[ف.م]

در بعضی از پستهای انتقال کشور به هنگام وصل یک ترانس از سمت فشار قوی ، ترانس مزبور همراه با ترانس دیگر این پست که از قبل در مدار بوده با عملکرد رله دیفرانسیل زمین R.E.F ( برای هر دو ترانس ) قطع و باعث خاموشی گسترده در منطقه میگردند .

عملکرد رله R.E.F ترانس اول با توجه به ایجاد جریان هجومی (inrush current ) در لحظه وصل و امکان اشکال در سیستم فیلترینگ رله مزبور و یا صحیح نبودن تنظیمات قابل توجیه است .
اما علت عملکرد رله R.E.F ترانس دوم چه میباشد ؟

Topic

[حمید]

جریان هجومی گردشی شکل گرفته ناشی از شرایط گذرای حاصل از وصل ترانس دوم به دلیل X/R قابل توجه مدار گردشی ( نسبت به X/R شبکه) سبب یک جریان هجومی پایدار تر (طولانی تر) در دو ترانس شده که در صورت عدم پیش بینی حفاظتی لازم می تواند تریپ نابجای هر دو ترانس را بدنبال داشت باشد. شرح مختصر پدیده فوق برگرقته از مرجع معروف The Art and science of protective relaying در زیر آمده است. When parallel transformer banks having individual breakers are located some distance away from any generating station, a possibly troublesome magnetizing-current-inrush problem may arise. If one bank is already energized and a second bank is then energized-magnetizing-current inrush will occur-not only to the bank being energized but also to the bank that is already energized. Moreover, the inrush current to both banks will decay at a much slower rate than when a single bank is energized with no other banks in parallel. The magnitude of the inrush to the bank already connected will not be as high as that to the bank being switched, but it can easily exceed twice the full-load-current rating of the bank; the presence of load on the bank will slightly reduce its inrush and increase its rate of decay. Briefly, the cause of the foregoing is as follows: The d-c component of the inrush current to the bank being energized flows through the resistance of transmission-line circuits between the transformer banks and the source of generation, thereby producing a d-c voltage-drop component in the voltage applied to the banks. This d-c component of voltage causes a build-up of d-c magnetizing current in the already-connected bank, the rate of which is the same as the rate at which the d-c component of magnetizing current is decreasing in the bank just energized. When the magnitudes of the d-c components in both banks become equal, there is no d–c component in the transmission-line circuit feeding the banks, but there is a d-c component circulating in the loop circuit between the banks. The time constant of this trapped d–c circulating current, depending only on the constants of the loop circuit, is much longer than the time constant of the d-c component in the transmission-line circuit feeding the banks. Above figure shows the circuits involved and the magnetizing-current components in each circuit. The significance of the foregoing is two-fold. First, desensitizing means already described for preventing differential-relay operation on magnetizing-current inrush are not effective in the bank that is already energized. Only time delay in the operation of the differential relay will be elective in preventing undesired tripping. However, if the banks are protected by separate relays having tripping suppression or harmonic restraint, no undesired tripping will occur. Second, if the banks are protected as a unit, even the harmonic-current restraint type may cause undesired tripping because, as shown in Fig, the total-current wave very shortly becomes symmetrical and does not contain the necessary even harmonics required for restraint.

[مجيد]

با توجه به شرايط و توضيحات فوق ، پیش بینی حفاظتی لازم جهت عدم تريپ ترانس (عدم عملكرد رله ref ترانس دوم ) چيست و چه راهكاري توصيه ميشود؟

[حمید]

مشکل مذکور ناشی از درگیر شدن اندوکتانس مغناطیسی دو ترانس هم جوار با پدیده جریان هجومی و افزایش L/R مدار بیش از مقادیر نرمال مورد انتظار حفاظتهای ترانس است. راه حل نیز افزایش زمان عملکرد حفاظت مذکور و یا استفاده از حفاظتهای مجزا برای هر ترانس است.

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