• Active Power and DC Voltage Coordinative Control for Cascaded DC–AC Converter With Bidirectional Power Application

      Tian, Yanjun; Chen, Zhe; Deng, Fujin; Sun, Xiaofeng; Hu, Yanting; University of Chester (IEEE, 2015-10-31)
      Two stage-cascaded converters are widely used in dc–ac hybrid systems to achieve the bidirectional power transmission. The topology of dual active bridge cascaded with inverter DABCI) is commonly used in this application. This paper proposes a coordinative control method for DABCI and it is able to reduce the dc-link voltage fluctuation between the DAB and inverter, then reduce the stress on the switching devices, as well as improve the system dynamic performance. In the proposed control method, the DAB and inverter are coordinated to control the dc-link voltage and the power, and this responsibility sharing control can effectively suppress the impact of the power variation on the dc-link voltage, without sacrificing stability. The proposed control method is also effective for DABCI in unidirectional power transmission. The effectiveness of the propose control has been validated by both simulations and experiments.
    • DC-Link Voltage Coordinated-Proportional Control for Cascaded Converter with Zero Steady-State Error and Reduced System Type

      Tian, Yanjun; Loh, Poh C.; Deng, Fujin; Chen, Zhe; Hu, Yanting; 1. Department of Energy Technology, Aalborg University, Denmark; 2. Faculty of Science and Engineering, University of Chester, UK (IEEE, 2015-06-11)
      Cascaded converter is formed by connecting two sub-converters together, sharing a common intermediate DC-link voltage. Regulation of this DC-link voltage is frequently realized with a Proportional-Integral (PI) controller, whose high gain at DC helps to force a zero steady-state tracking error. Such precise tracking is however at the expense of increasing the system type, caused by the extra pole at the origin introduced by the PI controller. The overall system may hence be tougher to control. To reduce the system type while preserving precise DC-link voltage tracking, this paper proposes a coordinated control scheme for the cascaded converter, which uses only a proportional DC-link voltage regulator. The resulting converter is thus dynamically faster, and when compared with the conventional PI-controlled converter, it is less affected by impedance interaction between its two sub-converters. The proposed scheme can be used with either unidirectional or bidirectional power flow, and has been verified by simulation and experimental results presented in the paper.
    • Enhanced design of an offgrid PV-battery-methanation hybrid energy system for power/gas supply

      Xu, Xiao; Hu, Weihao; Cao, Di; Liu, Wen; Huang, Qi; Hu, Yanting; Chen, Zhe
      Extensive studies have been carried out on various hybrid energy systems (HESs) for providing electricity to off-grid areas. However, a standalone HES that is capable of providing power and gas, has been less studied. In this paper, a standalone Photovoltaic (PV)-battery-methanation HES is proposed to provide adequate, reliable and cost-effective electricity and gas to the local consumers. Identifying a potential solution to maximize the reliability of the system, asked by consumers, and to minimize costs required by the investors is challenging. Bi-level programming is adopted in this study to tackle the pre-mentioned issue. In the outer layer, an optimal design is obtained by means of particle swarm optimization. In the inner layer, an optimal operation strategy is found under the optimal design of the outer layer using sequential quadratic programming. The results indicate that 1) The bi-level programming used in this study can find the optimal solution; 2) The proposed HES is proved to be able to supply power and gas simultaneously. 3) Compared with the right most and leftmost points on Pareto set, the total costs are reduced by 17.77% and 2.16%.
    • Flicker mitigation strategy for a doubly fed induction generator by torque control

      Zhang, Yunqian Q.; Hu, Weihao; Chen, Zhe; Cheng, Ming; Hu, Yanting (IET, 2014-03)
      Owing to the rotational sampling of turbulence, wind shear and tower shadow effects grid connected variable speed wind turbines could lead to the power fluctuations which may produce flicker during continuous operation. A model of an megawatt (MW)-level variable speed wind turbine with a doubly fed induction generator is presented to investigate the flicker mitigation. Taking advantage of the large inertia of the wind turbine rotor, a generator torque control (GTC) strategy is proposed, so that the power oscillation is stored as the kinetic energy of the wind turbine rotor, thus the flicker emission could be reduced. The GTC scheme is proposed and designed according to the generator rotational speed. The simulations are performed on the national renewable energy laboratory 1.5 MW upwind reference wind turbine model. Simulation results show that damping the generator active power by GTC is an effective means for flicker mitigation of variable speed wind turbines during continuous operation. keywords: {asynchronous generators;oscillations;power generation control;torque control;wind power plants;wind turbines;GTC strategy;continuous operation;doubly fed induction generator;flicker emission;flicker mitigation strategy;generator active power;generator torque control;kinetic energy;megawatt-level variable speed wind turbine;power oscillation;tower shadow effects grid connected variable speed wind turbines;turbulence;upwind reference wind turbine model;variable speed wind turbines;wind shear;wind turbine rotor
    • Impedance coordinative control for cascaded converter in bidirectional application

      Tian, Yanjun; Deng, Fujin; Chen, Zhe; Sun, Xiaofeng; Hu, Yanting; University of Chester (IEEE, 2015-06-30)
      Two stage cascaded converters are widely used in bidirectional applications, but the negative impedance may cause system instability. Actually the impedance interaction is much different between forward power flow and reversed power flow, which will introduce more uncertainty to the system stability. This paper proposes a control method for the constant power controlled converter in cascaded system, and consequently it can change the negative impedance of constant power converter into resistive impedance, which will improve the cascaded system stability, as well as merge the impedance difference between forward and reversed power flow. This paper addresses the analysis with the topology of cascaded dual-active-bridge converter (DAB) with inverter, and the proposed control method can also be implemented in unidirectional applications and other general cascaded converter system. The effectiveness has been validated by both simulation and experimental results.
    • Impedance interaction modeling and analysis for bidirectional cascaded converters

      Tian, Yanjun; Deng, Fujin; Chen, Zhe; Sun, Xiaofeng; Hu, Yanting (2015-06-30)
      For the cascaded converter system, the output impedance of source converter interacts with the input impedance of load converter, and the interaction may cause the system instability. In bidirectional applications, when the power flow is reversed, the impedance interaction also varies, which brings more uncertainty to the system stability. An investigation is performed here for showing that the forward and reverse interactions are prominently different in terms of dynamics and stability even though the cascaded converter control remains unchanged. An important guideline has been drawn for the control of the cascaded converter. That is when voltage mode converter working as the load converter; the constant power mode converter as the source converter, the system is more stable. The concluded findings have been verified by simulation and experimental results.
    • Power System with Variable Speed Wind Turbine and Diesel Generation Units

      Hu, Yanting; Chen, Zhe; Glyndwr; Aalborg University (EER, 2014-01-20)
      Thispaper presents a power system consisting of wind turbines, diesel generation units, and energy storage system. Both wind turbines and diesel engine adopt variable speed operation mode;and power electronic interface are used for the generation units which provide flexible and wide range of control on the power. The system configuration, characteristics, operation principles are presented. The controller and control strategies are discussed. The simulation studies have been performed and the results are presented.
    • Q-V droop control using fuzzy logic and reciprocal characteristic

      Wang, Lu; Hu, Yanting; Chen, Zhe; Glyndwr University; Aalborg University (Engineering and Technology Publishing, 2014-01-01)
      Today, the conventional power system is facing some global environmental problems, which is leading to a new trend of power grid by using “green and clear” energy sources. As the platform of smart grid technology, the microgrid associated with distributed energy resources (DERs) may provide electric power at distributed voltage level, which not only is an autonomous system, but also can be connected to the main grid. To improve the stability and controllability of the power grid, this paper presents an improved Q-V droop control strategy using fuzzy logic controller and reciprocal characteristic. Matlab/Simulink is used for analysing the performance of system. The feasibility of the improved droop control strategy has been verified and discussed. The results demonstrate the improved Q-V droop control strategy could have good effects in grid-connected and islanded mode, and during operation mode transitions.
    • Smart guaranteed time-slot allocation algorithm for industrial wireless sensor networks emergency message transmission

      Chen, Qinyin; Hu, Yanting; Chen, Zhe; Davies, John N.; Excell, Peter (IET, 2015-04-01)
      This paper presents investigation on application of wireless sensor networks (WSNs) in wind power generation systems and highlights an important issue associated with the deadline for the delivery of messages among nodes based on the IEEE 802.15.4E standard. Owing to the limits of standard and the power system application requirements, this research proposes a smart guaranteed time slot (S-GTS) allocation algorithm which is based on the urgent/important matrix. This proposed algorithm promotes the utilisation of contention free period in a superframe. Besides, over seven GTSs can be allocated in a superframe, there are only seven GTSs that can be used in the standard. In addition, this study proves the value of BO and SO upper bound is 6 for the WSN application in power systems. Moreover, the network delay of S-GTS performs better than the 16-time-slot mechanism and i-GAME mechanism.
    • Terahertz reading of ferroelectric domain wall dielectric switching

      Zhang, Man; Chen, Zhe; Yue, Yajun; Chen, Tao; Yan, Zhongna; Jiang, Qinghui; Yang, Bin; Eriksson, Mirva; Tang, Jianhua; Zhang, Dou; et al.
      Ferroelectric domain walls (DWs) are important nano scale interfaces between two domains. It is widely accepted that ferroelectric domain walls work idly at terahertz (THz) frequencies, consequently discouraging efforts to engineer the domain walls to create new applications that utilise THz radiation. However, the present work clearly demonstrates the activity of domain walls at THz frequencies in a lead free Aurivillius phase ferroelectric ceramic, Ca0.99Rb0.005Ce0.005Bi2Nb2O9, examined using THz time domain spectroscopy (THz-TDS). The dynamics of domain walls are different at kHz and THz frequencies. At low frequencies, domain walls work as a group to increase dielectric permittivity. At THz frequencies, the defective nature of domain walls serves to lower the overall dielectric permittivity. This is evidenced by higher dielectric permittivity in the THz band after poling, reflecting decreased domain wall density. An elastic vibrational model has also been used to verify that a single frustrated dipole in a domain wall represents a weaker contribution to the permittivity than its counterpart within a domain. The work represents a fundamental breakthrough in understanding dielectric contributions of domain walls at THz frequencies. It also demonstrates that THz probing can be used to read domain wall dielectric switching.