IEC 61850 Process Bus - It is Real!

Authors: Damien Tholomier and Denis Chatrefou, AREVA T&D

Benefits and Future of Process Bus

Process Bus Benefits
Process bus based applications offer some important advantages over conventional hard wired analog circuits. The first very important one is the significant reduction in the cost of the system due to the fact that multiple copper cables are replaced with a small number of fiber optic cables.
Another result is the practical elimination of CT saturation because of the elimination of the current leads resistance RL.

In this case the CT secondary is connected to the phase current inputs of the Merging Units and RL is practically equal to zero. The knee-voltage then will be only dependent on In this case the impedance of the merging unit current inputs RRP is very small, thus resulting in the elimination of CT saturation and all associated with it protection issues.

An additional benefit of process bus based solutions is the improvements of the safety of the substation by eliminating one of the main safety related problems - an open current circuit condition. Since the only current circuit is between the secondary of a current transformer and the input of the merging unit located right next to it, the probability for an open current circuit condition is very small. It becomes non-existent if optical current sensors are used.

Last, but not least, the process bus improves the flexibility of the protection system. Since current circuits can not be easily switched due to open circuit concerns, the application of bus differential protection, as well as some backup protection schemes becomes more complicated.

The above is not an issue with process bus, because any changes will only require modifications in the subscription of the protection IEDs receiving the sampled analog values over IEC 61850 9-2.

The Future
The process bus is a very promising technology. First experiences have proven its feasibility. The simultaneous experimentation of NCIT and process bus has first been driven by the NCIT in order to connect them to protection systems. The test of both technologies is probably one of the reason of the limited use today of the process bus. The focus on purely the process bus with conventional sensors is likely to develop the business, for instance for retrofit (replacement of cable) and voltage distribution (also in MV) applications.

The industrial optimization phase shall now start in order to bring fully cost effective solutions and be generalized. New IEDs based on IEC61850-9-2 (protective relays, Intelligent Merging Unit, etc.) will become available progressively, while in parallel, the utilities will gain confidence in "protection over process bus".

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