DIGITAL TRANSFORMER PROTECTION F ROM POWER PLANTS TO DISTRIBUTION SUBSTATIONS
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22-03-2010, 10:59 PM
Traditionally, the protection of power transformers has been relegated to the application of transformer differential and backup overcurrent relays to provide shortcircuit protection. With the advent of modern multifunction transformer packages, differential and overcurrent protection are only two of many protective and logic functions that can be incorporated into the transformer protection package to enhance value to the user. Other protective functions include overexcitation protection (24), neutral overvoltage (59G), ground differential (87GD) and underfrequency (81U)/undervoltage (27) load shedding. Presently, many of these functions are handled via separate discrete relays or not applied at all because of economic considerations. Transformer protection requirements also vary depending on the location of the transformer on the power system. This paper discusses transformer protection at various locations: power plants, industrial plants, transmission substations and distribution substations. Since transformer protection requirements vary depending on the application, users typically want only those functions which are needed for that specific application. To accommodate this customized functionality, the selection of transformer protective functions should be determined by users as opposed to manufacturersâ€ the same as is done with discrete relay applications. This paper discusses user-selectable functionality that tailors the relay to specific applications. In addition, the use of programmable logic extends the benefit of digital multifunction transformer protection especially for distribution transformer applications. Logic schemes have been developed to provide distribution bus fault protection, digital feeder relay backup protection and load shedding at two-bank distribution substations. This paper addresses these types of schemes and their benefits. The following specific areas of transformer protection are addressed in this paper. 1. Power Plant Transformer Protection Â¢ overexcitation and differential restraint Â¢ generator step-up unit (GSU) transformer ground fault protection Â¢ auxiliary / start-up and industrial transformer protection 2. Transmission Substation Transformer Protection Â¢ overexcitation (V/Hz) protection Â¢ sudden pressure relay (SPR) blocking 3. Distribution Substation Transformer Protection Â¢ underfrequency / undervoltage load shedding 4. Distribution Substation Logic Schemes Â¢ bus fault logic Â¢ feeder back-up logic Â¢ two-bank substation load shedding The application of transformer differential protection (87T) has been discussed in detail in many other papers and will therefore not be discussed here. Instead, this paper concentrates on other protective functions within digital multifunction transformer relays as well as the logic that can be applied with this technology.
Charles J. Mozina
Beckwith Electric Company
6190-118th Avenue North Â¢ Largo, FL 33773-3724 U.S.A.
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