Research On Problems In Protection Of Power Transformer And Converter Transformer

Power transformer is one of the most important equipment in power system, not only for its relatively high prize, but also because its normal operation is the key to the security and stability of the whole system. However, statistics show that the correct operation rate of the protection for power transformer stays low for years, which brings great hidden danger to power system. Especially when transformer is switched unloaded or recovered from out-of-zone fault, the generation of high inrush current usually causes mal-operation of the protection.Problems related to inrush of power transformer are studied in this paper, the change of characteristics of inrush in three-phase transformer is discussed, the influence of three-phase unsynchronous closing to protection is calculated, and the DC bias in converter transformer is categorized and calculated.First, the influencing factors in inrush of single phase transformer are analyzed. Currently existed constraining methods are reviewed, and the traditional methods of inrush constraining are compared on the basis of single phase inrush.The mechanism of mutual influence of three-phase's magnetizing circuits through circulating current of delta side in the changes of voltage of leakage reactance are analyzed, the change of characteristics of inrush caused by two compensating methods of Y→△and→Y are compared, the helping effect of circulating current and the phenomenon of symmetrical inrush are discussed, different kinds of inrush blocking principles are summarized.Three-phase's unsynchronous closing are usually caused by the aging of switches. The influence of this phenomenon to inrush is calculated. The conclusion that unsynchronous closing could bring one phase into severe saturation is driven. The compensating methods of current phase and blocking principles of inrush are compared. To balance between reliability and speed of protection, the original phase current could be used to in calculating second harmonics, and the principle of using 'or' gate constraining method and switching to phase-splitting method of constraining adaptively should be adopted.The problems existed in protection of converter transformer are reviewed, the DC bias phenomenon in converter transformer are categorized. The three-phase's DC bias caused by AC and DC lines running in parallel are calculated and analyzed in the last.