PI control based on fuzzy set-point weighting tracking for hydraulic crane boom system

A PI control strategy based on fuzzy set-point weighting following was proposed for the active damping control of a hydraulic crane boom system （HCBS）. Two valve-controlled PI controllers, which include a proportional feedforward controller based on fuzzy set-point weighting following and a limited semi-integrator（LSI）, are designed respectively. LSI is used to limit output signal and to prevent wind up at the low frequency of the spectrum. By using a range camera and an electronic feedback control, the tip damping on the HCBS can be adjusted artificially. A collaborative control simulation technique of HOPSAN and MATLAB/SIMULINK is applied to the controller design. Simulation results show that the proposed PI control system has less overshoot as well as faster response. The tip damping on the HCBS during operation is improved.

Asymmetrical Fault Current Calculation Method and Influencing Factors Analysis of Droop-Controlled Inverters

Since the fault dynamic of droop-controlled inverter is different from synchronous generators (SGs), protection devices may become invalid, and the fault overcurrent may damage power electronic devices and threaten the safety of the microgrid. Therefore, it is imperative to conduct a comprehensive fault analysis of the inverter to guide the design of protection schemes. However, due to the complexity of droop control strategy, existing literatures have simplified asymmetric fault analysis of droop-controlled inverters to varying degrees. Therefore, accurate fault analysis of a droop-controlled inverter is needed. In this paper, by analyzing the control system, an accurate fault model is established. Based on this, a calculation method for instantaneous asymmetrical fault current is proposed. In addition, the current components and current characteristics are analyzed. It was determined that fault currents are affected by control loops, fault types, fault distance and nonlinear limiters. In particular, the influences of limiters on the fault model, fault current calculation and fault current characteristics were analyzed. Through detailed analysis, it was found that dynamics of the control loop cannot be ignored, the fault type and fault distance determine fault current level, and part of the limiters will totally change the fault current trend. Finally, calculation and experimental results verify the correctness of the proposed method.

Coordinating optimization-based sliding mode variable structure control for electro-hydraulic servo system

A sliding mode variable structure control （SMVSC） based on a coordinating optimization algorithm has been developed. Steady state error and control switching frequency are used to constitute the system performance indexes in the coordinating optimization, while the tuning rate of boundary layer width （BLW） is employed as the optimization parameter. Based on the mathematical relationship between the BLW and steady-state error, an optimized BLW tuning rate is added to the nonlinear control term of SMVSC. Simulation experiment results applied to the positioning control of an electro-hydraulic servo system show the comprehensive superiority in dynamical and static state performance by using the proposed controller is better than that by using SMVSC without optimized BLW tuning rate. This succeeds in coordinately considering both chattering reduction and high-precision control realization in SMVSC.

Generalized coupling resonance modeling, analysis, and active damping of multi-parallel inverters in microgrid operating in grid-connected mode

This paper firstly presents an equivalent coupling circuit modeling of multi-parallel inverters in microgrid operating in grid-connected mode. By using the model, the coupling resonance phenomena are explicitly investigated through the mathematical approach, and the intrinsic and extrinsic resonances exist widely in microgrid.Considering the inverter own reference current, other inverters reference current, and grid harmonic voltage, the distributions of resonance peaks with the growth in the number of inverters are obtained. Then, an active damping control parameter design method is proposed to attenuate coupling resonance, and the most salient feature is that the optimal range of the damping parameter can be easily located through an initiatively graphic method. Finally,simulations and experiments verify the validity of the proposed modeling and method.

Overview of power quality analysis and control technology for smart grid

With the wide application of non-linear loads and the large-scale access of distributed energy generations based on power electronics equipments, power quality problems in the distribution network are increasingly serious with new characteristics. Further in-depth research is of great significance in theory and practice. This paper provides an overview of power quality analysis, compensators, and control technologies under the new situation of smart grid. It focuses on the topologies and control methods for power quality conditioners, especially new characteristics of power quality and applicable control technologies in microgrids and distributed power plants.Finally, trends and prospects of power quality control technology are introduced, which is important to achieve security and efficient operation of smart grid.

Constant temperature control of tundish induction heating power supply for metallurgical manufacturing

The tundish induction heating power supply(TIHPS)is one of the most important equipment in the continuous casting process for metallurgical manufacturing.Specially,the constant temperature control is greatly significant for metallurgical manufacturing.In terms of the relationship between TIH load temperature and output power of TIHPS,the constant temperature control can be realized by power control.In this paper,a TIHPS structure with three-phase PWM rectifiers and full-bridge cascaded inverter is proposed.Besides,an input harmonic current blocking strategy and a load voltage feedforward control are also proposed to realize constant temperature control.To meet the requirement of the system,controller parameters are designed properly.Experiments are con?ducted to validate the feasibility and effectiveness of the proposed TIHPS topology and the control methods.

Calculation Method for Commutation Failure Fault Level in LCC-HVDC System Under Single-line-to-ground Faults Considering DC Current Variation

Earlier studies have reported some calculation methods for commutation failure fault level(CFFL) in line-commutated-converter based high-voltage direct current(LCCHVDC) system under single-line-to-ground(SLG) faults. The accuracy of earlier methods is limited because they only consider the commutating voltage drop and phase shift, while neglecting the DC current variation. Hence, this paper proposes a CFFL calculation method under SLG faults considering DC current variation, for better planning and designing of LCC-HVDC systems. First, the fault commutating voltage magnitude and phase shift are calculated. Then, the fault DC voltage during different commutation processes is deduced. Based on the commutating voltage magnitude and phase shift, and DC voltage during different commutation processes under SLG faults, the characteristics of CFFL with different fault time are demonstrated and analyzed. Next, the transient time-domain response of the DC current after the fault is obtained based on the DC transmission line model. Discrete commutation processes are constructed based on the commutation voltage-time area rule to solve the extinction angle under different fault levels and fault time. Finally, the CFFL is calculated considering the fault time, commutating voltage drop, phase shift, and DC current variation. The accuracy of the proposed method compared with the traditional method is validated based on the CIGRE benchmark model in PSCAD/EMTDC.

Carrier dynamic overlapping switching frequency optimal pulse width modulation method for modular multilevel converters

Modular multilevel converters(MMCs) operate in the low modulation index region in many applications.However, when utilized at the low modulation index region, large harmonics appear in the output voltage, which degrade the performance of the MMC. To improve the harmonic characteristic in the low modulation index, the carrier dynamic overlapping switching frequency optimal pulse width modulation(CDOSFOPWM) method is proposed for the MMC here. The whole modulation index region is divided into three regions: high modulation index region, middle modulation index region, and low modulation index region. The carrier amplitude, carrier overlap ratio, and frequency of triangular carriers are chosen dynamically according to the modulation index region of the modulation signals, to achieve the optimal harmonic characteristic in the whole modulation index region and maintain the switching loss. The number of on-state submodules(SMs) can be calculated by CDOSFOPWM, and the selection of SMs is performed by a reducing switching frequency voltage balancing algorithm. Finally, the proposed method is verified by simulation and experimental results.

Intelligent Control Method of Power Supply for Tundish Electromagnetic Induction Heating System

An intelligent control method is proposed to improve the performance of power supply for tundish electromagnetic induction heating,which can adequately regulate the tundish temperature.The topology structure of power supply for tundish electromagnetic induction heating is presented,and its working principle is analyzed.The power supply consists of six power units,and each of them consists of a fore-stage threephase rectifier and back-stage single-phase inverter.The feedforward control DC voltage is used by three-phase rectifier to obtain the stable DC voltage supplied to the inverter.The cloud controller based intelligent temperature control algorithm is combined with the power feed-forward algorithm to obtain accurate tracking of the output current and constant temperature control of the tundish steel in the back-stage inverter.The simulation and experiment are performed to verify the accuracy and effectiveness of the proposed method.

Voltage control for static var compensator using novel optimal nonlinear PI

Static var compensators （SVCs） are used for voltage and reactive power control in power systems. In this paper, we will consider the structure of SVCs that mainly consists of Y-connection of mechanically-switched capacitor （MSC） and delta connection of thyrister-controlled reactor （TCR）. First, the control model of SVC was established in this paper. Then, a novel optimal nonlinear voltage controller for SVCs is proposed. The proposed SVC voltage controller consists of a nonlinear function and a conventional PI controller. The improved simplex method （SPX） is presented to adjust and optimize the parameters of the nonlinear PI controller in real-time, and to make transient state response procedure of SVC optimum. The integration of time multiplied absolute error （ITAE） is adopted as the optimized objective function of SPX. Simulation and engineering application results show that the proposed voltage control method is able to track reference voltage value of SVC immediately. It also demonstrates that the whole SVC control System can synthetically compensate reactive power.

A nonlinear repetitive controller

A nonlinear repetitive controller is proposed. The new method is mainly composed of a repetitive control part and a deadband relay. Whenever the input error goes beyond the range of the deadband relay, the control loop is driven dominantly by the deadband relay to obtain fast dynamic response and meanwhile to avoid the saturation of the repetitive control part. After the input error falls within the range of the deadband relay, the deadband relay automatically turns off and the repetitive control alone governs the current control to eliminate the steady state error. A systematic methodology is established and it is linked to the conventional control system design. The proposed scheme is practically applied to the current control of active filter. Experimental results verified the feasibility of the proposed method.

Guest Editorial:Special Issue on Power Quality

More and more distributed generations (DGs)such as solar,wind,and energy storage,etc,inject into the power grid mainly through the power electronics equipments.Large-scale power electronic equipments connected to the grid bring new challenges for traditional power quality.Power quality analysis theory and control technology for smart grid will provide the comprehensive and systematic solutions and approaches in the