Abstract:The operation of transmission systems with large share of wind power is specially challenging under storm conditions. Under the stormy wind speed conditions, wind turbine protection system is designed to shut down the turbine to avoid excessive mechanical load. The sudden loss of wind power from large offshore plants is difficult to forecast accurately, which results in a large amount of power imbalance. The severity of such a wind power imbalance towards frequency stability needs to be studied for the future power systems. In addition, the overhead transmission lines can also be affected during storms, thereby increasing their probability of failure in the operation of power system under the islanded conditions. This paper investigates how the stormy weather can threaten the frequency stability of future Danish power system with large share of wind power and how to avoid the frequency instability through proper control and defence strategies such as high-voltage direct current (HVDC) control and load shedding. Sensitivity studies are performed for ramp rates of HVDC control, load shedding strategies, inertia of the system with different volumes of disturbances to understand their impact on frequency stability.

Abstract:The gas-fired generation has recently become an important power source for power systems. The increasing integration of gas-fired units (GFUs) brings a problem of location allocation strategy for power system planners. This paper proposes a bi-level maximum-minimum optimal placement model of GFUs to improve the static voltage stability in the transmission network. In the first stage, the locations of installed GFUs are optimized to improve the static voltage stability margin. The optimal installed capacity of GFUs is determined to minimize the operation costs and power losses in the second stage. The proposed mixed-integer nonlinear programming (MINLP) model is solved by second-order cone programming relaxations. Numerical results in the IEEE 118-bus test system demonstrate the effectiveness of the proposed method and the static voltage stability can be improved.

Abstract:For the planning, operation and control of multi-terminal voltage source converter (VSC) based high-voltage direct current (HVDC) (VSC-MTDC) systems, an accurate power flow formulation is a key starting point. Conventional power flow formulations assume the constant frequencies for all asynchronous AC systems. Therefore, a new feature about the complex coupling relations between AC frequencies, DC voltages and the exchanged power via VSC stations cannot be characterized if VSC-MTDC systems are required to provide cross-regional frequency responses. To address this issue, this paper proposes a comprehensive frequency-dependent power flow formulation. The proposed approach takes the frequencies of asynchronous AC systems as explicit variables, and investigates the novel bus models of the interlinking buses of VSC stations. The proposed approach accommodates different operation modes and frequency droop strategies of VSC stations, and considers the power losses of VSC stations. The effectiveness and generality of the developed approach are validated by a 6-terminal VSC-HVDC test system. The test system presents the characteristics of the coexistence of numerous VSC operation modes, the absence of slack buses in both AC and DC subsystems, and diversified grid configurations such as point-to-point integration of renewable energy sources and one AC system integrated with multiple VSC stations.

Abstract:A comprehensive scheme based on decentralized control, partitioning, multi-agent systems, and fuzzy logic is presented in this paper for the voltage control of power systems. In our proposed smart self-healing method, two types of control agents are defined, namely master and local, which are applied in two steps. In the first step, the power system returns to the normal state after fault occurrence. Immediately after a fault detection in a power system, the system is divided into three subsystems using spectral graph partitioning. Partitioning is conducted based on reactive power flow in transmission lines. For each subsystem, a local control agent and a performance index (PI) are defined. Whenever the PI of a subsystem exceeds its threshold limit, the local control agent uses the Sugeno fuzzy system to intelligently select and apply control actions. In the second step as performed by the master control agent, the power system is transformed to an optimal state by solving the optimization problem. Simulations on a 39-bus New England reveal the effective performance of the proposed method.

Abstract:In recent years, the expansion of renewable energy in electric power systems has been increasing at such a rapid pace that it has started affecting frequency stability. Renewable generators connected to the grid produce variable amounts of power, and in most cases have no inherent inertia response (IR) to the system frequency. Therefore, the high penetration of renewable generators in the system results in low inertia and frequency distortion. If renewable generators account for a high proportion of the supply in a power system, the use of energy storage systems (ESSs) with frequency-support algorithms (in the place of synchronous generators) can stabilize the network. The participation of ESSs in frequency support must be organized precisely, so that they are fully devoted to their own purpose. In this paper, the frequency-support parameters of ESSs are calculated for achieving stable frequency response from a network. An estimation and calibration process is conducted during the active power-order change of the ESSs in the substation, and is verified through electromagnetic-transients-including-DC (EMTDC)-based simulations.

Abstract:This paper focuses on the optimal scheduling of the district energy system with multiple energy supply modes and flexible loads. For multi-energy system (MES), the energy hub (EH) model including energy storage system and integrated electric vehicle (EV) is established. Based on the model, the influence of pollutant trading market on total operation cost is analyzed, and the optimal scheduling strategy is further put forward to realize the minimum purchase cost and emission tax cost of the MES. Finally, this paper compares the economic benefit of the fixed mode and the response mode, and discusses the contribution of the energy storage device and the multi-energy complementary mode to energy utilization efficiency. The simulation results indicate that optimal scheduling strategy of the EH can coordinate various energy complementary modes reasonably. Meanwhile, the proposed strategy is able to improve the operation economy of the EH, and ensure the better response effect of the demand side. The sensitivity analysis demonstrates the impact of pollutant emission price change on emission reduction.

Abstract:Combined heat and power (CHP) generation is a valuable scheme for concurrent generation of electrical and thermal energies. The interdependency of power and heat productions in CHP units introduces complications and non-convexities in their modeling and optimization. This paper uses the stochastic fractal search (SFS) optimization technique to treat the highly non-linear CHP economic dispatch (CHPED) problem, where the objective is to minimize the total operation cost of both power and heat from generation units while fulfilling several operation interdependent limits and constraints. The CHPED problem has bounded feasible operation regions and many local minima. The SFS, which is a recent metaheuristic global optimization solver, outranks many current reputable solvers. Handling constraints of the CHPED is achieved by employing external penalty parameters, which penalize infeasible solution during the iterative process. To confirm the strength of this algorithm, it has been tested on two different test systems that are regularly used. The obtained outcomes are compared with former outcomes achieved by many different methods reported in literature of CHPED. The results of this work affirm that the SFS algorithm can achieve improved near-global solution and compare favorably with other commonly used global optimization techniques in terms of the quality of solution, handling of constraints and computation time.

Abstract:The valuation of premium power based on perceived utility (PU， i.e., perceived effectiveness or satisfaction degree of customers) and their willingness to pay has been researched for years. One of the remaining challenges is the accurate evaluation of PU. PU is the foundation of premium power evaluation, and it is essential for the decision-making process of investments on premium power by high-tech manufacturers (HTMs). This paper presents a framework to effectively evaluate PU. In this framework, factors influencing PU were classified to be quantitative and qualitative. An indication system (IS) was then constructed based on those influencing factors. Finally, PU was evaluated by the IS, considering psychological perception and decision-making behavior of HTMs. The effectiveness of the proposed PU-evaluation framework was demonstrated through field case studies.

Abstract:The increasing number of distributed energy resources (DERs), advancing communication and computation technologies, and reliability concerns of the customers have caused an intense interest in the concept of microgrid. Although DERs are the biggest motivation of the microgrids due to their intermittent generation characteristics, they constitute a risk for system reliability. Battery storage systems (BSSs) stand as one of the most effective solutions for this reliability problem. However, the inappropriate use of BSS creates other operational problems in power systems. In order to deal with these concerns explicitly in microgrids, an optimized microgrid central controller (MGCC) is the key factor, which controls the real-time operation of a microgrid. This work proposes a model predictive control (MPC) based MGCC that will provide optimal control of the microgrid, considering economic and operational constraints. The proposed system will minimize the energy cost of the microgrid by utilizing mixed-integer linear programming (MILP) assuming the presence of DERs and BSS as well as the bi-directional grid connection. Moreover, the aging effect of BSS will be considered in the proposed optimization problem which will provide an up-to-date system model. The proposed method is evaluated using real load and photovoltaic (PV) generation data.

Abstract:A bipolar hybrid microgrid is a new topology which benefits from the advantages of both alternating current (AC) and direct current (DC) microgrids. Interlinking AC/DC converter is the key of this topology which has the following characteristics: being able to provide two equal pole voltages in DC side; complying with the standards of current quality at AC side; being able to control active and reactive power independently in AC side, and transmitting bidirectional power. In this paper, two categories of power converters including single-stage and two-stage converters are proposed for this topology. A new cost-effective control strategy is added to the control of general grid-connected converter for each interlinking converter, and the control of autonomous DC-link pole voltage for both candidates is achieved. Detailed simulations based on the designed control strategies are conducted to validate the function of control strategies under the operation conditions of different DC sides. The performances of two selected interlinking converters with balanced and unbalanced DC loads are analyzed. Suggested power quality of microgrid and total harmonic distortion (THD) analysis are demonstrated in grid-tied and islanded modes. Eventually, semiconductor power loss simulations based on a closed-loop thermal network simulation are conducted. Thereby, the mutual effects of power loss and initial junction temperature are investigated.

Abstract:Accurate topology estimation is crucial for effectively operating modern distribution networks. Line outages in a distribution network change the network topology by disconnecting some parts of the network from the main grid. In this paper, an outage detection (or topology estimation) algorithm for radial distribution networks is presented. The algorithm utilizes noisy power flow measurements collected from a subset of lines in the network, and statistical information characterizing errors in forecasting load demands. Additionally, a sensor placement scheme is presented. The sensor placement provides critical sensing for the outage detection algorithm so that any number of possible outages in the network can be detected. The performance of the proposed outage detection algorithm using the proposed sensor placement is demonstrated through several numerical results on the IEEE 123-node test feeder.

Abstract:An analytical calculation method for the reliability sensitivity indexes of distribution systems is proposed to explicitly quantify the impact of various influence factors on system reliability. Firstly, the analytical calculation formulas for the reliability indexes of distribution systems are derived based on the fault incidence matrix (FIM). Secondly, the factors that affect system reliability are divided into two categories: quantifiable parameter factors and non-quantifiable network structure factors. The sensitivity indexes for the quantifiable parameter factors are derived using the direct partial derivation of the reliability calculation formulas. The sensitivity indexes for the non-quantifiable network structure factors are derived using the transformation of FIMs. Finally, the accuracy and efficiency of the proposed sensitivity calculation method are verified by applying them to an IEEE 6-bus RBTS system. This paper sums up the factors that influence system reliability in detail and gives the explicit analytical calculation method for the sensitivity of each factor. Repetitive calculation of the reliability index can be avoided during the sensitivity analysis. The bottleneck that affects the reliability level of distribution systems can be identified efficiently, and valuable information and guidance can be provided to enhance the reliability of distribution systems.

Abstract:Due to the advantages such as low line cost, low transmission loss, and high power supply reliability, DC distribution networks have become the main development trend for future distribution networks. In this paper, a typical DC distribution network with multiple voltage levels is considered as a research object. It is proposed that the interface converters between DC buses with different voltage levels be implemented through the series-parallel combination of full-bridge LLC resonant converters. To realize the decentralized self-discipline control of DC voltage under various working conditions, different slack buses are prepared according to the voltage ranges of the DC buses, and the voltage regulation modes of the DC distribution network are divided into main voltage regulation mode, backup voltage regulation mode, and off-grid voltage droop regulation mode. By introducing a voltage coefficient related to DC voltage deviation as a basis for mode switching, the voltage fluctuations caused by slow switching between control modes in the method of traditional voltage margin control is reduced, facilitating fast and smooth switching between different voltage regulation modes. Finally, a simulation model for DC distribution networks is constructed utilizing MATLAB/Simulink. Simulation results verify the effectiveness and feasibility of the proposed voltage regulation modes and switching methods for DC distribution networks. Finally, an experimental platform is also constructed to verify the feasibility of the mode switching method proposed in this paper.

Abstract:To reduce the probability of commutation failure (CF) of a line commutated converter based high-voltage direct current (LCC-HVDC) transmission, a DC chopper topology composed of power consumption sub-modules based on thyristor full-bridge module (TFB-PCSM) is proposed. Firstly, the mechanism of the proposed topology to mitigate CF is analyzed, and the working modes of TFB-PCSM in different operation states are introduced. Secondly, the coordinated control strategy between the proposed DC chopper and LCC-HVDC is designed, and the voltage-current stresses of the TFB-PCSMs are investigated. Finally, the ability to mitigate the CF issues and the fault recovery performance of LCC-HVDC system are studied in PSCAD/EMTDC. The results show that the probability of CF of LCC-HVDC is significantly reduced, and the performances of fault recovery are effectively improved by the proposed DC chopper.

Abstract:The increase in global electricity consumption has made energy efficiency a priority for governments. Consequently, there has been a focus on the efficient integration of a massive penetration of electric vehicles (EVs) into energy markets. This study presents an assessment of various strategies for EV aggregators. In this analysis, the smart charging methodology proposed in a previous study is considered. The smart charging technique employs charging power rate modulation and considers user preferences. To adopt several strategies, this study simulates the effect of these actions in a case study of a distribution system from the city of Quito, Ecuador. Different actions are simulated, and the EV aggregator costs and technical conditions are evaluated.

Abstract:A wide-area damping controller (WADC) is effective in damping inter-area low-frequency oscillation (LFO), if the time delay in a wide-area control loop can be properly handled. In order to simplify the WADC design and enlarge the delay adaptation range, the classic power system stabilizer (PSS) is adopted, and a new unified residue (UR) method is proposed for compact WADC design. The strategy of control loop selection is also improved by modifying the relative residue index based on a few dominant oscillation modes. The designed PSS-based compact WADC is as simple as classic PSS with no more than two lead-lag phase compensation units. Case studies are carried out on an IEEE 16-machine 68-bus power system. Simulation results demonstrate that the control loop selection before the WADC design is necessary and that the proposed selection strategy can easily pick out the suitable candidate control loops. In addition, it is feasible for the UR method to design WADCs with different time delays in the selected control loops. All the designed WADCs are effective in damping inter-area LFO and robust to time delay variations under operation conditions. Comparisons among five design methods for PSS-based WADC show that the proposed UR method is superior in delay adaptation, the conciseness of WADC structure and computation speed of parameters.

Abstract:Virtual synchronous generator (VSG) simulates the first-order motion equation of a synchronous generator (SG) with the algorithm. VSG can improve the system voltage and frequency support capabilities of a microgrid or a weak grid. It is now widely applied at a high penetration level of distributed generation (DG) systems. However, because there is a contradiction between active power steady-state deviation of VSG and dynamic impact regulation, the VSG running in grid-connected mode with existing strategies cannot meet the steady and dynamic control requirements. Thus, an improved virtual inertial control strategy of VSG is proposed in this paper. The active power impact is reduced effectively under the circumstance of damping coefficient D _ω equal to 0 and a large inertia, thus the dynamic characteristic of active power is improved and its steady-state characteristic is maintained. Firstly, based on the analysis of the damping coefficient effect on the system dynamic process, two forms of improved virtual inertia algorithms are put forward by cascading a differential link into different positions of the first-order virtual inertia forward channel. Then, by comparing the characteristics of the system with the two improved algorithms, the improved virtual inertial strategy based on differential compensation is proven to be better, and the design of its parameters is analyzed. Finally, simulation and experimental results verify the effectiveness of the proposed algorithm.

Abstract:Transformerless grid-connected inverters offer greater efficiencies when transferring power from renewable energy sources to the electrical grid. If the grid-inverter connection is done with an LCL filter, high attenuation of switching harmonics is achieved while preserving a small-size output filter. However, damping must be included in the controller to assure closed-loop stability. This paper proposes a reference computation methodology for the inverter-side current feedback in a photovoltaic (PV) generation system connected to the grid through an LCL filter. Theoretical analysis of the closed-loop system stability and of the steady-state performance are presented as well as experimental validation of the closed-loop performance. The feedback controller includes active damping and relies on a resonant control structure which improves the ability of dealing with grid harmonic distortion. The controller uses a reduced set of measurements, which requires the inverter-side current and grid voltage only, and assures a power factor close to unity.

Abstract:This paper focuses on the development of electric vehicle (EV) charging infrastructure in the UK, which is a vital part of the delivering ultra-low-emission vehicle (ULEV) and will transition into low emission energy systems in the near future. Following a brief introduction to global landscape of EV and its infrastructure, this paper presents the EV development in the UK. It then unveils the government policy in recent years, charging equipment protocols or standards, and existing EV charging facilities. Circuit topologies of charging infrastructure are reviewed. Next, three important factors to be considered in a typical site, i.e., design, location and cost, are discussed in detail. Furthermore, the management and operation of charging infrastructure including different types of business models are summarized. Last but not least, challenges and future trends are discussed.

Abstract:Pakistan is an energy-resourceful country with vast and untapped renewable energy sources (RESs). The wind, solar, and biomass of the country are practically capable of ending a power sector collapse caused by demand-supply variances. A significant percentage of Pakistan’s population resides in rural areas. For rural population, the lack of connection to the mainstream of national development is a direct consequence of frequent power blackouts and, in certain cases, a lack of grid connection altogether. Lucrative features of smart grid are not fully incorporated into the power network yet, but policy-makers are paying attention to increase RES reliance. A comprehensive study describing the renewable energy potential of Pakistan is of importance. This research work attempts to present a collective summary of Pakistan’s renewable energy potential. A statistical analysis of the proposed and installed projects in various districts are presented. This paper elaborates the pressing needs of renewable energy integration for resolving Pakistan’s energy crisis. Renewable energy projects are acclaimed in this paper for affording higher living standards and better job opportunities than the fossil fuel based industry in Pakistan. Integrating RESs into the national portfolio is guaranteed to offer profound socio-economic benefits to Pakistan’s rural population.