Final Thoughts


When discussing the communications network architecture for IEC 61850 based PAC systems one of the most commonly asked questions is “What should the communications architecture be?” And as usual the answer starts with “It depends!”

It depends on many, many things. The IEC 61850 90-4 TR Network engineering guidelines for substations includes a detailed check list of all the different criteria that need to be met by the substation’s communications network. But before the PAC specialists start going through the checklist they need to do some homework. 

First of all we need to consider if we are designing a fully digital substation (with MMS, GOOSE and sampled values) or a hybrid substation (with MMS, GOOSE and hard wired analog circuits.)

The second critical requirement is to determine the performance requirements. We need to know the maximum acceptable fault clearing time for the specific substation in case of breaker failure - the most challenging fault condition from the point of view of the stability of the electric power system. Once we know this time we can determine the maximum acceptable latencies between all IEDs exchanging messages as part of the protection scheme. This will help in the design of the communication network that will meet the performance requirements.

Another issue is the requirements for redundancy.
Many times I hear the argument that it is too expensive to use IEC 61850 in small distribution substations. But is that really true? If we think about the fact that such substations typically do not have redundant batteries, it means that a battery failure which brings down the complete PAC system in the substation is acceptable. If this is acceptable, the failure of a single switch used for the communication network start topology will be acceptable as well, meaning that we can replace without hesitation the copper wires between all IEDs with GOOSE messages between them thus taking advantage of all the benefits that this will bring us.





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