| In the current power grid environment,distribution generation(DG)and power electronic load present the characteristics of high penetration,diversification,and decentralization,thereby leading to serious power quality problems in distribution networks.The safe and stable operations of power generation and power utilization equipments are subjected to grave threats.Meanwhile,some precise digitized DGs are highly sensitive to power quality pollution and raise power quality demands,which make the prominent contradiction between power quality and equipments.The existing comprehensive assessment methods for power quality could not find the subject of quality evaluation from the perspective of the essence and demand,and this subject should be the equipments connected to power systems.Moreover,high penetrative,diversified,and decentralized DGs and load equipments casue complicated pollution distributions,the uncertainty of DG output powers and the variability of load equipment operations result in dynamic pollution scenarios,along with constantly changed and adjusted network structures,these make difficulties in building accurate models of the pollution and structure parameters.At present,the control strategies for harmonic distortion and voltage deviation based on mechanism modelings could not perfectly solve this problem.Therefore,this paper carries out a research that focuses on the subjective problem of comprehensive power quality assessments and the limitations of harmonic and voltage controls used mechanism modelings.The research contents are as follows:First of all,aiming at the the subjective problem of comprehensive assessments for power quality,considering the essence and demand of power quality,a method of a state space representation and a comprehensive assessment of compatibility level is proposed for power quality.The compatibility between power quality and equipments is analyzed.A power quality state space representation model is constructed to describe power quality pollution level in different pollution scenarios.A tolerance boundary surface is established to represent the power quality pollution tolerance of equipments.The compatibility/incompatibility degree,influence degree,and performance indices of power quality are defined to respectively reflect the severity degree of power quality affecting equipments,the magnitude and scope of power quality affecting the equipments in the incompatible status,and the quality level is to equipments.The above indices are used to implement the comprehensive assessment of compatibility level at each bus.A combined weighting model comprehensively taking into account quality level and equipment capacity is employed to assign the weights of assessment results of buses,thereby obtaining the assessment results of the whole region.Example analyses illustrate the rationality and validity of the defined indices.Secondly,on the basis of the state space representation and the comprehensive assessment of compatibility level for power quality,taking into account the practical situation of equipment tolerances,a comprehensive assessment method of compatibility level considering uncertainty is proposed for power quality.A tolerance boundary cloud is designed by utilizing cloud generators and a cloud fusion model to represent the uncertainty of the pollution tolerance of equipments.The power quality compatibile and incompatibile statuses are divided detailedly.The fuzzy membership function is employed torepresenttheuncertaintyofequipmentfaultstate.Theabsolute compatibility/incompatibility degree,fuzzy compatibility/incompatibility degree,fuzzy influence degree,and fuzzy performance indices of power quality are defined to more practically and fully implement the comprehensive assessments of compatibility levels for power quality of buses and the whole region.Example analyses verify the rationality and accuracy of the defined indices.Subsequently,utilizing the control basises of single pollution provided by the comprehensive assessment method of compatibility level considering uncertainty,focusing on the limitations of the control strategies based on mechanism modelings,this paper adopts the idea of data-driven modeling and proposes a partition control strategy based on pattern matching of univariate time series for single pollution.A piecewise linear approximation(PLA)taking into account turning points of critical trends is employed to extract major local trend features and their time features of univariate time series of total harmonic distortion of voltage(THD_V)and deviation of supply voltage(DSV),thereby establishing pattern feature matrices.A two dimensional feature distance(2DFD)method is developed to implement the pattern matching on the index series to measure the coupling degrees ofTHD_V or DSV among buses.The control targets ofTHD_V and DSV are respectively reset in the absolute compatible status of power quality.A partition criterion based the coupling degree is defined to divide control areas.Dominant control buses and control device compensation rates are determined in the identified areas.Example simulation analyses illustrate the feasibility and validity of the proposed strategy.The 2DFD method is compared to other pattern matching methods for univariate time series to verify the accuracy and applicability of this method.Finally,utilizing the control basises of combined pollution provided by the comprehensive assessment method of compatibility level considering uncertainty,using multifunctional active type control devices,this paper proposes a comprehensive partition control strategy based on pattern matching of multivariate time series for harmonic and voltage.A multidimensional piecewise linear approximation(MPLA)taking into account turning points of critical trends is employed to extract major local trend features,time features,and pollution severity features of multivariate time series ofTHD_V and DSV,thereby establishing pattern feature matrices.A three dimensional feature distance(3DFD)method is developed to implement the pattern matching on the index series to measure the overall coupling degrees ofTHD_V and DSV among buses.The comnprehensive improvement targets ofTHD_V and DSV are reset in the absolute compatible status of power quality.Comprehensive control areas are divided according to the partition criterion based the coupling degree.Dominant control buses and control device compensation rates are determined in the identified areas.Example simulation analyses illustrate the feasibility and validity of the proposed strategy.The 3DFD method is compared to other pattern matching methods for multivariate time series to verify the accuracy and applicability of this method. |
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