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1高壓差動保護(hù)基本原理
高阻抗保護(hù)系統(tǒng)是一種簡單的技術(shù)，它要求保護(hù)方案中使用的所有CT都具有相對較高的拐點(diǎn)電壓、相似的磁化特性和相同的比率。這些CT應(yīng)安裝在受保護(hù)對象的所有端部。為了制定方案，屬于一個相位的所有CT應(yīng)并聯(lián)連接。從CT連接點(diǎn)連接一個測量支路。測量支路是一個具有高歐姆值的可變設(shè)定電阻器（Rs）和過電流繼電器的串聯(lián)。因此，高阻抗擴(kuò)散保護(hù)對流過測量brunch的電流作出響應(yīng)。然而，該電流是由測量支路上的并聯(lián)CT連接引起的差分電壓的結(jié)果。此電流和電壓通過歐姆低相互關(guān)聯(lián)。典型的高阻抗差分方案如圖1所示。請注意，此圖中只顯示了一個階段。
由于并聯(lián)CT連接，高阻抗差動繼電器只能測量操作量。這意味著在這樣的方案中沒有任何穩(wěn)定的數(shù)量。因此，為了保證繼電器在外部故障期間的穩(wěn)定性，操作量不得超過設(shè)定的拾取值。因此，對于內(nèi)部故障，即使某些電流互感器嚴(yán)重飽和，CT并聯(lián)點(diǎn)上的電壓也不應(yīng)上升到繼電器拾取值以上。為了實現(xiàn)設(shè)置電阻器Rs的合適值的選擇，應(yīng)使飽和CT提供比測量支路低得多的阻抗路徑。在外部故障導(dǎo)致電流互感器飽和的情況下，非飽和電流互感器將驅(qū)動大部分溢出差動電流通過飽和電流互感機(jī)的二次繞組，而不是通過測量brunch。飽和電流變壓器次級繞組上的電壓降也會出現(xiàn)在測量brunch上，但通常會相對較小。因此，繼電器的拾取值必須設(shè)置為高于該錯誤操作電壓。
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.
