# Control of Distributed Generators and Direct Harmonic Voltage Controlled Active Power Filters for Accurate Current Sharing and Power Quality Improvement in Islanded Microgrids

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## Abstract

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## 1. Introduction

_{pcc}while realizing power sharing directly through PCC voltage measurement only. Furthermore, the APF adopted here mainly contains an embedded voltage harmonic compensator that directly takes the PCC voltage and compensates the corresponding voltage harmonics on either of the DGs. Shared compensation can be achieved simultaneously.

- We develop the harmonic current sharing accuracy control strategy for the islanded microgrid, considering nonlinear unbalanced loads and with unequal feeder impedances.
- We improve the PCC voltage quality and successfully eliminate the harmonic distortion levels produced at the output terminals of the DGs due to the whack-a-mole effect in an islanded microgrid with the compensation of APF.
- We implement a power sharing strategy of active power and reactive power control using a voltage controlled APF at the PCC in an islanded microgrid which allows the circulating current to be reduced considerably.

## 2. System Configuration

## 3. Local Control of DG

#### 3.1. PR Inner Voltage and Current Loops

_{pω}and k

_{pE}are the proportional voltage and current controllers respectively.

#### 3.2. Selective Virtual Impedance Using DSC-SOGI in an Islanded Microgrid

#### 3.3. Harmonic Power Sharing Compensation Controller

_{circ}

_{,aj}corresponds to the zero sequence, I

_{circ}

_{,bj}represents the positive sequence and

_{circ}

_{,cj}shows the negative sequence circulating current for the jth inverter in the islanded microgrid, respectively.

_{iHC}[8,13]. After that, the corresponding compensated harmonic currents are passed through a proportional controller, and then fed into multiple P

^{+}resonant controllers for the sake of the supplementary voltage reference set point. Hence, the harmonic current sharing will be shared proportionally. Once the harmonic power sharing is achieved at this level, it is assumed to have better voltage levels at the DG terminals as well as at the point of common coupling [29]. However, if it does not meet the requirement and serious voltage distortions found at the DG terminals as well as at the PCC still exist, then the cooperation of APF with the DGs can be made in the secondary control, which can solve this problem. This is discussed in Section 4.

_{iHC}denotes the compensation gains and I

^{i}

_{actual}. DG shows the actual current of the ith DG and the overall matrix is shown in Equation (7).

#### 3.4. Voltage and Frequency Controller

^{+}and Q

^{+}) [17]. P*

^{+}and Q*

^{+}are considered as the fundamental positive sequence of active power and reactive power. V

_{ref}and ω

_{ref}define the voltage and frequency references, whereas ω

_{sec}and V

_{sec}represent the resulted output frequency and voltage, respectively.

_{p}

_{i}and d

_{q}

_{i}are the droop coefficients.

#### 3.5. Droop Control

_{pi}and d

_{qi}are the values of active and reactive power droop coefficients.

#### 3.6. Power Calculation

## 4. Design of the Proposed Voltage Controlled APF

#### 4.1. Active Damping Compensation Control Loop

_{d}, the stability of the system can be achieved. The output voltage v

_{ad}for the active damping loop in is given as follows:

_{d}represents the active damping compensation loop gain and i

_{c}is the capacitor current feedback of the inductive-capacitive-inductive (LCL) filter.

#### 4.2. Current Control Loop and DC Voltage Controller

_{p}of the PI controller can be adjusted based on the selected cross over frequency.

_{dc}and makes a comparison with the DC voltage reference V

_{dcref}to create the active current reference ∇

_{id}.

#### 4.3. Harmonic Voltage Control Loop

_{resn}denotes the gain of the resonant controller, h shows the harmonic order and φ is the phase angle chosen at the resonance frequency.

## 5. Proposed Control Strategy for the APF and DGs in an Islanded Microgrid

_{pcc}at the PCC even without adopting the load current.

_{h}defines the feedback and the compensation gain for controlling the harmonics and its values can be adjusted from −1 to ∞. The value of K and its polarity can be adjusted for harmonic compensation of the converter. Normally the gain K is chosen as KV

_{pcch}< 0 for harmonic compensation or KV

_{pcch}> 0 for harmonic rejection [18,20,36].

## 6. Simulation Results

**.**that proper harmonic current sharing is achieved with respect to the DG rated power. Consequently, the output waveform voltages of the DGs are prone to more distortion. However, the PCC voltage is improved considerably. As previously shown in Figure 7, as soon as the harmonic compensation is activated, a small improvement in PCC is observed at the expense of DG output voltages. It can be observed in Figure 8b that due to the harmonic current compensation as well as the PCC, DG1 voltage is severely distorted and at the same time, DG3 output voltage is not even reasonable. This shows that DG1 and DG3 play a leading role in the compensation of PCC voltage. As can be seen in Figure 8a, without implementing the harmonic current compensation, the active and reactive power is not shared due to unequal feeder impedances and nonlinear loads and there exists large circulating current. As soon as the harmonic current sharing is enabled in step two, enhanced current sharing among the DGs can be realized and also proper sharing of active and reactive power can be achieved as it revealed by the comparison of Figure 8a,b and Figure 9a,b. However, the THD of the PCC voltage is still not in the range of International Electrotechnical Commission (IEC) standards. The harmonic distortion at the output of DGs is high. Therefore, in order to improve the power quality, our proposed APF is incorporated with the DGs in step three, which not only improves the power quality V

_{pcc}at the point of common coupling (PCC), but also eliminates the voltage imbalance at the output of the DG terminal. Figure 10 illustrates the circulating current of each DG unit after step two and step three.

## 7. Conclusions

## Author Contributions

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**Diagram of the simulated test setup with three inverters and one active power filter (APF). DG = distributed generators; PCC = point of common coupling; LCL filter = inductive-capacitive-inductive filter; DC power source = direct current power source.

**Figure 6.**Voltage waveforms of DGs in various steps for the harmonic compensation from left to right. (

**a**) First step (without harmonic current compensation); (

**b**) second step (with harmonic current compensation); (

**c**) third step (enabling the APF).

**Figure 7.**Voltage waveforms of DGs in various steps for the harmonic compensation from left to right. (

**a**) First step (without harmonic current compensation); (

**b**) second step (with harmonic current compensation); (

**c**) third step (enabling the APF).

**Figure 8.**Current waveforms of DGs in various steps for the harmonic compensation from left to right. (

**a**) First step (without harmonic current compensation); (

**b**) second step (with harmonic current compensation); (

**c**) third step (enabling the APF).

**Figure 9.**Power sharing among the DGs. (

**a**) First step (without harmonic current compensation); (

**b**) second step (with harmonic current compensation); (

**c**) third step (enabling the APF). Where P1, P2, P3 is the active power sharing of DG1, DG2 and DG3 respectively. Similarly Q1, Q2, Q3 is the corresponding reactive power sharing of DG1, DG2 and DG3 respectively.

**Figure 11.**The frequency spectrum of PCC voltage and output DG voltages of multiple DGs under different steps with and without APF in an islanded microgrid. (

**a**) First step (without harmonic current compensation); (

**b**) second step (with harmonic current compensation); (

**c**) third step (enabling the APF).

System Parameters | Value |

Switching frequency | 10 kHz |

DC link | 650 V |

Nominal Voltage | V = 230 V |

DGs | |

DGs feeder parameters | Feeder inductance of DG1 L_{DG1} = 5mH,DG1 feeder resistance R _{DG1} = 0.25 Ω;Feeder inductance of DG2 L _{DG2} = 3mH,DG2 feeder resistance R _{DG2} = 0.2 Ω;Feeder inductance of DG3 L _{DG3} = 4mH,DG3 feeder resistance R _{DG3} = 0.25Ω. |

Load Parameters | Value |

Linear Load | R = 1 × 10^{−2} Ω, L = 10 × 10^{−3} H |

Nonlinear Load | L_{NL} = 300 × 10^{−6} H, C_{NL} = 150 × 10^{−6} F, R_{NL} = 20 Ω |

Unbalanced Load | R = 230 Ω |

APF | |

System Parameters | Value |

LCL filter (APF converter) | L_{1} = 1.8 mH, Parasitic resistance of L_{1} (R_{1} = 200 mΩ)L _{2} = 0.9 mH, Parasitic resistance of L_{2} (R_{2} = 0.4 mΩ),Filter Capacitor C _{f} = 9 µF |

Load Parameters | Value |

Nonlinear Load | DC Smoothing inductor, L_{L} = 1.8 mHDC Load, R _{L} = 150 Ω |

Voltage and Current Loops | Value |

K_{p}, K_{i}, ω_{hi}, ω | 0.175,0.75, 4.75, 100 pi |

K_{ih} | 250 (h = 1), 50 (h = 5), 40 (h = 7), 20 (h = 11, 13) and 10 (h = 17) |

Power Control Parameters | Value |

Real power and reactive power droop coefficients | d_{p1} = 0.0001/3 rad/s/W, d_{q1} = 0.0001/3 V/Var; d_{p2} = 0.0001/2 rad/s/W,d _{q2} = 0.0001/2 V/Var; d_{p3} = 0.0001 rad/s/W, d_{q3} = 0.0001 V/Var |

K_{pω,} K_{iω} | 0.8, 10 s^{−1} |

K_{pE}, K_{iE} | 0.8, 10 s^{−1} |

τ | 50 ms |

Virtual Harmonic Resistance | R^{-1}_{v, f} = 6 Ω, R^{5}_{v} = 1 Ω, R^{7}_{v} = 2 Ω, R^{11}_{v} = 4 Ω, R^{13}_{v} = 4 Ω, R^{17}_{v} = 4 Ω.L ^{−1}_{v,f} = 6 mH, L^{5}_{v} = 2 mH, L^{7}_{v} = 1.5 mH,L ^{11}_{v} = 1.5 mH, L^{13}_{v} = 1.5 mH, L^{17}_{v} = 1.5 mH. |

Test Steps | Total Harmonic Distortion (THD) % | |||
---|---|---|---|---|

Step 1 (without harmonic current compensation) | DG1 voltage | DG2 voltage | DG3 voltage | V_{pcc} voltage |

3.54% | 5.82% | 8.00% | 10.10% | |

DG1 current | DG2 current | DG3 current | ||

5.98% | 9.04% | 12.73% | ||

Step 2 (with harmonic current compensation) | DG1 voltage | DG2 voltage | DG3 voltage | V_{pcc} voltage |

12.25% | 6.43% | 8.16% | 8.30% | |

DG1 current | DG2 current | DG3 current | ||

10.41% | 10.53% | 10.56% | ||

Step 3 (enabling active power filter) | DG1 voltage | DG2 voltage | DG3 voltage | V_{pcc} voltage |

1.05% | 1.35% | 2.31% | 2.30% | |

DG1 current | DG2 current | DG3 current | ||

0.58% | 0.94% | 1.83% |

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## Share and Cite

**MDPI and ACS Style**

Munir, H.M.; Ghannam, R.; Li, H.; Younas, T.; Golilarz, N.A.; Hassan, M.; Siddique, A.
Control of Distributed Generators and Direct Harmonic Voltage Controlled Active Power Filters for Accurate Current Sharing and Power Quality Improvement in Islanded Microgrids. *Inventions* **2019**, *4*, 27.
https://doi.org/10.3390/inventions4020027

**AMA Style**

Munir HM, Ghannam R, Li H, Younas T, Golilarz NA, Hassan M, Siddique A.
Control of Distributed Generators and Direct Harmonic Voltage Controlled Active Power Filters for Accurate Current Sharing and Power Quality Improvement in Islanded Microgrids. *Inventions*. 2019; 4(2):27.
https://doi.org/10.3390/inventions4020027

**Chicago/Turabian Style**

Munir, Hafiz Mudassir, Rami Ghannam, Hong Li, Talha Younas, Noorbakhsh Amiri Golilarz, Mannan Hassan, and Abubakar Siddique.
2019. "Control of Distributed Generators and Direct Harmonic Voltage Controlled Active Power Filters for Accurate Current Sharing and Power Quality Improvement in Islanded Microgrids" *Inventions* 4, no. 2: 27.
https://doi.org/10.3390/inventions4020027