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:

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

EXPECTED SIMULATION RESULTS:

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

reactive power components.

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

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Fig. 4. Instantaneous voltages at DG terminals during unbalanced load changes in feeder F1, (a) DG1and (b) DG2 .

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

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

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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.

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

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

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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.