0040_14.Seamless Switching Method Between Grid-Following and Grid-Forming Control for Renewable Energy Conversion Systems

摘要：IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 61, NO. 1, JANUARY/FEBRUARY 2025 ## Seamless Switching Method Between Grid-Following and Grid-Forming ## Control for Renewable Energy Conversion Systems ###, Dao Zhou ### Xian Gao, Member, IEEE ###, Senior Member, IEEE, Amjad Anvari-Moghaddam*, Senior

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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 61, NO. 1, JANUARY/FEBRUARY 2025

Seamless Switching Method Between Grid-Following and Grid-Forming

Control for Renewable Energy Conversion Systems

###, Dao Zhou

Xian Gao, Member, IEEE

###, Senior Member, IEEE,

Amjad Anvari-Moghaddam*, Senior Member, IEEE*, and Frede Blaabjerg*, Fellow, IEEE*

Abstract—In alignment with decarbonization efforts, there has been widespread global interest in renewable energy sources such

as wind and solar, which are connected to the grid via grid- connected inverters. The transition from traditional synchronous

generator-based power systems to power-electronic-based power systems has introduced increased complexity due to the stochas-

tic and intermittent nature of renewable energy outputs. Conse- quently, grid-connected inverters need to dynamically adapt their

control strategies to cope with varying external grid conditions and ensure high reliability. However, such transitions can cause abrupt

changes in the control loops (e.g., power loop or voltage loop), and lead to voltage and current distortions, potentially compro-

mising safe operation. To address this issue, this paper proposes a smooth switching method between the grid-following (GFL)

and grid-forming (GFM) control in grid-connected mode. This method can improve the control flexibility of the grid-connected

converters and broaden the stability boundary of the power system. The proposed method is verified in a case study of a 15.8 kVA grid-connected converter. Time-domain simulations carried out in

Matlab/Simulink and an established experimental prototype are applied to verify the effectiveness of the proposed control method.

The results demonstrate that the proposed control method effec- tively mitigates voltage and current distortions during transitions,

ensuring safer and more reliable operation.

Index Terms—Renewable energy sources, grid-connected inverters, seamless switching method, grid-following (GFL)

control, grid-forming (GFM) control.

energy into power grids is increasing year by year. The dis- tributed energy is usually connected to the power grid through power electronic inverters. They have two common control modes of the inverter [3]. One is the grid-following (GFL) control, regulating the active and reactive power injected into the power grid with a fast response but providing almost no moment of inertia for the system, and utilizing a phase-locked loop (PLL) for synchronization, which cannot operate in a stand-alone mode [4]. The other one is the grid-forming (GFM) control, regulating the frequency and voltages of the inverter and providing inertia and damping for the power system, which enables it to operate both in the grid-tied mode and the stand-alone mode [5]. Based on the prior art studies [6], it has been revealed that the GFL converter suffers from instability in power grids with low short-circuit ratios (SCR), while the GFM converter suf- fers from instability in power grids with high SCR [7], [8].It indicates that the GFL converter can be more suitable for the stiff power grid while the GFM converter is more suitable for the weak power grid. Considering the different performances of the GFL and GFM converters operation in the power grids with various SCRs, some coordination technologies are pro- posed. A secondary control scheme that coordinates the GFL and GFM converters for restoring frequency and voltage in a microgrid with 100% inverter-based generation is proposed in [9]. It adopts a leader-follower consensus framework, with GFL inverters acting as followers and GFM inverters acting as leaders. In [10], an optimal placement strategy of the GFM converters is proposed to enhance the small-signal stability of PLL-integrated power grids. However, they just consider the placement and power sharing among the GFL and GFM converters, neglecting the seamless transitions between them, which lack the control flexibility. In order to make full use of the GFL and GFM converters, numerous methods have been proposed for the three-phase in- verters to realize a smooth transition between the GFL and GFM control [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24].In[13], two semi-parallel control paths are proposed for the GFL and GFM modes respectively. The two control paths remain synchronized throughout the operation of the inverter to realize smooth switching. It is worth noting

that during the synchronization process, the inverters remain connected to the system without any power injection, named

I. INTRODUCTION ITH the intensification of the global energy crisis and en-

Wvironmental problems, renewable energy sources have

been developed vigorously [1], [2]. The integration of distributed

Received 29 November 2023; revised 21 March 2024 and 28 July 2024; accepted 19 August 2024. Date of publication 1 October 2024; date of current version 31 January 2025. Paper 2023-SECSC-1808.R2, presented at the 2023 11th International Conference on Power Electronics and ECCE Asia, Jeju Island, Republic of Korea, May 22–25, and approved for publication in the IEEE Transactions on Industry Applications by the Renewable and Sustainable Energy Conversion Systems Committee of the IEEE Industry Applications Society [DOI: 10.23919/ICPE2023-ECCEAsia54778.2023.10213821]. (Corre- sponding author: Xian Gao.) Xian Gao is with the College of Information Science and Technology & College of Artificial Intelligence, Nanjing Forestry University, Nanjing, China. Color versions of one or more figures in this article are available at

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Fig. 1. Typical configuration of a grid-connected voltage source converter.

standby mode. Following the completion of synchronization in standby mode, the inverter can change to its desired mode of operation. Authors of [16] introduce a method for the seamless restoration of power to critical infrastructure. Under grid-tied mode, the inverters don’t generate any active and reactive power, leaving the entire load to be supported by the grid, working in a standby mode. The primary objective is just to ensure the secure operation of critical infrastructures, with the GFM inverters functioning akin to a backup. A unified control method of the grid-connected inverters for a smooth transfer to stand-alone mode regardless of whether the system is exporting power to or importing power from the grid is proposed in [18]. The proposed scheme is designed to avoid any modifications in the control loops and the need to impose initial values on the compensators during the mode transfer. In [20], a seamless switching con- trol strategy based on model prediction is proposed to realize maximum power tracking under grid-tied mode and voltage regulation under islanded mode. An autonomous control strategy of inverters realizing smooth switching of sudden islanding and reconnection without the requirement of communication is pro- posed in [21] and [22].In[23], a compensation loop is designed and added to the excitation loop to realize the seamless transition without any external signals from the detection scheme of the islanding. A multifunctional converter is proposed in [24], which can operate either as a voltage- or current-controlled source and realize a smooth transition between the two operating modes. However, the majority of these research works focus on the transitions between the grid-tied mode and the stand-alone mode, where the GFL control is applied in the grid-tied mode, while the GFM control is applied in the stand-alone mode. Moreover, they do not allow any power exchange between the converters and the power grids during the transition. Thus, there is still a scarcity of papers addressing transitions specifically between the GFL and GFM converters within the grid-tied mode, which can improve the control flexibility of the grid-connected converters. In addition, during the transition, the inverters still can be allowed to inject non-zero power into the power grid. Notably, a weak power grid poses challenges to the PLL syn- chronization and will also adversely affect the stable operation of GFL inverters [25], [26]. The GFL inverters possess a weaker power transfer ability under a weak power grid [27]. Instability problems may occur when the generation capacity from renew- able energy sources is substantial, such as in scenarios involving

the stability of the power system under varying working con- ditions. During the grid-connected mode, transitions between the GFL and GFM control are indispensable for broadening the stability boundary of the power system. Furthermore, the seamless transitions in the situations in which the inverters inject non-zero power into the power grid also need to be considered. To achieve optimal performance across diverse operational conditions and leverage the benefits of both the GFL control and GFM control, this paper aims to introduce a straightforward switching control strategy, facilitating seamless transitions be- tween the GFL and GFM control. The main contributions can be summarized as follows. 1) Comprehensive illustrations of the mathematical models of a typical three-phase grid-connected system are given. 2) Each control loop of both the GFL and GFM control is discussed in detail. 3) A straightforward seamless switching method is presented to realize a smooth transition between the GFL and GFM control under the grid-connected mode no matter whether there is any power transfer between the converter and the power grid. The rest of this paper is organized as follows. Section II gives the modeling of a three-phase grid-connected voltage source converter. In Section III, each control loop of both the GFL and GFM control is presented in detail, and the seamless switching method is proposed. In Section IV, a time-domain simulation model is built in MATLAB/Simulink to verify the effectiveness of the proposed seamless switching method. Section V provides an experimental validation. Finally, conclusions are drawn in Section VI.

f f g dc a b c pcca, upccband upccc ga gb gc a b care the converter output ga gb gc Ca Cb Cc

GAO et al.: SEAMLESS SWITCHING METHOD BETWEEN GRID-FOLLOWING AND GRID-FORMING CONTROL

Fig. 2. Phase relationship between the PCC and power grid voltages.

Defining δ as the power angle, which is the phase angle difference between the PCC voltage vector Upccδ and the grid voltage vector Ug0. α represents the angle of the grid impedance. The phase relationship between the PCC and the power grid is shown in Fig. 2. The active power p and the reactive power q flowing from the PCC to the power grid can be given as: { ( 2 ) p = 3Upcccosα − 3UpccUgcos (α + δ) /Zg ( 2

) (1) q = 3Upccsinα − 3UpccUgsin (α + δ) /Zg

In this paper, the synchronizing frame is defined by ω, which is synchronized to the voltage phase angle at the PCC. According to Kirchhoff’s voltage law, the mathematical model of the main circuit in the ω-defined rotating d-q frame can be achieved as [30], [31]:

digd upccd− ugcosδ = Lg+ Rgigd− ωLgigq(2) dt digq upccq+ ugsinδ = Lg+ Rgigq+ ωLgigd(3) dt dupccd i d− igd= Cf− ωCfupccq(4) dt dupccq i q− igq= Cf+ ωCfupccd(5) dt did ud− upccd= Lf− ωLfiq(6) dt diq uq− upccq= Lf+ ωLfid(7) dt where subscripts d and q represent the d-axis and q-axis com- ponents of a variable, respectively. Under the d-q frame, the expression of the output active power and reactive power can be given as: { p = 1*.*5 (upccdigd+ upccqigq)

(8) q = 1*.5 (−upccdigq*+ upccqigd)

III. SMOOTH SWITCHING METHOD

In this paper, the active and reactive power control (PQ control) and the virtual synchronous generator control (VSG control) are adopted as the GFL control and the GFM control, respectively. The proposed control schemes of the proposed smooth switching method are shown in Fig. 3. The control system consists of a grid synchronization loop, a power loop, an excitation loop, a voltage loop and a current loop. The grid synchronization loop is composed of two parts:

the phase-locked loop (PLL) for the GFL control and the power synchronization loop for the GFM control. The control system is performed under the control synchronizing frame (defined by the grid synchronization loop), while the electrical system is performed under the actual system synchronizing frame (de- fined by the PCC voltage) [32]. While the two synchronizing frames align during the steady-state operation, a minor differ- ence arises during the dynamic state. To improve the model accuracy, this discrepancy is considered, and variables within the control synchronizing frame are denoted with a superscript c, while variables within the actual system synchronizing frame are denoted with a superscript s.

A. Grid-Following Control The GFL control consists of the PLL unit, a power loop and a current loop. The GFL control adopts a PLL unit to enable the inverter synchronized to the power grid. The outer power control loop regulates the active and reactive power injected into the power grid. The outer power loop generates references for the current loop, denoted as i ∗ GFLdand i ∗ GFLq, respectively. The outputs of the power loop can be given as follows: { i ∗ =(k + k /s)(P − P) GFLd pPQ iPQ ref e ∗

(9) i = − (kpPQ+ kiPQ/s)(Qref− Qe) GFMq

where kpPQand kiPQare the proportional and integral coeffi- cients of the power controller. To track the references set by the outer power loop, the inner current loop is adopted to adjust the converter currents. The outputs of the current loop can be given as follows: { c ∗ c c c ud=(kpc+ kic/s)(iGFLd− id) − ωgLfiq+ upccd () u c =(k + k /s) i ∗ − i c + ω L i c + u c q pc ic GFMq q g f d pccq (10) where kpcand kicare the proportional and integral coefficients of the current controller;ωgpresents the grid angular frequency.

B. Grid-Forming Control The GFM control is composed of a power synchronization loop, an excitation loop, a voltage loop and a current loop. Unlike the GFL control, the GFM control does not need a PLL unit for synchronization. Instead, it emulates the power synchronization characteristics of conventional synchronous generators, repre- sented by the swing equation: { dω P J vsg = ref − Pe − D(ωvsg− ωg) dt ωvsgωvsg (11) dθvsg/dt = ωvsg

where J denotes the moment of inertia; D denotes the damping coefficient; ωVSGdenotes the angular frequency of the VSG control. The excitation loop adopts the droop control using an inte- grator, which is also called droop-I control [33]. The excitation loop can be given as follows: ∫ Em= E₀ + kq(ku(UN − upcc)+Qref− Qe) (12)

Fig. 3. Control schemes of the smooth switching method between grid-following and grid-forming control.

where kqis an integral gain; kuis the voltage droop coefficient; E₀ is the no-load electromotive force of the converter; UN is the peak value of the rated grid voltage. The voltage loop regulates the PCC voltages to track the references set by the excitation loop, which can be given as follows: ⎧ () ⎨ c c c idref=(kpu+ kiu/s) upccdref− upccd− ω Cg fupccq ) + ⎩ c c c iqref=(kpu+ kiu/s) (0 − upccqω Cg fupccd (13) where kpuand kiuare the proportional and integral coefficients of the voltage controller. The current loop of the GFM control is almost the same as that of the GFL control. The only difference is that the references of the current loop under the GFM control are determined by the voltage control, while the references of the current loop under the GFL control are determined by the outer power control. Thus, it will not be described in details here. The references of the ∗GFMd Fig. 4. Flow chart of the proposed seamless switching method. current loop under the GFM control are defined as i and ∗ iGFMq.

C. Smooth Control Switching Method Between the energy conversion system may constantly encounter changes in Grid-Forming and Grid-Following Control operational scenarios, characterized by fluctuations in renewable In the case that both the GFL and GFM control schemes have energy sources generation, which leads to time-varying system the same inner current loop, only the outer loops should be stability. Consequently, a consensus has been drawn by both the regulated during the switching period. To ensure this smooth industry and academia that it is important to conduct real-time transition, it is crucial to maintain consistent references for monitoring of system strength within renewable energy con- the inner current loop of both control modes. In addition, the version systems [35]. A generalized SCR (gSCR) is proposed steady-state operation points before and after the transition to assess the stability of multi-infeed power electronic systems should remain unchanged [34]. The flow chart of the proposed [36]. Based on this, a distributed power method developed to seamless switching method is shown in Fig. 4. The switch signal calculate the gSCR in real time is proposed in [37], which can is given by the extra controller, which depends on the exter-be an efficient tool to identify the strength of the external grid nal grid strength. In practical implementation, the renewable and determine the switch signal.

TABLE I PARAMETERS OF A GRID-CONNECTED CONVERTER

= 0(14)

Because the change occurs under the grid-connected mode, when the Preffor both the GFL and GFM controls is set to the same value, the output of the power synchronization loop ωvsgis the same as that of the PLL unit ωpllduring the steady- state. Thus, it is possible to realize the smooth switching in the grid synchronization part. During the transition from the GFM control to the GFL control, the grid synchronization part should be changed from the power synchronization loop to the PLL unit. To achieve this, the output of the integrator in the power synchronization θvsg0should be set as the initial value of the integrator in the PLL unit during the transition, and the switch in the grid synchronization part is required to change from position ‘2’ to ‘1’. Conversely, when switching from the GFL control to the GFM control, the grid synchronization part should be altered from the PLL unit to the power synchronization loop. The output of the integrator in the PLL unit θpll0should be set as the initial value of the integrator in the power synchronization during the transition, and the switch in the grid synchronization part is required to change from position ‘1’ to ‘2’. To ensure consistent steady-state operation points before and after the transition from the GFM control to the GFL control, Prefand Qrefof the GFL control should be set as the same values asPe0andQe0of the GFM control under the steady-state operation. Because the converter is connected to the power grid, the frequency is equal to ωg. Hence, Prefand Pe0are the same. However, for theQe, due to the regulation of the excitation loop, there is a droop relationship between the reactive power and the voltages at the PCC during the steady-state operation, which can be expressed as:

ku(UN − Upcc)+Qref− Qe

It can be observed that when the PCC voltage deviates from the rated grid voltage UN,theQeunder the GFM control will not track the Qref, which may result in differences in the steady- state operation points before and after the transition. To address this issue, it is necessary to calculate and set the steady-state value Qe0of the GFM control as Qreffor the GFL control. By combining (1)–(8) and (14), the steady-state value of the output reactive powerQe0can be calculated. During the transition from the GFM control to the GFL control, the switch in the power control loop should be in position ‘3’ to ensure consistency in the operation point before and after the transition. The switch in the current control part is needed to switch from position ‘2’ to ‘1’. Once the converter can effectively track Prefand Qref,the switch in the power control loop can switch to position ‘4’ to regulate the Qetracking the Qref. Conversely, when changing from the GFL control to the GFM control, in order to ensure the same steady-state operation points before and after the transition, upccdrefand upccqrefof the GFM control should be set as the same values as Upccd0and Upccq0of the GFL control under steady-state operation. Notably, during the steady-state operation, the upccqrefand Upccq0are both equal to 0, so only upccdrefand Upccd0need to be considered. By combining (1)–(8) and assuming Peand Qecan well track the references Prefand Qrefwithout any steady state error, the Upccd0can be calculated. During the transition from the GFL control to the GFM control, the switch in the current control

part should switch from position ‘1’ to ‘2’ and the excitation loop is not activated where the switch should be in position ‘5’. After the switching, when the converter enters into steady-state operation, the excitation loop is enabled, and the switch should be in position ‘6’.

IV. SIMULATION MODEL AND RESULTS

In order to verify the effectiveness of the proposed smooth switching method, a case study system is established in MAT- LAB/Simulink [1]. The key parameters of the case study are listed in Table I [38]. Moreover, the proposed smooth switching method is applied in both the strong and weak power grids. The SCR is set as 10 and 1.5 for the study. When the converter changes from the GFM control to the GFL control, the steady-state value of the reactive power Qe0under the GFM control needs to be calculated. The output reactive power of the GFM control has a droop relationship with the PCC voltage, as expressed in (14). Because of the regulation of the voltage control loop, the steady-state value of the q-axis component of the PCC voltage Upccq0is 0, and the steady-state value of the PCC voltage Upcc0is equal to its d-axis component

Fig. 5. Simulation results of switching between grid-forming and grid-following control without and with proposed smooth switching control. (a) PCC voltages;

(b) converter output currents (SCR = 10). TABLE II THD OF VOLTAGE AND CURRENT WITH AND WITHOUT PROPOSED METHOD Upccd0. Based on the analysis above and the parameters shown in Table I, setting all the differential terms as 0 in (2)–(7) and combining (1), (8) and (14), the steady-state value of the reactive power Qe0can be calculated. In the case of SCR is 10, the Qe0 is *−*1.3 Var, and in the case of SCR is 1.5, the Qe0is −6Var. Similarly, when the converter changes from the GFL control to the GFM control, the steady-state value of the d-axis component and the q-axis component of the PCC voltage Upccd0and Upccq0 under the GFL control need to be calculated, respectively. In the case of the GFL converter, because of the regulation of the power control loop, the output active power Peand reactive power Qe are equal to Prefand Qref. Furthermore, due to the presence of the PLL unit, Upccq0is 0. By considering the parameters shown in Table I, setting all the differential terms as 0 in (2)–(7) and combining (1) and (8), the steady-state value of the d-axis component of the PCC voltage Upccd0can be calculated. In the case of SCR is 10, the Upccd0is 313 V., and in the case of SCR is 1.5, the Upccd0is 310 V. When the power grid is strong and the SCR is 10, the simu- lation results of switching between the GFM and GFL control without and with the proposed smooth switching control are shown in Fig. 5. The control mode changes from the GFM to GFL control at t = 1 s and changes from the GFL to GFM control at t = 1.5 s. The reference of reactive power control changes fromQe0to 0 at t = 1.04 s, and the excitation loop is activated at t = 1.54 s. Both the PCC voltage and converter current have some oscillations during the switching period without the proposed smooth switching control. However, with the smooth switching control, during the switching time, the oscillations of both the PCC voltages and converter currents are effectively reduced. In addition, the total harmonic distortions (THD) are also reduced. The THD of the PCC voltage and converter current without and

with the proposed seamless switching method is summarized in Table II. Similarly, the same comparisons are also adopted in the weak power grid. When the SCR is 1.5, the simulation results of switching between the GFM and GFL control without and with the proposed smooth switching control are shown in Fig. 6. During the switching period, the PCC voltages and converter currents undergo oscillations without the smooth switching con- trol. However, when the proposed method is adopted, both the PCC voltages and converter currents have realized a smooth switching. Therefore, through the simulation results, it is evident that the proposed straightforward strategy can realize seamless tran- sitions between the GFL and GFM control regardless of whether the power grid is strong or weak.

V. E XPERIMENTAL VALIDATION To validate the efficiency of the proposed seamless switching method, a three-phase grid-connected system setup is used as

Fig. 6. Simulation results of switching between grid-forming and grid-following control without and with proposed smooth switching control. (a) PCC voltages;

(b) converter output currents (SCR = 1.5). Fig. 7. Experimental setup of a three-phase grid-connected system. illustrated in Fig. 7. The parameters of the experimental setup are the same as those specified in Table I in Section IV.The three-phase grid-connected converter is based on the Imperix standard PEB-SiC-8024 module. The power grid is simulated by three high-fidelity linear amplifiers APS 15000. The con- verter currents and grid currents are measured by the LEM LAH50-P current sensors, while the PCC voltages are measured by the LEM LV20-P voltage sensors. All the measured data are transmitted to the B-BOX RCP control platform. The control algorithm is coded in a personal computer and loaded to the B-BOX RCP control platform via a patch cable. The real-time monitoring and the adjustment of control variables are carried out using the Imperix cockpit. Fig. 8. Experimental waveforms of PCC voltages and converter currents

changing from grid-forming to grid-following without smooth switching control (SCR = 1.5).

When the power grid is weak and the SCR is 1.5, the experi- mental results are as shown in Figs. 8–11. During the transition from the GFM to GFL control, the experimental waveforms of the PCC voltages and converter currents without the smooth switching control are shown in Fig. 8. During the switching time, the converter outputs have large oscillation, which leads to a shutdown of the converter. When the smooth switching control is applied, the experimental waveforms of PCC voltages and converter currents are shown in Fig. 9. In this case, both the PCC voltages and converter currents have realized a smooth transition. It is worth mentioning that when the SCR is 1.5, the waveforms of the currents are distorted under the GFL control which indicates that the GFL converter is not suitable for a very weak power grid. During the transition from the GFL to GFM control, the experimental waveforms of PCC voltages and converter currents

Fig. 12. Experimental waveforms of PCC voltages and converter currents

Fig. 9. Experimental waveforms of PCC voltages and converter currents

changing from grid-forming to grid-following without smooth switching control changing from grid-forming to grid-following with smooth switching control (SCR = 10). (SCR = 1.5).

Fig. 13. Experimental waveforms of PCC voltages and converter currents

Fig. 10. Experimental waveforms of PCC voltages and converter currents

changing from grid-forming to grid-following with smooth switching control changing from grid-following to grid-forming without smooth switching control (SCR = 10). (SCR = 1.5).

Fig. 14. Experimental waveforms of PCC voltages and converter currents

Fig. 11. Experimental waveforms of PCC voltages and converter currents

changing from grid-following to grid-forming without smooth switching control changing from grid-following to grid-forming with smooth switching control (SCR = 10). (SCR = 1.5).

without a smooth switching control are shown in Fig. 10.The experimental waveforms of PCC voltages and converter currents with the smooth switching control are shown in Fig. 11. Without the proposed method, the large oscillations during the transition leads to the trigger of the hardware protection. With the proposed control, it is clear that the proposed smooth switching control works well and gives obviously a much smoother transition between the GFL control and GFM control.

Similarly, the same experiments are carried out in a strong power grid. When the SCR is 10, the experimental results are shown in Figs. 12–15. When the control mode changes from the GFM to GFL control, the experimental waveforms of the PCC voltages and converter currents without and with the smooth switching control are shown in Figs. 12 and 13. When the control mode changes from the GFL to GFM control, the experimental waveforms of the PCC voltages and converter currents without

Fig. 15. Experimental waveforms of PCC voltages and converter currents

changing from grid-following to grid-forming with smooth switching control (SCR = 10).

and with the smooth switching control are shown in Figs. 14 and 15. It is evident that the implementation of the proposed smooth switching control effectively ensures the safe operation, and a smooth transition between the GFL and GFM control.

VI. CONCLUSION

To cope with the complex working conditions introduced by the stochastic and intermittent nature of renewable energy sources, grid-connected inverters need to dynamically adapt their control strategies to cope with varying external grid con- ditions. The GFL and GFM control are suitable for different levels of grid strength. Therefore, a collective control design can be implemented to optimize performance based on external grid conditions, thus achieving a unified control structure for grid-connected inverters. This paper proposes a straightforward smooth switching method to facilitate seamless transitions be- tween the GFL and GFM control in the grid-connected mode. The key to achieving seamless switching is to maintain consis- tent operation points before and after the transition. The proposed seamless switching method leverages the ad- vantages of both the GFL and GFM converters, enhancing the control flexibility of grid-connected converters and broadening the stability boundaries of power grids. In addition, the method allows inverters to inject non-zero power into the grid during transitions, ensuring a consistent power supply. Simulation and experimental results have verified the effectiveness of the pro- posed method, demonstrating its ability to mitigate voltage and current distortions during transitions. This ensures safer and more reliable operation of grid-connected inverters. Although the validation of the proposed method demonstrates its potential for industrial application, further research is re- quired to fully assess its wider applications and limitations. Furthermore, given the successful implementation of seamless transitions between the GFL and GFM control for a single grid-connected inverter, future research could focus on coor- dinating multiple grid-connected inverters within large-scale power systems. Ensuring effective collaboration among these inverters is crucial for enhancing the reliability and performance of power-electronic-based power systems in an increasingly renewable energy-dominated grid.

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Dao Zhou (Senior Member, IEEE) received the B.S. degree in electrical engineering from Beijing Jiaotong University, Beijing, China, in 2007, the M. S. degree in electrical engineering from Zhejiang University, Hangzhou, China, in 2010, and the Ph.D. degree in electrical engineering from Aalborg University, Aal- borg, Denmark, in 2014. Since 2014, he has been with the Department of Energy, Aalborg University, where he is currently an Associate Professor. His research interests include modeling, control, and reliability of power electronics in renewable energy applications. He serves as an Associate Editor for IEEE TRANSACTIONS ON INDUSTRY AP- PLICATIONS. He was the recipient of few IEEE prize paper awards.

Amjad Anvari-Moghaddam (Senior Member, IEEE) is an Associate Professor and Leader of Intelli- gent Energy Systems and Flexible Markets (iGRIDS) Research Group with the Department of Energy (AAU Energy), Aalborg University where he is also acting as the Vice-Leader of Power Electronic Con- trol, Reliability and System Optimization (PESYS) and the coordinator of Integrated Energy Systems Laboratory (IES-Lab). His research interests include planning, control and operation management of mi- crogrids, renewable/hybrid power systems and inte- grated energy systems with appropriate market mechanisms. He has coauthored more than 300 technical articles, eight books and 19 book chapters in the field. Dr. Anvari-Moghaddam is the Editor-in-Chief of Academia Green Energy journal and is the Associate Editor for several leading journals, such as IEEE TRANSACTIONS ON POWER SYSTEMS, IEEE SYSTEMS JOURNAL, IEEE OPEN ACCESS JOURNAL OF POWER AND ENERGY, and IEEE POWER ENGINEERING LETTERS. He is the Chair of IEEE Denmark, Member of IEC SC/8B- Working Group (WG3 & WG6) as well as Technical Committee Member of several IEEE PES/IES/PELS and CIGRE WGs. He was the recipient of 2020 and 2023 DUO–India and SPARC Fellowship Awards, DANIDA Research Fellowship grant from the Ministry of Foreign Affairs of Denmark in 2018 and 2021, IEEE-CS Outstanding Leadership Award 2018 (Halifax, Nova Scotia, Canada), and the 2017 IEEE-CS Outstanding Service Award (Exeter-U.K.).

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Xian Gao (Member, IEEE) received the B.S. degree in electrical engineering from the Nanjing University of Information Science and Technology, Nanjing, China, in 2017, the M.S. degree in electrical engineer- ing from the University of Chinese Academy of Sci- ences, Beijing, China, in 2020, and the Ph.D. degree in electrical engineering from Aalborg University, Aal- borg, Denmark, in 2024. She was a Visiting Scholar with Imperial College, London, United Kingdom, in

1. Since 2024, she has been with the College of Information Science and Technology & College of Artificial Intelligence, Nanjing Forestry University, Nanjing, China, where she is currently a Lecturer. Her current research interests include stability and control of power-electronics-based power systems. She was the recipient of Best Paper Award at IEEE ICPE 2023-ECCE Asia.

Frede Blaabjerg (Fellow, IEEE) received the Ph.D. degree in electrical engineering from Aalborg Uni- versity, Aalborg, Denmark, in 1995, and the honoris causa degree from University Politehnica Timisoara (UPT), Timișoara, Romania, in 2017, and Tallinn Technical University (TTU), Tallinn, Estonia, in

1. He was with ABB-Scandia, Randers, Denmark, from 1987 to 1988. He became an Assistant Professor in 1992, an Associate Professor in 1996, and a Full Professor of power electronics and drives in 1998 with AAU Energy, Aalborg. In 2017, he became a Villum Investigator. He has authored or coauthored more than 600 journal papers in the fields of power electronics and its applications. He is the coauthor of eight monographs and Editor of fourteen books in power electronics and its applications eg. the series (4 volumes) Control of Power Electronic Converters and Systems published by Academic Press/Elsevier. His research interests include power electronics and its applications such as in wind turbines, PV systems, reliability, power-2-X, power quality and adjustable speed drives. He was the recipient of the 38 IEEE Prize Paper Awards, IEEE PELS Distinguished Service Award in 2009, EPE-PEMC Council Award in 2010, IEEE William E. Newell Power Electronics Award 2014, Villum Kann Rasmussen Research Award 2014, Global Energy Prize in 2019 and 2020 IEEE Edison Medal. He was the Editor-in-Chief of IEEE TRANSACTIONS ON POWER ELECTRONICS from 2006 to 2012. He was a Distinguished Lecturer of the IEEE Power Electronics Society from 2005 to 2007 and the IEEE Industry Applications Society from 2010 to 2011 as well as 2017 to 2018. During 2019–2020, he was the President of IEEE Power Electronics Society. He is the Vice-President of the Danish Academy of Technical Sciences. He was nominated in 2014-2021 by Thomson Reuters to be between the most 250 cited researchers in Engineering in the world.

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