| The low voltage variable-frequency drive(VFD)is a typical device sensitive to voltage sags.Therefore,it is very concerned for users to study its tolerance characteristics and compatibility ability to voltage sags in distribution system.Consequently,a method is proposed by this paper to analyze the compatibility ability of VFDs with sags involving the compatibility probability of VFDs in uncertain region based on mechanism analysis,simulation,and experimental research.The performance and mechanism of how sags influence VFDs are deeply analyzed by theoretical and simulation analysis.Furtherly,universal voltage tolerance curves of VFDs under three-phase sags,two-phase sags,and single-phase sags are obtained based on experimental data.The main steps are as followings:First,sag characteristics that may influence voltage tolerance ability of VFDs are discussed.According to the circuit topology of the VFDs,the mathematical formulas of relative signals are deduced to analyze the influence mechanism of various parameters.Some of these parameters are related to the equipment including low voltage protection threshold,overcurrent protection threshold,load torque,and motor speed,while others are characteristics of voltage sags which include sag types,multiple sags,continuous sag,etc.Meanwhile,a simulation model is built to study the influence of the main factors on the DC voltage and motor speed of VFD.The mechanism analysis and simulation study are verified by each other,which lay a theoretical foundation for drawing up a suitable experimental scheme.Next,the Chroma Model 61860 is used as the voltage disturbance source to set up a test platform and a test scheme is presented.7 representative brands of VFDs in products domestic market with small and medium power(7.5 kW&18.5 kW)are selected as the sample.Then various parameters related to equipment and characteristics of voltage sags are tested on the sample VFDs.Based on data analysis,plenty of voltage tolerance curves of VFDs under different influence factors are drawn.Further,universal tolerance curves of VFDs under three-phase sags,two-phase sags,and single-phase sags are obtained by comprehensive analysis and treatment of more than 13000 groups of test data,making up for the ignorance of sag type and operating condition of equipment in exiting standard curves.The universal curves can provide data foundation for analysis of sag tolerance ability of VFDs and for solution strategies as well as reference resources for standard setting.Finally,the compatibility ability of low-voltage VFDs with sags in distribution system is studied.A method is proposed to analyze the compatibility ability of VFDs with sags combined with the maximum entropy method.The modified maximum entropy method by logarithmic transformation is used to construct the fault probability distribution functions of VFDs in uncertain regions based on the test data.Similarly,the probability distribution functions of amplitude and duration of sags in the distribution system are also formed by the modified maximum entropy method.According to the obtained probability distribution functions,the compatibility probability in uncertain area and total numbers for no trip are calculated under the three kinds of sags types.Further,the validity,accuracy,and adaptability of the presented method are testified by comparison with other probabilistic fitting methods and a traditional method of compatibility analysis. |
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