| With the accelerated depletion of fossil energy,it becomes inevitable to develop new energy generation technology,mainly including wind and solar power.More and more distributed generators have been connected to power grid,which causes many problems such as the low system efficiency,the poor power quality,the failure to access to DG effectively and safely.Meanwhile,DC distribution network is becoming the new trend of power grid development due to the advantages of low loss,low cost,high efficiency and high reliability.However,the research on DC power quality problems remains inadequate as DC distribution network is under planning and construction.This paper devotes to the prospective study on discovery,analysis and evaluation of DC power quality problems.It hopes to provide reference and theoretical support for DC power quality standardization,and provide method support for power quality comprehensive evaluation of DC distribution network.This paper first introduces the network topology and the converter link of DC distribution network.And according to different load requirements,it preliminarily discusses the choice of 3 DC distribution voltage levels,including low-voltage,medium-voltage and high-voltage.Secondly,the equivalent model and work principle of photovoltaic and battery energy storage modules in DC distribution network are emphatically analyzed.By referring to some studies about AC system power quality,considering the difference of AC and DC power quality problems,integrating the research present situation of DC power quality,the concept of some power quality problems such as voltage deviation,voltage fluctuations,harmonics and ripples,voltage sags and short supply interruptions are defined preliminarily.The causes of some DC power quality problems is discussed in this paper and the induced mechanism of harmonics is emphatically mathematically analyzed.The model of LVDC distribution network is built based on PSCAD/EMTDC.Simulation analysis for photovoltaic and energy storage modules is conducted to verify the rationality of the constructed model.By simulating the DG output power variation,its effects on DC bus voltage fluctuations are studied;three different operating conditions including system normal operation,three phase unbalance and harmonics existing in AC side are set to study the effects on harmonics in DC side;By simulating load Switching-in extensively and instantaneously,its effects on DC bus voltage sags are studied;various faults are set in DC side to study the effects of different fault types,faults locations and fault impedance on DC bus voltage sags;the single-phase ground fault and two-phase ground fault in AC grid side are set in AC side to study the effects on DC bus voltage sags.Thus the induced mechanism of DC power quality problems can be further clarified.The possible power quality problems in DC distribution network are divided into two types,events and variations,according to the characteristic difference of power quality disturbances.The event-type power quality problems include voltage sags and voltage short interruptions.The variation-type power quality problems for variations include voltage deviation,voltage fluctuations,harmonics and ripples.On basis of power quality indices particularity analysis in DC distribution network,a comprehensive evaluating method for DC power quality is proposed in this paper.The combination weighting method based on maximizing deviation is adopted to allocate the subjective and objective weight proportion fairly,thus making the decision of index weights more accurate.Fuzzy matter-element model is established to evaluate power quality of DC distribution network in order to solve the problems in DC power quality evaluation process,such as fuzziness and uncertainty.Finally,the rationality and superiority of the method is verified by a case study and methods comparison. |
