A low-power instrument transformer (LPIT) delivers a low-power analog or digital signal, normally 1 VA or less. IEC 61869-6 sets its rated burden at 2 MΩ in parallel with 50 pF, its rated delay at 50, 100 or 500 µs and its frequency range at 99 to 101 % for measuring and 96 to 102 % for protection.
What an LPIT is
IEC 61869-6, 2016 edition, is the product standard that adds general requirements for low-power instrument transformers. Its acronym is LPIT (low-power instrument transformer). It covers AC applications with rated frequencies from 15 Hz to 100 Hz at medium, high and extra-high voltage, and DC applications (clause 1).
Clause 3.1.601 defines an LPIT as an assembly of one or more current transformers or voltage transformers. They may be connected to transmitting systems and secondary converters. The assembly transmits a low-power signal, analog or digital, to meters, protection or control equipment. The output power is normally less than or equal to 1 VA. LPITs are also called non-conventional transformers.
The standard distinguishes current types (LPCT) from voltage types (LPVT), measuring, protection and multipurpose types, and electronic types, which include active components, from passive ones. An example from the standard itself: three current sensors and three voltage sensors connected to a merging unit that delivers a digital output are considered one LPIT.
IEC 61869-6 defines errors for both analog and digital output. The format of the digital output and its interface are in IEC 61869-9, not in this part.
The parts of an LPIT
Figure 601 of the standard shows the general diagram of a single-phase LPIT. Depending on the technology, not all parts are present. A passive LPIT, with no active electronic components, has only passive components and no power supply.
| Part | What it does (clauses 3.1.610 to 3.1.614) |
|---|---|
| Primary sensor | Provides the information of the input signal |
| Primary converter | Converts the sensor signal into one suitable for the transmitting system |
| Transmitting system | Couples the primary and secondary parts, over a short or long distance |
| Secondary converter | Converts the transmitted signal into one proportional to the input |
With analog output, the secondary converter feeds meters and protection directly. With digital output, it connects to a merging unit before feeding the secondary equipment.
Rated burden: 2 MΩ in parallel with 50 pF
In an LPIT the burden is defined differently from a conventional CT. The standard redefines “burden” as the impedance of the secondary analog circuit, expressed as a resistance in parallel with a capacitance (clause 3.4.6), and declares the rated output “not applicable” (clause 3.4.8).
The standard value of the rated burden (clause 5.5.601):
| Resistance | Capacitance |
|---|---|
| 2 MΩ | 50 pF |
For equipment compatible with the earlier standard IEC 60044-8, 2 kΩ with 5 000 pF and 20 kΩ with 500 pF are accepted. It is a very high resistance compared with the burden of a conventional CT, which is in the order of ohms.
The standard warns of two things about capacitance:
- Attention must be paid to the parallel capacitance of the connected measuring instruments and protection equipment.
- If the transmission cable is not part of the sensor, its capacitance must be added to the burden. The standard gives a typical cable capacitance between 15 pF/m and 100 pF/m.
With a rated burden of 50 pF, a long cable can exceed the standard value with its capacitance alone. For example, 10 m of 100 pF/m cable contributes 1 000 pF, twenty times the rated burden. That is why the cable length and type must be in the specification.
Rated delay: 50, 100 or 500 µs
The delay is the actual time between an event at the primary and its result at the output. It can be due, for example, to band-limiting filters and digital processing (clause 3.4.601). With analog output it must be constant, because any deviation becomes a phase error.
The standard values of the rated delay time are 50 µs, 100 µs and 500 µs (clause 5.5.602). The rated delay time is defined only for LPITs with analog output (clause 3.4.602). With digital output, the time stamp of the frame allows it to be compensated and its contribution to the phase displacement can be neglected (note 602 to clause 3.4.4); that case is covered in IEC 61869-9.
The rated delay time is equivalent to a phase displacement of −2π × f × tdr (clause 3.4.605). At 50 Hz, for example, 100 µs is 1.8 electrical degrees and 500 µs is 9 degrees. The standard does not count it as an error: the phase error is the difference between the actual displacement and the sum of the rated phase offset and the displacement due to the rated delay time.
There is another phase shift that is not an error either: the rated phase offset, specific to the technology and independent of frequency, such as that of a Rogowski coil (clauses 3.4.603 and 3.4.604).
Frequency range
Clause 5.4 adds the frequency range in which accuracy is guaranteed:
| Accuracy classes | Frequency range |
|---|---|
| Measuring | 99 % to 101 % of rated frequency |
| Protection | 96 % to 102 % of rated frequency |
Outside those ranges, accuracy, if specified, is defined in Annex 6B.
What to verify when specifying or testing
- Confirm the output type. The rated delay time exists only for analog output; in digital output the frame carries the time stamp.
- Review the total connected capacitance: that of the equipment and that of the cable.
- Record the rated delay and phase offset on the data sheet.
- Check the frequency range of the measuring and protection classes.
What not to do
- Do not connect to an LPIT a burden designed for a conventional CT. They are measured in different units.
- Do not ignore the capacitance of the transmission cable.
- Do not assume that a digital-output LPIT has a rated delay equal to the analog one.
Related topics
The rated burden and delay are related to the maximum processing delay of the merging unit and to the dual accuracy class. Accuracy with harmonics and insulation and power supply complete the LPIT requirements. The overview is in non-conventional instrument transformers.
When to call us
At Tiza Group we support digital substation projects with low-power transformers and merging units: specification in secondary engineering design, architecture in communications engineering, both worldwide, remotely, and protection testing with secondary injection in Colombia, or elsewhere on request. Write to us and we will quote it.
Related guides
- Maximum processing delay of a merging unit
- Dual accuracy class 0.2S/5P20 and INT32 scale
- Accuracy with harmonics and noise in an LPIT
- Non-Conventional Instrument Transformers (LPIT)
- Merging Unit: IEC 61869-9, 9-2LE and Tests
References
- IEC 61869-6, Instrument transformers - Part 6: Additional general requirements for low-power instrument transformers, 2016 edition, clauses 1, 3.1.601 to 3.1.614, 3.4.4, 3.4.6, 3.4.8, 3.4.601 to 3.4.605, 5.4, 5.5.601 and 5.5.602.
- IEC 61869-9, digital interface for instrument transformers, cited by IEC 61869-6 for the digital output.
Frequently asked questions
What is an LPIT?
An assembly of one or more current or voltage transformers, which may be connected to transmitting systems and secondary converters, and which delivers a low-power analog or digital signal to meters, protection or control. The output is normally 1 VA or less. They are also known as non-conventional transformers (IEC 61869-6, clause 3.1.601).
What is the rated burden of an LPIT?
A resistance of 2 MΩ in parallel with a capacitance of 50 pF. For equipment compatible with IEC 60044-8, 2 kΩ with 5 000 pF and 20 kΩ with 500 pF are also accepted (clause 5.5.601).
What rated delays does the standard define?
50 µs, 100 µs and 500 µs, only for LPITs with analog output. With digital output, the time stamp of the frame allows the delay to be compensated (clauses 3.4.602, 5.5.602 and note 602 to 3.4.4).