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The system equivalent inertia of the power grid is gradually decreased with the increasing penetration of renewable energy resources, which leads to a higher risk of frequency fluctuation after a major fault. For the frequency emergency control in the scenario of a high-voltage direct current (HVDC) line block fault, a novel criterion for enabling the fast response of user-end distributed loads is proposed in this study based on voltage feature identification and frequency drop detection. The reactive power to voltage sensitivity coefficient is combined with the Hausdorff algorithm to increase the accuracy of the calculation. The proposed criterion can realise the HVDC block fault identification at the end of the distribution grid, so as to achieve fast decentralised decision-making for distributed loads. This method may greatly reduce the need for high-speed communication and take actions faster than the primary and secondary frequency controls of the traditional units. The criterion makes it possible for low voltage small load in the user-end to be controlled precisely and selectively to fill the large power imbalanced caused by an HVDC block fault. The validity and superiority of the criterion are verified by simulation experiments using multiple test systems. [ABSTRACT FROM AUTHOR]

The sub‐synchronous control interaction (SSCI) has frequently occurred in the direct‐drive permanent magnetic synchronous generators (PMSGs) based wind farm integrated to the weak AC network. The system strength, i.e. short‐circuit ratio (SCR) has been believed as the critical factor so far. However, the impedance–frequency characteristics of the practical network exhibit a strong resonance that might have impacts on the SSCI as well. In this study, considering the network resonance, the eigenvalue‐ and impedance‐based analyses are conducted to study the SSCI phenomenon in the grid‐connected PMSG system. The impacts on SSCI from system strength, network resonance and network‐resonant frequency are investigated firstly. Then, the parameter limits of PMSG control for the stable region are calculated to discuss the effects of network resonance on the parameter design. The results show that the network resonance plays a critical role in the SSO mode. When the network resonance is considered in the PMSG system, the SSO would arise more easily, and the stable region of the control parameter will be reduced. Meanwhile, the frequency of the SSO mode matches well with the network resonance frequency in dq frame. The analytical results are all verified by simulation. [ABSTRACT FROM AUTHOR]

According to an accident analysis report, with regard to sub-synchronous oscillation (SSO) in Xinjiang, China, dynamic reactive compensation devices have great influence on the existence of sub-synchronous harmonics. This paper based on the practical phenomenon for further study, focuses on the interaction between direct-drive permanent synchronous generators (D-PMSGs)-based wind farm and static var compensator (SVC). Detailed electro-magnetic transient simulation model, such as D-PMSGs-based wind farm compensated by SVC, is established in PSCAD/EMTDC. Time-domain simulation results demonstrate that there is interaction between D-PMSGs-based wind farm and SVC, which is closely related with the control parameters of SVC and D-PMSGs-based wind farm. This type of control interaction will lead to sub-synchronous oscillation. Therefore, the influence of above control parameters on the oscillation characteristics is further studied here. The practical significance of this paper is indicating the necessity of coordinating and optimising control parameters in large-scale wind farm. [ABSTRACT FROM AUTHOR]

With the increase of DC project voltage level and transmission capacity, the security and stability risk caused by DC blocking is gradually increasing. Aiming at reserve dispatch after ultra-high voltage direct current (UHVDC) block, scenarios of DC power modulation participating in dispatching are presented. Mathematical models of generator re-dispatching incorporated control of DC in AC/DC hybrid grid in three situations in terms of enough reserve capacity in the disturbed province, regional reserve capacity sufficient and insufficient to make up for the power deficiency are presented. Then, a decision-aid system for optimal reserve dispatch after UHVDC block is proposed. Simulation results of Central China power grid indicate that the proposed scheduling scheme can reduce the operation risks of security and load loss if necessary. [ABSTRACT FROM AUTHOR]

With the development of wind energy conversion system (WECS), sub-synchronous oscillations (SSO) phenomena have occurred in direct-drive permanent magnetic synchronous generators (D-PMSGs)-based wind farms. There are several studies on the analysis of the SSO worldwide. However few researchers are concern about the interaction among D-PMSGs. Considering the interaction among D-PMSGs, especially in the weak grid condition, some researches are conducted in this study. The model of D-PMSG is shown firstly. Then the simulations are carried out to research the characteristics of interaction among D-PMSGs. Control parameters of grid side converter and wind turbine (WT) operating conditions are taken into consideration. At last, a proper mitigation method is introduced. Meanwhile, the impact of interaction among WTs on the effect of mitigation method is studied. The simulation result shows that there are distinct interactions among D-PMSGs. The interactions among D-PMSGs need to be considered when coordinating the SSO mitigation control among D-PMSGs. [ABSTRACT FROM AUTHOR]

In recent years, the subsynchronous oscillation (SSO), which occurs in direct‐drive permanent magnetic synchronous generators (D‐PMSGs)‐based wind farm, has great impact on the stability of power system. The different parameters and operation conditions of D‐PMSGs lead to the subsynchronous interaction (SSI) among D‐PMSGs. What is more, the SSI will influence the SSO characteristics of the system of D‐PMSGs‐based wind farm integrated to AC network. Here, considering the difference of parameters or operation conditions among D‐PMSGs in the wind farm, eigenvalue analysis, based on the small‐signal state‐space model of two aggregated D‐PMSGs which are equivalent to wind farm, is carried out. Then the stability and the SSI between D‐PMSGs of the equivalent system can be studied, and the dominant D‐PMSG can be determined based on the participation factors. Lastly, the detailed electromagnetic transient simulation model of D‐PMSGs equivalent system is built in PSCAD/EMTDC. The simulation is conducted to corroborate the eigenvalue analysis. The result shows that an obvious SSI exists between two aggregated D‐PMSGs. Furthermore, the different SSO characteristics among D‐PMSGs in the wind farm indicate the study of SSO problem based on an aggregated D‐PMSGs model may leave out some useful information. [ABSTRACT FROM AUTHOR]

This study presents a non‐linear adaptive control (NAC) for the energy storage system (ESS) embedded dynamic voltage restorer (DVR) in enhancing the low‐voltage ride through (LVRT) capability of wind farms. The proposed NAC features a perturbation observer to estimate and then compensate the real perturbation of the whole system, including parameter uncertainties, measurement noise, and external disturbances such as different grid faults and intermittent wind power. It can achieve an adaptive and robust control without requiring accurate system model and full‐state measurements. This control is then applied to the ESS embedded DVR (ESS‐DVR) system, in which the ESS can store the blocked wind power for potential power fluctuation suppression. Simulation studies have verified that the proposed control for ESS‐DVR can effectively enhance the LVRT capability of wind farms installed with different types of WTGs under different operating conditions. Moreover, its superiority has also been demonstrated by comparing with fixed gains‐based conventional vector control and accurate system model‐based feedback linearising control. [ABSTRACT FROM AUTHOR]

The Central China Power Grid (CCPG) and North China Power Grid (NCPG) are connected together by a single AC ultra‐high‐voltage (UHV) transmission link. When suffering power disturbances, power oscillation with high magnitude and long period would happen on the UHV tie‐line, which may affect the synchronous and stable operation of the interconnected two power grids. This study proposes a novel emergency control scheme for high‐voltage direct current (HVDC) in CCPG to suppress the high peak value of the power oscillation. Based on a two‐machine equivalent model, the maximum rising slope of power oscillat'ion is used to predict the peak value of power oscillation and power disturbance which is used as the reference of emergency power modulation. Simulation verification is carried out based on the full model of the CCPG and NCPG, using Power System Analysis Software Package. Moreover, field test has also been carried out in CCPG. Both simulation results and recorded wave curve show that the proposed HVDC emergency control scheme can effectively suppress the peak value under different power disturbances. This significantly improves the transient performance of the interconnected power system and the power transfer capacity of the UHV tie‐line. [ABSTRACT FROM AUTHOR]

This paper presents a study of the technical efficiency of industries in a transitional economy: China. Using data for 28 manufacturing industries across 29 provinces and the Data Envelopment Analysis approach, the technical efficiency of each industry is measured and compared across regions and provinces. The determinants of differential technical efficiency performance are analysed, with a particular focus on the impact of trade orientation and foreign investment. Trade openness is found generally to have a positive effect on technical efficiency. [ABSTRACT FROM AUTHOR]

Presents information on a study which investigated the impact of macroeconomic of direct foreign investment (DFI) in China during the 1979-96. Average growth rate of the Chinese economy during this period; What has attributed to the growth; Performance of DFI; What is DFI.

This paper investigates the impact of foreign direct investment (FDI) on the export performance of China at the provincial level. First, it presents a theoretical discussion of the impact of FDI on foreign trade, and then an empirical study of the impact of FDI on the export performance of regions in Chin. It has been found that the impact of FDI on exports differs across three macro-regions in China. The effect is stronger in the coastal region than in the inland regions. Although FDI shows a positive and significant impact on exports from the central region, its impact on the western region is found to be insignificant. [ABSTRACT FROM AUTHOR]