Study On On-line Monitoring And Assessment For Health Status Of Distribution Transformers

Transformer is one of the most important electrical equipment in power system, theThe distribution transformer (DT) are directly connected to the users with largeinstallation number, the health level and operation status of which determines thesecurity and reliability of power supply and the entire power grid. In order to get theinformation of DT’s online operation status and health condition, and to provide thetechnical support for the state overhaul of DT, according to the corresponding nationalstandards and industry standards, we get the online electrical parameters as the mainstatus indicators in this paper, and take other state information as assistance. The onlineevaluation method for DT’s health is studied. The research contents include thefollowing aspects:①A new online monitoring method for winding deformation of DT is proposed.Short-circuit reactance (SCR) of transformers in service is calculated, and thedeformation of transformer windings could be reflected through online measurement ofthe voltage and current at primary side and secondary side with different loads. Thisnew method does not need zeroing in no-load and is not affected by the excitationcurrent and variation of prime side current of distribution transformer in service. Themethod has been used in distribution transformers (DT) of different capacity whoseinductances changed with different variations for imitating different windingsdeformation in DT. The simulation results verified the feasibility and effectiveness ofthe proposed method for on-line monitoring of windings deformation in DT. Taking thelimiting value of electric power industry standards and national standard as basis, thevariation of SCR (vertical comparison) and the difference between three-phase SCR(horizontal comparison) are used to analyze the health condition of the transformerwindings.②A new online monitoring method for load loss and no-load loss of DT isproposed, which is used to on-line detect and analyze the health condition oftransformer winding and iron core, to judge whether existing windings short circuit andother overall defects. In the same way, short-circuit resistance of transformers in serviceis calculated through online measurement of the voltage and current at primary side andsecondary side with different loads. Then the copper loss and iron loss are obtained, andthen the load loss and no-load loss are calculated. Based on the national standard, respectively comparing to, including vertical comparison and horizontal comparison,the load loss and no-load loss, the DT health state is evaluated.③After the short circuit resistance and the resistance are online measured, theshort circuit impedance is obtained. Then with short circuit impedance, an onlinemethod for transformer capacity is proposed. This method is used to solve suchquestions like that the actual operation of nameplate is missing, nameplate capacity isnot match with the actual and so on. Then electric power company could charge correctfee, to adjust power supply load in power system, to avoid overload or light load, and tomake the transformer operate in the best status, which is of great significance.④An on-line data acquisition control module which is suitable for the methodabove is designed. Online assessment system and methods for DT health status arestudied, which takes electrical parameters as the main factors, auxiliary of the otherstate information’s. And DT health status online evaluation simulation platform is set up.In order to give full consideration to middle level between the transition states and tosolve the boundary between the fuzzy address level, the subjection function of electricalparameters and other auxiliary status information are established, in which quantitativeindex is dealed with half trapezoid and half ridge subjection function and qualitativeindex is dealed with half trapezoidal and triangular subjection function. The hierarchicalmodel of transformer state evaluation is established, and the comprehensive scoringrules and methods is designed. The final evaluation results of transformer can bedivided in to four conditions: in good condition, alert condition, abnormal condition,severely abnormal condition.Several distribution transformers with different capacitywhose windings are in severe deformation are taken as example. Their conditions areevaluated in simulation platform, then the feasibility and validity of the simulationplatform are proved by the results of test.By monitoring the electrical parameters of distribution transformer in service, thencomprehensive assessment of its healthy status can be obtained.