یکی از الزامات auto reclosing در سطوح ولتاژ 230 کیلو ولت و بالاتر، امکان اعمال سوئیچینگ تکفاز جهت بهبود شرایط پایداری سیستم به هنگام پاکسازی خطا است. با اینحال سوئیچینگ تکفاز روی دستگاه های حاوی راکتانس بالا نظیر ترانسفورماتور و راکتور می تواند مشکل آفرین باشد، دلیل آن چیست؟

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[محمد سعید جعفرپورKeymaster]

نظر به طراحی تجهیزات قدرت برای کارکرد عادی در شرایط برقراری ولتاژ متقارن سه فاز، وقوع برخی پدیده ها در شرایط غیر عادی محتمل است. راکتانس مغناطیسی سیم پیچی ترانسفورماتور از امکان رزونانس یا فرورزونانس با خازن پراکنده سیم پیچی در فرکانس 50 هرتز در صورت تیون موفقیت آمیز برخوردار است. با اینحال چنین انطباق امپدانسی در شرایط عادی رخ نمی دهد. در شرایط اتصال تکفاز یا دو فاز سمت اولیه زمین نشده یا مثلث یک ترانس قدرت به شبکه، ولتاژ نقطه صفر از حالت نوترال خارج شده و خازنهای پراکنده مرتبط با سیم پیچی یا خط انتقال ورودی در مدار معادل ظاهر می شوند. در شرایط اتصال سه فاز متقارن خازنهای مزبور عملا bypass بوده و مشکل زا نمی شوند.
<p dir=”ltr”>The transformer is the basic building block in stepping up or down voltages and currents in a power system. It is also one of the basic elements of a ferroresonant circuit. The two most widely used transformer connections are the wye and delta. Each one has its advantages and disadvantages, and depending on the particular application and engineering philosophies, one or the other or a combination of both will be used. First we will look at the delta connected (at primary) transformer, as shown in Fig. . Figure a shows a simple power system. The source of power is represented by a grounded equivalent power source because normally the system is grounded at the source and/or at some other points in the power system. The connection between the source and the transformer is made by an (underground) cable having a capacitance C.</p>

<p dir=”ltr”>There are four prerequisites for ferroresonance:</p>
<p dir=”ltr”>•
The first prerequisite for ferroresonance is the employment of cables with relatively large capacitances. The transformer primary is delta connected or in ungrounded wye connection, while the secondary can be either grounded or ungrounded wye or delta connected. This system is shown in Fig. a and the transformer is assumed to be unloaded (load provides damping).
•
The second prerequisite for ferroresonance is the transformer connection.
•
The third prerequisite for ferroresonance is an unloaded transformer. In order to simplify this discussion, the coupling capacitances of the transformer (from phase to ground, phase to phase, and from primary to secondary) will be assumed to be low enough compared with the shunt capacitance of the primary line (cables in populated areas) so that they can be neglected. However, it must be noted that at voltage levels above 15 kV the coupling capacitances (increased insulation) of the transformer become more important so that their contribution to the ferroresonant circuit cannot be neglected.
•
The fourth prerequisite is when either one or two phases are connected; then these circuit configurations are susceptible to ferroresonance. When the third phase is energized, the system will be balanced and the current will flow directly to and from the transformer, bypassing the capacitance to ground.
Adding a load to the secondary side of the transformer changes the ferroresonant circuit considerably: the effective load impedance ZL will be reflected back into the primary side by the square of the turns ratio of the primary (N1) to the secondary (N2). This reflected impedance ZL′=(N1/N2)2.ZL will in turn limit considerably the current in the ferroresonant circuit, thus reducing the possibility of this phenomenon significantly.</p>

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