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Differential protection for converter transformers

机译:换流变压器的差动保护

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This paper will present differential protection schemes used for special Converter Transformers. These transformers are typically used to feed MV drives, power electronic or FACTS devises. Converter transformers are industrial power transformers which may have phase angle shift different from standard 30º or a multiple of 30º. Typical example is 24-pulse converter transformer with additional phase angle shift Θ of ±7,5º. Such special industrial transformers typically have three windings, but sometimes even up to five windings. The converter transformer additional phase angle shift Θ is typically obtained by special connections of its HV winding (e.g. extended-delta or zig-zag). However, if numerical differential protection for standard three-phase power transformers is directly applied for differential protection of such converter transformer, it will not be possible to compensate for additional, non-standard phase angle shift Θ caused by special winding connections. As result a permanent false differential current would appear. For the worst case when Θ=15º the false differential current of up to 26% of the through-load current will appear. Thus, the minimum pickup of the differential protection must be increased to at least double of this value and as a consequence differential relay will not be sensitive for the low level internal faults within the protected converter transformer. Therefore a special differential protection solution is often required. The paper will provide information how to design such differential schemes and present current waveforms from DRs captured in actual installations.
机译:本文将介绍用于特殊转换器变压器的差动保护方案。这些变压器通常用于为中压变频器,电力电子设备或FACTS设备供电。换流变压器是工业用变压器,其相角偏移可能不同于标准的30º或30º的倍数。典型示例是具有±7,5º的附加相角偏移θ的24脉冲转换器变压器。这种特殊的工业变压器通常具有三个绕组,但有时甚至多达五个绕组。换流变压器的附加相角偏移Θ通常是通过其HV绕组的特殊连接(例如扩展三角形或之字形)获得的。但是,如果将标准三相电力变压器的数值差动保护直接用于这种变流器变压器的差动保护,则将无法补偿由特殊绕组连接引起的额外的非标准相角偏移Θ。结果将出现永久性的错误电流。在最坏的情况下,当Θ=15º时,会出现高达整个负载电流26%的虚假差分电流。因此,必须将差动保护的最小接点增加到该值的至少两倍,结果,差动继电器对受保护的换流变压器内的低电平内部故障将不敏感。因此,通常需要特殊的差动保护解决方案。本文将提供信息,说明如何设计这种差分方案以及如何从实际安装中捕获的DR中呈现电流波形。

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