The protective relay as a node in the substation control network

The microprocessor-based protective relay is increasingly being employed as an integral measurement and control node in the substation automation environment. This paper discusses those aspects of relay design that impact its use as a communications and substation node device. Elements of concern include communications and data response times, accuracy of measurements, use as a control output device and interaction between protection functions and automation functions. Once a relay has been integrated into an automation system, the commissioning and maintenance testing of the relay becomes much more complicated and can require extensive supplementary equipment and extended time periods for testing. Systems that integrate the relay as a system device require careful design to ensure the relay can be used without compromising its protection functions; and to ensure the integrated system will perform as expected and as intended under all conditions. When the relay becomes an integral part of a SCADA (supervisory control and data acquisition) system, additional functions, not normally considered a part of the relay itself, become critical and at the same time their implementation is most cost-effective if additional capabilities are designed into the relay at the outset