Low-power instrument transformers can replace bulky 1 A/5 A CT and 100/√3 V VT circuits with compact, linear analogue sensors or a sensor–merging-unit–Sampled Values chain—but the benefit comes only when the complete interface is engineered as one measurement system. Sensor ratio, cable, connector, relay input, integration, time synchronisation, data quality and failure logic are all protection parameters.
This guide distinguishes LPCTs, Rogowski coils and passive LPVTs; explains analogue and digital architectures; and provides selection, error-budget, EMC, interoperability, testing, maintenance and retrofit guidance for MV switchgear.
Executive rules
- “LPIT” describes a measurement category, not one sensor principle or interchangeable connector.
- A Rogowski coil output is proportional to di/dt and requires a defined integrator; a passive LPVT output is proportional to primary voltage.
- No iron-core saturation does not mean unlimited protection range: sensor insulation, conductor position, cable, input electronics, ADC and integrator can still limit performance.
- Treat the supplied secondary cable/connector/termination as part of the calibrated sensor system unless documented otherwise.
- For Sampled Values, engineer merging unit, stream, time, network, relay subscription and fail-safe behaviour end to end.
- Specify accuracy at load, minimum fault, maximum asymmetric fault, frequency, harmonics, temperature and required protection time—not one rated point.
- Test polarity, scaling, phase, channel mapping, data quality and complete trip behaviour with final firmware/configuration.
- Control vendor compatibility and replacement strategy before standardising a switchgear fleet.
1. Terminology: what is—and is not—an LPIT
| Term | Practical meaning | Do not assume |
|---|---|---|
| LPIT | Instrument transformer/sensor with a low-power analogue or digital secondary signal under the applicable IEC 61869 framework | That every LPIT is a Rogowski coil or Ethernet device |
| LPCT | Low-power current transformer; passive analogue products are covered by IEC 61869-10 | One universal output level, cable or relay input |
| Rogowski coil | Air-cored winding whose raw voltage is proportional to rate of change of primary current | Direct current measurement without integration |
| LPVT | Low-power voltage transformer; IEC 61869-11 covers passive analogue output proportional to primary voltage | All divider technologies have identical frequency/temperature behaviour |
| Merging unit (MU) | Samples/merges measurement channels and publishes digital values | That conformance alone proves system interoperability/performance |
| Sampled Values (SV) | IEC 61850 mapped sampled measurement communications | That the network is automatically deterministic, redundant or time-correct |
2. Current standards and edition compatibility
- IEC 61869-1:2023: current general requirements for analogue/digital instrument transformers; it cancelled/replaced IEC 61869-6:2016 and added/updated general LPIT, bandwidth and digital-signal concepts.
- IEC 61869-10:2017: low-power passive current transformers with analogue output for measurement/protection.
- IEC 61869-11:2017 + ISH1:2021: low-power passive voltage transformers with analogue output proportional to primary voltage.
- IEC 61869-9:2016: digital communications requirements for instrument-transformer measurements, based on IEC 61850.
- IEC 61869-13:2021: stand-alone merging units with specified conventional/low-power analogue inputs; digital output format is addressed by IEC 61869-9.
- IEC 61850-9-2:2011+A1:2020: mapping of Sampled Values over Ethernet.
- IEC TS 60255-216-1:2025: protection functions that subscribe to SV/GOOSE/time and publish GOOSE, including functional/interoperability test considerations.
Parts 10 and 11 were originally structured with IEC 61869-1:2007 and IEC 61869-6:2016, while IEC 61869-1:2023 now replaces the latter. A procurement specification must identify the exact normative combination accepted by the manufacturer/certification body; do not silently mix clauses from editions.
3. Rogowski-coil current sensing
For mutual inductance M, the ideal coil output is vs(t) = M·dip(t)/dt. An analogue or digital integrator reconstructs current: ip(t) = (1/M)∫vs(t)dt, subject to calibration, bandwidth and drift.
- Wide range: the air core has no ferromagnetic saturation/remanence, enabling one sensor to span load and large fault current.
- Compact/safe signal: millivolt-level voltage output avoids the conventional CT’s dangerous open-secondary mechanism, provided the qualified product is installed as specified.
- Phase/frequency: raw output scales with frequency for sinusoidal current; the integrator and relay must correct magnitude/phase across the specified band.
- No DC: a pure steady DC component produces no output. Reconstruction of fault decaying DC is limited by low-frequency response/integrator droop.
- Geometry: conductor centring, adjacent phases, return conductors, gap/joint and mounting position can affect some coil designs; use manufacturer tolerances.
- System limit: maximum range may be set by sensor voltage, cable, input protection, integrator or ADC clipping even though the coil core does not saturate.
ABB’s KECA product information is a practical example: it describes an air-cored Rogowski sensor with millivolt output proportional to current derivative and digital integration in the IED. That architecture is an example, not a universal pinout or scaling.
4. Other low-power current-sensor considerations
IEC 61869-10 covers low-power passive CTs and multi-purpose devices; product technology and output law must be read from the certificate/data sheet. Some current sensors produce a low-voltage signal proportional to current rather than a raw derivative. Never enable a relay’s Rogowski integration merely because the connector looks compatible.
- Confirm rated transformation ratio/output, reference primary current and extended accuracy/limit factor.
- Confirm whether integrator is inside sensor, relay input module or merging unit.
- Confirm input impedance/termination, cable type/length, shielding and connector coding.
- Confirm phase error at low current for power, directional and earth-fault functions.
- Confirm maximum fault waveform and duration at the relay/MU—not sensor alone.
5. Passive low-power voltage transformers
A passive LPVT under IEC 61869-11 provides an analogue secondary voltage proportional to primary voltage; derivative output is outside that document’s scope. Resistive dividers are common in MV switchgear, while other product constructions require their own error and transient analysis.
- Linearity: a resistive divider does not have an iron core to saturate, supporting measurement and protection over a broad voltage range.
- Temperature: high- and low-side resistor coefficients and gradients affect ratio/phase; use specified class across the switchgear temperature range.
- Input/cable: relay input impedance and cable capacitance/leakage form part of the divider transfer function. Changing cable or adding parallel devices may change accuracy.
- Frequency/transient: verify amplitude/phase for frequency, harmonics, ferroresonance-related studies, switching impulses and protection polarisation needs.
- Residual voltage: three sensors and relay calculation can derive 3U0 only if primary reference/connection, phase matching and grounding permit it.
- Isolation: low output does not reduce the primary sensor’s insulation, creepage, partial-discharge or internal-arc installation requirements.
Schneider Electric’s current product pages illustrate resistive-divider LPVTs qualified to IEC 61869-11 and magnetic LPCTs qualified to IEC 61869-10. Compatibility statements on one vendor platform should not be extrapolated to another relay without evidence.
6. Analogue LPIT architecture
Primary conductor/voltage → passive sensor → calibrated low-power cable/connector → relay or MU analogue input → scaling/integration/filter → protection function.
| Interface item | Specify | Failure effect |
|---|---|---|
| Sensor | Standard/edition, class, ratio/output, range, phase, temperature and insulation | Bias, loss, clipping or phase error |
| Cable | Part number, maximum length, capacitance/resistance, shielding, routing and bend/radius | Gain/phase/EMC error |
| Connector | Keying, pinout, contact resistance, sealing, retention and test adapter | Wrong channel/polarity or intermittent signal |
| Relay/MU input | Sensor mode, impedance, input range, integrator and scaling | Systematic factor error or waveform distortion |
| Configuration | Phase/channel mapping, reference values, polarity and checksum | False direction/differential/trip |
Do not megger/electrically test low-power signal electronics or cables with a conventional CT/VT procedure unless the manufacturer explicitly permits the test voltage and connections.
7. Digital process-bus architecture
Sensor → MU → IEC 61869-9/IEC 61850 SV publisher → Ethernet process network → subscribing protection IED → GOOSE/hardwired trip. Digitising measurements moves copper-burden problems into a time/data/network engineering problem.
- Dataset: exact currents/voltages, order, scaling, quality and neutral/residual derivation.
- Stream: source/destination identifiers, APPID/VLAN/priority, sample profile/rate, configuration revision and supervision.
- Time: clock source, PTP profile, grandmaster redundancy, accuracy/holdover and time-quality response. Phase error from timing is critical for differential/directional protection.
- Network: latency, jitter, bandwidth, multicast filtering, PRP/HSR or other approved redundancy, switch failure and maintenance isolation.
- IED: subscription mapping, channel/polarity, missing/out-of-order samples, invalid/questionable quality and stream switchover.
- Trip: hardwired or GOOSE architecture, breaker I/O, end-to-end delay and independent backup.
- Cybersecurity: access, secure engineering, configuration/firmware authenticity, logs and IEC 62351/project controls without compromising protection latency/availability.
8. Error budget and protection margin
Separate ratio and phase-angle error budgets. Include sensor, cable/termination, input/integrator, sampling/time, temperature, conductor position, frequency and calibration drift. For differential protection include errors at every end and synchronisation between streams.
If demonstrably independent uncertainties are combined statistically, root-sum-square may be justified; protection security often requires a bounded worst-case combination. Do not use √sum-of-squares automatically for common temperature, calibration or time errors. Convert the approved budget into pickup/restraint/directional-angle margins.
| Function | Critical LPIT performance | Verification focus |
|---|---|---|
| 50/51 | Wide range and fault transient reproduction | Clipping, integrator and operate time |
| 67/67N | Current/voltage phase and polarity at low residual | Angle boundary, time skew and topology |
| 87 | Matched ratio/phase/time at all ends | External-fault stability and lost stream |
| Power/metering | Low-current/voltage ratio and phase | Class across load/temperature/harmonics |
| Arc-flash current supervision | High-speed fault current assertion | Sensor/input/filter plus logic latency |
9. Advantages—stated without marketing shortcuts
- smaller/lighter primary sensors and reduced copper burden;
- broad linear current/voltage range for combined measurement and protection;
- no ferromagnetic saturation/remanence in Rogowski/resistive-divider principles;
- reduced conventional CT open-secondary and VT short-circuit energy hazards for qualified low-power outputs;
- fewer sensor variants/ratios may cover a switchgear family;
- digital sharing can reduce point-to-point analogue wiring and expose quality diagnostics;
- wideband measurement may support power-quality/transient applications when specifically qualified.
10. Limitations and lifecycle risks
- low-level analogue signals are sensitive to connector, cable, grounding and EMC errors;
- sensor/relay/MU pinout, scale and integration can be platform-specific despite standards compliance;
- electronic input/MU auxiliary power adds a common failure mode;
- Rogowski coils cannot measure steady DC and have low-frequency/integrator limitations;
- process-bus protection depends on time/network/configuration and skilled diagnostic tooling;
- replacement after 15–30 years may require an adapter, new calibration or matched IED platform;
- one shared sensor/MU can create a common-mode failure across protection, metering and control;
- testing teams need low-power/SV-capable calibrated equipment and safe adapters;
- digital cyber/configuration controls add lifecycle work that copper schemes did not require.
11. Selection workflow
- Define functions, redundancy, measurement class and protection response for all load/fault/topology cases.
- Choose analogue direct input or MU/SV architecture and define failure/backup philosophy.
- Specify governing IEC parts/editions and require certificates for the complete sensor/cable/input combination.
- Check primary insulation, PD, temperature, internal-arc/mechanical installation and conductor geometry.
- Check current/voltage range, ratio and phase at minimum signal, normal load, maximum fault and required frequency/bandwidth.
- Build ratio, phase and time error budgets and apply them to protection margins.
- Freeze cable, connector, pinout, input module, integrator and scaling; prohibit uncontrolled substitutions.
- For SV, produce stream/subscription/time/network design and redundancy/cyber failure matrix.
- Prove multi-vendor interoperability with exact firmware/configuration before procurement release.
- Define FAT/SAT, calibration, spares, replacement/revalidation and technician tooling/training.
12. FAT and interoperability test matrix
- ratio/polarity/phase at low, reference and high signal;
- maximum fault waveform including DC offset, frequency deviation and duration;
- sensor/cable/input temperature and permitted cable-length/termination extremes;
- adjacent-conductor/position influence where applicable;
- directional/differential operation at worst ratio/phase/time error;
- analogue lead open/short/cross-connect/shield-earth failure alarms and safe response;
- SV channel order, scale, polarity, quality and configuration revision;
- packet loss, burst loss, duplicate/out-of-order samples, latency/jitter and stream loss/recovery;
- PTP loss, clock step, grandmaster switchover, holdover and time-quality degradation;
- network/switch/link/redundancy failure and maintenance bypass;
- GOOSE or hardwired trip complete-path time and breaker/BF interaction;
- restart, firmware/configuration change, checksum and event/oscillography traceability.
IEC 61850 conformance testing and a supplier’s Protocol Implementation Conformance Statement are necessary but not sufficient. Project interoperability testing must combine the actual sensor/MU/IED/network/time devices and the exact protection scenario.
13. Site commissioning
- Inspect sensor type/serial, mounting, conductor orientation, cable/connector and screen/earth.
- Confirm relay/MU input hardware, sensor mode, scale, polarity and phase mapping against approved ICD/SCD/wiring.
- Primary-inject current or apply calibrated voltage to verify the complete analogue chain.
- Compare primary reference with relay/MU values and phase angle; test minimum and high-range points as practical.
- For Rogowski inputs, prove integration/frequency response and phase direction, not RMS magnitude alone.
- For SV, verify stream identifiers, dataset/order, quality, sample/time status and subscription redundancy.
- Inject/play back dynamic forward/reverse, external/internal fault and sensor/data-failure cases.
- Operate trip through final I/O/GOOSE to breaker and prove BF/backup.
- Record healthy live-load phasors/power direction and compare independent reference measurement.
- Archive calibration, raw results, configurations/checksums, firmware, network capture and as-built connector/cable details.
14. Retrofit and replacement decisions
| Question | Why it matters |
|---|---|
| Can the existing relay accept this exact low-power output? | A conventional 1 A/5 A input is not an LPCT input |
| Is the cable part of calibrated ratio/phase? | Extension/adapter can invalidate class |
| Can the sensor be replaced without opening the primary circuit? | Split-core convenience versus accuracy/installation qualification |
| Will one MU replace multiple independent CT/VT cores? | Common-mode availability and maintenance risk |
| Are spares/firmware/configuration tools supportable for asset life? | Connector and digital obsolescence can dominate lifecycle cost |
| Can tests be performed during staged outages? | Need safe injection, simulation and trip isolation points |
15. Maintenance and condition evidence
- trend healthy phase ratios, angles, residual quantities and cross-bay comparisons;
- inspect connector retention, contamination, moisture, cable damage and shielding;
- review MU/IED stream-loss, time-quality, input-range and self-monitoring alarms;
- verify firmware/configuration baselines and network/time redundancy periodically;
- repeat complete-chain calibration after sensor/cable/input/MU/firmware replacement;
- retain vendor adapters, calibrated test equipment and known-good spares;
- treat unexplained directional/differential phasor drift as a protection defect, not merely metering error.
16. Frequent mistakes
| Mistake | Consequence | Correction |
|---|---|---|
| “LPCT = Rogowski” | Wrong input/integration | Identify exact output law and product standard |
| RJ45 treated as Ethernet | Damage or wrong signals | Approved pinout/keyed adapter and labelling |
| No saturation = unlimited range | Input/ADC/integrator clips | End-to-end maximum waveform test |
| Cable extended freely | Ratio/phase/EMC error | Qualified cable/length/termination only |
| SV values visible = protection proven | Time/quality/failure logic latent | Dynamic complete-path testing |
| One MU without common-mode review | Multiple functions lost together | Redundancy/backup FMEA |
| Standard edition omitted | Contractual incompatibility | State exact parts/editions/certificates |
References and further reading
- IEC 61869-1:2023 — General requirements for instrument transformers
- IEC 61869-10:2017 — Low-power passive current transformers
- IEC 61869-11:2017 with ISH1:2021 — Low-power passive voltage transformers
- IEC 61869-9:2016 — Digital interface for instrument transformers
- IEC 61869-13:2021 — Stand-alone merging units
- IEC 61850-9-2:2011+A1:2020 — Sampled Values mapping
- IEC TS 60255-216-1:2025 — Protection functions using digital inputs/outputs
- ABB KECA current sensor — Rogowski implementation example
- Schneider Electric LPCT — IEC 61869-10 product example
- Schneider Electric LPVT — resistive-divider product example
Engineering note: Product examples show real implementations, not cross-vendor compatibility endorsements. Final selection must be based on the approved sensor–cable–input/MU–IED chain and project-specific protection tests.