Search

The thermal inertia of a building envelope endows a building with a heat storage capability, introducing scheduling flexibility to a building energy microgrid (BEM). The flexibility is usually modelled as virtual energy storage (VES) and used to optimize the operation of BEMs to reduce electricity costs. However, the VES capacity is impacted by and varies with variations in indoor/outdoor temperature. If only the building envelope's effect on heat transfer is considered, without proper quantification of scheduling flexibility provided by the building envelope, the scheduling scheme (especially VES charging/discharging schemes) will deviate from the actual VES operating conditions, which may affect the thermal comfort of individuals in buildings or bring high electricity costs. In this paper, a time‐varying building VES model (TVES) with three time‐varying parameters (virtual electric capacity (VEC), state of charge (SOC), and charge and discharge power) is proposed to quantify the electricity storage capability of the VES at different time periods for participating in operation of the BEM. Based on the TVES, a day‐ahead optimal scheduling strategy is developed for the BEM to simultaneously reduce the electricity cost and guarantee the user's thermal comfort, where the proposed time‐varying parameters of TVES are taken as constraints in the optimization. Numerical studies verify the effectiveness of the proposed TVES and optimal scheduling strategy. [ABSTRACT FROM AUTHOR]

With the increasing distribution network accidents caused by typhoon disasters, risk management in typhoon scenarios is necessary. A risk management framework for power distribution networks (PDNs) undergoing a typhoon disaster is proposed. First, a risk prediction model of the PDN under typhoon conditions based on the Batts model is developed to predict the loss of load risk (LLR) index and economic loss risk (ELR) index of the PDN before the typhoon lands. Then, a vulnerability index of overhead lines, considering the failure frequency of specific overhead lines and their load importance, is introduced for use in a weak link identification model to identify weak links in the PDN. Finally, a two‐time‐step risk relief model based on typhoon forecast information and the identified weak links is developed to allocate PDN relief materials to reduce the LLR and ELR during a typhoon disaster. An IEEE RBTS BUS6 system is employed as a test system to verify the risk management strategy. The vulnerability index proposed by the strategy considers both the importance of load and line failure. The risk management strategy adopts a two‐time‐step model, which reduces the impact of meteorological forecast errors and enables more efficient use of materials. The results show that the proposed framework can take more effective use of relief materials to reduce the loss of PDNs compared with the traditional strategy. [ABSTRACT FROM AUTHOR]

This study proposes to extend the numerical algorithm for subspace state space system identification (N4SID) for power grid electromechanical dynamics monitoring using the multi‐channel noisy synchrophasor measurements. The oscillation modes, mode shapes, participation factors and coherent groups are estimated in a comprehensive manner. It consists of five key steps: (i) estimation of power system state subspace model via N4SID; (ii) electromechanical modes discrimination from the trivial modes by developing the generalised inverse and mode assurance criterion; (iii) calculations of the mode shapes using the estimated states and output subspace matrices; (iv) estimation of participation factors estimated by the calculated mode shapes and (v) the generator coherency identification by developing the direction cosine denoted coherency inductor. Test results on the IEEE 16‐machine 68‐bus test system and the practical China Southern power Grid using field synchrophasor measurements are performed. Comparison results with other existing methods show that the proposed method achieves better accuracy and efficiency for power system electromechanical dynamics monitoring in the presence of measurement noise. [ABSTRACT FROM AUTHOR]

To schedule the distributed energy resources (DERs) and smart buildings of a microgrid in an optimal way and consider the uncertainties associated with forecasting data, a two‐stage scheduling framework is proposed in this study. In stage I, a day‐ahead dynamic optimal economic scheduling method is proposed to minimise the daily operating cost of the microgrid. In stage II, a model predictive control based intra‐hour adjustment method is proposed to reschedule the DERs and smart buildings to cope with the uncertainties. A virtual energy storage system is modelled and scheduled as a flexible unit using the inertia of building in both stages. The underlying electric network and the associated power flow and system operational constraints of the microgrid are considered in the proposed scheduling method. Numerical studies demonstrate that the proposed method can reduce the daily operating cost in stage I and smooth the fluctuations of the electric tie‐line power of the microgrid caused by the day‐ahead forecasting errors in stage II. Meanwhile, the fluctuations of the electric tie‐line power with the MPC based strategy are better smoothed compared with the traditional open‐loop and single‐period based optimisation methods, which demonstrates the better performance of the proposed scheduling method in a time‐varying context. [ABSTRACT FROM AUTHOR]

Improving the computational efficiency of reliability assessment is a long‐term goal for researchers. Regarding this issue, an impact‐increment‐based decoupled reliability assessment approach (IID) is proposed in this study. First, a new formulation of reliability indices is derived by decoupling the reliability index of composite systems into two parts: generation adequacy and transmission reliability. Then, an impact‐increment‐based state enumeration reliability assessment method is implemented to both the parts, which are more efficient than tradition methods. Thereafter, a reduction technique for the higher‐order contingency states is developed for the transmission reliability evaluation. With this technique, the number of analysed contingencies can be significantly reduced, which improves the computational speed significantly. Finally, the overall framework of the proposed IID approach is demonstrated. Case studies are performed on the RTS‐79, RTS‐96 and a provincial system of China. Results indicate that the proposed method has superior performance to the traditional reliability assessment methods in terms of accuracy and computational speed, particularly on large‐scale composite systems. [ABSTRACT FROM AUTHOR]

This study proposes a novel spatial‐temporal decomposition approach for systematically tracking dominant modes, mode shapes and coherent groups in bulk power systems using measurement data. First, components of the dominant oscillation modes, including frequencies and damping ratios, are identified from measurement data through a proposed recursive continuous wavelet transform. Second, cross‐wavelet transform is employed to estimate the mode shapes using the wavelet coefficient of the dominant modes. Furthermore, a reconstructed wavelet coefficient, which integrates the wavelet coefficients of all the estimated dominant modes, is used to identify the coherent groups of generators via the cross‐correlation coefficient. The proposed approach is evaluated on the simulation data from 16‐machine 68‐bus test system and China Southern Power Grid (CSG) as well as the field‐measurement data collected from phasor measurement units of CSG. It is demonstrated that the proposed approach performs with high accuracy, robustness and efficiency in tracking dominant modes, mode shapes and coherent groups in the bulk power systems. [ABSTRACT FROM AUTHOR]

In an interconnected bulk power system, wide‐area measurement system (WAMS) can provide abundant remote measurement for the coordinated control system. However, WAMS data have obvious time delays, which affect the performance of the control system and should be properly considered. Time‐delay stability margin (TDSM) is an index to reflect the maximum delay that a time‐delay system can sustain without losing its stability. To construct a coordinated control system based on WAMS data, it is crucial to accurately and rapidly determine the TDSM value. In this paper, a novel Jordan‐Taylor‐Schur (JTS) approach to determine the TDSM value of power system is proposed. It consists of three, Jordan standardization, Taylor separation and Schur simplification. With the three procedures of this method, the model dimension of time‐delay system can be reduced, and the stability criterion derivation can be rebuilt to omit some unnecessary variables so that both the computational efficiency and accuracy can be boosted. Further, it is worth highlighting that the proposed method can be applied to various Lyapunov‐based stability criteria to improve their efficiency with small errors. Finally, three power systems with time delays have been used to demonstrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This study presents a novel methodology to evaluate the QV interactions among buses and to partition the pilot voltage‐reactive power coupling areas (VRPCAs) using relative gain (RG). According to the concept of a multi‐input multi‐output system, the QV coupling RG is first calculated based on the QV matrix, which is extracted from power flow Jacobian matrix, to evaluate the QV interactions among different buses and then to determine the VRPCAs. The voltage stability critical buses are first identified through a modified loading margin. For each critical bus, the other buses that have strong QV coupling are detected via the cross RG and are clustered into a VRPCA piloted by the corresponding critical bus. New England 39‐bus system and Polish power system are used to test the performance of the proposed approach. Simulation results verify the effectiveness of the proposed approach in evaluating the QV interactions and partitioning the VRPCAs. [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]

A new partition‐composition method for online detection of transient stability of interconnected power system with the data from wide area measurement system (WAMS) is proposed. It consists of two parts: partition and composition. In the partition step, interconnected grids are divided into several local areas according to their geographical locations or ownership. Each area is modelled as an adjoint power system (APS) to determine its own synchronism. In the composition step, transient stability of entire grid is evaluated by the APS model of entire system composed with several key parameters of local areas, which can be directly derived from local WAMS. The proposed method only uses WAMS data, hence it is independent from the offline data, such as network topology, device model and system parameters. Another outstanding feature of the proposed method is that only a few key parameters are required to be transferred from the local control centres to the global control centre. These features are helpful for the interconnected grid with different owners since exhaustive data are unnecessary for the global control centre. Case studies on the 10‐generator New England system and one of 2007 operation cases of North China Power Grid validate its correctness and effectiveness. [ABSTRACT FROM AUTHOR]

This study proposes a novel synchrophasor measurement‐based correlation approach to identify the dominant oscillation modes in bulk power systems. In the proposed approach, the reference channel (RC) of cross‐correlation (CC) is optimally selected based on residue analysis. With the selected RC, the CC of field‐measurement data is formed to extract the free‐decay system responses. Then, eigensystem realisation algorithm (ERA) is applied to the extracted responses to estimate the system state‐space model. A practical model order selection strategy for ERA is proposed to determine the system model order. Further, cross‐coherence spectrum is employed to distinguish the dominant modes from the eigenvalues of the estimated state‐space model. The proposed method can achieve accurate and robust solutions in the presence of different levels of errors and noises in measurement data. The effectiveness of the proposed method has been verified in a 16‐generator, 68‐bus test system as well as the China Southern Power Grid system. [ABSTRACT FROM AUTHOR]

Zn(O,S) films were fabricated by oxidizing ZnS thin films deposited by electron beam evaporation method onto glass substrates at temperatures of 350-500℃ for 2 h in an atmosphere of oxygen. The XRD and EDX confirmed that the Zn(O,S) films were obtained successfully. The influence of the oxidization temperature on the optical and electrical properties of the Zn(O,S) thin films was investigated. The experimental results show that the Zn(O,S) thin film oxidized at the temperature of 400℃ exhibits better properties than others, with the transmittance of 86% in the visible region, the band gap energy of 3.36 eV and the resistivity of 3.22 × 103 Ω·cm, which makes it a potential buffer layer of solar cell. [ABSTRACT FROM AUTHOR]

This study proposes a novel adaptive identification method for the voltage stability critical injection region (VSCIR) in bulk power systems. From the viewpoint of a multi‐input multi‐output system, a power system can be partitioned into load and generator coupled subsystems, and the concept of voltage coupling relative gain (RG) is introduced to determine the VSCIRs. A two‐stage partitioning method of VSCIR is presented. In the first stage, the voltage stability weak buses are determined through loading margin and are identified as critical buses in the system. For each critical bus, the load and generator buses that have strong voltage coupling are identified through the cross‐RG and clustered into a VSCIR centred by the corresponding critical bus as pilot buses. In the second stage, practical VSCIRs are obtained through (i) merging VSCIRs that have strong coupled pilot buses, and (ii) allocating the buses that are shared by multiple regions to the most coupled VSCIRs. Further, this study proposes a voltage control strategy for improving the voltage stability of the system based on the practical VSCIRs. The proposed method is tested on the New England 39‐bus system, as well as the bulk 2383‐bus Polish power grid. [ABSTRACT FROM AUTHOR]

Stochastic subspace identification (SSI) methods have been widely employed for oscillatory mode identification on probing and ambient data and are reported to have good performances. This work proposes a novel SSI‐based approach for identifying dominant oscillatory mode from measurement data and extends the application of SSI to ringdown condition. The proposed approach first constructs an initial cluster of eigenvalues from SSI with repetitive calculations and then utilises a novel hierarchical clustering method to extract the dominant modes from the initial cluster. The repetitive calculations within the SSI are performed through varying the model order over a range defined by a novel initial order determination process. By doing so the challenge of model order determination for SSI‐based methods is resolved. Moreover, benefiting from the repetitive calculations and the clustering process, the proposed approach is highly immune to prevalent noises in the measurements. Finally, the proposed approach is applied and validated on the field‐measurement data from the phasor measurement units of China Southern Power Grid (CSG) through comparisons with Prony, ARMAX, and Monte Carlo methods. Test results demonstrate that the proposed approach performs with high accuracy, robustness, and efficiency in CSG. [ABSTRACT FROM AUTHOR]

In this paper, a new criterion to quickly evaluate the system transient stability based on wide area measurement system (WAMS) information, energy function method and adjoint power system (APS) model is proposed. APS is a two‐dimensional‐first‐order differential equation model, which can be considered as a virtual representation to the real power system. Meaning of APS is first explained by using the one machine infinite bus system. Then, a new criterion based on projection kinetic energy of APS is proposed to determine the transient stability of the original power system with multiple generators. Trajectory prediction method based on curve fitting technique is adopted to accelerate the evaluation process. The proposed method is sufficiently rapid since it does not rely on the network structure, system model, system parameters and the identification of the coherent generator groups. Therefore, it can be utilised for online transient stability assessment based on WAMS. Simulation results on the New England system and one of North China Power System validate the correctness and effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This study presents a general approach to identify dominant oscillation modes in bulk power system by using wide‐area measurement system. To automatically identify the dominant modes without artificial participation, spectral characteristic of power system oscillation mode is applied to distinguish electromechanical oscillation modes which are calculated by stochastic subspace method, and a proposed mode matching pursuit is adopted to discriminate the dominant modes from the trivial modes, then stepwise‐refinement scheme is developed to remove outliers of the dominant modes and the highly accurate dominant modes of identification are obtained. The method is implemented on the dominant modes of China Southern Power Grid which is one of the largest AC/DC paralleling grids in the world. Simulation data and field‐measurement data are used to demonstrate high accuracy and better robustness of the dominant modes identification approach. [ABSTRACT FROM AUTHOR]

This study presents a detailed performance evaluation of comfort‐constrained state‐queueing model for integration of thermostatically controlled appliances (TCAs) as virtual generators (VGs). The configuration of VG models is presented. The heating mode of heating, ventilating and air‐conditioning (HVAC) unit – heat pump is used as an example TCA load to demonstrate VG operation. First, an estimated baseline of the aggregated HVAC loads is obtained based on outdoor temperature forecasts using thermal dynamic models. Then, the control method of HVAC units for intra‐hour load balancing is proposed. The effects of different comfort‐constraints, outdoor temperature profiles and thermal deadbands are modelled to evaluate the performance of TCAs as VGs. The technical functionality of the TCA‐VG is described and demonstrated by the modelling results. [ABSTRACT FROM AUTHOR]

Partial cation substitution is an effective way to inhibit defects and carrier recombination, which can improve the efficiency of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Herein, flexible Cu2Zn1−xCdxSn(S,Se)4 (x = 0–15%) solar cells are fabricated on Mo foils with partial Cd substitution for Zn via a green solution‐process. The best device performance can be achieved when Cd/(Zn + Cd) = 8%, with an efficiency up to 6.49% and a significantly improved device repeatability. The EU decreases from 24 to 15 meV, indicating that antisite defects and band tailings are effectively suppressed. C–V data reveal that Wd and Vbi are enhanced after doping Cd, resulting in a stronger built‐in electric field which facilitates Fermi‐level splitting and hence increases band bending of the absorber toward the junction interface. Furthermore, the mechanism of current shunting is studied using an equivalent circuit model with three parallel current pathways to fit J–V curves. The key parameters for the solar cell diode such as A, J0, and Rsh are significantly improved by partially substituting Zn with Cd, demonstrating that current shunting loss is suppressed and the junction quality is improved, resulting in a significant improvement in device repeatability. [ABSTRACT FROM AUTHOR]

Highlights This article proposes a 6061‐T6 Al alloy and carbon fiber hybrid composite material pre‐curing and forming integrated new process, which can significantly reduce the hybrid composite material processing time, improve the production efficiency, and provide the possibility of hybrid composite materials in automotive parts applications. The process started by placing hand‐stacked carbon fiber reinforced plastic (CFRP) composites on the surface of 6061 Al alloy sheets, followed by bonding and pre‐curing of the CFRP to the Al alloy sheets through two different sets of dies in a hydraulic press, and ultimately full curing in an aging furnace. Mechanical properties of the CFRP/Al hybrid composite material were measured by flexural tests. The results show that the flexural strength of CFRP/Al hybrid composites with different layups ([90]12, [0/45/90/−45]3, and [0]12) was increased by 159%, 93.4%, and 67.9%, respectively, compared with the single 6061 Al alloy. According to the macro‐morphological observation, [90]12 lay‐up parts have the least thinning. According to the microstructure observation, no obvious cracks or voids were found in the [90]12 lay‐up parts. Elongated cracks and void defects could be found in the [0]12 lay‐up parts. For the [0/45/90/−45]3 lay‐up parts, voids are mainly exist in ±45°layups. The results of the spring‐back analysis show that the standard deviation of the formed parts obtained from the three lay‐up methods is within 1 mm, indicating that the forming effect is good. New process improves productivity and reduces production costs The flexural strength of CFRP/Al hybrid parts is greatly improved [90]°12 lay‐up parts have the least thinning and no cracks were found The forming precision of the three lay‐up parts is good [ABSTRACT FROM AUTHOR]