INTRODUCTION

The IEC 61850 Standard for Communication Networks and Systems for Utility Automation allows utilities to consider new designs for substations applicable for both new substation and refurbishments. The levels of functional integration and flexibility of communications based solutions bring significant advantages in costs in all stages of a project. This integration affects not only the design of the substation, but almost every component and/or system in it - protection, monitoring and control, by replacing the hardwired interfaces with communication links. The use of high-speed peer-to-peer communications using Generic Substation Event (GSE) messages and sampled values from non-conventional or conventional sensors, allows the development of distributed applications. In addition the use of optical local area networks leads in the direction of copper-less substations.

This article is addressing the issues only from the viewpoint of the communication network. The principles, applications and benefits of data models and the configuration language will be discussed in a future article.

Interoperability or Interchangeability

One of the main reasons for the success of IEC 61850 is that for the first time it introduced a set of tools that allow the development and engineering of state-of-the-art high performance, adaptive and self-healing substation protection, automation and control systems based on the latest communications technology and multifunctional devices from different suppliers. Substation automation systems (SAS) and distributed protection schemes are not something new. However, they have typically been designed and implemented as single-vendor solutions, using for the protection schemes hard wiring or proprietary vendor communication protocols. The requirements of the users for new communications based solutions without any degradation in functionality and performance while not being restricted to the use of single vendor's devices, led to the development of the peer-to-peer communications models and services known as GOOSE (Generic object oriented system event) and Sampled value transmission in the IEC 61850 domain. It is important to understand that the goal of IEC 61850 is to ensure interoperability - i.e. the ability of two or more IEDs from the same vendor, or from different vendors, to exchange information and use that information for correct execution of specified functions. In some cases it may be even possible to achieve interchangeability - the ability to replace a device supplied by one manufacturer with a device supplied by another manufacturer, without making changes to the other elements in the system.

The requirements of different applications resulted in the definition of IEC 61850 models and services that can support all substation protection, automation and control applications. One of the differentiating contributions of the standard compared to other communications protocols is that it goes beyond the typical SAS communication types and specifies methods that can be used for time-critical functionality typically provided through hard-wired signals exchange. Another important contribution is the definition of standardized object models where the information available from the different applications is defined both with syntax as well as with semantic.

An important issue to understand when considering the use of different devices and tools is that even though a device can be compliant to the definitions of the standard, it may not be able to communicate with devices from other suppliers. This is due to the fact that in order to support innovation and flexibility IEC 61850 limits the number of mandatory data objects and attributes, as well as it leaves the selection of implemented services to the supplier of any specific device. The price to pay is the issues with interoperability. That is why during the process of selection of devices and tools to be used in a substation protection, automation and control system, it is very important to perform interoperability testing between all devices intended for use in a project.