remote control equipment - Indian Railways Institute of Electrical ...

The use of hermetically sealed components, coated cirucit boards and corrosion protected
handware should be a consideration selecting SCADA equipment for outdoor installation.
The electrical environment of a power station is more servere than its physical
environment Large amounts of electrical noise and transients can be generated by the
operation of power equipment and their controls. These can be minimised through shielding
and surge reduction practices. Ground potentical during faults can also affect electronic
equipment in station making a solid ground mass essential.
7.2.3.2 Analog data acquisition
Data Acquisition in an electric utility SCADA system concentrates on the power sytem
performance quantities, line and transformer load currents, real and sometimes reactive power
flow and bus voltage. Secondary quanitites such as transformer temperature, transformer tap
position, or other multiple position quanitities are also transmitted in analog format.
The principal electrical quantities (volts, amps) are measured from power system instrument
transformers with transducers. There are convert to DC voltages or currents which can be readily
accepted by the SCADA system remote terminal unit.
The value received from scada system measurements is a funtion of the investment
made in their measurement sub systems.
7.2.3.2.1 Measuring system performance
The overall accuracy of transmitted quantities is affected by a number of factors. These
include instrument tranformer errors, transucer performance, analog to digital conversion
quantizing error, and offsets introduced in transducers and A/D converters.
7.2.3.2.2 Instrument Transformer Performance
Analog quantities can be no more accurate than the instrucment tranformers used for
their measurement. When metering grade tranformers are available they will provide the best
accuracy.
7.2.3.2.3 Transducers
The measuring interface element between a SCADA system and a power system is the
transducer. These provide scaled, low energy signal which represents a power system quantity
that the RTU can easily accept. Transducers also isolate and buffer the RTU from the power
system and station environments. Their outputs are usually D.C. voltages or currents which
range upwards of a few tens of volts or milliamperes.
Transducers meausing power sytem electical quintities are generally designed to be
compatible with instrument transformer outputs. Potential inputs are based around 110 to 120
volts A.C. and current inputs accept 0-5 Amps. These are for the most part, long time steady
state ratings. Input circuits are provided with short time, over range, capabilities so that they
may survive power system disturbances which may produce abnormally high voltage or
currents, many times higher than rated for short durations.
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