**4.4 Digital Protective Relay (DPR)**

Historically, different kinds of protective relays have been designed to isolate the area of faults and reduce the impacts of the faults from other parts of the system (Zhang, 2006). The first type of protective relays was electromechanical one. With the introduction of electronic devices e.g. the transistor in the 1950s, electronic protection relays were introduced in the 1960s and 1970s. Recently, digital (microprocessor-based) protective relays have been introduced to the power systems. A DPR uses an advanced microprocessor to analyze voltages and currents of a power system for the purpose of detection of faults in such a system (Hewitson et al., 2004).

Digital protective relays, in addition to performing the traditional relaying tasks, are capable of measuring and recording analog and status data, as well as communicating with a centralized location. They collect current and voltage signals from instrument transformers and digitize them. Due to the fact that relays should act very fast, the accuracy of measured data is not of major concern. Consequently, to speed up A/D conversion, lower sampling rates are normally applied. This implies that data obtained from DPRs are generally less accurate than from the other data resources (Kezunovic, 2008a).

The communication systems of SPMS may be similar to the SCADA communications in terms of technology, architecture and utilized media. Although these communication systems may be the same, their streamed data are different. The phasor data, which is provided by PMUs, have different nature in comparison with the data of RTUs. Phasor data is continuous and streaming in nature while RTU data is transmitted to the master station either in a specified time intervals or when master station requests it. Another difference between PMU and RTU data is their volume. In general, data of a PMU has more value than

As a result of the above facts, two aspects are of major importance in SPMS communications: communication bandwidth and communication latency (Phadke & Thorp, 2008). High bandwidth communications guarantee that all phasor data can be transmitted to PDCs without any packet drops. On the other hand, low latency communications provide

The Digital Fault Recorder (DFR) acts as the black box of a substation. It records highly accurate waveforms related to faults. The recorded data are huge amount of analog and status data for pre-fault, fault and post-fault conditions (Kezunovic, 2008a). These data may include maximum current, sequence of events, type of fault and the sequence of operation of

The sample rate of the DFR is normally very high and assumed to be 64 to 356 samples per cycle. The DFR data is normally formed in COMTRADE format (IEEE Inc., 2006). Additionally, the data captured by DFRs are offline and they are not used in real time. These data are stored as samples for further offline processing. Generally, the DFRs are installed in

Historically, different kinds of protective relays have been designed to isolate the area of faults and reduce the impacts of the faults from other parts of the system (Zhang, 2006). The first type of protective relays was electromechanical one. With the introduction of electronic devices e.g. the transistor in the 1950s, electronic protection relays were introduced in the 1960s and 1970s. Recently, digital (microprocessor-based) protective relays have been introduced to the power systems. A DPR uses an advanced microprocessor to analyze voltages and currents of a power system for the purpose of detection of faults in such a

Digital protective relays, in addition to performing the traditional relaying tasks, are capable of measuring and recording analog and status data, as well as communicating with a centralized location. They collect current and voltage signals from instrument transformers and digitize them. Due to the fact that relays should act very fast, the accuracy of measured data is not of major concern. Consequently, to speed up A/D conversion, lower sampling rates are normally applied. This implies that data obtained from DPRs are generally less

**4.2.3 Communication system of SPMS** 

real time streaming between PMUs and PDCs.

the most important substations (Kezunovic, 2008a).

accurate than from the other data resources (Kezunovic, 2008a).

**4.3 Digital Fault Recorder (DFR)** 

circuit breakers (Thomas et al., 2006).

**4.4 Digital Protective Relay (DPR)** 

system (Hewitson et al., 2004).

data provided by a RTU.

First DPRs sample rates were 4 to 20 samples per cycle, but nowadays, DPRs are available that sample at 64 to 128 samples per cycle (IEEE Inc., 2006).

#### **4.5 Circuit Breaker Monitor (CBM)**

The Circuit Breaker Monitor (CBM) is an electronic device that monitors circuit breakers. The CBM captures detailed information about each CB operation in real time; either the operation is initiated manually by the operator or it is initiated automatically by the protection and control equipments (Kezunovic, 2008b). The CBM data is also formed in COMTRADE format.
