Abstract:Wind and solar energy have seen significant decreases in the cost of these technologies over that last decade which has lead to increasing levels integrated into the grid. They also offer unique benefits such as no fuel costs, quick installation, and no pollution. But as more variable renewable energy (VRE) such as wind and solar is integrated into electrical power systems, a range of new technical challenges is becoming more prevalent in planning and operating the grid. Challenges include dealing with variable and uncertain power production as well as the unique physical characteristics that wind and solar generators have. This special section looks at some of the new techniques to solve these challenges with integrating ultra-high levels of VRE.

Abstract:As more variable renewable energy (VRE) such as wind and solar are integrated into electric power systems, technical challenges arise from the need to maintain the balance between load and generation at all timescales. This paper examines the challenges with integrating ultrahigh levels of VRE into electric power system, reviews a range of solutions to these challenges, and provides a description of several examples of ultra-high VRE systems that are in operation today.

Abstract:This paper applies double-uncertainty optimization theory to the operation of AC/DC hybrid microgrids to deal with uncertainties caused by a high proportion of intermittent energy sources. A fuzzy stochastic expectation economic model for day-ahead scheduling based on uncertain optimization theory is proposed to minimize the operational costs of hybrid AC/DC microgrids. The fuzzy stochastic alternating direction multiplier method is proposed to solve the double-uncertainty optimization problem. A real-time intra-day unbalanced power adjustment model is established to minimize real-time adjustment costs. Through comparative analysis of deterministic optimization, stochastic optimization and fuzzy stochastic optimization of day-ahead scheduling and real-time adjustment, the validity of fuzzy stochastic optimization based on a fuzzy stochastic expectation model is proved.

Abstract:The integration of continuously varying and not easily predictable wind power generation is affecting the stability of the power system and leads to increasing demand for balancing services. In this study, a short-term operation model of a district heating system is proposed to optimally schedule the production of both heat and power in a system with high wind power penetration. The application of the model in a case study system shows the increased flexibility offered by the coordination of power generation, consumption and heat storage units which are available in district heating systems.

Abstract:In traditional power systems, besides the conventional power plants that provide the necessary reactive power in transmission system, the shunt capacitors along with the tap changers of transformers are also employed in distribution networks. In future years, because of the high number of distributed resources integrated into the distribution networks, it will be essential to schedule complete active-reactive power at distribution level. In this research work, an economic framework based on the active-reactive power bids has been developed for complete active-reactive power dispatch scheduling of smart distribution networks. The economical complete active-reactive power scheduling approach suggested in this study motivates distributed energy resources (DERs) to cooperate in both active power markets and the Volt/Var control scheme. To this end, using DER’s reactive power capability, a generic framework of reactive power offers for DERs is extracted. A 22-bus distribution test system is implemented to verify the impressiveness of the suggested active-reactive power scheduling approach.

Abstract:Focused on the challenges raised by the largescale integration of renewable energy resources and the urgent goal of energy saving, a novel control scheme for the unified power flow controller (UPFC) series converter is proposed to achieve line loss reduction and security enhancement in distribution systems with a high penetration of renewable energy. Firstly, the line loss minimum conditions of a general distribution system with loop con- figurations are deduced. Secondly, security constraints including the permissible voltage range, the line loading limits and the UPFC ratings are considered. System security enhancement with the least increase in line loss is tackled by solving a much reduced optimal power flow (OPF) problem. The computational task of the OPF problem is reduced by deducing the security-constrained line loss minimum conditions and removing the equality constraints. Thirdly, a hybrid control scheme is proposed. Line loss minimization is achieved through a dynamic controller, while an OPF calculator is integrated to generate corrective action for the dynamic controller when the security constraints are violated. The validity of the proposed control strategies is verified in a modified IEEE 33 bus test system.

Abstract:Transient stability of doubly-fed induction generators (DFIGs) is a major concern in both AC and DC grids, and DFIGs must stay connected for a time during grid faults according to the power grid requirements. For this purpose, this work proposes an overcurrent and overvoltage protective device (OCV-PD) to ensure that DCbased DFIG system can stay connected and operate well during the faults. Compared with a series dynamic braking resistor (SDBR), two aspects are improved. First, a twolevel control strategy and DC inductor circuit are used to ensure that the OCV-PD can limit the current impulse to protect DFIG system during an overcurrent fault. Second, the OCV-PD can protect system from overvoltage fault which a SDBR cannot do. Simulation results verify its validity and feasibility, finding that for overcurrent protection the OCV-PD outperforms a SDBR with an average decreased index of 3.29%, and for overvoltage protection it achieves an average index of 1.02%.

Abstract:Recent advances in a power electronic device called an electric spring (ES) provide feasible solutions to meeting critical customers’ requirements for voltage quality. A new version of the ES was introduced based on a back-to-back converter (ESBC) configuration which extends the operating range and improves the voltage suppression performance to facilitate ultra-high renewable penetration. This paper proposes an efficient control method to facilitate the voltage regulation function of an ESBC with non-critical loads. Particularly, the proposed method is suitable for various load characteristics. We also develop a consensus algorithm to coordinate multiple ESs for maintaining critical bus voltage in distribution systems with ultra-high renewable penetration. The proposed operation of the ESBC is verified by simulation of a modified IEEE 15-bus distribution network. The results show that the ESBC can effectively regulate system voltage and is superior to the original version of the ES.

Abstract:As the power control technology of wind farms develops, the output power of wind farms can be constant, which makes it possible for wind farms to participate in power system restoration. However, due to the uncertainty of wind energy, the actual output power can’t reach a constant dispatch power in all time intervals, resulting in uncertain power sags which may induce the frequency of the system being restored to go outside the security limits. Therefore, it is necessary to optimize the dispatch of wind farms participating in power system restoration. Considering that the probability distribution function (PDF) of transient power sags is hard to obtain, a robust optimization model is proposed in this paper, which can maximize the output power of wind farms participating in power system restoration. Simulation results demonstrate that the security constraints of the restored system can be kept within security limits when wind farm dispatch is optimized by the proposed method.

Abstract:In recent years, the Middle East region countries have experienced rapid population and economy growth, which has resulted in large increase of energy and power demand. Although the traditional fossil fuels remain the majority for supplying the domestic demand, additional generating capacity and fuel supply are necessary according to current situation and future demand forecast. The renewable energy provides an alternative resource for satisfying demand, especially in this region with high potential of solar and wind energy. Besides the development of renewable energy, interconnected electricity networks also enable the cross-border power exchange to fulfil electricity demand. Many Middle East countries have already started developing renewable energy and reforming the national power sector for regional electricity integration. However, none of them has already implemented their targets and the challenges are still huge. This study reviews current conditions of electricity and energy interconnection development, and analyzes the process of regional electricity network integration and national power sector reforms and provides suggestion for regions’ plan. Finally, the technology developments for future power grid interconnection and renewable energy integration are also reviewed.

Abstract:Climate change has become one of the most important issues for the sustainable development of social well-being. China has made great efforts in reducing CO2 emissions and promoting clean energy. Pilot Emission Trading Systems (ETSs) have been launched in two provinces and five cities in China, and a national level ETS will be implemented in the third quarter of 2017, with preparations for China’s national ETS now well under way. In the meantime, a new round of China’s electric power system reform has entered the implementation stage. Policy variables from both electricity and emission markets will impose potential risks on the operation of generation companies (GenCos). Under this situation, by selecting key variables in each domain, this paper analyzes the combined effects of different allowance allocation methods and power dispatching models on power system emission. Key parameters are set based on a provincial power system in China, and the case studies are conducted based on dynamic simulation platform for macro-energy systems (DSMES) software developed by the authors. The selected power dispatching models include planned dispatch, energy saving power generation dispatch and economic dispatch. The selected initial allowance allocation methods in the emission market include the grandfathering method based on historical emissions and the benchmarking method based on actual output. Based on the simulation results and discussions, several policy implications are highlighted to help to design an effective emission market in China.

Abstract:Thermostatically controlled loads (TCLs) have great potentials to participate in the demand response programs due to their flexibility in storing thermal energy. The two-way communication infrastructure of smart grids provides opportunities for the smart buildings/houses equipped with TCLs to be aggregated in their participation in the electricity markets. This paper focuses on the realtime scheduling of TCL aggregators in the power market using the structure of the Nordic electricity markets a case study. An International Organization of Standardization (ISO) thermal comfort model is employed to well control the occupants’ thermal comfort, while a rolling horizon optimization (RHO) strategy is proposed for the TCL aggregator to maximize its profit in the regulation market and to mitigate the impacts of system uncertainties. The simulations are performed by means of a metaheuristic optimization algorithm, i.e., natural aggregation algorithm (NAA). A series of simulations are conducted to validate the effectiveness of proposed method.

Abstract:The reliability of power transformers is subject to service age and health condition. This paper proposes a practical model for the evaluation of two reliability indices: survival function (SF) and mean residual life (MRL). In the proposed model, the periodical modeling of power transformers are considered for collecting the information on health conditions. The corresponding health condition is assumed to follow a continuous semi-Markov process for representing a state transition. The proportional hazard model (PHM) is introduced to incorporate service age and health condition into hazard rate. In addition, the proposed model derives the analytical formulas for and offers the analytical evaluation of SF and MRL. SF and MRL are calculated for new components and old components, respectively. In both cases, the proposed model offers rational results which are compared with those obtained from comparative models. The results obtained by the contrast of the proposed analytical method and the Monte Carlo method. The impact of different model parameters and the coefficient of variation (CV) on reliability indices are discussed in the case studies.

Abstract:In order to enhance the stability of single-phase microgrid, virtual synchronous generator (VSG) control method is investigated in this paper. Its electromagnetic model and electromechanical model are established to illustrate the performance of VSG. Considering the 2nd fluctuation of fundamental-frequency in the output power, an instantaneous power calculation strategy is proposed based on the intrinsic frequency of single-phase VSG. Besides, a virtual power calculation method is presented to achieve islanded/grid-connected seamless transition. Stability analysis and comparison simulation results demonstrate the correctness of the presented power calculation method. At last, the effectiveness of the proposed approach is verified by comparison experiments of islanded/gridconnected operations in a 500 VA single-phase inverter.

Abstract:As renewable energy source (RES) tend to be integrated more and more at home and electric vehicles (EVs) take a greater share in the personal automobile market, users see a chance to charge their EVs by microRES. In this context, this paper presents a review about opportunities and challenges needed to be overcome by the EV users to implement a vehicle to home system in their homes, as well as the integration of micro-RES. Several practical scenarios are presented, with different demand profiles, by integrating renewable energy that could be used to charge the EV.

Abstract:This paper proposes a sectionalizing planning for parallel power system restoration after a complete system blackout. Parallel restoration is conducted in order to reduce the total restoration process time. Physical and operation knowledge of the system, operating personnel experience, and computer simulation are combined in this planning to improve the system restoration and serve as a guidance for system operators/planners. Sectionalizing planning is obtained using discrete evolutionary programming optimization method assisted by heuristic initialization and graph theory approach. Set of transmission lines that should not be restored during parallel restoration process (cut set) is determined in order to sectionalize the systeminto subsystems orislands. Each island with almost similar restoration time is set as an objective function so as to speed up the resynchronization of the islands. Restoration operation and constraints (black start generator availability, load-generation balance and maintaining acceptable voltage magnitude within each island) is also taken into account in the course of this planning. The method is validated using the IEEE 39-bus and 118-bus system. Promising results in terms of restoration time was comparedto other methods reported in the literature.

Abstract:Coordinated charging of electric vehicles (EVs) is critical to provide safe and cost effective operation of distribution systems where household single phase charging of EV could contribute to imbalance of the distribution system. To date, reported researches on optimization methods for coordinated charging aiming at minimizing power losses have the disadvantages of low calculation efficiency when applied to large systems or have not taken the voltage constraints into account. The phase component and polar coordinates power flow equations of an unbalanced distribution system are derived. Primal dual interior point dynamic programming is introduced for coordinated charging of EVs to minimize distribution system losses where charging demand, voltage and current constraints have been taken into account. The proposed optimization is evaluated using an actual 423-bus case as the test system. Results are promising with the proposed method having good convergence under time-efficient calculations while providing optimization of power losses, lower load variance, and improvement of voltage profile versus uncoordinated scenarios.