IEC 61850 Measurement Nodes: MMXU, MMTR and Sensors

What the standard says and what it is for

Edition 1 of IEC 61850-7-4 (2003) gathers metering in group M and sensors and monitoring in group S. The measurement nodes answer a practical question: where is each quantity in the relay or meter model and what is it called. The sensor nodes do the same for the state variables of primary equipment, such as the gas in a circuit breaker or the oil in a transformer. How CSWI, CILO and XCBR form the command chain is covered in IEC 61850 Control Logical Nodes.

Both groups are clear examples of how a common model reduces integration work: SCADA reads the same paths on any device, without manufacturer-specific register tables. The group classification is in the guide on logical node groups. How IEC 61850 models wind turbines and distributed resources is explained in IEC 61850 Logical Nodes for Wind and Generation.

This guide describes the model. The calculation of power factor or power quality indices is not covered here: each topic has its own guide, such as power factor in industry.

Group M nodes

MMXU and MMXN

MMXU calculates currents, voltages, powers and impedances in a three-phase system. Its data objects are optional and cover total active, reactive and apparent power, average power factor, frequency, phase-to-phase and phase-to-ground voltages, phase currents, per-phase powers, power factor and per-phase impedance. MMXN does the same for a single-phase system, where voltages and currents are not related to a phase.

When the device measures through an external instrument transformer, the source of the measurement is modeled with the primary equipment nodes.

MMTR

MMTR calculates three-phase energy and is used mainly for billing. It handles net apparent, real and reactive energy since the last reset, and supply and demand energy, with a default direction: supply is the flow into the busbar and demand is the flow out of it. To know which convention a meter uses, check that definition before interpreting the signs.

MSQI

MSQI describes the positive, negative and zero sequence components of current and voltage, the values of the DQ0 transformation and several unbalance indices, including the maximums. At least one of its sequence data objects must be used.

MHAI and MHAN

MHAI calculates harmonics or interharmonics in a three-phase system. Two settings define what is obtained: the base frequency and the evaluation window. When the window equals one cycle of the base frequency, the result is harmonics; with more cycles, subharmonics and their multiples also appear. The lowest analyzable frequency is the inverse of the window and the highest is half the sampling rate. The frequency can be given or calculated, for example with a phase-locked loop, though only when there is a dominant frequency. For the methods of calculating disturbances the standard refers to IEC 61000-4-7 and IEEE 519. See IEEE 519 harmonic study and the THD harmonic distortion tool.

MSTA

Many values are not used raw but as averages, minimums and maximums over an evaluation period. MSTA calculates them for current, voltage and apparent, active and reactive power. The period is adjustable and a control command starts the interval; the report can start when the period ends.

MDIF

It provides the calculated values that represent the other side of a line or other object, as used by differential protection (PDIF): the remote operating current or the current samples of each phase.

The measured value: range, quality and time

Measurement data objects use the MV (measured value) class of part 7-3. Each value carries:

• the value (mag), which is reported on change;
• a range indicator: normal, high, low, high-high or low-low;
• the quality and the timestamp, mandatory;
• as configuration, the unit, the deadband (db) and a parameter (zeroDb) that forces values very close to zero to zero;
• optionally, a sampling rate and substitution.

The deadband is a percentage of the range between maximum and minimum: a value is reported when it changes by more than that percentage. That is the reason for a frequent question: SCADA does not «see» small variations because they are filtered at the source. For a billing meter, which cannot lose resolution, the band is adjusted with care.

Group S nodes: sensors and monitoring

SIMG. The critical gas alarm (for refilling) is mandatory; others indicate unsafe gas (block operation of the equipment) or dangerous gas (trip to isolate the equipment), and at least one of the pressure, density or temperature alarms must be used according to what is supervised. This relates to the SF6 low pressure alarm and the SF6 density monitor. If there are more measuring points, they are added with numbered extensions of the data object (Tmp1, Tmp2…).

SIML. For oil-filled transformers and tap changers. Among its optional data objects is gas in the liquid, which according to the standard can be used for the Buchholz alarm or trip. It is the node where moisture, hydrogen and level live; the subject is expanded in online transformer monitoring. See also the Buchholz or sudden pressure trip.

SARC and SPDC. SARC distinguishes the arc fault (mandatory) from the switching arc; SPDC reports the acoustic level and the partial discharge alarm, with a mandatory operation counter.

How it is applied

• Signal list: define which data objects go to SCADA and with which deadband.
• Testing: inject a known quantity and compare the node value with the reference, also verifying the range and quality it reports.
• Quality metering: place in MHAI and MSQI what the analyzer or the relay calculates; metering and power quality have their own guide.
• Monitoring: tie SIMG and SIML to the alarms the operator sees.

We can help with your metering

At Tiza Group we verify measurements, scales, deadbands and sensor alarms in your IEDs, from the source to SCADA. We do it remotely with our remote relay configuration or on site, in Colombia or elsewhere on request, with our IEC 61850 engineering. How IEC 61850 models a measurement (value, deadband, range, vector and three phases) is explained in IEC 61850 Measurements.

Related guides

• Logical node groups and system nodes LLN0 and LPHD
• IEC 61850: what it is, parts of the standard and data model
• Energy meter and multifunction meter
• Power quality analyzer
• Primary equipment logical nodes
• IEC 61850 MMS: reports, SBO controls and files
• IEC 61850 Logical Nodes for Wind and Generation
• IEC 61850 Measurements: MV, CMV, WYE and DEL
• IEC 61850 Control Logical Nodes: CSWI, CILO, ATCC

References

• IEC 61850-7-4, edition 1 (2003): clauses 5.10 and 5.11, tables 7 and 8.
• IEC 61850-7-3, edition 1 (2003): MV data class.
• IEC 61000-4-7 and IEEE 519: harmonic calculation methods (cited by the standard).

Frequently asked questions