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The multi-infeed DC system can effectively transmit power and ensure the flexibility of the system’s operation mode, but the strong coupling among the converter stations makes the analysis of the commutation failure (CF) problem complicated. To this end, this paper firstly studies the propagation characteristics of CF among multiple stations, and the influence of the coupling impedances among these stations and the fault location on the CF. Then a method based on the decoupling idea is used to change the CF analysis problem of a multi-infeed DC system into that of the weakest single-infeed system. Finally, using the simulation analysis, the influence of fault type/wiring mode on CF is analyzed considering the relationship between the extinction angle, i.e., the CF criterion, and the influence of various variables on this factor is studied.

With the development of renewable energy generation, the increase of wind turbines makes stability analysis difficult. Aiming at this problem, this paper proposes a self- adaptive equivalent modeling of wind farm with various wind speed for small signal analysis. Firstly, the frequency domain model of wind farm is constructed, and the small signal impedance model of wind turbines is changed to the external circuit by Schur transform. Then, we decompose the dynamic characteristics of the wind turbines, compare the system’s eigenvalues under the dominant mode and categorize the turbines. The number of equivalent turbines is determined by turbine control parameters and network topology. Finally, the equivalent parameters are calculated to build the equivalent model. The simulation result, based on the MATLAB/Simulink platform, verifies the rationality of the proposed self-adaptive equivalent modeling method.

The neutral beam injection system in the Chinese Fusion Engineering Test Reactor(CFETR) is used for auxiliary heating of the plasma, it is necessary to create an acceleration grid power supply to provide energy for the plasma. The high voltage power supply adopts the inverter type power supply scheme, in which the step-up transformer plays the role of voltage isolation and power transformation between the primary and secondary sides, and is the key equipment to determine the voltage level and safety of the high voltage power supply. In addition to the general characteristics of large capacity and high voltage class, the transformer also needs to meet the requirements of operating under unusual frequency and high harmonic input. According to the requirements of step-up transformer, this paper reports the engineering design of the transformer, including the calculation of key parameters, the design of the main electromagnetic structure, such as core and winding of transformer, simulation of transformer insulation level for the standard input voltage, research about winding loss and temperature rise caused by higher harmonic. It lays the foundation for the future construction of CFETR N-NBI system.

Kinetic energy of DFIG-based wind turbines (WTs) can be released to provide frequency support for power systems, as the DFIG has a wide admissible range of rotor speed. In this paper, an adaptive frequency support (AFS) based speed control is proposed for DFIG-based WTs. In the case of grid frequency drop, the proposed control scheme can directly regulate the WT's rotor speed and release the kinetic energy for frequency support. Moreover, the rotor speed can be limited within the admissible range. Control design for the proposed scheme is elaborately presented. In addition, to analyze the WT's dynamic behavior, model reduction for the WT system is conducted, wherein the proposed control scheme is applied. The derived reduced-order model not only precisely replicates the dynamics of the full-order WT system, but also greatly relieves computation burden and thus enables simulations of large-scale wind farms. Based on the reduced-order model, a two-area test system, which contains three hundred DFIG-based WTs in three wind farms, is simulated. It's shown that the proposed control scheme can fully utilize the stored kinetic energy of large-scale wind farms for frequency support, which reduces frequency fluctuations greatly and thus improve the frequency stability of power systems.

When voltage source converters (VSCs) are connected to a weak AC grid, the constant reactive power control (CRPC) is more suitable for realizing optimal reactive power dispatch than the constant AC voltage control. This paper investigates the stability issues caused by the CRPC of VSCs. An impedance model of the VSC is developed which contributes to representing the CRPC dynamics characteristics. The impact of the CRPC parameters on the q-q channel impedance is analyzed. Then, a multiple-input multiple-output model is developed to analyze the small-signal stability of a grid-connected converter system. The eigenvalue analysis for the effects of both CRPC proportional gain and grid inductance on stability is presented. The impedance-based analysis reveals that the CRPC may cause medium-high frequency oscillations in the grid-connected converter system. Finally, time-domain simulations are conducted to validate the analysis.

In order to describe the uncertainties, such as distributed generation (DG) output, load fluctuation and contingencies, in active distribution networks comprehensively, this paper proposes an optimal DG planning model in the presence of active management schemes. A correlated sample matrix of wind speed, illumination intensity and load is generated using quasi Monte Carlo simulation and singular value decomposition. Fuzzy C-means clustering is utilized to classify scenarios of the sample matrix to improve the computation efficiency of optimal power flow. The optimal allocation model is mathematically formulated as a bi-level programming problem, which is solved by a hybrid algorithm combining dynamic niche differential evolution and primal-dual interior point algorithms. The solution provides the trade-off between system economy and security. Case study demonstrates the effectiveness of those techniques.

Lexical chain of continuity caused by semantic relations between words which provides clues about news structure and theme. On the basis of solving the problem of ambiguity resolution semantic knowledge fusion network use of lexical chain, combined with the feature of word frequency, location and agglomeration and a series features of Vietnamese news event to, build the lexical chain. By using semantic similarity for ambiguity resolution and eventually constitutes a lexical chain of the news events clue information.

A new isolated single-stage power factor correction (PFC) bridgeless forward converter is proposed in this paper. The proposed converter consists of two forward loops with a four-winding transformer. By eliminating the input bridge diodes, conduction losses can be decreased. An improved constant on-time control (COT) is applied to the proposed converter to achieve high power factor and then the input current harmonics can meet the IEC61000-3-2 Class C standard. The proposed converter operates in either discontinuous current mode (DCM) or boundary continuous current mode (BCM) according to the input voltage value. Detailed analysis and circuit operation stages are provided. An 80-W prototype is built up to verify the theoretical analysis of the proposed converter.

Demand for wireless communication is increasing significantly, while the frequency availability for wireless communication is quite limited. Currently, various countries are prompting the use of TV white spaces. IEEE 802.11 WG (Working group) has started a TG (Task Group), namely IEEE 802.11af to develop the international standard for WLAN in TV white spaces. In order to increase maximum throughput, a channel aggregation mechanism is introduced in the draft standard of TGaf. In Japan, ISDB-T based area-one-segment broadcasting system (Area-one-seg) which is a digital TV broadcast service in the limited area have been permitted to do actual services in TV white spaces from April 2012. The operation of the IEEE 802.11af system shall not jeopardize the Area-one-seg system due to the common operating frequency band. If the Area-one-seg partially overlaps with the IEEE 802.11af in some frequency, the IEEE 802.11af cannot use the channel aggregation mechanism due to lack of channels. As a result, the throughput of the IEEE 802.11af deteriorates. In this paper, the physical layer of IEEE 802.11af D2.0 is introduced briefly, and a partial subcarrier system for the IEEE 802.11af is proposed to effective use of TV white spaces and increase throughput. The IEEE 802.11af systems effectively coexist with the Area-one-seg by using null subcarriers. Computer simulation shows up to 60 % throughput gain is achieved with the proposed mechanism.

Driven by the progress of regulations of TVWS worldwide, IEEE 802.11 started a new Task Group af (TGaf) to pursue extension of traditional Wi-Fi to TV White Spaces (TVWS). Wi-Fi in TVWS offers longer distance and broader bandwidth; while maintaining the same user experience of traditional Wi-Fi systems. It is therefore regarded as one of most promising technologies to exploit the market derived from the new regulations of VHF/UHF bands. In this paper, we present the world's first prototype system of Wi-Fi in TVWS based on the TGaf draft specification. The prototype system verifies the PHY and MAC design of TGaf and the database query protocols that is needed for TVWS operation but not in the scope of TGaf. It is demonstrated that, with sufficiently defined MAC protocol and PHY that meets sharp transmission spectrum mask, the developed prototype supports video streaming on single TV channel while well coexisting with primary TV services in the vicinity.

A novel integrated Buck-Flyback non-isolated PFC converter is proposed in this paper. This new converter is an inherent integration of a Buck converter and a Flyback converter, which operates in either Flyback mode or Buck mode according to whether the input voltage is lower or higher than the output voltage. In this way, the dead zones of AC input current in traditional Buck PFC converter are eliminated. Therefore, the proposed integrated Buck-Flyback non-isolated PFC converter can achieve high power factor under universal AC input range and its input current harmonics can easily meet the IEC61000-3-2 Class C limits. The efficiency of the proposed converter is not deteriorated obviously compared to the conventional Buck converter. Detailed theoretical analysis and optimal design considerations are presented. A 100W prototype was built up to verify the theoretical analysis of the proposed integrated Buck-Flyback non-isolated PFC converter.

A new primary side controlled high power factor single-stage flyback LED driver with self-adjusted current reference proposed in this paper. The self-adjusted current reference is realized by a negative feedback circuit to achieve output current regulation and suitable for discontinuous conduction mode (DCM) or boundary conduction mode (BCM). The output of the self-adjusted current reference is a half-sine waveform and followed by the primary current, hence high power factor can be achieved. A DCM single-stage flyback LED driver with self-adjusted current reference is analyzed in this paper and detailed design considerations are presented. Finally, a DCM laboratory prototype for LED driver with 360mA/25V output based on the theoretical analyses was built up. Experimental results show that good output current regulation and high PF can be achieved.

Moving forward based on the current spectrum regulations and industrial standardization for unlicensed usage of TV white space, we postulate a potential deployment scenario and describe the management procedure of coexisting secondary user (SU) networks under the interference constraints to the primary user (PU) receivers. For the quantitative evaluation of coexistence we first introduce a new parameter named quality of coexistence (QoC), which is utilized to measure the benefits achieved by a novel coexistence scheme also proposed in this paper, namely, interference-constrained time slot splitting and allocation. Furthermore, on the basis of a system model we obtain the optimal SU networks time slot allocation ratios by maximizing the aforementioned QoC, therefore, leading to an efficient spectrum usage by SUs without incurring harmful interference to the PUs. Finally, we simulate a coexistence scenario in TV white space in order to compare the proposed scheme with other schemes that select a single SU network to operate based on simplistic criteria. It can be seen that when the SU network employs adaptive transmit power control (TPC), the medium value of QoC is doubled by using the optimal time slot allocation, resulting in significant improvement in coexistence.

This paper proposes a space-time scheduling algorithm that improves the throughput performance of millimeter-wave wireless personal/local area network(mmWave WPAN/WLAN) systems. The upcoming mmWave WPAN/WLAN systems are designed to support high definition TV (HDTV) transmission, high speed wireless docking and gaming, etc. These data-rate-greedy applications require Gbits/s or even multi-Gbits/s transmission capacity which can be provided by the mmWave bands. Directional antenna are preferred in these bands to compensate high path loss. However the narrowed transmission angle of directional antenna makes the system susceptible to blockage of human body or furniture. Consequently, the throughput performance is compromised and may not meet the requirement of the targeting applications. Cooperative relay with time slot sharing is a solution to guarantee throughput under such situation. Compared with lower frequency bands, it is easier in mmWave bands to share time slot between the direct path and relay path, due to less co-channel interference constrained by path loss. In this paper, we propose a space-time scheduling algorithm to assist cooperative relay. The algorithm is based on graph theory and designed to find optimized relay station and time slot to maximize the system throughput. Computer simulations demonstrate over 25 percent throughput improvement is achieved under conference room None-line-of-sight (NLOS) environment with human blockage. The achieved performance meets the requirement of uncompressed HDTV transmission.

This paper describes the challenges of frequency channel selection for WLANs operating in the TV White Space (TVWS) and proposes a dynamic frequency selection (DFS) technique as a solution. Based on the statistics gained from past usage experience of channels, of the TVWS channels' vacancies and channel occupation by the adjacent WLANs, each WLAN evaluates and prioritize the channels independently using a priority function. Then by selecting the channel of the highest priority among vacant channels, each WLAN can quickly find a channel in the low interference regime and whose vacancy is adaptive to its traffic load distribution as well. Performance of the proposed DFS is evaluated through simulation using QualNet, based on TVWS channels with realistic feature and by setting the weight factors of different metrics in the priority function to different values. The evaluation results show that compared to conventional DFS, the proposed DFS avoids adjacent APs selecting the same channel and reduces channel switch rate, and is more adaptive to diverse environments. As a consequence, it maximally reduces total time consumption of channel switch by 40% than random selection.

This paper presents the performance analysis of a realistic Smart Utility Network (SUN) system with low duty signaling design. The main focus of this paper is to evaluate the performance of the SUN system based on the specifications in an on-going industrial standardization activity, the IEEE 802.15.4g. For this purpose, a theoretical framework is constructed to model the transceiver with low duty signals over a realistic SUN channel based on actual measurements. The theoretical framework provides the models of multiple access interference (MAI) and inter-symbol interference (ISI). To align the analysis with actual industrial requirements, the developed framework is substituted with parameters from the IEEE 802.15.4g specifications. As a result, it is found that the low duty system (i.e. 0.1%) is capable of supporting up to 100 users in a single network. Additionally, by reducing the duty factor (e.g. from 1% to 0.1%), a 2-digit performance improvement (e.g. bit error rate (BER)=10–5 to BER=10–7) can be achieved as compared to the requirement of IEEE 802.15.4g specifications (i.e. BER ≈ 10–5).

To assess civil servants' performance involves a number of indices, each index weight is difficult to identify and the result is tend to be interfered by subjective factors. Therefore, the Projection Pursuit Classification Model (PPC) and a Real coded Accelerating Genetic Algorithm (RAGA) are presented to convert the problem from multi-dimensional to one-dimensional subspace and solve the multi-dimensional global optimization problem. According to projection values assess civil servants' performance. It is easy to make decision, the result is relatively objective, and the projection values are dispersed. PPC provides a new approach to assess the civil servants' performance.