9291_c002 - 2 The Protection of Synchronous Generators 2.1...

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2 The Protection of Synchronous Generators Gabriel Benmou yal Schweitz er Engineering Laboratorie s, Ltd. 2.1 Review of Functions. ......................................................... 2 -2 2.2 Differential Protection for Stator Faults (87G) . ............. 2 -2 2.3 Protection Against Stator Winding Ground Fault. ........ 2 -4 2.4 Field Ground Protection. .................................................. 2 -5 2.5 Loss-of-Excitation Protection (40) . ................................. 2 -6 2.6 Current Imbalance (46). ................................................... 2 -6 2.7 Anti-Motoring Protection (32). ....................................... 2 -8 2.8 Overexcitation Protection (24) . ....................................... 2 -9 2.9 Overvoltage (59). ............................................................. 2 -10 2.10 Voltage Imbalance Protection (60) . ............................... 2 -10 2.11 System Backup Protection (51V and 21) . ..................... 2 -12 2.12 Out-of-Step Protection. .................................................. 2 -13 2.13 Abnormal Frequency Operation of Turbine-Generator. .......................................................... 2 -15 2.14 Protection Against Accidental Energization. ................. 2 -16 2.15 Generator Breaker Failure . ............................................. 2 -17 2.16 Generator Tripping Principles. ....................................... 2 -17 2.17 Impact of Generator Digital Multifunction Relays . ..... 2 -18 Improvements in Signal Processing . Improvements in Protective Functions In an apparatus protection perspective, generators constitute a special class of power network equipment because faults are very rare but can be highly destructive and therefore very costly when they occur. If for most utilities, generation integrity must be preserved by avoiding erroneous tripping, removing a generator in case of a serious fault is also a primary if not an absolute requirement. Furthermore, protection has to be provided for out-of-range operation normally not found in other types of equipment such as overvoltage, overexcitation, limited frequency or speed range, etc. It should be borne in mind that, similar to all protective schmes, there is to a certain extent a ‘‘philosophical approach’’ to generator protection and all utilities and all protective engineers do not have the same approach. For instance, some functions like overexcitation, backup impedance elements, loss-of-synchronism, and even protection against inadvertant energization may not be applied by some organizations and engineers. It should be said, however, that with the digital multifunction generator protective packages presently available, a complete and extensive range of functions exists within the same ‘‘relay’’: and economic reasons for not installing an additional protective element is a tendancy which must disappear. ß 2006 by Taylor & Francis Group, LLC.
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The nature of the prime mover will have some definite impact on the protective functions imple- mented into the system. For instance, little or no concern at all will emerge when dealing with the abnormal frequency operation of hydaulic generators. On the contrary, protection against underfre- quency operation of steam turbines is a primary concern. The sensitivity of the motoring protection (the capacity to measure very low levels of negative real power) becomes an issue when dealing with both hydro and steam turbines. Finally, the nature of the prime mover will have an impact on the generator tripping scheme. When delayed tripping has no detrimental effect on the generator, it is common practice to implement sequential tripping with steam turbines as described later.
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9291_c002 - 2 The Protection of Synchronous Generators 2.1...

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