The dependable fix is to give a conventional high-impedance restricted earth fault (REF) scheme a dedicated CT core, or use a numerical transformer differential relay whose integrated REF element is explicitly designed to reuse its phase-current inputs. Placing separate low- and high-impedance relays in one external CT secondary circuit is acceptable only after both manufacturers approve the complete connection and its calculated voltage, burden, saturation, and insulation duties.
Secondary Voltage and Saturation Limits
The number that matters is the secondary voltage demanded from the CT during the maximum external through fault. CT excitation increases as secondary voltage rises. When the required voltage passes the usable region of the excitation characteristic, magnetizing current rises sharply and the reproduced current becomes distorted.
A low-impedance transformer differential element expects usable current samples from every winding. It applies bias or restraint to remain stable when large through current produces CT mismatch. A high-impedance REF circuit uses a different stability principle: impedance in the relay branch limits spill current when one CT is treated as saturated during an external fault. During an internal earth fault, the CT group can develop a high secondary voltage across that branch.
This is heat, not logic. Added relay burden requires additional CT secondary voltage, increases excitation, and raises thermal duty in CT windings, leads, relay inputs, stabilizing resistors, and voltage-limiting components. A voltage limiter can protect insulation without restoring the missing current waveform needed by the low-impedance differential element.
| Quantity or limit | Why it decides the design | Where to obtain it |
|---|---|---|
| Maximum through-fault secondary current | Sets the worst external-fault voltage and saturation duty | Fault study and selected CT ratio |
| CT excitation characteristic | Shows the relationship between secondary voltage and excitation current | CT test certificate or excitation test |
| CT winding resistance | Adds to the voltage required to circulate fault current | CT certificate or measured resistance corrected as required by the design method |
| Lead-loop resistance | Long leads increase secondary voltage and copper heating | Conductor data or end-to-end measurement |
| Relay input burden | A series input adds directly to loop burden | Relay manual for the configured input |
| High-impedance stability setting | Controls external-fault stability and internal-fault sensitivity | Protection calculation and relay manual |
| Peak circuit voltage and withstand | Determines insulation and voltage-limiter requirements | Manufacturer calculation method and equipment ratings |
Available Protection Arrangements
| Arrangement | CT behavior | Design burden | Recommendation |
|---|---|---|---|
| Dedicated core for high-impedance REF | REF CTs can be selected and matched for the high-impedance duty | Requires another suitable core and secondary circuit | Preferred for a conventional standalone high-impedance REF scheme |
| Integrated REF in a numerical transformer differential relay | Phase currents are measured once and processed by coordinated software elements | Requires a relay with the required REF function and compatible neutral-current input | Preferred when the relay manual explicitly defines the arrangement |
| Separate low- and high-impedance relays sharing one external CT circuit | The series burden and high-voltage duty can affect both functions | Requires a complete saturation, stability, sensitivity, thermal, and insulation study | Use only with written manufacturer approval for the exact circuit |
The evidence includes numerical transformer relays such as the SEL-387 and GE T60 as examples with REF capability. That architecture is materially different from wiring two independent protection schemes in series. An electromechanical ABB HU example has internal restraint-transformer connections that are not externally accessible for the proposed interconnection.
A reviewed arrangement using an SEL-351S ahead of an SEL-587Z used MOVs to limit high voltage, but it involved directional overcurrent and high-impedance bus differential protection. It does not establish compatibility for transformer differential plus high-impedance REF because the CT topology, operating criteria, and consequences of saturation differ.
Shared-Circuit Compatibility Checks
A common core is not merely a question of whether two relay inputs can carry the same current. The complete secondary network must satisfy both protection principles under internal faults, external phase faults, external earth faults, energization, and CT-circuit abnormalities.
- Trace the secondary circuit from every CT terminal through test switches, terminals, relay inputs, stabilizing components, and the neutral connection. Mark which elements are series-connected and which CTs are paralleled at the REF summation point.
- Confirm that the low-impedance relay exposes suitable current-circuit terminals. Internal paralleling, restraint transformers, or hidden common points can make sharing impossible.
- Add CT winding resistance, measured lead-loop resistance, terminal resistance, test-switch resistance, and every series input burden using the manufacturer’s specified basis.
- Calculate external-fault stability using the high-impedance relay manual’s method and the maximum through-fault current. Check the result against the actual CT excitation curve.
- Calculate internal-fault secondary voltage and compare it with relay-input, CT-secondary, terminal-block, test-switch, wiring, and stabilizing-component withstand ratings.
- Check whether a voltage limiter changes current available to the low-impedance relay. Its clamping action protects against excessive voltage but can deepen waveform distortion.
- Confirm that simultaneous operation, CT saturation, or loss of one protection function cannot extend the outage beyond the protection philosophy.
Recommended Design Procedure
Start with the dedicated-core arrangement. Select the phase and neutral CT circuits to meet the high-impedance relay manufacturer’s matching, resistance, excitation, stability, sensitivity, and voltage requirements. Keep the low-impedance transformer differential element on its own CT cores.
If cores are unavailable, evaluate an integrated numerical differential-and-REF relay before considering external series sharing. Verify in the relay manual that the REF element uses the same phase inputs as the biased differential element and identify the required neutral CT connection. The relay’s internal reuse of sampled currents avoids placing a separate high-impedance branch in series with another relay input.
Use external sharing only as an engineered exception:
- Produce a single-line and detailed CT-secondary schematic showing polarity, earthing point, shorting facilities, and component ratings.
- Complete the through-fault saturation and high-impedance stability calculation with measured circuit resistance.
- Check internal-fault peak voltage and energy against all connected devices. Size any MOV or other limiter by the applicable relay-manufacturer method.
- Give both relay manufacturers the schematic, CT excitation data, resistance values, maximum fault currents, settings, and voltage calculation.
- Record their acceptance of the exact connection before commissioning it.
Commissioning and Verification
Prove each protection function independently and then prove their interaction. Before injection, isolate the primary equipment according to the site switching and protection-testing procedure; an open CT secondary on an energized primary can generate hazardous voltage.
- Verify CT ratio, polarity, winding resistance, insulation condition, single-point secondary earthing, and continuity through every series device.
- Compare the measured excitation curve with the curve used in the calculation.
- Inject balanced phase currents and confirm that the transformer differential and REF operating quantities remain restrained.
- Simulate an external fault with the specified worst-case CT mismatch or saturation test method. Confirm that neither element trips outside its intended zone.
- Inject an in-zone earth-fault condition and confirm REF pickup, trip output, target indication, event recording, and breaker path.
- Test the transformer differential operating and biased-restraint characteristic separately.
- Where a voltage limiter is installed, verify its connection and rating without applying an uncontrolled overvoltage to relay inputs.
- Repeat end-to-end trip checks after returning test switches and shorting links to service positions.
Recurring Design Pitfalls
Low steady-state burden does not prove fault compatibility. A numerical input may draw little load at rated current yet still face excessive voltage or a severely distorted waveform during a high-impedance scheme operation.
Matching ratios alone is also insufficient. CT excitation characteristics, secondary resistance, lead resistance, polarity, and physical location all affect REF stability. Heavy phase faults deserve explicit study because unequal saturation can create residual spill current and unwanted REF operation.
Finally, built-in REF and standalone high-impedance REF are not interchangeable labels. Confirm the actual algorithm and wiring in the selected relay manual. Software reuse of phase samples can be suitable even when connecting two physical relay inputs to one core is not.
Frequently Asked Questions
How do I decide whether one CT core can feed both relays?
Calculate maximum through-fault voltage and burden, check the CT excitation curve, and verify the withstand of every series device. Use the arrangement only when both manufacturers approve the exact schematic and calculation.
How do I avoid a separate CT core for REF?
Select a numerical transformer differential relay with an integrated REF element that explicitly reuses its phase-current inputs. Confirm the required neutral CT and connection in that relay’s manual.
How do I check high-impedance REF stability?
Use the relay manufacturer’s calculation with maximum external-fault secondary current, CT winding resistance, measured lead-loop resistance, relay burden, and the CT excitation characteristic. Then reproduce the specified mismatch or saturation case during commissioning.
How do I use an MOV in a shared CT circuit?
Apply the manufacturer’s voltage and energy sizing method, then check the clamped waveform’s effect on the other relay. An MOV limits voltage; it does not correct CT saturation or recover an accurate current sample.
How do I know when to stop and contact official support?
Stop when the manual does not define the topology, internal relay connections are inaccessible, or any saturation, stability, voltage, thermal, or insulation limit remains unresolved. Send the official manufacturer support channel the full CT schematic, excitation data, resistance measurements, fault-current study, settings, and limiter calculation, and request written approval before energization.