How reliable are modern substation automation systems?

Protection, control and monitoring

Typical power substations are supposed to receive, transform the electricity either by stepping up or down the voltage and then send it forward. Substations designed in the past made use of protection and control schemes implemented with a single-function, electromechanical or static devices and hard-wired relay logic.

With the advent of microprocessor based multi-function Intelligent Electronic Devices (IEDs), we get the opportunity to move more functionality into fewer devices; as well as resulting in simpler designs with reduced wiring.

Beginning in early 1990s, industry developed a communications architecture that would facilitate the design of systems for protection, control, monitoring and diagnostics in the substation, which was the Substation Automation Systems (SAS).

IEC 61850 Based SAS

The IEC 61850 protocols provide the utilities and manufacturing entities with a new standard and design for the modern electric substation.

Since the approval of IEC 61850 by UCA (Utility Communication Architecture) International Users Group in 2004, many European and Chinese utilities have implemented IEC 61850 standards on the newly installed smart high voltage substations, i.e. 110/10kV air insulated substation.

Figure 1 depicts the IEC 61850 based substation architecture; the different components are described below.

Regarding to the substation protection, IEC 61850 protocols standardized the data exchange rules over the Process and Station bus in high-speed peer-to-peer communications. Generic Object Oriented Substation Event (GOOSE) message is introduced through the entire system to make sure the high priority of the signals from Circuit Breaker IEDs.

Some critical devices and structures are described in the following parts.

IEC 61850 Critical Devices

1) Merging Unit (MU)

A merging unit combined with the instrument transformer (CT/VT) monitors the working status of lines and devices. Unlike the conventional analog signal, MU can digitize the original current and voltage signals and send them over the Ethernet network in the form of sampled values.

Merging unit and these input/output units are usually combined into single devices and recognized as Merging Unit.

2) Intelligent Electronic Devices (IEDs)

Intelligent Electronic Devices (IEDs) are devices that perform electric protection or control functions in power systems, with advanced local control intelligence and have the ability to monitor processes.

IEDs receive data from sensors and power equipment, and can issue control commands, such as tripping circuit breakers if they sense voltage, current, or frequency anomalies, or raise/lower voltage levels in order to maintain the desired level.

Common types of IEDs include protective relaying devices, load tap changer controllers, circuit breaker controllers, capacitor bank switches, reclose controllers, voltage regulators, etc.