Number of wires from the secondary connection busbar to the transformer used

Typically, three to four wires are used from the secondary busbar to a transformer, depending on whether the CT secondary is star-connected and grounded.

Standard Wiring Practices

In a substation, the secondary side of current transformers (CTs) is usually connected to the busbar and then to the transformer protection or metering circuits. For a three-phase system: Star (Wye) Connection: The secondary terminals of the R, Y, and B phase CTs are connected in a star configuration. The three S1 terminals of each phase go to the respective meter or relay inputs, while the S2 terminals are linked together to form a common star point.

A single wire from this star point is then grounded and connected to the transformer secondary busbar, resulting in four wires in total: three phase wires plus one neutral/ground wire ( ). Delta Connection: Less common for CT secondaries, as it complicates metering. In this case, only three wires are used, one per phase, but additional calculations are required to convert phase current to line current ( ).

Grounding ConsiderationsOne-point grounding is critical to prevent floating potentials and ensure safety. Typically, the star point of the CT secondary is grounded at the CT terminal box before connecting to the transformer busbar ( ). Proper grounding ensures that the current flowing through the meter or relay reflects the line current accurately without introducing measurement errors.

SummaryThree-phase star-connected CT secondary: 4 wires (R, Y, B, neutral/ground) from the busbar to the transformer. Three-phase delta-connected CT secondary: 3 wires (R, Y, B) from the busbar to the transformer. The exact number may vary depending on the substation design, protection scheme, and whether the CTs are used for metering, protection, or both ( ).

This wiring ensures accurate current measurement, proper relay operation, and safe grounding practices in transformer connections.