Summary

Due to large quantity of cabling needed for implanting a monitoring and control system using conventional equipment, Escelsa Energias do Brasil adopted a system with a communication bus, connecting various sensors to a remote unit, thus allowing complete monitoring of various parts of a substation, mainly power transformers.

The monitoring system aims, primarily, to eliminate the large amount of cables that turn out when a conventional centralized system is adopted, where the information of each sensor must be taken to the control room through independent cablings, at various meters from the substation yard.

In 2007, increase of installed power of substation “SE Ibes” in Vitória – ES was carried out, with the replacement of one of its two 33 MVA power transformers by a new 138/13.8 kV – 25/33/41.5 MVA three phase transformer, called 7TR1, shown in the single-line diagram of figure 1.

With the change of transformer, modernization of the supervision and control system was also decided. However, due to the large amount of cabling needed for implementing a monitoring and control system by using conventional equipment, Escelsa Energias do Brasil opted for a decentralized architecture, using a system with a communication bus. It connects various sensors to a remote terminal unit, thus allowing complete monitoring of various parts of a substation, mainly power transformers, described as follows.

Figure 1 – Single-Line Diagram of Transformer 7TR1

The monitoring system aims, mainly, to eliminate the large amount of cables that turn out when a conventional centralized system is adopted, where the information from each sensor must be taken to the control room through independent cablings, at various meters from the substation’s yard.

In order to meet this requirements, a control system was developed with a modular and decentralized architecture, based on IEDs (Intelligent Electronic Devices) supplied by Treetech, installed at the operating unit of the power transformer, at the substation yard. These IEDs are developed specifically for the substation’s yard conditions, like high temperatures and electromagnetic interferences. They receive information from various sensors, such as temperatures, voltages, currents, TAP position, alarms, etc., as well as, to send control signals, such as increase and decrease TAP, turn on and off forced ventilation, block switch, etc., so as to create a system with local autonomy for decision making.

All IEDs are interconnected to a rmote terminal unit manufactured by Foxboro, model C50, installed at the control room and interconnected through optical fiber cables. The chosen communication protocol was DNP3.0, because its robustness ensures that the data from the sensors are sent to a remote unit in a complete and safe way, aside from being the protocol used in the existing remote unit.

The choice of architecture took in consideration the comparison of characteristics of the centralized and decentralized options, as table 1 shows, where the advantages of the decentralized architecture chosen for this application are listed.

Comparison of Centralized and Decentralized Architectures

To ensure the robustness, reliability and local autonomy of the system, the IEDs used must be specifically designed and built for the extreme conditions found in substation yards, such as extreme temperatures, voltage impulses and electromagnetic interferences. For this case, they are submitted to type tests performed in official laboratories, such as IPT and by INPE, including.

• Climatic Test: (IEC 60068-2-14): (-40 to +85°C);
• Voltage Impulse (IEC 60255-5);
• Applied Voltage (IEC 60255-5);
• Surge Immunity (IEC 61000-4-5);
• Radiated Electromagnetic Field Immunity (IEC 61000-4-3);
• Immunity to Conducted Electromagnetic Disturbances (IEC 61000-4-6);
• Electrostatic Discharges (IEC 60255-22-2);
• Electrical Fast Transient Immunity (IEC61000-4-4);
• Electrical Transient Immunity (IEC 60255-22-1);
• Vibration response: (IEC 255-21-1);
• Vibration resistance: (IEC 255-21-1).

The DNP3.0 protocol (Distributed Network Protocol version 3.0) was chosen for application because it is an open and non-proprietary communication protocol, based on IEC (International Eletrotechnical Commission) specifications, adapted to be used in highly safe applications, at moderate speed and amount of data. It is very flexible and can be used in any hardware platform.

The model specified by ISO-OSI (International Standards Organization – Open System Interconnection) establishes seven layers for a network protocol. Now the IEC specifies a simplified model, which consists in physical layers, data link and application only. Such model is called EPA (Enhanced Performance Architecture). Figure 2 below shows the EPA structure and its communication system.

Figure 2 – Simplified ISO-OSI model

The User Layer can be defined as the place where the user will handle the data, after all communications. The user layer is used for sending / receiving complete messages from / to a device.

The Application Layeris responsible for specifying in details the requests from the user layer, and sending back to it the messages proceeding from the Data Link layer. In other words, it groups the messages from the user layer, calls from fragments, in a message from multiple fragments with the complete information to be processed and sent to a station through the Data Link layer.

The Data Link Layeris used for the transmission of messages between the primary (originator) and secondary (receptor) stations. It also packs the data, checks occasional transmission errors through CRC (Cyclic redundancy Check) checking and sends them to the network.

The DNP 3.0 protocol permits a large variety of commands that can be used according to the need of each application. In the application described here the subset of commands described in table 3 was used, which also corresponds to DNP functions that are available in the IEDs:

The detailed architecture of this system can be seen in Figure 3 which follows. The remote C50 performs the entire reading logic of the IEDs, as well as their command and control sequences, such as turn on / turn off the groups of fans, increase / decrease the TAP position, etc.

Figure 3 – Adopted Decentralized System Architecture

Table 3 describes the IEDs used at the transformer’s operating unit, the acquired information and its functionalities performed locally. In Figures 4 and 5 the transformer 7TR1 and the installation details of the IEDs in its operating unit can be seen.

IEDs associated to the monitoring system

Figure 4 – Transformer 7TR1

Figure 5 – Photograph of the Installed IEDs

The system described here is found in operation since November 2007. As reported by the field operation personnel, the communication with all IEDs was maintained stable and functional, both for acquisition and for command and control of all monitored variables.

The digital monitoring and control system has shown to be effective, because when adopting a decentralized architecture for this type of project, various benefits can be confirmed such as the reduction in the amount of cabling, reduction of costs and minimization of failures, that is, in case any module of the system fails, the system as a whole will not be compromised.

The system description, as well as the reported field experience with the use of this architecture, will allow an evolution of benefits and acquired experience with the presented monitoring system, which can generate applicabilities in highly important substations for the electrical network. Escelsa Energias do Brasil intends, at a near future, to adopt this solution to its other substations, thus standardizing this architecture so as to unify and simplify the entire monitoring and control process.

There is furthermore the possibility of future expansions, with the inclusion of new IEDs to aggregate new functions to the system, as for example the monitoring of moisture in oil and monitoring of capacitance graded bushings. Therefore, it is sufficient to connect the new parallel sensors in the existing communication network.