| Strengthening and encouraging the growth of distributed generation technology and its industry(GDGTI)is an important approach to prompt the structural reform of the energy supply front in our country.GDGTI has ablity to effectively solve the bottleneck of the growth of renewable energy in current developmenting stage,to optimize the structure of energy industry,to coordinate the renovation of rural grid,to promote the developmenting efficiency of energy coordination,and to implement the complementary of multiple energies in time and space.Three topics about the distribution generation(DG)system operating in intentional islanding mode and grid-connecting mode have been discussed in paper: 1)high-quality voltage supply to guarantte the voltage stable of PCC is urgently needed in the autonomous system due to the lack of the voltage support from the utility grid;2)A varirty of coupling resonance phenomena among lumped DG units operating in grid-connecting mode greatly deteriorate the power qualities of low-voltage distribution networks;3)A large quantity of harmonic resonance interacting problems in wide-band frequency domain challenge the security of the power system.The specific works and innovations in this paper can be summarized as follows:(1)An adaptive sliding-mode global robust voltage controller is proposed and implemented to the inverter control system of DG units operating in intentional islanding mode so as to fulfill the urgent needs of high-quality voltage supplies in autonomous systems.The main reasons for the inverter control system of the DG unit failing to provide high-quality PCC voltage support are the internal perturbations of filter paramerters of the inverter control system and the external disturbances from non-linear load current.Therefore,the adaptive algorithms to observe internal perturbations and external disturbances are seperatively designed in order to eliminate the reaching phase of state trajectories,and further to achieve the global robustness of the control system.Besides,the adaptive algorithm greatly reduces the chattering of input voltage.Comparative simulations and experiments are conducted.And the results indicate that total harmonic distortion(THD),chattering,and steady-state error of output voltages are effectively reduced and the dynamic performances and the capability of perturbation rejection for the control system of inverters are effectively enhanced and the circulating current among paralleled inverters is attenuated effectively by the use of the proposed controller.(2)A generalized harmonic resonance model of the lumped DG system is established and used to explore the phenomena and the pattern of coupling resonances among lumped DG units operating in grid-connected mode.Firstly,both the deadbeat control and proportional quasi-resonance(PR)control method for inverter control system are choosen to explore the universal phenomena and the pattern of coupling resonances.Secondly,the generalized harmonic resonance model is established,which can be widely implemented without relying on specific control schemes.Thirdly,enlightened from the mathematical description of an extreme value,the distribution of resonance peaks for inverter systems using both two current control methods are figured out.Then,the coupling resonances can be classified into two types: intrinisic resonance band and extrinisic resonance band.Also,the model reveals that the output current of the inverter contains three resonance peaks: individual resonance peak,parallel resonance peak,and series resonance peak.Especially,the intrinisic non-fixed resonance peaks will migrate toward a low frequency domain along with the growth in number of parallel inverters.(3)The mechanism of the resonance peak migrating toward a low frequency domain along with the growth in number of parallel inverters in a lumped DG system is explained from the perspective of physical circuit.And a proposed 3-D graphical approach can be used to optimize the damping control parameters in an intuitive way,which provides a high-efficiency scheme to tackle the power quality problems in low-voltage distribution network for lumped DG units.The essential reason for the resonance peak migrating toward a low frequency domain is the decrease in value of the equivalent output impedance of the lumped DG system.The practical physical meaning of active damping schemes based on both deadbeat current control and PR control are carefully derived.Besides,a proposed 3-D graphical approach based on damping control parameters-harmonic frequency-resonance amplitude can be used to determine the optimal value of damping control parameters in an intuitive way.(4)A lumped equivalent model and an overall equivalent model are built for distributed power plant,which can be utilized to explore the mechanism and distributing pattern of resonance bands in wide-band frequency domain.The lumped equivalent model and overall equivalent model are taken following elements into account: multiple collecting system structure,the Norton's equivalent circuit of the DG unit,stray capacitance of distributed transformers,and distributed capacitance of the overhead transmission lines.The lumped equivalent model is used to analyze the coupling resonances among multiply transimission lines in the power plant;on the contrary,an overall equivalent model is used to discuss the power quality of the total output current from the plant to the utility grid.Then,the filter resonance band,the ring resonance bands are discovered.Furthermore,ring resonance band will be in collusion with filter resonance band at specific length of transmission lines.As a result,a superimposition resonance band emerges and high harmonic currents are produced.Besides,the identity of the resonance peaks explored from the two models is discussed.(5)A lumped equivalent model and an overall equivalent model integrating with hybrid active power filter(HAPF)are built for distributed power plant,which is used to analyze the resonance damping effectiveness in wide-band frequency domain.Besides,based on the theoretical analysis,the HAPF with customized damping effects is suggest to install at the 35 kV bus in a distributed power plant to overcome the harmonic resonance problems.The harmonic feature of the bus-side current of each transmission line in a practical 48 MW wind farm located in a mountainous area is analyzed.Based on the practical harmonic data in the distributed wind farm,the model of customized designed HAPF integrating into the lumped equivalent model and the overall equivalent model is figured out,and then used to testify the attenuating effectiveness of resonance bands in wide-band frequency domain.Finally,some comparative simulations and experiments are conducted to verify the correctness of the harmonic resonance modeling,also the feasibility of proposed hybrid active power filter(HAPF)installed at the 35 kV bus to overcome the harmonic resonance problems is testified in a wind farm.(6)The prototype of inverter for low-voltage distributed generation and the prototype of hybrid active power filter for large distributed power plant are developed.(1)The prototype of inverter for low-voltage distributed generation is developed and the plantform of distributed generation system for photovaltic house roof is established in our laboratory.Different control algorithms(deadbeat current control and PR control current)are testified by implemented into inverter control system in the plantform.Also,both photovoltaic DC voltage source and diode-rectifier DC voltage source are used as primary energies for inverter power stage in the plantform.(2)The prototype of hybrid active power filter for large distributed power plant is developed and the plantforms of conventional cabined-based HAPF and modularized box-based HAPF(MBB-HAPF)are established in our laboratory,respectively.Finally,three cases of utilizing HAPF to control harmonic current in 35 kV,6kV,380 V are introduced and the test results show that the developed HAPF series have capacity to dynamically control harmonic currents in wide frequency domain in low-,middle-,and high-voltage level distribution network. |
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