SUBSTATION AUTOMATION FROM RUS BULLETIN 1724E-300

SUBSTATION AUTOMATION FROM RUS BULLETIN 1724E-300
What is Substation Automation in the Electrical Power System?

Substation automation is the use of state-of-the-art computers, communications, and networking equipment to optimize substation operations and to facilitate remote monitoring and control of substations cost effectively.

Substation automation uses intelligent electronic devices in the substation to provide enhanced integrated and coordinated monitoring and control capabilities. Substation automation may include traditional SCADA equipment, but more often encompasses traditional SCADA functionality while providing extended monitoring and control capabilities through the use of non-traditional system elements.

In the traditional SCADA system (legacy system), a host computer system (master station) located at the energy control center communicates with remote terminal units located in the substations. RTUs are traditionally “dumb” (non-intelligent) devices with very limited or no capability to perform local unsupervised control. Control decisions are processed in the master station and then carried out by the RTU through the use of discrete electromechanical control relays in the RTU.

Analog telemetry information (watts, VARs, volts, amps, etc.) is generated by discrete transducers whose outputs are wired into the RTU. Device status (breaker position, load tap changer position, etc.) is monitored by the RTU through sensing of discrete contacts on these devices. Monitored data is multiplexed by the RTU and communicated back to the master station computer in the form of asynchronous serial data. Substation automation systems do include many of the same basic elements as the legacy SCADA system but with significant enhancements.

A central operations computer system generally provides the master station function. Legacy RTUs may be incorporated into the automation scheme, particularly in retrofit situations, but are generally replaced with intelligent programmable RTUs and other IEDs in an integrated LAN. Legacy transducers are replaced by IEDs that provide not only the traditional analog signals, but a number of additional data values that can be useful to operations, engineering, and management personnel. IEDs communicate with RTUs and local processors via a substation LAN with an open communications protocol, thereby eliminating discrete transducer analog signals.

Programmable logic controllers (PLCs) may be included, discretely or integrated into the intelligent RTU, to provide closed loop control and control functions, thereby eliminating the need for many electromechanical relays and interlocks. The integration of IEDs in the substation has been a major challenge for electric utilities and equipment suppliers.

The primary obstacle has been the lack of standards for LAN communications protocols, with manufacturers opting for proprietary protocols that require costly interface modules for protocol conversion. Strides have been made in recent years to resolve the protocol standardization problems, and some de facto standards have emerged. The trend will continue toward more vendor-independent substation network environments as these standardization efforts move forward and as the level of standards support improves among IED manufacturers.