High-Impedance Busbar Differential Protection: CT Design, Stability Voltage, Resistors and Commissioning

A calculation-led guide to high-impedance 87B protection for MV busbars: CT matching, stability voltage, pickup sensitivity, stabilizing resistors, MOV sizing, trip logic and field testing.

Technical review date: 19 August 2026 · Audience: busbar protection engineers, MV switchgear designers and commissioning specialists

The short answer

High-impedance busbar differential protection (87B or 87Z) is a fast unit scheme for a fixed bus zone. For each phase, all boundary CT secondaries are connected in parallel to a high-impedance relay branch. Correctly matched CT currents circulate during load and external faults. An internal bus fault produces spill current and a high relay voltage that trips every breaker feeding the zone.

The high impedance is not added to improve sensitivity. Its main purpose is security during an external fault when one CT saturates: most spill current is forced through the low resistance of the saturated CT circuit instead of through the relay branch. The relay pickup voltage must be above the worst voltage created by that condition, yet low enough to detect the minimum internal fault.

Application boundary: high-Z 87B is attractive for a simple, fixed bus with matched dedicated CTs. It becomes difficult where CT ratios differ, bays move dynamically between buses, or frequent CT switching is required.

1. Protection zone and trip objective

Install one CT boundary on every circuit connected to the bus: incomers, transformer feeders, generators, outgoing feeders and bus couplers/sections as applicable. The protected primary zone is the metalwork and conductors between these CTs.

Incomer CTs + feeder CTs + coupler CT → phase-segregated parallel CT circuit → high-Z 87B → 86 lockout → trip all sources in faulted zone

For normal load and external faults, the vector sum of all primary currents at the bus is approximately zero. For an internal fault, sources feed toward the fault and the vector sum is nonzero. Bus faults normally require rapid clearing because all sources contribute and internal-arc energy can rise quickly.

2. How the high-impedance principle remains stable

During the worst external fault, the CT on the faulted circuit carries the sum of all source contributions and is usually the most likely to saturate. In the conservative stability model, that CT produces no secondary current and becomes its winding resistance plus lead resistance. Healthy CTs drive current toward this path.

Because the relay branch has very high resistance, the resulting spill current mainly circulates through the saturated CT. Relay voltage is essentially the voltage drop across the saturated CT and its leads. Set the relay above this voltage with the specified security margin.

Vs = (IF,max / CTR) × (RCT + kRlead)

Use k = 1 only when the actual three-phase equivalent uses one-way lead resistance; use the appropriate return-path factor for phase-to-earth faults. Calculate every bay because CT resistance, lead length and maximum external fault contribution differ.

3. Relay pickup and security margin

Vpickup = Ks × max(Vs,bay)

Ks is the relay-manufacturer security factor. Some established applications use 1.5 or more to cover CT differences and future fault-level growth, but no value is universal. Do not set pickup above the effective CT accuracy/knee-point capability without a validated manufacturer method: an internal fault may then produce an undefined or insufficient relay voltage.

Where the relay uses an external stabilizing resistor:

Rstab ≈ Vpickup / Ielement,pickup − Relement

Use actual relay impedance and tolerance. Modern high-Z IEDs may contain internal resistance and be calibrated directly in volts; do not add a resistor unless the manual requires it.

4. Worked screening calculation

An 11 kV bus has maximum external three-phase fault current 25 kA. All CTs are 2000/1 A. The worst CT secondary resistance is 4.0 Ω and the one-way lead resistance to the common junction is 0.5 Ω.

IF,sec = 25,000 / 2,000 = 12.5 A
Vs,3φ = 12.5 × (4.0 + 0.5) = 56.25 V
With illustrative Ks = 1.5: Vpickup ≥ 84.4 V

Now check a maximum 18 kA phase-to-earth fault using round-trip lead resistance:

IF,sec = 18,000 / 2,000 = 9 A
Vs,1φ-E = 9 × (4.0 + 2×0.5) = 45 V

The three-phase case governs in this example. A preliminary 90 V pickup can be examined, but only after CT excitation, minimum sensitivity, relay range, MOV energy and insulation checks.

5. Minimum internal-fault sensitivity

During an internal fault, part of the secondary current excites every paralleled CT and part flows through the relay and nonlinear resistor. Minimum primary operating current is approximately:

Imin,pri = CTR × [ΣIexc,i(Vpickup) + Irelay + IMOV]

For eight CTs, if each takes 15 mA at 90 V, relay current is 30 mA and MOV leakage is 5 mA:

Imin,pri = 2,000 × [8(0.015)+0.030+0.005] = 310 A

Compare 310 A with the minimum internal phase and earth fault for every operating mode. In a resistance-grounded MV system the earth-fault current may be lower, so a high-Z setting secure for a 25 kA external fault can be insufficiently sensitive to the limited internal earth fault. A separate sensitive earth-fault scheme may be needed.

If pickup voltage is above the CT knee point, sinusoidal excitation curves may not predict actual pulsed operation accurately. Complete-circuit pickup testing is then important.

6. CT specification

Requirement Engineering reason
Same ratio on every CT in one phase circuit Currents are physically summed; numerical ratio correction is not available.
Compatible core/excitation performance Reduces normal spill and makes stability/sensitivity calculations meaningful.
Adequate knee-point/accuracy voltage CTs must develop relay voltage for an internal fault.
Known secondary winding resistance Direct input to worst external-fault stability voltage.
Dedicated protection cores Other burdens or shorting links can disable or alter the high-Z circuit.
Individual leads to common junction Simplifies resistance calculation, testing and fault analysis.
Correct polarity and phase identity One reversed CT can operate on load or restrain for an internal fault.
Single controlled secondary earth Avoids parallel paths and circulating currents.

Mixed ratios and interposing CTs are strongly discouraged unless the relay manufacturer provides a complete application method. Using a lower tap on a multi-ratio CT reduces effective accuracy voltage and can create high voltage on unused turns through autotransformer action.

7. Stabilizing resistor, MOV and insulation duty

An internal fault attempts to drive a near-open high-impedance secondary circuit, producing hazardous voltage. A nonlinear resistor—Metrosil, MOV or relay-specific high-impedance module—must clamp the voltage within the insulation capability of CTs, terminal blocks, test switches, wiring and relay.

Check:

  • maximum unclamped peak voltage;
  • clamping voltage at expected current;
  • energy for breaker clearing plus breaker-failure backup time;
  • repetitive duty and cooling;
  • fault duration at minimum DC voltage;
  • failure mode and alarm/backup element;
  • touch-safe enclosure and warning labels; and
  • 86 contact or other post-trip bypass arrangement if required by the design.

A varistor sized only for normal breaker clearing may fail if a breaker does not interrupt and backup clearing takes 20–30 cycles.

8. Bus configurations

Arrangement High-Z application note
Single bus Best application: one fixed CT zone and one trip matrix.
Sectionalized bus Use separate zone circuits; coupler CT and breaker position determine overlap/trip logic.
Double bus with selector disconnectors Requires CT switching or multiple circuits; operational and CT-open-circuit risk increases.
Transfer/bypass bus CT assignment during bypass is complex; low-Z numerical protection is often preferable.
Future expansion New CT must match ratio/performance and be incorporated into sensitivity and MOV calculations.

If CT circuits are switched by disconnector auxiliary contacts, the contacts and operating sequence must never open an energized CT. Early-make/late-break arrangements and strict breaker-before-disconnector operating rules may be required. Avoid dynamic CT switching where a numerical low-Z solution is practicable.

9. Equipment inside the bus zone

Bus-connected VTs, station-service transformers, grounding transformers and surge arresters can produce legitimate differential current because they are shunt devices inside the CT boundary. A failed device should trip the bus, but temporary arrester conduction, auxiliary-transformer inrush or a secondary fault may challenge a very sensitive scheme.

Options include an application-specific short delay, higher pickup, separate CTs that exclude an auxiliary transformer, or dedicated primary protection. Any delay must be evaluated against bus damage and arc-flash energy.

10. Open and shorted CT supervision

An open CT secondary can create dangerous voltage and false differential operation. A shorted CT or test link can remove that bay’s contribution and desensitize the whole high-Z zone. Supervise the bus wire/CT circuit with a sensitive alarm stage where possible.

A low AC-voltage continuity test across the complete parallel CT circuit can reveal a short: healthy CT magnetizing branches and relay impedance allow little current, whereas a short produces a clear increase. Perform only using the approved de-energized test procedure.

Seal CT shorting and test switches, provide visible service position, label each bay/phase and include their status in maintenance release/return checklists.

11. Maintenance grounding

Grounding chains or jumpers on both sides of a breaker can create a primary bypass through the ground grid and CT window. With breaker contacts closed, this can desensitize high-Z protection or produce unexpected current during an external earth fault. Connect maintenance grounds to a common ground-grid point where practicable and keep breaker contacts open during the relevant maintenance state. Analyze the exact dead-tank or switchgear CT geometry in the work procedure.

12. Trip matrix, breaker failure and end zone

87B must trip every breaker that can energize the faulted zone, including incomers, generators, couplers and back-fed feeders. Operate 86 lockout, initiate breaker failure and block automatic transfer/reclose. Verify trip-coil contact duty and DC supply.

A fault between a circuit CT and breaker may be inside the bus zone but remain supplied from a remote source after local bus breakers open. Provide end-zone logic or transfer trip. CT location determines which remote terminal must clear this fault.

13. FAT and SAT programme

  1. Review zone boundary, CT schedule, maximum external faults and complete high-Z calculations.
  2. Test CT ratio, polarity, winding resistance and excitation/knee point.
  3. Measure each installed lead resistance and complete loop.
  4. Verify one secondary earth point and absence of unintended shorts.
  5. Test relay pickup voltage, stabilizing resistor and sensitive alarm stage.
  6. Test MOV/Metrosil connection, rating evidence and insulation safeguards.
  7. Apply balanced through current and prove stability.
  8. Simulate the worst saturated-CT external-fault equivalent.
  9. Perform minimum internal-fault pickup test on the complete circuit.
  10. Test every zone trip, 86, breaker failure, end-zone intertrip and automation block.
  11. Retrieve event records and preserve measured as-left values.
Acceptance criterion: the test report must reconcile measured pickup and CT/lead data with the approved stability and sensitivity calculations. “Relay operated during injection” is not adequate evidence.

14. Advantages and limitations

Strength Limitation
Fast, proven and highly secure against external CT saturation Requires matched dedicated CTs and controlled wiring.
Economical for a simple fixed zone Difficult for dynamic multi-zone buses and mixed ratios.
Simple operating principle High secondary voltage and nonlinear resistor need specialist design.
High sensitivity can be achieved More parallel CTs increase excitation current and reduce sensitivity.
Limited software dependence A shorted CT/test link can silently disable the scheme.

Related guides

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 arrangements and application guidance
  2. IEEE C37.110-2023 — CT selection, saturation and distorted secondary current
  3. IEC 61869-2:2012 — requirements for inductive CTs
  4. SEL — High- versus Low-Impedance Bus Differential — stability voltage, sensitivity, CT and maintenance considerations
  5. SEL-587Z High-Impedance Differential Relay — high-impedance relay and CT saturation application
  6. ABB REB611 Product Guide — phase-segregated high-impedance bus protection and CT supervision

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.

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