In IEC 61869-9 the accuracy of a transformer with digital output is measured end to end, from the primary to the message with its time stamp, and includes synchronization errors. Protection outputs are labeled with a dual class, for example 0.2S/5P20: the measuring class first and then the protection class with its accuracy limit factor. Samples are INT32 integers with a fixed scale of 1 mA for current and 10 mV for voltage.
Short answer
When an instrument transformer has a digital output, or when a merging unit digitizes its signals, accuracy is measured at the output, not at the terminals. IEC 61869-9, 2016 edition, defines it end to end: all the errors between the primary and the message with its time stamp, including the synchronization error.
To make use of the 32-bit format of the messages, the standard requires protection transformers to carry a dual class, for example 0.2S/5P20, and fixes a sample scale: 1 mA for current and 10 mV for voltage.
End-to-end accuracy
Clause 5.6 of IEC 61869-9 refers the accuracy classes to the product standards of the 61869 series (parts 7, 8, 12, 13, 14 and 15). It clarifies three things:
- The class specifications hold end to end. They cover all the errors introduced between the primary and the properly time-stamped message at the digital output.
- They directly incorporate the time synchronization errors, whose requirements are in clause 6.904.
- Transformers with digital output are classified into two groups: measuring and protection.
Clause 6.902.1 adds that the frequency response and accuracy with harmonics follow Annex 6A of IEC 61869-6, between the primary and the digital output.
For protection tests, accuracy, which travels in sampled values, is verified over one full period of the power-frequency signal, for example 20 ms in a 50 Hz system. For measuring, several cycles can be used or several measurements averaged.
The dual class
The standard requires all protection transformers and the protection channels of a stand-alone merging unit (SAMU) to have a dual class. Its reason is to document precisely the measuring class and the protection class that apply to each channel. It recognizes a field fact: protection transformers are often also used for measuring and indication.
The label is written with a slash ”/”. The measuring class comes first. Then comes the protection class followed by the accuracy limit factor KALF. The dual class must appear on the nameplate of the device.
| Label | What it is |
|---|---|
| 0.2S | Class 0.2S measuring transformer, with no protection class |
| 0.2S/5P20 | 5P20 protection transformer that also meets measuring class 0.2S |
How a conventional protection class, with its limit factor, is interpreted is explained in the current transformer guide.
- Is the channel used for protection?
- No: Measuring class only, for example 0.2S
- Yes: Dual class with a slash: measuring first, protection and KALF after, for example 0.2S/5P20
The INT32 scale
Each sample travels as a signed 32-bit integer, INT32, inside the IEC 61850-9-2 sampled values message. The standard fixes its scale for all manufacturers. It is the content of Annex 9A, which is informative:
- Current: the least significant bit (LSB) is worth 1 mA, as an instantaneous value.
- Voltage: the LSB is worth 10 mV, as an instantaneous value.
- One bit is used for the sign.
With 31 bits of magnitude, the largest possible value is 2,147,483,647. In current, that is 2,147 kA peak. In voltage, 21,475 kV peak. These are values far larger than anything that is measured, and that is the advantage: the same format serves currents of milliamperes and of tens of kiloamperes without changing scale.
How the standard justifies that scale
Annex 9A explains the criterion with an example of a class 0.1 current with application to protection and disturbance recording. The LSB is chosen about 4 times smaller than the minimum permitted error.
The example takes a 100 A rated transformer. Those 100 A rms are equivalent to 141,421 mA peak in a pure sinusoidal signal. In class 0.1, at 5 % of rated current, the tolerated current error is 0.4 %, that is, 0.0002 times the rated current, which is 20 mA. An LSB of 1 mA is far below that error.
On the high side, the maximum expected current is 65 times the rated one for busbar and line protection. The standard doubles it to cover a full DC component of the short-circuit current, and obtains a limit factor of 130. For generator and transformer protection it uses 25 times, which gives a factor of 50. In the 100 A example, the maximum peak stays within the 32-bit scale with margin.
For voltage, the maximum factor assumed is 2.5 times the rated voltage, which covers most applications, instead of the 1.9 of the earlier standard IEC 60044-7.
The nomograms
Figure 9A.1 is a current nomogram and 9A.2 a voltage one. They work like a slide rule. The reference line is moved to the rated current, or rated voltage, of the device. The two arrows must stay within the ±31 bit scale.
The current nomogram marks classes 0.1, 0.2, 0.5 and 1, classes 0.2S and 0.5S, the values for busbar and line protection, for generator and transformer protection, and the requirements of distance functions. The voltage one marks measuring classes 0.1 to 1 and protection classes 3P and 6P.
- Rated current or voltage of the object. For example 100 A rms
- Peak value. 141,421 mA in the example, for a sinusoidal signal
- Maximum expected current. 65 or 25 times the rated one, doubled for the DC component
- Minimum error permitted by the class. LSB chosen about 4 times smaller
- Both ends within ±31 bits. The same format covers the whole application
What to check in a purchase
- Ask for the class of each channel as a dual class, with KALF, if the channel is used for protection.
- Check that the nameplate and the accuracy statement mention the end-to-end class, not only that of the sensor.
- Verify that the project’s sampling rate and synchronization are those that were considered for that class.
- When subscribing to the samples, the relay must scale with 1 mA and 10 mV per bit. A wrongly declared scale gives measurements with an incorrect factor.
What not to do
- Do not compare two merging units by the class of the sensor. Compare the class of the digital output.
- Do not label a protection channel with the 5P20 class alone without the measuring one.
- Do not assume a scale other than 1 mA and 10 mV on an output that declares IEC 61869-9.
- Do not forget that synchronization error is part of accuracy.
Related topics
The dual accuracy class and the scale combine with accuracy with harmonics and noise and with the sampling rate. The conformance classes indicate which variants of the interface the device meets. The complete function of the unit is described in merging unit.
When to call us
At Tiza Group we review the digital output specification of your transformers, the scale in the relay and the real accuracy of the whole chain, and we check them on the installed equipment. We offer current and voltage transformer testing, in Colombia or elsewhere on request, and IEC 61850 engineering. We quote the work according to scope.
Related guides
- Accuracy with harmonics and noise in an LPIT
- Merging unit sampling rates: F4800S2I8U0
- Conformance classes a, b, c and d of a merging unit
- Merging unit: IEC 61869-9, 9-2LE and testing
- Current transformer (CT): parts, functions and testing
References
- IEC 61869-9, Instrument transformers - Part 9: Digital interface for instrument transformers, edition 1.0 of 2016, clauses 5.6, 6.902.1, 7.2.6 and Annex 9A (informative, figures 9A.1 and 9A.2).
- IEC 61869-6, edition 1.0 of 2016, Annex 6A.
Frequently asked questions
What does 0.2S/5P20 mean on the nameplate of a transformer with digital output?
It is a dual class. The 0.2S part is the measuring accuracy class and 5P20 is the protection class with an accuracy limit factor of 20. The device meets both. A measuring-only transformer is labeled with the measuring class alone, for example 0.2S.
Why does the standard fix the sample scale at 1 mA and 10 mV?
Because a signed 32-bit integer with that scale covers the dynamic range required by the measuring and protection classes, from small currents up to fault currents with a DC component. Annex 9A shows it with nomograms. The fixed scale prevents each manufacturer from using a different one.
Does the accuracy of a merging unit include time synchronization?
Yes. IEC 61869-9 says the accuracy classes apply end to end and directly incorporate the errors associated with time synchronization. The time error of the synchronization source is part of the phase error that is evaluated.