (FC) as main and supercapacitors (SC) as complementary power sources. The SC energy storage compensates the slow transient response of the FC stack and supports the FC to meet the grid power demand. The proposed control strategy of the MHPCS comprises three control loops; dc-link voltage controller, power management controller, and load current sharing controller. Each distributed generation (DG) unit uses an adaptive proportional resonance (PR) controller for regulating the load voltage, and a droop control strategy for average power sharing among the DG units. The performance of the proposed control strategy is verified by using digital time-domain simulation studies in the PSCAD/EMTDC software environment.

KEYWORDS:

1.      Fuel cell (FC)

2.       Multihybrid power conversion system (MHPCS)

3.       MV microgrid

4.       Supercapacitor (SC)

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1. (a) MV microgrid consisting of two DG units. (b) Proposed structure

of hybrid FC/SC power conversion system.

CONTROL SYSTEM:

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

EXPECTED SIMULATION RESULTS:

Fig. 3. Balanced load changes in feeders F₃ and  F₁. (a) Instantaneous real and

(b) instantaneous reactive powers of the feeders.

Fig. 4. Instantaneous voltages at the DG unit terminals during balanced load

changes in feeder F₁, (a) DG₁ and (b) DG₂ .

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

Fig. 6. Dynamic response of the DG units to balanced load changes: (a) real

power, and (b) reactive power components.

Fig. 7. Dynamic response of DG₁ units to balanced load changes: (a) FC

stack and SC module power of first hybrid unit; (b) FC stack and SC module

power of second hybrid unit; and (c) dc-link voltage.

Fig. 8. Unbalanced load change in feeder F₁. (a) Instantaneous real and (b)

instantaneous reactive powers of the feeders.

Fig. 9. Dynamic response of the DG units to unbalanced load change with

conventional PR controller: (a) real power, and (b) reactive power components

Fig. 10. Dynamic response of the DG units to unbalanced load change with

adaptive PR controller: (a) real, and (b) reactive power.

Fig. 11. Dynamic response of DG₁ units to unbalanced load change: (a) FC

stack and SC module power of first hybrid unit; (b) FC stack and SC module

power of second hybrid unit; and (c) dc-link voltage.

CONCLUSION:

This paper presents a hierarchical active power management strategy for a MV islanded microgrid considering the MHPCS. The proposed strategy includes power management of the FC/SC hybrid system, current sharing among the MHPCS components, voltage control of the ac-side, and power sharing among the DG units. The SC energy storage compensates the slow transient response of the FC stack. An adaptive PR controller and a droop controller are, respectively, used to effectively regulate the load voltage and to share the average power among the DG units. The performance of the proposed control strategy in both balanced and unbalanced load switching is investigated using PSCAD/EMTDC software. The results show that the proposed strategy:

• enhances the dynamic response of the microgrid in fast transients;

• accurately shares the load current among the FC/SC hybrid units;

• robustly regulates voltage and frequency of the microgrid;

• is able to share the average power among theDGunits even under unbalanced conditions;

• effectively eliminates the low frequency transient of power components; and

• locally compensates the unbalanced loads.

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] Z. Jiang and R. Dougal, “A hybrid fuel cell power supply with rapid dynamic response and high peak-power capacity,” in Proc. IEEE APEC, 2006, pp. 1250–1255.

[4] H. Nikkhajoei and R. Lasseter, “Distributed generation interface to the certs microgrid,” IEEE Trans. Power Del., vol. 24, pp. 1598–1608, Jul. 2009.

[5] M. Zandi, A. Payman, J.Martin, S. Pierfederici, B.Davat, and F. Meibody- Tabar, “Energy management of a fuel cell/supercapacitor/battery power source for electric vehicular applications,” IEEE Trans. Veh.Technol., vol. 60, pp. 433–443, Feb. 2011.