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Existing research on the measurement of the valuation effect mainly follows the residual method proposed by Lane and Milesi‐Ferretti (2001). This cannot be used to perform structural decomposition. We propose an aggregation approach rather than the residual method to measure structurally the investment flow and valuation effect of China's external assets. The results indicate that the valuation effect of China's external assets has been highly volatile and it was negative during the pandemic period. The structural decomposition shows that portfolio investment and direct investment made the main contributions to the valuation effect. The impact of exchange rates on the valuation effect has generally been higher than that of asset price in terms of direct investment and total external assets but the opposite has been true for portfolio investment. China's outward investments are currently more inclined to Asian countries and a few European countries but inflows to China still mainly come from developed countries. [ABSTRACT FROM AUTHOR]

The integration of high proportion renewable energy not only brings many benefits, but also increases the difficulty of power system stability analysis. One of the reasons for this problem is the unclear internal characteristics of new energy equipment. Therefore, there is an urgent need for a more detailed analysis of the characteristics of new energy equipment. Taking a double‐fed induction generator (DFIG), which has complex operation characteristics and is widely used, as an example, a detailed energy function (DEF) of DFIG for small signal stability analysis is proposed in this paper. This method performs DFIG's internal energy analysis, making the traditional energy structure more comprehensive. On this basis, the internal parameters of DFIG with high sensitivity to stability are obtained, and the damping evaluation index is constructed to evaluate the damping of DFIG. Finally, the proposed model's correctness and method are verified in the 4‐generator 2‐area system. [ABSTRACT FROM AUTHOR]

In the microgrid, virtual synchronous generator technology can significantly enhance the anti‐interference characteristics of the system frequency and bus voltage, as well as solve the problems of insufficient damping and low inertia. However, the system frequency and active power oscillation caused by power fluctuations and grid faults threaten the stable operation of the grid seriously. Therefore, for an alternating current (AC) microgrid multi‐virtual synchronous generator (VSG) parallel system, an improved virtual synchronous generator control algorithm based on a fuzzy inference system is proposed, which adjusts the values of virtual inertia and damping coefficient dynamically through fuzzy logic rules to realize the coordinated control of the two. The enhanced VSG algorithm described in this research has a substantial influence on power‐frequency oscillation suppression, decreases active power and frequency overshoot, shortens the adjustment time, and improves system frequency stability active power, according to simulation and experimental findings. [ABSTRACT FROM AUTHOR]

This paper proposes a power system multimode generator energy loss factor (GELF) based on energy flow method under environmental excitation. This index can evaluate generator damping changes in different oscillation modes in power systems, timely discover the damping deterioration of the generator, and track the source of low‐frequency oscillation. A signal expression method for electrical frequency domain under environmental excitation is derived. This method is combined with dissipated energy flow method to obtain a new type of GELF. In terms of signal processing, dynamic mode decomposition and Morlet wavelet filter extraction system mode components are used to improve computing accuracy. The proposed index enables online calculation with the support of phasor measurement unit and wide area measurement system. Simulation analysis is conducted in a standard 39 node system and wind‐integrated power system. Test results show the effectiveness and generality of the proposed index and method. [ABSTRACT FROM AUTHOR]

Inertial support for hybrid AC/DC microgrid systems is provided by the virtual synchronous generator (VSG). However, the fast performance of the system is neglected while enhancing the stability of the system. To address this problem, an adaptive virtual inertia control strategy based on VSG technology is proposed. This control approach adaptively adjusts the system virtual inertia when the system operation deviates from the nominal value, by slowing down the rate of change of AC frequency and DC voltage and improving the deviation of AC frequency and DC voltage. Meanwhile, when the AC frequency and DC voltage restore back to the rated value, the rate of change of AC frequency is accelerated and the DC voltage fluctuation is reduced by dynamically adjusting the virtual inertia of the system. The proposed adaptive virtual inertia control method combines the advantages of large and small inertia to effectively improve the dynamic response of the system voltage and frequency in both rectifier and inverter modes. Finally, the simulation and experimental results verify the effectiveness of the proposed control algorithm. [ABSTRACT FROM AUTHOR]

Virtual synchronous generator technology can effectively improve the anti‐interference characteristics of the system frequency and bus voltage in the microgrid, and solve the problems of insufficient damping and low inertia of the system. However, in an islanded microgrid with multiple distributed generation, the difference in line impedance will cause local voltage deviations, which in turn leads to a series of problems, such as reducing power distribution accuracy and increasing bus voltage drop. Therefore, for the island‐type microgrid multi‐inverter distributed power generation parallel system, in order to solve the problem of low power distribution accuracy and large frequency oscillation caused by system parameters in virtual synchronous generator control, an improved virtual synchronous generator control algorithm based on adaptive droop coefficient is proposed in this paper, which not only eliminates the resistance component of the line impedance, makes the system impedance characteristic present a purely inductive nature, but also realises real‐time adjustment of active and reactive power. While maintaining the stability of the bus voltage and system frequency, it maintains high power distribution accuracy and improves the dynamic performance and operational stability of the power grid system. [ABSTRACT FROM AUTHOR]

The low and variable inertia due to the integration of large-scale wind generation poses severe challenges to the frequency control of the power grid. In this study, a novel model predictive control (MPC)-based load frequency controller (LFC), which can adapt to the changes in system inertia, is proposed to improve the frequency control effect. First, the inertia estimation scheme based on measurements is designed. Then, the state-space model, which is the key that determines the control effect of MPC, is corrected in time based on the estimated inertia. The proposed method in this study can avoid the problem of MPC-based LFC failure caused by inaccurate model parameters. The performance of the proposed strategy is demonstrated in two scenarios, i.e. wind power fluctuation and load multi-step, using a two-area interconnected power system with time-variant inertia. [ABSTRACT FROM AUTHOR]

Ambient‐based oscillation modes extraction is an effective means of monitoring the small signal stability of a power system on‐line. In this study, a data‐driven approach based on output‐only observer/Kalman filter identification (O3KID) combined with the eigensystem realisation algorithm (ERA) was proposed to extract electromechanical modes (frequency, damping ratio and mode shape) from synchronised ambient data. As a key for extracting a reduced‐order state matrix using the ERA, the Markov parameters (impulse responses) are first estimated by employing O3KID on multi‐channel ambient data. O3KID makes it possible to identify the modes with the ERA using only output ambient data, while ensuring the reliability of the extractions of frequencies, damping ratios and mode shapes. The performance of the proposed method was evaluated by employing the IEEE 16‐generator 5‐area system and measured data from a real power system. The estimation results in all cases as well as comparison results with the RDT‐ERA method and NExT‐ERA method indicate that the O3KID‐ERA method is a promising method for ambient data‐driven electromechanical oscillation modes extraction. [ABSTRACT FROM AUTHOR]

Considering the high spatial complexity of signal processing methods and the low diversity between the base classifiers in the ensemble classifier, a new method with optimal multi‐resolution fast S‐transform (OMFST) with low spatial complexity and rotation forest (ROF) was proposed. Firstly, Gini importance of features is evaluated by random forest (RF), and the sequence forward search method was adopted for feature selection. Then, the intermediate matrix was constructed on the optimal feature set. The results of inverse fast Fourier transform in the main frequency points of signals based on OMFST are compressed in time domain. The intermediate matrix was used to extract features for power quality (PQ) disturbances recognition. Finally, the ROF was adopted to encourage simultaneously individual accuracy and diversity within the ensemble. The optimal ROF was applied to identify 17 kinds of PQ disturbance signals. The simulation results show that the new method can effectively compress the time–frequency matrix of the existing S‐transform (ST) method; the space complexity of the ST modular matrix is reduced significantly and has higher accuracy. Besides, the results of the experiment with real PQ data prove that the new method was effective for practical industrial applications. [ABSTRACT FROM AUTHOR]

Microgrid systems, such as solar photovoltaic (PV) and wind turbine (WT), integrated with diesel generator can provide adequate energy to supply increased demands and are economically feasible for current and future use considering depletion of conventional sources. It is, thus, important to determine the appropriate sizes of PV, WT, diesel generator, and associated energy storage system (ESS) for efficient, economic, and reliable operation of electric power system in microgrid. Stochastic nature of intermittent renewable energy (RE) resources complicate their planning, integration, and operation of electric power system. Therefore, it is critical to generate typical scenarios of wind speed, irradiation, and load time series to reflect their stochastic characteristic for microgrid system planning and operation. In this study, a wind‐irradiation‐load typical scenarios generation method is proposed for optimal sizing RE resources of microgrid. The teaching‐learning‐based optimisation (TLBO) method is used to find the best configuration of the microgrid system. Simulation results show that scenarios generated by the proposed model have ability to approximate the original scenarios and reduce planning data effectively. [ABSTRACT FROM AUTHOR]

With regards to the problems in which the power‐oscillation characteristics of power systems with high penetration of renewable power generation are complex and difficult to effectively analyse, a novel network virtual‐inertia method is proposed to evaluate the power oscillation in such systems. Firstly, the network virtual inertia is defined and developed. Secondly, the network virtual inertia is calculated and obtained based on the measurement information. Thirdly, the relationship between the network virtual inertia and generator inertia is constructed. Finally, the network virtual inertia of power systems with high penetration of renewable power generation is analysed. The evaluation method is effective and easy to implement. Simulations involving four‐generator two‐area containing renewable power generation were conducted under various disturbances to prove the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The quality of electric power has become an important issue for electric utility companies and their customers. With the extensive application of micro‐grid technologies, power quality (PQ) disturbances are more likely to affect users; thus, research on the recognition of PQ disturbances has attracted increased attention. This study presents a novel PQ disturbance‐recognition algorithm, based on time‐frequency (TF) analysis and a decision tree (DT) classifier. The proposed method requires fewer feature statistics compared to the S‐transform‐based approach for PQ disturbance identification. In this study, feature statistics extracted using TF analysis are trained by a DT classifier to perform the automatic classification of PQ disturbances. As the proposed methodology can efficiently identify PQ disturbances, the performance of the DT classifier can be ensured. In addition, the influence of noise is investigated, and 12 types of noisy disturbance, with signal‐to‐noise ratios of 30–50 dB, are considered for the classification problem. Finally, the proposed method is compared with other popular proposed disturbance‐recognition algorithms in terms of detection accuracy. The experimental results reveal that the proposed method can effectively detect and classify different PQ disturbances. [ABSTRACT FROM AUTHOR]

Summary: The transient stability problem of a single‐machine infinite‐bus system with static var compensator is solved in this paper, where the static var compensator controller is designed by an improved backstepping method combining error compensation, adaptive backstepping control, and sliding mode variable structure control. Crucially, the error compensation term, which chooses in the step of virtual control by the adaptive backstepping method, is introduced to ensure that the system states are bounded, maintaining the nonlinearity of the power systems while also improving the speed of parameter identification. Meanwhile, the Lyapunov function is constituted step by step to achieve stability of the subsystem. In addition, a parameter updating law and a nonlinear control law are explicitly given to asymptotically stabilize the closed‐loop system. Finally, a simulation is used to illustrate the effectiveness and the practicality of the proposed control approach. [ABSTRACT FROM AUTHOR]

The stability problem of steam valves in a single machine infinite bus system with thyristor controlled series compensation is addressed through the use of an improved adaptive backstepping method based on error compensation. The method combines sliding mode variable structure control, backstepping control and adaptive control. Initially, an excitation control input and error compensation are simultaneously obtained in the design of virtual control by adaptive backstepping and the Lyapunov method. The Lyapunov function is then constituted in stages to achieve subsystem stability, before obtaining a parameter updating law and a non‐linear control law for the closed‐loop system. The results for both systems show that the improved non‐linear backstepping sliding mode variable structure control approach can have four benefits. It can ensure that the system states are bounded, retain the faster stability of power systems, improve the speed of parameter identification and achieve real‐time parameter estimation. Finally, a simulation is presented to illustrate both the effectiveness and the feasibility of the improved control approach. [ABSTRACT FROM AUTHOR]

A new approach to the analysis of the transient energy interaction in power systems is presented in this work on the basis of an analogous elastic network. A power system is first rigorously mapped to an elastic network consisting of elastic branches with different elasticity coefficients. On the basis of elasticity mechanics and the analogous elastic network, the transient energy of the different parts, such as the synchronous generators, branches, and loads, is defined and has been proved to agree with the law of energy conservation. Subsequently, based on the law of energy conservation and energy feature analysis, the concept of the power system energy structure (PSES) is built to exhibit the distribution and interaction of transient energy in the power system. Wide area and local area transient energy interaction can be analysed using the measurement‐based energy structure, which shows obvious potential in on‐line monitoring and analysis of power system oscillation. Finally, the simulations of Western Systems Coordinating Council (WSCC) three‐generator test system and a practical power system are performed and analysed to verify the rationality of the analogous elastic network and the validity of the proposed PSES. [ABSTRACT FROM AUTHOR]

This study presents H∞ control synthesis of a miniature unmanned aerial vehicle to improve its hovering capability in the presence of wind disturbances. By time‐scale techniques, the six‐degree‐freedom flight system can be decoupled into subsystems in different time‐scales: one including slow‐varying translational dynamics and Euler angles, and another composed of fast‐varying angular velocities. To avoid contradictions between subsystems, control specifications for subsystems are classified, respectively, in disjoint frequency ranges. Robust H∞ controllers, applied directly to the original flight system, are formulated by composing the sub‐controllers. To achieve desired handling and flying qualities, wind characteristics are considered, and a novel flexible strategy is then developed to ensure an adequate response in all flight modes under different weather conditions. The effectiveness and merits of the proposed methods are illustrated with a simulation example. [ABSTRACT FROM AUTHOR]

An efficient bisphosphine-catalysed sequential [4+2]/[4+2] annulation reaction of a γ-benzyl allenoate with benzylidenepyrazolones has been developed. Under the catalysis of DPPB [1,3-bis(diphenylphosphanyl)butane], the [4+2]/[4+2] annulation proceeded smoothly to give spiro-pyrazolidinone derivatives in moderate yields. This protocol provides a simple and practical strategy for the construction of highly substituted 2 H-spiro[naphthalene-pyrazole] skeletons. [ABSTRACT FROM AUTHOR]

In this paper, we apply a so-called robust and perfect tracking ( RPT) control technique to the design and implementation of the flight control system of a miniature unmanned rotorcraft, named He Lion. To make the presented work self-contained, we will first outline some background knowledge, including mainly the nonlinear flight dynamics model and the inner-loop flight control system design. Next, the highlight of this paper, that is, the outer-loop flight control system design procedure using RPT control technique, will be detailed. Generally speaking, RPT control technique aims to design a controller such that (i) the resulting closed-loop system is asymptotically stable, and (ii) the controlled output almost perfectly tracks a given reference signal in the presence of any initial conditions and external disturbances. Since it makes use of all possible information including the system measurement output and the command reference signal together with all its derivatives (if available) for control, RPT control technique is particularly useful for the outer-loop layer of an unmanned aircraft. Both simulation and flight-test results will be presented and analyzed at the end of this paper, and the efficiency of the RPT control approach will be evaluated comprehensively. [ABSTRACT FROM AUTHOR]

A graphical abstract of the article "Highly Diastereoselective Synthesis of Spiro[tetrahydrothiophene-3,3′-pyrazol] with an All-Carbon Quaternary Stereocenter via [3 + 2] Cascade Michael/Michael Cyclization Catalyzed by DABCO" is presented.