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Techato, K. Hybrid HVAC–HVDC Grids. Encyclopedia. Available online: https://encyclopedia.pub/entry/14354 (accessed on 01 December 2023).

Techato K. Hybrid HVAC–HVDC Grids. Encyclopedia. Available at: https://encyclopedia.pub/entry/14354. Accessed December 01, 2023.

Techato, Kuaanan. "Hybrid HVAC–HVDC Grids" *Encyclopedia*, https://encyclopedia.pub/entry/14354 (accessed December 01, 2023).

Techato, K.(2021, September 20). Hybrid HVAC–HVDC Grids. In *Encyclopedia*. https://encyclopedia.pub/entry/14354

Techato, Kuaanan. "Hybrid HVAC–HVDC Grids." *Encyclopedia*. Web. 20 September, 2021.

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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.

hybrid grids
frequency deviations
active and reactive power flow
supervisory control
contingency analysis

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 ^{[1]}.

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 ^{[2]}^{[3]}^{[4]}. 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 ^{[5]}. 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 ^{[6]}^{[7]}^{[8]}^{[9]}. 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 ^{[10]}^{[11]}^{[12]}. 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 ^{[13]}^{[14]}

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 ^{[15]}.

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 ^{[16]}^{[17]}^{[18]}. 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 ^{[19]}. 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 ^{[20]}^{[21]}^{[22]}^{[23]}. Several droop control and persistent DC voltage control schemes are used for supervisory control ^{[24]}^{[25]}^{[26]}^{[27]}^{[28]}.

Optimization algorithms are quite necessary for the effective integration of asynchronous grids into the MTDC network ^{[28]}. 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 ^{[29]}^{[30]}^{[31]}. 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 ^{[32]}.

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 ^{[33]}^{[34]}. 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 ^{[35]}^{[36]}^{[37]}^{[38]}. 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 ^{[39]}^{[40]}^{[41]}^{[42]}.

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 ^{[43]}^{[44]}. 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 ^{[45]}^{[46]}^{[47]}.

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 ^{[6]}. 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 ^{[7]}^{[8]}.

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 ^{[9]}^{[10]}. 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 ^{[11]}.

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 ^{[12]}^{[48]}.

LCC convertors alone are incapable of handling the bi-directional flow of active and reactive power support during an outage of a transmission line or overload condition. It results in the worst transient response, reduction in usage of remaining lines, and system blackout. Therefore, for attaining the system’s reliability during contingency conditions, the flexibility of the existing LCC–MTDC system can be modified to a hybrid MTDC system by incorporating an extended VSC terminal between generation and load areas **Figure 2**a. The extended VSC terminal functions as an inverter during generation side contingency and contrarily as a rectifier during load side contingency. The MTDC controller and CRPS controller are responsible for regulating active and reactive power, respectively (**Figure 2**b) ^{[15]}.

The grounding conformations in an MTDC network ensure safety and protection during contingency conditions. A critical analysis of grounding resistors values in different conformations of (a) monopole/bipolar ideal earth return (R_{gnd} = 0), (b) monopole/bipolar earth return (R_{gnd} = 1Ω, 2Ω, 3Ω), (c) monopole/bipolar no-ground return is performed, and results reveal that grounding resistance’s increment has a significant influence on highest post-contingency DC voltages (**Figure 3**a,b). Therefore, the grounding conformations nearer to ideal-earth return should be opted for to achieve the stable operation of MTDC networks ^{[49]}.

The MTDC network should be responsive with fault ride through, e.g., (FRT) capacity in onshore AC grid faults. Communication-free control strategies (a) wind-turbine power set-point alteration, (b) offshore grid frequency alteration, (c) offshore AC grid voltage-controlled reduction, have been formulated, which provides FRT capability during AC faults and contingencies. They deliver a fast regulation of the turbine’s power output without the use of conventional chopper resistors, which is considered a conventional solution for providing FRT capabilities during contingencies ^{[50]}^{[51]}^{[52]}.

The incorporation of wind, renewable energy resources, and remote grids have produced numerous frequency stability challenges. Therefore, stabilizing the MTDC grid frequency swings during abnormal conditions is necessary, as it impacts power flow regulation. Voltage-based load control provides a controlled voltage amplitude variation and regulating active power flow consumption as per demand by frequency support service at each HVDC terminal. This control scheme manages the active power requirement for frequency deviation reduction and DC voltage regulation from both generator and load ^{[19]}, since there is a risk of backflow of excessive power flow consumption as per demand by frequency support service at each HVDC terminal. This control scheme manages the active power requirement for frequency deviation reduction and DC voltage regulation from both generator and load ^{[19]}. Since there is a risk of backflow of excessive power to grids in isolated working grid networks, a control strategy is needed to optimize power share-out between AC and DC transmission lines. The modular multilevel converter (MMC) uses a power feedback loop control scheme that not only follows AC power flow value but also minimizes constraints and losses ^{[20]}.

Weak AC grids or isolated working grids in the hybrid HVDC system require a control strategy for stable operation during normal and fault conditions. A coordinating control scheme known as Vernier has been proposed, which comprises a series combination of the two VSC–HVDC and a capacitor-commutated converter, and is capable of sharing control burden and keeping constant margin and firing angle at inverter and rectifier ^{[24]}. HVDC and HVAC systems are used separately for bulk power transfer, but we need to model their frequency at some specified level for their parallel operation using the transmission network having two systems, one with 110 kV and 380 kV HVAC and converted to the other with 400 km hybrid HVAC–VDC current line, with bipolar MMC HVDC line link. Here, DBC gives more profound results over CCC in terms of control capabilities ^{[25]}^{[53]}. A supervisory control strategy based on LQR control theory assists in a stable, synchronous, and parallel operation of a coordinated network of HVDC link and FACT devices. The proposed model integrates VSC HVDC link, static VAR compensator (SVC), and thyristor-controlled series capacitor (TCSC) devices. TCSC produces desired values of line reactance and power flow, and SVC helps out in reactive power injection for a stable operation of the power system. The proposed supervisory methodology proficiently mitigates power system blackouts ^{[54]}.

In the coming years, everyone will witness the evolution of the current HVAC power grids because of increasing expectations from the power systems. The whole world will embrace the new concept of hybridization of power grids along with their changed operation and composition. This acceptance means addressing the technical, protection, modeling, social and economic, climate, and generation-source-based integration challenges that have been discussed in this paper.

Moreover, the satisfactory operation and control flexibility of MTDC networks requires stable, optimal, and balanced power in HVDC grids. Therefore, a comprehensive overview of various PFC strategies of MTDC networks has been technically explored. OPF, HMIDC, droop control, adaptive droop control, LCC–CIA, and LCC–CEA control are distinguished methodologies that address solutions for the establishment of independent and self-sustaining control of active and reactive power flow and power-sharing in an MTDC grid. The ion-flow field characteristics analysis contributes to the reasonable corridor arrangement of parallel HVAC–HVDC transmission lines.

The contingency and post-contingency control methodologies and analyses such as D2SRF PLL and grounding schemes and HMIDC have also been explored in real-time to ensure a stable, fluctuation-less, and bi-directional PFC and reactive power support during sudden outages or AC contingencies. Moreover, some of the fault-ride-through control configurations and capabilities have also been highlighted. The paper has also reviewed the various hybrid HVAC and HVDC systems’ control schemes such as voltage source converter and current coupled converter for reducing the power and frequency deviations. Generation and supervisory control of hybrid HVAC–HVDC grids are quite necessary for fast and robust frequency regulation for optimal power flow. Voltage-based load control, frequency support, MMC, model reference adaptive control designed using a hybrid approach, and power oscillation damping (POD) designed using pole placement and supplementary controllers are the control strategies proposed for generation control. Whereas, Vernier, CCC, DBC, VSCs integrating TCSC and SVC devices, and several droop control schemes and persistent DC voltage control schemes are proposed for supervisory control of hybrid HVAC–HVDC systems.

There is also a complete assessment of the different types of optimization techniques that are being used in hybrid HVAC–HVDC power systems. A brief explanation is provided to demonstrate how optimization influences the OPF, economic dispatch, LFC, and generation control of the hybrid HVDC–HVAC grid. Furthermore, the optimization approaches in the field of offshore and onshore power systems are also elaborated in detail. A review of algorithms that produces OPF in DGs and DSs has also been discussed in detail. After analyzing the various research articles, it is clear that by using the modern state-of-the-art optimization algorithms, the cost function can be minimized to achieve the objective function of economic dispatch and AGC.

For security challenges, classical security-constrained optimal power flow shows some disadvantages, i.e., the complexity and inability to handle large power systems. An insight of new techniques incorporating PSCOPF and CSCOPF as a control method for meshed systems security considering all types of contingencies of the system is also elaborated.

For expediting the upgradation of current power grids, the current HVAC transmission systems are popularly being converted to hybrid HVAC–HVDC power transmission systems. Apart from the integration and security problems noted previously, the acceptability of such hybrid systems is expected to be stronger since there is no need for developing completely new construction. The visual appearance is similar to that of existing HVAC systems.

For future works, manufacturers, utilities, stakeholders, and third parties need to find a balance between the performance and cost of installation of the hybrid HVAC–HVDC systems. The researchers should look for solutions for technical challenges, reliable and fast HVDC protection schemes, the best models for real-time simulation, agencies to get funding for the installation costs, ways to save the environment, and beneficial generation sources.

The impending hybrid HVAC–HVDC grid should be able to surpass the present HVAC grid system. A security algorithm can be formulated by using both the preventive and corrective control methods, which can fulfill all the security constraints of line, generator, and equipment outages. It can also allow the hybrid grid to exchange power with minimum losses. This dynamic model can encounter all contingencies instead of a static security model.

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