Process Bus - more than just CT/VT

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Process Bus is arguably the next biggest evolution of the impact that IEC 61850 has on the engineering, implementation, construction and operation of electrical facilities for substations, generators and switchboards.

You might be surprised to know that you may already have it or could have it without too much fuss, bother or controversy.

Plant Status and Controls - General I/O Interface

The K group Logical Nodes for the controls of standard items of plant such as Pumps, KPMP), Fans (KFAN) etc.

In the case of switchgear, the Logical Nodes are defined as XCBR and XSWI.

However it may be that the actual primary plant does not have its own direct IEC 61850 LAN communication port interface yet it would be advantageous to use the LAN to replace or avoid extensive wire-based solutions in favour of the LAN.  This is particularly applicable to SAS refurbishment to existing primary equipment.

In such cases, it is necessary to use an I/O interface that can be located in proximity to the plant with very short cabling compared to the extensive numbers and length of conventional cabling, marshalling boxes, isolating links etc  to the secondary equipment.

It is perfectly in accordance with the Standard and the intent of GGIO for Vendor's to implement GGIO as a generic I/O mapping for their opto inputs and contact outputs simply because at the time of manufacture of the IED, they do not have any specific definition of the physical world to which they will be connected.

However it is also useful if those GGIO have the capability of being renamed to a standard IEC 61850 Logical Node once the application is known.  An example of this is providing an I/O unit as the direct interface from the SAS to the switchgear

Click to enlarge

Sampled Values of Pressure, Temperature, Rotation, Humidity ....

There are many 'real-world' quantities which are measured in substations, and indeed any process plant.  In the past these signals were provided as 0-10 mA, 4‑20 mA or -10/0/+10 mA transducer values.

These analogue values can be transmitted as Sampled Values using the T Group Logical Nodes.

Their sampling rate may be much slower than say for a current or voltage measurement of the power system, but the mechanisms and data models are defined in IEC 61850.

IEC 61850 defines sampling rates as INT16U integer and qualified as samples/second or seconds/sample i.e. from as fast as 65535 samples/second (one sample every 0.015 milliseconds) to 65535 seconds/sample (one sample every 18.2 hours) so it can be just an hourly log of transformer temperature or height of the water in the dam!

Combining the T group with S and H group Logical Nodes enables the IEC 61850 technology to revolutionise other SCADA and Industrial control in other areas such as water, gas etc

Replacing 1 A / 110 V Analog wires with IEC 61850-9-2 Sampled Values

Most instrument transformers in use have been wound-secondary construction with 1 A or 5 A CT or 110 V VT outputs (so-called "conventional" CT/VT) which of course can still be expected to have a long operating life (~50 years).

However these involve hundreds of wires and terminations per bay creating huge engineering and operational issues, especially when multiple devices need to have measurement of the current and voltage on the line/busbar.

The conversion of the measurements into a digital message is carried out in the Merging Unit - so called since it combines several measurements into a single message containing all the instantaneous values of the measured analogue values into one single message.  These instantaneous measurements are the Sampled Values defined in IEC 61850‑9‑2 and the Guideline document from UCAIUG as IEC 61850‑9‑2LE.

Sample Values can be taken at whatever rate is appropriate for the particular analogue quantity being measured.  In the case of CT/VT sensors according to IEC 61850‑9‑2, this is defined as 80 samples per cycle (0.25 ms @ 50 Hz) for protection functions or 256 samples per cycle (0.078 ms) for power quality and synchrophasors. As the sampling rate is defined as samples per cycle, the sampling frequency as samples per second varies according to the power system frequency and hence the time between samples varies (sad).

As an example the Schniewindt Stand Alone Merging Unit (SAMU) is a useful solution to convert conventional 1 A / 110 V CT/VT signals to IEC 61850‑9‑2LE (many other vendors now offer IEC 61850‑9‑2LE SAMU as well).
SAMU can be installed next to the CT/VT in the yard (in a suitable enclosure) or immediately where the yard cabling enters the control room.

To note that IEC 61850‑9‑2LE is not an official Standard  ... but the title states it as "Implementation Guideline for Digital Interface to Instrument Transformers using IEC 61850‑9‑2", i.e. it is a "Guideline" to help the various vendors implement Sampled Values in a consistent method - it is an interim solution to describe how IEC 61850‑9‑2 generic interfaces should be configured to suit CT and VT applications in a substation ... imagine if one relay or MU vendor assumed sampling at 4000 Hz, another at 3200 Hz, another at 80 samples/cycle and another at 96 samples per cycle ..... all chance of vendor-agnostic interoperability would be lost.  IEC 61850-9-2LE Guideline therefore allowed all the MU and relay vendors to focus on some common definitions rather that arbitrary different sampling rates that may cause interoperability problems that would hinder adoption.

However, IEC 61850‑9‑2LE is now effectively defunct

In fact, on a broader scale, IEC 60044 series for CT/VT specification has been usurped by the official IEC 61869 series - in particular IEC 61869‑9.

IEC 61869 now defines sampling rates as samples/second : preferred 4800 samples per second preferred for protection, preferred 14400 samples per second for power quality, but lowest 4000 samples per second for legacy reference, and as high as 96000 samples/second for HV d.c.

There are some other variances between IEC 61850‑9‑2LE and IEC 61869‑9 - notably

  • IEC 61850‑9‑2LE defined time synchronisation input as 1PPS over glass fibre, but some vendors simply continued with coaxial cable.  To note that in reality 1PPS and IRIG‑B s nowhere near accurate enough to suit the one microsecond time coherency requirement for CT/VT Sampled Values ... but IEEE 1588 v2 PTP was not available when they wrote the guideline.
  • IEC 61869‑9 provides for the far more suitable IEEE 1588 v2 PTP over fibre of any sort - although IEC 61850‑90‑3 recommends use of LC connectors on the fibre. 

Low Power Instrument Transformers (LPIT)

LPIT is NOT a new technology with optical CT/VT trials even in 220 kV substations dating back to the early 1980's.

They have been used extensively for revenue metering applications for more than two decades due to their exceptional linearity and accuracy range from importing to exporting power levels for independent power producers.
Risk reduction is a significant benefit compared to the potential for conventional CT explosion.
LPIT obviously needs some form of electronics to convert the sensor signal into a usable form either as an analogue signal e.g. 1 A or 10 mA, or as a digital message. IEC 60044‑8 defined an early standard message format for Merging Units. This is now handled more universally as IEC 61850‑9‑2 and the interim guideline from UCA IUG as “9‑2LE”

Low Power Instrument Transformers (LPIT) are also known as Non Conventional Instrument Transformers (NCIT) and have two parts of the unit:

  • Primary Sensor based on different "electro-physical" principles
    • Optical Faraday Effect CT
    • Optical Pockels Effect VT
    • Rogowski Coil CT
  • Sensor Interface Unit (SIU) output options
    • If the SIU combines two or more SVs into one message, the SIU would be referred to as a Merging Unit
    • Analogue 1 A / 110 V 'conventional' outputs per phase
    • Analogue 0-10 mA outputs per phase
    • IEC 61850‑9‑2 or more specifically IEC 61869‑9 Sampled Value output

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Extra Notes:

Process Bus is that part of the LAN that specifically carries all the signals to/from the primary "process" plant and equipment.  Process Bus therefore carries BOTH IEC 61850 8-1 and IEC 61850 9-2 messages if there are any intelligent devices associated with the primary plant.  It may be that a VLAN is dedicated on the fibre for Process Bus signals only.

Station Bus is the section of LAN that carries messages between the Bay IEDs and the Station Equipment.  Depending on the LAN architecture, the fibre that carries Station Bus signals may also carry Process Bus signals.

Play "Spot the Process Bus"

a)    which part of this diagram is Process Bus?

b)    Does the answer change depending on where the RSTP loop is open?

click to enlarge:


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