Winding Inductance and Performance Prediction of a Switched Reluctance Motor with an Exterior-rotor Considering the Magnetic Saturation

This paper deals with an analytical method to effectively calculate the inductance of an exterior-rotor switched reluctance motor(SRM),which evaluates the winding inductance of both the active section and the end section,accounting for the influence of core saturation.According to the inductance calculated by the analytical model,the flux linkage table and torque table can be established,and the steady state performance such as phase current,flux linkage,copper loss and core loss can be predicted.Effectiveness of this method is verified by the finite element method as well as by experimental results of a 12/8 SRM prototype.

Experimental investigation on ablation characteristics of coated and uncoated steel under 30/80 μs impulse current

The ablation tests on coated and uncoated Q235 B steel sheets were conducted under 30/80 μs impulse current simulating the lightning first return stroke current, aimed at further understanding the ablation characteristics of steel and investigating the impact of anti-corrosion coating on these characteristics. Ablation characteristics were investigated through the macroscopic morphology and x-ray diffraction patterns on the surface of damaged zones, the microstructure and micro Vickers hardness on the cross-section of damaged zones, and the maximum rear-face temperature of sample sheets. It can be concluded that the ablation areas of uncoated sheet consist of the melted layer and the heat-affect layer. These ablation areas include not only the area ablated directly by the arc root, of which the depth is deeper, but also the area forming due to the splashing of molten steel, of which the depth is shallower and decreases when the area’s distance from the arc attachment point increases. For coated sheet, coating materials have decomposed and evaporated forming an ablation pit on the sheet surface, in which the steel surface is exposed, and zinc filler of coating primer has infused into the exposed surface. The ablation diameter of uncoated sheet relates to the amplitude of the 30/80 μs impulse current in quadratic function, while for coated sheet, the relation is linear. In general, under the 30/80 μs impulse current, the coating can decrease the energy injected from the arc to the steel sheet and reduce the melting and splashing of steel. As a result, the ablation severity of uncoated sheet is severer than that of coated sheet.

Efficient Probabilistic Load Flow Calculation Considering Vine Copula⁃Based Dependence Structure of Renewable Energy Generation

Correlations among random variables make significant impacts on probabilistic load flow(PLF)calculation results.In the existing studies,correlation coefficients or Gaussian copula are usually used to model the correlations,while vine copula,which describes the complex dependence structure(DS)of random variables,is seldom discussed since it brings in much heavier computational burdens.To overcome this problem,this paper proposes an efficient PLF method considering input random variables with complex DS.Specifically,the Rosenblatt transformation(RT)is used to transform vine copula⁃based correlated variables into independent ones;and then the sparse polynomial chaos expansion(SPCE)evaluates output random variables of PLF calculation.The effectiveness of the proposed method is verified using the IEEE 123⁃bus system.

Resilience-oriented Hardening and Expansion Planning of Transmission System Under Hurricane Impact

In this paper,we propose a two-stage transmission hardening and planning(TH&P)model that can meet the load growth demand of normal scenarios and the resilience requirements of hurricane-induced damage scenarios.To better measure the resilience requirements,the proposed TH&P model includes two resilience assessment indexes,namely,the load shedding(LS)under the damage scenario and the average connectivity degree(ACD)at different stages.The first-stage model,which aims to meet the load growth demand while minimizing the LS,is formulated as a mixed-integer linear program(MILP)to minimize the total planning and hardening cost of transmission lines,the operating cost of generators,and the penalty cost of wind power and load shedding in both normal and damage scenarios.The second-stage model aims to further improve the ACD when the ACD of the scheme obtained from the first-stage model cannot reach the target.Specifically,the contribution of each transmission line to the ACD is calculated,and the next hardened line is determined to increase the ACD.This process is repeated until the ACD meets the requirements.Case studies of the modified IEEE RTS-24 and two-area IEEE reliability test system-1996 indicate the proposed TH&P model can meet the requirements for both normal and damage scenarios.

Power and Voltage Control Based on DC Offset Injection for Bipolar Low-voltage DC Distribution System

The bipolar low-voltage DC(LVDC) distribution system has become a prospective solution to better integration of renewables and improvement of system efficiency and reliability. However, it also faces the challenge of power and voltage imbalance between two poles. To solve this problem, an interface converter with bipolar asymmetrical operating capabilities is applied in this paper. The steady-state models of the bipolar LVDC distribution system equipped with this interface converter in the gridconnected mode and off-grid mode are analyzed. A control scheme based on DC offset injection at the secondary side of the interface converter is proposed, enabling the bipolar LVDC distribution system to realize the unbalanced power transfer between two poles in the grid-connected mode and maintain the inherentpole voltage balance in the off-grid mode. Furthermore, this paper also proposes a primary-side DC offset injection control scheme according to the analysis of the magnetic circuit model, which can eliminate the DC bias flux caused by the secondaryside DC offset. Thereby, the potential core magnetic saturation and overcurrent issues can be prevented, ensuring the safety of the interface converter and distribution system. Detailed simulations based on the proposed control scheme are conducted to validate the function of power and voltage balance under the operation conditions of different DC loads.

Characterization and Trading of Energy Level and Energy Shift Considering Virtual Power Plant

The capability of shifting the electricity generation or consumption to proper time of the day,also defined as energy shift(ES),is the key factor to ensure the power balance,especially under high penetration of variable renewable energy(VRE).However,the ES is not characterized and traded as an independent product in current market mechanisms.In this letter,the marginal utility of an ES is assessed and leveraged to characterize the effective ES,while a novel market scheme is proposed considering the trading of both ES and energy level(EL).The proposed scheme can well integrate ES producers such as virtual power plants that cannot be rewarded sufficiently to actively participate in the current market because they are principally labeled as EL consumers.Finally,the novel concept and mechanism are illustrated by a numerical study and verified to outperform the existing price schemes on integrating the ES resources and VRE.

Distributionally Robust Co-optimization of Transmission Network Expansion Planning and Penetration Level of Renewable Generation

Transmission network expansion can significantly improve the penetration level of renewable generation.However,existing studies have not explicitly revealed and quantified the trade-off between the investment cost and penetration level of renewable generation.This paper proposes a distributionally robust optimization model to minimize the cost of transmission network expansion under uncertainty and maximize the penetration level of renewable generation.The proposed model includes distributionally robust joint chance constraints,which maximize the minimum expectation of the renewable utilization probability among a set of certain probability distributions within an ambiguity set.The proposed formulation yields a twostage robust optimization model with variable bounds of the uncertain sets,which is hard to solve.By applying the affine decision rule,second-order conic reformulation,and duality,we reformulate it into a single-stage standard robust optimization model and solve it efficiently via commercial solvers.Case studies are carried on the Garver 6-bus and IEEE 118-bus systems to illustrate the validity of the proposed method.

C-Vine Pair Copula Based Wind Power Correlation Modelling in Probabilistic Small Signal Stability Analysis

The increasing integration of wind power generation brings more uncertainty into the power system. Since the correlation may have a notable influence on the power system,the output powers of wind farms are generally considered as correlated random variables in uncertainty analysis. In this paper, the C-vine pair copula theory is introduced to describe the complicated dependence of multidimensional wind power injection, and samples obeying this dependence structure are generated. Monte Carlo simulation is performed to analyze the small signal stability of a test system. The probabilistic stability under different correlation models and different operating conditions scenarios is investigated. The results indicate that the probabilistic small signal stability analysis adopting pair copula model is more accurate and stable than other dependence models under different conditions.

Short-Term Wind Power Prediction Based on ICEEMDAN-SE-LSTM Neural Network Model with Classifying Seasonal

Wind power prediction is very important for the economic dispatching of power systems containing wind power.In this work,a novel short-term wind power prediction method based on improved complete ensemble empirical mode decomposition with adaptive noise(ICEEMDAN)and(long short-term memory)LSTM neural network is proposed and studied.First,the original data is prepossessed including removing outliers and filling in the gaps.Then,the random forest algorithm is used to sort the importance of each meteorological factor and determine the input climate characteristics of the forecast model.In addition,this study conducts seasonal classification of the annual data where ICEEMDAN is adopted to divide the original wind power sequence into numerous modal components according to different seasons.On this basis,sample entropy is used to calculate the complexity of each component and reconstruct them into trend components,oscillation components,and random components.Then,these three components are input into the LSTM neural network,respectively.Combined with the predicted values of the three components,the overall power prediction results are obtained.The simulation shows that ICEEMDAN-SE-LSTM achieves higher prediction accuracy ranging from 1.57%to 9.46%than other traditional models,which indicates the reliability and effectiveness of the proposed method for power prediction.

Experimental and analytical investigation on metal damage suffered from simulated lightning currents

The damage of two typical metal materials, Al alloy 3003 and steel alloy Q235 B, subjected to four representative lightning current components are investigated by laboratory and analytical studies to provide fundamental data for lightning protection. The four lightning components simulating the natural lightning consist of the first return stroke, the continuing current of interval stroke, the long continuing current, and the subsequent stroke, with amplitudes 200 k A, 8 k A,400 A, and 100 k A, respectively. The damage depth and area suffered from different lightning components are measured by the ultrasonic scanning system. And the temperature rise is measured by the thermal imaging camera. The results show that, for both Al 3003 and steel Q235 B, the first return stroke component results in the largest damage area with damage depth0.02 mm uttermost. The long continuing current component leads to the deepest damage depth of 3.3 mm for Al 3003 and much higher temperature rise than other components. The correlation analysis between damage results and lightning parameters indicates that the damage depth has a positive correlation with charge transfer. The damage area is mainly determined by the current amplitude and the temperature rise increases linearly with the charge transfer larger.

Analysis and stabilization control of a voltage source controlled wind farm under weak grid conditions

This paper investigates and discusses the interaction stability issues of a wind farm with weak grid connections,where the wind turbines(WTs)are controlled by a new type of converter control strategy referred to as the voltage source(VS)control.The primary intention of the VS control method is to achieve the high-quality inertial response capability of a single WT.However,when it is applied to multiple WTs within a wind farm,its weakgrid performance regarding the stability remains concealed and needs to be clarified.To this end,a frequency domain model of the wind farm under the VS control is first developed.Based on this model and the application of a stability margin quantification index,not only the interactions between the wind farm and the weak grid but also those among WTs will be systematically assessed in this paper.A crucial finding is that the inertial response of VS control has negative impacts on the stability margin of the system,and the dominant instability mode is more related to the interactions among the WTs rather than the typical grid-wind farm interaction.Based on this knowledge,a stabilization control strategy is then proposed,aiming for stability improvements of VS control while fulfilling the demand of inertial responses.Finally,all the results are verified by time-domain simulations in power systems computer aided design/electromagnetic transients including DC(PSCAD/EMTDC).

Sensorless Control on a Dual-Fed Flux Modulated Electric Motor

This paper proposes the operation principle and a new flux estimation method for sensorless control strategy for the dual-fed flux modulated electric motor(DFFM).The DFFM is designed based on the flux modulation theory,it includes two stator windings and one rotor which simplify the mechanical structure.The rotor has only modulation iron and no permanent magnets on it,so there is no cogging torque problem in this motor.With adjustment of the outer and inner stator flux rotating frequency and amplitude,different rotation speed and torque of the sandwiched rotor can be gained for the DFFM.Furthermore,an improved flux estimation based sensorless control strategy is performed on the proposed machine to fit the two winding set control situation.The startup and performance of the proposed control strategy is verified by the simulation and experiments.

Frequency-constrained Co-planning of Generation and Energy Storage with High-penetration Renewable Energy

Large-scale renewable energy integration decreases the system inertia and restricts frequency regulation. To maintain the frequency stability, allocating adequate frequency-support sources poses a critical challenge to planners. In this context, we propose a frequency-constrained coordination planning model of thermal units, wind farms, and battery energy storage systems (BESSs) to provide satisfactory frequency supports. Firstly, a modified multi-machine system frequency response (MSFR) model that accounts for the dynamic responses from both synchronous generators and grid-connected inverters is constructed with preset power-headroom. Secondly, the rate-of-change-of-frequency (ROCOF) and frequency response power are deduced to construct frequency constraints. A data-driven piecewise linearization (DDPWL) method based on hyperplane fitting and data classification is applied to linearize the highly nonlinear frequency response power. Thirdly, frequency constraints are inserted into our planning model, while the unit commitment based on the coordinated operation of the thermal-hydro-wind-BESS hybrid system is implemented. At last, the proposed model is applied to the IEEE RTS-79 test system. The results demonstrate the effectiveness of our co-planning model to keep the frequency stability.

Optimal Siting and Sizing of Distributed Renewable Energy in an Active Distribution Network

The distributed generation (DG) plays an important role in the context of the environmental problems and sustain- able development throughout the world. This paper proposes a DG siting and sizing model in an active distribution network (ADN). The objective is to minimize the total cost, including investment, operation and maintenance costs. The proposed model is transferred to a Mixed Integer Second-Order Cone Programming (MISOCP) model based on a distribution network forward backward-sweep power flow and constraint relaxation. The CVX platform and GUROBI solver are used for the solution. The scenario analysis is used for the uncertainties of load and DG. Different numbers of operational scenarios are considered in order to analyze the effect of a non-network solution to the final planning result and total investment. The planning results with and without consideration of active managements, and the planning results with and without taking environmental profits into consideration, are compared and analyzed. The proposed methodology is verified with a modified IEEE 33 example.

Contemporary Trends in Power Electronics Converters for Charging Solutions of Electric Vehicles

Electrifying the transport sector requires new possibilities for power electronics converters to attain reliable and efficient charging solutions for electric vehicles(EVs).With the continuous development in power electronics converters,the desire to reduce gasoline consumption and to increase the battery capacity for more electric range is achievable for EVs in the near future.The main interface between the power network and EV battery system is a power electronics converter,therefore,there is a considerable need of new power converters with low cost and high reliability for the advance charging mechanism of EVs.The rapid growth in power converter topologies brings substantial opportunities in EV charging process.In view of this fact,this paper investigates the significant aspects,current progress,and challenges associated with several power converters to suggest further improvements in charging systems of EVs.In particular,an extensive analysis of front-end as well as back-end converter configurations is presented.Moreover,the comparative properties of resonant converter topologies along with other DCDC converters are discussed in detail.Additionally,isolated,and non-isolated topologies with soft switching techniques are classified and rigorously analyzed with a view to their respective issues and benefits.It is foreseen that this paper would be a valuable addition and a worthy source of information for researchers exploring the area of power converter topologies for charging solutions of EVs.

Impact of Different AC Voltage Control Modes of Wind-farm-side MMC on Stability of MMC-HVDC with Offshore Wind Farms

Wind-farm-side modular multilevel converters(WFMMCs) used in modular multilevel converter based highvoltage direct current(MMC-HVDC) transmission systems must be able to control the AC grid voltage in offshore wind farms. Different AC voltage control strategies can significantly affect the dynamic characteristics of WFMMCs. However, existing studies have not provided a general methodology of controller parameter design, and few comparative studies have been conducted on control performance under varying operating conditions as well as the effects of different AC voltage control modes(AVCMs) on the stability of MMC-HVDCs with offshore wind farms. This paper provides a controller parameter design method for AVCMs, which is tested in various operating scenarios. Sequence impedance models of offshore wind farms and WFMMCs under different AVCMs are then developed. The effects of AVCMs on the small-signal stability of the interconnected system are then analyzed and compared using the impedance-based method. Finally, case studies are conducted on a practical MMC-HVDC system with offshore wind farms to verify the theoretical analysis.

Unified Control Strategy of Heterogeneous Thermostatically Controlled Loads with Market-based Mechanism

This paper studies the coordination of heterogeneous thermostatically controlled loads(TCLs)to provide the real-time ancillary services.A market-based control framework is adopted for its advantages.The first advantage is that the demand curve-oriented approach makes it possible to form a unified control scheme for heterogeneous loads without identifying their different characteristics.The second one is that the broadcast price signal helps simplify the downlink control and reduce the implementation cost.Then,the separate demand curve construction strategies based on a virtual price for different types of TCLs are presented.The flexibility of each TCL is reflected through the curve,and its practical constraints,i.e.,comfort requirements of users and operation constraints of devices,are satisfied explicitly.To ensure the control fairness and full utilization for the regulation ability of TCL cluster,a comfort-levelequality principle is applied in demand curve construction.Simulations are carried out to verify the effectiveness of the proposed method in providing frequency regulation services,for which a regulation capacity estimation method is developed.Finally,a series of case studies are conducted considering the practical situations,e.g.,model errors,imperfect communication and sudden load change after the end of services.

Density-based Global Sensitivity Analysis of Islanded Microgrid Loadability Considering Distributed Energy Resource Integration

With the proliferation of renewable energy and electric vehicles(EVs),there have been increasing uncertainties in power systems.Identifying the influencing random variables will reduce the effort in uncertainty modeling and improve the controllability of power systems.In this paper,a density-based global sensitivity analysis(GSA)method is proposed to evaluate the influence of uncertainties on islanded microgrids(IMGs).Firstly,the maximum IMG loadability evaluation model is established to assess the distance from the current operation point to the critical operation point.Secondly,the Borgonovo method,which is a density-based GSA method,is used to evaluate the influence of input variables on IMG loadability.Thirdly,to improve GSA efficiency,a modified Kriging model is used to obtain a surrogate model of IMG loadability,and Borgonovo indices are calculated based on the surrogate model.Finally,the proposed method is tested on a 38-bus IMG system.Simulation results are compared with those considering other methods to validate the effectiveness of the proposed method.Energy storage systems are considered to diminish the influence of critical uncertainties on IMG operation.

Demand-side Response Strategy of Multi-microgrids Based on an Improved Co-evolution Algorithm

An effective modeling and optimization method,which takes into account source-load-storage coordination,and full-time collaborative optimization within and outside micro-grids,is introduced.Considering the operational conditions of various resources and their interactions,an energy management model for microgrids is proposed aiming at maximization of renewable energy utilization and minimization of overall system costs.The model is suitable for both real-time pricing and time-of-use mechanisms.In microgrids,demand response and economic energy storage dispatch are introduced to enhance selfcoordination and self-balancing ability among different resources.Depending on whether there is still an imbalance between supply and demand after coordination within a niicrogrid,trade between it and external microgrids are optimized in a orderly manner by considering different transaction prices and usage rights.Finally,three different schemes are designed,where the Lagrangian multiplier method as well as a co-evolution algorithm are used to solve and analyze different examples,verifying the reliability and validity of the method proposed in this paper.

Transient Angle Stability of Inverters Equipped with Robust Droop Control

Transient angle stability of inverters equipped with the robust droop controller is investigated in this work.At first,the conditions on the control references to guarantee the existence of a feasible post-disturbance operating point are derived.Then,the post-disturbance equilibrium points are found and their stability properties are characterized.Furthermore,the attraction regions of the stable equilibrium points are accurately depicted by calculating the stable and unstable manifolds of the surrounding unstable equilibrium points,which presents an explanation to system transient stability.Finally,the transient control considerations are provided to help the inverter ridethrough the disturbance and maintain its stability characteristics.It is shown that the transient angle stability is not a serious problem for droop controlled inverters with proper control settings.