SCADA

In this modern world electricity plays a vital role. As electrical needs are increasing in our day-to-day life, the quality and continuity of power becomes most important.

Substations undoubtedly are the backbones of power transmission. So a good network between these substations will provide undisturbed power supply of good quality.

My Article deals with study of SCADA. It includes the study of components and applications of SCADA.

SCADA stands for Supervisory Control And Data Acquisition System. Supervisory and controlling is done using load dispatcher and the data acquisition is made possible through FEP and RTU.

1. INTRODUCTION  

In developing countries like our India our energy needs are increasing rapidly and doubled for every seven years. It is difficult to achieve the increased demand through additional capacity generates to better capacity utilization. Nearly we are losing 5% of power due to technical problems in transmission and distribution system. Employing several techniques like SCADA, HVDC, and FACTS can reduce this. So scada plays a major role in achieving objective like better plant management, higher availability, improved load management, reduced losses, Energy management, Energy audit.

SCADA stands for supervisory control and data acquisition system. It refers to a central system that monitors and controls the complete site in real time mode from a remote location with acquisition of data for analysis and planning from one control location. Data acquisition begins at RTU level. Includes several data like meter readings status position of devices are communicated to the SCADA. Data is then compiled and formatted in such a way that a control room operating using the scada can make appropriate supervision decisions.

2. ARCHITECTURE OF SCADA

The primary function of scada is data acquisitions. In the power system SCADA, required data will be acquired from the remote station \power houses to the master computer center. For this one computer called Remote Terminal Unit (RTU) is used in the remote sub-station \power house. It scans following data from the yard at fast periodic intervals of milliseconds.

Analog: MW, MVAR, Ampere, KV, Frequency, etc.

Digital:  Status indication of breakers, isolators, etc.

One computer at the master station called FEP (Front End Processor) collects data from different Ruts installed at various sub- stations sequentially polling one by one. The communication media may be P&T, PLCC, OFC, and microwave. FEP or another computer called sever\ Workstation may process the data. The processed data is displayed on single line or shift engineer. This computer (Dispatcher’s Console) is also called man machine Interface (MMI) through which the operator can interface or view the real time data of any selected station by using simple commands. The FEP, Server, Workstation, Dispatcher’s console, Printers etc are connected in LAN with redundancy.  The single line diagram illustrating the principle of operation of SCADA is enclosed below:

At Remote Location: 

         (a)Transducers (Analog Inputs):

                          In conventional systems the sub-station/power house metering of the analog parameters such as voltage, current, MW, MVAR, frequency etc are achieved making use of instrument transformers. The secondary of voltage transformers are rated for 110 volts and current transformers of 5A or 1A are commonly used to take above inputs and give out puts with proportionate (4-20 mA).

Transducers are named depending on the parameters for which they are used. Some of them are:

Frequency transducers

Power transducers (active/reactive)

Current transducers

Voltage transducers

Power factor transducers

Transducers are also classified as self- powered or auxiliary powered depending upon the usage of supply required for the functioning of the units. Self-powered transducers do not need separate voltage for normal functioning. They use pt secondary voltage given for it’s functioning the normal output will be (0-5 mA), (0-5v) etc. in respect to auxiliary powered transducers, they need extra input voltage say 220 v ac or 48 v dc etc for functioning. The outputs will be 4 to 20 mA where 4 mA corresponds to live zero value.

(b) Contact multiplier relays (digital input):

Contact multiplier relays (CMR) are used for providing potential free contacts from the used contacts to acquire the status of circuit breakers etc. these are used where it is difficult to get spare auxiliary contacts of devices from field. 

(c) Interposing relays: 

Interposing relays are used for carrying the commands from the yard to the master station to the remote equipments like CBs, Isolators and OLTS.separate relays are used for closing and opening of circuit breakers or Isolators.

    

(d) Remote Terminal Unit (RTU):

Remote Terminal Unit is nothing but a computer user for the remote station to scan all the analog and digital data from the yard RTU also accepts and faith fully executes the commands issued by the master station.  RTU consist of following cards along with microprocessor. A \D converted and signal conditioning units. A simplified block diagram is shown below:

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Article By:
K Praveen Kumar
Asst.Professor
EEE Department
Sphoorthy Engineering College

Sphoorthy Engineering College