Substation Automation

Electric utilities, especially in developed countries, continuously encounter the challenge of providing reliable power to the end-users at competitive prices. Due to several reasons such as equipment failures, lightning strikes, accidents and natural catastrophes, power disturbances and outages in substations occur and often result in long service interruptions. Thus, the substations should be properly controlled and monitored in order to take the necessary precautions accurately and timely. In this respect, substation automation, which is the creation of a highly reliable, self healing power system that rapidly responds to real time events with appropriate actions, ensures to maintain uninterrupted power services to the end users.

Substation Automation enable an electric utility to remotely control and monitoring of the switch yard, recording, protection of the power equipment, revenue metering and automation functions for energy management and assert management. Conventional substation is composed with interlocking logic, RTU (Remote Terminal Unit), Relays, conventional switchgear and CT/PT (current/potential transformers). Substation automation system is structured in three basic levels. The station level provides an overview across the whole station and is located in a shielded control room. Station level includes HMI Workstation, Master Station Computer, Backup Station Computer and GPS (Global Positioning System) receiver, etc. The bay level conducts maintenance work only within one bay and it is usually close to the switchgear. Bay level includes protection and control IEDs (intelligent electrical devices) of different bays such as circuit breakers, transformers, and capacitor banks. Equipment in bay level and station level are called secondary equipment. Process level provides the interface between the substation automation system and the switchgear. Process level includes switchyard equipment (also primary equipment) such as CTs/PTs, remote I/O, actuators, merging units etc.

Process level is designed primarily to interface with nonconventional CTs and VTs, a process level communication will also include “transitional” hardware that will interface with existing copper CTs and VTs. The benefits of the process near implementation of the IEC 61850-based technology include elimination of copper, the elimination of CT saturation, and avoidance of CT open circuits, which are a serious safety hazard.

With this solution, new designs become possible, where electronic transformers are used
instead of conventional transformers in the switchyard. The voltage and current signals are captured at the primary side, converted to the optic signals by an merging unit (MU), and transferred to the protection and control devices via optical fibers. This can lower the requirement of transformer
insulation and reduce the conducted and radiated interference suffered in the analog signal transmitted through legacy wiring.

Substation automation benefits :

• Increased performance and reliability of electrical protection.
• Advanced disturbance and event recording capabilities, aiding in detailed electrical fault analysis.
• Display of real time substation information in a control center.
• Remote switching and advanced supervisory control.
• Increased integrity and safety of the electrical power network including advanced interlocking functions.
• Advanced automation functions like intelligent load-shedding.

 

Functioning of Substation Automation System (SAS)

Bus voltages and frequencies, line loading, transformer loading, power factor, real and reactive power flow, temperature, etc. are the basic variables related with substation control and instrumentation. The various supervision, control and protection functions are performed in the substation control room.

The relays, protection and control panels are installed in the controlled room. These panels along with PC aids in automatic operation of various circuit breakers, tap changers, autoreclosers, sectionalizing switches and other devices during faults and abnormal conditions.

Thus, primary control in substation is of two categories:

• Normal routine operation by operator’s command with the aid of analog and digital control system.
• Automatic operation by action of protective relays , control systems and PC.

 

The automated substation functioning can be treated as integration of two subsystems, as discussed below :

(a) Control System

The task of control system in a substation includes data collection, scanning, event reporting and recording; voltage control, power control, frequency control, other automatic and semiautomatic controls etc.

The various switching actions like auto reclosing of line circuit breakers, operation of sectionalizing switches, on-load tap changers are performed by remote command from control room. The other sequential operations like load transfer from one bus to another, load shedding etc. are also taken care by control center.

(b) Protective System

The task of protective system includes sensing abnormal condition, annunciation of abnormal condition, alarm, automatic tripping, back-up protection, protective signaling.

The above two systems work in close co-operation with each other. Most of the above functions i.e. automatic switching sequences, sequential event recording, compiling of energy and other reports, etc. are integrated in software in the substation computer. This software is of modular design, which facilitates addition of new functions.

The communication between circuit breakers, autoreclosers and sectionalizing switches in the primary and secondary distribution circuits located in the field and the PC in distribution substation control room is through radio telecontrol or fibre optic channel or power line carrier channel as is feasible.