1高压差动保护基本原理
高阻抗保护系统是一种简单的技术，它要求保护方案中使用的所有CT都具有相对较高的拐点电压、相似的磁化特性和相同的比率。这些CT应安装在受保护对象的所有端部。为了制定方案，属于一个相位的所有CT应并联连接。从CT连接点连接一个测量支路。测量支路是一个具有高欧姆值的可变设定电阻器（Rs）和过电流继电器的串联。因此，高阻抗扩散保护对流过测量brunch的电流作出响应。然而，该电流是由测量支路上的并联CT连接引起的差分电压的结果。此电流和电压通过欧姆低相互关联。典型的高阻抗差分方案如图1所示。请注意，此图中只显示了一个阶段。
由于并联CT连接，高阻抗差动继电器只能测量操作量。这意味着在这样的方案中没有任何稳定的数量。因此，为了保证继电器在外部故障期间的稳定性，操作量不得超过设定的拾取值。因此，对于内部故障，即使某些电流互感器严重饱和，CT并联点上的电压也不应上升到继电器拾取值以上。为了实现设置电阻器Rs的合适值的选择，应使饱和CT提供比测量支路低得多的阻抗路径。在外部故障导致电流互感器饱和的情况下，非饱和电流互感器将驱动大部分溢出差动电流通过饱和电流互感机的二次绕组，而不是通过测量brunch。饱和电流变压器次级绕组上的电压降也会出现在测量brunch上，但通常会相对较小。因此，继电器的拾取值必须设置为高于该错误操作电压。

1 Highlmpedance Differential Protection Basic Principles
High impedance protection system is a simple technique which requires that all CTs.used in the protection scheme, have relatively high knee point voltage, similarmagnetizing characteristic and the same ratio. These CT shall be installed in all endsof the protected object. in order to make a scheme all CTs belonging to one phaseshall be connected in parallel. From the CT junction points a measuring branch isconnected. The measuring branch is a series connection of one variable settingresistor (Rs) with high ohmic value and overcurrent relay. Thus, the high impedancediferential protection responds to a current flowing through the measuring brunch.However, this current is result of a differential voltage caused by this parallel CTconnection across the measuring branch. This current and voltage are interrelated byOhms Low. Typical high impedance differential scheme is shown in Figure 1. Notethat only one phase is shown in this figure.
Due to the parallel CT connections the high impedance differential relay can onlymeasure the operating quantity. That means that there is no any stabilizing quantityin such schemes. Therefore in order to guaranty the stability of the relay duringexternal faults the operating quantity must not exceed the set pickup value. Thus, forexternal faults, even with severe saturation of some of the current transformers, thevoltage across the CT paralleling point shall not rise above the relay pickup value. Toachieve that a suitable value for setting resistor Rs shall be selected in such a waythat the saturated CT provides a much lower impedance path as compared with themeasuring branch. In case of an external fault causing current transformer saturationthe non-saturated current transformers will drive most of the spill differential currentthrough the secondary winding of the saturated current transformer and not throughthe measuring brunch. The voltage drop across the saturated current transformersecondary winding will appear also across the measuring brunch, however it wiltypically be relatively small. Therefore, the pick-up value of the relay has to be setabove this false operating voltage.