HIGH-Z
Three circuits on a 220 kV bus, Class PX CTs paralleled onto a high-impedance circulating-current zone. Shunt resistors turn spill current into voltage. 87BZ and the supervisory element measure that voltage.
Stable — no operate
Load current cancels
Infeeds and outfeeds sum to zero at the bus. Secondary currents circulate between CTs and almost none flows in the shunt resistors, so the voltage across them stays near zero.
Swipe the scheme to follow the wiring
Differential current
0 A
Σ secondary (S1 sense)
Voltage across shunts
0 V
Idiff × 2000 Ω ∥ 2000 Ω
Relay states
Both idle
87BZ 150 V · 87S 25 V
Every circuit on the protected bus has a dedicated Class PX core, same ratio, P1 toward the bus. All S1 terminals land on one polarity bus; all S2 terminals land on one residual bus. Kirchhoff then does the comparison: any current that enters the bus must leave it, so the secondary currents cancel and nothing is left to push through the shunts.
The paralleled secondaries are closed through two high-value resistors — Rstab for the bus-zone differential and Rsup for the supervisory. Spill current Idiff develops V = Idiff × (Rstab ∥ Rsup). 87BZ and 87S are voltage-operated elements measuring across those resistors. They are not series current coils.
External fault current circulates around the CT secondaries and barely enters the high-impedance branch. Even if one CT saturates (its magnetising branch collapses toward a short), the healthy CTs still cannot force enough current through Rs to reach the 150 V setting. Typical rule of thumb: knee-point Vk ≥ 2 Vs, with Vs sized from Ifault (Rct + 2 Rl).
On a bus fault every infeed adds. Idiff is large, the theoretical voltage is kilovolts, and the Metrosil (silicon-carbide VDR) conducts to clip the voltage so the CTs and wiring survive. Both relays see the clamped voltage and operate.
87S is set well below 87BZ (25 V vs 150 V here). An open CT secondary, a high-resistance joint, or a wiring earth leaves a standing voltage from load current. Supervisory alarms without tripping the bus. Many HV boards also AND a check zone — a second high-Z circuit on a different core set — before issuing a trip.