It will work till you get a fault. Bcoz, 5P20 class have lower kneepoint voltage than PS class and so, it will get saturated soon for higher fault currents. So it will be giving nuisance trippings for external faults too. PS class have higher saturation point and your protection will be stable even for external faults which draw large currents. So its preferable to use only PS class cores for special protection for better stability and sensitivity.
-Vijay
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Multifunction Numerical Relays beeing used these days receive CT secondary input from a CT core having 5P20 class to provide all types of relevant protections including differential protection. According to Relay manufacturers this is OK.What are your comments/observations ?
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I beg to differ. a 5P Class CT's composite error is + / - 5%. That means a + 5% CT also will be labelled as 5P and a - 5% CT too. If for the differential protection, these CTs are used, it may so happen that the Primary Side CT is 5P with +5% error and the Secondary Side CT is also 5P, but with a -5% error. Under such a condition, if the relay bias setting is 10% or less, even during normal operations, there can be 10% differential current and the relay would operate. This is not preferable. So, one has to go for only PS Class CTs for differential.
Differential CT
Re: Differential CT
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