A current transformer (CT) can be included in a transformer’s neutral connection by wrapping it around the neutral conductor that connects the transformer’s neutral bushing to the earth (ground). This configuration is generally employed for restricted earth fault (REF) (or) residual earth fault protection.
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The CT is installed on the neutral-to-earth wire, thus it only detects neutral current flowing to the ground during a fault.
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It must be as close to the transformer neutral bushing as feasible to avoid interfering with other return lines.
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The CT ratio is calculated using the transformer’s neutral current under fault conditions, not full load current.
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The CT secondary is linked to the appropriate protective relay (e.g., earth fault relay or REF relay).
This configuration detects earth faults in the transformer winding zone that shall not be seen from phase CTs, resulting in faster and more sensitive protection.
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