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The technological development in the area of power electronics has paved the way for the construction of High Voltage Direct Current (HVDC) systems. The utilization of HVDC grids alongside conventional High Voltage Alternating Current (HVAC) grids poses several challenges, especially, from stability and control points of view. Indeed, moving towards such systems in the context of conventional Alternating Current (AC) power systems cannot be possible without ensuring the overall stability of hybrid HVAC/HVDC grids. This paper analyzes different aspects of the stability of Voltage Source Converter (VSC)-based HVDC grids and presents various methods of improving stability based on a systematic and comprehensive review. In addition, this paper provides a concise classification of various control methods to improve the operation of such grids and the advantages of each method.

A power system stabilizer (PSS) is a control system integrated into the control structure of specific generation units within AC grids. It monitors current, voltage, and machine shaft speed. Analysing these variables, the PSS generates appropriate control signals to the voltage regulator unit, aiming to damp system oscillations. With the advancement of high-voltage direct current (HVDC) overlaid high-voltage alternative current (HVAC) grids, it is anticipated that direct current power system stabilizers (DC-PSS) will be developed to perform a similar role as their AC counterparts. DC-PSS will be responsible for monitoring and controlling DC voltage levels, ensuring stable operations. This paper focuses on DC-PSS in HVDC grids, designed to ensure stable operation and mitigate voltage fluctuations. Unlike conventional AC power systems, HVDC includes only DC voltage and power. The input signal for DC-PSS is the variations in DC voltage, and the output signal is proportional to the power changes at the specific bus where the DC-PSS is installed, aiming to minimize DC voltage oscillations. These characteristics pose significant challenges in DC-PSS. The paper addresses the challenges and highlights issues such as inertia and low-frequency oscillations associated with DC-PSS. Various control methods are presented and a comparison is made among these methods.

In high voltage direct current (HVDC) grids, the reference values of power controllers are normally adjusted based on the DC-link voltage as the DC voltage is regulated by the droop controllers. As a matter of fact, except for the DC slack bus, there is a trade-off between power-sharing and voltage regulation of all the remaining HVDC buses. The main issue with this control strategy is that in case of contingencies, significant voltage and power oscillations throughout HVDC grids can be expected. To address such issues, the integration of a battery energy storage system into HVDC grids through a multi-port DC/DC power converter is investigated in this paper. The DC/DC converter used in this paper consists of three ports: (1) two ports are connected in cascade with the intended DC transmission line, formerly named cascaded power flow controller, and (2) the third port is for the integration of the battery energy storage system to form a multi-functional device. Therefore, the proposed framework can potentially achieve small-signal stability enhancement and regulate/adjust the power throughout the HVDC grids.

The primary goal of grid-connected microgrids is to control the active and reactive power, which is reachable by the inner current control loop in the control structure of power converters. However, when facing unbalanced conditions, the inner current control loop implemented in the dq frame does not function properly. In such conditions, the popular current control loop malfunctions since there is an oscillatory coupling between harmonic components. Therefore, in this study, a new harmonic detector based on decoupled double synchronous reference frame within the current control loop is proposed in which the oscillatory coupling between harmonic components is eliminated, and the overall performance of the power converter control system is significantly improved. The performance of the precisely developed mathematical models is verified by Matlab simulations, and the simulation results confirm the accuracy and proper operation of the proposed strategy.

Future high-voltage direct-current (HVDC) networks based on voltage source converters (VSCs) will have different structures (asymmetric monopolar, bipolar, or symmetric monopolar), voltage levels, control, and protection schemes. Therefore, dc-dc converters are needed to interconnect those VSC-HVDC grids and several technical issues on their control and operational systems must be adequately addressed. A dc-dc converter based on a modular-dual active bridge (M-DAB) converter is suggested to reach a desirable interconnection of the HVDC grids and regulate power flow (PF) between them. A dynamic averaged model is proposed for the M-DAB converter and its stability is analyzed using the Lyapunov function. Moreover, a new local controller based on nonlinear control theory is proposed for the M-DAB. The new M-DAB local controller is integrated with the energy management system (EMS), by updating the PF equations, to create a complete control structure. Considering the CIGRE DCS3 HVDC test system and the studied M-DAB, static, dynamic simulation, and experimental studies are conducted and the dc-dc converter and the performance of the designed controllers and the EMS are examined and validated., Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Intelligent Electrical Power Grids

This paper presents a robust model predictive control (RMPC) method with a new mixed H2/H∞ linear time-varying state feedback design. In addition, we propose a linear parameter-varying model for inverters in a microgrid (MG), in which disturbances and uncertainty are considered, where the inverters connect in parallel to renewable energy sources (RES). The proposed RMPC can use the gain-scheduled control law and satisfy both the H2 and H∞ proficiency requirements under various conditions, such as disturbance and load variation. A multistep control method is proposed to reduce the conservativeness caused by the unique feedback control law, enhance the control proficiency, and strengthen the RMPC feasible area. Furthermore, a practical and efficient RMPC is designed to reduce the online computational burden. The presented controller can implement load sharing among distributed generators (DGs) to stabilize the frequency and voltage of an entire smart island. The proposed strategy is implemented and studied in a MG with two DG types and various load types. Specifically, through converters, one type of DGs is used to control frequency and voltage, and the other type is used to control current. These two types of DGs operate in a parallel mode. Simulation results show that the proposed RMPCs are input-to-state practically stable (ISpS). Compared with other controllers in the literature, the proposed strategy can lead to minor total harmonic distortion (THD), lower steady-state error, and faster response to system disturbance and load variation.

Expansion of electric vehicles’ (EVs’) charging stations is an unavoidable requirement in sustainable cities. Provision of safe protection and control infrastructure is one of the most important requirements for a reliable charging station. In this direction, a centralized protection, control and monitoring unit (PCMU) for smart charging stations is proposed here. PCMU communicates with chargers of the station, protective devices (e.g. relays and circuit breakers), local generating units and all other devices installed at the charging station to determine the station status and to detect/locate faults that may occur in the supply grid of the station. The PCMU includes a self-healing technique which is one of the specialties of smart systems that assists network reliability to increase. One of the major contributions of this research is to propose a protocol for data exchange between the PCMU and the other components similar to the logic used in standard of IEC 61850. Also, potential impacts of implementation of the proposed unit on the most-widely used reliability indices are discussed. The proposed protection strategy is examined via two case studies.

With increasing energy demand, the use of renewable energy resources is increasing. Renewable energy sources require power electronic converters to get integrated into power grids. Multi-terminal high voltage direct current (MT-HVDC) systems are a promising solution for their grid integration. Using droop-based controller strategies in power converter stations of MT-HVDC grid, in addition, to providing power-sharing, can support the frequency of the connected AC grids. However, power changes lead to direct voltage deviations, thereby disrupting the stability of the MT-HVDC system. Here, a new control structure for the droop controller of the voltage source converter (VSC) controller is proposed which is named modified droop controller (M-Droop). This method, in addition to power-sharing, uses an appropriate control action to provide the required voltage stability. Here, analytical studies of the modified control strategy are reported. Moreover, through a developed MATLAB Simulink platform, its performance is verified in the events of transients, consisting of fault, variable load, and change in power generation.

Several types of high voltage direct current (HVDC) breakers have been introduced and commercialized. Each of them has advantages and disadvantages. Among them, the hybrid HVDC breaker is highly successful. One of the most important concerns that the hybrid HVDC breaker has faced is high power loss throughout its fault current breaking process. The hybrid HVDC breaker comprises a high voltage bidirectional main HVDC breaker. A significant number of electronic switches need to be connected in a series where anti-parallel diodes are essentially embraced. During fault inception, a number of series solid-state switches and a number of series diodes dramatically increase the power loss of the main breaker. This study, firstly, studies the power loss of the hybrid HVDC breaker and later develops a structure of a full-bridge hybrid breaker (FBHB) to reduce the losses of the current structure both in the normal and fault protection states. In this paper simulations are done based on PSCAD. In addition to the analytical study and simulations, we show that the developed structure substantially decreases the amount of power lost during the normal operation and fault current breaking stage.

Direct voltage fluctuations due to the presence of relatively large DC reactors (as an essen-tial part of HVDC breakers), lack of inertia, and unwanted frequency fluctuations in theAC side of HVDC grids, have major consequences on the stability of HVAC/HVDC grids.The use of the DC Power System Stabilizer (DC-PSS) can damp and eliminate voltageoscillations caused by the presence of the DC reactors. However, DC-PSS cannot addressthe issues of inertia and unwanted frequency fluctuations. A method to improve inertiais proposed here that can operate well with the droop controller, and DC-PSS does notinterfere with power-sharing and does not interact with any of these elements. Since thepresence of a droop controller in HVAC/HVDC grids associates with power and directvoltage, the method proposed here can improve direct voltage fluctuations by eliminatingsevere power peaks. Moreover, this method does not change the voltage level of the entiresystem, so there is no need to change the set-points of controllers. In addition, all param-eters of the controllers are tuned by an intelligent algorithm, and the Participation factor(PF) scheme is used to find the proper placement of the proposed controller.

The integration of offshore wind farms has revitalized the interest in multi–terminal highvoltage direct current (M–HVdc) transmission grids. HVdc breakers’ importance has increased asM–HVdc grids are now a commercial truth. Several HVdc circuit breaker technologies have beendeveloped, published, and appeared as prototypes for HVdc networks. This paper summarizes theHVdc breaker technologies from the last two decades, distributed mainly in literature. A comparisonof various state–of–the–art HVdc breakers is presented. Further, areas are identified where furtherresearch and development are required. The goal is to provide primary challenges and prospects inthe HVdc breaker field.

High-voltage direct current breaker is among the essential components of high-voltage direct current grids. Such a breaker generally needs a direct current reactor to reduce the fault currents rate. However, direct current reactors have destructive effects on the multi terminal high-voltage direct current grid dynamic stability, and in such a system, despite the variety of controllers, the system dynamics are highly sensitive to the operating point. Therefore, additional damping control will be needed. This paper proposes a modification to be applied to the traditional droop controller of high-voltage direct current grids to cope with the influence of these large reactors, improving the direct voltage stability and decreas ing power variations in the transient events by introducing a direct current power system stabilizer. The proposed method for direct voltage control has been investigated through the analytical model of the system. Stability improvement has been studied following the application of the proposed method by investigating zeros, poles, and frequency response analysis. Moreover, a method is proposed for optimal design and optimal placement of direct current power system stabilizer. The system analysis and time-domain simulations demonstrate a decent damping improvement attained by the proposed method. All simu lations and analytical studies are conducted on Cigré DCS3 test high-voltage direct current grid in MATLAB/Simulink

The growing relevance of voltage source converters (VSCs), and the deep impact they have on the development and maintenance of the electrical grid, increase the necessity of further research on how to deal with nonideal grid conditions from the VSCs control. This paper is aimed to summarize the basic techniques and schemes that might be required for a grid-connected VSC to work under these conditions: grid synchronization schemes, sequence decomposition, current reference generation, and current controllers. At the same time, some alternative schemes that improves the basic ones are cited. Modelling and the two typical current controllers design and tuning under stationary and synchronous reference frames are also exhibited. Given the importance of the current control stage in the VSC behaviour, five control schemes, designed to track negative sequence currents, are shown and tested in simulation and experiments. According to the experiments, it is shown that the standard proportional-resonant controller achieves the best performance in negative sequence tracking due to the robustness of its non-ideal version, the improved implementation thanks to the delta operator, and the non-dependence on grid-synchronization schemes. Alternatively, one approach based on dual synchronous reference frame is also highlighted for easiness of implementation and good performance

High voltage direct current (HVDC) systems are now well integrated into AC systems in many jurisdictions. The integration of renewable energy sources (RESs) is a major focus and the role of HVDC systems is expanding. However, the protection of HVDC systems against DC faults is a challenging issue that can have negative impacts on the reliable and safe operation of power systems. Practical solutions to protect HVDC grids against DC faults without a widespread power outage include: 1) using DC circuit breakers (CBs) to isolate the faulty DC-link, 2) using a proper converter topology to interrupt the DC fault current, and/or 3) using high-power DC transformers and DC hubs at strategic points within DC grids. The application of HVDC CBs is identified as the best approach that satisfies both DC grids and connected AC grids’ requirements. This article reports a comprehensive review of HVDC CBs technologies, including recent significant attempts in the development of modern HVDC CBs. The functional analysis of each technology is presented. Additionally, different technologies based on information obtained from literature are compared. Finally, recommendations for the improvement of CBs are presented.