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Circulating current vs balanced voltage differential

Short answer

In circulating current, CTs supply current into a low-burden loop and the relay is shunted across pilots. In balanced voltage, CT secondaries are effectively open-circuited, their emfs oppose and cancel, relays are in series; air-gapped CTs are needed to avoid saturation.

State the difference between circulating current differential protection and balanced voltage differential protection with reference to behavior of CTs.

Both are differential (unit) protection schemes that compare quantities at the two ends of a protected zone. In the circulating current scheme the CT secondary currents circulate through the pilots, and the relay is connected across the pilots at the equipotential point. In the balanced (opposed) voltage scheme the CT secondary voltages are opposed in series through the pilots, so no current flows normally, and relays are connected in series with the pilots.

Circulating current:          Balanced voltage:
 CT1 ──── pilot ──── CT2       CT1 ──R1── pilot ──R2── CT2
  │        │          │         │                      │
  └──── pilot ────────┘         └────── pilot ─────────┘
          [R] (shunt)             relays in series;
     relay across pilots          CTs oppose each other
PointCirculating currentBalanced voltage
CT secondary stateCTs supply current into a low-burden loop (normal CT duty)CT secondaries are effectively open-circuited for through current
CT type neededNormal CTs; must be matched to avoid spill currentAir-gapped or special CTs (linear couplers) so that the secondary emf is proportional to primary current without saturation
Pilot current (normal / external fault)Large current circulates in pilotsIdeally zero current in pilots
Relay connectionShunt, across the pilots at the mid-pointSeries with the pilots at each end
Effect of CT mismatch/saturationSpill current through relay; needs bias or high-impedance relayUnequal voltages drive current; needs CTs with identical linear characteristics
Effect of pilot capacitanceLittle effectCharging current through pilot capacitance can cause false trip on long lines
Pilot lengthBest for short distances (generators, transformers, busbars)Suits longer feeders
Action on internal faultUnbalanced current flows through the relayVoltages no longer balance; current flows through relays in series

Summary: in the circulating current scheme the CTs work as normal current sources in a closed loop, while in the balanced voltage scheme the CTs work like open-circuited voltage sources whose emfs cancel, so they must be air-gapped to avoid saturation and overheating.

✓ Answer checked by Claude Opus 5.5 (high effort)

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