What are Summation CT (Current Transformers) ?


WA Summation Current Transformer (Summation CT) combines the secondary currents from multiple main CTs (each monitoring a different circuit) into a single output current. This summed current is proportional to the total current flowing across those circuits and is used for centralized measurement or protection.

It works by having multiple primary windings (one connected to each main CT's secondary) and a single secondary winding. The turns ratios of the Summation CT's primaries are designed to match the main CTs' ratios for accurate summation. The input currents must be in phase for correct arithmetic addition.

Summation CTs are used for:

  • Metering multiple feeders with a single meter.
  • Triggering protective relays based on the total current in multiple circuits.
  • Converting three-phase currents into a single-phase equivalent for measurement.

Essentially, they provide a consolidated view of current flow across several circuits, simplifying monitoring and protection strategies.

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