# Design of Decentralized Hybrid Microgrid Integrating Multiple Renewable Energy Sources with Power Quality Improvement

^{1}

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## Abstract

**:**

## 1. Introduction

- Designing a control technique for an interlinking converter for efficient power-sharing among the AC and DC microgrid and power-quality improvement. The proposed control effectively coordinates the power exchange between the AC and DC hybrid microgrid.
- Integration and efficient utilization of renewable energy sources by the superior operation friendliness of the AC and DC microgrids.
- The proposed control supports the bidirectional power flow between DC and AC microgrids without much deviation in the frequency and a seamless transition between grid-connected and islanded mode with minimal dependence on additional sources.

## 2. Microgrid Configuration

#### 2.1. Load

#### 2.2. PV Array Design

_{oc}, I

_{mp}, and P

_{mp}represent the open-circuit voltage, maximum current, and maximum power of the PV module, respectively. Thus, six modules are connected in a series to form a string. Four strings are connected in parallel to obtain a power of 7 kW with a maximum voltage of 328.2 V (54.7 × 6 = 328.2).

#### 2.3. Boost Converter Design

_{in}represents the PV output voltage at maximum power condition; “f” represents the switching frequency of the boost converter, which is considered 10 kHz; and ΔI

_{PV}represents the ripple current.

#### 2.4. PMSG Wind Turbine

_{m}is the mechanical power extractable from the wind, ρ is the air density, A is the rotor-swept area, V

_{w}is the speed of the wind, and C

_{p}(λ,β) is the coefficient of power, a function of λ,β (tip-speed ratio, pitch angle). The wind turbine is designed for 12 kW at a nominal wind speed of 12 m/s.

#### 2.5. Fuel Cell

_{2}and O

_{2}as fuel. The reaction of hydrogen and oxygen between the anode and cathode produces electric power along with heat and water. The fuel-cell output voltage is given by [40]:

_{FC}is the output voltage of the fuel cell, E

_{fc}is the internal voltage of the fuel cell, ${\eta}_{act}$ is the fuel cell voltage drop due to activation, ${\eta}_{ohm}$ is the voltage drop due to ohmic polarization, and ${\eta}_{con}$ is a voltage drop due to concentration polarization. The power produced by the fuel cell is given by:

_{FC}is the power produced by a stack of fuel cells, N

_{0}is the number of cells in the stack, and I

_{FC}is the stack current. A fuel cell of 30 kW at 350 V is used in this work.

#### 2.6. Inverter

_{LL}is the RMS line voltage and “m” is the modulation index.

_{DC}is the DC link capacitor. For 20% of the voltage ripple, the DC link capacitance for the fuel cell and wind turbine generator is considered 5000 μF and 2000 μF, respectively.

#### 2.7. Buck—Boost Converter

#### 2.8. BESS

_{g}is the maximum generation from DC DG units, P

_{l}is the critical load, and V

_{b}is the battery terminal voltage. A buck–boost charge controller is used for charging and discharging the battery, which is connected to a 700 V DC bus.

#### 2.9. Diesel Generator

#### 2.10. Interlinking Converter

#### 2.11. Utility Grid

## 3. Control Algorithm

#### 3.1. Solar PV Control

_{PV}, and V

_{PV}is observed as per the flowchart given in Figure 2. Figure 3 shows the control logic of the P&O MPPT-based boost converter. Based on the observed changes, the duty cycle is increased or decreased. The duty cycle is passed to a PWM generator, which generates the switching pulses for the boost converter. This process is repeated to achieve maximum output power.

#### 3.2. Inverter Control

#### 3.3. BESS Control

#### 3.4. IC Control

_{o}is eliminated in Equation (14).

#### 3.5. Control in Multi-Microgrid Approach

## 4. Simulation Results and Analysis

#### 4.1. Mode 1: Grid-Connected Mode

_{rms}and 50 Hz). The DC sub-grid consists of 25 kW loads. In the DC sub-grid, the PV array generates a power of 6.6 kW and the PMSG wind turbine generates a power of 10 kW, the battery supplies power of 4.6 kW, and the remaining 3.8 kW power for the DC load is supplied by the AC sub-grid through the IC. It is observed between 0 s < t < 1 s in the simulation results, as shown in Figure 7.

#### 4.2. Mode 2: Islanded Mode

#### 4.3. Mode 3: Battery-Charging Mode

#### 4.4. Mode 4: DC-to-AC Power Flow

#### 4.5. Virtual APF

_{rms}and the current amplitude varies in the grid and islanded mode due to the change in load.

_{ph}load current, R

_{ph}grid current, and R

_{ph}voltage, respectively. Figure 14 shows the performance of the IC in maintaining the THD of the grid current when it is operated as an APF and power-exchange converter. The superior performance of the IC as an APF in maintaining the THD below 5% is evident from the chart in Figure 14.

#### 4.6. Performance of HMG with a Reduction in Load

_{PCC}and frequency of the AC sub-grid.

#### 4.7. Performance of HMG with Increment in Load

_{PCC}and frequency of the AC sub-grid.

## 5. Conclusions

_{PCC,}and frequency of the AC grid.

- The proposed controller efficiently coordinates the AC/DC hybrid microgrid in all four modes of operation.
- The required power is transferred between the AC and DC microgrid via the interlinking converter. With an energy-storage system, the power exchange between the microgrids is efficiently managed by the controller and only the excess power demand is obtained from the utility grid.
- The modified control technique for the interlinking converter improves the power quality under unbalanced and non-linear load conditions.
- The interlinking converter supports AC/DC voltage bidirectionally during the islanded mode of operation. This reduces the need for additional voltage sources.
- The proposed controller helps in the seamless transfer between grid-connected and isolated modes.
- The future works to be carried out are:
- The proposed controller can be extended to a multi-microgrid approach.
- The multi-parallel interlinking converter can be utilized in place of the interlinking converter, and an analysis can be carried out.
- The proposed controller can be applied for real-time applications.
- Economic analysis and the impact of the proposed microgrid on the present microgrid setup can be analyzed through HOMER software.
- Degradation of the hybrid components can be included in the analysis.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## Abbreviations

APF | Active power filter |

BESS | Battery energy storage system |

DG | Distributed generation |

HMG | Hybrid microgrid |

IC | Interlinking converter |

IRPT | Instantaneous reactive power theory |

MG | Microgrid |

MGCC | Microgrid centralized controller |

MPPT | Maximum power point tracking |

PCC | Point of common coupling |

PI | Proportional integral |

PLL | Phase locked loop |

PMSG | Permanent magnet synchronous generator |

P&O | Perturb and observe |

RES | Renewable energy sources |

STS | Static transfer switch |

THD | Total harmonic distortion |

WT | Wind turbine |

## Appendix A

Utility Grid | |

Three-phase four-wire system with balanced voltages | 415 V, 50 Hz |

Source impedance | R = 1 Ω, L= 6 mH |

PMSG Wind Turbine | |

Nominal mechanical power—P_{m} | 12 kW |

Nominal generator electrical power—P_{g} | 12/0.9 kVA |

Nominal wind speed—V_{m} | 12 m/s |

Maximum power at base speed | 0.8 (p.u) |

Wind Turbine Inverter | |

DC link voltage—V_{DC} | 677.49~700 V |

DC link capacitor—C_{DC} | 4685 μF~4700 μF |

Coupling inductor—(R + L) | 0.026 + 8.22 mH |

Ripple filter—(P + Q) | 20 W + 1 kVAr |

Fuel Cell | |

Voltage at (0 A, 1 A) | (450.442.5) V |

Nominal current—I_{nom} | 40 A |

Nominal voltage—V_{nom} | 350 V |

Maximum current—I_{end} | 140 A |

Power obtained—P_{obt} | 27 kW |

Boost Converter | |

Inductor—L | 3.9 mH |

Capacitor—C | 70 µF |

Switching frequency—f_{s} | 10 kHz |

Duty cycle—D | 50% |

Buck–Boost Converter | |

Inductor—L | 3 mH |

Capacitor—C | 70 µF |

Switching frequency—f_{s} | 10 kHz |

Duty cycle—D | 50% |

Fuel Cell Inverter | |

DC link voltage—V_{DC} | 677.49~700 V |

DC link capacitor—C_{DC} | 4685~4700 µF |

Coupling inductor—(R + L) | 0.01722 + 5.48 mH |

Ripple filter—(P + Q) | 30 W + 1.5 kVAr |

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**Figure 18.**AC sub-grid V

_{PCC}, frequency, and power flow of battery with 10% load reduction in the HMG.

**Figure 21.**AC sub-grid V

_{PCC}, frequency, and power flow of battery with 10% load increment in the HMG.

Control Strategies | ||||||
---|---|---|---|---|---|---|

Conditions | Proposed | Ref. [34] | Ref. [35] | Ref. [36] | Ref. [37] | Ref. [38] |

Support of DC voltage | yes | No | yes | yes | No | yes |

Support of AC voltage | yes | yes | yes | yes | yes | No |

Frequency deviation | yes | No | No | No | No | No |

Continuous operation of voltage sources | No | yes | yes | yes | yes | yes |

Implementation to parallel interlinking converter | yes | No | No | yes | yes | yes |

Seamless operation between grids | yes | No | No | No | No | No |

Power-quality Improvement | yes | No | yes | No | No | No |

Solar PV Array | |
---|---|

Model | SunPower SPR—305WHT |

Number of cells—N_{c} | 96 |

Open-circuit voltage—V_{oc} | 64.2 V |

Short-circuit current—I_{sc} | 5.96 A |

Voltage at maximum power point—V_{MP} | 54.7 V |

Current at maximum power point—I_{MP} | 5.58 A |

No. of series modules per string—NS | 6 |

No. of parallel strings—NP | 4 |

Maximum power extractable—Po | 7 kW |

Sl. No. | Mode | Time Interval in Seconds | Description |
---|---|---|---|

1 | Mode 1: grid-connected mode | 0 to 1 s and 2.5 s to 4 s | AC DG units are synchronized with grid voltage and frequency (three-phase four-wire balanced system—415 V, 50 Hz) |

2 | Mode 2: islanded mode | 1 s to 2.5 s | The system is isolated from the utility grid. The diesel generator acts as the voltage and frequency reference in the AC sub-grid. Non-critical loads of DC and AC sub-grids are turned off. |

3 | Mode 3: battery-charging mode | 4 s to 5 s | The load in the DC sub-grid is lesser than the DG unit’s generation and the battery gets charged. |

4 | Mode 4: DC-o-AC power-flow mode | 5 s to 6 s | Power transfer takes place from the DC to the AC sub-grid. The battery is presumed to be fully charged. |

Mode | Details of Load Connected | P_{PV*}(kW) | P_{WT-DC*}(kW) | P_{B*}(kW) | P_{IC*}(kW) | P_{WT-AC*}(kW) | P_{FC*}(kW) | P_{G*}(kW) | P_{DG*}(kW) |
---|---|---|---|---|---|---|---|---|---|

Mode 1 | DC sub grid load—25 kW | 6.6 | 10 | 4.6 | 3.8 | - | - | - | |

Non-linear loads in AC sub grid—P = 72.5 kW | - | - | - | −3.8 | 10 | 26 | 40.3 | - | |

Non-linear loads in AC sub-grid—Q = 11 kVAr | - | - | - | 12 | - | - | - | −1 | |

Mode 2 | DC sub-grid critical load—20 kW | 6.6 | 10 | 3.4 | - | - | - | - | - |

Non-linear critical loads in AC sub-grid—P = 50 kW | - | - | - | - | 10 | 26 | - | 14 | |

Non-linear critical loads in AC sub-grid—Q = 2.5 kVAr | - | - | - | - | - | - | - | 2.5 | |

Mode 3 | DC sub-grid load—12.5 kW | 6.6 | 10 | −7 | 2.9 | - | - | - | |

Non-linear loads in AC sub-grid—P = 72.5 kW | - | - | - | −2.9 | 10 | 26 | 39.4 | - | |

Mode 4 | DC sub-grid load—12.5 kW | 6.6 | 10 | - | −4.1 | - | - | - | - |

Non-linear loads in AC sub-grid—P = 72.5 kW | 4.1 | 10 | 26 | 32.4 | - |

_{PV*}—photovoltaic, P

_{WT-DC*}—wind turbine in DC grid, P

_{B*}—battery, P

_{IC*}—interlinking converter, P

_{WT-AC*}—wind turbine in AC grid, P

_{FC*}—fuel cell, P

_{G*}—grid, P

_{DG*}—diesel generator.

Phase | IC as APF in HMG | IC for Power Exchange in HMG | ||||
---|---|---|---|---|---|---|

AC Sub-Grid Voltage | Grid Current | Load Current | AC Sub-Grid Voltage | Grid Current | Load Current | |

%THD | %THD | %THD | %THD | %THD | %THD | |

R_{ph} | 0.07 | 4.33 | 14.29 | 7.50% | 15.4 | 14.29 |

Y_{ph} | 0.07 | 4.64 | 15.77 | 8.30% | 16.2 | 15.77 |

B_{ph} | 0.07 | 4.24 | 13.29 | 8.50% | 15.9 | 13.29 |

Mode | Details of Load Connected | P_{PV*}(kW) | P_{WT-DC*}(kW) | P_{B*}(kW) | P_{IC*}(kW) | P_{WT-AC*}(kW) | P_{FC*}(kW) | P_{G*}(kW) | P_{DG*}(kW) |
---|---|---|---|---|---|---|---|---|---|

Mode 1 | DC sub-grid load—22.5 kW | 6.6 | 10 | 2.5 | 3.4 | - | - | - | |

Non-linear loads in AC sub grid—P = 72.5 kW | - | - | - | −3.4 | 10 | 26 | 40 | - | |

Non-linear loads in AC sub grid—Q = 11 kVAr | 12 | −1 | |||||||

Mode 2 | DC sub-grid critical load—18 kW | 6.6 | 10 | 1.4 | - | - | - | - | - |

Non-linear critical loads in AC sub-grid—P = 50 kW | - | - | - | - | 10 | 26 | - | 14 | |

Non-linear critical loads in AC sub-grid—Q = 2.5 kVAr | - | - | - | - | - | - | - | 2.5 | |

Mode 3 | DC sub-grid load—10 kW | 6.6 | 10 | −9 | 2.4 | - | - | - | |

Non-linear loads in AC sub grid—P = 72.5 kW | - | - | - | −2.4 | 10 | 26 | 38.9 | - | |

Mode 4 | DC sub-grid load—11 kW | 6.6 | 10 | - | −5.6 | - | - | - | - |

Non-linear loads in AC sub-grid—P = 72.5 kW | 5.6 | 10 | 26 | 30.9 | - |

_{PV*}—photovoltaic, P

_{WT-DC*}—wind turbine in DC grid, P

_{B*}—battery, P

_{IC*}—interlinking converter, P

_{WT-AC*}—wind turbine in AC grid, P

_{FC*}—fuel cell, P

_{G*}—grid, P

_{DG*}—diesel generator.

Mode | Details of Load Connected | P_{PV*}(kW) | P_{WT-DC*}(kW) | P_{B*}(kW) | P_{IC*}(kW) | P_{WT-AC*}(kW) | P_{FC*}(kW) | P_{G*}(kW) | P_{DG*}(kW) |
---|---|---|---|---|---|---|---|---|---|

Mode 1 | DC sub-grid load—27.5 kW | 6.6 | 10 | 7.1 | 3.8 | - | - | - | |

Non-linear loads in AC sub-grid—P = 72.5 kW | - | - | - | −3.8 | 10 | 26 | 40.3 | - | |

Non-linear loads in AC sub-grid—Q = 11 kVAr | 12 | −1 | |||||||

Mode 2 | DC sub-grid critical load—22 kW | 6.6 | 10 | 5.4 | - | - | - | - | - |

Non-linear critical loads in AC sub-grid—P = 50 kW | - | - | - | - | 10 | 26 | - | 14 | |

Non-linear critical loads in AC sub-grid—Q = 2.5 kVAr | - | - | - | - | - | - | - | 2.5 | |

Mode 3 | DC sub-grid load- 12.5 kW | 6.6 | 10 | −7 | 2.9 | - | - | - | |

Non-linear loads in AC sub-grid—P = 72.5 kW | - | - | - | −2.9 | 10 | 26 | 39.4 | - | |

Mode 4 | DC sub-grid load—12.5 kW | 6.6 | 10 | - | −4.1 | - | - | - | - |

Non-linear loads in AC sub-grid—P = 72.5 kW | 4.1 | 10 | 26 | 32.4 | - |

_{PV*}—photovoltaic, P

_{WT-DC*}—wind turbine in DC grid, P

_{B*}—battery, P

_{IC*}—interlinking converter, P

_{WT-AC*}—wind turbine in AC grid, P

_{FC*}—fuel cell, P

_{G*}—grid, P

_{DG*}—diesel generator.

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**MDPI and ACS Style**

Jayaram, J.; Srinivasan, M.; Prabaharan, N.; Senjyu, T.
Design of Decentralized Hybrid Microgrid Integrating Multiple Renewable Energy Sources with Power Quality Improvement. *Sustainability* **2022**, *14*, 7777.
https://doi.org/10.3390/su14137777

**AMA Style**

Jayaram J, Srinivasan M, Prabaharan N, Senjyu T.
Design of Decentralized Hybrid Microgrid Integrating Multiple Renewable Energy Sources with Power Quality Improvement. *Sustainability*. 2022; 14(13):7777.
https://doi.org/10.3390/su14137777

**Chicago/Turabian Style**

Jayaram, Jayachandran, Malathi Srinivasan, Natarajan Prabaharan, and Tomonobu Senjyu.
2022. "Design of Decentralized Hybrid Microgrid Integrating Multiple Renewable Energy Sources with Power Quality Improvement" *Sustainability* 14, no. 13: 7777.
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