Journal of Power Electronics

The Korean Institute of Power Electronics (전력전자학회)
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Power Flow Control at the Subnetwork-Level in Microgrids

  • Liu, Kun (Department of Electrical Engineering, Shanghai Jiao Tong University) ;
  • Khan, Muhammad Mansoor (Department of Electrical Engineering, Shanghai Jiao Tong University) ;
  • Rana, Ahmad (Department of Mechanical Engineering, Villanova University) ;
  • Fei, Dong (State Grid Shanghai Procurement Company)
  • Received : 2016.10.06
  • Accepted : 2017.11.02
  • Published : 2018.03.20
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Abstract

This paper presents the idea of a smart load that can adjust the input power flow based on the intermittent power available from RESs (Renewable Energy Resources) to regulate the line voltage, and draw a constant power from the grid. To this effect, an innovative power flow controller is presented based on a Resistive ES (Electric Spring) in combination with a PEAT (Power Electronics based Adjustable Transformer), which can effectively shape the load power flow at the subnetwork level. With a PEAT incorporated in the step down transformer at the grid side, the proposed controller can supply non-critical loads through local RESs, and the critical loads can draw a relatively constant power from the grid. If there is an abundance of power produced by the RESs, the controller can supply both non-critical loads and critical loads through the RES, which significantly reduces the power demand from the grid. The principle, practicality, stability analysis, and controller design are presented. In addition, simulation results show that the power flow controller performs well in shaping the load power flow at the subnetwork level, which decreases the power demand on the grid. Experimental results are also provided to show that the controller can be realized.

Keywords

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Fig. 1. Electric Spring.

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Fig. 2. (a) Single phase full bridge AC/AC converter. (b) PEAT topology with a buck/boost transformer. (c) Modified PEAT design forintegration with a step down transformer, and an AC/AC module installed at secondary side. (d) Modified PEAT design for integrationwith a step down transformer, and an AC/AC module installed at the primary side. (e) PEAT physical construction.

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Fig. 3. (a) Simplified network with a PEAT; (b) inductive ES control method; (c) resistive ES control method.

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Fig. 4. (a) Line voltage Vl curve when the source voltage Vs changes ¡¾10% by applying the inductive ES method; (b) active power Pscurve when the source voltage Vs changes ¡¾10% by applying the inductive ES method.

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Fig. 5. (a) Line voltage Vl curve when the source voltage Vschanges ¡¾10% by applying the resistive ES method; (b) activepower Ps curve when source voltage Vs changes ¡¾10% byapplying the resistive ES method.

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Fig. 6. Simplified system.

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Fig. 7. A simplified power network with both ES and PEAT.

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Fig. 8. Dual mode control structure.

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Fig. 9. Active power curves for the simulations: (a) only the ES is applied; (b) both the ES and the PEAT are applied, the first powerflow controller is activated; (c) both the ES and the PEAT are applied, the second power flow controller is activated.

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Fig. 10. Mains voltage curves: (a) only an ES is applied; (b) bothan ES and a PEAT are applied, the first power flow controller isactivated; (c) both an ES and a PEAT are applied, the secondpower flow controller is activated.

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Fig. 11. PEAT topology.

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Fig. 12. Photograph of a practical PEAT.

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Fig. 13. Voltage curves of the output voltage of an AC/ACconverter before and after applying a low pass LC filter whereCH1- Vfiltered, 100V/Div, CH2- VAC/AC 100V/Div, Time Base5ms/Div: (a) duty cycle is 0.55; (b) duty cycle is 0.3.

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Fig. 14. Topology of an experimental simulation of ES operation.

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Fig. 15. Photograph of the AC/AC Buck converter.

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Fig. 16. Voltage and current curves of a smart load: (a) the ESgain is 0.05, CH1- I1 15A/Div., CH2- Vl 150V/Div., Time Base100ms/Div; (b) the ES gain is 0.2, CH1- I110A/Div., CH2- Vl150V/Div., Time Base 100ms/Div.

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Fig. 17. Experiment setup schematic.

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Fig. 18. Photograph of the practical experiment setup.

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Fig. 19. Measured waveforms where CH1 - wind powergenerator current I2 50A/Div., CH2 - Line voltage Vl 500V/Div.,CH3 - Line current I3 50A/Div., CH4 - Grid current I1 50A/Div.,Time Base 250ms/Div.

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Fig. 20. RMS values of the currents with the grid current I1, windpower generator current I2 and line current I3.

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Fig. 21. Simulated RMS values of the currents with the gridcurrent I1, wind power generator current I2 and line current I3.

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Fig. 22. RMS values of the line voltage.

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Fig. 23. Simulated RMS values of the line voltage.

TABLE I SYSTEM STABILITY ANALYSIS TEST DATA

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TABLE II CASE STUDIES RESULTS

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