Forums » Electrical Engineering

zero sequence Ct

    • 1 posts
    September 8, 2019 11:08 AM PDT

    How the zero sequence Ct works and its protection relay operation if there's a fault current.

    • 85 posts
    September 8, 2019 7:26 PM PDT

    It works just like an AC Clamp on Ammeter.  A GFCI, you may call them an RDC, uses this same technology to detect an imbalance of current between conductors that should be balanced.

    It is a single CT usually low ratio, 100:5 or 50:5 here in North America.

    The phase conductors feeding a load are routed through the CT window. The CT will detect any imbalance in current, similar to a residual CT connection.

    The advantage over the residual connection is that the ZST can be much more sensitive. 

    The residual connection uses the phase CTs, so they must be sized for full load current. 

    The ZST sees only the imbalance current & so the ratio can be much lower.

    For a solidly grounded system a ground fault on one phase will result in a large imbalance on the faulted conductor & the current will return to the source using a path that is outside the CT window.  The ZST will output a proportional current to the protective relay which in turn signals a breaker to open. The relay must be set above any normal imbalance in current. 

    The old name for this transformer is BYZ or Balanced Wye Zero Sequence Current Transformer.

    Find a clamp on ammeter & experiment, you do not need three phase. A single phase load is sufficient.

    1. Run all conductors through the window & verify the reading is zero.  Electricians use this technique to check for ground faults or leakage currents on circuit conductors & loads.  

    2. Run a single ungrounded conductor through the window & verify the meter is reading full current. 

    The ZST is used as a permanent installation of experiment 1.