Annex 6A of IEC 61869-6 requires an LPIT with digital output to have an anti-aliasing filter, because every frequency above half the sampling rate is reflected down to lower frequencies, and the dangerous one is the one that lands on the fundamental. It also sets harmonic error limits for measuring and protection classes, defines a special wide-bandwidth class for traveling waves and asks the manufacturer for noise information.
Short answer
A low-power instrument transformer (LPIT), part of the family of non-conventional instrument transformers, with digital output does not see the continuous primary signal. It sees samples taken at fixed intervals. That conversion introduces two effects the standard regulates: the noise of the electronics themselves and aliasing, the reflection of high frequencies down to low frequencies.
Annex 6A of IEC 61869-6, 2016 edition, asks for an anti-aliasing filter, sets error limits with harmonics and requires the manufacturer to supply noise information. An LPIT that is good at 50 or 60 Hz can fail in a substation with converters, filters or electric traction if nobody reviewed this annex. The total harmonic distortion (THD) of your network is the first figure worth knowing.
Noise: what the standard asks
Clause 6.603 recognizes that there is no single set of noise requirements for all LPITs, because they depend on the application. The manufacturer must therefore supply spectral noise information, preferably as equations or spectral density plots as a function of frequency.
White, random noise averages to zero over time. Its effect on a measurement depends on the integration time and the application. It is expressed as equivalent primary current per root hertz.
The standard gives an example with an optical current transformer rated 400 A, with 4 V rated output and 10 mV of output noise in a 10 kHz bandwidth. That noise is equivalent to 1 A in the primary. It can be written as 0.01 A per root hertz, or 0.000025 per unit per root hertz, or −92 dB.
As a guide, noise levels between −50 and −70 dB per root hertz, referred to the rated current, are usually enough for most applications.
Annex 6A.2 adds two useful manufacturer acceptance measurements, both with a spectrum analyzer:
- With no primary signal, the output is measured. It shows the noise the equipment itself generates, for example from converter clocks, multiplexer switching or DC converters.
- With a pure sinusoidal primary signal, at rated frequency and magnitude, the output is measured. It shows the harmonic distortion the equipment introduces.
Aliasing and the anti-aliasing filter
Clause 6A.3 applies to LPITs with digital processing or transmission, even if they have an analog output. A digital system only represents frequencies up to half the sampling rate, which the standard calls fs. If there are several rates along the chain, the lowest governs, normally the output rate.
Frequencies above fs/2 are reflected downward. The most critical are those that land on the system frequency, fr. The first one to land on fr is fs − fr.
To avoid this, the LPIT includes an anti-aliasing filter. The samples that leave that filter then travel in a merging unit as sampled values. Table 6A.1 gives the minimum attenuation for frequencies equal to or greater than fs − fr:
| Accuracy class | Minimum attenuation |
|---|---|
| 0.1 | 34 dB |
| 0.2 | 28 dB |
| 0.5 | 20 dB |
| All protection classes | 20 dB |
The attenuation compares the signal entering at that frequency with the signal that appears reflected at fs − fr at the output, both referred to their rated values. For a low-power voltage transformer, voltages are used instead of currents.
Frequency response mask
The standard illustrates the set of requirements with a diagram. For measuring class 1, with fr of 60 Hz and fs of 4,800 Hz, the diagram plots the allowed gain against frequency. It marks the fundamental frequency, the harmonics, the aliasing frequencies and a forbidden region for the response.
- Harmonic or disturbance in the primary. Its frequency is above half the sampling rate
- Anti-aliasing filter. It must attenuate it before sampling, according to the class
- Sampling. What got through is reflected toward lower frequencies
- Frequency reflected onto the fundamental. The most harmful: it adds to the 50 or 60 Hz measurement
- Digital output error. Outside the class if the filter did not attenuate enough
Harmonic accuracy by class
Clause 6A.4 starts from a fact: non-linear loads, compensators and electric traction inject harmonics into the network, and these matter for metering, power quality and protection.
- Measuring classes (Table 6A.2). It sets ratio and phase error limits at low frequency and at harmonics, by order ranges. The limits grow with harmonic order and are tighter in classes with a lower number. The transition between points is a straight line on a logarithmic scale on both axes. The value at 0 Hz means DC coupling is permitted but not required.
- Power quality extension (Tables 6A.3 and 6A.4). For measuring harmonics up to order 40, and sometimes up to 50, in line with EN 50160 and IEC 61000-4-7. It can be applied to any class to declare better performance at high frequencies. There is one version for low-bandwidth DC applications and another for high-bandwidth ones.
- Protection classes (Table 6A.5). They cover the second to fifth harmonics and the 16.7 or 20 Hz component, which appears because of the influence of railway traction. The limits are the same for current and voltage transformers.
To see the exact limits of each range, consult the table in the current edition of the standard. This guide does not reproduce them because they depend on the class and the harmonic order.
Special wide-bandwidth classes
Clause 6A.4.5 describes a special protection class for traveling-wave relays, which need frequencies of up to 500 kHz. Table 6A.6 gives a maximum instantaneous error of 10 % from the rated frequency up to 50 kHz, and 30 % (3 dB) at 500 kHz.
The standard clarifies that the values are given for information only, because there is no consensus for general requirements. It also warns that, because of its wide bandwidth, this class is not suitable for the standardized digital outputs. Traveling-wave relays are usually supplied together with the transformers and the associated electronics.
Clause 6A.4.6 adds a special class for DC-coupled low-power voltage transformers. Its purpose is to indicate the polarity of the residual DC voltage on the line, with a maximum instantaneous error of 10 % from 0 Hz up to the rated frequency. With an analog output, take care that the input transformers of the connected relays do not saturate.
How it is tested
Clause 6A.5 says the ideal test applies the rated signal at rated frequency plus a percentage of each harmonic. Since that circuit is hard to build, applying a single harmonic per measurement is accepted. For equipment with a ferromagnetic core, tests below the nominal frequency are done with the signal magnitude reduced in inverse proportion to the frequency, to avoid saturating the core.
What not to do
- Do not buy an LPIT with digital output without asking for the harmonic accuracy class and the output sampling rate.
- Do not assume the anti-aliasing is the same in all classes. It changes from 34 to 20 dB.
- Do not use the traveling-wave class as if it were a standardized protection function.
- Do not compare the noise of two LPITs without the same reference and the same bandwidth.
Related topics
Harmonic accuracy is related to the dual accuracy class and to the sampling rate. Insulation and auxiliary power and electromagnetic immunity complete the requirements. The overview is in non-conventional instrument transformers and merging unit.
When to call us
At Tiza Group we review which class and frequency response your application needs, especially when there are converters, harmonic filters or traction load. If you need to study the power quality of your installation, see our power quality service. To verify installed equipment, we offer current and voltage transformer testing, in Colombia or elsewhere on request. We quote the analysis.
Related guides
- Dual accuracy class 0.2S/5P20 and INT32 scale
- Merging unit sampling rates: F4800S2I8U0
- Insulation and auxiliary power of a low-power LPIT
- Non-conventional instrument transformers (LPIT)
- THD: what it is, how it is calculated and IEEE 519 limits
References
- IEC 61869-6, Instrument transformers - Part 6: Additional general requirements for low-power instrument transformers, edition 1.0 of 2016, clauses 6.603, 6A.2 to 6A.6 (Annex 6A, normative) and Tables 6A.1 to 6A.7.
- IEC 61000-4-7 and EN 50160, to which Annex 6A refers for harmonic measurement and power quality.
Frequently asked questions
What is aliasing and why does it matter in an LPIT with digital output?
It is the effect by which a frequency above half the sampling rate appears reflected as a lower frequency. If that reflected frequency coincides with the fundamental, the LPIT measures wrongly even though the sensor works well. That is why IEC 61869-6 requires an anti-aliasing filter with a minimum attenuation depending on the class.
What is the first frequency that folds onto the fundamental?
It is the sampling rate minus the rated system frequency. With 4,800 samples per second in a 60 Hz system, it is 4,740 Hz. The standard uses exactly that case in its frequency response mask example for measuring class 1.
Can an LPIT be used as a traveling-wave sensor under IEC 61869-6?
The standard includes a special protection class with bandwidth up to 500 kHz, but for information only. There is no consensus to set general requirements, and the class is not suitable for the standardized digital outputs. This equipment usually comes designed together with the relay or the fault locator.