Achieving the future-proof architecture of a distribution substation automation

Substation automation architecture

This thesis investigates how station-level data processing can be utilized to help in creating a future-proof architecture for the secondary system of a distribution substation. The needed technology is evaluated, and an overall life-cycle cost analysis is performed showing the cost benefit of a centralized architecture.

The thesis shows that the larger the substation, the greater the benefits of a centralized architecture. It also shows how great is the impact of increased reliability. The outage costs of a network exceed all the other life-cycle costs of the secondary system, and illustrates how focusing on substation automation is a cost-efficient way to improve the reliability of the network.

The question is how to proceed with substation automation so that it is both interoperable and able to utilize new algorithms during the life-span of an IED, without increasing the life-cycle costs or shortening the life-span of the physical device itself.

At the same time, utilities want to discourage vendors from creating monolithic secondary systems, which can only be sold and maintained by a single vendor.

Various concept-level proposals for the secondary system of a substation have been proposed which address the conflicting requirements for low life-cycle costs and the speedy utilization of new technology.

New features have also called for substantial changes in the substation’s entire secondary system, requiring maintenance breaks or the time-consuming planning of back-up connections

An overview of this set-up, along with the two others, is presented in Figure 1 below, where the set-up is described as ‘Decentralized’.

An alternative approach has been to fully centralize the functionality in a distribution substation. By moving all the functionality to a centralized station computer, the life cycle of the bay-level measurement devices has been greatly extended, up to the lifetime of the primary equipment.

Furthermore, the upgrade measures needed to implement new features have been simplified, because only the centralized station computer requires updating. This, however, creates a single point of failure in the substation. When the central station computer is out of operation, the protection for the whole substation is lost.

The same maintenance problem as with a fully decentralized solution also exists. When an upgrade is needed in an environment where all the functionality resides in the same computer, the upgrade affects the whole protection system.

Therefore, maintenance breaks and extensive testing are required.

A third approach addresses the challenge by combining these two methods. In this approach, only a part of the bay-level functionality is moved to a new substation-level centralized station computer.

The functionality is divided so that the most critical and important features, such as earth fault or overcurrent protection, would remain in the bay-level devices, thus ensuring network safety in all situations. This forms the backbone of a network protection system with a long life cycle.

The functionality defined for the substation level would consist of value-added applications and other “nice-to-have” features, for which a faster update cycle is both necessary and acceptable.

This set-up also has a natural in-built back-up scheme, as the bay-level and station-level devices provide a redundant protection system. The measures for updating the central unit are cheaper and safer, allowing the smooth utilization of new functions.

One hypothesis of this thesis is that this combined approach will provide the most future-proof platform for the secondary system of an electricity distribution substation, resulting in the lowest overall life-cycle costs.