Future trends


The question was asked whether IEC 61850-9-2 Process Bus will be a future trend in the Utilities market, even for indoor substations? My response was:

I believe there is a strong benefit for indoor GIS substations too. 9-2 process buses facilitate vendor independent connections to non-conventional instrument transformers (NCITs), such as Rogowski coils. Rogowski coils are used in ABB’s combi-sensor for GIS and iPASS, and these have been converted to 9-2LE operation in Queensland.

The cable runs are shorter in a GIS substation, but the process bus advantages of combining more sensors and monitoring into a single fibre optic pair from each bay can still be realised. The largest GIS switchroom I’ve seen was around 320m long and had 345kV and 169kV GIS buses at the Taichung Power Station in Taiwan. The linear design (compared to a star/radial design in an AIS yard) means that reducing multi-core copper cabling is still worthwhile.

You have some preconceptions there that are not entirely correct:

  1. Yes, redundancy is important. This can be solved two ways. The first is to use PRP or HSR to have redundant network connections from the MU to the IEDs. The second is to use two MUs (from different vendors). This is similar to the Main1/Main2 or X/Y approached used now. All an MU is doing in a conventional substation is moving the ADCs into the switchyar. As Maciej pointed out, this has a BIG safety benefit.
  2. Signal delay does affect performance, but this is why the performance classes are defined in IEC61850-5. For a transmission substation there is an allowance of 3 ms to get the measurement into the ‘thinking’ part of the IED. This includes the digitising and publishing time (40%), network transmission (20%) and receiving and processing (40%). My research found that a transformer relay that could use CTs or SV as inputs responded 0.4 ms slower with SV than with CTs. This paper has just been accepted and I have linked to it below.
  3. Yes, LAN design is required, just as proper cable design (selecting cross sectional area, terminal types etc) is required for a network based solution. Provided the network traffic does not exceed the LAN limit (e.g. 100Mb/s) the ‘slow down’ is simply due to queuing effects and the largest delay will be just under 250 µs (50 Hz) or 208 µs (60 Hz). Even with a ‘plug and pray’ approach process bus networks are surprisingly robust. I tried really hard to break my test network and overloading traffic (injecting at 1 Gb/s) was really the only way I achieved it. At the risk of blatant self promotion, I would like to link to my research papers, which are experimental assessments of process bus operation:
    • Performance analysis of IEC 61850 sampled value process bus networks
    • Network interactions and performance of a multi-function IEC 61850 process bus
    • System level tests of transformer differential protection using an IEC 61850 process bus

@Michael, You are quite correct in that the NCIT to MU is vendor specific. In some cases, such as SDO’s Optical CT the secondary converter with the light source is in the same box as the merging unit. ABB’s iPASS solution is a retrofit and uses the MVB synchronous serial protocol that used to go to ABB’s relays to supply data to the MU. The benefit of using 61850-9-2 as the IED interface is that the selection of instrument transformer and protection IEDs can be decoupled. In the past you had to select matched pairs, but this is no longer the case. There will always be a vendor-dependent section, because that is how things are made. I doubt anybody would expect an SDO/Arteche secondary converter to drive a NxtPhase/Alstom sensor head. There is nothing to stop a utility installing sensor heads from two different vendors, running to two separate secondary converters, then to two separate merging units, and then onto two separate LANs. With conventional CTs the secondary torroids for X & Y are independent, they’re just in the same bushing. With OCTs the HV apparatus is much smaller, so you’re able to achive the same thing. As for analogue interfaces, it seems that very few people adopted them. IEEE Std C37.92 defines a 4 Vrms nominal for 1 pu voltage and 200 mVrms nominal for 1 pu current. It is certainly easier to generate these signals than 110 Vrms and 1 Arms, but when the OCT and NCIT measurement engines are digital it makes sense to publish the information in true digital form. This reduces noise in the system and allows the use of fibre optic cables for communication. The NCITs that support high power outputs generally do so with an external amplifier module, and this increases the cost of the system.

You might see more of this technology in Oil & Gas. I’m now in this industry and we have 132/33 kV substations to power electrical driven gas compressors. Sometimes it is more economical to buy power from the grid than to burn valuable fuel gas in a turbine to drive a compressor.

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