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In this revision, it is possible to find different examples of publicly available MatLab Graphical User Interfaces (GUI’s) developed to help researchers and students that have to deal with the challenging problem phase equilibrium data correlation (e.g. LLE, LVE). These examples are: GMcal_TieLinesLL: http://hdl.handle.net/10045/51725 GMcal_TieLinesVL: http://hdl.handle.net/10045/122857 Boundaries_LL_NRTL: http://hdl.handle.net/10045/121471 ParamIni_LL_NRTL: http://hdl.handle.net/10045/130017

Over the past decades, there has been a dramatic increase in the frequency of natural disasters, which are the leading causes of large‐scale power outages. This paper, therefore, assesses the significance and role of optimal tie‐line construction in improving the service restoration performance of unbalanced power distribution systems in the aftermath of high‐impact low‐probability incidents. In doing so, a restoration process aware stochastic mixed‐integer linear programming model is developed to find the optimal locations for new tie‐line construction in unbalanced three‐phase distribution systems. In particular, the restoration process of distribution systems, including the fault isolation and system reconfiguration, is contemplated to place tie‐lines in the most proper locations to enhance the manoeuvring capability of distribution systems and reducing the customer interruption time. Furthermore, the model is a stochastic one wherein uncertainty associated with potential damages alongside demand uncertainty is captured via a set of likely scenarios. To validate the effectiveness of the proposed stochastic mixed‐integer linear programming model, it is tested and verified on the IEEE 13‐ and 123‐bus test systems.

The high temperature superconducting (HTS) cable can be used as a low-voltage tie line between substations for its characteristic of high currentdensity. In this way, the remaining capacity on the low-voltage side of the transformer can be fully utilized. Moreover, The HTS cable can run in parallel with the original tie line, forming an electromagnetic ring network, to ensure the power supply of important loads. However, the short circuit current of the distribution network is large, and the electromagnetic ring network operation results in the quench of HTS cable when the short circuit fault is outside the HTS cable. It is important to ensure the safety of HTS cable under various short circuit faults. To ensure the safe operation of HTS cable in Shenzhen Power Grid, this paper built a distribution network model with HTS cable in Matlab/Simulink. The fault current and thermal process in different fault situation were analyzed. The design of fault current limiter (FCL) which is composed of the resistive superconducting fault current limiter (R-SFCL) and a shunt reactor was completed. The model of R-SFCL using R - Q method is build, and the effectiveness of the R-SFCL design is verified.

Sectionalizing switches (SSs) and tie lines play essential roles in reducing the duration of customer interruptions in electricity distribution networks. The effectiveness of such assets is strongly influenced by their placement in the grid. Operation of SSs and tie lines is also inherently interdependent. Due to the structural complexities regarding the mathematical modeling of such dependencies, optimization of the planning and operation of switches and tie lines has typically required either leveraging heuristic and metaheuristic approaches or oversimplifying the network topology. To tackle such issues, this paper presents a computationally-efficient model for reliability-oriented concurrent switch and tie line placement in distribution networks with complex topologies. The proposed model can be applied to grids with several tie lines and laterals per feeder, and yields the optimal location of tie lines, type of tie switches, namely manual or remote-controlled, and the location and type of SSs. Being cast as a mixed integer linear programming (MILP) problem, the model can be efficiently solved with guaranteed convergence to global optimality using off-the-shelf optimization software. The efficiency and scalability of the proposed model are demonstrated through implementation on five networks and the outcomes are thoroughly discussed.

With increasing demand for load, the generation of electrical power should also be increased and the interconnection of the network must be carried out in order to ensure the quality of the power generated and to feed the load. Load Frequency control (LFC) of three unrivalled interconnected thermal, hydro and solar thermal power plants has been designed with PI, PID and Fuzzy Logic controllers. AGC is used to stabilize disturbances in the power system with the use of different controllers. Classical conventional and Smart controllers are used as additional controllers independently to improve the performance of the Load Frequency Control (LFC) system by suppressing the oscillation of three uneven area systems, including Solar Thermal, Hydro and Nuclear Thermal, Systems in Area 1, 2 and 3 respectively. The simulation result shows that Fuzzy controller gives excellent dynamic time-setting responses and effectively mitigates oscillations against both step load and random disturbances, PID controllers are simple and cost-effective. The system using three generation sources is used and simulated in MATLAB2015.

This paper demonstrates a Power Hardware-in-the-Loop (PHIL) implementation of a decentralized optimal power flow (D-OPF) algorithm embedded into the operations of two microgrids connected by a tie line. To integrate the static behavior of the optimization model, a two layer control architecture is introduced. Underneath the dispatch commands from the D-OPF, a primary control scheme provides instantaneous reaction to the load dynamics. This setup is tested in the PHIL environment of the CoSES Lab in TU Munich. In the experiment, the two microgrids cooperatively optimize their operation through an ADMM based unbalanced D-OPF. The operations is then benchmarked against the exclusive use of primary control, without D-OPF. The decentralized approach outperforms, but also shows minor inefficiencies of integrating optimization methods into the real-time operation of the system.

This paper proposes an online data splicing power flow analysis method based on sensitivity analysis. Since the step-down transformer in the whole network data is usually equivalent to the 220 kV side, the equipment below 220 kV is not modeled in detail, and the provincial network data is modeled to 110kV and 35 kV voltage level equipment in detail. Different levels of modeling have a greater impact on the accuracy of stability analysis and calculation. Therefore, the purpose of improving data accuracy is achieved by splicing the entire network data (external network) and provincial network data (internal network). This paper is based on the sensitivity analysis method to stabilize the tie-line power by adjusting the output of the external network units, and form the whole network data that can accurately analyze the stability characteristics of the provincial network. The problem of power flow deviation caused by the voltage and angle deviation of the nodes at both ends of the regional tie line during the data splicing process is solved. Through the actual application of splicing the entire network and Hunan Provincial Network data, the effectiveness and necessity of the proposed method have been verified.

A tie-line security region provides all possible tie-line power injections with which constraints in a regional network will not be violated. With the high penetration of renewables, a sharp increase or decrease of renewable power requires a good depiction of the tie-line security region considering time coupling. However, the tie-line power security region with time coupling is a high-dimension polytope, which leads to a considerable computational burden. This paper presents a fast algorithm to approximate the original high-dimension tie-line security region considering time coupling. A few lower-dimension polytopes associated with combinations of different periods are calculated for the approximation. An evaluation method with clear physical explanations is established to measure the accuracy of the approximation. The method converts the evaluation to two linear programming problems based on the mathematical relationship between optimal points and polytope vertices. Under a Monte Carlo framework, the statistical analysis is employed to measure the accuracy using five presented indices. The performance of the proposed methods is verified by the case studies on the IEEE 118-bus test system and a 661-bus utility system.

This article deals with the Automatic Generation Control (AGC) mechanism for an interconnected, deregulated hybrid power system. The physical generation constraints such as boiler dynamics, governor dead band (GDB) and generation rate constraints (GRC) are included for a more realistic approach. The integration of distributed generation (DG) systems is also considered to match with present scenario. The intermittency nature of the DGs ruins the system's inertia and dynamic behaviour. A novel cascaded fractional order controller configuration 2-DOF (FOPIλDN)-PDN is proposed for the AGC mechanism and a new quasi opposition based volleyball premier league (QVPL) algorithm is employed to optimize the controller gains. On the other hand, the inertia emulation strategy (IES) based modified HVDC tie-line is used parallel to the AC tie-line to retrace the power system's inertia. The impact of the proposed controller and IES based modified HVDC tie-line has been analysed by subjecting to several case studies. The effectiveness of the QVPL based proposed controller has also been tested by comparing it with the previously published works on the same platform.

The inverter’s end of ultra high voltage DC (UHVDC) system under hierarchical connection mode (HCM) is connected to 500 and 1000 kV AC buses through high end converter and low end converter transformers, respectively. The two AC buses are interconnected via coupling transformer and the tie line. This complex structure of interconnection at inverter’s end can make the understanding unclear about each AC system’s strength of UHVDC-HCM. Therefore, this paper covers the system’s strength evaluation by developing the “Hierarchical mode effective short circuit ratio” (HMESCR) index. The HMESCR incorporates the impact of neighborhood AC networks, impedance of coupling transformer, the tie line and strength of the objective AC network. Based on the proposed index, an equivalent model for UHVDC-HCM system is developed. An equivalent model is verified under the steady state conditions such as maximum available power and the transient conditions such as commutation failure immunity index under AC faults. In addition, three different cases are considered in order to get the in-depth and comparative performance analysis of an equivalent model with UHVDC-HCM system. MATLAB and PSCAD/EMTDC are used as analysis tools for illustrating the steady state and transient state conditions. The analysis indicates that the presented equivalent model has similar behavior as UHVDC-HCM system under both steady state and transient state conditions, which verifies the accuracy of proposed index in assessing the strength of both AC systems of UHVDC-HCM.

In China, a growing number of regional power grids are interconnected by the line-commutated converter based high voltage direct current (LCC-HVDC) tie-line, forming a large-scale interconnected power grid. As the operation and control of the interconnected power grid are undertaken by multiple control centers, there is a demand for a distributed alternating current (AC)/direct current (DC) power flow algorithm. In this paper, a distributed power flow algorithm considering the LCC-HVDC tie-line is implemented based on the existing work of the author. The paper proposed some technologies to implement the algorithm. In the fixed-point iteration scheme of this paper, the combined boundary bus states are used to calculate the power flow of the DC system, thus the coordination of discrete state variables of the DC system is avoided. Also, the idea that converting the DC tie-line to an AC line is proposed to calculate the extended WARD equivalent of the external network of subsystems. Moreover, the unbalanced power distribution method is improved to make it more in line with the practical situation. Tests were carried out on the IEEE 118-bus system and an interconnected power grid constructed by six IEEE 118-bus systems, the correctness and effectiveness of the algorithm were proved.

Interconnection of two autonomous swarm grids (DC clusters) which are designed for energy access can increase power supply availability for power hungry appliances such as water pumps. Here, a surplus or a deficit power in one cluster is balanced by a second cluster through power exchange. Usually, the clusters are connected by a tie line if bus voltages in both clusters are equal and constant. If the bus voltages are different and constant, a single converter is used to interconnect the clusters and to control the power flow between them. In both interconnections, power flow is triggered by bus voltage variations from their constant values. However, if the bus voltages are different and are not constant, the coordination of the power exchange between the clusters is a challenge. In this paper, two buck-boost converters are proposed to interconnect the clusters that have different and unregulated bus voltages. To control the power exchange while reducing transmission line loss, a decentralized control approach which is composed of a voltage droop control method and a mid-point voltage control method is proposed. Simulations of two interconnected clusters carried out in MATLAB/Simulink software show how the proposed control method can efficiently coordinate the power exchange.

Overhead transmission line faults are mostly single‐phase grounding faults, and to ensure the power supply reliability of photovoltaic power plants in the cases of tie line transient faults, a feasible solution is to employ the single‐phase reclosing. However, for the single‐phase reclosing of the tie line, photovoltaic power source may face the over‐voltage problem during the non‐full phase operation, which will seriously affect the success rate of the single‐phase reclosing. This paper, according to different non‐full phase operation control strategies of photovoltaic power source, establishes the corresponding sequence network analysis models, and proposes the theoretical calculation method of non‐full phase operation voltage, which takes account of different neutral point grounding modes of step‐up transformer and different load levels. Meanwhile, the closing impulse problem of the single‐phase reclosing is discussed. The correctness of the theoretical method is verified by digital simulation results. On this basis, the applicable scope and application suggestions of the single‐phase reclosing are given. The research conclusions can provide important guidance and reference for the application of single‐phase reclosing technology in tie line of renewable energy power plant.

Unlike traditional microgrids, Provisional Microgrid (PMG) utilizes only renewable generation and small energy storage units; thus, it does not have an inherent self‐islanding capability and instead, relies on importing power from any coupled microgrid (CMG) for islanding purposes. Therefore, assuring adequate power sharing between interconnected provisional and coupled MGs is crucial for assuring self‐islanding capability and reliable operation. The optimal power that can be transferred between the coupled MG and the PMG is mainly restricted by the size of the interconnecting tie‐line; hence, determining the tie‐line optimal size becomes a crucial task that must be tackled. Furthermore, the amount of power transferred is significantly influenced by the flexibility level of each microgrid. Since battery storage systems (BSS) is the main source of flexibility in PMGs, it becomes also vital to obtain the optimal size of the BSS for planning islanded PMG system. This paper presents an optimization model to jointly determine the optimal size of the BSS in each MG and the tie‐line size to assure optimal power sharing and minimum system cost (tie‐line investment cost, BSS investment cost, and interconnected system’s operation cost). Numerical results demonstrate the effectiveness of the proposed model.

Multi-area economic dispatch (MAED) deals with the optimal dispatch of multiple areas. MAED with tie line constraints, transmission losses, multiple fuel option and valve point effects is considered as a large scale non-linear optimization problem. An accurate optimization method to solve this problem is of great interest. However the conventional particle swarm optimization (PSO) has been applied to solve many optimization problems with success, it suffers from some drawbacks. Therefore, chaotic PSO (CPSO) has been used to treat these drawbacks. Choosing the parameters of CPSO has a great effect on its performance. So, a hybrid CPSO and genetic algorithm (HCPSOGA) method is employed in this paper to solve this problem. The hybrid method is derived by combining CPSO and GA where GA is used to optimize the parameters of CPSO. To show the effectiveness of the hybrid method, two test systems are used. The results show the superiority of the hybrid method over some published methods based on same test systems.

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.

The short-circuit current characteristics fed by the renewable energy source are quite different from those of the synchronous generator, which may lead to the phase selection error or failure of the traditional fault phase selection component, and then affect the operating reliability of the distance protection and the automatic reclosing. This study analyses the existing issues in the application of traditional phase selection components to the tie-line of renewable energy power, according to the fault characteristics of different renewable energy sources such as photovoltaic power source, direct-drive wind turbine and doubly-fed wind turbine. Furthermore, the sequence voltage expressions in the case of different types of faults, which takes account of the influence of the renewable energy type and the transition resistance, are theoretically derived. Hereby, a novel phase selection method by using sequence-voltage phase comparison and phase-voltage amplitude comparison is proposed. This method is applicable to the tie line of different types of renewable energy power station, and possesses the superiorities of high reliability and strong tolerance towards large fault transition resistance. Digital simulation results verify the favourable performance of the proposed method.

The identification of a tie-line power transmission region is crucial for the secure operation of interconnected power grids. The characterization of a tie-line power transmission region with regional operational constraints is inherently a multi-parametric programming problem, which greatly suffers from a heavy computational burden. In this paper, we propose a fast boundary search approach to efficiently characterize the tie-line power transmission region in AC/DC hybrid grids. The multi-parametric programming method is improved in the following three aspects: (1) the search direction is fixed along the boundary of a region, which avoids the search inside a region; (2) the abundant search of subregions that share a boundary facet is avoided; and (3) a fast guess of the Karush-Kuhn-Tucker (KKT) condition is presented, which reduces the number of optimization calculations. Furthermore, an expansion strategy is presented to enlarge the tie-line power transmission region via quantifying the impact of system parameters on the transmission region. Simulations based on the IEEE 118-bus test system and a 661-bus utility system show that (1) the proposed method can be over 30 times faster than existing methods and (2) the proposed method successfully expands the tie-line power transmission region by identifying the key parameters.

An identification algorithm is proposed for generator outages in external systems given tie-line flow measurements and pre-outage linear sensitivity factors. The problem is challenging due to limited information available to operators in interconnected systems. To overcome the underdetermined nature of the problem (the number of tie-line measurements is smaller than the number of generators), a clustering method based on pivoted QR decomposition is implemented so that the outage location can be identified to the area of origination. Two test systems, the 68-bus New England system and the 500-bus synthetic system, were used for validation. The results demonstrate that with limited knowledge of the external system, the algorithm is able to identify the correct generator cluster in all cases. Another advantage of the proposed algorithm is its high efficiency, which enables detection within sub-milliseconds. In addition, an estimation of the cluster injection change is also provided by the algorithm.

The present article addresses robust control of both frequency and tie line power of a two area six unit diverse energy source based non linear power network. In large interconnected network, deviation in frequency and tie line power occurs due to uncertainty in load variation. In order to keep both frequency and tie line power flow to their nominal values, Automatic Generation Control (AGC) is required. AGC aims proper balance between power generation and electrical demand. In AGC concern the present article proposes a robust type-II fuzzy PID controller to diminish all local error signals (change in frequency ‘Δf’ and change in tie line power flow ‘ΔPtie’) of each control area. Controller tries to keep network frequency and interline power flow within nominal values. To justify supremacy of proposed type-II fuzzy controller a comparative study with fuzzy type-I and conventional PID controllers has been carried out for this analysis. To obtain precious gain parameters of above implemented controllers, a novel hybridized Gray Wolf Optimization and Sine Cosine algorithm (hyGWO-SCA) has been proposed for the study. In fact of this to show supremacy of proposed hyGWO-SCA technique, its performances are compared with host GWO and SCA technique. To verify the robust nature of type-II fuzzy PID controller the work goes through different sensitive analysis. Overall critical assessment of this study concludes, proposed hyGWO-SCA tuned type-II fuzzy controller exhibits superior performance in regard of AGC analysis. Finally the work is extended to four area eight unit interconnected system. Keywords: Automatic Generation Control (AGC), Integral of Time multiplied with Absolute Error (ITAE), Sine cosine algorithm (SCA), Gray wolf optimizer (GWO), Type-II fuzzy controller, Generation rate constraint (GRC)

In this paper, a high-voltage direct current (HVDC) tie-line has been accurately modeled and proposed for multi-area interconnected power system to enhance load frequency control and automatic generation control (AGC). HVDC tie-line has been modeled based on a simple first-order transfer function for AGC system. Time constant of this transfer function indicates the required time by HVDC to set DC current because of load perturbation in multi-area interconnected power systems. It is worth mentioning that the inexact and very simple model of HVDC-line cannot yield a reliable and correct response. Hence, an accurate and interpretable model is proposed for HVDC-link to control the AGC system. To verify the accuracy of the proposed model, it has been analytically compared with the conventional HVDC model. The integrated multi-area power system has also implemented the inertia emulation controller technique which enables AGC system to tackle the accumulated energy in HVDC-line’s capacitance.