SCADA Application On Power Transmission System On The Grid.

The acronym SCADA stands for Supervisory Control and Data Acquisition. SCADA is an industrial monitoring measurement and control system. It consists of a central station, field data gathering elements (Remote Terminal Unit RTUs) and a collection of standard and custom software. Used to monitor and control remotely located data elements. Contemporarily SCADA system exhibit predominantly open loop control characteristics and utilize predominantly long-distance communication. Similar system to SCADA systems are routinely seen in factories, treatment plants etc. These are often referred to as distributed control systems (DCS). They have similar functions to SCADA systems, but the field data gathering or control units are usually located within a more confined area. Communications may be via a local area network (LAN) and will normally be reliable and high speed integrated. A DCS system usually employs significant amounts of closed loop control. SCADA systems on the other hand generally cover larger geographic areas and rely on a variety of communication systems that are normally less reliable than a LAN.

Supervisory refers to the actual seeing and monitoring of the system. Control is the real operation. It involves the sending and receiving of commands. Data acquisition is the most important part of SCADA system. It involves the collection of data. Data collected includes:

· Measurements i.e. voltage, frequency, current and reactive power.

· Indications i.e. ON or OFF switches, auto or manual transformers, open or closed CBs.

· Alarms i.e. for system operation.

HOW SCADA WORKS.

In most SCADA systems, the master unit sequentially scans the remote terminal units (RTUs), by sending a message to each RTU to inquire whether the RTU has anything to report. If it does, the RTU will send a message back to the master, and the data received will be put into the memory of the computer for processing. The scan of all RTUs in the systems will be completed in approximately 2s. However, in the event of trouble at a remote station, a message will be sent from the remote unit to the master. The normal scan will be interrupted long enough for the master to receive the message and provide an alarm so that the master unit can automatically perform predetermined control actions. In any event, in most cases, the status of all stations equipped with RTUs can be monitored every few seconds, providing the operations at the control center with-up-to-date reviews of the system conditions.

Data Acquisition

The basic information with regard to the power system is collected by equipment in the various substations and power plants. The distributed control system equipment enables remote data acquisition. Data may also be entered manually or calculated. These data are treated exactly like the automatically collected data.

Data acquisition operation is required to:

· Read power system measurement data from RTUs into the control computer under program control.

· Detect and handle data error conditions due to RTU and communication system malfunctions and noise

· Scale and convert analogue data into binary form directly usable by the computer programs.

· Interface with database manager (DBM) that generates data base addresses, and store data in database.

· Store only error free data, quality indicators should be set to denote error conditions.

· Complete the scan in minimum possible time before the next scan begins.

Performance monitoring.

Performance monitoring function is required to enhance the operational availability of the SCADA system.Response time is the manifestation of control system performance as seen from the operator’s point of view. Response time is defined as the time from the request of a function until the complete result of the function is ready. The requirements on system response time of showing data to the operators, or of performing control commands, greatly affect the design of the system and need to be looked into deeply.

The reason why response time is important is that long delays cause stress and are very annoying to the operator. Stress is caused by disruption of the operator’s thought process. Humans in a problem solving situation use the short term memory of their brains. This short-term memory is characterized by low capacity and high volatility.

A basic requirement (and design goal) is that the availability of SCADA system, its communication system and RTUs must be greater than the corresponding availability of the power system network. That is, to be of value, the SCADA system must remain in operation during power system outages, faults, and failures caused by electrical and mechanical malfunctions or hostile environment conditions.

Basic philosophy of performance monitoring is that any single fault within the SCADA system shall neither stop the system from operating nor disturb the power system.

Remote terminal unit (RTU)

The remote terminal units (RTUs) read status and information from the power system process, report changes and information to the central system and execute commands received from the central system. They have developed in steps from pure hardware units to flexible microprocessor-based units. The basic feature which has made it possible to realize a remote terminal unit is storage of data. The collected data may then be processed in different ways leading to advanced RTUs in terms of functional content. The RTU may serve as a filter performing functions which refine data aimed for the central system.

The acquisition and control normally found are:

· Collection of: indications, analog values and digital values

· Collection and accumulation of energy counting pulses

· Output of: On/Off controls, Increase/decrease controls, set point values

By using the basic features, data storage and data queuing more advanced functions can be realized. More autonomous RTUs and those having a large functional content have more software, and software which can be changed, giving the RTU a certain functional content or adapting the RTU to various environments.

The remote terminal unit (RTU) with power supply, meter sensors, actuators, controlling electronics and communication interface are directly interfaced to the power line through a PLM.

Features

• Has a modular and streamlined architecture which simplifies both trouble-shooting and expansion in the field.
• They generate analogue and digital signals that will be monitored by the central station.
• They are normally installed in the meter box and store data in a non-volatile memory. Version for both single and three phases are available.
• In a place where meters are nucleated multi-meter interface unit (MMIU) is used. This is an interface that can manage up-to 16 electric meters each with dedicated line, and all
• with a shared earth.
• ü They are intelligent devices, which collect, process and record power consumption data
• from electric meter y picking up the meter-pulse output, converting it to suitable digital
• format for data processing. They have relay capability in that they resume normal
• operation when power returns after a failure.
• ü They have alarm to warn of an impending power interruption.

Communication network

The communications network is intended to provide the means by which data can be transferred

between the central host computer servers and the field-based RTUs. The

Communication Network refers to the equipment needed to transfer data to and from different

sites. The medium used can either be cable, telephone or radio.

The use of cable is usually implemented in a factory. This is not practical for systems covering

large geographical areas because of the high cost of the cables, conduits and the extensive labor

in installing them. The use of telephone lines (i.e., leased or dial-up) is a more economical

solution for systems with large coverage. The leased line is used for systems requiring on-line

connection with the remote stations. This is expensive since one telephone line will be needed

per site. Dial-up lines can be used on systems requiring updates at regular intervals (e.g., hourly

updates). Here ordinary telephone lines can be used. The host can dial a particular number of a

remote site to get the readings and send commands.

Remote sites are usually not accessible by telephone lines. The use of radio offers an economical

solution. Radio modems are used to connect the remote sites to the host. An on-line operation

can also be implemented on the radio system. For locations where a direct radio link cannot be

established, a radio repeater is used to link these sites.

Historically, SCADA networks have been dedicated networks; however, with the increased

deployment of office LANs and WANs as a solution for interoffice computer networking, there

exists the possibility to integrate SCADA LANs into everyday office computer networks.

The foremost advantage of this arrangement is that there is no need to invest in a separate

computer network for SCADA operator terminals. In addition, there is an easy path to integrating

SCADA data with existing office applications, such as spreadsheets, work management systems,

data history databases, Geographic Information System (GIS) systems, and water distribution

modeling systems.