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The Kriegers Flak combined grid solution (KF CGS) will interconnect the eastern synchronous area of Denmark and Germany by extending the existing high‐voltage alternating current (HVAC) offshore wind farm infrastructure in the Baltic Sea. In contrast to conventional point‐to‐point interconnectors, the extension creates a meshed submarine grid (MSG) which includes an interconnector and wind farm collectors to the countries using the same equipment. Denmark East as part of the Nordic system and 50 Hertz as part of the synchronous Continental European grid are asynchronous to each other, which makes a frequency transformation necessary. The interconnection will be realised by a high‐voltage direct current (HVDC) back‐to‐back (BtB) converter in voltage source converter technology located at the German end of the interconnector. The BtB and the entire HVAC MSG between Denmark and Germany will be controlled by a so‐called master controller for interconnector operation (MIO). The KF CGS will integrate offshore wind power generation, increase the security of supply and enhance integration of regionally produced renewable energy. This study describes the overall infrastructure of the KF CGS and the MIO functions. A model‐based evaluation of those shows the behaviour of the HVAC/HVDC MSG in normal operation and in the case of contingencies. [ABSTRACT FROM AUTHOR]

In multi‐terminal voltage source converter based high‐voltage direct‐current (VSC‐HVDC) systems, droop controllers are commonly adopted to regulate the active power and voltage. Thus, the droop controller coefficients inevitably affect the system stability. In this study, a method to design droop control coefficient is proposed considering the system stability. The state‐space model of the multi‐terminal VSC‐HVDC system with droop controllers is established in which the droop controllers are considered as feedback controllers to reflect the control characteristics. Based on the model, the determination of droop control coefficients can be equivalent to an optimisation problem. The objective is to minimise the Frobenius‐Hankel norm of the system S/T mixed sensitivity. The constraint conditions include closed‐loop system eigenvalues constraint and steady‐state errors constraint. By doing so, the optimal system stability and robustness are achieved. To verify the proposal, simulations on a four‐terminal VSC‐HVDC system are performed with two another sets of droop control coefficients for comparison. Simulation results show that the proposed droop controller can ensure system stability and robustness under various disturbances and faulty conditions. [ABSTRACT FROM AUTHOR]

Short‐term frequency stability (STFS) is becoming a great concern for a regional receiving‐end power system such as the East China Power Grid (ECPG) mainly for two reasons: (i) the increasing percentage of power injected from multi‐infeed (ultra‐) high‐voltage DCs (HVDCs) with the skyrocketed capacities; (ii) the replacement of traditional generation by the continuously growing inertia‐less renewables. However, the existing emergency control is not adaptive enough to maintain STFS under varying operation conditions. In this study, a real‐time optimised control is proposed to solve the issue more efficiently. It combines emergency power boosting (EPB) of HVDC and emergency demand response (EDR) in a coordinated way. The optimal control problem is formulated based on the online‐updated models that reflect the current state of the system and the dynamic responses of multiple types of generators. By converting the non‐linear constraint into a linear matrix inequality, the allocation of EPB and EDR can be optimised in a real‐time and coordinated way. The performance of the proposed control and its advantages over the existing one are verified by simulation studies on the model of ECPG with low, normal and high penetrations of renewables. [ABSTRACT FROM AUTHOR]

High‐voltage direct current (HVDC) can speed up the recovery of a power system. However, both rectifier and inverter sides of HVDC are weak systems during a black start. To coordinate frequency characteristics of both sides within the frequency deviation constraint, a frequency regulation strategy based on variable‐parameter frequency limit control (FLC) during a black start is proposed. First, the mathematical models of asynchronous interconnected AC–DC system are established, and the optimal FLC parameters leading to minimum frequency deviation of both sides are obtained. Subsequently, based on the characteristics of the optimal FLC parameters with the parameters of the equivalent generator and the capacity of the AC system, a frequency regulation strategy is proposed to adapt to the continuously changing parameters and configurations of the AC power system during a black start. Finally, the simulation model is established using PSCAD/EMTDC software to verify the control strategy and the correctness of the optimal FLC parameters. [ABSTRACT FROM AUTHOR]

The rising share of renewable generation increases uncertainty in power system operation. New methods are needed for secure operation of power systems under uncertainties. We present a robust optimal power flow algorithm for mixed ac and dc power transmission systems which considers uncertain renewable generation and/or load. The uncertain infeeds are assumed to lie inside a known norm-bounded set. The presented algorithm guarantees satisfaction of various steady-state power system constraints for all uncertainties within this set. In order to satisfy these constraints at minimal cost, the algorithm modifies both the redispatch and primary reserve. This is done through the optimization of setpoints and droop gains of generators and high-voltage dc converters. The algorithm is evaluated on a test system, the Cigre B4 DC grid, the IEEE RTS 96 grid, and on the Danish transmission system. In these examples, we show that redispatch costs can be reduced by additionally optimizing primary reserve, and demonstrate primary reserve sharing between asynchronous ac grids through dc grids. We illustrate the impact of the uncertainty set choice on the worst-case redispatch costs, perform a sensitivity analysis of the approach, and evaluate the computation time as a function of the number of uncertainties. [ABSTRACT FROM AUTHOR]

Optimal transmission switching (OTS) has been proposed as a new control paradigm to improve the economics of electric power systems. The problem is formulated as mixed integer linear programming in which binary decision variables are used to represent the on and off state of transmission lines. In this paper, we propose an approximate model for the OTS problem to achieve better results in terms of solution quality and computation time. We prove the feasibility and optimality properties of our proposed approximate model through theorems. Through numerical studies we show that our proposed approximate model finds OTS solutions with the same electricity generation cost but with lower number of switched transmission lines in the same or less amount of time. Therefore, number of switched transmission lines is reduced to half, one-third, or even less, depending on the case study. [ABSTRACT FROM AUTHOR]

Y‐connected modular multilevel converter (Y‐MMC) is a new topology of direct AC/AC power conversion, with broad application prospects in fractional‐frequency transmission system. This study studies the mathematical model of Y‐MMC and builds the port‐controlled Hamiltonian model. Since vector control scheme can hardly ensure global stability, the interconnection and damping assignment passivity‐based control (IDA‐PBC) method is applied for controller design of Y‐MMC. Three typical IDA‐PBC strategies are then proposed, featuring the asymptotical stability of the desired equilibrium. To eliminate steady‐state error, integrators are further added to the IDA‐PB controller. Different from the previous research, the proposed method enables the decoupling of different‐frequency components and the suppression of frequency leakage. Besides, the reactive power distribution coefficients are introduced to characterise the optimisation allocation of reactive power between arms. Finally, the effectiveness and superiority of the proposed control strategy are verified by both the simulation and experiment results. [ABSTRACT FROM AUTHOR]

With the increasing installation of the high‐voltage direct current device and flexible AC transmission system, the power transfer capability increases significantly. However, the risk of line overloads or even cascading failures caused by DC blocking or line outage also augments, which requires accurate and effective post‐contingency corrective control (CC) actions. Traditional sensitivity‐based methods are fast, but are less accurate and may suffer from the seesaw problem. Artificial intelligence optimisation‐based methods usually exhibit better CC performance but are time‐consuming. For fastly performing accurate CC actions, a second‐order cone programming (SOCP)‐based CC optimisation model for line overloads alleviation in meshed AC/DC power systems is proposed. Initially, to reduce the number of CC actions and optimisation variables (OVs), a heuristic search method is proposed to automatically select the most effective generators/loads for the overloaded lines. Subsequently, thyristor controlled series capacitor (TCSC) compensation levels and multi‐terminal direct current (MTDC) power are treated as additional OVs to minimise load shedding/generation rescheduling. Additionally, to boost computational efficiency, SOCP format power flow equations for systems with MTDCs and TCSCs are proposed. Eventually, the effectiveness, accuracy, and superiority in computational efficiency of the proposed method are verified based on the modified IEEE 30‐bus test system. [ABSTRACT FROM AUTHOR]

This study presents a generic linearised modelling method and a virtual parallel resistor damping control for a voltage source converter‐based high‐voltage direct current (HVDC) system including a direct current (DC) grid. The generic modelling method is composed of DC network's equivalent node‐admittance matrix and the Thévenin equivalent matrix of all converters. A simplified two‐terminal HVDC system is used to identify the key factors influencing the stability of the DC grid easily. Those key factors are proved by two methods, namely, root locus and participation factors analysis. The parameters of the virtual parallel resistor damping control are designed using the simplified HVDC system. The performance of the proposed damping control is studied employing the linearised model of the DC grid. Time simulations such as instability suppressing, power‐controlled station N − 1 fault, and DC system faults are carried out to validate the effectiveness of the damping control. [ABSTRACT FROM AUTHOR]

This study presents a detailed performance analysis of multi‐phase (six‐phase) induction generator in conjunction with different types of wind energy conversion systems (WECS). It targets to emphasise the advantages of considering an asymmetrical six‐phase induction generator (ASIG) in stand‐alone, grid‐connected fixed speed and grid‐connected variable speed operation. Various aspects such as efficiency, reliability and productivity are considered while performing the analysis. In stand‐alone mode, reliability and efficiency of the self‐excited ASIG are ascertained by disconnecting one of the two three‐phase sets connected to a resistive load. In grid‐connected fixed speed mode, two different scenarios are implemented to pursue the applicability of ASIG. In the first scenario, only one three‐phase winding set is connected to the grid and another set is connected to a local resistive load. In the second scenario, both three‐phase windings are connected to the grid via Y−Δ/Y three‐winding transformer. Variable speed operation in grid‐connected mode is accomplished by employing back‐to‐back pulse‐width modulation converters between the generator and grid. In this configuration, ASIG is operated with indirect field oriented control to obtain maximum aerodynamic efficiency. Transmission of active and reactive power is monitored with vector‐oriented control. Experimental results of considered scenarios are presented to verify the viability of ASIG in various configurations of WECS. [ABSTRACT FROM AUTHOR]

This study presents a two‐stage method to compute the passive reduced‐order models of multi‐input multi‐output electrical networks containing distributed‐parameter transmission lines (TLs). The method is based on the discretised TL models and a Krylov subspace model‐order reduction method called passive reduced‐order interconnect macromodelling algorithm. The section numbers of the TLs are adaptively determined using an innovative and fast algorithm. In stage 1, the high‐order cascaded Π circuit models of the TLs are reduced and synthesised into low‐order RLC two ports; stage 2 further reduces the entire lumped network. System passivity is preserved during the reduction. Useful postprocesses and a practical passivity test technique are given. Numerical tests confirm the superiority of the proposed method in reliable passivity preservation with decent accuracy and efficiency. [ABSTRACT FROM AUTHOR]

Renewable generation is mainly connected through converters. Even if they provide more and more ancillary services to the grid, these may not be sufficient for extremely high penetrations. As the share of such generating units is growing rapidly, some synchronous areas could in the future occasionally be operated without synchronous machines. In such conditions, system behaviour will dramatically change, but stability will still have to be ensured with the same level of reliability as today. To reach this ambitious goal, the control of inverters will have to be changed radically. Inverters will need to move from following the grid to leading the grid behaviour, both in steady state and during transients. This new type of control brings additional issues on converters that are addressed in this study. A solution is proposed to allow a stable operation of the system together with a limited solicitation of inverters during transients. [ABSTRACT FROM AUTHOR]

Changing dynamics of power systems caused by the migration from conventional to distributed energy sources increase the risk of blackouts due to voltage instability, especially in case of unforeseen network conditions (e.g. (N‐k)‐cases). To enable both secure and efficient power supply, novel monitoring and emergency control systems for the identification of voltage emergency situations as well as the execution of control actions are required that react reliably in due time and adaptively in the case of changing network situations. This study presents a distributed agent‐based approach to counteract voltage instability that is based solely on local measurements and limited inter‐agent communication. Distributed agents located at substations in the (sub‐)transmission network monitor distribution and transmission voltages, as well as, load tap changer positions and are able to autonomously curtail load in case system stability is endangered. The applicability of the approach is demonstrated in a co‐simulation environment interfacing the multi‐agent system with a dynamic power system simulation. The presented approach allows for an early detection of voltage instability as well as a coordinated execution of available control actions. [ABSTRACT FROM AUTHOR]

The combined AC/DC grids have, due to their technological features of high voltage and large capacity as well as operating flexibility and reliability, become one of the most potential solutions to connect large‐scale wind farms to power systems. The traditional centralised control lacks flexibility and robustness. In contrast, the distributed control is unable to simultaneously consider global optimum and response speed. A scheme of intelligent distributed control for combined AC/DC grids is proposed to constitute a multi‐agent system by equipping each terminal with an agent. While adopting the proposed control method, the optimisation of the upper layer is similar to the traditional grid dispatch and the lower layer system achieves the group compromise through the interaction and coordination among the agents and between the agents and the system. Based on the state information such as the voltage and the frequency of the neighbouring converter stations, the combined AC/DC grids can allocate the active power reasonably through the consistency control and provide the frequency support of the AC grid. Finally, four operating scenarios, including wind speed jump, wind power plant cluster disconnection, converter station failure and load demand response, are verified in a typical model of combined AC/DC grids by using the PSCAD/EMTDC software. [ABSTRACT FROM AUTHOR]

This study focuses on the optimal gain scheduling of the robust non‐linear sliding mode control (SMC) for the voltage source converters high‐voltage direct current (VSC‐HVDC) systems, for ensuring the desired dynamic behaviour improvement and stability enhancement. First, the development of the system steady‐state mathematical model of non‐linear character is explored in order to control the active power, the reactive power, and the DC link voltage towards their desired set point values. Their measured signals, as the outputs, are controlled via the optimal SMC‐based controllers, using their reference signals directly. Both artificial bee colony and mine blast algorithm are considered to optimally select the optimal gains of SMC controller that will minimise the steady‐state error of the obtained dynamic behaviour. Therefore, the DC voltage, the active and reactive powers can be properly maintained at their reference values. Both optimal SMC‐based techniques are compared in terms of the behavioural analysis and robustness. Using MATLAB™‐Simulink, the results significantly verify that the designed controllers based on SMC can achieve favourable tracking performance and contribute efficiently towards improving the system dynamic behaviour. [ABSTRACT FROM AUTHOR]

Effective control measures taken on the transient potential energy distribution in power transmission network can not only improve the stability of power system but also reduce the probability of cascading failures occurrence. By researching the distribution variation rules of voltage, phase‐angle difference, potential energy and its margin in the three‐machine system, the relationship is revealed further between transient potential energy distribution and network structure. The potential energy distribution mainly depends on the topology structure and parameter of the network, the potential energy is distributed most easily in the vulnerable links where equivalent reactance is bigger and voltage is lower. Three control measures are proposed to relieve the accumulation of transient potential energy. Changing network structure and parameter in vulnerable links can perfect the internal structure of power network from the root, and then improve the resisting disturbance capacity of the power system. Whereas decreasing the transmission power in the vulnerable links is an emergency control measure for improving the operation stability of power system on‐line. Simulation results on wepri‐36 system testify the validation of the proposed method. [ABSTRACT FROM AUTHOR]

This study proposes a novel interactive teaching–learning optimiser (ITLO) for voltage source converter based high voltage direct current (VSC‐HVDC) systems with offshore wind farm integration. Conventional vector control strategy is adopted, in which the parameters of eight proportional–integral loops are optimally tuned to achieve a reliable and satisfactory control performance under various operation conditions. Multiple classes are employed to realise a wider exploration compared with that of original teaching–learning based optimisation. Moreover, a small world network is incorporated for an interactive learning among the teachers or students from different classes, such that a deeper exploitation can be resulted in. Hence, ITLO can effectively avoid a local optimum due to the appropriate trade‐off between explorations and exploitations. Three case studies are carried out, such as active and reactive power tracking, short‐circuit fault at power grid, and offshore wind farm connection. Simulation results verify the effectiveness and advantages of ITLO over that of existing meta‐heuristic optimisation algorithms. [ABSTRACT FROM AUTHOR]

This study highlights the significance of dish‐Stirling solar thermal system (DSTS) and high voltage direct current (HVDC) link in the combined automatic load frequency control (ALFC) and automatic voltage regulator (AVR) model of the multi‐area thermal‐diesel plant. Appropriate generation rate constraints and governor dead band for the thermal plant are considered. A maiden attempt has been made to apply fractional order integral double derivative controller with derivative filter (FOIDDF) as a secondary controller for both ALFC and AVR loops. The performance of the FOIDDF controller is compared with some commonly used classical controllers. The lightning search algorithm is implemented for simultaneous optimisation of the controller parameters. The comparison shows the better performance of FOIDDF than others. The effect of the AVR loop on the ALFC loop is also analysed for the first time in the combined model. The investigation of the effect of DSTS and HVDC links reveals that their inclusion improves the system dynamics. The superiority of the proposed controller has been established for variable insolation of the DSTS. The rigorous sensitivity analysis of the different position and magnitude of disturbance, change in tie‐line synchronising coefficient and different condition of the DSTS reflects the robustness of the proposed controller parameters obtained at the nominal condition. [ABSTRACT FROM AUTHOR]

This manuscript presents a decentralised control scheme for coordinated SVC (CSVC) of large‐scale power networks. In this way, for each area of the power grid, a model predictive controller (MPC) which modifies the set points of reactive power compensators participating in CSVC algorithm is designed. The proposed controller takes into account reactive power limits of these compensation devices. The novelty of the method lies in the consideration of measured reactive power deviation on tie‐lines between neighbouring areas as measured disturbance and compensation of the disturbance by regional MPC controllers. As another contribution of this work, the validation of the proposed algorithm is done in real‐time simulation environment in which the decentralised MPC controllers are run in parallel on separate computational cores. The stability and robustness of the presented algorithm is validated for a large‐scale realistic transmission network with 5000 buses considering standard communication protocols to send and receive the data. Simulation results show that the proposed method can regulate the voltages on the pilot buses at the desired values in the presence of load variations and communication delays. Finally, the computational burden of the proposed method is evaluated in real time. [ABSTRACT FROM AUTHOR]

Hydro‐Québec has identified wide‐area control system as a key initiative able to significantly increase the voltage stability of its grid. In this context, an innovative project for wide‐area and local voltage control of shunt compensators was initiated. Its main objective is to implement a closed‐loop control system on Hydro‐Québec's network to optimise the reactive power support from the dynamic shunt compensators. The outcome of this project is the 'global and local control of compensators' (GLCCs) system which is based on synchrophasor technology and intelligent electronics devices. Applied to each shunt compensator of Hydro‐Québec's network, this robust voltage control system measures voltage variations in the load area and adjusts the operation set point of each shunt compensator accordingly, thus avoiding voltage collapse resulting from extreme contingencies. The GLCC control solution was intensively tested in simulation using PSSE software. Moreover, a pilot project was commissioned on a test bench replica of the system and also tested in real time using Hydro‐Québec's Hypersim digital simulator. Field tests of the GLCC were conducted on a −230/ + 660 MVars static vars compensator. The results of this pilot project were deemed conclusive and the deployment of the new voltage control system has been initiated by Hydro‐Québec. [ABSTRACT FROM AUTHOR]

A shunt active DC filter (ADF) with current injection suppresses current harmonics from high-voltage DC (HVDC) transmission systems. This paper proposes an ADF based on artificial neural networks (ANNs). The ANNs are very generic, accurate and convenient statistical models. They can emulate numerical model components, which are of complicated non-linear input/output relationships. The neural network in the control system of the ADF estimates the harmonic components of the load current and voltage. The ANN is trained offline using databases from the conventional controller. The simulation is carried out by MATLAB/Simulink and its results confirm that the proposed controller can offer better performance such as suppression of the current variations (current harmonics) in the DC-transmission line.

This study presents a backup protection scheme for transmission line compensated with unified power flow controller (UPFC) using power differential protection algorithm. In this scheme, power differential feature is evaluated from the apparent power flow in each phase at the local and remote bus to discriminate between the fault and normal situation. The proposed protection algorithm can be integrated with the conventional distance protection to supervise its operation during stressed conditions. Extensive fault simulation studies have been performed for different types of fault with several conditions and critical dynamic situations/issues such as load encroachment, power swing and high impedance fault (HIF). A 500 kV three machines four bus transmission system with UPFC has been used to validate the performance of the proposed scheme using MATLAB software. The test results depict that the proposed scheme is robust against HIF, power swing and load encroachment. It is not affected by the variation in pre‐fault power flow angle and synchronisation delay. The proposed scheme has also been tested for current transformer (CT) and coupling capacitor voltage transformer (CCVT) transient error, which brings out the superiority of the scheme. [ABSTRACT FROM AUTHOR]

This study presents a novel interfacing setup for testing and validating wide‐area monitoring and control (WAMC) techniques used in smart grids. The main purpose of this study is to provide a realistic approach for conducting WAMC studies. In here, a wide‐area controller (WAC) for a flexible AC transmission system (FACTS) device is implemented. The measurements for the WAC are collected using phasor measurements units (PMUs). Three main segments are being interfaced in this study. First, the real‐power grid, the local area controller of the actual FACTS device, and the PMUs are simulated using real‐time digital simulator. This simulation represents the central simulation and is interfaced with the WAC, which is realised using a MATLAB‐based program. This interface represents a novel software‐in‐the‐loop (SIL) scheme. On the other hand, an actual FACTS device is designed and interfaced to the central simulation via hardware‐in‐the‐loop (HIL) scheme. This SIL and HIL combination makes the experimental testbed more realistic and closer to the industrial standard. Various tests are conducted to examine the performance of the developed testbed. [ABSTRACT FROM AUTHOR]

A coordinated fast primary frequency control scheme from offshore wind power plants (OWPPs) integrated to a three terminal high voltage DC (HVDC) system is proposed in this study. The impact of wind speed variation on the OWPP active power output and thus on the AC grid frequency and DC grid voltage is analysed. The removal of active power support from OWPP after the frequency control action may result in second frequency (and DC voltage) dips. Three different methods to mitigate these secondary effects are proposed, such as, (i) Varying the droop gains of the HVDC converter (ii) Releasing the active power support from OWPP with a ramp rate limiter and (iii) An alternative method for the wind turbine overloading considering rotor speed. The effectiveness of the proposed control scheme is demonstrated on a wind power plant integrated into a three terminal HVDC system developed in DIgSILIENT PowerFactory. The results show that the proposed coordinated frequency control method performs effectively at different wind speeds and minimises the secondary effects on frequency and DC voltage. [ABSTRACT FROM AUTHOR]

Unified power flow controller (UPFC) has impact on the performance of distance relay during power swing. Generally, power system's parameters oscillate during power swing and since UPFC operation depends on some of these parameters (bus voltage, active and reactive power), the UPFC injects oscillated series voltage and draws oscillated shunt current. These oscillating series voltage and shunt current influence distance relays, as the impedance seen by a distance relay during power swing changes in UPFC‐compensated lines. This study shows how and why series voltage and shunt current change during power swing. Moreover, the admittance swing characteristic is proposed in this study and effects of series and shunt branches of UPFC on the admittance seen by a distance relay during power swing have been examined. [ABSTRACT FROM AUTHOR]

Transmission expansion planning (TEP) is generally determined by peak demands. To improve the efficiency and sustainability of energy systems, attention has been paid to demand response programs (DRPs) and distributed generation (DG). DRPs and DG will also have significant impacts on the controllability and economics of power systems, from short‐term scheduling to long‐term planning. In this study, a non‐linear economic design for responsive loads is introduced, based on the price flexibility of demand and the customers' benefit function. Moreover, a probabilistic multi‐objective TEP model which considers DRPs is also proposed. A probabilistic analysis method, the so‐called Monte–Carlo simulation method, is implemented to handle the uncertainty of the loads, DRPs and DG in the TEP problems. Due to the problems' non‐convex formulations, a non‐dominated sorting differential evolution program is used to solve the TEP problems. The proposed TEP model can find the optimal trade‐off between transmission investment and demand response expenses. The planning methodology is then demonstrated on an IEEE 118‐bus system in order to show the feasibility of the proposed algorithm. [ABSTRACT FROM AUTHOR]

This study proposes a novel method to partition volt/VAR control areas in bulk power systems using spectral clustering. By revealing the internal connectivity structure of the network considering the power‐flow‐based volt/VAR model, an undirected edge‐weighed graph is formed according to the defined electrical distance. On the basis of the graph theory, a novel approach is proposed through appropriate volt/VAR control area number selection, quality factor design, subspace projection, and spectral clustering. The approach is to partition the system into multiple volt/VAR control areas that have strong internal connections but weak external connections. Meanwhile, the proposed approach maintains the reactive power balances in each individual area and the minimum reactive power flow between those areas to facilitate area voltage regulations. Simulation results based on the New England 39 and IEEE‐118 test systems demonstrate the effectiveness of the proposed method. In addition, the Union for the Coordination of the Transmission of Electricity system is further employed to evaluate the performance in practical bulk systems. These results show that the proposed approach can partition volt/VAR control areas appropriately in the power system. [ABSTRACT FROM AUTHOR]

Being able to estimate the AC grid strength using short circuit capacity (SCC) from the perspective of a connected high‐voltage direct current (HVDC) station allows adjustment of converter control parameters or to select the converter's operational control mode. The SCC can be calculated by estimation of the grid's impedance and its equivalent voltage. This study presents the operational and practical challenges in real‐time implementation of the grid estimation algorithms especially for HVDC applications. This study shows that the recursive least‐square algorithm can be very efficiently used for the real‐time estimation of SCC. This technique forms a regression problem using algebraic complex equations with an objective to minimise the error between estimated and measured parameters. The algorithm has been reformulated and simplified to make it non‐complex without the use of matrices, in order to facilitate further implementation on an industrial real‐time controller. The performance of the real‐time implementation has been evaluated using a hardware‐in‐the‐loop platform. A sensitivity analysis has been also carried out to study the impact of different parameters and operational conditions on the performance of the estimation algorithm. Finally, the actual application of real‐time SCC estimation for the HVDC systems has been demonstrated. [ABSTRACT FROM AUTHOR]

System operators are required to serve the load in the most cost-effective way while maintaining the integrity of the system and heeding reliability requirements. In the day-ahead market, operators acquire reserves in an attempt to guarantee N-1 reliability; yet, reserve deliverability is not guaranteed. Prior research has shown that the use of transmission switching, or topology control, may help improve reserve deliverability. In this paper, transmission switching is used as a corrective mechanism to help the system achieve N-1-1 reliability, where not only has the system lost a single element, but also it experiences the loss of a second major element after an adjustment period. In an attempt to preserve N-1-1 reliability, for this paper, a day-ahead unit commitment model that acquires supplementary reserves is solved. The day-ahead market solution is then tested for N-1-1 reliability using contingency analysis models with and without transmission switching. The methodology can be employed at the day-ahead time stage to ensure the system has acquired sufficient supplemental reserves. The results demonstrate that not only can corrective transmission switching be beneficial post-contingency without inhibiting the ability to return to N-1 reliability, but it can also help obtain an N-1-1 reliable solution. [ABSTRACT FROM AUTHOR]

Multi‐terminal dc (MTDC) grids are expected to be built and experience rapid expansion in the near future as they have emerged as a competitive solution for transmitting offshore wind generation and overlaying their ac counterpart. The concept of inertia sharing for the control and operation of MTDC grids, which can be achieved by the proposed unified reference controller. The control objectives of the MTDC grids voltage source converter (VSC) stations are no longer limited to the stabilisation of MTDC grid, instead, the requirements of ac side are also met. The interaction dynamics between the ac and dc grid is analysed to illustrate the proposed concept. In addition, the voltage source converter stations can work in different operation modes based on the proposed unified control structure, and can switch among the operation modes smoothly following the secondary control commands. Simulation results exhibit the merits and satisfactory performance of the proposed control strategy for stable MTDC grid operation. [ABSTRACT FROM AUTHOR]

This research presents a model of a utility‐scale photovoltaic unit (USPVU) enhanced with an embedded hybrid energy storage system (HESS), suitable for stability studies in transmission systems. The main goal of this model is the simultaneous provision of primary frequency control and dynamic grid support. The primary frequency control includes both droop response (achieved by the frequency sensitive mode [FSM] operation) and inertial response (IR). To obtain these grid support functions, the research designed a suitable voltage and frequency (V–f) control, which coordinates the photovoltaic (PV) maximum power point tracking control, HESS converter control, and PV inverter control. Firstly, a midterm assessment of energy requirements in the sized HESS, based on frequency data, validated the energy availability of the enhanced USPVU for primary frequency control, according to new prequalification rules for energy‐constrained units. Then, transient stability assessments were performed on a representative transmission system to check the performance of the added FSM and IR in USPVUs with dynamic grid support. The results of the frequency phenomena in the IEEE 39‐bus system showed that the enhanced USPVU shared primary frequency control responsibilities with the conventional generation. This was achieved with two criteria to dispatch and commit conventional units by USPVUs. [ABSTRACT FROM AUTHOR]

Modern wind power plants (WPPs) are equipped with sophisticated controllers that allow various schemes to be used by manufacturers to meet grid code requirements. This includes active and reactive power controls as well as voltage control. Voltage control can be achieved in a number of different ways to satisfy the different grid codes, but also the project specific needs. Hydro‐Québec TransÉnergie (HQT) has performed numerous dynamic studies related to the integration of WPPs and optimisation of their control schemes including voltage control. This study presents the HQT experience with various voltage control issues including tuning of the WPP controller and study cases of angle and voltage stability issues. [ABSTRACT FROM AUTHOR]

In modern power systems the smooth control of active power flow is one of the major concerns for power industry. In the current study the authors devise an adaptive fractional order robust damping control system for static series synchronous compensator connected to an infinite power transmission network. A detailed non‐linear state model is presented with inclusion of parametric uncertainties, disturbances and other non‐linearities. A novel fractional order sliding manifold is proposed and based on it an adaptive fractional order controller is derived. The uncertainty in the state model is estimated online using the adaptive control system. The stability and the convergence proof of the closed loop system is verified using fractional order Lyapunov theorem. Furthermore, the proposed control scheme is compared with the classical proportional integral derivative, integer order sliding mode and fractional order controllers under different scenarios. The effectiveness of the proposed control scheme is verified using numerical simulations. [ABSTRACT FROM AUTHOR]

This study presents automatic generation control (AGC) in the presence of a gate controlled series capacitor (GCSC) to provide an effective frequency control ancillary service for a two‐area diverse‐GENCOs multi‐DISCOs power system. First, the contribution of the GCSC in tie‐line power exchange is formulated mathematically and then a tilt‐integral‐derivative (TID)‐based damping controller is proposed. The effectiveness of the proposed TID‐based GCSC‐AGC coordinated controller is compared with just AGC, proportional‐integral‐derivative‐based GCSC‐AGC, thyristor controlled series capacitor‐AGC, static synchronous series compensator‐AGC, and thyristor controlled phase shifter‐AGC. A modified group search optimisation (MGSO) algorithm is employed to adjust the considered controllers. Physical limitations of generation rate constraint (GRC) non‐linearity and governor dead‐band (GDB) effect are taken into account in the evaluations to obtain realistic results. Bilateral contracts and pool‐co transactions are considered concurrently for the realisation of a competitive environment. Simulations reveal that the proposed controller outperforms the other controllers in providing better dynamic responses. Additional evaluations are performed under the un‐contracted step, sinusoidal and random load demands seen as contract violations to demonstrate the superiority of the proposed controller over other controllers. Sensitivity analyses are performed for different uncertainty scenarios to show the robustness of the proposed control approach. [ABSTRACT FROM AUTHOR]

Stochastic security constrained unit commitment (SSCUC) based on a DC model could be problematic in AC networks because the DC model is potentially inaccurate. However, solving SSCUC problems using an AC model is still very challenging. Accordingly, an improved linearised AC optimal power flow (ILACOPF) model is provided in order to optimise the SSCUC problem. The proposed SSCUC model includes a linear representation of network losses and reactive power and bus voltage magnitudes. Moreover, in this study, transmission switching (TS), a powerful tool for grid side flexibility, is introduced and utilised to facilitate the mitigation of the uncertainty of wind power generation. Nevertheless, solving the SSCUC problem with TS in a full AC network is still one of the challenges in practical implementation, which is facilitated by the proposed ILACOPF model. Additionally, the aim of this study is to develop a more accurate power flow model to obtain a more realistic SSCUC solution using TS. The proposed ILAC‐SSCUC model using TS is formulated as a mixed‐integer linear programme, being solved by the proposed effective solution approach based on Benders' decomposition. Numerical simulations on a 6‐bus and IEEE 118‐bus systems have been performed to evaluate the effectiveness of the method. [ABSTRACT FROM AUTHOR]

This study presents a comprehensive analysis and a suppression method of sub‐modules' (SMs') voltage stress in the hybrid modular multilevel converters when riding through zero DC voltage faults in high‐voltage direct‐current (HVDC) systems. First, the general DC fault ride through (FRT) strategy considering the redundancy of the arm voltage generation scheme under a reduced dc‐link voltage is derived. Then full‐bridge sub‐modules (FBSMs') voltage stress for the conventional DC‐FRT schemes with and without common‐mode voltage injection are analysed. Finally, on the basis of the available full‐bridge SM capacitor energy control strategy, an improved method implemented by energy interaction between half‐bridge sub‐modules and FBSMs is presented. The proposed DC‐FRT scheme can make all SM capacitor voltages balanced at their rated values during the zero DC voltage conditions. Simulated results are provided to demonstrate the validity of the analytical results and the feasibility of the proposed DC‐FRT scheme. [ABSTRACT FROM AUTHOR]

AC/DC hybrid system plays an important role in large‐capacity and long‐distance power transmission. Moreover, load frequency control (LFC) of AC/DC hybrid system is very essential for the safe operation of power grid. Since LFC in essence stems from the power imbalance of the area caused by the variations of the generating and consuming party, this study presents a novel disturbance rejection‐based (DRB) LFC scheme to eliminate this power imbalance, thus achieving frequency and AC tie‐lie interchange power stability. This study assumes only part of the LFC system states information is measurable and utilises extended states observer to estimate the unmeasurable states including the disturbances. Moreover, then a composite feedback controller is designed utilising both the measurable and observed states information. The proposed DRB LFC schemes retain the structure of the LFC model and sufficiently utilise measurable states information for controller design, which has good engineering applicability compared with robust or intelligent LFC scheme. A multi‐area system containing three subareas with parallel interconnected AC/DC link is studied and the simulation results show the effectiveness of the proposed methods. [ABSTRACT FROM AUTHOR]

When the energy storage battery (ESB) is introduced into the DC microgrid, the DC microgrid can perform demand side management well. To achieve flexible charge and discharge controls of the ESB, the grid‐connected device of the ESB needs to have a bidirectional power transmission control function with constant power. Based on this, this Letter first designs circuit topology of bidirectional boost converter for high‐power transmission between the ESB and the DC microgrid. Secondly, the control system based on the staggered timing trigger mechanism is developed, and the working process of the converter in the forward and reverse transmission of power is analysed. Finally, the bidirectional boost converter and its control system are verified by real‐time digital simulator platform. Simulation results show that the proposed converter and its control system can realise stable high‐power bidirectional transmission between the ESB and the DC microgrid, and achieve accurate tracking of the power reference value. [ABSTRACT FROM AUTHOR]

In practical engineering application, trial method is widely used to determine the control value of unified power flow controller (UPFC in short). Since the calculation is based on offline typical data, the trial method is inefficient and less accurate, which could not adjust to the changing operating conditions. In order to make up for the deficiency, this paper proposes a novel UPFC control strategy based on power transfer distribution factors. First, the anticipated line tripping fault throughout the grid is scanned and the N − 1 overload lines are marked. Then, in regard to each overload condition, a certain UPFC control value is calculated based on power transfer distribution factors. The anticipated control list could be formed and updated according to online EMS data. When a line tripping fault is detected, simply by searching the control list, a condition could be matched and the UPFC could immediately be activated to eliminate the line overload. This method mentioned above could provide decision-making support to power-grid dispatching institutions. The results of a simulation example shows that the method is efficient and utilisable. [ABSTRACT FROM AUTHOR]

There may be a commutation failure during a fault or start-up of the UHVDC inverter side. The law of commutation failure process in UHVDC converter is studied here. First, the cause of inverter commutation failures is briefly introduced. Then, the general law for the bridge arm conduction of the inverter in the process of the commutation failure is revealed. Finally, the system debugging recorded data of some commutation failures of Taizhou UHVDC inverter in China is used to verify the law. Research shows that the commutation failure will go through three stages which has obvious features: (i) the commutation failure stage, (ii) the bypass circuit formation stage, and (iii) the commutation failure recovery stage. In the last stage, the AC currents of two conducted phases increase instantly in opposite directions, which are easy to recognise. In the second stage, only the DC current increases, and the AC currents equal 0 approximately. Because of the three stages, the commutation failure valve number is easily recognised. The study on the law of commutation failure process lay the foundation for commutation failure process analysis and identification for relay protection. [ABSTRACT FROM AUTHOR]

As is well known, the high installation and operational costs of the unified power flow controller (UPFC) restrict its widespread applications in power grid. An economical “Sen” transformer (ST) that is composed of a three-phase four-winding transformer and three sets of tap changers provides similar power flow control functionality as the UPFC. Unfortunately, the ST is not suitable for high-voltage systems because of its high-voltage insulation requirement on all secondary windings and tap changers. To overcome this problem, a high-voltage “Sen” transformer that consists of a three-phase two-winding transformer and two sets of tap changers is proposed in this paper. In high-voltage power system applications, the high-voltage “Sen” transformer (HVST) offers several advantages over the ST, such as fewer windings, fewer tap changers, bigger phase-shift range, and larger power flow control region. This paper focuses on the configuration, principles, and applications of the HVST. The contrastive simulation results in a two-area test system and an IEEE 39 bus system prove the validity and advantages of the proposed HVST. Moreover, a 10 kV HVST device and a corresponding experiment platform are developed to verify the feasibility and effectiveness of the HVST. The HVST can provide a flexible voltage and power flow control.

This study presents gravitational search algorithm (GSA)‐based optimisation technique for the optimum co‐ordination of multi‐type flexible AC transmission system (FACTS) devices with the existing reactive power sources present in a connected power network. Three types of FACTS devices: namely, unified power flow controller, static var compensator and thyristor controlled series capacitor are used in the present problem and IEEE‐30 and IEEE‐57 bus systems are taken as standard test systems. The existing reactive power sources are reactive generations of the generators and transformer tap setting arrangements. The main objective of the present study is to reduce active power loss, system operating cost including the cost of FACTS devices and congestion in transmission network. The result obtained with the proposed GSA‐based approach is compared with the results obtained by genetic algorithm, differential evolution and particle swarm optimisation techniques. It is observed that by optimal placement of FACTS devices along with the optimal setting of the transformer tap arrangements and control of reactive generations of the generators line loss, system operating cost and transmission congestion is reduced significantly by the GSA‐based method compared with other methods. [ABSTRACT FROM AUTHOR]

The efficient operation of the present day power system is an important issue to satisfy the customer needs. To improve the performance of the existing power system, the flexible alternating current transmission system (FACTS) devices have been attracted by an engineering community with the expertise in power system. This article proposes the self‐adaptive firefly algorithm (SAFA) for placement of FACTS devices, which identifies the appropriate type, best possible locations and optimal parameters of FACTS devices. Static var compensator, thyristor controlled series compensator and unified power flow controller are considers as FACTS devices for their placement. The objectives are to improve the power system performance by placement of FACTS devices through minimising real power loss, improving voltage profile and enhancing the voltage stability. Effectiveness of the proposed SAFA is tested on standard IEEE 30 and IEEE 57 bus systems with different objectives. The results are compared with other approaches, which clearly indicate the effectiveness and usefulness of the proposed method. [ABSTRACT FROM AUTHOR]

Transmission congestion is one of the most challenging issues in operation of deregulated electricity markets of today. This study investigates into benefit of unified power flow controller (UPFC) as a viable solution to the problem of congestion management. A comprehensive load flow model for the UPFC is presented which has the capability to control active and reactive powers and voltage magnitude simultaneously. This study discusses about two different approaches for determination of suitable location of UPFC. One of them is sensitivity‐based approach and the other is pricing‐based method. Reduction in total system losses has been taken as objective in sensitivity‐based approach. Furthermore, sensitivity of total system losses with respect to control parameters of UPFC have been evaluated using two different methods. A pricing‐based approach which makes use of locational marginal price difference and congestion rent for determination of suitable location of UPFC. Minimisation of cost of rescheduling subjected to system operating constraints is the objective to be met for congestion management. Interior point method, a mathematical tool for non‐linear optimisation, is used to obtain the solution of above‐mentioned problem. [ABSTRACT FROM AUTHOR]

The generation of offshore wind power is less predictable. This can cause the overload of offshore DC transmission system and thus requires the curtailment of wind power. To reduce the amount of wind power curtailment, a method of optimising DC power flow using DC power flow controller (DC‐PFC) is proposed. The analytical expression of coordinating DC‐PFCs and converters in controlling the power flow of the DC system has been created. Method has been developed to optimise the power flow of DC grids within which control setting changes automatically in different wind conditions to reduce both the power curtailment and power losses. The proposed method has been demonstrated and validated on a 9‐port DC system. It is concluded that both the curtailment of wind power and power losses are effectively reduced by inserting DC‐PFCs into DC grids. [ABSTRACT FROM AUTHOR]

This study discusses the technical requirements of line‐commutated converter‐based high‐voltage direct current (LCC‐HVDC) power transmission participating in a receiving end AC system's black start. The control and start‐up modes of LCC‐HVDC suitable for black start are discussed. Through short‐circuit capacity analysis and frequency estimation, the appropriate strength of the receiving end AC system to start the LCC‐HVDC is studied. A reactive power coordination strategy is proposed to reduce overvoltage caused by the HVDC filter. An innovative additional frequency control at the inverter side is proposed to suppress the frequency oscillation in the black‐start procedure. This control is realised through the constant voltage at rectifier and constant current at inverter control mode. Compared with the conventional rectifier‐side additional frequency control method, the inverter‐side additional frequency control method requires no remote communication and is thus more practical and effective when the receiving end is in a blackout. On the basis of the analysis, the conventional LCC‐HVDC is started successfully under the proposed requirements of black start and the overvoltage and frequency oscillation are also reduced effectively by the proposed control strategies, both of which are validated through electromagnetic transient simulation based on PSCAD/EMTDC software. [ABSTRACT FROM AUTHOR]

Transmission switching, as a power flow control mechanism that can reduce costs and improve reliability, has gained a lot of attention during the last decade. Despite its benefits, industry adoption has been very limited due to its computational complexity, stability and AC performance concerns. Pennsylvania New Jersey Maryland (PJM) has published a list of corrective switching solutions to relieve actual and post‐contingency network violations. The list is developed based on operators' prior knowledge and offline studies. This study employs a fast AC‐based real‐time contingency analysis (RTCA) tool with corrective transmission switching (CTS) functionality to analyse PJM's switching solutions with actual PJM data. The results show that the RTCA CTS tool is almost always able to find solutions that perform better than, or equally as well as, PJM's solutions. The results also show that PJM's list identifies solutions for only 3% of the problematic contingencies with post‐contingency violations over the course of one week. The RTCA CTS tool, however, is able to find corrective TS solutions for almost all the cases. The results suggest that CTS is ripe for industry adoption. The tool provides significant savings and would pave the road towards a smarter transmission network as an essential ingredient of the future smart grid. [ABSTRACT FROM AUTHOR]

The objective of this study is to determine the critical receiving power ratio (CRPR) of a receiving system fed only by high‐voltage DC transmission lines. The approach is to check the voltage response when a severe fault occurs in a simplified asynchronously connected receiving system. The DC power limit is found by continuously increasing the DC power until the voltage cannot recover from the severe fault. The topic is expanded as follows. First, a simplified system model is established under a few reasonable assumptions. Then, the analytical voltage response expressions when a three‐phase short‐circuit fault occurs are developed, taking the effect of commutation failure, fast AC voltage control and load characteristics into consideration. The expressions of the DC power limits under different load characteristics are deduced from the analytical voltage response expressions. Besides, several crucial factors that limit the infeed power capacity of an asynchronously connected receiving system are discussed in this study based on the sensitivity analysis of the DC power limits. The CRPRs of a receiving system with different crucial characteristics are found through time‐domain simulation with detailed models. The validity of the analytical voltage response expressions and the crucial factor analysis is well proved by the simulation results. [ABSTRACT FROM AUTHOR]

Alleviating dynamic oscillations of the tie‐line power exchange and area frequencies in affected interconnected power systems caused by loading condition changes is the prominent responsibility of automatic generation control (AGC). To compensate such the load‐changes and also suppress these oscillations, the application of the flexible AC transmission systems (FACTS) can be assigned as one of the practical and effective approaches. In this study, the static synchronous series compensator (SSSC) which is one the series FACTS families is engaged to enhance the overall dynamic performance of multi‐area multi‐source interconnected power system. Likewise, accomplished endeavours in this regard are led to exude the hierarchical adaptive neuro fuzzy inference system controller–SSSC (HANFISC–SSSC) to suppress both prominent issues in multi‐area interconnected power systems i.e.: the low frequency oscillations and the tie‐line power exchange deviations. Hereof also, implementation of multi‐objective optimisation technique is not avoidable. Due to high efficiency of multi objective particle swarm optimisation (MOPSO) to unravel the non‐linear objectives, it has been used to solve the optimisation problem. To verify the high performance of suggested HANFISC–SSSC, two different multi‐area interconnected power systems i.e.: two‐area hydro–thermal–diesel and three‐area hydro–thermal power systems have been considered for this study. In the interim, the capability of HANFISC–SSSC has been perfectly and precisely evaluated and compared with conventional SSSC by incidence of the step load perturbation in all areas of both the test power systems. To sum up, the simulations results obtained from both the power systems have transparently corroborated high performance of HANFISC–SSSC compared with conventional SSSC. [ABSTRACT FROM AUTHOR]