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Current transformer

    • 15 posts
    May 15, 2019 10:45 PM PDT

    having a current transformer 500:5, 5P5, 30VA for a motor with a full current of 700A. The short circuit current is estimated around 25kA. If the there is a fault, will the Current transformers will go to saturation?

    • 200 posts
    May 25, 2019 10:41 PM PDT

    Thanks for the suggestion of this topic, Pedro. Sorry for the late reply.

    Let's start with CT codes for the sake of knowledge of all Z4E members, the Current Transformer you presented better in this way:

    500:5, 30VA, 5P 5 - meaning:
    500 - 500A rated primary current;
    ___ 5 - 5A rated secondary current;
    _____ 30VA - accuracy power or load;
    __________ 5P - accuracy class: 5%;
    _____________ 5 - accuracy limit factor (ALF), meaning the ratio 500:5 will have an error less than 5% at 5In and at 30VA load;

    Reading of a question with similar CT was made at "CT saturation during unbalanced faults" would be valuable. At site:
    https://www.eng-tips.com/viewthread.cfm?qid=448600


    under the name "WHiPCPL".

    Not sure which application the CT is being used so let's see some:
    - A Protection CT must saturate sufficiently high to allow a relatively accurate measurement of the fault current. Protection CTs are expected to have an Accuracy Limit Factor (ALF) that is usually fairly high (supposedly is not your case, ALF=5);

    - An Instrument CT requires good accuracy around the nominal current value, do not need to withstand currents as
    high as other protection devices. The Instrument CTs, unlike the Protection CTs, have the lowest possible Safety Factor (SF) in order to protect the instruments through earlier saturation, in order words, low saturation to protect the Ammeter/Ampere-meter;

    - Non-magnetic CTs, also known as ROGOWSKI coils, is a voltage proportional to the variation of the primary current and they do not saturate and their response is linear, so they can be used over wide current ranges - hence they have limitations concern protection;

    Some notes on saturation:
    - To ensure that the CT will not affect the operating accuracy of the protection, it is necessary to have no saturation up to the setting point;
    - In the 3 CT arrangement, it is preferable, whenever possible, to use a toroidal CT around the 3 phases, the 3 phases must be placed in the center of the toroid to prevent local saturation of the magnetic material;
    - Just as for the earth fault protections, DC components may result in transient saturation of the CTs and generate a false differential current;
    - A high fault current flowing through the monitored conductors can cause CT saturation, and the risk of nuisance tripping of the protection (non stability);

    I hope this helps.

    Regards.


    This post was edited by Alex de Moura at May 25, 2019 10:45 PM PDT