What changes with low-power sensors?
Low-power instrument transformers (LPITs) deliver a low-energy signal rather than the traditional 1 A/5 A CT or 100/110 V VT secondary. Current sensing may use an air-core Rogowski coil or another low-power passive current transformer; voltage sensing commonly uses a resistive or capacitive divider. The smaller signal reduces secondary energy and can make MV switchgear more compact, but it shifts engineering attention from heavy secondary wiring to interface compatibility, scaling, shielding, bandwidth and digital data quality.
An LPIT is not automatically a process-bus device. It may feed a relay through a low-energy analogue input, or feed a merging unit that publishes IEC 61850 Sampled Values. These architectures have different failure modes and test methods.
1. Architecture options
or
Primary conductor → sensor/SAMU or merging unit → IEC 61850-9-2 Sampled Values → Ethernet network → protection IED
| Architecture | Advantages | Engineering obligations |
|---|---|---|
| Direct low-energy analogue input | Simple signal path; no process-bus network or time source required for basic measurement. | Sensor/relay interface must match exactly; cable type, length, connector and scaling are part of accuracy. |
| Sensor plus merging unit | Multiple IEDs can subscribe; fewer long copper analogue circuits; easier functional replication. | Network design, Sampled Value profile, time synchronization, redundancy, data quality and cyber controls become protection requirements. |
| SAMU on conventional CT/VT | Digitizes existing 1 A/5 A and 100/110 V signals for migration. | Retains CT/VT safety and burden issues while adding digital interface and timing requirements. |
2. Rogowski coil behavior
An air-core Rogowski coil produces an output related to the rate of change of primary current. Integration in the sensor electronics or relay reconstructs current. With no ferromagnetic core, it does not saturate in the conventional CT sense and can provide a wide dynamic range from load to high fault current. However, low-frequency response, integrator accuracy, external fields, conductor position, coil closure and cable/connector integrity matter.
“No saturation” does not mean “no limit.” The relay input and analogue-to-digital chain still have voltage range, bandwidth and clipping limits. Verify maximum prospective fault current, DC-offset response and the performance claimed for the complete sensor-plus-IED combination.
3. LPVT behavior
Passive voltage dividers draw very little power and can reduce VT size and ferroresonance exposure, but ratio and phase performance depend on divider design, connected cable and input impedance. Confirm phase-to-earth insulation, overvoltage factor, discharge behavior, temperature dependence and whether the output supports measurement, protection or both. Residual-voltage and synchronism-check applications require reliable phase angle and scaling on all three phases.
4. Standards map
| Document | Role |
|---|---|
| IEC 61869-1:2023 | General requirements for instrument transformers with analogue or digital secondary signals. |
| IEC 61869-10:2017 | Additional requirements for low-power passive current transformers with analogue output. |
| IEC 61869-11:2017 | Additional requirements for low-power passive voltage transformers with analogue output. |
| IEC 61869-9:2016 | Digital interface requirements for instrument transformers using IEC 61850 architecture. |
| IEC 61869-13:2021 | Requirements for stand-alone merging units accepting standardized conventional analogue inputs. |
| IEEE C37.92-2023 | Low-energy analogue interfaces between relays and power-system signal sources. |
| IEEE C37.235-2021 | Application of Rogowski coils for protective relaying. |
Compliance by individual components does not prove system accuracy. The specification must identify the sensor, cable, connector, burden/input, scaling data and IED firmware as a tested chain.
5. Protection advantages and limitations
- Wide dynamic range: one sensor can often serve metering and protection without the metering-core saturation compromise of a conventional CT.
- Reduced secondary hazard: a disconnected low-power sensor does not create the same high open-circuit voltage hazard as an energized conventional CT; follow the manufacturer’s safety instructions nevertheless.
- Compact switchgear: less core and copper can reduce footprint and weight.
- Standardization potential: digital values can be shared by multiple protection and monitoring functions.
Limitations include vendor-specific connectors/scaling in some analogue implementations, damage-prone small-signal cables, dependence on electronics and auxiliary power for active paths, process-bus common-mode risks and a skills gap in testing. A conventional secondary-injection set connected to 1 A terminals cannot directly test an LPIT input without the correct adapter or digital test method.
6. Specification checklist
| Topic | Specify explicitly |
|---|---|
| Primary performance | Rated current/voltage, extended range, short-time withstand, overvoltage factor, frequency and insulation level. |
| Accuracy | Protection and measurement classes, ratio/phase error over temperature and operating range, transient response and bandwidth. |
| Interface | Analogue output type/rated value or SV profile, connector pinout, cable type/maximum length, input impedance and grounding/shielding. |
| Scaling | Nameplate data, electronic data sheet/SCL values, polarity, phase designation and change-control method. |
| Reliability | Self-supervision, auxiliary-power behavior, redundant sources/streams, sensor replacement and obsolescence strategy. |
| Testing | Primary comparison, end-to-end injection, SV simulation/subscription, time sync, failover and as-left configuration evidence. |
7. Commissioning workflow
- Verify sensor identity, direction arrow/polarity, phase and mechanical installation.
- Confirm cable/connector is the approved type and has not been extended or repaired outside instructions.
- Check configured primary/secondary scaling in every subscribing IED and merging unit.
- Apply a known primary current/voltage or approved simulator and compare each phase magnitude and angle.
- For SV, test stream identity, sample rate/profile, quality flags, time synchronization, network redundancy and loss-of-stream behavior.
- Test protection pickup and trip end-to-end, including the breaker.
- Store the electronic sensor data, SCL files, firmware and test results under revision control.
8. When conventional CTs/VTs remain preferable
Conventional transformers may remain the better choice where several legacy devices require standard high-energy outputs, the owner lacks digital test capability, a long proven lifecycle is more important than footprint, or the application requires a performance class not demonstrated by the offered sensor chain. A hybrid design is valid, but it should be chosen consciously and should avoid creating two unsupervised measurement truths for the same bay.
Related protection 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
- IEC 61869-1:2023 — general instrument-transformer requirements
- IEC 61869-10:2017 — low-power passive CTs
- IEC 61869-11:2017 — low-power passive VTs
- IEC 61869-9:2016 — digital instrument-transformer interface
- IEC 61869-13:2021 — stand-alone merging units
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.