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Bennia, I.; Elbouchikhi, E.; Harrag, A.; Daili, Y.; Saim, A.; Bouzid, A.E.M.; Kanouni, B. Grid-Forming Inverters. Encyclopedia. Available online: https://encyclopedia.pub/entry/53248 (accessed on 28 April 2024).
Bennia I, Elbouchikhi E, Harrag A, Daili Y, Saim A, Bouzid AEM, et al. Grid-Forming Inverters. Encyclopedia. Available at: https://encyclopedia.pub/entry/53248. Accessed April 28, 2024.
Bennia, Ilyas, Elhoussin Elbouchikhi, Abdelghani Harrag, Yacine Daili, Abdelhakim Saim, Allal El Moubarek Bouzid, Badreddine Kanouni. "Grid-Forming Inverters" Encyclopedia, https://encyclopedia.pub/entry/53248 (accessed April 28, 2024).
Bennia, I., Elbouchikhi, E., Harrag, A., Daili, Y., Saim, A., Bouzid, A.E.M., & Kanouni, B. (2023, December 28). Grid-Forming Inverters. In Encyclopedia. https://encyclopedia.pub/entry/53248
Bennia, Ilyas, et al. "Grid-Forming Inverters." Encyclopedia. Web. 28 December, 2023.
Grid-Forming Inverters
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This entry is adapted from the peer-reviewed paper 10.3390/en17010059

The electrical grid is predominantly characterized by synchronous generators (SGs). These generators provide a consistent voltage and frequency, facilitating the synchronization of VSIs and their connection at the point of common coupling (PCC) for injecting power into the main grid. These inverters operate as current sources and are known as grid-following inverters. The imperfections of grid-following inverters arise when the grid is absent, as this concept fails to produce instantaneous voltage and frequency set points. Therefore, this operating mode has been significantly modified, from following the grid to leading it. This concept is known as grid-forming (GFM) inverters, which can independently set up and keep the voltage and frequency within their nominal ranges without the need for the main grid.

grid-forming microgrid droop control decentralized control synchronization frequency

1. Introduction

The amount of renewable energy generation systems that are connected to the grid is continuously increasing. Consequently, the presence of power electronics devices in power systems has also increased. At present, the electrical grid is predominantly characterized by synchronous generators (SGs). These generators provide a consistent voltage and frequency, facilitating the synchronization of VSIs and their connection at the point of common coupling (PCC) for injecting power into the main grid. These inverters operate as current sources and are known as grid-following inverters. The imperfections of grid-following inverters arise when the grid is absent, as this concept fails to produce instantaneous voltage and frequency set points. Therefore, this operating mode has been significantly modified, from following the grid to leading it. This concept is known as grid-forming (GFM) inverters, which can independently set up and keep the voltage and frequency within their nominal ranges without the need for the main grid [1]. The block diagram of both concepts is depicted in Figure 1.
Energies 17 00059 g001
Figure 1. Power converter classification: (a) grid-forming and (b) grid-following inverters.
GFM inverter control consists of a voltage control path that regulates the output voltage to a specified voltage reference and a phase-angle reference path that integrates a predetermined frequency to obtain the phase-angle reference. These references (voltage and frequency) are commonly achieved through decentralized control such as droop control [2][3], virtual synchronous machines [4], virtual SG [5], synchronverter [6], power synchronization control [7], generalized virtual synchronous control [8], and virtual oscillator control [9]. Presently, the biggest challenges for decentralized control schemes are the seamless reconnection and black-start of GFM inverters [10].
The decentralized control for GFM inverters is generally composed of inner control loops for current and voltage, a virtual impedance loop, and a controller employing a droop mechanism [11]. The primary control objectives for GFM inverters involve stabilizing amplitudes and ensuring power sharing among them. The stabilization of frequency and voltage is typically accomplished through current and voltage control loops, while the droop control mechanism facilitates power sharing [12].

2. GFM Inverter Control Methods

The decentralized control design is very important for the GFM inverter’s performance. Various control approaches for GFM inverters are examined and compared with the GFM characteristics exhibited by synchronous machines in [13]. A hybrid grid-forming/following controller was proposed in [14]. Voltage synchronization is achieved by incorporating a phase-locked loop (PLL), and load sharing is facilitated through the utilization of a power-frequency droop. A droop-based GFM adaptive virtual resistance control was suggested in [15] for postfault oscillation mitigation in GFM inverters. In [16], authors investigated the use of impedance-based analysis to define, assess, and enhance the performance of GFM inverter controllers in various ways. In [17], a multivariable controller was proposed with the aim of decoupling P and Q loops in GFM inverters using droop control and other methods. A decentralized control approach for multiparallel GFM-distributed generators on island MG was developed in [18]. It offers the advantage of decoupling the frequency from load conditions. The authors in [19] explored GFM inverter modeling and control methodology, focusing on droop control. Embedding voltage and current loops within a single controller is a strategy that can significantly enhance the dynamic performance of the GFM system, as discussed in [20]. In MG’s with mismatched inductive/resistive feeder impedance, the virtual impedance loop, acting as an optional loop, plays a role in improving the power quality and power-sharing accuracy of GFM systems [21]. Drawbacks such as steady-state error and deviations in frequency and voltage amplitude are associated with the GFM droop control mechanism [22]. Consequently, a secondary control is introduced to mitigate these deviations [23]. Despite the deviations introduced by the total demand for active and reactive power from the loads, the GFM inverter system effectively restores both frequency and amplitude [24]. Additionally, a threshold-based method for frequency and voltage restoration in islanded MGs fed by droop-controlled GFM is proposed in [25]. It aims to restore the system when pulse load and plug-in events occur. A relevant solution for enhancing the efficiency and power quality of GFM inverters can be found in compact inverter structures, as demonstrated in [26].
The primary focus of the above studies is on the dynamics of GFM microgrids. This focus intensifies specifically after a disturbance in the system when GFM inverters are operated in parallel. Less attention has been given to the synchronization and reconnection of GFM inverters in MG or the synchronization of GFM microgrids to the main grid. The main difficulty is the lack of information about voltage, frequency, and phase angle at the PCC. Hence, the synchronization with other sources and proportional contribution to power-sharing in the (V-f) mode becomes challenging tasks for the incoming inverter. Likewise, when transitioning from islanded operation to grid-connected mode, the synchronization of the MG with the main grid is a necessary step. Thus, a synchronization control algorithm is necessary. A seamless connection to the MG/grid is essential for both GFM and grid-following inverters. A synchronization method for inverters in GFM, known as the controller-sync, is proposed in [27]. In this approach, the inverter initially aligns with the MG frequency without contributing to power sharing. Subsequently, the controller transitions to power-sharing mode. A communication-based method to synchronize the GFM microgrid to the utility grid is proposed in [28]. This approach is based on exchanging data between two proposed devices: synchronization data sender and a synchronization data controller.
In summary, the current state of research reveals certain gaps:
  • The comprehensive applicability of droop-based control in GFM inverters remains inadequately addressed.
  • The analysis of the decentralized control’s performance is lacking, particularly in the context of off-grid systems and varying PQ load demands.
Furthermore, when it comes to synchronization techniques, several gaps exist:
  • While many techniques address the connection of inverters to the utility grid, there is a lack of exploration regarding the (re)connection of an inverter to an islanded MG.
  • The majority of techniques are designed for inverters operating in (P-Q) control mode, leaving a gap in the understanding of inverters in (V-f) control mode post-(re)connection.
  • Certain methods require communication infrastructure, leading to cost implications for the seamless plug-and-play operation of inverters and exposing them to potential cyber-attacks.

References

  1. Gursoy, M.; Mirafzal, B. Direct vs. Indirect control schemes for grid-forming inverters–unveiling a performance comparison in a microgrid. IEEE Access 2023, 11, 75023–75036.
  2. Guerrero, J.M.; Vasquez, J.C.; Matas, J.; De Vicuña, L.G.; Castilla, M. Hierarchical control of droop-controlled AC and DC microgrids—A general approach toward standardization. IEEE Trans. Ind. Electron. 2010, 58, 158–172.
  3. Issa, W.; Al-Naemi, F.; Konstantopoulos, G.; Sharkh, S.; Abusara, M. Stability analysis and control of a microgrid against circulating power between parallel inverters. Energy Procedia 2019, 157, 1061–1070.
  4. Alsiraji, H.A.; Guerrero, J.M. A new hybrid virtual synchronous machine control structure combined with voltage source converters in islanded ac microgrids. Electr. Power Syst. Res. 2021, 193, 106976.
  5. Bevrani, H.; Ise, T.; Miura, Y. Virtual synchronous generators: A survey and new perspectives. Int. J. Electr. Power Energy Syst. 2014, 54, 244–254.
  6. Shuai, Z.; Hu, Y.; Peng, Y.; Tu, C.; Shen, Z.J. Dynamic stability analysis of synchronverter-dominated microgrid based on bifurcation theory. IEEE Trans. Ind. Electron. 2017, 64, 7467–7477.
  7. Wu, H.; Wang, X. Design-oriented transient stability analysis of grid-connected converters with power synchronization control. IEEE Trans. Ind. Electron. 2018, 66, 6473–6482.
  8. Rathnayake, D.B.; Razzaghi, R.; Bahrani, B. Generalized virtual synchronous generator control design for renewable power systems. IEEE Trans. Sustain. Energy 2022, 13, 1021–1036.
  9. Gurugubelli, V.; Ghosh, A.; Panda, A.K. Design and implementation of optimized virtual oscillatory controllers for grid-forming inverters. ISA Trans. 2023, 139, 685–712.
  10. Unruh, P.; Nuschke, M.; Strauß, P.; Welck, F. Overview on grid-forming inverter control methods. Energies 2020, 13, 2589.
  11. Rathnayake, D.B.; Akrami, M.; Phurailatpam, C.; Me, S.P.; Hadavi, S.; Jayasinghe, G.; Zabihi, S.; Bahrani, B. Grid forming inverter modeling, control, and applications. IEEE Access 2021, 9, 114781–114807.
  12. Guerrero, J.M.; Matas, J.; Garcia De Vicunagarcia De Vicuna, L.; Castilla, M.; Miret, J. Wireless-Control Strategy for Parallel Operation of Distributed-Generation Inverters. IEEE Trans. Ind. Electron. 2006, 53, 1461–1470.
  13. Anttila, S.; Döhler, J.S.; Oliveira, J.G.; Boström, C. Grid forming inverters: A review of the state of the art of key elements for microgrid operation. Energies 2022, 15, 5517.
  14. Geng, S.; Hiskens, I.A. Unified grid-forming/following inverter control. IEEE Open Access J. Power Energy 2022, 9, 489–500.
  15. Me, S.P.; Zabihi, S.; Blaabjerg, F.; Bahrani, B. Adaptive Virtual Resistance for Postfault Oscillation Damping in Grid-Forming Inverters. IEEE Trans. Power Electron. 2022, 37, 3813–3824.
  16. Ray, I. Review of impedance-based analysis methods applied to grid-forming inverters in inverter-dominated grids. Energies 2021, 14, 2686.
  17. Rathnayake, D.B.; Bahrani, B. Multivariable control design for grid-forming inverters with decoupled active and reactive power loops. IEEE Trans. Power Electron. 2022, 38, 1635–1649.
  18. Huang, X.; Wang, K.; Qiu, J.; Hang, L.; Li, G.; Wang, X. Decentralized control of multi-parallel grid-forming DGs in islanded microgrids for enhanced transient performance. IEEE Access 2019, 7, 17958–17968.
  19. Salem, Q.; Aljarrah, R.; Karimi, M.; Al-Quraan, A. Grid-Forming Inverter Control for Power Sharing in Microgrids Based on P/f and Q/V Droop Characteristics. Sustainability 2023, 15, 11712.
  20. Dragičević, T. Model predictive control of power converters for robust and fast operation of AC microgrids. IEEE Trans. Power Electron. 2017, 33, 6304–6317.
  21. Wang, X.; Li, Y.W.; Blaabjerg, F.; Loh, P.C. Virtual-impedance-based control for voltage-source and current-source converters. IEEE Trans. Power Electron. 2014, 30, 7019–7037.
  22. Bennia, I.; Daili, Y.; Harrag, A. Hierarchical Control of Paralleled Voltage Source Inverters in Islanded Single Phase Microgrids. In Proceedings of the International Conference in Artificial Intelligence in Renewable Energetic Systems, Tipaza, Algeria, 22–24 November 2020; Springer: Berlin/Heidelberg, Germany, 2020; pp. 302–313.
  23. Bekker, I.; Hofmann, L.; Mertens, A. Secondary control with grid-forming inverters for an island grid restoration approach without communication. Electr. Power Syst. Res. 2022, 213, 108498.
  24. Bennia, I.; Harrag, A.; Daili, Y.; Bouzid, A.; Guerrero, J.M. Decentralized secondary control for frequency regulation based on fuzzy logic control in islanded microgrid. Indones. J. Electr. Eng. Comput. Sci. 2023, 29, 85–100.
  25. Sadeque, F.; Mirafzal, B. Frequency restoration of grid-forming inverters in pulse load and plug-in events. IEEE J. Emerg. Sel. Top. Ind. Electron. 2022, 4, 580–588.
  26. Aouichak, I.; Jacques, S.; Bissey, S.; Reymond, C.; Besson, T.; Le Bunetel, J.-C. A bidirectional grid-connected DC–AC converter for autonomous and intelligent electricity storage in the residential sector. Energies 2022, 15, 1194.
  27. Sharma, D.; Sadeque, F.; Mirafzal, B. Synchronization of inverters in grid forming mode. IEEE Access 2022, 10, 41341–41351.
  28. Litwin, M.; Zieliński, D.; Gopakumar, K. Remote micro-grid synchronization without measurements at the point of common coupling. IEEE Access 2020, 8, 212753–212764.
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Subjects: Automation & Control Systems; Engineering, Electrical & Electronic
Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :
Ilyas Bennia
,
Elhoussin Elbouchikhi
,
Abdelghani Harrag
,
Yacine Daili
,
Abdelhakim Saim
,
Allal El Moubarek Bouzid
,
Badreddine Kanouni
View Times: 143
Revisions: 2 times (View History)
Update Date: 29 Dec 2023
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