آنجا که مانند واحدهای گازی نیاز به استفاده از راه اندازهای SFC است، استفاده از GCB به دلیل ضرورت ایزولاسیون ژنراتور از شبکه در دوره راه اندازی غیر قابل اجتناب است. در موارد دیگر استفاده از GCB یک سری مزایای فنی دارد که در مقالات به آنها پرداخته شده است. استفاده از GCB به دلیل امکان کنترل مستقل تر نیروگاه از مزیتهای فنی نظیر امکان پاسخ بهتر به حفاظتهای SELECTIVEمخصوص ترانس واحد و ژنراتور همچنین امکان پاسخ سریع به فالتهای این ناحیه برخوردار است. کلیدهای مزبور قادرند فالتهای ترانس واحد را در کمتر از 4 سیکل از مدار خارج کنند، اما در شرایط عدم حضور GCB با وجود قطع کلید فشار قوی پست ، محل خطا همچنان از طریق ژنراتور DE-EXCITE شده که می تواند چند ثانیه طول بکشد، تغذیه می شود، این موضوع می تواند سبب تنشهای حرارتی و مکانیکی در ترانس واحد و حتی خود ژنراتور در شرایط خطاهای نامتقارن گردد. در هر حال در شرایطی که جریان و سطح اتصال کوتاه ژنراتور اجازه استفاده از GCB های موجود در بازار را می دهد، استفاده از آن می تواند مزایای فنی به شرح زیر را در پی داشته باشد. با اینحال در یک ارزیابی کامل می باید تبعات مالی و اقتصادی استفاده از GCB مزبور را نیز مد نظر داشت.
در زیر برخی مزایای فنی و ارجاعات مربوطه آمده است.
A major objective of all operators of power plants is the achievement of the highest possible plant availability at the lowest possible cost. As already mentioned, the greatly improved functionality of modern generator switchgear allows the realisation of simpler and more economic power plant layouts. The use of modern SF6 generator circuit-breakers further helps the operator of a power plant in reaching the aforementioned target in the following ways:
Simplification of operating procedures:
• During the starting-up or shutting-down of a generator only one circuit-breaker needs to be operated thus reducing the number of the switching operations necessary.
• In the normal case the automatic rapid changeover switching equipment required to transfer the unit auxiliary supplies from the station to the unit transformer (and vice versa) is not needed.
• The division of responsibility for the operation of the power plant and for the operation of the high-voltage system is clearly defined.
Improved protection of the generator and the main and unit transformers:
• The differential protection zones of the generator, the main transformer and the unit transformer can be arranged to achieve maximum selectivity.
• Generator-fed short-circuit currents are interrupted within a maximum of four cycles whereas the reduction of the fault current by the rapid de-excitation equipment may require a number of seconds.
Increased security and higher power plant availability:
• Simplified operational procedures and clearly defined operational responsibilities reduce the likelihood of operational errors.
• The application of a generator circuit-breaker increases the general availability of the power plant auxiliary equipment.
• Synchronising at the generator voltage level with the help of a generator circuit-breaker is considerably more secure than synchronising with a high-voltage circuit-breaker [3>.
• The rapid changeover of the auxiliary supplies during the starting-up and shutting-down of the unit with the associated high inrush currents and resulting stresses is eliminated and possible damage to the drive motors of pumps, fans, etc. is thus avoided.
• The rapid and selective clearance of all types of faults avoids expensive secondary damage and the consequently long down times for repair. A case of serious secondary damage being caused when the generator-fed fault is not immediately interrupted is the bursting of the transformer tank following an internal fault in the main or unit transformer. Another case is the thermal destruction of the generator damper winding due to short-time unbalanced load conditions. Such conditions can arise due to single or two phase faults within the main transformer or on its connections to the high-voltage circuit breaker.
Common causes of main transformer internal failures are the flashover of a bushing, winding interturn faults, failures of the tap-changer and carbonisation and/or excessive moisture content of the transformer oil. Even if the system-fed component of the fault current is interrupted by the high-voltage circuit-breaker within approximately 3 to 4 cycles, in a layout without a generator circuit-breaker the generator continues to supply a fault current throughout the de-excitation time interval which may last for several seconds. The internal pressure resulting from the vaporisation of the transformer oil is a function of the product of arc current and time. This pressure stresses the transformer tank, and, if it rises above a certain value, will cause the tank to rupture, with a resulting oil spillage and possibly an oil fire. Tank rupture may occur after 4.5 to 5 cycles. The presence of a generator circuit-breaker which allows a rapid clearance also of the generator-fed component of the fault current can therefore make up the difference between a repairable damage and a catastrophic event with severe environmental pollution and possible personnel jeopardy [4>, [5>.
Short-Time Unbalanced Load Conditions:
A three-phase short-circuit represents a symmetrical loading of a generator. Single- and two-phase faults on the other hand represent a short-time unbalanced load condition with critical mechanical and thermal stresses for a generator [6>. The thermal stresses result from the negative sequence component of the fault current which interacts with the generator damper windings. Unbalanced load conditions can give rise, within a very short time, to critically high temperatures in the damper windings. These temperatures are particularly critical for turbo generators and in the worst case may cause the rotor to jam in the stator. If a generator circuit-breaker is present it will separate the generator from the fault within four cycles and thus effectively prevent damage to the generator. If no generator circuit-breaker is fitted, the generator will continue to supply a negative sequence current until de-excitation is completed. The de-excitation may take several seconds, during which time the generator may suffer severe damage.
Specifically, the use of modern SF6 generator circuit-breakers positively affects power plant availability in three ways:
• The use of generator circuit-breakers allows the plant auxiliary supplies to be drawn directly from the high-voltage transmission system at all times, i.e. also during the critical start-up and shut-down phases of the plant operation. Supply from this source is considerably more reliable than that from a local sub-transmission network and results in an improved plant auxiliary equipment availability.
• The rapid interruption of generator-fed short-circuit currents reduces the resulting fault damage and shortens repair times, thereby also contributing to an increased power plant availability. Although they have a low probability of occurrence such outages have a substantial effect on the availability of a generating unit and on the overall performance of an operating utility.
• Compared to high-voltage circuit-breakers modern SF6 generator circuit-breakers exhibit higher maintenance intervals as they are especially designed for a high mechanical and electrical endurance. Depending on the application the down-time of an unit due to circuit-breaker maintenance can therefore be significantly reduced when a generator circuit-breaker is used.
A higher availability leads to an increased number of the operating hours and hence to a higher profit for the operator of the power plant. Substantial surplus of receipts can be achieved in this way and the payback time for the expenditures of a generator circuit-breaker is generally very low [7>.
[1> IEC-Publication 60056-1987: High-Voltage Alternating Current Circuit-Breakers.
[2> IEEE Std. C37.013-1997: IEEE Standard for AC High-Voltage Generator Circuit
Breakers Rated on a Symmetrical Current Basis.
[3> I. M. Canay; D. Braun; G. S. Köppl: Delayed Current Zeros Due to Out-of-Phase
Synchronizing. IEEE Transactions on Energy Conversion, 13 (1998) 2, pp. 124-132.
[4> L. Widenhorn; K. Froehlich; B. Culver: Minimised Outage Time of Power Plant Units
after Step-up Transformer Failure. Conference Proceedings of POWER GEN Asia '94,
Hong Kong, 1994, pp.145-150.
[5> Electric Power Research Institute: Power Transformer Tank Rupture: Risk Assessment
and Mitigation. EPRI Report TR-104994, 1995.
[6> I. M. Canay; L. Werren: Unbalanced Load Stresses in Generators due to Switching
Failures, Faults in Power Transformers, Instrument Transformers and Lightning
Arresters. ABB Technical Report ASB 88/200, 1988.
[7> D. Braun; L. Widenhorn; J. Ischi: Impact of the Electrical Layout on the Availability of a
Power Plant. Conference Proceedings of 11th CEPSI, Kuala Lumpur, 1996, pp. 330-336.