Power Quality Analysis And The Compensation System Study Of Large Scale Grid-Connected Photovoltaic Power Plant

Climate and environment change and lack of fossil fuels in recent years shows that we have to gradually change the structure of energy consumption, vigorously develop renewable energy, and take the way of sustainable development in the field of energy supply. Solar energy is an important part of new energy, installed capacity increased gradually globally. In China, several MW-level Grid-connected Photovoltaic power plants(GCPV) are constructing or operating one after another in Qinghai, Gansu, Ningxia and other places, and the enterprise(State Grid) of our country has made corresponding grid-connected provision for large scale GCPV, but in today GCPV is still in the demonstration phase, for gathering our photovoltaic business experiences. But more than hundred 10 MW level large-scale GCPVs have been built overseas, and the related standard and the standard of grid connected photovoltaic power has formed a relatively complete system.This paper relies on the National High Technology Research and Development of China( 863!Program)(2011AA05A301), "Key technology research o f grid-connected large scale photovoltaic power plant# and Hunan Provincial Innovation Foundation for Postgraduate(CX2014B130) " Analysis of Large-scale photovoltaic plant harmonic characteristics and suppression for resonances#, this paper carried out the following research work: mathematical modeling of large scale GCPV in harmonic domain, harmonic interaction between large scale GCPV and the grid, harmonic and resonances suppression for GCPV. This paper is trying to provide theoretical basis and technical support for large scale GCPV of our country, and provide a useful reference for the practical process of promoting photovoltaic industry. The research content mainly includes four aspects: harmonic domain model for large scale GCPV, the harmonic interaction between GCPV and the grid, harmonic control scheme for GCPV and parallel resonance suppression strategy.(1)The harmonic domain modeling method of large scale GCPV are proposed, the special characteristics of the harmonics generated from GCPV are revealed.In order to analyze the generation mechanism of harmonic and accurately estimate the harmonics generated from GCPV under different conditions, this paper proposed a dynamic model of PV station in harmonic domain. Took dead-time effect, modulation section and the efficiency curve into account, equivalent non-ideal model of inverter in harmonic domain was established; discussed Thevenin!s equivalent circuit of 1 MW power unit; established the impedance network model that approaches real grid-connected system. All the above constitutes the mathematical model of GCPV.Based on the developed model, the relationship among harmonics of GCPV, irradiance, temperature of photovoltaic panel was analyzed. The comparison of simulation data and measured data verify the model. The mathematical model is an effective method to analyze the operating characteristics of GCPV, could provide the theoretical basis for estimation the ability of grid to accept PV and the development of power quality control equipments.(2)The mathematical model of power transmission and power distribution system for GCPV are established, the mechanism of interaction between GCPV and the grid are revealed.Mathematic model of power transmission and power distribution system for GCPV is established, transmission line with distributed capacitance, loads in different location, reactive power compensation. On the basis, we defined the harmonic voltage amplification factor and harmonic current amplification factor, qualitative and quantitative analyzed the mechanism, relationships among harmonic amplification factor and transmission distance, harmonic order as well as type were analyzed.The analysis of the harmonic amplification factors show that: just one resonance curve for harmonic voltage series resonance, i.e., the transmission line with fixed distance may resonance a harmonic voltage once. But for harmonic current parallel resonance, double resonance curves were shown, that means the transmission line with fixed distance may resonance harmonic currents twice or more. This will undoubtedly increase the probability of harmonic current resonance.(3) Harmonic control device which is suitable for large-scale GCPV is proposed, realized the harmonic suppression of GCPV.Be aimed at the characteristics of GCPV and for the purpose of engineering application, IHAPF(Hybrid Active Power Filter with Injection circuit) is proposed for suppressing the harmonics of GCPV. The injection circuit is composed of a reactor, a isolation transformer and a single tuned passive filter, this topology structure makes it has more practical value in engineering compared with conventional Hybrid Active Power Filters. The efficiency and reliability of the compensation system are depended on the parameter design for the passive filters and the injection circuit. We focus on the harmonic injection ratio, the fundamental voltage and harmonic voltage of the reactor, design the parameters. Simulations based on PSIM are carried out, an experimental prototype is developed, and simulation and experimental results verify the correctness and effectiveness of the IHAPF system.(4)Based on IHAPF, paralleled resonance suppression strategy is proposed, which could effectively suppress the parallel resonance of harmonic current while evolutes in the transmission system.While harmonic current transmitted in the network, parallel resonance may be caused, and will affect the power quality level of the gird. Base on the model of APF(Active Power Filter), the mechanism of resonance suppression strategy is researched. Matlab calculation results verify the theoretical feasibility. For high voltage IHAPF, paralleled resonance suppression strategy is proposed. On the basis of simulations of harmonic amplification factors, the optimal control factors were given. The simulation and experimental results show that the control strategy could improve the harmonic resonance phenomenon between GCPV and the grid.In the background of the rapid development of large scale centralized GCPV, on the basis of mathematical model of GCPV and its power transmission system and power distribution system, the special characteristics of GCPV harmonics and the mechanisms of the interaction between GCPV and the grid are revealed, and then put forward the harmonic control device and harmonic suppression strategy. This paper has formed a relatively complete GCPV power quality analysis and control scheme, the mathematical modeling methods, the engineering design and the contro l strategy, which has been instructive for large scale new energy grid connected characteristics research, evaluation of the ability that the grid to accept new energy, and development of other power quality control systems.