Renxian Cao

Hysteresis Model Predictive Control for High-Power Grid-Connected Inverters With Output LCL Filter

Hysteresis model predictive control (HMPC) is a variant of finite-control-set model predictive control, which is mainly developed for high-power applications. In this paper, the HMPC scheme is proposed to control a high-power grid-connected two-level voltage source inverter with an inductive-capacitive-inductive (LCL) filter. To ensure the stable operation of the control system, the resonance characteristics of the system with an LCL filter based on HMPC are analyzed, and an active damping scheme is then given. On the other hand, the impacts of the modeling error on system stability and average switching frequency are analyzed in detail. Then, a new switching frequency-adaptive control scheme that uses the average switching frequency to adjust the parameter value of the predictive model online is presented. The new control scheme can keep the average switching frequency within an acceptable operational margin, while improving the stability of the control system. For demonstration, the proposed control scheme has been implemented on a small-scale grid-connected inverter system, and the results show that the theoretical analysis is correct and that the proposed control scheme is effective.

A Model Predictive Control-Based Common-Mode Voltage Suppression Strategy for Voltage-Source Inverter

Model predictive control (MPC) method has been developed as a simple and effective current control technique for voltage-source inverters (VSI). The conventional MPC method applied to VSI adopts all the seven voltage vectors (VVs), including one zero VV and six nonzero VVs, to implement the predictive current control in order to improve the current control performance. However, the common-mode voltage (CMV) is large due to the use of zero VV. Although the MPC strategy only using six nonzero VVs can reduce the CMV, its switching frequency is relatively high due to too many switches between nonadjacent VVs. In this paper, a new MPC-based CMV suppression strategy for VSI is proposed and applied to a permanent-magnet synchronous generator (PMSG)-based control system. Only four nonzero VVs, including three adjacent nonzero VVs and one nonadjacent nonzero VV, are utilized to perform the optimization in every sampling period, resulting in switching frequency and calculation effort reduction with no adverse effect on the current response. Moreover, the influence of the MPC-based CMV reduction algorithm on the speed control of PMSG is tested in this paper. The experimental results confirm the effectiveness of the proposed strategy.

Response and Protection of DFIG System under Grid Fault

The use of doubly fed induction generators (DFIG) in large wind turbines has become quite common over the last few years. However, DFIG are very sensitive to grid disturbances, especially to voltage dips, therefore low voltage ridethrough(LVRT) technology for DFIG becomes one of the critical technology in the uninterrupted operation of wind power generation. In order to help the study of low voltage ride-through(LVRT) technology for DFIG, a detailed thoretical analysis of the dynamic behavior of DFIG during grid voltage drop and recovery has been proposed, and obtained the maximum instantaneous rotor voltage when the voltage drop and the maximum and minimum rotor voltage when voltage recovery, then given the relevant simulation waveform. At last, the method on crowbar design is proposed.

Test-Bed of Doubly Fed Induction Generator for Variable-Speed Constant-Frequency Wind Power Generation

In this paper, a test-bed built for the experiments of variable-speed constant-frequency (VSCF) wind power generation is described. The test-bed is composed of a doubly fed induction generator (DFIG), a VVVF inverter driven squirrel motor for wind turbine simulation, dual-DSP-based controllers, as well as a PC-based data-acquisition and control system. In this system, a pair of bidirectional PWM voltage-source converters in a back-to-back configuration is inserted between the rotor windings and the grid lines to control the rotor currents in order to vary the slip power thereby implementing the VSCF operation. Vector-control is used to independently control the flow of active and reactive power between the system and grid line, and to make it easy for the system to implement maximal power point tracking (MPPT) and optimal reactive power splitting in wind power plants

Dynamic analysis of doubly fed induction generator during symmetrical voltage swells

With the rapid increase of wind farms especially large offshore wind farms, a new problem associated with the response of doubly fed induction generator to temporary overvoltage has arisen. This paper develops a theoretical analysis of the dynamic behavior during symmetrical three phase voltage swell. Overvoltage may lead to reversal of power in the line side converter. Simulation results are in good agreement with those obtained theoretically.

A Study on Dynamic Model And Analyse of Wind Turbine Generation System

In order to study dynamic performances of wind turbine generator system (WTGS) in laboratory, a dynamic model of WTGS is necessary. Firstly, a dynamic wind turbine emulator (WTE) is designed, a novel dynamic torque compensation scheme is developed, which consider wind shear effect, tower shadow effect and big inertia of the actual wind turbine all together. Then, the mechanical transmission system of variable speed WTGS set based on doubly-fed induction generator is fully studied in this paper. For the dynamic model of the mechanical transmission, we take account of wind turbine and generator inertia, damping and stiffness coefficient of the shaft chain all together, which are used in studying the influence on the wind power generation system dynamic performances detailed. Then it points out that optimization of parameters between the wind turbine and the mechanical transmission is very important and necessary for designing of WTGS. After that, the results of the simulation are used to prove the correctness of theoretical analysis. The simulation results strongly support the theoretical analysis and simulation results.

A Reactive Power Sharing Strategy of VSG Based on Virtual Capacitor Algorithm

In the islanded microgrid, distributed generators are controlled with virtual synchronous generator (VSG) strategy to simulate rotor inertia and droop characteristics of synchronous generators, in order to enhance the voltage and frequency support capabilities. Since the capacity and location distribution of each VSG is random, the VSG output impedance, line impedance and its capacity are mismatched, resulting in inaccurate sharing of reactive power. Based on the study of reactive power sharing schemes without communication and system parameters detection, and aiming at the contradiction between reactive power sharing error and voltage control accuracy of existing schemes, a reactive power sharing strategy based on virtual capacitor is proposed. The strategy simulates characteristics of paralleling capacitor at the VSG output terminal, and compensates the output voltage according to adaptive control of VSG output reactive power, thus to reduce reactive power sharing error, and improve the voltage control accuracy meanwhile. The design of virtual capacitor parameters and a two VSG parallel system stability with proposed strategy are analyzed in this paper. The correctness and effectiveness of the proposed strategy is verified by experiments.

Resonance characteristics analysis of paralleled inverters based on Resonance Stability Margin

With increasing penetration of renewable energy sources into modern power systems, multi-parallel inverters are commonly employed in the interface to the utility grid, giving rise to potential resonance problems. Firstly, 2-parallel inverters equivalent circuit model is established based on its control. In order to decouple inverter output current, the components characteristics of inverter output are analyzed in different frequencies, and one of 2-parallel inverter output current under condition of synchronized and asynchronized carriers are worked out. An idea “Resonance Stability Margin” is raised to investigate and evaluate resonance characteristics, variables that represent the stability of the system are found in control system of single inverter, and are used to record when the system achieves the same closed-loop dominant pole position or open-loop phase margin under different conditions. Values of those variables recorded indirectly reflect the degree of resonance in the paralleled inverters system. Experimental results validate the developed resonance characteristics and the proposed resonance stability margin.

Speed sensorless torque closed-loop vector control of IPMSM

The speed sensorless control method, which is based on the extended electromotive force (EEMF) model of interior permanent magnet synchronous motor, is difficult to make into use and its parameter dependence is strong. So a method to simplify the EEMF model was proposed and a simplified full order state and sliding mode observer (FOSSMO) was designed in order to achieve sensorless control. In addition, a simplified stator flux and electromagnetic torque observation method was presented. Then, torque closed-loop control system without speed sensor is established. The simulation and experimental research show that the proposed sensorless stator flux and electromagnetic torque observation method is independent of all the inductance and permanent magnet flux linkage parameters and the transient characteristic of the sensorless torque closed-loop control system is good.

Notice of Retraction The Analyse of Maximum Power Point Tracking's Energy Losses in PV System

Increasing efficiency is the most critical issues for photovoltaic systems, this paper introduces the views of the energy losses to the maximum power point tracking, and analyzes the maximum power point trackings dynamic and static energy losses detailedly.Then proved by the simulation of static and dynamic characteristics for incremental method (IncCond). The simulation and the theory proved that IncConds energy losses in the process of tracking the maximum power point.