A Comprehensive VSM Control Strategy Designed for Unbalanced Grids

Doubly fed induction generator (DFIG) still shares a large part in today's wind power market. It provides the benefits of variable speed operation cost-effectively, and can control its active and reactive power independently. Crowbar protection is often adopted to protect the rotor-side voltage source converter (VSC) from transient overcurrent during grid voltage dip. But under unbalanced grid voltage condition, the severe problems are not the transient overcurrent, but the electric torque pulsation and dc voltage ripple in the back-to-back VSCs. This paper develops dynamic models in MATLAB/Simulink, validates it through experiments, investigates the behavior of DFIG during unbalanced grid voltage condition, and proposes new controllers in separated positive and negative sequence. Methods to separate positive and negative sequence components in real time are also developed, and their responses to unsymmetrical voltage dip are compared. Simulation results prove that the separated positive and negative sequence controllers limit the torque pulsation and dc voltage ripple effectively.

Single-phase cascaded H-bridge rectifier(CHBR) of power electronic traction transformer(PETT) has disadvantage of secondary ripple power. To solve this problem, a CHBR with the active power decoupling(APD) topology is used, and a comprehensive control strategy is proposed, which includes dc-side capacitor voltage control and the APD control strategy based on real-time calculating conduction duty cycle. Firstly, the causes of secondary ripple power and the working mechanism of the APD circuit are analyzed. Then, based on the theoretical analysis, the control strategy of the APD of the CHBR is established to realize rectifier voltage control. Based on real-time calculating conduction duty cycle of each APD arm, the secondary ripple power during rectifier conversion can be suppressed, and the circuit works in the current intermittent mode. So that,the system reliability and power density can be improved. Finally, simulation and experiment results are carried out to verify the of correctness and effectiveness of the proposed method.

This paper investigates the fixed-time tracking control strategy for a wearable upper-limb exoskeleton under bounded disturbances. A 5 DOF (degree of freedom) model is firstly derived by Lagrange method. Then the fixed-time controller with a novel sliding surface is constructed which can ensure that both the reaching time and settling time of the closed-system are independent of the initial system conditions. In order to guarantee the tracking performance under bounded disturbances, a nonlinear disturbance observer is utilized to estimate the exact values of the disturbances and compensate the controller in fixed-time. Finally, the comprehensive control strategy is utilized to achieve the excellent tracking performance and simulations are presented to illustrate the effectiveness of the control strategies.

Abstract. To control severe air pollution in China, comprehensive pollution control strategies have been implemented throughout the country in recent years. To evaluate the effectiveness of these strategies, the influence of meteorological conditions on levels of air pollution needs to be determined. Using the intensive air pollution control strategies implemented during the Asia-Pacific Economic Cooperation Forum in 2014 (APEC 2014) and the 2015 China Victory Day Parade (Victory Parade 2015) as examples, we estimated the role of meteorological conditions and pollution control strategies in reducing air pollution levels in Beijing. Atmospheric particulate matter of aerodynamic diameter ≤ 2.5 µm (PM2.5) samples were collected and gaseous pollutants (SO2, NO, NOx, and O3) were measured online at a site in Peking University (PKU). To determine the influence of meteorological conditions on the levels of air pollution, we first compared the air pollutant concentrations during days with stable meteorological conditions. However, there were few days with stable meteorological conditions during the Victory Parade. As such, we were unable to estimate the level of emission reduction efforts during this period. Finally, a generalized linear regression model (GLM) based only on meteorological parameters was built to predict air pollutant concentrations, which could explain more than 70 % of the variation in air pollutant concentration levels, after incorporating the nonlinear relationships between certain meteorological parameters and the concentrations of air pollutants. Evaluation of the GLM performance revealed that the GLM, even based only on meteorological parameters, could be satisfactory to estimate the contribution of meteorological conditions in reducing air pollution and, hence, the contribution of control strategies in reducing air pollution. Using the GLM, we found that the meteorological conditions and pollution control strategies contributed 30 and 28 % to the reduction of the PM2.5 concentration during APEC and 38 and 25 % during the Victory Parade, respectively, based on the assumption that the concentrations of air pollutants are only determined by meteorological conditions and emission intensities. We also estimated the contribution of meteorological conditions and control strategies in reducing the concentrations of gaseous pollutants and PM2.5 components with the GLMs, revealing the effective control of anthropogenic emissions.

A hybrid power plant combining a solid oxide fuel cell (SOFC) and a micro gas turbine (MGT) is a suitable technology solution for decentralized energy production utilizing natural gas and biogas. Despite having high electrical efficiency and low emissions, the dynamic interactions between components can lead to damages of the system if a comprehensive control strategy is not applied. Before building a coupled hybrid power plant demonstrator, the “hybrid system emulators” approach is followed to solve any integration issues. A test rig consisting of an MGT and emulated SOFC is developed. The dynamics of the SOFC are reproduced by a real time model. The created cyber-physical system provides an effective platform to validate and optimize the control concepts for the future hybrid demonstrator by adding the complexity of the hybrid plant to the MGT test rig. The ability to develop and test the control strategy on such a system dramatically reduces the technology risk and increases the chances of success for the demonstrator operation.

Clinical outcomes for young people with screening-detected and clinically-diagnosed rheumatic heart disease in Fiji

Three phase pulse width modulation (PWM) rectifiers have excellent performance when the grid voltages are balanced, and it is widely adopted in the industry. However, the asymmetrical loads or grid faults will lead to the unbalanced grid voltages. In addition, the negative sequence (NS) voltages and currents in the unbalanced grid result in the fluctuation of the dc bus voltage. In this article, the intrinsic mechanism between the voltage ripple amplitude and the NS currents is revealed. Furthermore, this article proposes a data-driven adaptive current control method. By shaping the NS currents, the corresponding amplitudes of the second-order harmonic voltage on the dc bus are observed. Based on the observation data, the reference currents that can minimize the dc bus voltage ripple are adaptively derived online. As a result, the dc bus voltage ripple can be suppressed effectively. In addition, the proposed method does not need the NS decomposition or the parameters such as the grid voltages and inductances. Therefore, it has a strong robustness against parameter mismatch. Finally, the accuracy of the theoretical analysis and the effectiveness of the proposed method are validated by experiments.

When a grid voltage is unbalanced, the unbalanced grid voltages act as an external disturbance for the grid-connected PWM inverter systems, resulting in an unbalanced grid current. Firstly, the mathematical model of the grid-connected converter, in view of unbalanced grid voltages, is proposed and analyzed in detail. In order to achieve the decomposition of positive and negative sequence voltage, this paper adopts the software phase-locked loop to decouple based on double synchronous reference frame. This method has good dynamic performance. In order to suppress negative sequence current on the AC side, the positive and negative sequence components of the grid current are controlled in the double synchronous reference frame respectively. Finally, Simulation results verify the correctness of the control strategy.

Global tobacco prevention and control in relation to a cardiovascular health promotion and disease prevention framework: A narrative review

To explore the law and distribution characteristics of malaria prevalence in Pingguo County, Guangxi Zhuang Autonomous Region, so as to provide the evidence for formulating the targeted control strategy and measures after malaria elimination. The data of malaria epidemic situation in Pingguo County from 1951 to 2014 were collected and analyzed with the retrospective epidemiological method. From 1951 to 2014, there were 71 365 malaria cases reported in the county. The malaria incidence dropped from 429.43/10 000 in 1960 to 0 in 1993. Since 2002, there was no local endogenous infection case reported. In 1993, this county achieved the standard of malaria elimination basically, and in 2014, passed the examination and evaluation of malaria elimination. In Pingguo County, the comprehensive malarial control strategy is effective in different stages. In the future, the monitoring of mobile population and dealing with imported malaria cases timely is the key of consolidation of malaria control achievements.

A path to cooperation between China and Mongolia towards the control of echinococcosis under the Belt and Road Initiative

The low‐frequency dynamics of virtual synchronous machine (VSM) depends on multiple factors. In this study, the oscillation mode of a single VSM is first identified by exploring the evolution of oscillation from synchronous mode to sub‐synchronous mode with the variation of structure and parameters. The inter‐oscillation modes among multiple VSMs modelled in quasi‐state phasor domain are then studied by decomposing into mean motion and relative motion. Inspired by and weak‐coupling‐based coherency identification, the associated VSM group can be fast identified and used for effective stabilizer location. The approximate eigenvalues can be fast obtained by system decomposition in uniform‐damping scenarios or by extended‐equal‐area‐criterion approach in non‐uniform‐damping scenario. An improved design for lead–lag compensation is proposed to damp both synchronous and sub‐synchronous oscillation of VSMs. Effectiveness of the proposed control strategy in grid‐connected/islanded mode is verified with real‐time simulation.

The low voltage ride-through (LVRT) capability of the doubly-fed induction generator (DFIG) significantly impacts upon the integration of wind power into the power grid. This paper develops a novel comprehensive control strategy to enhance the LVRT and reactive power support capacities of the DFIG by installing the energy storage system (ESS). The ESS is connected to the DC-link capacitor of the DFIG and used to regulate the DC-link voltage during normal or fault operations. The unbalanced power between the captured wind power and the power injected to the grid during the transient process is absorbed or compensated by the ESS. The rotor-side converter (RSC) is used to control the maximum power production and the grid-side converter (GSC) is used to control the reactive power before participating in the voltage support. When the supply voltage continues to drop, the rotor speed is increased by controlling the RSC to realize the LVRT capability and help the GSC further enhance the reactive power support capability. The capacity of the GSC is dedicated to injecting the reactive power to the grid. An auxiliary transient pitch angle controller is proposed to protect the generator’s over speed. Both RSC and GSC act as reactive power sources to further enhance the voltage support capability with serious voltage sags. Simulations based on a single-machine infinite-bus power system verify the effectiveness of the developed comprehensive control strategy.

This paper proposes a novel finite control set model predictive control (FCS-MPC) strategy with merely grid-injected current sensors for an inductance-capacitance-inductance (LCL)-filtered grid-tied inverter, which can obtain a sinusoidal grid-injected current whether three-phase grid voltages are balanced or not. Compared with the conventional FCS-MPC method, four compositions are added in the proposed FCS-MPC algorithm, where the grid voltage observer (GVO) and Luenberger observer are combined together to achieve full status estimations (including grid voltage, capacitor voltage, inverter-side current, and grid-injected current), while the sequence extractor and the reference generator are applied to eliminate the double frequency ripples of the active or reactive power, or the negative sequence component (NSC) of the grid-injected current caused by the unbalanced grid voltage. Simulation model and experimental platform are established to verify the effectiveness of the proposed FCS-MPC strategy, with full status estimations under both balanced and unbalanced grid voltage conditions.

With the increase of distributed power generation integrated into the power grid through power electronic converters, the problem of inertia reduction in power system has been caused, a new control technology is needed to enable distributed power sources to actively participate in the voltage and frequency regulation of the grid. In this paper, a model predictive control (MPC) strategy based on virtual synchronous machine (VSM) for the grid-connected converter is proposed. The method does not require traditional pulse width modulation and current inner loop control. When the voltage and frequency of power grid change, the active power and reactive power can respond in time and simulate characteristics of traditional synchronous machine. The simulation results are given to verify the effectiveness of the control strategy presented above.