Coordinated Hybrid VAR Compensation Strategy with Grid-Forming BESS and Solar PV for Enhanced Stability in Inverter-Dominated Power Systems

Abstract

This paper proposes a coordinated hybrid VAR compensation strategy that leverages the dynamic support capabilities of a grid-forming (GFM) battery energy storage system (BESS) and solar photovoltaic (PV) plant to enhance the stability of inverter-dominated power systems. The hybrid compensator integrates a VSC-based static synchronous compensator (STATCOM) with a thyristor-switched capacitor (TSC), combining the fast dynamic response of the STATCOM with the high reactive power capacity of the TSC. A coordinated control framework is developed to enable seamless interaction between the hybrid VAR compensator and the GFM-controlled PV and BESS units, ensuring improved voltage regulation and transient stability under varying operating conditions. The PV plant operates at maximum power Point while maintaining its grid-forming capability, thereby maximizing renewable energy utilization while contributing to frequency and voltage support. The effectiveness of the proposed strategy is validated through FPGA-based real-time simulations under scenarios including large load variations, solar irradiance fluctuations, and grid disturbances. Results show that the coordinated operation enhances voltage stability, strengthens reactive power support, mitigates low-frequency oscillations, and significantly improves the dynamic performance of low-inertia, inverter-dominated grids.

1. Introduction

The increasing penetration of renewable energy sources (RESs), particularly solar photovoltaic (PV) inverters, is transforming conventional power systems into inverter-dominated grids with reduced inertia and limited reactive power support. In such environments, preserving voltage stability and controlling reactive power become critical challenges, especially under dynamic conditions such as load variations, renewable power changes, and grid perturbations [1]. Conventional synchronous generators (SGs), which inherently support voltage and frequency control, are gradually being replaced by inverter-based resources (IBRs) that lack these intrinsic capabilities unless specifically designed for grid-forming (GFM) operation [2,3].

Conventional reactive power compensators, including synchronous condensers and shunt capacitor banks, provide steady-state voltage support but do not have the quick dynamic operation needed in low-inertia grids. However, despite their superior dynamic control, power electronic solutions like static synchronous compensators (STATCOMs) are restricted by their reactive power capacity [4,5,6,7,8]. In [4] proposes a hybrid shunt compensator topology combining a static VAR compensator (SVC) and a STATCOM for EHV transmission systems to prevent voltage collapse caused by reactive power imbalance. The use of SVC for improving power system stability is discussed in [5], while STATCOM-based enhancement is presented in [6]. Furthermore, the coordination of D-STATCOM and SVC for improving the voltage stability and reactive power control in AC grids is reported in [7]. However, these approaches are limited in addressing stability challenges of low-inertia, inverter-dominated grids with high renewable penetration. Hybrid VAR compensators that combine VSC-based STATCOMs with thyristor-switched capacitors (TSCs) have shown promise in addressing these complementary shortcomings. The STATCOM combines the advantages of both technologies by providing continuous fine-tuning, while the TSC gives high-capacity, discrete reactive power steps [8]. Most hybrid VAR compensation techniques currently in use, however, were not specifically designed to address the difficulties faced by inverter-dominated grids; rather, they were designed for traditional grids with adequate inertia.

Meanwhile, the progression of newly discovered GFM control strategies has enabled PV and BESS inverters to provide inertia, frequency control, and voltage support, making them effectual participants in grid stability [9]. Compared to the grid-following controller, GFM-controlled inverters can set up voltage and frequency references like SGs, thereby contributing to system strength and resilience under varied operating environments. Integrating GFM-enabled PV and BESS units with hybrid VAR compensators presents a novel opportunity for coordinated voltage and reactive power management in low-inertia grids [10,11]. In [12], a flexible AC power flow control system is proposed for medium-voltage distribution networks, demonstrating through simulations that coordinated control of BESS and solar PV can optimize power flow, enhance grid stability, and support multiple grid services. In [13] presents a real-time, model-driven controller that effectively coordinates BESS and grid devices for voltage regulation in PV-integrated distribution grids, achieving fast and accurate voltage support without disrupting the BESS charging cycle. In [14], a self-adaptive voltage controller is proposed for large-scale solar PV plants, accounting for real-life factors such as ambient conditions, thereby enhancing grid code compliance and voltage support. A reactive power flow control strategy is introduced in [15] for PV inverters in low-voltage distribution networks, enabling effective RMS voltage regulation and seamless transitions between grid-connected and islanded modes using localized control. Reference [16] presents a dynamic voltage support scheme that enables single-phase grid-connected PV systems to meet low-voltage ride-through requirements during voltage sags by regulating active and reactive power. The studies in [12,13,14,15,16] focus on the reactive power support capabilities of IBRs in grid-connected and grid-following modes, where PV units operate at maximum power Point.

Furthermore, a synchronverter-based PV-STATCOM is introduced in [17], which enhances voltage and reactive power regulation by emulating system inertia, thereby offering improved stability and low-voltage ride-through (LVRT) performance compared to conventional PV-STATCOMs, particularly under weak grid conditions. A large PV farm operating as a smart PV-STATCOM with intelligent MPPT-based DC-link voltage control is presented in [18], effectively mitigating power system oscillations and sub-synchronous resonance during both power generation and non-generation periods. A hybrid power system comprising parabolic trough solar, Dish-Stirling, and diesel generation is investigated in [19], demonstrating improved voltage stability and transient performance through dynamic voltage restorer-based compensation and the integration of FACTS devices, especially under dynamic load conditions. In [20], a PV-STATCOM control strategy is proposed in which a solar PV system suspends real power generation to function as a STATCOM for power oscillation damping, thereby enhancing power transfer capability and providing continuous 24/7 grid support, including during the nighttime. In [21] presents a novel frequency regulation scheme based on reactive power control for large-scale solar PV systems, which enhances frequency stability by manipulating active power demand through voltage variation without compromising voltage stability. An improved mathematical methodology is proposed in [22] to derive the P–Q capability curve of grid-integrated PV systems operating under maximum power Point conditions, facilitating effective reactive power management.

Main features of the proposed work and review of the recent literature are given in Table 1. IBRs typically work with limited or no reactive power capability unless specially controlled through modern GFM control strategies. Even with GFM inverters that emulate voltage source behavior, there remains a need for rapid and bulk reactive power compensation to maintain voltage profiles and support fault ride-through. To address this, a hybrid VAR compensator, consisting of a STATCOM and a TSC, offers a balanced and cost-effective solution. VSC-based STATCOM provides continuous and rapid dynamic reactive power regulation. It is efficacious for diminishing voltage sags, stabilizing voltage during transients, and providing fine-tuned voltage control under different operating grid situations. However, it is costly for handling utility-scale reactive power compensation. TSC, composed of thyristor-controlled capacitor banks, offers high-capacity reactive support in discrete steps. It is cost-effective and energy-efficient but slower and less precise in its response compared to STATCOM. By combining these two control strategies, the hybrid VAR compensator leverages the strengths of both: (a) high-speed control and fine regulation from the STATCOM, (b) high reactive power capacity and cost efficiency from the TSC. This hybrid arrangement is especially crucial in IBR-dominated and low-inertia power grids where SGs inertia and excitation systems are absent, voltage variations are more pronounced and require speedy correction, renewable intermittency demands coordinated voltage support, and reactive power is not inherently accessible from IBRs. Moreover, coordinating the hybrid VAR compensator with GFM-controlled PV and BESS allows for improved voltage recovery during and after faults, enhanced dynamic response during load and irradiance changes, and reliable support during system islanding or black-start conditions. Thus, the integration of a VSC-based STATCOM–TSC hybrid VAR compensator into such systems is not only profitable but increasingly necessary for reliable and stable grid operation in the context of high renewable energy penetration.

Table 1. Main features of the proposed work and review of the recent literature.

Ref.	Type of Coordination	Renewable Power Sources	Energy Storage System	GFM Control	Controller Design Technique	Work Carried Out
[23]	STATCOM + TSC	Wind	✘	✘	Conventional	Combining a 3-level 4.16 kV NPC-based STATCOM with TSC
[24]	STATCOM + TSC	Wind	✘	✘	Fuzzy-PI controller	Overcome voltage fluctuations at the POI
[7]	STATCOM + SVC	PV	✘	✘	Grid-following (PV operates at MPPT)	Power quality problems that SVC alone cannot resolve, particularly when a load fluctuates, can be resolved by the cooperative method
[9]	✘	Constant DC Source	✘	✔	Droop control	Improve transient stability
[4]	STATCOM + SVC	✘	✘	✘	Conventional	Hybrid VAR compensator provides voltage support in HV transmission systems
[12]	✘	PV	✔	✘	Grid-following	BESSs to provide a flexible AC power flow control in the distribution system
[17]	✘	PV	✘	✔	Synchronverter-based	Synchronverter-based PV system as STATCOM
[19]	STATCOM + SVC	PV	✘	✘	Conventional	Improve voltage stability
[20]	✘	PV	✘	✘	Conventional	A control of transmission line connected to a large PV solar system as a STATCOM
[21]	✘	PV	✘	✘	Conventional (frequency regulation via reactive power control)	A reactive power assisted frequency control solar PV system
[25]	✘	✘	✘	✘	Optimization framework	Optimal placement for shunt capacitors and SVC units in distribution networks
Our Study	STATCOM + TSC	PV	✔	✔	PV and BESS with droop-based GFM control	A novel coordinated hybrid VAR compensation strategy leveraging the dynamic capabilities of a GFM BESS and GFM PV to significantly enhance the voltage regulation and reactive power management in inverter-dominated power systems

This paper proposes a novel VSC-based hybrid VAR compensator comprising one STATCOM and one TSC, coordinated with GFM-enabled PV and BESS units for enhanced voltage and reactive power regulation in inverter-dominated modern power systems. The STATCOM provides fine-grained, fast-reactive power control, while the TSC delivers discrete reactive compensation during high demand periods. A coordinated control strategy is developed to integrate the VAR compensator with GFM-controlled PV and BESS resources, ensuring stable operation across various disturbances.

The main contributions of this paper are as follows:

• This work introduces the first coordinated control strategy that integrates a VSC-based STATCOM and a TSC with GFM-controlled PV and BESS units for improving voltage and reactive power regulation in inverter-dominated grids.
• A GFM control framework is implemented for both solar PV and BESS, enabling them to contribute actively to voltage and frequency stabilization while the solar PV operates at maximum power Point (MPP).
• The dynamic performance of the BESS and PV plant integrated with the hybrid VAR compensator is assessed under both normal operating conditions and different disturbances, including a significant drop in PV generation, large load variations, grid outages, and fault events as a large disturbance. This paper verifies the effectiveness of the proposed coordinated control strategy using FPGA-based real-time simulator.

The remainder of the paper is organized as follows: Section 2 describes the proposed hybrid VAR compensator, including the mathematical modeling of the VSC-based STATCOM and TSC. Section 3 presents the GFM-controlled PV and BESS units along with their control strategies. Section 4 details the test system. Section 5 discusses the results obtained from various case studies. Finally, Section 6 concludes the paper.

2. Proposed Hybrid VAR Compensator

The proposed coordinated hybrid VAR compensator model consists of a STATCOM and a TSC, as shown in Figure 1. The basic configuration of a VSC-based STATCOM comprises a voltage source converter (VSC), a DC-link capacitor, and a coupling transformer, as illustrated in Figure 2. In this study, a balanced three-phase system is considered, and the STATCOM is modeled using the synchronous rotating reference frame (dq-frame). Switching harmonics are neglected in control-level modeling to simplify the analysis.

Figure 1. General scheme of hybrid VAR compensator model.

Figure 2. Proposed STATCOM controller.

The differential equations for STATCOM dynamics in the abc-frame are [26]

$$L_c{\displaystyle \frac{d{i}_{s,abc}}{dt}}=v_{s,abc}-v_{p,abc}-R_c{i}_{s,abc}$$

(1)

Here, $i_{s,abc}$ is STATCOM output currents, $v_{s,abc}$ is converter output voltages, $v_{p,abc}$ is grid substation (GSS) voltage, and $R_c \mathrm{a}\mathrm{n}\mathrm{d} L_c$ are coupling transformer resistance and inductance.

The STATCOM dynamics (AC-side) in the dq-frame are expressed by the following equations:

$$L_c{\displaystyle \frac{d{i}_{s,d}}{dt}}=-R_c{i}_{s,d}+\omega L_c{i}_{s,q}+v_{s,d}-v_{p,d}$$

(2)

$$L_c{\displaystyle \frac{d{i}_{s,q}}{dt}}=-R_c{i}_{s,q}+\omega L_c{i}_{s,d}+v_{s,q}-v_{p,q}$$

(3)

Here, $i_{s,d} \mathrm{a}\mathrm{n}\mathrm{d} i_{s,q}$ are dq-axis components of STATCOM output current, $v_{s,d} \mathrm{a}\mathrm{n}\mathrm{d} v_{s,q}$ are dq-axis converter output voltages, $v_{p,d} \mathrm{a}\mathrm{n}\mathrm{d} v_{p,q}$ are dq-axis GSS voltages, and ω is angular frequency of the rotating frame.

The DC-link dynamics are expressed by the following equation (ignore loss):

$$C_{statcom}{\displaystyle \frac{d{v}_{statcom}}{dt}}={\displaystyle \frac{3}{2}}\left(v_{s,d}{i}_{s,d}+v_{s,q}{i}_{s,q}\right)$$

(4)

In steady state, the reactive power $Q_{statcom}$ injected by the STATCOM is $\left(v_{s,q}=0\right)$

$$Q_{statcom}={\displaystyle \frac{3}{2}}{v}_{s,d}{i}_{s,q}$$

(5)

Hence, controlling the $i_{s,q}$ component allows reactive power regulation, while the $i_{s,d}$ component can be used for active power flow or DC-link voltage regulation.

A. 

Control Strategy of VSC-Based STATCOM

To regulate voltage and reactive power in an inverter-dominated power system, a vector control strategy is implemented for the VSC-based STATCOM using a synchronous reference frame (dq-frame). The control system includes voltage and current control loops, implemented through the following sequential stages:

i. 

Signal Measurement and Synchronization

The control process begins with the measurement of three-phase voltages and currents at the point of common coupling (PCC). A phase-locked loop (PLL), specifically the sinusoidal measurement (PLL, three-phase), is used to extract the frequency and phase angle (θ) of the GSS voltage. This ensures synchronization with the grid and facilitates transformation into the rotating reference frame. Using the Park transformation, the measured three-phase quantities in the abc-frame are converted into the dq-frame.

ii. 

Voltage Control Loops

The reference currents $i_{s,d}^{\ast } \mathrm{a}\mathrm{n}\mathrm{d} i_{s,q}^{\ast }$ are generated through DC voltage and AC voltage control loops, respectively, as follows:

$$i_{s,d}^{\ast }=k_{p,statcom,dc}\left(V_{statcom}^{ref}-V_{statcom}\right)+k_{i,statcom,dc}{\displaystyle \int \left(V_{statcom}^{ref}-V_{statcom}\right)dt}$$

(6)

$$i_{s,q}^{\ast }=k_{p,statcom,ac}\left(V_{grid}^{ref}-k_{d}Q-V_{p,d}\right)+k_{i,statcom,ac}{\displaystyle \int \left(V_{grid}^{ref}-k_{d}Q-V_{p,d}\right)dt}$$

(7)

Here, $V_{statcom}^{ref}$ and $V_{statcom}$ are STATCOM reference and measured DC-link voltage, respectively, $V_{grid}^{ref}$ and $V_{p,d}$ are grid reference and measured voltage, respectively, and $k_{p,statcom,dc}$, $k_{i,statcom,dc}$, $k_{p,statcom,ac},$ and $k_{i,statcom,ac}$ are PI controller gain. $k_d$ is reactive drop gain and $Q$ is reactive power flowing into the AC grid.

iii. 

Inner Current Control Loops

The reference voltages $v_{s,d}^{\ast } \mathrm{a}\mathrm{n}\mathrm{d} v_{s,q}^{\ast }$ for the VSC are generated using current controllers that compare the measured dq-currents with their references:

$$v_{s,d}^{\ast }=k_{p,d,current}\left(i_{s,d}^{\ast }-i_{s,d}\right)+k_{i,d,current}{\displaystyle \int \left(i_{s,d}^{\ast }-i_{s,d}\right)dt}+\omega L_{cp}{i}_{s,q}+v_{s,d}$$

(8)

$$v_{s,q}^{\ast }=k_{p,q,current}\left(i_{s,q}^{\ast }-i_{s,q}\right)+k_{i,q,current}{\displaystyle \int \left(i_{s,q}^{\ast }-i_{s,q}\right)dt}+\omega L_{cp}{i}_{s,d}$$

(9)

Here, $k_{p,d,current}$, $k_{i,d,current}$, $k_{p,q,current},$ and $k_{i,q,current}$ are PI controller gain. $\omega$ is grid angular frequency and $L_{cp}$ is coupling transformer inductance.

The reference voltages in the dq-frame are transformed back to the abc-frame using the inverse Park transformation.

iv. 

PWM Generation

The three-phase reference voltages $v_a^{\ast },{ v}_b^{\ast }, \mathrm{a}\mathrm{n}\mathrm{d} v_c^{\ast }$ are fed into a three-phase, three-level sinusoidal PWM generator, which produces the corresponding gating signals for the power semiconductor switches (IGBT) in the VSC. This generator modulates the switching pattern to synthesize the desired output voltage waveform. The switching frequency is 10 kHz.

B. 

TSC Model

A TSC is a shunt-connected reactive power compensator used in power systems to provide discrete capacitive VAR support. It consists of a capacitor bank connected in series with an anti-parallel thyristor pair, allowing the capacitor to be switched in or out of the system without mechanical switching. The TSC operates in steps depending on the firing angle of the thyristors, providing dynamic voltage and reactive power support.

When the thyristors are gated at the zero crossing of the AC voltage (i.e., firing angle α = 90°), the entire capacitor bank is connected to the system and provides maximum reactive power. The instantaneous current through the capacitor is given by

$$i_C\left(t\right)=C_{TSC}{\displaystyle \frac{d{v}_C\left(t\right)}{dt}}$$

(10)

Here, $i_C\left(t\right)$ is the capacitor current, $C_{TSC}$ is the capacitance, and $v_C\left(t\right)$ is the voltage across the capacitor.

In sinusoidal steady state, the reactive power $Q_{TSC}$ provided by the TSC is expressed as

$$Q_{TSC}=V_{rms}^2\omega C_{TSC}$$

(11)

Here, $V_{rms}$ is the RMS voltage across the capacitor and $\omega =2\pi f$ is the angular frequency of the system.

Thyristors are controlled by their firing angle (α). In TSC, the thyristors are either fully conducting (switched ON at zero crossing) or not conducting (OFF), providing stepwise changes in reactive power, not continuous. The equivalent susceptance $B_{eq}$ of the TSC is

$$B_{eq}=\left\{\begin{array}{ll}\omega C_{TSC},& when thyristors are ON (conducting)\\ 0,& when thyristors are OFF\end{array}\right.$$

(12)

Thus, the TSC behaves as a stepwise variable susceptance. For control purposes,

$$Q_{TSC}=\left\{\begin{array}{ll}{Q}_{cap}=V_{rms}^2\omega C_{TSC},& if switched ON\\ 0,& if switched OFF\end{array}\right.$$

(13)

where switching is based on control logic, as shown in Figure 3. When reactive power flowing into the AC grid is greater than $Q_{min},$ then TSC is switched on; otherwise, TSC is switched off.

Figure 3. TSC model.

The capacitor bank is designed by following the expression

$$C_{tsc}=\left({\displaystyle \frac{Q_{tsc,rated}}{3V_{c,rms}^2{\omega }_{rated}}}\right)\left({\displaystyle \frac{n^2-1}{n^2}}\right)$$

(14)

Here, $Q_{tsc,rated}$ is the rated reactive power of capacitor bank, $V_{c,rms}$ is rms line-to-line primary rated voltage of coupling transformer, and n is harmonic order of the tuned frequency. L and R are designed using $L=1/\left(C_{tsc}{\left(n{\omega }_{rated}\right)}^2\right)$ and $R=n{\omega }_{rated}L{Q}_f$. $Q_f$ is quality factor capacitor bank.

C. 

Proposed coordinated hybrid VAR compensator

A TSC and a VSC-based STATCOM are used in the coordinated hybrid VAR compensator to provide the best possible balance between high-capacity reactive power supply and quick dynamic response. The coordinated strategy is shown in Figure 4. In this set up, the STATCOM ensures fine voltage regulation under dynamic and transient operating situations by providing quick and continuous reactive power control. On the other hand, in steady-state conditions, the TSC, which is made up of banks of capacitors controlled by thyristors, effectively provides large-step reactive power compensation with low switching losses. While the TSC manages the bulk reactive demand and the STATCOM corrects for rapid voltage fluctuations, the two systems work in tandem to provide smooth reactive power management across a broad working range. In addition to improving transient response and system voltage stability, this hybrid coordination lowers converter stress and boosts overall efficiency. As a result, in contemporary low-inertia, inverter-dominated power systems, the coordinated STATCOM + TSC structure provides a practical and affordable means of preserving voltage and reactive power balance. The coordinated hybrid VAR compensator works according to the following cases:

$$Case-1: \left\{\begin{array}{c}{Q}_{demand}=Q_{TSC}+Q_{STATCOM}\Rightarrow \mathrm{Grid} \mathrm{Operate} \mathrm{at} \mathrm{Nominal} \mathrm{Voltage}\\ \Downarrow \\ \left[STATCOM absorb reactive power\right] \& \left[TSC Switched ON\right]\end{array}\right.$$

$$Case-2: \left\{\begin{array}{c}if Q_{demand}\ge Q_{TSC}+Q_{STATCOM}\Rightarrow \mathrm{Grid} \mathrm{Voltage} \mathrm{Increase}\\ \Downarrow \\ \left[STATCOM generate reactive power\right] \& \left[TSC Switched OFF\right]\end{array}\right.$$

$$Case-3: \left\{\begin{array}{c}if Q_{demand}\le Q_{TSC}+Q_{STATCOM}\Rightarrow \mathrm{Grid} \mathrm{Voltage} \mathrm{decrease}\\ \Downarrow \\ \left[STATCOM absorb less reactive power\right] \& \left[TSC Switched ON\right]\end{array}\right.$$

Figure 4. Proposed coordinated hybrid VAR compensator (STATCOM + TSC).

D. 

VSC–STATCOM Controller Design and Tuning

All proportional–integral (PI) controllers for the STATCOM voltage loops, STATCOM current loops, and GFM PV/BESS units were tuned using MATLAB’s (R2023) Model Linearizer tool based on frequency–response analysis. The Bode plots were used to verify the adequate gain margin (GM), phase margin (PM), and crossover frequencies to ensure robust stability while maintaining a fast transient response.

Figure 5a shows the Bode response of the STATCOM AC voltage controller with a gain margin of 6.24 dB and a phase margin of 13.6° at 5.44 rad/s. These values indicate stable operation with a relatively high bandwidth suitable for dynamic voltage regulation during fast grid disturbances. Figure 5b illustrates the DC-link voltage controller response, which exhibits an infinite gain margin and a phase margin of 28.6° at 120 rad/s, demonstrating a highly stable response with sufficient damping and rapid DC voltage recovery.

Figure 5. Bode plot of STATCOM voltage and current controllers. (a) AC voltage controller; (b) DC voltage controller; (c) Current controller (I_d); (d) Current controller (I_q).

The inner current control loops, shown in Figure 5c,d, exhibit significantly higher stability margins compared to voltage loops, as expected due to their faster dynamic response. The i_d current controller achieves a gain margin of 46 dB at 3.16 rad/s and a phase margin of 78.9° at 0.09 rad/s, while the i_q controller shows a gain margin of 23.5 dB at 1 rad/s and a phase margin of 22.5° at 0.54 rad/s. The positive and sufficiently large gain and phase margins across all loops confirm that the designed controllers provide stable operation across a wide range of operating conditions, including variations in solar irradiance, load changes, and reactive power demand.

Overall, the frequency–response-based tuning ensures well-damped STATCOM dynamics, strong current loop stability, and coordinated performance with GFM-controlled PV and BESS units, facilitating reliable voltage support in inverter-dominated grids.

E. 

Test of proposed coordinated hybrid VAR compensator

The proposed coordinated hybrid VAR compensator model is tested on a conventional high voltage transmission grid. STATCOM and TSC are integrated into the transmission system, as illustrated in Figure 1. The model is implemented and simulated using MATLAB/Simulink (R2023) and validated through FPGA-based real-time simulation on the OPAL-RT platform. The system parameters, including STATCOM, grid, transmission line, and transformer, are given in Appendix A.

An 1100 MW load is initially connected to the GSS. The grid voltage is maintained at 1.0 per unit (nominal voltage). The proposed controller regulates the STATCOM to absorb the reactive power while the TSC operates simultaneously, ensuring that the combined reactive power absorbed by the STATCOM and supplied by the TSC meets the system’s reactive power demand, as shown in Figure 6. At 0.5 s into the simulation, an additional 450 MW load is connected, causing the grid voltage to drop below 0.97 per unit, as shown in Figure 6a,b. In response, the reactive power absorption by the STATCOM decreases, resulting in a voltage recovery to approximately 0.985 per unit and increased reactive power injection into the AC grid. Subsequently, an 850 MW load is disconnected at 1.0 s of simulation time, leading to a voltage rise above 1.02 per unit. In response, the TSC is deactivated and the STATCOM starts injecting reactive power. This action stabilizes the grid voltage, bringing it down to approximately 1.006 per unit. Figure 6a,b shows the results of the reactive power flowing into the AC grid, and the grid voltage using the MATLAB Simulink and FPGA real-time simulator, respectively.

Figure 6. Reactive power flowing into AC grid, and grid voltage in p.u.; (a) MATLAB Simulink results, (b) real-time simulator results.

Figure 7 presents a comparative analysis of grid voltage and reactive power flow under three operational scenarios: (i) coordinated hybrid VAR compensation using both STATCOM and TSC, (ii) STATCOM-only operation, and (iii) TSC-only operation. When both STATCOM and TSC operate in coordination, the system exhibits superior voltage regulation and dynamic response. At t = 1 s, the connection of a 450 MW load on the secondary side causes the grid voltage to drop below 0.97 p.u.; in response, the STATCOM reduces reactive absorption and the reactive power flow into the grid increases, enabling a rapid voltage recovery to approximately 0.985 p.u. Subsequently, at t = 2 s, the disconnection of the 400 MW primary-side load and the 450 MW secondary-side load causes the grid voltage to rise above 1.02 p.u.; the TSC is switched out and the STATCOM begins injecting reactive power, stabilizing the grid voltage at about 1.006 p.u. In contrast, when only the STATCOM is in operation, the system exhibits faster transient response but insufficient reactive capacity during large disturbances, leading to deeper voltage dips and slower post-disturbance voltage recovery. Conversely, when only the TSC is connected, the system maintains adequate reactive support during steady-state conditions but suffers from slower dynamic stabilization, increased overshoot, and reduced damping capability due to its limited fast-response characteristics. Overall, the coordinated operation effectively combines the rapid dynamic performance of the STATCOM with the bulk reactive support of the TSC, ensuring improved voltage stability, reduced overshoot, and enhanced reactive power management compared to standalone cases.

Figure 7. Comparative results (case 1: coordinated STATCOM + TSC, case 2: STATCOM only, case 3: TSC only) showing (a) grid voltage (p.u.), (b) reactive power flow into the AC grid.

The coordinated operation of the STATCOM and TSC effectively regulates voltage under dynamic load conditions. The hybrid VAR compensator maintains voltage within acceptable limits by adaptively managing reactive power exchange. The proposed hybrid VAR compensator is further validated on a renewable-integrated power system comprising a grid-connected or islanded utility-scale BESS and PV power plant.

3. Grid-Forming PV and BESS

A topology of a grid-connected IBRs system with GFM control is illustrated in Figure 8. In this configuration, the IBRs, comprising a PV system and a BESS, are connected to the grid via a circuit breaker. The GFM-controlled IBRs and the grid are interfaced at the point of interconnection (POI). A three-phase voltage source inverter (VSI), governed by a droop-based GFM control scheme, is employed to regulate the power exchange and maintain system stability. $R_f, L_f, \mathrm{a}\mathrm{n}\mathrm{d} C_f$ are filter resistance, inductance, and capacitance, and $R_l \mathrm{a}\mathrm{n}\mathrm{d} L_l$ are grid resistance and inductance. The load is connected at the POI.

Figure 8. Grid-connected IBRs topology with GFMI control.

A. 

Proposed Grid-forming Inverter Control Model

As shown in Figure 9, a multi-loop control diagram is developed for the GFMI, incorporating an inner current control loop, a voltage control loop, and a droop control loop. When the voltage quantity is aligned with the d-axis for the GFMI, it produces

$${\omega }_{inv}={\omega }_n-d_p\left(P_{meas}-P_{ref}\right)$$

(15)

$$v_{od}^{\ast }=V_n-d_q\left(Q_{ref}-Q_{meas}\right), v_{oq}^{\ast }=0$$

(16)

Figure 9. PV and BESS GFMI controller with droop control loop.

As a result, the following sums up the appropriate illustrations for δ_inv, P_meas, and Q_meas:

$$\Delta {\dot{X}}_P^{\prime }=A_P^{\prime }{X}_P^{\prime }+B_P^{\prime }{X}_{Fil}+B_{P\omega }^{\prime }\Delta {\omega }_{com}$$

(17)

$$\Delta V_{odq}^{\ast }=C_P^{\prime }\Delta X_P^{\prime }, \Delta {\omega }_{inv}=C_{P\omega }^{\prime }\Delta X_P$$

(18)

Here, $X_P^{\prime }={\left[{\delta }_{inv}, P_{meas}, Q_{meas}\right]}^T, V_{odq}^{\prime }={\left[v_{od}^{\ast }, v_{oq}^{\ast }\right]}^T$.

The PI regulators in the dq-axis are modeled for the voltage control process using intermediate parameters φ_d and φ_q

$${\dot{\phi }}_d=v_{od}^{\ast }-v_{od}, {\dot{\phi }}_q=v_{oq}^{\ast }-v_{oq}$$

(19)

$$i_{id}^{\ast }=k_{pv}\left(v_{od}^{\ast }-v_{od}\right)+k_{iv}{\phi }_d, i_{iq}^{\ast }=k_{pv}\left(v_{oq}^{\ast }-v_{oq}\right)+k_{iv}{\phi }_q$$

(20)

As a result, the voltage regulator loop’s expression is

$$\Delta {\dot{X}}_{\phi }=0_{2\times 2}\Delta X\phi +B_{V1}\Delta V_{odq}^{\ast }+B_{V2}\Delta X_{Fil}$$

(21)

$$\Delta {\mathrm{I}}_{idq}^{\ast }=C_{V1}\Delta X_{\phi }+D_{V1}\Delta V_{odq}^{\ast }+D_{V2}\Delta X_{Fil}$$

(22)

Lastly, the GFMI’s entire model is written as follows:

$$\Delta {\dot{X}}_{inv}=A_{inv}\Delta X_{inv}+B_{inv}\Delta V_{cDQ}+B_{inv\omega }\Delta {\omega }_{com}$$

(23)

$$\Delta {\mathrm{I}}_{oDQ}=C_{inv}\Delta X_{inv}$$

(24)

Here, $X_{inv}={\left[X_P, X_{\phi }, X_{\tau }, V_i, X_{Fil}\right]}^T$.

B. 

Networks and Load Model

GFMI IBRs and loads are connected at the POI, as shown in Figure 6. Modeling the network and the loads is performed with fixed R-L impedance, whereas local load on the GFMI ends is performed with fixed impedance load. The expressions are given as

$$L{\dot{i}}_{DQ}={\omega }_b\left(v_{gDQ}-v_{LDQ}-R{i}_{t1DQ}\pm {\omega }_{com}L{i}_{QD}\right)$$

(25)

$$L{\dot{i}}_{DQ}={\omega }_b\left(v_{cDQ}-v_{LDQ}-R{i}_{DQ}\pm {\omega }_{com}L{i}_{QD}\right)$$

(26)

$$L{\dot{i}}_{DQ}={\omega }_b\left(v_{LDQ}-R{i}_{LDQ}\pm {\omega }_{com}L{i}_{LQD}\right)$$

(27)

Here, v_DQ are the POI voltages, where load is connected. The simplified model is given as

$$\Delta {\dot{\mathrm{I}}}_{NetDQ}=A_{Ner}\Delta {\mathrm{I}}_{NetDQ}+B_{Net}\Delta V_{bDQ}+B_{Net\omega }\Delta {\omega }_{com}$$

(28)

$$\Delta {\dot{\mathrm{I}}}_{LoadDQ}=A_{Load}\Delta {\mathrm{I}}_{LoadDQ}+B_{Load}\Delta V_{bDQ}+B_{Load\omega }\Delta {\omega }_{com}$$

(29)

Here, ${\mathrm{I}}_{NetDQ}={\left[i_D, i_Q\right]}^T$, ${\mathrm{I}}_{LoadDQ}={\left[i_{LD}, i_{LQ}\right]}^T$

$$V_{bDQ}={\left[v_{gD}, v_{gQ}, v_{LD}, v_{LQ}, v_{cD}, v_{cQ}\right]}^T$$

(30)

Therefore, the overall system model (GFMI IBRs, networks, and load) is expressed as

$${\dot{X}}_{sys}=A_{sys}{X}_{sys}$$

(31)

Here, $X_{sys}=\left[\Delta X_{inv}, \Delta X_{Load}, \Delta X_{Net}\right]$.

Furthermore, the operational point that is provided for the dynamical modeling examination is derived from the power flowing estimation.

4. Test System

The proposed coordinated hybrid VAR compensator (STATCOM + TSC) model is integrated with IBRs, as illustrated in Figure 10. The system comprises an 80 MW solar PV plant and a 35 MW BESS, both operating in droop-based grid-forming mode and connected to a 230 kV grid via a circuit breaker. The PV plant operates at its maximum power point (MPP) using the incremental conductance (INC) MPPT method, while the BESS dynamically supplies or stores power based on load demand. When the load demand is lower than the total power generated by the PV plant, the surplus power is stored in the BESS. Conversely, during high load conditions, the BESS discharges to support the system. All system parameters, including those of the transmission line, transformer, PV, BESS, STATCOM, TSC, and their associated control settings, are provided in Appendix A. Using this test system, the capabilities of a GFMI-BESS and high-penetration inverter-based GFMI-PV sources are evaluated for maintaining power system stability in conjunction with the proposed coordinated hybrid VAR compensator. The dynamic performance of the BESS and PV plant integrated with the hybrid VAR compensator is assessed under both normal operating conditions and various contingencies, including a significant drop in PV generation, large load variations, grid outages, and fault events. The coordinated operation of the STATCOM and TSC effectively regulates voltage, ensuring that the hybrid VAR compensator maintains voltage within acceptable limits by adaptively managing reactive power exchange. The simulation validates whether the designed PV and BESS units, along with their respective controllers, comply with the performance requirements outlined in the current IEEE 2800 standard.

Figure 10. System diagram with IBRs and hybrid VAR compensator.

5. Results and Discussion

To verify the effectiveness of the proposed hybrid VAR compensator and assess the capabilities of the GFMI-BESS and GFMI-PV, a series of case studies were conducted. These case studies included the following: (i) variation in solar irradiance, (ii) changes in load demand, (iii) grid outage or islanding conditions, and (iv) three-phase faults on the transmission line. Each scenario represents a critical operating condition to evaluate the voltage regulation, reactive power management, and dynamic support provided by the coordinated control of the hybrid VAR compensator and GFMI units. The entire model, including the GFMI-PV, GFMI-BESS, STATCOM, and TSC, was implemented and simulated using an FPGA-based real-time simulator to ensure accurate evaluation of the control response and transient performance under real-time operational constraints. These real-time simulations validate the robustness and effectiveness of the proposed system in maintaining voltage stability and system resilience in inverter-dominated power systems. The coordinated control operation of STATCOM + TSC, GFMI-PV, and GFMI-BESS is shown in Figure 11.

Figure 11. Proposed coordinated control of STATCOM + TSC + GFMI-PV + GFMI-BESS.

A. 

Case Study 1: Change in Solar Irradiation

The results of the first case study, which examines the system’s response to a sudden reduction in solar irradiance, are illustrated in Figure 12a,b. Figure 12a depicts the real power output of the GFMI-PV and GFMI-BESS, the total real power delivered at the point of interconnection (POI), and the corresponding frequency variation at the POI. Under normal irradiance conditions, the GFMI-PV plant operates in MPPT mode and delivers approximately 78.4 MW of power. The connected load at the POI is initially met by the PV generation and the GFMI-BESS, which discharges during this period. A step change in solar irradiance from 800 $\mathrm{W}/{\mathrm{m}}^2$ to 560 $\mathrm{W}/{\mathrm{m}}^2$ is introduced, while the connected loads remain unchanged and are supplied continuously throughout the simulation.

Figure 12. Output response in case study 1, (a) real power output of the GFMI-PV and GFMI-BESS, the total real power delivered at POI, and the frequency at POI; (b) voltage at the POI, the total reactive power at the POI, and reactive power contributions from the GFMI-PV and GFMI-BESS.

Following the reduction in solar irradiance, the output power $\left(P_{pv}\right)$ of the GFMI-PV drops below the total load demand. In response, the GFMI-BESS discharges more to compensate for the power shortfall and maintains power balance at the POI. This active power $\left(P_{bess}\right)$ support from the BESS effectively mitigates the frequency dip caused by the sudden power imbalance. Although a temporary frequency reduction is observed immediately after the drop in irradiance, the frequency is restored to its nominal value within a short time frame, demonstrating the effectiveness and responsiveness of the proposed grid-forming control strategy implemented in the GFMI-BESS and GFMI-PV units.

Figure 12b illustrates the voltage at the POI, the total reactive power $\left(Q_{POI}\right)$ at the POI, and the individual reactive power $\left(Q_{pv} \mathrm{a}\mathrm{n}\mathrm{d} Q_{bess}\right)$ contributions from the GFMI-PV and GFMI-BESS. When solar irradiance is reduced, a voltage $\left(V_{POI}\right)$ drop at the POI is observed. However, the proposed hybrid VAR compensator responds effectively by dynamically adjusting its reactive power exchange. Specifically, both the GFMI-PV and GFMI-BESS reduce their reactive power absorption, which results in a net increase in reactive power injection into the grid $\left(Q_{grid}\right)$. At the same time, STATCOM starts absorbing less reactive power and TSC provides reactive power. Consequently, the voltage $\left(V_{POI}\right)$ at the POI begins to rise and stabilizes near its nominal value. $(Q_{grid}=Q_{TSC}+Q_{STATCOM}^{absorb}+Q_{BESS}+Q_{PV})$. The steady-state outputs are given in Table 2.

Table 2. Case study 3 steady-state outputs: solar irradiance variation, grid outage, and load changes.

$\begin{array}{c}{\mathit{P}}_{\mathit{P}\mathit{O}\mathit{I}}\\ \mathbf{\downarrow }\end{array}$	$\begin{array}{c}{\mathit{P}}_{\mathit{b}\mathit{e}\mathit{s}\mathit{s}}\\ \mathbf{\downarrow }\end{array}$	$\begin{array}{c}{\mathit{P}}_{\mathit{p}\mathit{v}}\\ \mathbf{\downarrow }\end{array}$	$\begin{array}{c}{\mathit{Q}}_{\mathit{P}\mathit{O}\mathit{I}}\\ \mathbf{\downarrow }\end{array}$	$\begin{array}{c}{\mathit{P}}_{\mathit{g}\mathit{r}\mathit{i}\mathit{d}}\\ \mathbf{\downarrow }\end{array}$	$\begin{array}{c}{\mathit{Q}}_{\mathit{g}\mathit{r}\mathit{i}\mathit{d}}\\ \mathbf{\downarrow }\end{array}$	$\begin{array}{c}{\mathit{V}}_{\mathit{P}\mathit{O}\mathit{I}}\left(\mathit{p}\mathit{u}\right)\\ \mathbf{\downarrow }\end{array}$	$\begin{array}{c}\mathit{f}\left(\mathit{H}\mathit{z}\right)\\ \mathbf{\downarrow }\end{array}$
Before Disturbance (P in MW and Q in MVAr)
72.8	−3.9	78.4	−4.1	−7.4	−66.8	1.02	60.05
After Irradiation Change
60.4	4.9	53.5	−0.5	0.8	−71	1.01	60.01
After Grid Outage or Islanded Mode
59.3	4.1	53.5	1.2	0	0	0.99	59.98
After Load Change in Islanded Mode
70.7	12.2	53.5	2.3	0	0	0.86	59.95

These results validate the efficacy of the proposed hybrid VAR compensator and the coordinated control of the GFMI-PV and GFMI-BESS in maintaining both voltage and frequency stability under varying solar generation conditions. The system’s ability to transition smoothly between charging and discharging modes and to modulate reactive power exchange highlights its robustness and suitability for grid-forming applications in low-inertia, high-penetration renewable energy scenarios.

B. 

Case Study 2: Change in Load

The second case study investigates the performance of the proposed hybrid VAR compensator and the coordinated control of GFMI-BESS and GFMI-PV under a sudden step change in load. An additional load (load-2) is connected at the POI using a circuit breaker as shown in Figure 10, to simulate a sudden increase in demand. The corresponding results are shown in Figure 13a,b.

Figure 13. Output response in case study 2, (a) real power output of the GFMI-PV and GFMI-BESS, the total real power delivered at POI, and the frequency at POI; (b) voltage at the POI, the total reactive power at the POI, and reactive power of the GFMI-PV and GFMI-BESS.

Figure 13a illustrates the real power contributions from the GFMI-PV $\left(P_{pv}\right)$ and GFMI-BESS $\left(P_{bess}\right)$, the total real power $\left(P_{POI}\right)$ at the POI, and the frequency at the POI. Before the load is switched in, the system is operating under steady-state conditions with the GFMI-PV supplying real power at its MPPT output, and the GFMI-BESS is in charging mode. When the additional load is suddenly connected, the total load demand at the POI exceeds the power generated by the PV system, resulting in an immediate power imbalance. In response, the GFMI-BESS quickly transitions from its prior state to discharge mode, injecting real power into the grid to support the increased load. This immediate injection of real power helps to mitigate the frequency dip that occurs due to the increased load demand. Although a transient frequency deviation is observed, the system frequency is rapidly restored to its nominal value, highlighting the capability of the proposed control strategy to provide fast frequency support and maintain system stability.

Figure 13b presents the voltage $\left(V_{POI}\right)$ at the POI, the total reactive power $\left(Q_{POI}\right)$ at the POI, and the individual reactive power contributions from the GFMI-PV and GFMI-BESS. The step increase in load introduces an additional reactive power demand, which causes an initial voltage dip at the POI. The hybrid VAR compensator responds dynamically to this voltage deviation. Both the GFMI-PV and GFMI-BESS increase their reactive power injection (or reduce absorption) to compensate for the increased demand. As a result, the voltage at the POI begins to recover and stabilizes close to its nominal value. The steady-state outputs are given in Table 2.

The effective sharing of reactive power between the GFMI-PV and GFMI-BESS, along with the contribution from the VSC-based STATCOM and TSC, ensures that the voltage deviation is quickly corrected. The coordinated control of the hybrid compensator under a step change in load confirms its robustness and adaptability in responding to sudden changes in power system operating conditions.

These results demonstrate that the proposed control strategy and hybrid compensator not only support real power balancing through BESS but also dynamically regulate reactive power to maintain voltage stability. This makes the system highly effective for grid-forming applications in weak or dynamic distribution networks with high penetration of IBRs.

C. 

Case Study 3: Grid Outage or Islanded Mode

This case study evaluates the performance of the proposed GFMI-based control and hybrid VAR compensator under a sequence of critical disturbances: solar irradiance variation, grid outage, and load changes in the islanded mode. The results are presented in Figure 14a,b.

Figure 14. Output response in case study 3, (a) real power output of the GFMI-PV and GFMI-BESS, the total real power delivered at POI, and the frequency at POI; (b) voltage at the POI, the total reactive power at the POI, and reactive power of the GFMI-PV and GFMI-BESS.

Initially, the system operates under normal conditions with the GFMI-PV generating power at MPPT. A step change in solar irradiance is then introduced, reducing the available solar resource. As a result, the output power from the GFMI-PV decreases accordingly. To maintain power balance at the POI, the GFMI-BESS responds by discharging to compensate for the power deficit and support the connected load.

Following the irradiance reduction, a grid outage is simulated by opening a circuit breaker, effectively transitioning the system into islanded operation. The system’s ability to remain stable during this transition is critical, as GFMIs must now assume the role of maintaining voltage and frequency in the absence of the main grid. After entering islanded mode, an additional load change is introduced. Despite the disturbance, both GFMI-PV and GFMI-BESS coordinate seamlessly to maintain voltage and frequency stability. The GFMI-BESS plays a key role by adjusting its power output to counteract the effects of the load variation, while the hybrid VAR compensator dynamically regulates the reactive power to stabilize the POI voltage.

Figure 14a shows the real power outputs of the PV $\left(P_{pv}\right)$ and BESS $\left(P_{bess}\right)$ units, the total power at the POI $\left(P_{POI}\right)$, and the frequency (f) in Hz. A temporary dip in frequency is observed following the irradiance reduction and load change, but the system recovers quickly, demonstrating effective frequency support and inertial response from the GFMI-based controllers. Figure 14b illustrates the POI voltage $\left(V_{POI}\right)$, reactive power exchanged at the POI $\left(Q_{POI}\right)$, and contributions from both the PV and BESS in terms of reactive power. Upon grid outage, the hybrid VAR compensator adjusts its behavior to ensure voltage stability. The reactive power injection is managed such that the POI voltage remains within acceptable limits throughout the islanded operation, even under fluctuating load conditions. The steady-state outputs are given in Table 2.

Overall, this case study confirms that the proposed control scheme, incorporating GFMI-PV, GFMI-BESS, and the hybrid VAR compensator, provides robust and stable operation under multiple challenging scenarios. The system successfully maintains voltage and frequency within safe margins, demonstrating strong potential for autonomous microgrid operation during grid contingencies.

D. 

Case Study 4: Three-Phase Fault in Transmission Line

This case study investigates the dynamic response and fault-handling capabilities of the proposed hybrid VAR compensator and GFMI-based PV and BESS during a severe grid disturbance, namely, a three-phase fault on the transmission line. The objective is to assess the system’s voltage and frequency stability, power-sharing capability, and fault ride-through performance under extreme fault conditions. The simulation results are presented in Figure 15a,b. At the start of the case study, the system operates under normal conditions with the GFMI-PV generating power at the MPP, and the GFMI-BESS operating in support mode to balance any fluctuations in demand and generation. A three-phase fault is then introduced on the transmission line.

Figure 15. Output response in case study 4, (a) real power output of the GFMI-PV and GFMI-BESS, the total real power delivered at POI, and the frequency at POI; (b) voltage at the POI, the total reactive power at the POI, and reactive power of the GFMI-PV and GFMI-BESS.

Figure 15a shows the real power outputs from the GFMI-PV $\left(P_{pv}\right)$ and GFMI-BESS $\left(P_{bess}\right)$, the total real power $\left(P_{POI}\right)$ at the POI, and the system frequency. Immediately after fault inception, a sharp drop in real power output is observed due to the sudden voltage depression and protective action of the inverter-based resources. The fault causes a temporary reduction in the power delivered to the POI. However, once the fault is cleared, the GFMI-PV and GFMI-BESS controllers rapidly stabilize their outputs. The BESS contributes significantly to frequency support during and after the fault by injecting or absorbing active power as needed. Although there is a brief deviation in frequency, the proposed grid-forming control strategies ensure a fast and stable frequency recovery.

Figure 15b presents the POI voltage $\left(V_{POI}\right)$, total reactive power $\left(Q_{POI}\right)$ exchanged, and individual contributions from the PV and BESS units. As expected, the POI voltage drops sharply during the fault due to the sudden increase in fault current and the voltage sag in the faulted line. The hybrid VAR compensator dynamically responds by supplying a substantial amount of reactive power to mitigate the voltage dip. The reactive power contribution from both the GFMI-PV and GFMI-BESS increases significantly during the fault and remains elevated briefly after the fault is cleared, ensuring smooth voltage recovery.

Figure 16 and Figure 17 present the harmonic analysis of the three-phase grid voltage and current at the POI. The measured total harmonic distortion (THD) values are 0.3% for voltage and 0.13% for current, respectively. These values are significantly lower than typical grid code limits (e.g., IEEE-519), which generally permit voltage THD up to 5% and current THD up to 8% for distribution-level systems. The low harmonic content confirms that the coordinated operation of the STATCOM, TSC, and GFM-controlled PV/BESS does not introduce adverse switching or resonance effects into the network despite the high-power converter switching activity.

Figure 16. THD % of output three-phase voltage at POI.

Figure 17. THD % of output three-phase current at POI.

The results also demonstrate that the STATCOM’s current-regulated control loops, combined with GFM droop control in PV and BESS, effectively suppress harmonic components during steady-state and dynamic conditions. Even when TSC switching occurs or the system undergoes load changes, the voltage waveform remains nearly sinusoidal with minimal harmonic distortion. This verifies that the proposed hybrid VAR compensator can enhance reactive power support and voltage stability without compromising power quality, making it suitable for deployment in sensitive renewable-rich and inverter-dominated grids.

This case study demonstrates the fault ride-through capability of the proposed GFMI-based PV and BESSs in conjunction with the hybrid VAR compensator. Despite the three-phase fault, the system maintains operational stability and rapidly recovers post-fault voltage and frequency to nominal values. The results validate the effectiveness of the coordinated control strategy in enhancing system resilience and ensuring uninterrupted power quality during severe grid disturbances.

6. Conclusions

(a)

Technical Findings

In order to improve voltage regulation and dynamic reactive power support in inverter-dominated power systems, this study proposed a coordinated hybrid VAR compensation strategy that combines grid-forming inverter-controlled PV and BESS units with a thyristor-switched capacitor (TSC) and a VSC-based STATCOM. The suggested compensator provides both quick dynamic support and long-term voltage regulation by fusing the large steady-state reactive power capacity of the TSC with the quick transient reaction capability of the STATCOM. By offering independent voltage and frequency support without depending on traditional synchronous generation, the grid-forming function of PV and BESS significantly enhances system stability.

Using FPGA-based execution, extensive real-time validation was performed under a variety of operational conditions, including variations in solar irradiation, load fluctuations, grid-connected to islanded transitions, and severe fault disruptions. The outcomes showed greater low-frequency oscillation damping, quicker voltage recovery, less overshoot, robust fault ride-through capabilities, and improved transient performance while preserving power quality. Coordinated operation achieves greater stability through the efficient distribution of dynamic and steady-state VAR support, according to comparative assessments versus STATCOM-only and TSC-only systems.

(b)

Practical and Future Outlook

Practical deployment considerations were also looked at in addition to technical performance. The coordinated control greatly reduces continuous converter loading and minimizes frequent TSC switching operations, thereby lowering the thermal stress on semiconductor devices and extending capacitor bank lifespan, even though the hybrid STATCOM–TSC architecture requires a larger initial investment than standalone compensators. For large-scale renewable-rich grids where steady VAR demand and transient events coexist, this results in lower long-term maintenance costs and better lifespan performance, making the strategy economically advantageous.

Due to hardware limitations and the high-power requirements of multi-source GFM implementation, the current work is restricted to MATLAB/Simulink modeling and FPGA-based real-time simulations. In order to assess controller performance under real-world nonlinearities, communication delays, and switching interactions, future work will concentrate on hardware-in-the-loop (HIL) tests using OPAL-RT/RTDS platforms, followed by a scaled laboratory prototype. A thorough techno-economic analysis utilizing actual utility deployment scenarios will also be the focus of future research.

All things considered, the suggested coordinated hybrid VAR compensation framework provides a reliable, scalable, and cost-effective solution for next-generation low-inertia power systems, facilitating utility-scale renewable integration, microgrids, and smart grid applications while opening the door for realistic field-level implementation.