**4.2 Synchronized Phasor Measurement System (SPMS)**

The Synchronized Phasor Measurement System (SPMS) was firstly developed and introduced into the power system in mid-1980s. These systems have the ability of measuring currents and voltages, and calculating the angle between them. This ability has been made possible by the availability of Global Positioning System (GPS); on the one hand, and the sampled data processing techniques; on the other hand. In order to synchronize measured angles, SPMS uses time received from GPS as its sampling clock. In addition to measuring angles of voltages and currents, these systems can also measure local frequency and rates of frequency changes, and may be customized to measure harmonics, negative and zero sequence quantities (Phadke & Thorp, 2008).

A SPMS consists of three main parts: Phasor Measurement Unit (PMU), Phasor Data Concentrator (PDC), and communication system. PMUs are normally installed at remote sites. They calculate phasors of voltages and currents and stamp measured phasors with the time received from GPS. A PDC gathers data from several PMUs, rejects bad data, and aligns the time stamps. Communication system of SPMS is responsible for data delivery between PMUs and a PDC or multiple PDCs (Phadke & Thorp, 2008). Fig. 3 shows the hierarchy of SPMS. The next three sub-sections are going to describe each part of SPMS, separately.

#### **4.2.1 Phasor Measurement Unit (PMU)**

The Phasor Measurement Unit (PMU) is a microprocessor based device that uses the ability of digital signal processors in order to measure 50/60Hz AC waveforms (voltages and currents) at a typical rate of 48 samples per cycle (2400/2880 samples per second). To do

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

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

The communication systems provide communication routes between the master station and the remote sites. This can be done through private transmission media (e.g. fiber optic or

There are three main physical communication architectures used in SCADA communications: point-to-point, multipoint and relay station architectures (Clarke et al.,

The Synchronized Phasor Measurement System (SPMS) was firstly developed and introduced into the power system in mid-1980s. These systems have the ability of measuring currents and voltages, and calculating the angle between them. This ability has been made possible by the availability of Global Positioning System (GPS); on the one hand, and the sampled data processing techniques; on the other hand. In order to synchronize measured angles, SPMS uses time received from GPS as its sampling clock. In addition to measuring angles of voltages and currents, these systems can also measure local frequency and rates of frequency changes, and may be customized to measure harmonics, negative and zero

A SPMS consists of three main parts: Phasor Measurement Unit (PMU), Phasor Data Concentrator (PDC), and communication system. PMUs are normally installed at remote sites. They calculate phasors of voltages and currents and stamp measured phasors with the time received from GPS. A PDC gathers data from several PMUs, rejects bad data, and aligns the time stamps. Communication system of SPMS is responsible for data delivery between PMUs and a PDC or multiple PDCs (Phadke & Thorp, 2008). Fig. 3 shows the hierarchy of SPMS. The next three sub-sections are going to describe each part of SPMS,

The Phasor Measurement Unit (PMU) is a microprocessor based device that uses the ability of digital signal processors in order to measure 50/60Hz AC waveforms (voltages and currents) at a typical rate of 48 samples per cycle (2400/2880 samples per second). To do

(Synchrony, 2001).

2004).

separately.

applications (Clarke et al., 2004).

**4.1.4 Communication system of SCADA** 

sequence quantities (Phadke & Thorp, 2008).

**4.2.1 Phasor Measurement Unit (PMU)** 

leased line) or atmospheric means (wireless or satellite).

**4.2 Synchronized Phasor Measurement System (SPMS)** 

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

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

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