中压系统保护用电流互感器参数选择方法

通过实际试验和理论分析,阐明了电流互感器严重饱和时的特性。在通过互感器电流达到准确限值电流后继续增大,二次电流虽然不能与一次电流成比例变化,但其值(含有效值、平均值及由傅氏算法求出的基波值)却还是呈上升趁势。电流保护的整定值如小于电流互感器准确限值电流时能可靠动作,当一次电流继续增大,保护仍能可靠动作。故可按保护最大整定值确定电流保护用的电流互感器准确限值系数,以代替按短路电流选择电流互感器参数,解决长期困扰的因中压系统短路电流过大而选择电流互感器参数困难的问题。此外,还对饱和电流严重畸变时微机保护不同采样率所受影响进行了试验分析,提出在采样率较低时,应对过饱和系数有所限制,使在保护定值不超过电流互感器准确限值时,可保证在电流互感器过饱和情况下保护动作要求,并留有足够裕度。

Parameter selection of protective current transformer for medium-voltage system

Characteristics of CT(Current Transformer) in extreme saturation is illustrated by test and theoretical analysis.When the primary CT current exceeds accurate limit value,the secondary current is not proportional to it,but values such as virtual current,mean current and fundamental current calculated by Fourier algorithm are still on the increase.The current protection relay can operate properly when its setting value is less than the accurate limit current.When the primary current is continuously increasing,the relay can still properly act.Therefore the CT accurate limit coefficient can be determined by the maximal setting value of current relay,instead of the short-circuit current.Thus the difficulty in CT parameter selection due to the large short-circuit current in medium voltage system is settled.In addition,the effect of the extreme saturation current upon the different sampling rates of microprocessor protection is analyzed through experiments.For low sampling rate,the over saturation coefficient should be limited to ensure the proper operation of relay and remain enough margin when its setting value is not greater than the CT accurate limit value.