Grid Interfaced Invertion System With LCL Filter And Its Control Strategy Adaptive To Complicated Grid Conditions

With the exhausting of conventional resources such as coal, oil and naturalgas, the increasing of energy and electricity demands and the deteriorating ofglobal ecological environment, the grid interfaced power generation of newenergy has become a hotspot in the field of energy and electrical engineering.The grid interfaced invertion system, as the interface unit of generation sourceand grid, the performance of which directly impacts the energy quality injectedinto grid, is a key component of the entire generation equipment. This thesis willtake the three-phase voltage source grid interfaced invertion system as researchobject, conducting research on parameter design and optimization of LCL filter,control strategy of system with LCL filter, grid voltage information synchronousdetection and continuous operation ability enhancement of system undercomplicated grid conditions. And also, combined with the project background,an experimental platform of the hardware-in-the-loop generation system ofmicro gas turbine for grid integration is constructed to verify the research resultsof this thesis.Firstly, for the grid interfaced invertion system with LCL filter, a generalmathematical model and models under ideal, unbalance, distortion various gridvoltage constraints are built. Based on this, limitations of the conventional gridvoltage information detection algorithm under unbalanced and distorted gridconditions are indicated, and then a novel grid voltage information synchronousdetection algorithm based on AC amplitude integral is proposed, which has awide grid condition adaptation range. The proposed algorithm is compatible tothe unbalanced and distorted grid conditions, which can restrain the harmonicinterference, extract the fundamental positive and negative components of gridvoltage, consequently detect their information of phase angle, frequency andamplitude synchronously. The built system model and proposed grid voltageinformation detection algorithm lay a foundation for the subsequent researches.The equivalent model of LCL filter is built and its operating characteristicsis analyzed, revealing the relationship between the filter parameters, theirrelationships and the filter operating performance. Based on this, anoptimization design program of LCL filter parameters is proposed. In theprogram, the basis and idea of parameter calculation are demonstrated, and thenare the criteria and steps of the filter parameter selection procedure, at last, thefilter parameters are further optimized regarding to the harmonic suppression performance, parameters and their cooperation relationships, power loss on thedamping resistor, etc. The proposed program can reflect the impact trend andstrength of the filter parameters and their relationships on filter operatingperformance, in addition, both the harmonic suppression performance anddamping loss problem are considered. A better filtering performance can beobtained with the same total inductance value, and the power loss on thedamping resistor reduced, leading to the system efficiency enhanced.For the control strategy of grid interfaced invertion system with LCL filter,the system model of complex frequency domain is built, and the feedbackvariable degrees of freedom of the current control loop are analyzed, pointingout problems of the control strategy based on grid and inverter side currentclosed-loop respectively. In order to solve the problem of phase angle lag of gridcurrent of the conventional inverter side current closed-loop control strategy, thecause of which is analyzed, and then a concept of controlled complex impedanceis defined to update the reference signal of inverter output current, thus acompounded control strategy of indirect current based on feedforward ofcontrolled complex impedance is proposed. With the proposed control strategy,the grid current can be controlled indirectly through inverter side current closed-loop. The proposed control strategy has a good stability, facilitates the systemintegration, and needs no more additional sensors, what’s more, can eliminatethe phase angle lag of grid current in the conventional inverter side currentclosed-loop control strategy.In order to improve the system adaptability to unbalanced, distorted gridconditions, problems of the current control loop in fundamental positivesequence single synchronous rotating frame (FPS-SSRF) is analyzed, and aconcept of proportion plus hybrid integration is defined. Aiming at the impact ofunbalance in grid voltage, a grid power pulsation suppression strategy based onproportion plus negative sequence hybrid integration is proposed. Afterwards, alow frequency grid current harmonic suppression strategy based on proportionplus harmonic hybrid integration is proposed to enhance the system rejectionability to grid voltage background harmonics. Finally, it is the system controlstrategy with high adaptability to grid conditions based on proportion plusnegative sequence and harmonic hybrid integration. All current control loops inthe proposed control strategies above are only implemented in FPS-SSRF, andno extraction of fundamental positive, negative sequence or harmoniccomponents of current feedback signal is needed, leading to the complexity ofsystem control reduced. With the proposed control strategy, the fundamentalpositive, negative sequence and harmonic components of inverter output currentcan be precisely controlled with no static error, the grid power pulsation is suppressed and the waveform quality of grid current is improved, leading tocontinuous operation ability of system with high adaptability to complicated gridconditions elevated effectively.Finally, combined with the theoretical and simulation researches in thisthesis, an experimental platform of the hardware-in-the-loop generation systemof micro gas turbine for grid integration is constructed, and experimentalresearch is conducted on the platform to verify and evaluate the proposedalgorithms, programs, and control strategies. Experimental results demonstratethe correctness and effectiveness of the theoretical analyses and proposedmethods in this thesis.