21, rue d'artois, F-75008 Paris http://www.cigre.org B3-214 Session 2004 CIGRÉ Improvements on High Voltage Electrical Substations to face the new Brazilian Legislation G. M. Bastos (*) R. C. Valente E. Leser J. L. F de Almeida O. J. M. da Motta Furnas Centrais Elétricas S.A. Rio de Janeiro - Brazil Abstract: The objective of this work is to present the improvements implemented in high voltage substations to attenuate the effects of the new structuring of the Brazilian Electric Section. The modifications and improvements were done in the substations with the purpose of quickly replacing defective transformers by its spare units, minimizing unavailability of the facilities. It will also be presented the on line monitoring system that is being installed in some substations with the purpose of on line monitor equipment. Keywords: Improvements, On line monitoring, Switching system, Transformers, Substations, Plug Cubicle, Transfer bars, Equipment 1- Introduction The deregulation of the Brazilian Electric System led the Brazilian utilities, private or public, to adapt themselves to this new reality. All the new electrical installations, such as substations or overhead transmission lines, are bid by ANEEL, the National Regulating Agency of the Brazilian System. In this way the electrical market became very competitive. Legislation created by the Agency foresees heavy penalties for unavailability of electrical equipment. In the last biddings for concession of transmission facilities made by ANEEL, penalties corresponding to 10 times the revenue are foreseen for scheduled outages. For non-scheduled outages the penalties are 150 times the revenue for the first 5 hours and 10 times the revenue for the subsequent hours. A quick analysis of these figures shows that a not scheduled outage of 5 hours, means a revenue loss of up to 750 hours (150 x 5 hours), which corresponds to one month revenue. According to the legislation scheduled outages are all those requested, in advance, to the ONS - The Company in charge of the operation of Brazilian System. All other outages are classified as nonscheduled ones. For example, the preventive and corrective maintenances are considered as scheduled outages while the failures or defects that remove from operation the transformers are examples of non scheduled ones. * Gilson Machado Bastos Furnas Centrais Elétricas S.A. Email: gbastos@furnas.gov.br
As transformers are the kind of equipment associated with the largest revenues, some measures has been taken in order to adapt itself to the new demands, minimize penalties and remain competitive. The taken measures have the purpose to reduce the number of scheduled and non-scheduled outages and, since outages are unavoidable, replacing the transformer by the spare unit as fast as possible. To reduce the unavailability of an unit, it was projected a group of improvements that all together reduced the time of a transformer replacement from some hours to 20 minutes. To decrease the number of outages intelligent systems capable to monitor on-line the main equipment are being installed. On-line monitoring systems can be used continuously during the operation of the main substation equipment and offer in that way a possibility to record different relevant stresses which can affect their lifetime. The automatic evaluation of these data allows the early detection of incoming failures and performing conditioned based maintenance. 2- Transformer Replacement System As a generation and transmission power utility responsible for the transmission of large blocks of energy in voltages that vary from 138 to 765 kv, the power transformers installed in our system are rather great and heavy which require a large structure for movement them. The replacement of such great transformers as single-phase units of 765/345kV, 550 MVA may last more than one day. This time has an enormous cost, if one considers the loss of revenue due to the unavailability of the bank, besides the foreseen penalties. With the purpose of replacing a transformer installed in the smaller time possible a set of measures that combined can reduce the time of unavailability was developed. The system developed for quickly replacing a transformer is based on the philosophy adopted in our facilities of using bank of transformers formed by three single phases units instead of three phase units and the use of one spare unit for each Substation. The designed measures comprise the use of transfer bars, the installation of switches for the transfer of the units and the use of plug cubicles, which will be described below. 2.1 - Transfer bars In order to allow the replacement of a transformer by a spare unit without the need of moving it, transfer bars are installed in all voltage sides of the transformer bank. These bars consist of rigid bus bar, installed in front of the terminals of the transformers, to which the spare transformer is permanently connected. 2.2 Switching system In the past the replacement of a single-phase transformer by its spare was made manually, through removable links previously designed for this purpose. The substitution of a transformer could last some hours, mainly in case of extra high voltage transformers, and didn't attend the demands imposed by the new model. Thus, it was necessary to develop a system that would reduce the time of replacement of the transformers. An automatic system that consists of the installation of single phase switches, motorized, connecting the terminal bushings of the transformers to the transfer bars was then designed. These switches are commanded locally or remotely, manually or automatically, through the digital control of the Substation. 2
An example of a scheme used in the Substations is shown in the Figure 1 below. Figure 1 - Single-phase electrical scheme showing the switching system used for transformers bank The switches used for the switching system are single-phase conventional ones. It should be pointed out that the switches that connect the phase unit to the transfer bar are subject to phase-phase voltage across their terminals when carrying out thir function of placing the spare unit in operation. For this purpose, the manufacturers supplied switches with larger gap distances. Special additional tests are foreseen for this case, such as the application of switching surges at both ends of the switches. Considering that transformers tertiary windings are usually delta connected, they deserve a special treatment. Initially, a total replacement of the transformer was foreseen, in order to maintain in operation the load connected to the tertiary. However, this option was shown onerous and complex due to the need to settle a great amount of switches in an area where the space is usually reduced. Thus, it was decided that the tertiary windings would not be substituted automatically and only grounded through switches at one end and connected to surge arresters at the other. In case of necessity, the load connected to the tertiary can be restored manually through links in more appropriate time. As for the neutral terminals, they are permanently connected to the neutral bar, grounded in one only point. Switching systems like these have been installed in some facilities, the first one was put into operation on the middle of 2003. The use of this same philosophy for single-phase shunt reactor banks is only justified in the cases where this equipment is essential for the operation of the electric system. This may happen in reactors installed at the end of some transmission lines. As the switches of the system have a very important function they needs high reliability. Then, they are equipped with a monitoring system. 2.3- Plug Cubicles The use of automatic switching system reduced the time of replacement of the transformers drastically. However the total replacement time is yet limited by the time spent for the connection of the whole control, supervision and protection systems, formed by hundreds of cables. 3
The task of substituting and testing these hundreds of control, supervision and protection cables could last some hours. In order to reduce this time, a cubicle was designed with multi pole plugs, armored, for the connection of all cables from control protection and supervision circuits. The plug cubicles can be installed inside or adjacent to the transformer common cubicles and they have sockets for the three phases of transformers and for the spare unit. Inside the panel and to make the connections easier, the circuits are combined by its purpose and clearly identified as current circuits, alarm circuits, trip circuits temperature circuits, etc. The philosophy of the plug cubicle is presented in the figure 2 where a connection diagram for a bank of transformers formed by three single-phase units and an spare unit is shown. Figure 2- Plug cubicle used to facilitate transformer replacement A drawing of a plug cubicle is shown in the Figure 3 bellow. Figure 3 - Outline drawing of a plug cubicle 4
The following photos show a plug cubicle installed in the common cubicle of three single phase transformers bank, 525/138 kv, 133 MVA. Figure 4 - Photographs showing the internal view of a transformer plug cubicle When it is necessary to replace a transformer unit by its spare, the operator of the substation can easily and very quickly disconnect the plugs of the phase to be substituted and connect them to the proper sockets of the spare unit. The installation of the plug cubicles has reduced the total transformer replacement time, no matter how large its dimensions are, to 20 minutes, the time necessary for the connection of all plugs, task still done manually. However, we are evaluating the use of PLC - Programmable Logic Controller - to execute this task which could reduce the time of substitution to just a few minutes. One additional advantage of this system is to assure that all transformers have the same operation time if the replacement is done from time to time 2.3 - Economical Evaluation The automatic switching is not very expensive and can be easily installed in new substations. To install a new 765/525 kv transformers bank, which annual revenue is US$ 4.600.000,00 the cost of the switching system is US$ 300.000,00. If one applies the penalties imposed by ANEEL for this installation, one non-scheduled outage of 5 hours would lead to a penalty of US 360.000,00. Only one quick replacement of the transformer by its spare unit will save, in terms of penalty reduction, an amount equivalent to all costs related to the switching system. 3 - On Line Monitoring System The monitoring system has the purpose of "on line" monitor the state of the equipment, supplying means and conditions so that the maintenance people can modify its routine of preventive maintenance for a corrective one and, consequently, to decrease the time and the number of outages for maintenance. Besides, the system helps to detect incipient defects, decreasing the risk of failures and, in consequence, reducing the non-schedule outages. It was decided to monitor transformers, reactors, breakers, switches and series capacitors banks implementing a monitoring network where monitoring servers from different substations are connected to. The necessary information to the monitoring system is acquired through sensors, installed in the equipments and all monitoring data are directly accessed through web servers. The 5
utility internal network (Intranet) is connected to the monitoring network, facilitating the remote access to a group of users previously authorized. Through the monitoring system it can be verified, graphically, the evolution of the variables monitored, analyzing its tendencies and correlating them in such a way to establish diagnostics of the situation of the equipment and to establish prognostics of the future behavior of the equipment on the grounds of the present values. The installation of monitoring systems was a strategic decision to adapt to the new market, when, more than never, readiness and reliability became imperatives. The typical architecture of a monitoring system is shown in the following illustration. Figure 5 - Monitoring system architecture Through the study of the data obtained by the on line monitoring system, the periodic maintenance should be well established and the outages can be scheduled. In the future, the monitoring system shall be integrated and shall operate altogether with the supervision system, in order to allow a more consistent analysis of equipment's defects. The monitoring system can be divided in the following sub-systems: - Data Acquisition: Sensors - Data Treatment: Prognostics and Diagnostics; - Data Transmission: Architecture of the System; - Data Presentation. 3.1- Data Acquisition: Sensors The data acquisition is done through sensors installed in the body of the equipment to be monitored. The sensors are responsible for the collection of several informations about the monitored equipment. The choice of the variables to be monitored was based on the operative experience, in the historical of maintenance and in the sensors available on the market. 6
3.2- Data Treatment: Prognostics and Diagnostics. Diagnostics is the identification of the probable cause of a failure or defect, while prognostic is the indication of its tendency that may lead to deterioration of components or cause failures or defects. For the identification of a diagnostic and/or calculation of a prognostic it is necessary the manipulation of a specific group of values and the use of some mathematical equations or models, elaborated according to the operative needs and together with the expertise of the manufacturers. 3.3- Data Transmission: Architecture of the System. One of the largest challenges of this system was to create one system that can work together with the products developed by the different manufacturers, with varied communication protocols and data storage. The monitoring system specification establishes an open system, using worldly known communication protocols and data storage. The main technical characteristics of the architecture of the system, unique in the world are listed bellow. - Monitoring network: Systems of different manufacturers are to be directly connected to the network to allow easy accesses, avoiding that the substation control room be overcrowded with monitors and computers dedicated to each supplied equipment; - Connection of the monitoring network with the internal INTRANET is controlled by an access device to the network (fire-wall); - WEB pages: Monitoring data will be stored according to a standard format of database and accessible through WEB pages similar to the INTERNET standard; - Standard database format, commercially available on the market, is used so that applications can be developed for different maintenance purposes helping decisions. 3.4- Data Presentation The collected data, as well as the result of the diagnostics and prognostics of the system, are shown through tables and graphs in any personal computer connected to the internal net. Besides, tendency curves can be traced combining more than one variable to provide technical information of the behavior of the equipment 3.5- Economical analysis The first facilities with a monitoring system was put into operation in 2003 and a more detailed analysis of the economic advantages in function of both reduction of the maintenance costs and reduction of the number of defects will only be possible after some years of utilization. However, the reduction of the number of outages for preventive maintenance already justifies the investment in extra high voltage transformers due to revenue associated to them and also by the duration of their replacement. Regarding at the cost of installation of this system, using the example of the transformer 765/525 kv mentioned previously, the cost of the system was approximately of 5 % in relation to the cost of the equipment. 4- Conclusion The improvements introduced to face the new model have shown to be effective to minimize the time of transformers replacement for up to 20 minutes, reducing thus the penalties imposed by the National Agency of Energy. They also bring benefits to the system, since they allow the fast restoration of the system if a substitution of a transformer for its spare is needed. The use of Programmable Logic 7
Controllers - PLC - instead of the plug cubicles is a natural evolution and it should be implemented in the future, increasing even more the reliability of the system and reducing the time for replacement of transformers. The Monitoring System facilitates the maintenance of the main equipment of the substations, optimizes the periods between maintenances, formerly accomplished periodically. Also, it will allow knowledge of the state of the equipment in order to foresee the occurrence of failures or defects, to minimize the unavailability and to improve the operation of the Brazilian electric system. The justification for on-line monitoring of power transformers is driven by the needs of electrical utilities to reduce operating costs and enhance the availability and reliability of their equipment. The evaluation of data acquired by an on-line monitoring system shows the capability to detect incoming failures within active part, bushings, on-load tap changer and cooling unit. Using the benefits of modern Internet technology the distribution of information about the condition of the equipment can easily be done by means of standardized web browser technology. When considering the installation of on-line monitoring systems, the size, importance and condition of a power transformer should be analyzed. Especially for old transformers and in general at strategic locations in the electrical network on-line monitoring is necessary and recommended, because it saves the costs related to outages, repair and associated collateral damages due to major failures. 5- Bibliography [1] S. Tenbohlen, G. Bastos and others: " Experienced-based Evaluation of Economical Benefits of On-line Monitoring Systems for Power Transformers" CIGRE Session 2002, Paris, 2002. [2] Cláudio Semprine, G Bastos and others: Como Furnas enfrenta os desafios impostos pelo novo modelo do setor elétrico - Melhorias e Modernizações" Cigrè - SNPTEE- Brazil 2003 [3] Orlando M. da Mota, and others: Sistemas de monitoramento para equipamentos de subestações de energia elétrica, Cigrè - SNPTEE- Brazil 2003 [4] G. Bastos:" Major Electrical Equipments Monitoring Systems - The Brazilian experience" T & D July 2003 8