The term's station bus and process bus used in IEC 61850 are history. IEC 61850 as it exists today does not make any prescriptions with regard to substation architecture.

When the IEC standardization work with regards to substation communication started, the basic idea was, to have an architecture with 3 levels (see Figure 1): station level, bay level and process level. The station bus (IEC 61850, part 8) was intended for the communication between station and bay level, the process bus (IEC 61850, part 9) for the communication between bay and process level.

Figure 1 - original architecture for the substation communication

The further progress with the standardization work and in particular the harmonization with UCA showed that this was not the only architecture to be supported. It was therefore decided that IEC 61850 shall not make any specifications with regards to communication topology. As a consequence, IEC 61850 does not restrict any physical communication interfaces; it only specifies communication services that may be combined on a (physical) communication link according the requirements. It was further decided, that all communication shall use Ethernet (ISO/IEC 8802-3) as the basic communication technology.

In IEC 61850, the communication is specified with abstract services and models in part 7 and with a mapping of these abstract services and models to a concrete communication protocol in part 8 and part 9. There is one model that is only used for the communication between bay level and process level equipment: The model for the transmission of sampled values. It was therefore decided, that the mapping of that model shall be defined in part 9, while the mapping of all the other models shall be defined in part 8 (see Figure 2).

Figure 2 - Mappings defined in IEC 61850

To summarize: the difference between part 8 and part 9 is in the abstract models from part 7 that are mapped. Part 8 and part 9 only specify concrete communication models and services. They may be used over the same communication interface.

Keywords:
mappings, part 8, part 9, station bus, process bus, sampled values

provided by Christoph Brunner
ABB Switzerland Ltd
Convenor of TC57 / WG12