# Off-Grid System Configurations for Coordinated Control of Renewable Energy Sources

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## Abstract

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## 1. Introduction

## 2. Off-Grid Configurations

#### 2.1. Off-Grid Configurations Having Two DESs

#### 2.1.1. Off-Grid Configurations Having SPVA and DG

#### 2.1.2. Off-Grid Configurations Having DG and MHP

#### 2.1.3. Off-Grid Configurations Having DG and WT

#### 2.2. Off-Grid Configurations Having Three DESs

#### 2.2.1. Configurations Having SPVA, WT, and DG

#### 2.2.2. Configurations Having WT, MHP, and DG

#### 2.2.3. Configurations Having SPVA, DG, and MHP

#### 2.3. Off-Grid Configurations Having Four DESs

## 3. Control Approaches for Off-Grid Systems

#### 3.1. Control for Boost Converter of SPVA

_{out}) and input voltages (v

_{pv}), as well as, the inductor current (i

_{L}), are mandatory to obtain the control (d), which is equal to the sum of equivalent control (d

_{eq}) and switching control (d

_{s}). SPVA current (i

_{mpv}) is calculated using the classical P&O technique.

#### 3.2. Control for DC Dump Load

#### 3.3. Control Approach for DG Selector Switch

_{RESs}) is smaller than the load power demand (P

_{L}) as,

_{PDG}) and PCC voltage (V

_{p}) should be equal, and (2) difference between phase angles of the PCC voltage (θ) and DG voltage (θ

_{DG}) should be zero (Δθ = 0), where, θ and θ

_{DG}denote the phase angle of the PCC and generator terminal voltages, respectively.

#### 3.4. Control for Interfacing Inverter

_{(abc)1}or d

_{(abc)}

_{2}). However, if the generated power from DESs, which are connected to PCC, is zero ($\sum {P}_{WT}+{P}_{DG}+{P}_{MHP}=0$), the selector control (d1(

_{abc})), is obtained as shown in Figure 10.

_{s}is the angular frequency, described as,

_{s}is the system frequency.

_{abc}*) are compared with sensed PCC voltages (v

_{abc}), the errors are fed to AWPRC controller, the outputs of outer loops, which represent the output filter currents (i

_{C}(

_{abc})*), are added with load currents (i

_{L}(

_{abc})). The sum of output filter (i

_{inv}(

_{abc})) currents and load currents represent the reference interfacing inverter currents (i

_{inv}(

_{abc})*). The obtained currents are compared with the sensed output interfacing inverter currents (i

_{inv}(

_{abc})). The errors are fed to AWPRC and the output signals (d

_{1}(

_{abc})) are used to get the switching pulses. If ($\sum {P}_{WT}+{P}_{DG}+{P}_{MHP}\ne 0$), selector selects the second control (d

_{2}(

_{abc})). For this level of control, the value of the PCC voltage amplitude is calculated using (4) to compare it with its reference (V

_{p}*), and the error is fed to the AWPI controller. The output signal represents the reactive power required to maintain the amplitude of the PCC constant (Q

_{v}) during the transition. However, V

_{p}is calculated as,

_{abc}denotes the PCC phase voltage.

_{L}) from (Q

_{v}) as,

_{L}is calculated as,

_{α}, V

_{β}, i

_{Lα}, and i

_{Lβ}denote the PCC voltages and load currents in α-β transformation.

_{v}.

_{loss}) with P

_{L}as,

_{L}) is calculated as,

_{loss}.

_{sα}*, i

_{sβ}*), are transformed into three-phase currents (i

_{sa}*, i

_{sb}*, and i

_{sc}*), and compared with the sensed currents of DESs, which are connected to the PCC. The errors of currents are fed to AWPI and the output signals are used to obtain the gating pulses for interfacing inverter.

#### 3.5. Control for BES

## 4. Results and Discussion

#### 4.1. Performance of Off-Grid Configuration Based on SPVA, DG, and WT

_{s}≠ 0, where i

_{s}= (i

_{DG}+ i

_{WT}), the second level of control selected. One observes in Figure 13a that the AC voltage (v

_{L}) and system frequency (f

_{s}) are well regulated, and the reference output PV current (i

_{pv}) follows its reference (i

_{mpv}).

_{inv}) is increasing. This is because BES helps to balance the power in the system by providing the difference of power to the connected loads. One can see clearly that the AC voltage and system frequency are kept constant without any deviation, which confirms the robustness of the proposed control strategy based on AWPI controller for voltage and frequency regulation.

_{WT}) are sinusoidal and balanced, which confirms that tasks related to the power quality improvement at the PCC are achieved and the generators operate safely with their optimal performance.

#### 4.2. Performance of Off-Grid Configuration Based on SPVA and DG

_{L}), stator currents of SG (i

_{DG})m load currents (i

_{L}), output inverter current (i

_{inv}), SPVA current (i

_{pv}) and its reference current (i

_{pvref}), DC link voltage (V

_{dc}), SOC of the BES and the system frequency (f

_{s}), are demonstrated in Figure 16a. The objective of this test is to validate the performance of the MPPT, frequency, and voltage regulation at the PCC and the behavior of the AWPRC controllers during transitions. This test is conducted using a dynamic linear load and under solar irradiation. One observes that the SOC% is 50% ≤ SOC% ≤ 100% and i

_{pv}< i

_{L}that is why the BES is discharging. Based on these conditions, off-grid configuration operates in mode 2, for this reason, the DG is turned off (i

_{DG}= 0). One observes that the PCC voltage and the system frequency are regulated and the SPVA current follows it a reference, which is estimated using the enhanced P&O technique detailed in Figure 8. One sees under sudden decreasing of the SPVA current and increasing of load current, the voltage and frequency are maintained constant, which confirms that the AWPRC controllers perform well under steady-state and dynamic operations without any saturation issue. In this operating mode as well as in modes 2 and 4, DG loads are supplied from SPVA and BES, which allows to reduce significantly the excessive use of fossil fuel, as well as to increase the DG lifespan.

_{L}> P

_{PV}and SOC% ≤ 50% should be satisfied. Moreover, to connect DG to the PCC by switching on the STS, the phase’s shift of the stator terminals and that of PCC as well as the AC-voltage amplitude on both sides should be equal. One observes that all these conditions are satisfied. The obtained performance confirms that the selected enhanced locked loop performs well for these tests to achieve the desired tasks in the presence of disturbance and noise. Furthermore, to maintain constant and sinusoidal the AC voltage at the PCC, as well as, regulated frequency during transitions and in presence of nonlinear load, confirms the robustness of the second level of control and its AWPI controllers under the presence of severs conditions.

_{DGa1}) and after a transformer (v

_{DGa1}), excitation current (i

_{exc}), and stator current of phase “a” (i

_{DGa}), are shown in Figure 16a,b. Performances of the stator voltage of the phase “a” before (v

_{DGa1}) and after a transformer (v

_{DGa1}), as well as the stator current of phase “a” (i

_{DGa}) and “b” (i

_{DGb}), are shown. The objective of this test is to validate the steady-state performance of the second level of the control under the presence of linear load when off-grid configuration operates in mode 3. One observes that the system performs well, and the PCC voltage and the frequency are well regulated.

_{DGa}, v

_{DGb}, and v

_{DGc}), as well as the excitation current are shown. The performances of PCC (v

_{DGa}) and stator voltages (v

_{La}) of phase “a”, and DG (i

_{DGa}) and the load current (i

_{La}) are shown in Figure 18b. One observes in Figure 18b, that at t = 0.8 ms, conditions to synchronize DG with PCC are satisfied. At this time, conditions (Δθ = 0 and ΔV = 0) as demonstrated in Figure 9 are fulfilled, which allows the STS to switch on. One observes that transition is hard, but the voltage and frequency are kept constant, which confirms the robustness of the proposed control to achieve a transition to mode 3 without any saturation issue of the AWPI controllers and in the presence of noise.

_{dc}), BES current (i

_{bat}), load current (i

_{La}), and output inverter current of phase “a” are shown in Figure 20a, and the performances under decreasing of load at t = 300 ms and switched off of the phase a at t = 800 ms are shown in Figure 20b. One observes that charge and discharge of BES to balance the power in the system vary with the variation of the loads and the available power, which has provided from SPVA. This confirms the importance of the BES in this type of installation to maintain the system operation stable, ensuring an uninterruptible supply to the connected loads and compensating the intermittency of the SPVA.

_{inva}), load current (i

_{La}), DG current (i

_{DGa}), and DC link voltage (V

_{dc}) under the presence of nonlinear type RL, are presented in Figure 21a, and real-time performances under the presence of nonlinear type RC, are presented in Figure 21b. The performances under sudden increasing and decreasing of nonlinear type RL are presented in Figure 18c. The objectives of this test are to validate the proposed technique to improve the power quality at the PCC in the presence of nonlinear loads. One observes from these test results that the interfacing inverter acts as an active filter. It compensates harmonics and balances the DG currents. Moreover, the AWPI controllers perform well under sudden variation of loads without any saturation issue, which confirms the robustness of the developed control strategies for operating mode 3. This is proving that DG can operate in the presence of severe conditions with optimal performance.

## 5. Potential Applications

## 6. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## Abbreviations

DERs | Distributed energy resources |

NERS | Natural energy resources |

DGs | Diesel generators |

DESs | Distributed Energy sources |

BES | Battery energy storage |

SPVA | Solar photovoltaic arrays |

DC | Direct current |

AC | Alternative current |

PCC | Point of common coupling |

WT | Wind turbine |

MHP | Micro-hydropower |

MPPT | Maximum power point tracking |

P&O | Perturb and observe technique |

V/f | Control voltage/frequency |

SyRG | Synchronous reluctance generator |

PMSG | Permanent magnet synchronous generator |

SG | Synchronous generator |

SCIG | Squirrel cage induction generator |

DEs | Diesel engines |

AVR | Automatic voltage regulator |

STATCOM | Static compensator |

v_{out} | Boost converter output voltage, V |

v_{pv} | Boost converter input voltage, V |

i_{L} | Inductor current, A |

d | Control signal |

d_{eq} | Equivalent control signal |

i_{mpv} | Reference maximum current, A |

SOC% | State of charge of battery |

P_{L} | Load power demand, W |

P_{RESs} | Generated power from renewable energy sources, W |

V_{PDG} | Amplitudes of DG terminal voltage, V |

V_{p} | Amplitudes of PCC voltage, V |

θ | Phase angles of the PCC voltage, rad |

θ_{DG} | Phase angles of DG voltage, rad |

EPLL | Enhanced phased locked loop |

AWPRC | Proportional resonant with anti-windup feedback |

AWPI | Proportional integral with anti-windup feedback |

d1(abc) | Control signals generated by the first level of coordinated control |

d2(abc) | Control signal generated by the second level of coordinated control |

P_{WT} | Generated power from wind turbine, W |

P_{DG} | Generated power from diesel generator, W |

P_{MHP} | Generated power from micro-hydropower, W |

f_{s} | System frequency, Hz |

V_{abc}* | PCC reference voltages, V |

V_{abc} | Sensed PCC voltages, V |

i_{c(abc)}* | Reference RC output filter current, A |

i_{L}(_{abc}) | Sensed load current, A |

i_{inv}(_{abc}) | Sensed output inverter currents, A |

Q* | Reactive load reference power, Var |

Q_{L} | Reactive load power, Var |

V_{α}, V_{β} | PCC voltage in -β transformation |

i_{Lα,} i_{Lβ} | Load currents in -β transformation |

P* | Reference active load power, W |

P_{loss} | Active power loss, W |

P_{L} | Active load power, W |

P_{bat} | Battery power, W |

i_{sα}*, i_{sβ}* | Estimated source currents reference in α-β transformation |

i_{sa}*, i_{sb}*, and i_{sc}* | Estimated source currents in the natural reference frame |

i_{exc} | Excitation current in the synchronous generator, A |

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**Figure 1.**Off-grid configurations having SPVA and DE driven; (

**a**) SG, (

**b**) SyRG, (

**c**) SCIG, and (

**d**) PMSG.

**Figure 2.**Off-grid configurations having MHP driven SyRG and DE driven: (

**a**) SG, (

**b**) SyRG, (

**c**) SCIG, and (

**d**) PMSG.

**Figure 3.**Off-grid configurations having DE driven SG and WT driven: (

**a**) SG, (

**b**) SyRG, (

**c**) SCIG, and (

**d**) PMSG.

**Figure 4.**Off-grid configurations having SPVA, DE driven SG and WT driven: (

**a**) SG, (

**b**) SyRG, (

**c**) SCIG, and (

**d**) PMSG.

**Figure 5.**Off-grid configurations having MHP driven SyRG, DE driven SG and WT driven: (

**a**) SG, (

**b**) SyRG, (

**c**) SCIG, and (

**d**) PMSG.

**Figure 6.**Off-grid configurations having SPVA, MHP, DE driven SG, and MHP driven: (

**a**) SG, (

**b**) SyRG, (

**c**) SCIG, and (

**d**) PMSG.

**Figure 7.**Off-grid configurations having SPVA, MHP driven SyRG, DE driven SG, and WT driven: (

**a**) SG, (

**b**) SyRG, (

**c**) SCIG, and (

**d**) PMSG.

**Figure 13.**Dynamic performance off-grid configuration based on SPVA, WT, and DG under (

**a**) variation of wind speed with greater than 50%, (

**b**) balanced nonlinear load, (

**c**) unbalanced nonlinear load, and (

**d**) when SOC% equal to 100%.

**Figure 14.**Performance of WT-SPVA based off-grid system with (

**a**) decrease in solar irradiation, and (

**b**) increase in solar irradiation.

**Figure 16.**The dynamic performance of the PV-DG based off-grid configuration under loads and solar irradiation change with (

**a**) SOC of BES greater than 50%, and (

**b**) less than 50%.

**Figure 17.**Experimental results under the presence of linear load at (

**a**) excitation circuit and, (

**b**) PCC.

**Figure 19.**Test results of AWPRC for PCC voltage regulation under (

**a**) sudden load variation, and (

**b**) unbalanced linear load.

**Figure 20.**Experimental results under fixed solar irradiation and load change when SOC% of BES is greater than 50%: (

**a**) increasing and decreasing of linear load, (

**b**) decreasing of nonlinear load, and switched off the phase ‘a’.

**Figure 21.**Experimental results during the presence of (

**a**,

**b**) balanced nonlinear load, and (

**c**) sudden connected and disconnected of the load.

Modes | Conditions | DESs | SOC |
---|---|---|---|

Mode 1 (Without DG) | P_{L} < P_{RES}, 50% ≤ SOC ≤ 100% | WT, SPVA, BES | Charging the BES |

Mode 2 (Without DG) | P_{L} ≥ P_{RES}, 50% ≤ SOC ≤ 100% | WT, SPVA, BES | Discharging BES |

Mode 3 (With DG) | P_{L} ≤ P_{RES}, SOC ≤ 50% | WT, SPVA, BES, and DG | Charging the BES |

Mode 4 (Without DG) | P_{L} ≥ P_{RES}, SOC ≥ 100% | WT, SPVA, BES | Stop charging BES and turn on the dump load |

Modes | Conditions | DESs | SOC |
---|---|---|---|

Mode 1 (Without DG) | P_{L} < P_{pv}, 50% ≤ SOC ≤ 100% | SPVA, BES | Charging the BES |

Mode 2 (Without DG) | P_{L} ≥ P_{PV}_{,} 50% ≤ SOC ≤ 100% | SPVA, BES | Discharging BES |

Mode 3 (With DG) | P_{L} ≤ P_{PV}_{,} SOC ≤ 50% | SPVA, BES, and DG | Charging the BES |

Mode 4 (Without DG) | P_{L} ≥ P_{PV}_{,} SOC ≥ 100% | SPVA, BES | Stop charging BES and turn on the dump load |

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## Share and Cite

**MDPI and ACS Style**

Rezkallah, M.; Singh, S.; Chandra, A.; Singh, B.; Ibrahim, H.
Off-Grid System Configurations for Coordinated Control of Renewable Energy Sources. *Energies* **2020**, *13*, 4950.
https://doi.org/10.3390/en13184950

**AMA Style**

Rezkallah M, Singh S, Chandra A, Singh B, Ibrahim H.
Off-Grid System Configurations for Coordinated Control of Renewable Energy Sources. *Energies*. 2020; 13(18):4950.
https://doi.org/10.3390/en13184950

**Chicago/Turabian Style**

Rezkallah, Miloud, Sanjeev Singh, Ambrish Chandra, Bhim Singh, and Hussein Ibrahim.
2020. "Off-Grid System Configurations for Coordinated Control of Renewable Energy Sources" *Energies* 13, no. 18: 4950.
https://doi.org/10.3390/en13184950