Low-Impedance Busbar Differential Protection: Zones, CT Saturation, Bus Replica and Trip Logic

The short answer

Low-impedance busbar differential protection (87B) measures every bay CT separately, normalizes the currents and applies a restrained numerical differential characteristic. It is the preferred architecture for sectionalized, double and transfer buses, mixed CT ratios, dynamic zone selection, integrated breaker failure and detailed event records.

“Low impedance” describes the relay current inputs, not weak protection. Security comes from percentage restraint, CT-saturation detection, external-fault logic, minimum pickup, bus-replica supervision and—in complex schemes—an overall check zone.

1. Differential operating quantities

Each CT input is converted to a common primary or per-unit base and corrected for configured polarity. A typical operate current is:

A common restraint definition is:

The relay operates when I_op exceeds both a minimum pickup and the selected single- or dual-slope restraint characteristic. Some relays use maximum, average or other restraint equations, so settings are not portable between manufacturers.

2. Why CT saturation is the central security challenge

For an external fault, the CT on the faulted circuit carries the sum of contributions from all sources and may saturate severely due to fault magnitude, DC offset, system X/R, remanence, ratio, core area and secondary burden. Its secondary current then fails to balance the healthy source CTs, creating false differential current.

Modern relays may use the first unsaturated samples after fault inception to classify the event as external, then switch to a high-security slope while remaining able to trip if the fault evolves inside the zone. Others use waveform, directional or saturation-detector logic. Check the minimum replica time required from each CT and prove that the selected CTs can deliver it.

Do not accept “the relay handles saturation” without a calculation using maximum fault, X/R, CT excitation data, remanence assumption, lead burden and relay algorithm requirements.

3. Worked external-fault illustration

Two normalized inputs carry +12.5 A and −12.5 A for an external fault:

If the faulted-circuit CT saturates and reproduces only −7 A:

The percentage characteristic and external-fault security logic must restrain this spill. An internal fault is different: source currents rise in the same zone-entering direction, so operate current rises immediately with restraint current.

4. Minimum pickup, slopes and unrestrained stage

• Minimum operate pickup: above CT ratio error, standing spill, noise and maximum credible non-fault leakage, but below the minimum internal fault.
• Slope 1: sensitivity and security at load and moderate through current.
• Slope 2/high-security region: stability for severe external faults and CT saturation.
• Unrestrained high-set: very fast operation for large internal faults, if secure against worst external spill and relay-specific transient conditions.
• Earth-fault sensitivity: in resistance-grounded MV systems, verify pickup below limited bus earth-fault current; do not select settings only from the maximum three-phase fault.

Settings must be demonstrated on the complete I_op/I_rest plane for load, minimum internal, maximum external, CT saturation, bay outage and future configurations.

5. CT ratio normalization is not CT replacement

Low-Z relays can compensate different CT ratios and sometimes different nominal secondary currents. This is valuable for retrofit and mixed feeders, but each CT must still meet:

• continuous and fault-current input ratings;
• required transient replica time;
• accuracy and excitation performance;
• lead/burden limit;
• minimum-fault resolution; and
• anti-saturation security requirements.

Enter actual CT tap, nominal secondary and polarity independently for every bay. A 1200/1 CT configured as 1000/1 creates 20% scaling error that can look like an internal fault during through load.

6. Selective zones

A sectionalized or double bus requires one differential zone per bus section. The relay assigns each bay current to the zone selected by the physical disconnector arrangement. A coupler or section breaker can belong to two adjacent boundaries and requires carefully defined CT and trip logic.

7. Check zone

The check zone generally sums all CTs around the complete bus arrangement and ignores some selective disconnector assignment. Requiring both the selective zone and check zone can prevent a trip caused solely by a wrong bus replica.

Check-zone implementation is project- and manufacturer-specific. Establish whether it:

• uses the same or independent CT inputs;
• covers coupler and transfer arrangements;
• is required for every trip or only selected configurations;
• remains available during maintenance;
• has independent settings and saturation logic; and
• shares common configuration or processor failure modes.

Two elements using the same incorrect CT ratios and configuration are not fully independent merely because they have different names.

8. Bus replica and disconnector status

For a double bus, a feeder current must be assigned to Bus 1, Bus 2 or a transition/combined zone. Use double-point status—both open and closed auxiliary contacts—and supervise invalid states.

Auxiliary contacts must represent main-contact timing with appropriate early-make/late-break behavior. Test the entire disconnector travel sequence, not only stable end positions.

9. Coupler and section breaker design

The coupler CT location determines zone overlap. With one CT, a fault between CT and breaker may be assigned to only one zone and remain energized from the other. Two CT sets can create overlapping zones but add inputs and complexity.

For each coupler state define:

• which zones include its current;
• which breakers trip for each zone;
• what happens if the coupler fails;
• breaker-failure expansion into both buses;
• dead-bus and transfer logic blocking; and
• which end-zone/intertrip action clears the CT-to-breaker section.

10. End-zone and stub protection

A fault between CT and breaker may be inside the bus differential zone, but opening local bus breakers may not remove remote infeed through the circuit. End-zone logic monitors breaker status and current after the bus trip and sends transfer trip or trips the remote source if current persists.

When a breaker is open and its line disconnector remains closed, the section between CT and open breaker may become a stub. The relay must know whether to keep that current in the bus zone, move it to a stub zone or apply separate overcurrent logic. Draw physical conductor and CT locations; a single-line symbol often hides the real zone.

11. Breaker-failure and trip matrix

Busbar protection should initiate 50BF for every commanded breaker. A failed feeder breaker may require upstream/remote intertrip; a failed coupler may require tripping both bus sections. Trip all sources, including generators, transformer secondaries and reverse-fed feeders. Operate lockout and block ATS/autoreclose.

Use current plus 52a/52b supervision appropriate to the breaker and CT position. Timer calculation must include trip output, maximum breaker interruption, detector reset, communications and margin. Validate at minimum DC voltage.

12. Centralized, distributed and process-bus architectures

For digital inputs, define behavior on missing, invalid, test or unsynchronized SV streams. A stale or substituted stream must never silently remain in a live differential zone.

13. CT-circuit and measurement supervision

Individual inputs allow the relay to detect persistent differential current inconsistent with voltage, breaker status or operating conditions. Configure CT-open alarms below trip sensitivity where practical. Supervision must avoid blocking a real fault indefinitely.

Verify test switches short CTs before disconnecting relay inputs, maintain a single secondary earth and provide bay/phase identification. If CTs are shared, an error or maintenance action in another protection panel becomes a busbar-protection risk.

14. Equipment inside the bus zone

VTs, station-service transformers, grounding transformers and surge arresters connected inside the CT boundary draw current that does not leave through another boundary CT. A permanent failure is correctly internal, but temporary arrester conduction or transformer inrush can challenge sensitive settings.

Assess transient magnitude and duration. Use pickup, short delay, waveform logic, dedicated CTs or equipment-zone exclusion only when justified against the required bus-fault clearing time.

15. Setting workflow

1. Draw every CT boundary, breaker, disconnector and bus operating mode.
2. Calculate maximum external fault at each bay and minimum internal phase/earth fault for every source state.
3. Collect CT ratios, class/excitation, resistance, burden, X/R and remanence assumptions.
4. Normalize ratios and establish relay base quantities.
5. Set minimum pickup from maximum spill and minimum-fault sensitivity.
6. Set slopes and external-fault security using the manufacturer’s CT saturation method.
7. Assess unrestrained high-set security.
8. Configure selective zones, check zone, transition and invalid-state logic.
9. Build trip, breaker-failure, end-zone, lockout and automation-block matrices.
10. Validate spare inputs, future bays and worst single-failure states.

16. FAT and SAT programme

1. Verify CT ratio/polarity and every normalized channel.
2. Apply through current for each bay and prove near-zero I_op.
3. Test minimum internal faults in every zone.
4. Replay maximum external faults with severe asymmetric CT saturation.
5. Test an external fault evolving into an internal fault.
6. Exercise every disconnector transition and status-discrepancy condition.
7. Test selective/check-zone coordination and all fallback states.
8. Test coupler open/closed, end zone, stub and breaker failure.
9. Fail one bay unit, communication path, SV stream, clock and DC supply at a time.
10. Operate the actual breakers and verify current interruption, not only output contact.
11. Confirm event records, time quality, SCADA and alarm clarity.
12. Preserve as-left SCL/settings, firmware and test evidence.

17. Advantages and limitations

18. When low impedance is the right choice

Select low-Z 87B when the bus is sectionalized, double or transferable; CT ratios differ; bay currents must be recorded; breaker failure/end-zone integration is required; or future expansion/process bus is planned. For a simple fixed bus with matched dedicated CTs, high impedance can remain more economical and easier to understand.

Do not decide from relay purchase price alone. Compare CT requirements, engineering, bay units, communication, testing, spares, owner skills and outage consequence over the full switchgear lifecycle.

Related guides

• MV Busbar Protection: High-Z, Low-Z and Zone-Selective Interlocking
• High-Impedance Busbar Differential Protection: CT Design and Calculations
• Breaker-Failure Protection in MV Switchgear
• Process-Bus Protection: Sampled Values and Merging Units

Engineering limitation

This guide explains a defensible engineering workflow; it is not a project setting calculation. Final protection functions, settings, wiring and trip logic must be based on the approved single-line diagram, short-circuit and coordination studies, equipment data, grid code, relay manual, and verified commissioning results. Changes require formal protection-management control.

References and further reading

1. IEEE C37.234-2021 — bus protection principles, CT/breaker location and switching arrangements
2. IEEE C37.110-2023 — CT selection, saturation and relay impact
3. SEL — High- versus Low-Impedance Bus Differential — percentage restraint, CT saturation, zones and application comparison
4. SEL — Low-Impedance Bus Differential Security and Reliability — zone switching and CT saturation security
5. Siemens SIPROTEC 5 7SS85 Manual — low-impedance busbar protection implementation
6. Hitachi Energy REB670 Product Guide — multi-zone bus protection, check zone and breaker failure
7. IEC TS 60255-216-1:2025 — functional requirements for protection with digital SV/GOOSE inputs and outputs

Standards must be applied using the edition required by the project, utility and local law. Standards summaries on public pages are not substitutes for the controlled documents.