Bus protection — ANSI 87B differential

Reference notes

Substation bus protection demands the fastest, most-selective protection in the substation. Bus faults are rare but catastrophic — every connected feeder feeds fault current into a single point. ANSI 87B differential is the standard primary scheme. Use Next → to walk through high-impedance vs low-impedance numerical 87B, zone-interlocked OC alternatives, and 87B + 50BF coordination.

The bus-protection problem

• Bus = node where many feeders connect.
• Bus faults are rare (modern switchgear has low failure rate) but catastrophic — cumulative fault current 50-100+ kA from all feeders simultaneously.
• Mechanical forces (∝ I²) can shatter porcelain and propagate damage.
• Must clear in 1-2 cycles, selectively (no false trips on through-faults).

ANSI 87B principle

Kirchhoff's Current Law applied to the bus node:

All feeder CT secondary currents are summed algebraically. External (through) faults balance (current in = current out), so the sum stays at zero. Internal bus faults break the balance.

High-impedance 87B (classical)

• All CT secondaries paralleled and connected to a high-impedance voltage relay (typically a stabilizing resistor of several kΩ).
• Through-fault: parallel CTs balance, current circulates among them rather than flowing through the high-Z element. Relay stable.
• Internal fault: CTs lose balancing partners; fault current MUST flow through the high-Z relay element, developing voltage. Trip if voltage exceeds setting.
• Pro: extremely robust against CT saturation during severe through-faults — the high-Z forces voltage, decoupling from CT linearity.
• Pro: cheap, simple, no communications needed.
• Con: ALL CTs must have identical ratios.
• Con: topology changes (adding a feeder, switching CT into service) require re-engineering.
• Best for STATIC bus topologies.

Low-impedance numerical 87B (modern)

• Each CT to a separate relay analog input. No paralleling.
• Microprocessor computes differential and restraint quantities per zone in software.
• Software accommodates: different CT ratios per feeder, switching status changes (disconnector/breaker positions), multi-zone topology (ring bus, double-bus single-breaker, breaker-and-a-half).
• Dynamic-zone logic: as bus topology changes, differential zones reconfigure automatically.
• Products: SEL-487B, ABB REB670, GE Multilin B30, Siemens SIPROTEC 7SS8.
• Modern variant uses IEC 61850 sampled values to bring CT data over Ethernet from per-feeder merging units — eliminates copper CT runs.

Zone-interlocked overcurrent (cheaper alternative)

For lower-cost distribution buses (4-25 kV) where dedicated 87B isn't economic:

• Each feeder relay sends a BLOCK signal to the bus relay when it sees forward fault current.
• Bus relay (high-set OC on source side) waits 50-100 ms; if no block arrives, the fault must be on the bus itself → trip.
• Pro: no dedicated bus CTs, much cheaper.
• Con: slower (6-10 cycles vs 87B's 1-2).
• Con: relies on every feeder relay being properly configured.
• Con: doesn't catch high-impedance bus faults below feeder pickup.
• Modern relays implementing this: SEL-451, GE F60, ABB REF615.

87B + 50BF coordination

When 87B trips, it commands ALL breakers on the bus to open. 50BF monitors each breaker:

• If a breaker fails to clear current within 100-200 ms, 50BF escalates.
• Typical action: trip source-side breakers on the adjacent bus zone to isolate the fault through a different path.
• Or send Direct Transfer Trip (DTT) to remote line ends to remove generation infeed.
• 50BF is implemented within modern 87B relays or as separate per-breaker devices.

Multi-zone configurations

Complex bus topologies — ring bus, breaker-and-a-half, double-bus single-breaker — have multiple bus zones connected in series via bus-tie breakers. Modern numerical 87B supports up to ~20 zones simultaneously. Each zone has its own differential element; only the breakers around the faulted zone are tripped, leaving other zones in service.

Redundancy for critical buses

Per NERC PRC standards on bulk-electric-system buses (typically 230 kV+, universal at 500 kV and 765 kV): TWO complete independent 87B schemes from different manufacturers, with separate CT cores (often dual-core CTs), separate trip paths to dual breaker trip coils (TC1 / TC2), separate DC supplies. Trip outputs OR'd.

Testing & commissioning

Secondary current injection at each CT simulates a fault to verify the differential element operates correctly. Annual to biannual cycle per IEEE C37.230.