The concept of hybrid high-voltage alternating current (HVAC) and high-voltage direct current (HVDC) grid systems brings a massive advantage to reduce AC line loading, increased utilization of network infrastructure, and lower operational costs. However, it comes with issues, such as integration challenges, control strategies, optimization control, and security. The combined objectives in hybrid HVAC–HVDC grids are to achieve the fast regulation of DC voltage and frequency, optimal power flow, and stable operation during normal and abnormal conditions.
The power grids are growing both in terms of complexity and size. The added complexity is due to the regulations that aim to integrate renewable energy systems to make sustainable power systems and due to the increase in efficiency. The advanced energy storage techniques and power electronics technology is also contributing towards making the current high-voltage alternating current (HVAC) grids more complex. The high-voltage direct current (HVDC) systems are becoming equally important due to the involvement of onshore and offshore renewable energy sources, such as solar and wind power, since the output from their AC asynchronous grid system has to be transformed into HVDC for long-distance power transmission. This is because of fewer expenses, increased power transfer capability, and fewer dielectric losses. Therefore, the advanced research is focused on hybrid HVAC–HVDC grids that led to developing improved power transmission capability, introducing back-to-back systems for interconnected regions with different frequencies, and making power transmission systems more efficient for long distances .
The complete shift of HVAC grids into HVDC grids is more unlikely due to technical issues, such as power reversal and communication network for coordination between grids, as these problems are not found in HVAC systems . Therefore, it is more likely that the existing power grids will transform into hybrid HVAC–HVDC systems. The hybrid grids will receive the benefits of both the HVAC and the HVDC systems. It is worth mentioning that these hybrid grids are also facing some major integration challenges related to reliable and adequate modeling of high-speed power converters and protection from large fault currents that have been identified and discussed in this paper . This survey has highlighted the control strategies and optimization algorithms for fast, accurate, and robust control of generation, power-flow, supervisory, and contingency of hybrid HVAC–HVDC grids for their normal and abnormal operation. The security of the hybrid power system relevant to physical malfunctioning of power system equipment, line-outage, and contingencies, and various other physical reasons that sources variations and uncertainties in the MTDC networks causing a security breach has also been analyzed in detail for the stable and cost-effective operation of hybrid grids.
Power flow control (PFC) is considered one of the major issues in hybrid systems, which has not been comprehensively addressed earlier. The integration of asynchronous AC systems in multi-terminal direct current (MTDC) networks has also provoked numerous and critical challenges related to DC voltage control, power-sharing, and power flow that restrains the establishment of independent and self-sustaining regulation of active and reactive power conservation and flow . Therefore, appropriate control strategies should be modeled considering the converter (voltage source converter (VSC) or line-commutated converter (LCC)) losses and DC transmission line losses for a fast, accurate, and robust DC voltage regulation . Moreover, reactive power support is essential for the dispersal and dispatch of a stable, optimal, and instantaneous balanced power in HVDC grids under normal and fault circumstances 
The distribution of hybrid fields, formed through the combination of HVDC and HVAC electric fields originating from HVDC–HVAC overhead transmission lines, offers comprehensive and extensive research in the field of electromagnetic environment of transmission line engineering based on the analysis of ion-flow field characteristics. Therefore, appropriate control measures incorporating reasonable corridors arrangement must be analyzed explicitly to lower ion current density for a safer environment 
The operational strategy of the MTDC networks becomes quite troublesome in contingency conditions when the system is prone to a sudden outage of converters and various transmission equipment 
. Therefore, a stable bi-directional power flow regulation and reactive power support in post-contingency conditions should be necessarily addressed.
Generation and supervisory control of hybrid HVAC–HVDC grids are quite essential for a fast and robust frequency regulation with optimal power flow 
. This review paper also presents a comprehensive review of generation and supervisory control topologies that have not been addressed inclusively in other review articles. The voltage-based load control, frequency support, and modular multilevel converters (MMC) are the most profound control schemes used in generation control 
. Several droop control and persistent DC voltage control schemes are used for supervisory control 
Optimization algorithms are quite necessary for the effective integration of asynchronous grids into the MTDC network 
. Therefore, a comprehensive review of optimization algorithms is presented that ensures the successful and optimal inclusion of renewable energy resources having asynchronous grids in MTDC networks. These optimal solutions ensure a cost-effective economic dispatch in hybrid HVAC–HVDC networks. Several state-of-the-art algorithms have been analyzed in detail for a cost-effective optimal economic dispatch inclusive of optimal fuel cost and reduction in transmission line losses 
. Moreover, algorithms proposing optimal solutions for optimal power flow (OPF) and voltage stability in multi-area objective dynamic economic dispatch (MAMODED) systems have been analyzed in detail 
Optimal generation control is a basic requirement for the deviation-less frequency of the hybrid grids. The load flow control (LFC) in a deregulated environment consisting of multi-source generating units power systems should be implemented optimally to maintain an adequate balance between generation and demand. Therefore, optimal control of automatic generation control (AGC) of a multi-source system is quite essential in this regard, when frequency fluctuation occurs due to sudden contingencies of AC asynchronous grids in an MTDC grid 
. Therefore, a comprehensive review of modified hybrid optimization algorithms is presented that is more proficient than other traditional algorithms for the tuning of existing controllers 
. Moreover, an overview of hybrid algorithms is presented for calculating the optimal pricing capital cost of integrating offshore and onshore renewable energy sources into an MTDC network. This optimal solution ensures the essential security level of the power system at the expected operation cost 
Robust optimization algorithms are quite necessary for establishing reactive power support in hybrid HVDC grids that ensures DC voltage stability and power loss reduction. The cost-effective integration of distributed generations (DGs) into a hybrid HVAC–HVDC network requires optimal approaches. The advancements in renewable DGs, electric vehicles (EVs), and photovoltaic energy (PVs) have made the integration of renewable energy sources into the system complex and have also forecasted the need for integration of battery storage devices with DGs, and future distributed systems (DSs).
The security of hybrid HVAC–HVDC networks is quite important for their stable cost-effective operation under normal, abnormal, and contingency conditions. The paper has emphasized two major control techniques, i.e., corrective and preventive control. In the corrective control security-constrained optimal power flow (CSCOPF) scheme, a control action is required for each set of possible contingencies 
. Whereas in the preventive control security-constrained optimal power flow (PSCOPF) scheme, all possible sets of contingencies are considered, and it satisfies all security requirements without any extra control action. The maximum-security of the hybrid HVDC grids system can be achieved through these control schemes 
2. Control Strategies of Hybrid HVAC–HVDC Systems
2.1. Power Flow Control
The OPF in a hybrid grid is necessary for establishing an independent regulation of active and reactive power conservation between both sides of VSC–HVDC through rectifiers at both ends, i.e., offshore and onshore. Sequential and unified approaches are the two widely used strategies for this purpose. Still, the unified approach is mostly preferred for OPF tools because it solves all the equations together leading to less complex computation that requires a much smaller number of iterative loops 
. Using the same unified method, VSCs are significantly preferred over CSCs in MTDC systems, since they offer an AC side voltage control by regulating reactive power demand. They are capable of handling converter control modes and converter loss modeling irrespective of the extension or expansion of the system and the number of VSCs been installed 
Moreover, for optimal power flow calculations in a combined AC and MTDC system, a steady-state model has also been designed, which develops full power flow equations and nonlinear mathematical optimization by considering DC line losses and VSC terminal losses 
. LCC is still considered a dominant technology over VSC since it offers an economical solution for transmitting bulk power. The two widely used control methodologies for the stability of LCC–MTDC networks are: (a) inverter topology with constant extinction angle (CEA) control, (b) rectifier topology with constant ignition angle (CIA) control. LCC–MTDC networks with the CIA control method offer well stable and faster control response in initialization and DC power transfer conditions 
DC voltage droop control is a widely used strategy in DC voltage regulation in achieving stability and the dynamic response of an MTDC Figure 1
. The steady-state analysis chooses droop parameters by incorporating maximum current limits and desired voltage errors of wind farm side converters (WFCs) and grid side converters (GSCs). Droop control in AC GSCs regulates voltage deviations due to the wind’s stochastic nature. In contrast, WFCs regulate DC voltage by tackling AC Grid faults when the currents are injected by WFCs and extracted by the GSCs. So, the distribution of power among various terminals of MTDC terminals without communication channels during normal and fault conditions is possible using this scheme