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Curernt transformer accuracy

    • 2 posts
    July 8, 2019 7:36 AM PDT

    Hi All,

    I want to know the curernt transformer accuracy.
    1. if the secondary voltage is increased, does this effect the CT accuracy
    2. Suppose a CT with 20/5 and the current is inscreased from 25 ampere to 38 ampere and thermal factor is about 1.9, is this in accuracy range?

    • 103 posts
    July 8, 2019 9:35 PM PDT

    Would you like to tell us a little more about the CT ? 

    With the supplied information:

    You are probably still in the accuracy range if it has a 10A secondary rating.

    Since the CT was not selected based on the new current the conductor may be over loaded & push the CT beyond it's thermal limit.  The CTs we use in switchgear have a 10A secondary.  Due to the ambient temperature (50 degrees C) they can only be pushed to 1.5x.  I have not yet in nearly 40 years seen any used at more than the 5A secondary rating. 

    • 200 posts
    July 10, 2019 7:00 PM PDT

    Thanks for the suggestion of this topic, Aldo.

    Before try to answer your questions, it is suggested to take a look on:
    Current transformer

    Check if this topic could be of any help.

    Aldo Foley said:

    1. if the secondary voltage is increased, does this effect the CT accuracy

    Yes, if the voltage is higher than Knee-point core-saturation voltage, this knee-point is the Secondary voltage above which the Secondary current is NOT linear anymore in relation to the Primary current, making the CT away from the rated accuracy.

    The relation between Primary current (Ipri) and Secondary current (Isec) is considered LINEAR if the mathematical equation or function Isec = f(Ipri):

    - is of the type: Isec = f(Ipri) = Ipri x a + b ;
    - All other equations and functions Isec = f(Ipri) different from the type above are considered NON-LINEAR;

    By standard, the Knee-point core-saturation voltage is the voltage at which a 10% increase in Secondary voltage increases the Secondary current by 50%. Since application was not given:

    - Metering CTs - linear measurement usage over a range: high accuracy but inside of a small range of the CT rating, usually from 1.2 to 1.5 times rated current established 5A by Industry standards as the Secondary rating for CTs, as your example, the Metering CT (Secondary) rating probably would be something between 6A (= 1.2 x 5A) to 7.5A (= 1.5 x 5A). If we assume, just for this example, 120V as Secondary voltage across a 24 ohms resistive load (120V = 5A x 24ohms), just like Industry standards for Potential Transformers with Vpri <= 24kV that sets 120V on Secondary, the knee-point core-saturation voltage would be at 144V to 180V - 20% to 50% increment on Secondary voltage, and therefore Secondary current, and Primary current. Using your example: 25A to 38A in Primary of 20:5 CT, assuming it is a Metering CT, the "normal" 25A is already high for Metering and 38A means 52% from this "normal" rate - probably this CT already reach knee-point core-saturation voltage depending on the load connected to Secondary - if the Metering CT is correctly sized this saturation point is rarely a concern;

    - Protection CTs - on/off monitoring usage around a set-point: these CTs are often exposed to high Fault currents on Primary side sometimes 20 to 30 times rated current, or 2000% to 3000% - therefore the knee-point is frequently reached. In such cases, the set-points of instruments and relays connected to Secondary shall be inside of the linear region of Protection CTs curves as best as possible. Even so depending on the Fault current the accuracy decreases too much to have a precise measure of a trip point during a Fault event. 


    Aldo Foley said:

    2. Suppose a CT with 20/5 and the current is increased from 25 ampere to 38 ampere and thermal factor is about 1.9, is this in accuracy range?

    The Industry set standard ambient temperature at 30oC or at 55oC. As example: a typical rating factor 2.0, assuming 30oC standard, then the rating would be "RF 2.0 @ 30oC", meaning at 30oC ambient, the CT will safely carry a current 2 times the nameplate rating current in continuous duty.

    If we assume your (rare) example of RF = 1.9, and the 20:5 CT was correctly sized (maximized for Metering at 1.5, please see item above) for 30A rating (= 1.5 x 20A) on nameplate, then the CT can carry 57A (= 1.9 x 30A) at 30oC ambient in continuous duty, no problem.

    However, again, an increment of 25A to 38A for a 20:5 Metering CT would affect the accuracy. It is assumed your CT is a Metering CT because 20:5 would be a relatively low rating for Protection CT, a typical Protection CT would be 200:5 or 400:5 to monitor high Fault currents.

    Anyway, if your example 20:5 CT is a Protection CT, 38A is not a big difference from 20A or 25A to reach a protection/trip set-point, the accuracy can be reduced after (higher than) the set-point of the instrument connected to the Secondary.

    I hope this helps. Regards.

    This post was edited by Alex de Moura at July 11, 2019 4:35 AM PDT
    • 1 posts
    April 12, 2020 6:31 AM PDT

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