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Abstract This paper presents various control strategies to improve operations in two interconnected areas connected by a VSC‐HVDC transmission line. The main focus is on designing a central control system (CCS) that coordinates control units in both areas. In area 1, an AC voltage control unit is connected to the CCS. In area 2, three control units including a load power control unit, a fault detection unit, and an AC voltage control unit are also connected to the CCS. The CCS receives inputs from these units and generates commands for the DC voltage and active/reactive power control units on both sides of the DC line. The first proposed strategy addresses permanent voltage drops caused by load fluctuations in area 2. It adjusts the transmitted power from area 1 based on voltage variations in area 2. The second strategy focuses on mitigating faults in area 2 by injecting active and reactive power from area 1 during such events. The third strategy resolves transient voltage oscillations in both areas by controlling the reactive power of stations on either side of the DC line. Simulations using MATLAB‐SIMULINK demonstrate that these mechanisms successfully achieve their objectives.

This paper presents various control strategies to improve operations in two interconnected areas connected by a VSC‐HVDC transmission line. The main focus is on designing a central control system (CCS) that coordinates control units in both areas. In area 1, an AC voltage control unit is connected to the CCS. In area 2, three control units including a load power control unit, a fault detection unit, and an AC voltage control unit are also connected to the CCS. The CCS receives inputs from these units and generates commands for the DC voltage and active/reactive power control units on both sides of the DC line. The first proposed strategy addresses permanent voltage drops caused by load fluctuations in area 2. It adjusts the transmitted power from area 1 based on voltage variations in area 2. The second strategy focuses on mitigating faults in area 2 by injecting active and reactive power from area 1 during such events. The third strategy resolves transient voltage oscillations in both areas by controlling the reactive power of stations on either side of the DC line. Simulations using MATLAB‐SIMULINK demonstrate that these mechanisms successfully achieve their objectives. [ABSTRACT FROM AUTHOR]

Abstract In the case of dispersed load consumption, Ultra‐high Voltage Direct Current (UHVDC) system in cascaded connection mode can significantly reduce the cost by building DC lines between different ends, but the communication delay caused by it brings challenges to the original control strategy. Therefore, the control strategy suitable for cascaded multi‐terminal UHVDC (CMT‐UHVDC) system is developed. In this paper, first of all, the operation characteristics, mathematical model, and adaptability of the original control strategy are discussed. Next, an autonomous master‐slave control strategy is proposed to maintain voltage balance without real‐time communication between high‐end and low‐end valves. Finally, based on the planning parameters of an actual project, the electromagnetic transient simulation model of the CMT‐UHVDC transmission system is established. Several working conditions and the practical influence of sampling deviation on the control performance are discussed and analyzed, which verifies the proposed control strategy's effectiveness.

Abstract Multi‐input converters play an important role in integrating the independent energy sources utilized in the grid‐connected system and electric vehicle (EV) applications. In this scenario, several types of multi‐input converters are presented in the literature. Most of the MICs are operated using a time‐sharing scheme. This leads to a restricted duty cycle which limits the energy sources utilization and output voltage. To overcome the aforementioned limitations, a multi‐input single‐output (MISO) boost converter is proposed. It can improve the utilization of energy sources and output voltage with a reduction in the part count. To verify the feasibility of the proposed scheme, a 200 W prototype circuit is developed; simulation and experimental results are validated.

Abstract Conventional phase‐shifting transformers (PST) have been utilized for power flow control in transmission networks for many years. Compared to the slow reacting PSTs, recently trending unified power flow controllers (UPFC) enable much faster power flow control. This paper introduces a decentralized power flow control scheme for power flow controllers (PFCs) without the need for communication between the PFCs. For each PFC, an influence area is defined. The line loading of the transmission lines in this influence area is monitored in real‐time operation and if there is an overloading of a line, a feedback control scheme is used to mitigate the overloading of the line. As the power flow control of UPFC is dependent of the network topology and the UPFC converter limit, additional control measures are introduced to circumvent potential critical scenarios. Supplementary control measures are also developed to increase the robustness of control scheme for the simultaneous operation of UPFC with slow reacting PST. Using RMS simulation in DIgSILENT PowerFactory, the developed control method is validated in the IEEE 68‐Bus NETS/NYPS test bench network. The method is also validated for UPFCs with overlapping influence areas and the combination of fast acting UPFCs and slow acting PST devices.

In the case of dispersed load consumption, Ultra‐high Voltage Direct Current (UHVDC) system in cascaded connection mode can significantly reduce the cost by building DC lines between different ends, but the communication delay caused by it brings challenges to the original control strategy. Therefore, the control strategy suitable for cascaded multi‐terminal UHVDC (CMT‐UHVDC) system is developed. In this paper, first of all, the operation characteristics, mathematical model, and adaptability of the original control strategy are discussed. Next, an autonomous master‐slave control strategy is proposed to maintain voltage balance without real‐time communication between high‐end and low‐end valves. Finally, based on the planning parameters of an actual project, the electromagnetic transient simulation model of the CMT‐UHVDC transmission system is established. Several working conditions and the practical influence of sampling deviation on the control performance are discussed and analyzed, which verifies the proposed control strategy's effectiveness. [ABSTRACT FROM AUTHOR]

Multi‐input converters play an important role in integrating the independent energy sources utilized in the grid‐connected system and electric vehicle (EV) applications. In this scenario, several types of multi‐input converters are presented in the literature. Most of the MICs are operated using a time‐sharing scheme. This leads to a restricted duty cycle which limits the energy sources utilization and output voltage. To overcome the aforementioned limitations, a multi‐input single‐output (MISO) boost converter is proposed. It can improve the utilization of energy sources and output voltage with a reduction in the part count. To verify the feasibility of the proposed scheme, a 200 W prototype circuit is developed; simulation and experimental results are validated. [ABSTRACT FROM AUTHOR]

Conventional phase‐shifting transformers (PST) have been utilized for power flow control in transmission networks for many years. Compared to the slow reacting PSTs, recently trending unified power flow controllers (UPFC) enable much faster power flow control. This paper introduces a decentralized power flow control scheme for power flow controllers (PFCs) without the need for communication between the PFCs. For each PFC, an influence area is defined. The line loading of the transmission lines in this influence area is monitored in real‐time operation and if there is an overloading of a line, a feedback control scheme is used to mitigate the overloading of the line. As the power flow control of UPFC is dependent of the network topology and the UPFC converter limit, additional control measures are introduced to circumvent potential critical scenarios. Supplementary control measures are also developed to increase the robustness of control scheme for the simultaneous operation of UPFC with slow reacting PST. Using RMS simulation in DIgSILENT PowerFactory, the developed control method is validated in the IEEE 68‐Bus NETS/NYPS test bench network. The method is also validated for UPFCs with overlapping influence areas and the combination of fast acting UPFCs and slow acting PST devices. [ABSTRACT FROM AUTHOR]

Abstract The classical formulation of the transmission switching problem as a mixed‐integer problem is intractable for large systems in real‐time control settings. Several heuristics have been proposed in the past to speed up the computation time, which only limits the number of switchable lines. In this paper, a real‐time switching heuristic based on neural networks that provides almost instantaneous switching actions, are presented. The findings are shown on case studies of the IEEE 118‐bus test system, and the results show that the proposed heuristic is robust to out of distribution data. Additionally, the proposed heuristic has significant computational savings while all other performance metrics like accuracy are similar to state‐of‐the‐art machine learning methods proposed for transmission switching.

As conventional power generation units are being replaced with distributed energy resources, operational practices such as voltage regulation and congestion management are expected to be challenging. To address these challenges, regular and automated intercontrol center operational coordination will be needed between transmissions and distributions system operators (TSO and DSO). In this study, the data exchange required for the near-real-time operational coordination is investigated for a reactive power management use case. A realistic ICCP/TASE 2 protocol is implemented in a laboratory environment where the physical network is simulated in a real-time simulator while optimal set-points are communicated from the control centers to simulated assets being communicated through the IEC 60870-5-104. In addition, the sufficiency of the common information model (CIM) and common grid model exchange standard (CGMES) is evaluated for TSO-DSO network data exchange. The results from the cyber-physical test setup show that the detailed DSO grid model knowledge by the TSO results in lower system losses. Such data can be difficult to prepare, exchange, and compute. However, results show that simplified equivalent models can be acceptable if they are properly tailored to the specific use case. The experiences from the CIM implementation with the CGMES profile are found out to be sufficient for such operational data exchange.

A unified power flow controller (UPFC) is a versatile compensator providing multi-variable control for the critical power systems (CPS). In an attempt to reduce the search space for optimal allocation of UPFC thereby standardizing the deliberation over UPFC placement, zone-wise segregation of the network into weak and healthy zones is logically executed by an advanced network power stability indicator (NPSI). The UPFC performance is ascertained by a new line flow margin index (LFMI), power loss index and its economic benefit. The latter is derived by the cash outflow reduction (COR) and the internal rate of return (IRR), calculated by practical aspects like installation and maintenance cost, considering inflation and interest on amortization. For optimization, an improved version of the wind-driven optimization algorithm is proposed with finite-mass air parcels and intra-particle forces. An improved normalization technique is adopted for the multi-ranged, multi-objective problem. To simulate the CPS, the 57-bus test system with enhanced arbitrary load is considered. A significant reduction in power losses is observed when optimizing in reduced search space and the optimal location of G-UPFC is found to be in bus 13–15, which is also a part of the weaker zone. Several other results confirmatively approve the proposed approach of isolating the most appropriate zone for UPFC allocation based on the healthy and weak regions along with the proposed techno-economical evaluators.

The classical formulation of the transmission switching problem as a mixed‐integer problem is intractable for large systems in real‐time control settings. Several heuristics have been proposed in the past to speed up the computation time, which only limits the number of switchable lines. In this paper, a real‐time switching heuristic based on neural networks that provides almost instantaneous switching actions, are presented. The findings are shown on case studies of the IEEE 118‐bus test system, and the results show that the proposed heuristic is robust to out of distribution data. Additionally, the proposed heuristic has significant computational savings while all other performance metrics like accuracy are similar to state‐of‐the‐art machine learning methods proposed for transmission switching. [ABSTRACT FROM AUTHOR]

Abstract The automated identification and localisation of grid disturbances is a major research area and key technology for the monitoring and control of future power systems. Current recognition systems rely on sufficient training data and are very error‐prone to disturbance events, which are unseen during training. This study introduces a robust Siamese recurrent neural network using attention‐based embedding functions to simultaneously identify and locate disturbances from synchrophasor data. Additionally, a novel double‐sigmoid classifier is introduced for reliable differentiation between known and unknown disturbance types and locations. Different models are evaluated within an open‐set classification problem for a generic power transmission system considering different unknown disturbance events. A detailed analysis of the results is provided and classification results are compared with a state‐of‐the‐art open‐set classifier.

The large‐scale integration of remote renewable energy resources into existing AC grids is one of the main drivers for the construction of multi‐terminal direct current (MTDC) systems. Today, a few MTDC systems are already in execution stage or even in operation. However, with a growing number of extended MTDC systems, the probability of high‐voltage DC (HVDC) insulation faults is likely to increase and therefore, reliable solutions for handling of such disturbances are crucial in order to maintain high availability. Since most HVDC converters are realised by modular multilevel converters (MMC) converters are based on half‐bridge (HB) modules, they lack an inherent capability to control DC fault currents. Therefore, additional equipment like HVDC breakers needs to be installed as well. Among the various breaker types, the so‐called 'hybrid' DC circuit breaker (DCCB) is considered most popular. This contribution provides a more holistic view on the overall system behaviour, represented AC grid, MMC HVDC converter, DC reactor and DCCB altogether. It discusses the physical interactions of these main components in MTDC systems. In particular, the influence of the electrical parameters of the hybrid DCCB and its required DC reactors on the MMC converter's internal current and voltage conditions is outlined. [ABSTRACT FROM AUTHOR]

The controlled islanding problem is typically considered only for pure ac power systems, with the ultimate objective of either minimising power imbalance or power-flow among islands. However, as ac/dc hybrid power systems are becoming popular worldwide, current controlled islanding schemes should be redesigned to take into account the presence of high-voltage dc (HVDC) links, possibly connecting different islands. Accordingly, in this study, the authors solve the classic islanding problem combining HVDC power modulation with the conventional ac cutset search. The flexibility of the HVDC allows the authors' strategy to jointly capture the previously mentioned objectives, and provides even lower power imbalances than the minimum imbalance strategy, at a relatively small cost of increasing power-flow impact. Then, the optimisation problem is formulated as a mixed-integer linear programming problem, which can be conveniently solved with existing commercial solvers. Finally, they validate their proposed strategy in two case studies, corresponding to a modified IEEE-118 system and to the China Southern Power Grid system. In both cases, dynamic simulations show that their proposed approach outperforms classic algorithms where the presence of HVDC power modulation is not explicitly taken into account. [ABSTRACT FROM AUTHOR]

A tool to visualise real-time phasor measurement unit (PMU) data of a transmission line is proposed, named PQ chart. It is a simple and intuitive visualisation aid for operator situational awareness. The use of PMU data helps in capturing both steady-state and dynamic behavioural aspects of the transmission line. The tool is based on basic active and reactive power equations of a transmission line. The locus of active and reactive power flow corresponds to a circle in which the radius depends on voltage regulation. If the circle is imagined as a wheel, then the spokes of it correspond to the voltage angular difference across the transmission line. The PMU data is plotted on this canvas and its progression in time is observed. The occurrence of power swings, angular and voltage instabilities lead to a trajectory on the chart. A system operator can be alerted when any such movement occurs. The tool provides a single view from which deciphering active power flow, reactive power flow, voltage regulation, and voltage angle difference across a line becomes easy for the operator. Case studies with PMU data of 400 kV transmission lines in the Indian grid are presented to illustrate the tool's capabilities. [ABSTRACT FROM AUTHOR]

This study introduces a techno-economic planning strategy of reactive power (VAr) in power transmission systems. This planning strategy primarily has been focused on reactive power planning (RPP) through operating cost minimisation ensuring cost-oriented system security. The objective function (operating cost) is blended with four different components viz. cost due to real power loss, VAr generation cost, additional VAr compensation devices cost and line charging cost. FACTS (flexible AC transmission system) devices are installed at weak positions through different echelons to improve the system voltage stability. The authors also implemented the probabilistic hybridisation of crow search algorithm and JAYA to find out the optimal set of controlling parameters related to VAr. Finally, a sharp and minute analysis of the results has been done to validate the proposed strategy under multi-loading conditions. To understand the efficiency and efficacy the code profiling and simulation has been rendered on IEEE 30 bus and UPSEB 75 bus test system. [ABSTRACT FROM AUTHOR]

With the rapid development of voltage source converter based high-voltage direct current, power flow analysis becomes increasingly important for AC/DC hybrid systems. This study extends the holomorphic embedding (HE) method to the VSC-based AC/DC power flow problem. With an appropriate embedding technique, a general embedded model is established for AC/DC power flow with VSCs, which can not only preserve the deterministic property of HE that enables it to derive the desired solution in an explicit manner but also flexibly accommodate various bus types, VSC configurations, control strategies and operating conditions. Moreover, the sparsity of the recursive matrix in power series calculations and the introduction of the iterative calculation of the inverse matrix (ICIM) to rational approximants contribute to the high computational efficiency of the model. The proposed method is shown to be feasible and efficient by illustrative cases and is further extended to multi-terminal (VSC-MTDC) configurations to verify its flexibility. [ABSTRACT FROM AUTHOR]

High‐voltage direct current (HVDC) power modulation can effectively improve the system frequency stability after high power shortage fault disturbance. This study presents an emergency control method by coordinating the active powers of multiple HVDC lines for enhancing the frequency stability. Firstly, the system frequency response model with HVDC modulation is built, and then a simplified method for calculating the sensitivity of maximum frequency to HVDC modulation is proposed based on the first‐order differential method. Next, based on a functional relationship of the AC bus voltage and the actual HVDC modulation capacity, the methodology for calculating the HVDC real‐time modulation capacity is proposed. Finally, the coordinated multiple HVDC modulation emergency control model is established which aims to minimise the modulation cost considering the maximum frequency threshold. The proposed methodology has been tested in a real‐power system. The simulation results of different scenarios have verified its effectiveness in suppressing the rise of maximum frequency and enhancing frequency stability. [ABSTRACT FROM AUTHOR]

Recently large‐scale integration of power‐electronic‐based devices has made system‐level oscillations occur frequently, which posed a big challenge for modern power grids. Previous studies mainly treated sustained oscillations under the framework of (linear) small‐signal stability and/or by incorporating the impact of current saturation, but seldom considered system non‐linearity like phase‐locked‐loop. In this study, the authors established a sixth‐order non‐linear model for a three‐phase voltage‐source converter tied to AC grid, and studied its dynamics completely from non‐linear system theory and signal analysis technique, with all parameters including current controllers, phase‐locked‐loop, line inductance, and grid voltage. They found that the operation point usually becomes unstable by Hopf bifurcation, and the sustained oscillations are possible only for super‐critical Hopf bifurcation. Based on these observations, sustained periodic oscillation should be treated as a limit cycle even without saturation. With saturation, some other phenomena have also been found, such as saturation‐induced instability and saturation‐restricted oscillation. In addition, they discovered that for sustained oscillations, the usual purely sinusoidal three‐phase currents exhibit the form of sin⁡[2πf1t+csin⁡(2πfdqt)] or cos⁡[2πf1t+csin⁡(2πfdqt)], where f1 (fdq) denotes basic (modal oscillation) frequency with c a constant. This work could provide an improved understanding of sustained oscillations and associated system dynamics in an overall perspective manner. [ABSTRACT FROM AUTHOR]

This study aims to improve oscillation damping and the rate of integration of wind energy into the grid. To this end, this study focuses on selecting the optimal location of flexible alternating current transmission system (FACTS) devices, such as the static synchronous series compensator, the static synchronous parallel compensator, and the unified power flow controller (UPFC). This study proposes new methods to determine the optimal locations of FACTS devices in the transmission system. To further improve power oscillation damping and the rate of integration of wind energy, FACTS devices were combined with a multiband power system stabiliser (MBPSS) at each synchronous machine. The MBPSS is an advanced power system damping controller that offers clear advantages over conventional power system stabilisers in the damping of low‐frequency oscillatory modes. A comparative study was performed on the two‐zone, four‐machine (Kundur) multi‐machine test network containing a wind farm to select the best combination of FACTS devices with the MBPSS. The results indicate that a combination of UPFC with MBPSS is favourable in terms of improving stability and increasing the rate of integration of wind energy into the grid. [ABSTRACT FROM AUTHOR]

This study proposes a hybrid control (HC) strategy for improved frequency regulation and power sharing in multi‐terminal HVDC (MTDC) integrated AC grids. The proposed method uses the topology of bi‐polar converters in the MTDC system to improve power sharing and frequency regulation. In this methodology, the two voltage source converters (VSCs) in a bi‐polar topology are operated with individual specified control methods. In the proposed HC method, one of the converters operate in voltage square frequency droop (PV2f) and another in power frequency droop (Pf) control methods. This HC strategy is implemented for grid side voltage source converters in MTDC grids. Further, the performance of frequency regulation and autonomous power sharing of the proposed HC technique is compared with the conventional voltage and frequency droop (PVf) and improved PV2f control methodologies. In order to validate the proposed method, two AC‐MTDC systems are considered. In both the systems, mesh typed MTDC CIGRE B4 DC test system is in common, it is integrated with two‐area power system and New England IEEE 39‐bus system to form two different AC‐MTDC systems. [ABSTRACT FROM AUTHOR]

Short circuit fault occurrence in high-voltage DC (HVDC) systems causes extremely high currents in a fast raising time that does not experience current zero-crossing. To protect HVDC systems/grids against fault current, fast HVDC breaker is an essential equipment. This study presents the design procedure of a novel HVDC breaker based on solid-state controllable reactor which is able to reduce the fault current's rate of rise and fault current amplitude to less than grid nominal current in the breaking process. The main achievement of the proposed HVDC breaker is that not only breaker does not encounter fault current, but also none of the series HVDC equipment is influenced by the fault. The designed breaker performance is studied by PSCAD/EMTPS, and then the simulation results are validated by the developed laboratory experimental setup.

Currently, there is a national push for a smarter electric grid, one that is more controllable and flexible. Only limited control and flexibility of electric assets is currently built into electric network optimization models. Optimal transmission switching is a low cost way to leverage grid controllability: to make better use of the existing system and meet growing demand with existing infrastructure. Such control and flexibility can be categorized as a “smart grid application” where there is a co-optimization of both generators or loads and transmission topology. In this paper we form the dual problem and examine the multi-period N-1 reliable unit commitment and transmission switching problem with integer variables fixed to their optimal values. Results including LMPs and marginal cost distributions are presented for the IEEE RTS 96 test problem. The applications of this analysis in improving the efficiency of ISO and RTO markets are discussed.

The physical mechanism of the instability of the low-inertia voltage source converter (VSC) caused by the phase-locked loop (PLL) in a weak system is clearly revealed in a simplified way. Based on the complex torque method, the joint influence of the PLL and the system strength on the VSC is studied through the defined key stable component. To keep the VSC stable, the value of the key stable component must be positive. The component is decided by AC voltage, short-circuit-ratio (SCR), reactive power, and PLL parameters. Improper PLL parameters and weak AC system could make the key stable component negative. To keep the VSC stable in a weak system, the PLL parameters can be adjusted according to the curve of the key stable component, which means suitably decreasing kp and increasing ki. Finally, simulations are implemented to validate the theory, which matches well with the findings. [ABSTRACT FROM AUTHOR]

To relieve congestion due to open‐access feature in competitive power system framework, utilities plan flexible alternating current transmission systems (FACTS), ensuring improved utilization and performance of the transmission infrastructure. However, high‐investment cost restricts implementation to single type of FACTS' planning in a time. Therefore, successive planning of another FACTS to delay transmission expansion results in multi‐type FACTS planning. Consequently, to optimise performance, subsequent planning must coordinate with existing FACTS. The main objective of this study is to implement coordination of multi‐type FACTS for available transfer capability (ATC) enhancement. A hybrid real power flow performance index (PI) and particle swarm optimisation (PSO), coordinate thyristor control series compensator (TCSC) in the first planning horizon with static synchronous series compensator (SSSC) and unified power flow controller (UPFC), in the second horizon. The PI–PSO‐based multi‐type FACTS coordination improves ATC of multilateral power transfers in a standard 9‐buses test network. Results show that multi‐type FACTS achieved higher ATC; such that enhanced ATC by TCSC–SSSC ranges between 8.06–69.34% while the TCSC–UPFC ranges between 11.85–71.59% for various power transfer transactions. A comparison of the three coordination schemes shows that the scheme with more decision parameters provides superior loadability and transfer capability improvement. [ABSTRACT FROM AUTHOR]

The increased penetration of distributed energy resources (DERs) installed in distribution networks via power electronic converters reduces the overall rotating inertia of the power system, causing faster frequency dynamics after a large disturbance. However, these DERs coupled with energy storage systems (ESSs) can be managed to provide valuable grid support functions such as fast frequency response (FFR) and assist the transmission system operator (TSO) in frequency control. In this study, single‐ and three‐phase clusters of electric vehicles (EVs), acting as mobile‐ESSs, are grouped together as virtual power plants (VPPs) to centralise the provision of FFR for the distribution system operator (DSO) whilst considering network unbalance. Furthermore, the parameters of the EV clusters within each VPP are optimised to minimise the inertia‐weighed maximum frequency deviation following a disturbance whilst adhering to network security and power quality constraints. To this aim, a variant of the metaheuristic optimisation algorithm called improved harmony search is used for the optimisation process. Finally, the merits of the proposed methodology are shown with an illustrative example of an unbalanced three‐phase transmission and distribution network modelled in DIgSILENT PowerFactory. The results show that clusters of EVs grouped as VPPs effectively improve frequency support via the TSO–DSO interaction. [ABSTRACT FROM AUTHOR]

The stability of the modular multilevel converter‐based multi‐terminal direct current (MMC‐based MTDC) transmission system is strongly affected by the coupling among multiple elements and the interaction between the control systems. Improper designed control parameters may deteriorate the dynamic response or even reduce the stability margin of the MMC‐based MTDC. This study proposes an optimal control parameters design approach to enhance the stability of MMC‐based MTDC. The dynamic model of an MMC‐based MTDC consisting of main circuit, control system, AC system and DC system is established, and the small‐signal stability model is derived. The impact of control systems on the stability of MMC‐based MTDC is studied through participation factor analysis. An optimal control parameters design problem with an eigenvalue‐based objective function is formulated and a sensitivity analysis‐based optimisation algorithm is proposed to optimise the control parameters jointly. The backtracking line search is applied to determine the feasible step size in each iteration. Stability assessment and time‐domain simulations are performed to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The conventional first zone characteristic of distance protection is constructed mainly based on engineering experience, which limits its coverage of internal faults. The zone coverage is also adversely affected by the installation of series compensation devices with over‐voltage protection. Despite research efforts to overcome these problems, developing an algorithm for constructing an optimal first zone characteristic that encloses only all separable internal faults remains a challenging task. This study addresses this issue by presenting an optimisation‐based algorithm for constructing an adaptive optimal first zone characteristic for power lines. The algorithm takes into account different locations of the compensation devices along the line, their non‐linear protection, and the inaccuracies in measurements and considered grid model. The proposed and conventional zone construction algorithms are compared using a grid model in MATLAB Simulink. The results demonstrate that the proposed algorithm is robust to input parameters and thus able to construct an optimal zone characteristic for a specified range of grid operating points and fault parameters. Moreover, the first zone obtained by the proposed algorithm covers substantially more internal faults than the conventional zone with optimised settings. [ABSTRACT FROM AUTHOR]

Power systems are a class of typical cyber‐physical systems with switching‐hybrid dynamics. With the innovation of power electronics technology and the implementation of power grid development strategy, the application of voltage source converter‐based high voltage direct current (VSC‐HVDC) transmission has been paid more and more attention. Firstly, the general small‐signal model of the VSC‐HVDC system is established, and its stability analysis is carried out by integrated modal analysis, participation factor analysis, and sensitivity analysis methods. It is clarified that the system controller parameters will have a significant impact on the stability of the VSC‐HVDC system. Then, based on the switching system theory, the stabilising switching law of the bidirectional converter is derived, which makes the system energy attenuate fastest, and the stabilising switching controller is designed according to the switching law. Last, the effectiveness of the controller is verified by simulation, and the superiority of the stabilising switching control strategy is proved from the theoretical and simulation aspects by comparison with the traditional current vector control strategy. [ABSTRACT FROM AUTHOR]

This study proposes a modal online differential fault detection and localization (MODF) scheme for VSC-MTDC cable transmission. Modal theory is used to remove the coupling between layers of two-core extruded high-voltage direct current cable. As a result, the decoupled modes are determined and the modal matrix resulted. In high frequencies, the modes have different velocities, which result in a time difference between modal initial peaks. This time delay is used to design modal fault detection and localization scheme, which can work in a variety of strategies like pilot or non-communicated protection systems, one-sided or two-sided relaying, and single-zone or double-zone relay settings. Novel modal time–distance characteristics are introduced for adjusting the zone setting of relays in the MODF scheme. Multiple faults are applied to the CIGRE VSC-MTDC model in the real-time digital simulator with a Beckhoff CX5130 controller in the hardware-in-the-loop setup for testing the proposed MODF scheme. The advantages and weak points of each strategy are analyzed and compared for the modelled faults. Hilbert–Huang transform is used for the accurate detection of modal initial peaks. It is shown that the method gives accurate and selective results for a wide range of VSC-MTDC configurations. [ABSTRACT FROM AUTHOR]

This study presents a dq impedance‐decoupled network modelling method of offshore wind power plant (OWPP) for stability analysis. DQ impedance frequency responses of grid‐connected inverters (GCIs) and long transmission cables (LTCs) are first measured by frequency scanning method, which are fitted as transfer function matrices using matrix fitting algorithm. Then, the GCIs are modelled as Norton equivalent circuits. In addition, per‐unit‐length electrical parameters of the LTCs are extracted from the measured dq impedance frequency responses of a specific LTC, based on which dq impedance‐decoupled two port network models of the LTCs are established. DQ impedance‐decoupled network model of the whole OWPP is then established based on connection relationships of these GCIs and LTCs. Finally, generalised Nyquist criterion (GNC) is performed in all the dq impedance‐decoupled subsystems, and the subsystems where the GNC is not satisfied are identified as instability sources. Compared with conventional impedance‐based stability analysis methods of OWPP, the proposed dq impedance‐decoupled modelling method is able to facilitate the application of GNC and further instability source identification based on partitioning the whole OWPP into several decoupled subsystems. The effectiveness of the network modelling method is validated based on time‐domain simulation results in Matlab/Simulink and real‐time verification results in OPAL‐RT. [ABSTRACT FROM AUTHOR]

The voltage oscillation of islanded DC-power systems is probably caused by negative impedance, produced by the converter from conventional power control structure. In this study, the small disturbance linearised state equation of a DC-power grid with constant power load is derived to estimate the damping characteristics of DC voltage. To enhance the transient voltage stability, a novel DC-voltage damping control method is proposed, where both DC voltage and DC current are used as state-feedback variables. Furthermore, two state-feedback control loops are designed to regulate the duty cycle of buck–boost converter added to the energy-storage device. Using the root-locus method, the principle of system pole placement is further discussed to determine the optimal control parameters in the proposed feedback loops, so that the damping characteristics of power system can be optimised. Finally, experimental tests are conducted on a hardware-in-the-loop platform of a typical islanded DC-power system to validate the new control structure. The results demonstrate that the DC-voltage oscillation can be reduced rapidly, which greatly improves the transient stability of DC-power system. [ABSTRACT FROM AUTHOR]

High-power DC–DC converters are integrated into multi-terminal high-voltage DC (MTDC) networks, typically for voltage matching or flexible DC transmission. This study addresses droop-controlled radial and mesh MTDC networks with transmission loss minimisation considering the impact of integrating a high-power DC–DC converter. DC voltage droop control can achieve minimum power loss in radial MTDC transmission network by controlling the power-sharing among the receiving nodes inversely proportional to their transmission line resistances. Updating the droop characteristics by an optimisation algorithm guarantees optimal power flow (OPF) during renewable source power generation fluctuation. Meanwhile, the mesh network DC voltage references can be updated by the OPF for transmission loss minimisation. This study presents the impact of incorporating a high-power DC–DC converter at different system access lines of a radial MTDC network while targeting efficient power transfer and DC transmission loss minimisation. The effect of introducing a high-power DC–DC converter on the droop characteristic settings during steady-state operation in a radial MTDC network is investigated. Moreover, the impact of a high-power DC–DC converter in mesh MTDC network with OPF operation is explored. To validate the presented concepts, the CIGRE B4 MTDC network is used as a benchmark system with a metaheuristic optimisation technique. [ABSTRACT FROM AUTHOR]

The transition from conventional towards renewable energy systems requires additional transmission capacities and higher operational flexibility to overcome contingency situations. This study presents a novel transmission expansion planning approach for the simultaneous placement of AC systems and power flow controlling technologies including high voltage direct current, phase shifting transformers (PSTs) and thyristor controlled series compensators (TCSCs). The approach is formulated as a mixed‐integer linear programming optimisation problem and the operating point of power flow controlling devices is endogenously determined by minimising expansion costs. Both PSTs and TCSCs are integrated into the optimisation problem by means of new mathematical formulations. PSTs are presented in more detail considering the PST impedance, which is modelled as an equivalent phase shift. The controllable series impedance of TCSCs is also represented as an equivalent phase shift. Exemplary results based on a synthetic 120 bus system show a significant potential of PSTs and TCSCs to reduce expansion costs as well as to lower the requirement for new transmission capacities. The simultaneous employment of these technologies reduces the expansion costs within the test system between 18.7 and 32.4% compared to those costs generated by the sole reinforcement by AC systems. [ABSTRACT FROM AUTHOR]

This study investigates the impact of embedded voltage source converter‐based high voltage direct current (VSC‐HVDC) links on AC grids transient stability. Firstly, using transient energy functions, it is demonstrated that VSC‐HVDC links controlled to track constant power references, do not inherently improve transient stability of the surrounding AC grid as an AC line naturally does. Then, a control law using the feedback linearisation technique on a simple but representative power system is derived. The control law highlights and combines the three main actions the VSC‐HVDC link can offer to enhance rotor angle stability: fast power reallocation, injection of synchronising power and injection of damping power. The control law is implemented and validated in EMT simulation. It is then shown that an HVDC link can assure the synchronisation of different AC areas even if no AC transmission lines interconnect them. Through another case study, it is shown how the HVDC link can help to share dynamic frequency reserves to not jeopardise the stability of the system. The last example investigates the effect of a DC fault on AC transient stability and how the control can help improving the system response. [ABSTRACT FROM AUTHOR]

Nowadays power transmission losses in multi‐terminal high‐voltage DC (MT–HVDC) systems of large offshore wind farms are the main concerns to wind farm operators. Maintaining the DC voltages and currents within pre‐specified limits and simultaneously optimising DC power flows of MT‐HVDC systems using optimum droop control settings becomes a complex optimisation problem. This droop control scheme was less explored with power loss minimisation applications due to the difficulties in the optimisation of these droop gain settings. This study presents a new mathematical programming plus meta‐heuristic‐based optimisation approach to solve this optimisation problem. This hybrid optimisation methodology is also termed as 'matheuristic approach'. The proposed matheuristic method optimises the droop control settings for different operating conditions of offshore wind farms and simultaneously minimises the DC power losses in the MT‐HVDC system. The proposed optimisation method has been tested upon a six‐terminal HVDC system connected with large offshore wind farms. The superiority of the proposed optimisation method is demonstrated using the steady state as well as dynamic simulation studies. The simulation results show that DC power losses could be reduced significantly by adopting the proposed method to facilitate optimal power flows in the MT‐HVDC system of large offshore wind farms. [ABSTRACT FROM AUTHOR]

This study aims at validating the simultaneous operation of grid following and grid forming wind power plants when connected to a common diode rectifier‐based HVDC link. The controllers for both grid forming and grid following wind turbines include fault‐ride‐through capability with soft restoration of the off‐shore AC‐grid. Current and voltage controllers are developed in the stationary αβ reference frame. The presented control and soft restoration strategies are based on local measurements only and do not require communication between wind turbine generators. The small‐signal stability analysis of the mixed system in multiple d–q axis using detailed string models is carried out in order to show the sensitivity to grid following phase‐locked‐loop gains and to grid forming droop gains. Interoperability between grid forming and grid following wind turbines during transients is shown by means of detailed simulation during symmetric faults in different locations of the off‐shore grid. [ABSTRACT FROM AUTHOR]

In the modular multilevel converter (MMC)-based multi-terminal high-voltage direct current (HVDC) system, the fault current rises rapidly due to sub-module capacitors discharging when a DC-side short-circuit fault occurs, which makes the MMC easily blocked for self-protection. To avoid the blocking of the converter station and lower the DC fault current, this study proposes a novel fault current limiter (FCL) for MMC-based multi-terminal DC grids. First, the fault current limiting circuit, which consists of resistance, capacitance and power electronic components is designed. Then, the operation principle of the FCL is analysed in detail. Afterwards, the startup criterion is constructed, and the selection method of the key parameters in the fault current limiting circuit is also introduced in order to prevent the converter station from blocking in the case of DC short-circuit fault. Finally, a simulation model was built using PSCAD/EMTDC software, and extensive simulation results verified the effectiveness of the proposed FCL. [ABSTRACT FROM AUTHOR]

Currently, transmission grids are subject to experience more operation and control difficulties. The use of power flow controllers (PFCs) can ease such difficulties. However, most competent PFCs need to be used. Cost‐wise, the power transistor assisted Sen transformer (TAST) is attractive as compared to the unified PFC (UPFC). However, while the UPFC has three control parameters, the TAST has only two. The UPFC can regulate the voltage of its shunt bus while the TAST cannot. The main objective of this study is to investigate the extent to which the TAST and UPFC are comparable when used for power flow control. Power flow control is performed in this study for congestion management using a TAST/UPFC in a modified IEEE‐118 bus system. A TAST/UPFC is optimally located using sensitivity analysis. For the sake of validation, results are obtained using both load flow analysis and voltage vector analysis. With only two control parameters, the performance of the TAST is found very closely comparable to that of the UPFC. Accordingly, the TAST can competently replace the UPFC for application of power flow control, at a far lower cost. Nonetheless, appropriate limited angle TAST is a suitable substitute for the TAST at a further lower cost. [ABSTRACT FROM AUTHOR]

Multiterminal high‐voltage direct current (HVDC) systems still need advances in terms of protection in order to improve their reliability. In this context, the distance protection can play a major role by adding selectivity to the existing DC fault detection algorithms. Hence, the present work proposes a non‐unit DC distance protection algorithm that uses the frequency of the DC voltage transient oscillation to estimate the distance of the fault. The DC voltage transient frequency is extracted using the Hilbert–Huang transform and compared with a pre‐defined frequency/distance curve. The technique was evaluated by simulating faults in a four‐terminal symmetric monopole multiterminal HVDC system. In the simulation environment the algorithm was fully selective for faults within the first protection zone and had a correct operation rate of 94% or more for faults located in the second protection zone. To further validate the presented technique, the proposed algorithm was embedded in a digital signal controller, running in real‐time. In all performed tests in hardware, the faults were correctly detected and identified as being internal or external. The results indicate that the proposed algorithm could be used in real‐world applications, in conjunction with fault detection techniques, adding selectivity to multiterminal DC protection schemes. [ABSTRACT FROM AUTHOR]

Traditional way to enable frequency support provided by MTDC system for the connected weak AC systems is to modify the power reference of each converter under standard power‐sharing droop control based on the weak grid's frequency deviations. However, this scheme artificially couples all asynchronous AC systems to tackle with the frequency disturbances of any connected AC systems, which might result in broader frequency disturbance propagations. To overcome this, this study proposes a decoupled frequency control (DFC) scheme to improve weak AC systems frequency stabilities. First, the relationship between the active power and power set points of each converter is derived by the linearization of MTDC power flow equations. Then, the frequency coupling properties of each AC system with traditional frequency control are derived, and it shows that the frequency disturbance will be propagated to all the connected AC systems with traditional control. Furthermore, the control law of each converter in the proposed DFC is developed through the defined decoupled matrix, and the independent frequency regulation characteristic of each converter can be obtained. Finally, numerical simulations have demonstrated the effectiveness of the proposed DFC scheme in events of load changes and converter loss in the main and weak AC grid. [ABSTRACT FROM AUTHOR]

In this study, a maiden application of cascade proportional–integral–derivative with filter (PIDF) and one plus fractional-order derivative (1+FOD), i.e. PIDF(1+FOD) controller is proposed for the load frequency control mechanism. The main purpose of the cascade and fractional order controller is to increase the degree of freedom and reject the disturbances faster. A novel attempt has also been made to model a static synchronous series compensator (SSSC) with AC tie-line and HVDC tie-line. Here SSSC and HVDC tie-line are used to regulate and increase the power transfer capabilities between interconnected areas. Furthermore stored energy in the HVDC tie-line is utilised to improve the system dynamics by inertia emulation control strategy. An evolutionary salp swarm algorithm-based optimisation technique is adopted to optimise control parameters. The performance of the proposed controller is tested on a two-area hydro-thermal interconnected power system in deregulated environment. First, the effectiveness AC–DC-SSSC tie line is shown over AC-SSSC and alone AC tie-line with proposed PIDF(1+FOD) controller followed by the supremacy of proposed PIDF(1+FOD) over classical controllers. Finally, sensitivity is analysed by variation in system parameters, which shows the robustness of the proposed controller. [ABSTRACT FROM AUTHOR]

This article describes a novel power system architecture and operational concept which is founded on the idea of organising sets of distributed generators, loads, storages and multimodal interfaces at distribution level in supervised and controlled grid subsections called smart power cells, which are controllable entities intended to enforce the stability of the transmission network and participate in its operation. Further, the article addresses a control scheme which can be implemented within a smart power cell to control its active and reactive power exchange with the transmission network following set points provided by a superimposed control system, and thus participate in the operation of the transmission grid (e.g. participation in congestion management, system optimisation, balancing of generation and load). The dynamic behaviour of a future power system under consideration of the proposed system architecture including several smart power cells controlled with the developed control scheme is demonstrated through time‐domain simulation using a combined transmission–distribution power system model. [ABSTRACT FROM AUTHOR]

Half‐bridge modular multilevel converter‐based high‐voltage direct‐current (MMC‐HVDC) grids are technically and economically viable solutions to transmit bulk electrical energy generated from geographically dispersed and remote renewable resources. However, due to the half‐bridge converter's inability to control fault currents, these grids need specialised protection schemes to increase reliability and equipment safety. This study proposes a non‐unit non‐differential protection scheme founded on a comparative orientation measure of fault components for buses of half‐bridge MMC‐HVDC grids. Considering the selected protection strategy, the protection zone of this scheme can also cover a part of the converter station's ac side; in which severe short‐circuit faults should be quickly detected and isolated as well since they may cause the entire grid outage. This scheme can be used as the main protection or a backup of the conventional differential current protection, and subsequently, increase the protection system dependability by increasing the functional diversity. The test results disclose that the proposed scheme has superior sensitivity to identify single‐pole and double‐pole faults with resistances up to 1500 and 2000 Ω, respectively, at the protected dc bus with the maximum delay of 1 ms, while it is robust against all ac and dc faults outside the designated protection zone. [ABSTRACT FROM AUTHOR]

Voltage droop control is considered as a reliable control approach for multi‐terminal DC (MTDC) grid system. A low value of droop constant may result in a high dynamic variation associated with oscillations for power and DC voltage deviation. Thus, the stability of the MTDC grid is at risk. This study presents a linear quadratic regulator (LQR) control design with an adaptive droop to achieve appropriate power sharing among the converter terminals. A control strategy with the combination of adaptive droop and LQR is designed in this study. The weights of LQR are tuned using the Gramian analysis. The closed‐loop stability analysis is carried out using Lyapunov's stability approach. The proposed control scheme improves the closed‐loop system dynamics by reducing the overshoots/undershoots and time to reach steady‐state condition. In addition, power sharing according to converter rating is appropriately achieved. The performance of the proposed control scheme is validated on five terminal MTDC grid system simulated in MATLAB®. [ABSTRACT FROM AUTHOR]

This study presents a bi‐level optimised emergency control strategy, including load and generators shedding following DC blocking. Due to different constraints for networks with different voltages, a bi‐level optimised model spanning from the high voltages networks to the lower voltages is put forward. For the higher voltage level network, the overload area is identified and the seriousness of the overload is classified after DC blocking with the help of wide‐area measurement systems. Then, the objective function is built with the minimum total amount of load shedding and generator tripping to be solved by improved particle swarm optimisation (IPSO), considering the constraints of frequency and voltage stability, respectively. For the low voltage level networks, the least cost as the objective function with the constraints of load shedding and generator tripping strategy based on the analytic hierarchy process‐fuzzy comprehensive evaluation method. The modified 10‐machine 39‐bus system with HVDC lines is used as the test system. The simulation results demonstrated that the proposed strategy can give good control performance with consideration of different optimisation constraints following the DC line blocking. [ABSTRACT FROM AUTHOR]

In this study, a new kind of hybrid high‐voltage direct current (HVDC) system is proposed. Each terminal of the proposed system consists of one line commutated converter (LCC) and one full‐bridge modular multilevel converter (FB‐MMC). The LCC and FB‐MMC are connected in parallel so that they can share the same transmission line. The active–reactive power capability of the hybrid HVDC system is extended compared with the conventional LCC‐HVDC system, and power reversal control without power interruption can be achieved by the coordination control of LCC and FB‐MMC. Besides, the proposed hybrid HVDC system is capable of handling DC fault, because both LCC and FB‐MMC have DC fault blocking capability. Moreover, the power rating of FB‐MMC can be designed to low value while keeping the bulk‐power transmission capability of LCC. A two‐terminal bipolar hybrid HVDC system is built in PSCAD/EMTDC. The simulation results verify the effectiveness and feasibility of the proposed hybrid topology and corresponding control strategies. [ABSTRACT FROM AUTHOR]

Frequency stability control in multiple asynchronous grids is a challenging and complex issue. An adaptive droop control strategy to improve the frequency regulation of asynchronous AC areas connected by a multi‐terminal DC grid is proposed here. The droop coefficients are adjusted to share the active power adaptively among multiple asynchronous AC grids based on the characteristics of frequency deviation and rate of change of frequency. This results in a cogent allocation of imbalance power in multiple asynchronous grids from the frequency variation perspective. The performance of the proposed scheme is evaluated in a modified multi‐machine power system using DIgSILENT Power Factory. Simulation results under significant frequency disturbances caused by credible contingencies are presented to demonstrate the effectiveness of the proposed approach. It is found that the proposed adaptive control ensures an excellent and robust frequency response under different operative conditions. [ABSTRACT FROM AUTHOR]