Strengthening Power Systems for Net Zero: A Review of the Role of Synchronous Condensers and Emerging Challenges

Abstract

System strength is both supplied and demanded in a power system during normal operations and in the presence of disturbances. This is characterised by stable voltage and frequency, supporting renewable generation such as wind and solar. Because the retirement of synchronous generators reduces system strength supply, and the connection of new inverter-based resource (IBR) generators increases demand, there is an urgent need for new sources of system strength. This paper provides an overview of the challenges brought about by grid modernisation. It highlights tangible solutions provided by synchronous condensers (SCs) to bolster grid strength, stability, and reliability while accommodating the rising influx of renewable energy sources (RESs). Furthermore, this paper examines the role of SCs in improving weak grids, voltage control, power quality, short-circuit levels, and inertia management. It introduces the role of innovative hybrid synchronous condenser (HSC) systems to boost grid reliability and resilience. It also elaborates on the optimisation strategies for SC sizing, placement, and control and outlines economic aspects of their deployment. The review also highlights future directions and challenges in SC technology, emphasising the need for ongoing research and development to enhance system design and operation.

1. Introduction

Adding renewable energy sources to power systems worldwide is an effective move to cut greenhouse gas emissions and tackle the large CO₂ output from the energy sector, while also providing a sustainable and eco-friendly source of electricity generation to power electronic systems [1]. Existing investigations have shown that this is feasible, but that the variable and intermittent nature of RESs, such as wind and solar power, pose substantial challenges with regard to grid stability and reliability, underlined by the notion that achieving 100% net zero remains a big challenge for the future [2,3,4,5,6]. Achieving net-zero emissions is one of the main targets for limiting global temperature rises, where the global energy landscape is undergoing a significant transformation, driven by the urgent need to achieve net-zero emissions by 2050 [7]. The power sector plays a pivotal role in this transition, offering zero or negative-carbon electricity, which can significantly lower emissions in various sectors through electrification [8]. Technological advancements provide numerous pathways for decarbonising electricity, encompassing a mix of renewable and low-carbon energy sources, energy storage, and demand management [9,10]. Though variable renewables like wind and solar are projected to become the dominant energy source by 2050, amounting to a predicted 70% increase in global electricity generation, their intermittent nature poses challenges that demand innovative solutions to maintain grid stability, including advanced energy storage and flexible demand management [7,11,12,13].

Electrification trends, such as transportation and building heating, can reshape consumption patterns and necessitate flexible and reliable power system planning [14,15,16]. As we progress towards net-zero goals, regional strategies will likely differ based on resources, infrastructure, and socio-political factors [17]. Though advanced technologies and diversified portfolios can mitigate decarbonisation costs, effective system management remains crucial for ensuring reliability in a highly variable renewable energy landscape [18,19,20,21,22].

Though switching to renewable energy offers a sustainable future, its intermittent nature can cause grid instability issues, like voltage and frequency instability, as well as sub-synchronous oscillations (SSOs) [23]. Different innovative technologies have been presented to address these challenges. Software strategies to address SSOs involve adjusting converter controller settings, implementing digital filters, using supplementary damping controllers (SDCs), and applying grid-forming (GFM) control concepts. Flexible AC transmission system (FACTS) devices such as static var compensators (SVCs) and static synchronous compensators (STATCOMs) are commonly used to mitigate the effects of increasing penetration of RESs [24]. In recent years, synchronous condensers (SCs) have been implemented to address the above-mentioned issues due to their ability to provide voltage control and inject reactive power simultaneously [25,26]. SCs are synchronous machines that operate without generating active power, where after a prime mover to initial spinning, they continue to spin without needing an external source. SCs supply reactive power by adjusting their field voltage, allowing them to address SSOs without needing additional improvements or extra controllers [27]. Several studies have highlighted the benefits of SCs over other hardware solutions, such as STATCOMs and SVCs [27,28,29]. Further, SCs have been investigated in terms of optimisation techniques for allocating synchronous condensers to increase effectiveness within a grid [30,31,32,33]. Some studies have explored the influence of SCs on short-circuit ratio (SCR), suggesting a method to enhance the SCR and frequency response by repurposing decommissioned synchronous machines as SCs [34], because long transmission lines that are usually used to connect RESs to power systems decrease SCR and can lead to SSO issues [35,36,37]. Therefore, SCs have been introduced as a solution for increasing SCR in power systems, where their effects have been investigated [38]. As a result, the role of grid support technologies in different solutions has been presented, including synchronous condensers, which have received more attention in recent research [30,39,40,41,42].

Synchronous condensers traditionally support inertia in power systems, mainly for reactive power compensation and short-circuit current injection [26,32]. Although devices such as STATCOMs and SVCs, as a developed form of the FACTS, are introduced for replacing synchronous condensers for reactive power compensation [43,44], SCs offer an efficient solution for low-inertia, short-circuit power, and reactive power needs [45]. On the one hand, strategic installation locations, such as near-renewable generating stations and high-voltage direct current (HVDC) links, enhance system performance to match conventional power generation technologies, addressing requirements such as system inertia and voltage support [46].

On the other hand, the battery energy storage system (BESS) offers fast response and bidirectional charging, presenting as a vital tool for frequency regulation alongside conventional units, especially in microgrid applications [47,48,49,50,51]. Therefore, microgrids include only inverter-based resources (IBRs) or synchronous generators, whereas for IBRs, different forms of control could be implemented [52]. BESSs can support frequency through sag control, virtual inertial control, and virtual synchroniser control [26,50,53]. Although virtual sag control improves frequency stability [54], virtual inertia control reduces frequency deviation change in a power system [55].

Furthermore, hybrid synchronous condensers (HSCs) are emerging as crucial solutions [56,57]. These technologies offer grid support and stability through their unique functionalities, paving the way for a smooth transition towards a sustainable future. Additionally, the strategic combination of SCs and BESSs presents a promising hybrid solution, offering enhanced grid performance and reliability by effectively managing the challenges of integrating intermittent renewable energy sources, ultimately contributing to a cleaner and more sustainable future [7]. The primary advantage of the BESS to a power system is its capability to rapidly supply a significant amount of active power during frequency fluctuations with a fast response because of its power converters [58]. In power systems with high integration of renewable energy, these types of frequency variations are unavoidable, making BESSs an essential component [59,60,61]. In a hybrid system, a BESS is expected to have characteristics like a fast response, large capacity, and good power density, so it can inject sufficient power into the power system during frequency fluctuation events [60,61].

The main contribution of this paper is a comprehensive review of the challenges and solutions associated with integrating RESs into power systems. This paper provides valuable insights into the complexities of grid stability enhancement by examining issues related to weak grids, voltage control, power quality, short-circuit levels, and inertia management. Furthermore, the paper investigates the role of SCs in addressing these challenges, highlighting their advantages over other compensators. Additionally, the paper explores the concept of HSC systems by offering novel solutions for improving grid reliability and resilience.

This paper contributes significantly towards dealing with the challenges involved in modernising the grid. It reviews the problems in integrating RESs into power systems and considers the growing demand for system strength due to the retirement of synchronous generators (SGs) and the rise of IBR generators. The role of SCs in enhancing grid stability, voltage control, and power quality is highlighted, along with an introduction of a hybrid form of SC to improve grid performance. The paper also studies different optimisation methods for the sizing and placement of SCs and economic considerations for their deployment.

The rest of this paper is organised as follows: Section 2 elaborates on the concept of system strength, detailing its challenges. Section 3 investigates the various unique futures of SCs. Different parameters for grid stability enhancement are reviewed, and a hybrid form of SC is introduced in Section 4. Section 5 studies the optimisation of the size and placement of HSC systems and the control strategy for SCs. Section 6 highlights the economic considerations of HSC integration. Section 7 outlines some future directions for this research, and finally, some conclusions from this research are drawn in Section 8.

2. Weak Grid, Challenges and Solutions

A weak grid is characterised by low SCR, inertia, and impedance. Other definitions of this have been introduced in various references, some of which are noted in Table 1. These factors significantly impact the stability parameters of the grid, leading to voltage and frequency deviations. Furthermore, weak grids lack resilience, resulting in delayed or potentially catastrophic recovery from faults that can result in unreliable service to customers [62,63].

Table 1. Some definitions of system strength.

Refs.	Definitions
[64]	The ability of the power system to maintain core characteristics like voltage and frequency as consistently as possible under diverse operating conditions.
[65]	System strength is linked to the sensitivity of voltages following changes in active and reactive powers in the system (dV/dP, dV/dQ).
[66]	The power system can withstand changes in voltage magnitude, phase angle, and waveform at any given location, with or without disturbances.
[67]	A quality related to the overall stability of the voltage waveform, including its ability to return to a stable state after disturbance events.

Table 1. Some definitions of system strength.

Refs.	Definitions
[64]	The ability of the power system to maintain core characteristics like voltage and frequency as consistently as possible under diverse operating conditions.
[65]	System strength is linked to the sensitivity of voltages following changes in active and reactive powers in the system (dV/dP, dV/dQ).
[66]	The power system can withstand changes in voltage magnitude, phase angle, and waveform at any given location, with or without disturbances.
[67]	A quality related to the overall stability of the voltage waveform, including its ability to return to a stable state after disturbance events.

Various solutions have been proposed to address identified inertia deficiencies, including physical inertia from SGs, fast frequency response (FFR) sources like batteries, and the potential optimisation of SCs equipped with flywheels [68]. Accordingly, SCs can increase SCR to increase the strength of a grid [69].

Table 2. Key issues of weak grids with low system strength.

Problems	Description
Plant disconnection after credible faults	Weak grid systems, particularly in remote areas, can lead to the disconnection of power plants.
Adverse interactions among inverter-based plants	Inverter-based plants may interact negatively with each other, causing operational issues.
Insufficient active and reactive power	Low system strength can result in insufficient provision of active and reactive power after fault clearance.
Management of voltage control	Weak grid systems may struggle to manage voltage control, leading to voltage-related challenges.
Operation of protection equipment	Low system strength can impact the operation of protection equipment, affecting system reliability.

summarises the challenges associated with weak grid systems and low system strength [70,71].

Enhancing system strength involves minimising electrical distances to large SGs. Solutions to voltage control issues in weak grids encompass network reinforcements through additional lines/transformers and the deployment of SVCs, STATCOMs, thyristor-controlled series capacitors (TCSCs), and synchronous condensers [72]. To optimise the cost of improving system strength, new IBRs may need to operate with lower SCRs [73]. A categorised overview of the pros and cons of solutions for system strength deficiencies is presented in Table 3, detailing different types of solutions to improve the strength parameters of a system [74].

Table 3. Advantages and disadvantages of using different kinds of compensators.

Compensator	Benefits	Drawbacks	Refs.
IBRs like GFM	Increased system strength Decreased interactions between IBGs Lowest cost	Difficult to coordinate different vendors Limited fault current	[75,76,77,78,79]
TCSCs	Improved voltage stability Improved fault current Improved active power damping	No inertia	[76,80,81,82]
SGs	System strength enhancement Improved voltage and frequency control Improved system stability Inertia enhancement	Higher cost than synchronous condenser Against NetZero plan	[74,83,84,85]
SVCs and STATCOMs	Improved voltage stability Improved fault current	No inertia	[86,87,88,89]

Table 3. Advantages and disadvantages of using different kinds of compensators.

Compensator	Benefits	Drawbacks	Refs.
IBRs like GFM	Increased system strength Decreased interactions between IBGs Lowest cost	Difficult to coordinate different vendors Limited fault current	[75,76,77,78,79]
TCSCs	Improved voltage stability Improved fault current Improved active power damping	No inertia	[76,80,81,82]
SGs	System strength enhancement Improved voltage and frequency control Improved system stability Inertia enhancement	Higher cost than synchronous condenser Against NetZero plan	[74,83,84,85]
SVCs and STATCOMs	Improved voltage stability Improved fault current	No inertia	[86,87,88,89]

3. Synchronous Condensers

Synchronous condensers, first introduced in the 1930s [90], initially contributed to grid stability but also created challenges like high costs and limited exposure to renewable energy [91]. Some technologies such as STATCOMs partly replaced SCs, but they fall short in compensatory capacity, especially concerning grid inertia and stability [92]. Though static VAR devices could supply reactive power, this solution possesses voltage regulation challenges. STATCOMs, another solution, offer continuous adjustment, although they have limitations during grid faults. Though operational SCs can provide up to six times their rated capacity during faults [93], they experience inherent system inertia, a crucial feature for improved voltage stiffness and effective prevention of frequency instability during network incidents [69]. Additionally, the inertia provided by SCs has the potential to enhance the overall behaviour of the power system, particularly when augmented with flywheels, offering a valuable measure to address uncertain and unpredictable system conditions [40,53,94].

In this process, the SC’s rotor is connected to a DC-excited armature that rotates synchronously with the grid, creating a magnetic field that supplies reactive power [95]. The power factor of the connected network is thus controlled through field excitation, allowing manipulation of the power factor by adjusting the field current, either increasing or decreasing it as needed [96,97]. A SC operates without a prime mover in steady-state condition at constant speed, regardless of the load. This characteristic ensures synchronisation with the electrical network, proper power factor correction, and reactive power compensation. In a no-load situation, the condenser exhibits a leading power factor, like a capacitor, whereby adjusting field excitation allows for control over reactive power absorption or injection [98].

An over-excited synchronous machine, acting as a power factor correction device, can regulate the power factor by adjusting field excitation. To operate efficiently as a doubly fed induction machine, a synchronous condenser requires a converter that facilitates bidirectional power flow regulation, ensuring optimal operation without real power absorption or delivery [99]. Accordingly, the converter’s role is crucial in managing power flow, allowing the synchronous condenser to effectively fulfil its intended function [41,100].

The short-term overload capability of the grid, depending on the type of SC used, is increased, where this can provide more than six times its rating in up to a few seconds [101], which in turn enhances system support during emergencies or contingencies. Therefore, SCs are a practical choice for reactive power supply in wind/solar farms [102,103,104].

Low-voltage ride-through is another distinctive feature of SCs that maintains connection and provides smooth and reliable operation, even under extremely low voltage contingencies [99,105]. In this way, inherent inertia, with the help of modern excitation systems, can provide reliable support. However, compensators based on power electronics cannot have mechanical inertia and therefore cannot deliver ride-through performance comparable to SCs [58]. Although SCs are not the fastest compensators to respond, particularly in events that may happen in the grid, with the help of modern excitation technology and control systems, a synchronous condenser is fast enough to meet dynamic response requirements [97].

A synchronous condenser can help improve the power factor and stabilise the voltage [106]. A short-circuit incident in the power system can provide reactive power support and help maintain system voltage stability by absorbing or injecting reactive power as needed. Therefore, this provides real short-circuit strength to the grid, which improves system stability with weak interconnections and enhances system protection [32].

A synchronous condenser is not a source of harmonics, where, contrastingly, it can improve the stability of the power system by absorbing harmonic currents [96,100,107]. This feature enables ease of integration into existing networks. This way, connecting inverters in a grid could increase the need for AC harmonic filters, whereby usage of SCs could be valuable in a grid that is fed by IBRs [41,108].

4. Grid Stability Enhancement

In response to an increasing presence of RESs in power systems, the role of SCs has gained prominence. Numerous studies have highlighted the critical role of synchronous condensers in enhancing grid stability [39,42,101,109,110]. The asynchronous nature of RESs can lead to frequency and voltage fluctuations, where, in turn, the stability of power systems becomes affected by these [101,109]. In [110], SCs’ support for stable system frequency is introduced. In [111], it is stated that several countries have explored and already integrated synchronous condensers enhanced with flywheels as part of their efforts to enhance the reliability of their power systems in recent years. SCs, emulating the behaviour of SGs, offer an acceptable solution by providing system inertia and having the capability to inject or absorb reactive power rapidly to stabilise a grid. One study [42] has investigated different scenarios to check the abilities of SCs to provide reactive power in a grid, showing they can contribute to secondary control by returning the voltage to nominal values. These attributes are precious in regions with a high penetration of RESs [39,42]. SCs are desired for their ability to improve grid stability, enhance voltage control, and ensure power quality [62]. Further, SCs contribute to increasing the short-circuit level, which is vital for grid reliability, where their optimal location and control strategies are key subjects of study. However, economic considerations must also be evaluated to ensure their cost-effectiveness [63]. Separate from these, other challenges remain, where research into these areas can guide future directions about utilising SCs to enhance grid stability and integrate renewable energy effectively.

4.1. Voltage Control and Power Quality

Incorporating synchronous condensers into power systems addresses voltage control and power quality issues associated with RES integration. The ability of these SCs to regulate voltage levels with absorption or injection of reactive power is well documented [33,102,112]. Accordingly, they act as a buffer, smoothing out voltage fluctuations and enhancing the overall reliability of electricity supply [42]. In [42], SCs were employed in rural regions with fragile power grids due to their innate inertia response voltage regulation, challenges that are typically difficult to address solely with power electronic systems. Further, [113] presents a methodology for optimising the configuration of SCs in power grids with voltage support issues, with an approach centred on preventing and managing grid emergencies. In [42,95,114], researchers have explored constructive collaboration between synchronous condensers and battery energy resources to manage voltage under specific circumstances and investigated the distinct contributions of each component in maintaining voltage stability. The challenges associated with voltage control and power quality in power systems with high renewable energy integration are outlined in Table 4. Additionally, the role of SCs in mitigating these challenges is elaborated in the table.

Incorporating synchronous condensers into power systems offers a holistic solution to various voltage control and power quality challenges [12], complementing power electronic systems and battery energy resources and enhancing the stability, reliability, and resilience of power grids.

Table 4. Voltage control and power quality issues and the role of SCs in addressing these.

Refs.	Issue Description	Synchronous Condensers’ Effect
[115]	Poor voltage regulation	SCs can absorb or inject reactive power to regulate voltage levels, ensuring a stable and consistent voltage supply.
[116]	Undesired interaction between control systems	SCs act as buffers, smoothing out voltage fluctuations caused by control system interactions and enhancing overall system reliability.
[117]	Hunting of network voltage control schemes	SCs provide inertia response voltage regulation, preventing overreactions of voltage regulators and synchronisation voltage oscillations.
[118]	Wider propagation of voltage dip during a disturbance	SCs improve grid resilience by mitigating the spread of voltage dips during disturbances, ensuring synchronisation impacts.
[119]	Large change in steady-state and dynamic voltage following a reactive plant switching	SCs work in conjunction with battery energy resources to manage voltage during reactive power changes, maintaining stability.
[120]	Dynamic over-voltages following FRT causing plant disconnection	SCs can provide rapid voltage support during fault ride-through (FRT) events, preventing voltage spikes that lead to disconnections.
[121]	Potential susceptibility of phase-locked loop (PLL)	SCs enhance signal quality and synchronisation, reducing interference risks and improving the performance of PLL circuits.
[122]	Maloperation of the protection system	SCs contribute to system resilience by providing stable voltage support, reducing the frequency of nuisance tripping and enhancing safety.

Table 4. Voltage control and power quality issues and the role of SCs in addressing these.

Refs.	Issue Description	Synchronous Condensers’ Effect
[115]	Poor voltage regulation	SCs can absorb or inject reactive power to regulate voltage levels, ensuring a stable and consistent voltage supply.
[116]	Undesired interaction between control systems	SCs act as buffers, smoothing out voltage fluctuations caused by control system interactions and enhancing overall system reliability.
[117]	Hunting of network voltage control schemes	SCs provide inertia response voltage regulation, preventing overreactions of voltage regulators and synchronisation voltage oscillations.
[118]	Wider propagation of voltage dip during a disturbance	SCs improve grid resilience by mitigating the spread of voltage dips during disturbances, ensuring synchronisation impacts.
[119]	Large change in steady-state and dynamic voltage following a reactive plant switching	SCs work in conjunction with battery energy resources to manage voltage during reactive power changes, maintaining stability.
[120]	Dynamic over-voltages following FRT causing plant disconnection	SCs can provide rapid voltage support during fault ride-through (FRT) events, preventing voltage spikes that lead to disconnections.
[121]	Potential susceptibility of phase-locked loop (PLL)	SCs enhance signal quality and synchronisation, reducing interference risks and improving the performance of PLL circuits.
[122]	Maloperation of the protection system	SCs contribute to system resilience by providing stable voltage support, reducing the frequency of nuisance tripping and enhancing safety.

4.2. Short-Circuit Level and Power Quality

Integrating RESs into power systems introduces dynamic fluctuations in power generation, which can impact short-circuit levels and power quality. This has received considerable attention in the literature due to its implications for grid reliability and overall system performance [123,124]. Table 5 illustrates different SCR types and definitions that provide various perspectives on assessing the strength and interactions within power systems, tailored to specific analytical needs and considerations [125,126].

Table 5. Different definitions of short-circuit ratio.

SCR Type	Definition	Formulation	Components	Refs.
Basic SCR (BSCR)	The ratio of short-circuit apparent power from three-phase to ground fault at the Point of Connection (POC) and the rating of the IBRs at that location.	$\frac{S_{POC}\left[MVA\right]}{P_{POC}\left[MW\right]}$	$S_{POC}$: rated power without IBRs $P_{POC}$: after connecting IBRs	[127,128]
Weighted SCR (WSCR)	Defines operational difficulties of power transmission over key interfaces of IBRs by considering the weighted short-circuit capacities and ratings of IBRs connected on a common bus.	$\frac{\sum _i^N{SC}_{POCi}\left[MVA\right]\times P_{POCi}\left[MW\right]}{{\left(\sum _i^N{P}_{POCi}\left[MW\right]\right)}^2}$	${SC}_{POC}$: the SCR capacity at bus i without IRBs $P_{POCi}$: active power of connected IBRs	[129,130]
Composite SCR (CSCR)	Provides an estimate of the equivalent system impedance represented by multiple RESs by creating a general medium bus voltage.	$\frac{{SC}_{POC}\left[MVA\right]}{\sum _i^N{P}_{IBRs}\left[MW\right]}$	${SC}_{POC}$: the SCR capacity without RESs Denominator: sum of connected IBRs	[12,128]
Inverter Interaction Level SCR (IILSCR)	Measures interactions among IBRs by considering power output from RESs and their power inflow from nearby IBRs, using a line power flow method.	$\frac{{SC}_{POC}\left[MVA\right]}{P_{POC}+\sum _{m=1,m\ne i}^N{P}_{POC(m-i)}}$	${SC}_{POC}$: the SCR capacity without RESs $P_{POC}$: active power of connected IBRs	[131,132]
SCR with Interaction Factors (SCRIF)	Directly accounts for impedances measured among considered RESs, determining changes in voltage at one bus in response to changes at another.	$\frac{S_i\left[MVA\right]}{P_i+\sum _j\left(\frac{{∆V}_i}{{∆V}_j}\times P_j\right)}$	$S_i$: SCR level of the bus $P_{i/j}$: IBRs rate ${∆V}_{i/j}$: bus voltage variation	[126,133]
Multi-Infeed Effective SCR (MESCR)	Considers the interaction among DC/AC rectifier stations to calculate an effective SCR, accounting for power inflow from nearby IBRs.	$\frac{S_{POC,i}-Q_{POC,i}}{P_{DC,i}+\sum _{j=1,j\ne i}^K\left|\frac{Z_{ij}}{Z_{jj}}\right|\times P_{DC,j}}$	$S_{POC,i}$: SCR of AC system $Q_{POC,i}$: compensated reactive power $P_{DC}$: active power DC system $\left|\frac{Z_{ij}}{Z_{jj}}\right|$: equivalent mutual impedance	[129,134]
Site-Dependent SCR (SDSCR)	Accounts for the impact of multiple RESs’ interactions installed at multiple buses individually, considering operating conditions and voltage stability boundaries.	$\frac{{\left|V_{POC}\right|}^2}{\left(P_{POCi}+\sum _{j\ne i}{P}_{POCj}{w}_{ij}\right)\left|Z_{POCii}\right|}$	$V_{POC},P_{POC},Z_{POC}$: voltage power and equal impedance of IBRs $w_{ij}$: the ratio of impedances and voltages of $i$ and $j$ bus	[130,135]

Table 5. Different definitions of short-circuit ratio.

SCR Type	Definition	Formulation	Components	Refs.
Basic SCR (BSCR)	The ratio of short-circuit apparent power from three-phase to ground fault at the Point of Connection (POC) and the rating of the IBRs at that location.	$\frac{S_{POC}\left[MVA\right]}{P_{POC}\left[MW\right]}$	$S_{POC}$: rated power without IBRs $P_{POC}$: after connecting IBRs	[127,128]
Weighted SCR (WSCR)	Defines operational difficulties of power transmission over key interfaces of IBRs by considering the weighted short-circuit capacities and ratings of IBRs connected on a common bus.	$\frac{\sum _i^N{SC}_{POCi}\left[MVA\right]\times P_{POCi}\left[MW\right]}{{\left(\sum _i^N{P}_{POCi}\left[MW\right]\right)}^2}$	${SC}_{POC}$: the SCR capacity at bus i without IRBs $P_{POCi}$: active power of connected IBRs	[129,130]
Composite SCR (CSCR)	Provides an estimate of the equivalent system impedance represented by multiple RESs by creating a general medium bus voltage.	$\frac{{SC}_{POC}\left[MVA\right]}{\sum _i^N{P}_{IBRs}\left[MW\right]}$	${SC}_{POC}$: the SCR capacity without RESs Denominator: sum of connected IBRs	[12,128]
Inverter Interaction Level SCR (IILSCR)	Measures interactions among IBRs by considering power output from RESs and their power inflow from nearby IBRs, using a line power flow method.	$\frac{{SC}_{POC}\left[MVA\right]}{P_{POC}+\sum _{m=1,m\ne i}^N{P}_{POC(m-i)}}$	${SC}_{POC}$: the SCR capacity without RESs $P_{POC}$: active power of connected IBRs	[131,132]
SCR with Interaction Factors (SCRIF)	Directly accounts for impedances measured among considered RESs, determining changes in voltage at one bus in response to changes at another.	$\frac{S_i\left[MVA\right]}{P_i+\sum _j\left(\frac{{∆V}_i}{{∆V}_j}\times P_j\right)}$	$S_i$: SCR level of the bus $P_{i/j}$: IBRs rate ${∆V}_{i/j}$: bus voltage variation	[126,133]
Multi-Infeed Effective SCR (MESCR)	Considers the interaction among DC/AC rectifier stations to calculate an effective SCR, accounting for power inflow from nearby IBRs.	$\frac{S_{POC,i}-Q_{POC,i}}{P_{DC,i}+\sum _{j=1,j\ne i}^K\left|\frac{Z_{ij}}{Z_{jj}}\right|\times P_{DC,j}}$	$S_{POC,i}$: SCR of AC system $Q_{POC,i}$: compensated reactive power $P_{DC}$: active power DC system $\left|\frac{Z_{ij}}{Z_{jj}}\right|$: equivalent mutual impedance	[129,134]
Site-Dependent SCR (SDSCR)	Accounts for the impact of multiple RESs’ interactions installed at multiple buses individually, considering operating conditions and voltage stability boundaries.	$\frac{{\left|V_{POC}\right|}^2}{\left(P_{POCi}+\sum _{j\ne i}{P}_{POCj}{w}_{ij}\right)\left|Z_{POCii}\right|}$	$V_{POC},P_{POC},Z_{POC}$: voltage power and equal impedance of IBRs $w_{ij}$: the ratio of impedances and voltages of $i$ and $j$ bus	[130,135]

Synchronous condensers play a vital role in enhancing the short-circuit level of power systems [62]. Accordingly, as power grids incorporate more RESs, the available short-circuit current, which is essential for fault clearing and system protection, may decrease [136]. SCs can provide rapid fault current support by injecting reactive power during fault conditions, thereby reinforcing the short-circuit level and ensuring the reliable operation of protective devices [32]. A SC can also enhance short-circuit capacity, voltage control and system inertia, as indicated in [73,137]. In [98], a conventional method is performed to determine the short circuit in power systems.

4.3. Inertia Issue and Management

Inertia’s role in maintaining grid stability and ensuring reliable power system operation is apparent. Synchronous generators have traditionally provided substantial inertia support to power grids, helping to stabilise frequency. However, with the increasing integration of RESs, which typically lack inherent rotational inertia, managing inertia has become a significant challenge for power system operators worldwide.

Table 6 outlines various strategies in different countries to address the issue of inertia. Each listed country employs a combination of measures tailored to its specific power system needs, from setting regional inertia floors to modifying grid codes and procuring inertia products [138]. Though most systems determine inertia floors for the entire grid, Australia calculates requirements for regional sub-networks with secure inertia levels across different regions [139]. Additionally, modifications to grid codes regarding the Rate of Change of Frequency (RoCoF) are now being considered to address diminishing inertia levels. For instance, the UK is contemplating relaxation of the current 0.125 Hz/s grid code to 0.5 Hz/s or even 1 Hz/s, and Ireland and Northern Ireland are testing adjustments to their protection systems to relax RoCoF limits to 1 Hz/s [140,141]. Furthermore, monitoring and forecasting system inertia is crucial, with transmission power systems developing methods based on dynamic simulations or online assessments to assess and predict inertia levels [139]. Lastly, the provision of inertia through market-based approaches is gaining traction, with some countries enhancing inertia levels [141,142].

Table 6. Management of inertia issues in different countries.

Country	Inertia Management Strategies	Refs.
Australia	Setting regional inertia floors Considering modifications to RoCoF limits in grid code Developing methods for monitoring and forecasting inertia Procuring inertia products	[74,139,141,142,143,144,145,146,147]
Canada	Procuring inertia products via synthetic inertia services Setting requirements for Wind Power Plants (WPPs) to support frequency response	[147,148,149]
China	Requiring renewable generation stations to provide necessary inertia support	[146,150]
Ireland	Introducing Synchronous Inertia Response (SIR) service through auctions Adjusting grid code to relax RoCoF limits	[147,151,152]
UK	Modifying grid code to relax RoCoF limits Procuring inertia services through tenders	[140,150,151,153]
US	Setting critical inertia requirements Requiring wind and solar resources to provide a governor-like response	[149,154,155]

Table 6. Management of inertia issues in different countries.

Country	Inertia Management Strategies	Refs.
Australia	Setting regional inertia floors Considering modifications to RoCoF limits in grid code Developing methods for monitoring and forecasting inertia Procuring inertia products	[74,139,141,142,143,144,145,146,147]
Canada	Procuring inertia products via synthetic inertia services Setting requirements for Wind Power Plants (WPPs) to support frequency response	[147,148,149]
China	Requiring renewable generation stations to provide necessary inertia support	[146,150]
Ireland	Introducing Synchronous Inertia Response (SIR) service through auctions Adjusting grid code to relax RoCoF limits	[147,151,152]
UK	Modifying grid code to relax RoCoF limits Procuring inertia services through tenders	[140,150,151,153]
US	Setting critical inertia requirements Requiring wind and solar resources to provide a governor-like response	[149,154,155]

4.4. Hybrid Synchronous Condenser System

SCs, like other components in a network, have their disadvantages, with most having drawbacks common to other rotating machines [156,157]. The most common problems include losses in spinning elements, mechanical wear, and slower response time compared to power electronic technologies usually found in grids based on IBRs [43]. Other drawbacks that can be noted are high maintenance cost, high noise production, and lack of self-starting torque, where accordingly, auxiliary equipment should be used. Additionally, the cost of small-sized SCs is higher than that of static capacitors of the same rating [158]. SCs and IBRs each offer unique features that are beneficial to the grid. Table 7 outlines the key differences between SCs and power electronic converters [99,159]. The combination of two technologies, IBRs and SCs, limits some of the mentioned benefits in Table 7 [160]. Drawbacks of using newer technology over a SC with the same rating relate to diminished contribution in overloading capability, more complex installation and area, and complex coordinated control [161,162]. HSCs combine the strengths of SCs with modern power electronic converters or BESSs. These hybrid systems address grid stability challenges by leveraging the inertia and short-circuit current contribution of SCs with the fast response and control flexibility of power electronics. Different configurations can be explored, such as integrating SC with FACTS like STATCOM, either in series or parallel, to enhance voltage support, and fault ride-through capabilities have been evaluated to utilise their complementary characteristics in mitigating overshoots during fault recovery processes in a weak grid [163,164]. Alternatively, SCs can be combined with BESSs, using hybrid inverter systems that provide rapid frequency and voltage regulation. These configurations offer a range of benefits, including improved renewable energy integration, enhanced grid stability, and reliable active and reactive power support [165].

Integrating BESSs with SCs to form HSCs offers a range of compelling advantages [166]. Different combinations could be considered, with SCs accompanied by a Grid-Forming (GFM) or a Grid-Following (GFL) inverter [69]. The primary benefits of GFM converters are their efficient and faster methods for voltage and frequency support [167], where they can enhance the overall stability of the grid and ensure a more consistent energy output, particularly in the presence of intermittent renewable resources [77]. Table 8 summarises the comparison of GFL and GFM technologies in grid stability [168,169,170]. Additionally, the combined use of SCs and BESSs facilitates rapid responses to grid disturbances, ensuring quick voltage regulation and system strength enhancement [165]. Furthermore, these hybrid systems can be a reliable source of active and reactive power, crucial for maintaining grid voltage and frequency levels and improving power quality [58].

The integration of advanced inverter technologies enhances the functionality and stability of HSC systems [56]. GFL control is a mature technology that relies on a stable grid voltage for synchronisation and operation, functioning as a constant current source. It is widely used commercially, with well-defined standards [171]. However, GFL technology cannot provide black start capability and is limited in performance under weak grid conditions [172,173].

On the other hand, GFM control is an emerging technology capable of maintaining its voltage and frequency as a constant voltage source, thus enabling black start capability and islanding operation GFM technology. It offers superior performance in weak grid conditions and does not rely on a PLL for synchronisation, although it may use one for overall system response [174]. Despite its advantages, GFM technology has limited deployment experience in interconnected power systems [175].

By optimising the coordination between SC and BESS, HSCs can effectively address grid stability challenges, enhance renewable energy integration, and contribute to transitions towards cleaner and more reliable energy systems [176]. However, a notable drawback arises when these converters need to be shifted to current control when the current exceeds specified limits [167]. Figure 1 shows a scheme of a HSC as the combination of SCs and BESSs connecting to a grid. To connect the BESS to the grid, power electronic converters are used that can operate as either GFM or GFL inverters.

Table 7. Key differences between SCs and power electronic converters.

Features	Synchronous Condensers	Power Electronic Converters	Refs.
Overloading capability	Significant overloading capability and short-circuit current contribution	Limited overloading capability for short periods	[159]
Inertia support	Possess kinetic energy in rotating mass for inertia support	Lacks inherent inertia support	[99]
Control bandwidth	Lower bandwidth	Higher bandwidth	[177]
Frequency and voltage control	Slower frequency and voltage control	Fast frequency and voltage control	[159]
Control strategies	Limited implementation possibilities	Various control strategies can be implemented	[99]
Current handling	Absorbs a portion of full-rated current	Provides full-rated current in both inductive and capacitive modes	[177]

Table 7. Key differences between SCs and power electronic converters.

Features	Synchronous Condensers	Power Electronic Converters	Refs.
Overloading capability	Significant overloading capability and short-circuit current contribution	Limited overloading capability for short periods	[159]
Inertia support	Possess kinetic energy in rotating mass for inertia support	Lacks inherent inertia support	[99]
Control bandwidth	Lower bandwidth	Higher bandwidth	[177]
Frequency and voltage control	Slower frequency and voltage control	Fast frequency and voltage control	[159]
Control strategies	Limited implementation possibilities	Various control strategies can be implemented	[99]
Current handling	Absorbs a portion of full-rated current	Provides full-rated current in both inductive and capacitive modes	[177]

Table 8. Comparison of GFL and GFM inverters.

Refs.	Feature	GFL Mode	GFM Mode
[177]	Control mode [177]	Follows grid voltage	Forms grid voltage
[170]	Voltage stability	High stability	Very high stability
[169]	Frequency stability	High stability	Ultra-high stability
[177]	Grid synchronisation	Grid connection need	Can operate islanded
[168]	Reactive power support	Limited support	High reactive power support
[77]	Control flexibility	Less flexible	More flexible
[174]	Power quality	Good	Excellent

Table 8. Comparison of GFL and GFM inverters.

Refs.	Feature	GFL Mode	GFM Mode
[177]	Control mode [177]	Follows grid voltage	Forms grid voltage
[170]	Voltage stability	High stability	Very high stability
[169]	Frequency stability	High stability	Ultra-high stability
[177]	Grid synchronisation	Grid connection need	Can operate islanded
[168]	Reactive power support	Limited support	High reactive power support
[77]	Control flexibility	Less flexible	More flexible
[174]	Power quality	Good	Excellent

Despite the disadvantages of synchronous condensers, it is imperative to study their role in the future of grids. SCs offer unique advantages that make them indispensable in evolving energy landscapes. Their ability to provide critical grid support functions, such as inertia and reactive power support, is crucial for maintaining grid stability, especially in regions with a high penetration of renewable energy sources. When combined with BESSs, HSCs can effectively contribute to addressing issues related to deviating active power [58,167,178]. As power systems transition toward cleaner and more sustainable energy solutions, HSCs can play a pivotal role in facilitating renewables’ integration and ensuring grid reliability. Understanding how to optimise their deployment, mitigate their drawbacks, and benefit from their strengths is essential for building resilient and efficient grids that meet future challenges while achieving sustainability and net-zero emissions goals. Table 9 outlines published research that has considered technical aspects of the effectiveness of SCs.

Table 9. Summary of published research papers with a technical perspective.

Refs.	Year	Summary	Case Study	Software
[99]	2024	The potential of utilising static frequency converters to initiate synchronous condensers is explored, including investigating their start-up principles, coordinating control, and configuring protective devices.	SC	PSS/E
[40]	2023	A quantitative stability index for analysing systems with black-box IBRs and SCs is assessed. This is derived from impedance-based stability analysis and the influence of SCs on this stability index within a single-machine infinite-bus system.	SC	PSCAD/EMTDC
[75]	2023	A GFM control strategy is introduced that incorporates a simulated SC operating alongside a controlled current source. The strategy focuses on accurately representing the swing equation in the control scheme.	SC, BESS, GFM	MATLAB
[179]	2022	A Voltage and Speed Tracking Excitation Control (VSTEC) strategy is proposed using fuzzy control to achieve an increase in the stability and reactive power consumption of different synchronous condensers under varying conditions as sudden voltage rises.	Dual-excited SC (DESC)	PSCAD/EMTDC
[42]	2022	A distributed secondary control system was designed, and SCs play a role in voltage support. Frequency and voltage saw rapid improvement. Though the BESS responds quickly, the rotating machines significantly enhance the RoCoF and overall system stability.	SC, BESS	MATLAB
[113]	2022	Concentrating on transient voltage instability, this study suggests converting a power unit into a SC operation. Additionally, an optimal configuration method for this retrofitting process is introduced.	SC, UHVDC	DIgSILENT
[39]	2021	The voltage support capabilities of STATCOMs and SCs in a 3-phase fault are compared. The results indicate that using only STATCOMs requires significantly fewer dynamic reactive power compensation devices for stable voltage recovery than using only synchronous condensers, which is attributed to differences in device physics.	SC, STATCOMs	PSCAD
[56]	2020	The deployment and assessment of a HSC, combining a SC and STATCOM, investigates the potential of integrating SCs with a BESSs while evaluating their individual and combined grid support functionalities, including using GFM control for BESSs to enhance grid response characteristics.	SC, BESS, GFM, GFL	DIgSILENT
[103]	2020	Small-signal analysis revealed that a system with SC support can accommodate a higher share of photovoltaic generation. SCs expand the system’s operational capabilities, reducing the necessity for curtailing renewable energy.	SC	N/A
[167]	2019	The study focuses on a hybrid system featuring a SC and a BESS, with a central theme of coordinated reactive power sharing. It aims to highlight its benefits and discover potential applications in offering ancillary services.	SC, BESS	DIgSILENT
[180]	2018	A methodical evaluation approach is provided to simulate and analyse the impacts of SCs on a grid, considering the optimisation of technical parameters.	SC	MATLAB

Table 9. Summary of published research papers with a technical perspective.

Refs.	Year	Summary	Case Study	Software
[99]	2024	The potential of utilising static frequency converters to initiate synchronous condensers is explored, including investigating their start-up principles, coordinating control, and configuring protective devices.	SC	PSS/E
[40]	2023	A quantitative stability index for analysing systems with black-box IBRs and SCs is assessed. This is derived from impedance-based stability analysis and the influence of SCs on this stability index within a single-machine infinite-bus system.	SC	PSCAD/EMTDC
[75]	2023	A GFM control strategy is introduced that incorporates a simulated SC operating alongside a controlled current source. The strategy focuses on accurately representing the swing equation in the control scheme.	SC, BESS, GFM	MATLAB
[179]	2022	A Voltage and Speed Tracking Excitation Control (VSTEC) strategy is proposed using fuzzy control to achieve an increase in the stability and reactive power consumption of different synchronous condensers under varying conditions as sudden voltage rises.	Dual-excited SC (DESC)	PSCAD/EMTDC
[42]	2022	A distributed secondary control system was designed, and SCs play a role in voltage support. Frequency and voltage saw rapid improvement. Though the BESS responds quickly, the rotating machines significantly enhance the RoCoF and overall system stability.	SC, BESS	MATLAB
[113]	2022	Concentrating on transient voltage instability, this study suggests converting a power unit into a SC operation. Additionally, an optimal configuration method for this retrofitting process is introduced.	SC, UHVDC	DIgSILENT
[39]	2021	The voltage support capabilities of STATCOMs and SCs in a 3-phase fault are compared. The results indicate that using only STATCOMs requires significantly fewer dynamic reactive power compensation devices for stable voltage recovery than using only synchronous condensers, which is attributed to differences in device physics.	SC, STATCOMs	PSCAD
[56]	2020	The deployment and assessment of a HSC, combining a SC and STATCOM, investigates the potential of integrating SCs with a BESSs while evaluating their individual and combined grid support functionalities, including using GFM control for BESSs to enhance grid response characteristics.	SC, BESS, GFM, GFL	DIgSILENT
[103]	2020	Small-signal analysis revealed that a system with SC support can accommodate a higher share of photovoltaic generation. SCs expand the system’s operational capabilities, reducing the necessity for curtailing renewable energy.	SC	N/A
[167]	2019	The study focuses on a hybrid system featuring a SC and a BESS, with a central theme of coordinated reactive power sharing. It aims to highlight its benefits and discover potential applications in offering ancillary services.	SC, BESS	DIgSILENT
[180]	2018	A methodical evaluation approach is provided to simulate and analyse the impacts of SCs on a grid, considering the optimisation of technical parameters.	SC	MATLAB

5. Optimisation of Size, Location, and Control Strategies

Efforts towards SC deployment for short-circuit level enhancement and power quality improvement have gained more attention in recent years, with researchers developing control strategies that enable swift responses to dynamic grid conditions, which in turn ensure optimal performance, even with varying renewable energy source outputs [30,181].

Accordingly, efforts to optimise the deployment of SCs have been a focal point of research [34,125]. Studies have explored various sizing and siting to maximise their effectiveness [32], where optimisation models have also been developed to determine the optimal location and capacity of synchronous condensers within power systems [182]. In [183], the effectiveness of the SC’s location regarding voltage stability was investigated in a medium-voltage (MV) electric power line. Further, in [180], SC efficiency was evaluated through its installation at various locations within a network encompassing a wind farm. This installation aimed to provide both reactive compensation and active power injection.

Though the conventional proportional and integral (PI) control method offers a straightforward means of regulating terminal voltage to its reference values in SC control, this tends to introduce post-fault oscillations due to its inadequate regulation properties [184]. Furthermore, the effectiveness of linear controllers in controlling non-linear systems is hindered by their reliance on current operating conditions, prompting the adoption of non-linear controllers, notably employing differential geometry principles, as seen in traditional synchronous generators, to enhance control performance [90,185,186]. One study [187] introduced a hybrid excitation control technique for a recently developed SC, where the technique combined a conventional linear controller with an advanced non-linear controller rooted in the principles of differential geometry. Table 10 states some research gaps that have been identified in optimisation and control.

Further, SCs inject or observe reactive power to regulate voltage and share a system’s reactive power. Moreover, given its rotating nature, a SC can offer an inherent inertial response and contribute short-circuit current to aid in controlling the RoCoF and bolster system stability.

$$V_{{SC}_i}=V_{{SC}_i}^{\ast }-K_{{SC}_i}^Q\times (Q_{{SC}_i}^m-Q_{{SC}_i}^{\ast })$$

(1)

where

• $V_{{SC}_i}^{\ast }$: is the nominal voltage amplitude;
• $K_{{SC}_i}^Q$: is the droop factor;
• $Q_{{SC}_i}^m$: is the SG-measured reactive power output.

The SC can be represented as a synchronous motor without a mechanical load, where the voltage is regulated through an Automatic Voltage Regulator (AVR), and the control system integrates an overexcitation limiter (OEL) [95].

A control strategy for managing reactive power is usually suggested for SCs with a dual-loop control mechanism, where a closed-loop control system can help to increase response. Further, the weighting coefficient could determine the behaviour of the external control loop during steady-state conditions to control voltage and reactive power. This process is depicted in a simplified block diagram of the excitation control system shown in Figure 2 [93,188]. In [189], different standards for excitation control systems are introduced.

Table 10. Summary of the published research papers with an optimisation perspective.

Refs.	Year	Done	Method	Software
[76]	2023	A Genetic Algorithm-based optimisation method is proposed for optimally sizing and placing SCs to strengthen the grid, with implementation in MATLAB.	Genetic Algorithm	MATLAB
[190]	2023	Introduces a capacity optimisation technique for SCs utilising a specific algorithm. It involves the initial establishment of a capacity optimisation model for SCs. Subsequently, the algorithm is employed to address and solve the optimisation model.	Particle Swarm Optimisation (PSO) algorithm	MATLAB/DIgSILENT
[33]	2022	SCs’ optimal size and location are considered through mixed-integer convex optimisation. The optimisation is focused on minimising the costs of installation, maintenance, and operation while guaranteeing technical parameters.	Semi-Definite Program (SDP)	PSCAD/EMTDC
[31]	2020	Introduces an optimisation algorithm designed to place SCs to enhance system strength in a power network, including wind sources.	Genetic Algorithm	PSS/E
[191]	2020	This paper focuses on the excitation control of SCs during grid integration, with three main contributions to improving reliability. It also suggests a control strategy to dampen oscillations during grid integration.	PI/PID Control	PSCAD/EMTDC
[34]	2016	Presents an idea of using decommissioned SGs in the SC mode. This secondary application of retired SGs aims to enhance both frequency response and SCR, considering their strategic placement.	Post-Retirement Scheme (PRS)	PSS/E
[183]	2016	The simulation model presented in this study centres around the placement of the synchronous condenser in specific, restricted locations within a MV network.	PI/PID Control	MATLAB

Table 10. Summary of the published research papers with an optimisation perspective.

Refs.	Year	Done	Method	Software
[76]	2023	A Genetic Algorithm-based optimisation method is proposed for optimally sizing and placing SCs to strengthen the grid, with implementation in MATLAB.	Genetic Algorithm	MATLAB
[190]	2023	Introduces a capacity optimisation technique for SCs utilising a specific algorithm. It involves the initial establishment of a capacity optimisation model for SCs. Subsequently, the algorithm is employed to address and solve the optimisation model.	Particle Swarm Optimisation (PSO) algorithm	MATLAB/DIgSILENT
[33]	2022	SCs’ optimal size and location are considered through mixed-integer convex optimisation. The optimisation is focused on minimising the costs of installation, maintenance, and operation while guaranteeing technical parameters.	Semi-Definite Program (SDP)	PSCAD/EMTDC
[31]	2020	Introduces an optimisation algorithm designed to place SCs to enhance system strength in a power network, including wind sources.	Genetic Algorithm	PSS/E
[191]	2020	This paper focuses on the excitation control of SCs during grid integration, with three main contributions to improving reliability. It also suggests a control strategy to dampen oscillations during grid integration.	PI/PID Control	PSCAD/EMTDC
[34]	2016	Presents an idea of using decommissioned SGs in the SC mode. This secondary application of retired SGs aims to enhance both frequency response and SCR, considering their strategic placement.	Post-Retirement Scheme (PRS)	PSS/E
[183]	2016	The simulation model presented in this study centres around the placement of the synchronous condenser in specific, restricted locations within a MV network.	PI/PID Control	MATLAB

A BESS unit comprises a battery system, a DC/AC inverter, and an LCL filter, enabling the conversion of power from DC to AC and providing grid-related functions. In this system, the inverter functions in a voltage control mode and can function as a GFM inverter. The control system should be composed of dual loops: one for frequency control (P-loop) and another for voltage control (Q-loop).

$${\omega }_{{BESS}_i}={\omega }_{{BESS}_i}^{\ast }-K_{{BESS}_i}^P(P_{{BESS}_i}^m-P_{{BESS}_i}^{\ast })$$

(2)

$$V_{{BESS}_i}=V_{{BESS}_i}^{\ast }-K_{{BESS}_i}^Q(Q_{{BESS}_i}^m-Q_{{BESS}_i}^{\ast })$$

(3)

where

• ${\omega }_{{BESS}_i}^{\ast }$, $V_{{BESS}_i}^{\ast }$: are the nominal amplitude values of frequency and voltage;
• $K_{{BESS}_i}^P$, $K_{{BESS}_i}^Q$: are droop factors;
• $P_{{BESS}_i}^m$, $Q_{{BESS}_i}^m$: are the measured active and reactive powers;
• $P_{{BESS}_i}^{\ast }$, $Q_{{BESS}_i}^{\ast }$: are the reference values of the power output.

In the comprehensive hybrid control system, the internal controllers are coordinated by sharing data. These controllers receive data from local sensors, communicate with other controllers, and perform calculations. The main goals of the HSC control system are to fix grid frequency, keep voltage levels stable, ensure active power is distributed correctly among different sources, and handle reactive power distribution.

6. Economic Considerations

Economic feasibility is a significant aspect when integrating SCs. Several studies have conducted cost-benefit analyses to assess the economic viability of SCs, showing that although initial capital costs can be high, the long-term benefits in terms of grid stability, reliability, and integration of renewables outweigh the investments [33,69,182]. In [125], a comprehensive analysis is conducted to assess the advantages and disadvantages of system strength providers, considering both economic and technical aspects. The study investigated the complex balance between these providers’ cost-effectiveness and technical capabilities, offering valuable insights into their role in enhancing the stability and performance of power systems. The study in [180] explores the economic viability of combining SCs with wind turbines, highlighting the cost-effectiveness of a particular wind plant and SC configuration. Table 11 presents some references that have investigated economic issues related to SCs in grids.

Table 11. Summary of the published research papers with an economic perspective.

Refs.	Year	Done	Target
[192]	2024	An economic evaluation using a special method study was conducted on a PV farm system designed to enhance grid strength. SC sets play a key role in providing grid stability and strength.	Levelised Cost of Electricity (LCOE) reduction
[193]	2023	An optimisation method including RESs, BESSs, and SCs was used, and its effectiveness demonstrated through simulations and a production model, showing improved renewable energy transmission capabilities and economic benefits.	Reduction in system operation costs and improvement of the system operation economy
[33]	2022	Mixed-integer convex optimisation is a method for determining the best size and placement of SCs. The optimisation process minimises the expenses associated with installing, maintaining, and operating SCs while maintaining a specified SCR at connection points.	Installation, operation, and maintenance cost reduction
[176]	2022	This paper provides a comprehensive review of energy storage systems to assist power utilities and researchers in making informed decisions about selecting energy storage devices based on effectiveness and economic feasibility.	Reducing power generation costs, especially renewable energy
[194]	2022	Examining the limitations and difficulties associated with integrating renewable energy sources, particularly concerning the system strength and inertia of the power grid in the Australian NEM.	Decreasing the costs of securing power system operation
[30]	2021	Focuses on addressing weak grid integration issues and high installation and operating costs associated with SCs to maintain system SCR levels, minimise investment and operating costs of SCs, and reduce voltage variations in the power system.	Control the installation and operation costs
[32]	2018	This paper presents an optimal allocation method for synchronous condensers that minimises installation costs while maintaining system short-circuit ratios above a specified level at the converter point of common coupling.	Minimising the cost of installing SCs

Table 11. Summary of the published research papers with an economic perspective.

Refs.	Year	Done	Target
[192]	2024	An economic evaluation using a special method study was conducted on a PV farm system designed to enhance grid strength. SC sets play a key role in providing grid stability and strength.	Levelised Cost of Electricity (LCOE) reduction
[193]	2023	An optimisation method including RESs, BESSs, and SCs was used, and its effectiveness demonstrated through simulations and a production model, showing improved renewable energy transmission capabilities and economic benefits.	Reduction in system operation costs and improvement of the system operation economy
[33]	2022	Mixed-integer convex optimisation is a method for determining the best size and placement of SCs. The optimisation process minimises the expenses associated with installing, maintaining, and operating SCs while maintaining a specified SCR at connection points.	Installation, operation, and maintenance cost reduction
[176]	2022	This paper provides a comprehensive review of energy storage systems to assist power utilities and researchers in making informed decisions about selecting energy storage devices based on effectiveness and economic feasibility.	Reducing power generation costs, especially renewable energy
[194]	2022	Examining the limitations and difficulties associated with integrating renewable energy sources, particularly concerning the system strength and inertia of the power grid in the Australian NEM.	Decreasing the costs of securing power system operation
[30]	2021	Focuses on addressing weak grid integration issues and high installation and operating costs associated with SCs to maintain system SCR levels, minimise investment and operating costs of SCs, and reduce voltage variations in the power system.	Control the installation and operation costs
[32]	2018	This paper presents an optimal allocation method for synchronous condensers that minimises installation costs while maintaining system short-circuit ratios above a specified level at the converter point of common coupling.	Minimising the cost of installing SCs

Economic models could be developed to assess the financial implications of implementing SCs with the power to maximise the utilisation of RESs and facilitate the transition to net zero. The objective must be to minimise expenses associated with the installation, operation, and maintenance of SCs, while ensuring that the SCR for all PoCs remains at or above the desired thresholds. Table 12 compares some optimisation methods, such as heuristic algorithms, meta-heuristic algorithms, Semi-Definite Programs, and mixed-integer convex optimisation, providing insights into the most effective strategies for optimising HSC placement and operation. Heuristic and meta-heuristic algorithms, such as Genetic Algorithms, are commonly used due to their simplicity and ease of implementation, but they cannot guarantee global optimality, as they may converge to local optima [30,33]. Semi-Definite Programming and mixed-integer convex optimisation, on the other hand, offer a more robust framework for optimisation, providing guarantees of global optimality and better handling of complex constraints [195]. Metaheuristic algorithms provide efficient solutions for Transmission Expansion Planning (TEP) and optimal equipment placement [196]. TEP aims to minimise investment and operation costs while ensuring acceptable voltage and loading profiles. Though heuristic and meta-heuristic algorithms are useful for initial approximations, the Semi-Definite Programming (SDP) optimisation method is more effective in optimising SC placement [33,190]. Therefore, adopting a similar approach for optimising HSCs would be beneficial for achieving efficient network operations.

Additionally, economic considerations of HSC deployment should account for long-term benefits, such as enhanced grid stability, reduced energy losses, and improved power quality, leading to significant cost savings. Investments in advanced control strategies and optimisation techniques ensure efficient operation and enhance system resilience and sustainability.

Table 12. A comparison table of different optimisation methods.

Refs.	Method	Description	Advantages	Objective Function	Components
[197,198]	Heuristic algorithms	Rule-based methods to find satisfactory solutions	Simple and fast to implement	$min{\displaystyle \sum _{i=1}^N}\left(C_F^i+C_V^i{S}_n^i\right)BU{S}_i$	$S_n^i$: the rated power of SC $C_F^i$: the fixed installation cost of SC $C_V^i$: the variable installation cost of SC $BU{S}_i$: a flag of the installation allocation of SC
[32,199]	Meta-heuristic algorithms	High-level procedures (e.g., Genetic Algorithms, Particle Swarm Optimisation)	Can escape local optima, good for complex problems	$min{\displaystyle \sum _{i=1}^N}\left(C_F^i+C_V^i\frac{S_i}{y_i}\right)x_i$	$S_i$: the rated power of the newly installed SC $y_i$: an integer decision variable-rated SC $x_i$: installation allocation of SC
[33,200]	Semi-Definite Programming	A type of convex optimisation for handling certain classes of optimisation problems	Can provide global optimum for specific convex problems	$\mathit{min}{\displaystyle \sum _{i=1}^n}{d}_i{x}_i$	$x_i$: a decision variable of SC size/capacity $d_i$: a fixed vector of the same-sized SC
[195]	Mixed-integer convex optimisation	Combines integer constraints with convex optimisation	Flexibility in modelling discrete decisions and continuous variables	$min\left(\delta T{\displaystyle \sum _{i,j=1}^N}\left(S_{lt}^i+S_{lt}^j\right)∆t+\epsilon T{\displaystyle \sum _{i=1}^N}\left({\alpha S}_i^2+\beta S_i+\gamma \right)\right)$	$S_{lt}^i+S_{lt}^j$: the power flows of the line connected between i and j $\alpha ,\beta ,\gamma$: the cost coefficients of the cubic function $S_i$: the optimal-sized SC $T$: the time under analysis

Table 12. A comparison table of different optimisation methods.

Refs.	Method	Description	Advantages	Objective Function	Components
[197,198]	Heuristic algorithms	Rule-based methods to find satisfactory solutions	Simple and fast to implement	$min{\displaystyle \sum _{i=1}^N}\left(C_F^i+C_V^i{S}_n^i\right)BU{S}_i$	$S_n^i$: the rated power of SC $C_F^i$: the fixed installation cost of SC $C_V^i$: the variable installation cost of SC $BU{S}_i$: a flag of the installation allocation of SC
[32,199]	Meta-heuristic algorithms	High-level procedures (e.g., Genetic Algorithms, Particle Swarm Optimisation)	Can escape local optima, good for complex problems	$min{\displaystyle \sum _{i=1}^N}\left(C_F^i+C_V^i\frac{S_i}{y_i}\right)x_i$	$S_i$: the rated power of the newly installed SC $y_i$: an integer decision variable-rated SC $x_i$: installation allocation of SC
[33,200]	Semi-Definite Programming	A type of convex optimisation for handling certain classes of optimisation problems	Can provide global optimum for specific convex problems	$\mathit{min}{\displaystyle \sum _{i=1}^n}{d}_i{x}_i$	$x_i$: a decision variable of SC size/capacity $d_i$: a fixed vector of the same-sized SC
[195]	Mixed-integer convex optimisation	Combines integer constraints with convex optimisation	Flexibility in modelling discrete decisions and continuous variables	$min\left(\delta T{\displaystyle \sum _{i,j=1}^N}\left(S_{lt}^i+S_{lt}^j\right)∆t+\epsilon T{\displaystyle \sum _{i=1}^N}\left({\alpha S}_i^2+\beta S_i+\gamma \right)\right)$	$S_{lt}^i+S_{lt}^j$: the power flows of the line connected between i and j $\alpha ,\beta ,\gamma$: the cost coefficients of the cubic function $S_i$: the optimal-sized SC $T$: the time under analysis

7. Challenges and Future Directions

Future directions in SC technology and its integration with BESSs present both opportunities and challenges [201]. Addressing the challenges associated with SC technology, such as mechanical wear, high maintenance costs, and noise generation, remains a priority [42]. Continued research and development efforts should focus on devising innovative solutions to mitigate these limitations, thereby enhancing the overall reliability and performance of SCs [104,202]. The next opportunity is the combination of SCs with BESSs, which is a promising approach to overcoming the limitations of each technology individually. Hybrid SC–BESS systems have the potential to provide enhanced solutions for grid operation by leveraging the strengths of both technologies [56,58,167,176]. Further research is needed to optimise the integration and operation of these hybrid systems, considering factors such as control strategies, sizing, and coordination [30,95]. Additionally, exploring diverse types of inverters for use with BESSs can contribute to the development of more efficient and flexible grid solutions [203]. Different inverter technologies offer unique capabilities and performance characteristics, whereby understanding how they interact with BESSs and SCs is essential for maximising their potential in grid applications [204]. Furthermore, optimising the deployment and operation of SCs requires a comprehensive understanding of their technical and economic implications. Research efforts should focus on developing optimisation models and algorithms for determining the optimal size, location, and control strategies of SCs within power systems. Interdisciplinary research initiatives can integrate expertise from different fields, leading to holistic solutions to address the complex challenges of modern power systems. In summary, the future of SC and BESS integration holds great promise for enhancing grid stability, reliability, and sustainability. By addressing technical limitations, optimising system design and operation, and fostering interdisciplinary collaboration, researchers can contribute to developing robust and resilient power systems capable of accommodating increasing renewable energy penetration [56].

8. Conclusions

The transition towards net-zero emissions requires innovative approaches to address the intermittent nature of RESs, such as wind and solar power, particularly through IBR systems, which present significant challenges for developing grids. Renewable energy sources rely on system strength, typically provided by synchronous generators. At the same time, in a transition towards net zero, the retirement of synchronous generators is inevitable.

This study has comprehensively reviewed the challenges and solutions associated with integrating RESs into power systems, focusing on enhancing grid stability and reliability. The key areas discussed in this review include the role of SCs in improving grid stability, voltage control, power quality, short-circuit level management, and inertia issues. The role of SCs and HSCs in supporting the transition to net-zero emissions and bolstering grid stability and reliability has been reviewed. The summary findings of this review are as follows:

• In dealing with key issues of weak grids with low system strength, we have found that although different compensators could be used for enhancing system strength, SCs prove to provide one of the most effective solutions.
• It has been found that SCs can provide critical system inertia and effectively manage voltage levels and reactive power. They also enhance overall power quality and reduce voltage fluctuations.
• Hybrid synchronous condensers (HSCs) combine SCs and BESSs and are very effective in improving grid performance and reliability. HSCs demonstrate the inertia and SCR contributions of SCs with a fast response.
• Power electronics converters in HSCs offer fast response, virtual inertia, extra active and reactive support, and participation in voltage and frequency stability. Additionally, it is advantageous to use both GFM and GFL inverters within HSC systems. The GFM inverters are more effective for voltage and frequency support, especially in grids where there is a high possibility of islanding, and GFL inverters offer reliable performance under stable grid conditions.
• It is importance to optimise the sizes and placements of HSCs to maximise their effectiveness in enhancing grid stability. Among advanced optimisation models, SDP is suitable for this purpose.