**4.1.2 Remote Terminal Unit (RTU)**

The Remote Terminal Unit (RTU) is a stand-alone data acquisition and control unit. RTUs are generally microprocessor based and they monitor and control equipments at remote sites. Their main tasks are twofold: to control and acquire data from process equipments at the remote sites, and to communicate obtained data to a master station (MTU) (Stouffer et al., 2008).

It is useful to mention that traditional RTUs only communicated with a MTU. But nowadays, modern RTUs may also communicate among together. In some cases, an RTU can be configured as a relay. Such an RTU relays data obtained from lower RTUs to an MTU (Clarke et al., 2004).

In general, small-size RTUs include less than 10 to 20 analog and digital signals; mediumsize RTUs have 100 digital and 30 to 40 analog inputs. Other RTUs with more inputs are known as large-sized ones (Clarke et al., 2004).

#### **4.1.3 Programmable Logic Controller (PLC)**

Programmable Logic Controller (PLC) is a small industrial computer, which is initially designed to perform the logic functions that are carried out by electrical equipments e.g. relays, drum switches, and mechanical timer/counters (Jelatis, 2001). Nowadays, analog control is a standard part of the PLC operation as well (Clarke et al., 2004).

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this, first, the analog AC waveforms are synchronously sampled by an A/D converter for each phase. In order to provide synchronous clock for the entire system, the time from GPS satellites are used as input for a phase-lock oscillator and thereby, waveforms of the entire system are sampled with 1 microsecond accuracy. In the next step, PMU uses digital signal processing techniques to calculate the voltage and current phasors. Also, line frequencies can be calculated by PMU at each site. By using this technique, a high degree of resolution and accuracy will be achieved. The measured phasors are tagged by GPS time stamps and are transmitted to a PDC at the rates 30-60 samples per second (EPG & CERTS, 2006). Phasor

A study of RTU tasks in SCADA system indicates that PMU and RTU have almost the same

The main functions of a PDC are: to gather data from several PMUs, to reject bad data, to align the time stamps, and to create a coherent record of simultaneously recorded data. As a consequence, a snap-shot of phasors of the measured area can be obtained (Phadke & Thorp,

In some cases, a central PDC may concentrate the area data received from other PDCs and

A study of MTU tasks in a SCADA system indicates that the tasks and functions of PDC in

Fig. 3. SPMS sub-systems (Phadke & Thorp, 2008).

tasks in the SPMS and SCADA systems.

**4.2.2 Phasor Data Concentrator (PDC)** 

may provide phasors of the entire system.

2008).

data is formed in COMTRADE format (Phadke & Thorp, 2008).

SPMS systems are almost the same as those in SCADA systems.

In general, PLCs are modular in nature. They can be expanded to monitor and control additional field devices in remote sites. Since PLCs have built in microprocessor, they can be programmed to function locally even if communication with the master station is lost (Synchrony, 2001).

The PLCs have two main advantages over commercial RTUs. Firstly, they are generalpurpose devices and can easily perform variety of different functions. Secondly, PLCs are physically compact and require less space than alternative solutions (Clarke et al., 2004). As a result of these facts, in SCADA systems, PLCs are preferred to special-purpose RTUs because they are more economical, versatile, flexible, and configurable (Stouffer et al., 2008). However, PLCs may not be suitable for specialized requirements e.g. radio telemetry applications (Clarke et al., 2004).
