Process Bus Architectures for Substation Automation with consideration of Life Cycle Cost

The replacement and maintenance of substation secondary equipment may lead to downtime, which negatively affects the overall availability of the substation. One factor which contributes to this problem is the complex wiring required for the installation of a relay. Another possible contributing factor is relay obsolescence. If a new relay has to be installed, it may require a new set of configurations in order to communicate with the existing substation equipment, and this may not be possible without the use of expensive protocol converters. The IEC 61850 standard provides a means of reducing the downtime by using a standardized Ethernet bus, called the process bus, to provide an interface between protection Intelligent Electronic Devices (IEDs) and the primary equipment. The Ethernet connections are less complex than the original copper wiring and the standardized solution ensures IEDs from different manufacturers are interoperable without requiring converters. The implication of this 'plug and play' solution is that the time required to commission or replace IEDs is reduced. Substation Automation leads to the division of the substation into 3 logical levels - (i) the station level where high level control operations are carried out; (ii) the bay level, where the protection and control functions for different types of primary bays are located: and (iii) the process level, which provides the interfaces to the primary equipment. The station bus is the interface between (i) and (ii); the process bus is the interface between (ii) and (iii). A number of studies and site trials have been carried out at the station level, and consequently implementation of the station bus can now be considered mature technology. The maturity of the process bus is not considered to be at the same level as the station bus; a possible reason for this is that the process bus requires time critical digitized data, whereas the station bus can cope with a delayed response time. This presentation focusses on the IEC 61850 process bus architecture design for a feeder bay within the National Grid. A range of architectures can be applied, depending on the requirement for each substation such as reliability, availability and cost. Different possible architectures for the feeder bay are designed and compared by using a decision matrix that compares a list of factors, weighted according to the relative importance of reliability, availabilty and cost. The minimum life cycle cost method (where the choice with the lowest life cycle cost is deemed the optimum) is used for this purpose. In this presentation, minimum life cycle cost is used to determine the optimal process bus architecture deisgn. The system reliability and availability figures are estimated for each architecture. The life cycle costs based on these reliability and availability figures are then estimated. The one with the lowest life cycle cost is determined and this is the optimal process bus architecture.