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معمای تابلو شماره 35 – جریان اولیه ترانسهای جریان

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  • #833
    احسان آرزومند

      به چه صورت مي توان جريان ورودي ترانس جريان را بدست آورد.در صورتي كه در IEC60044-1 هم به اين مطلب اشاره كوچكي شده چون فقط مضارب 10-12.5-15-20-25-30-40-50-60-75 A را آورده كه جاي ابهام دارد. …
      به چه صورت مي توان جريان ورودي ترانس جريان را بدست آورد.در صورتي كه در IEC60044-1 هم به اين مطلب اشاره كوچكي شده چون فقط مضارب 10-12.5-15-20-25-30-40-50-60-75 A را آورده كه جاي ابهام دارد.براي مثال اگر ما جريان 1300آمپر داشته باشيم.كدام مضرب بهتر است.اين 1300امپراز اوليه CTعبور مي كند.وبه چه صورت مي توان كلاس دقت و BURDEN آنرا چك كرد كه درست است يا نه؟

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    • Author
    • #2743

        جریان اولیه ترانس جریان بر اساس جریان نامی فیدری که جریان آن باید سنجیده شود انتخاب می شود. ترانسهای جریان مطابق استاندارد از تحمل حرارتی برای جریانی تا 20 درصد بیشتر از جریان نامی برخوردارند. این موضوع انعطاف کافی برای انتخاب جریان اولیه مناسب از مجموعه مقادیر استاندارد شده را تأمین می نماید. بنابراین بطور مثال برای فیدری با جریان نامی حدود 1100 آمپر می توان ترانس جریان …/1000 یا …/1200 را انتخاب نمود. کنترل حد پائین جریان نامی به ملاحظات حرارتی و تحمل ترانس جریان بر می گردد در حالیکه چک حد بالا به حدود دقت اندازه گیری مربوط می شود. زیرا دقت قید شده در پلاک ترانسهای جریان اندازه گیری مربوط به 20 تا 120 درصد جریان نامی در بار نامی است، بنابراین اگر بی دلیل جریان نامی ترانس را خیلی بزرگ منظور کنیم در بار گذاری متغیر فیدر ممکن است جریان گذرنده از فیدر از 20 درصد جریان نامی نیز کمتر شود که همراه با افزایش خطای اندازه گیری می شود. انتخاب هر یک از دیگر پارامترهای ترانسهای اندازه گیری نظیر ولت آمپر، کلاس دقت و ضریب محدوده دقت (ALF) از دقدقه های مهم مهندسان طراح می باشد. Rated primary current: The value of current which is to be transformed to a lower value. In CT parlance, the “load” of the CT refers to the primary current. Rated secondary current: The current in the secondary circuit and on which the performance of the CT is based. Typical values of secondary current are 1 A or 5 A. In the case of transformer differential protection, secondary currents of 1/ root 3 A and 5/ root 3 A are also specified. Rated burden: The apparent power of the secondary circuit in Volt-amperes expressed at the rated secondary current and at a specific power factor (0.8 for almost all standards) Accuracy class: In the case of metering CT s, accuracy class is typically, 0.2, 0.5, 1 or 3. This means that the errors have to be within the limits specified in the standards for that particular accuracy class. The metering CT has to be accurate from 5% to 120% of the rated primary current, at 25% and 100% of the rated burden at the specified power factor. In the case of protection CT s, the CT s should pass both the ratio and phase errors at the specified accuracy class, usually 5P or 10P, as well as composite error at the accuracy limit factor of the CT. Composite error: The rms value of the difference between the instantaneous primary current and the instantaneous secondary current multiplied by the turns ratio, under steady state conditions. Accuracy limit factor: The value of primary current upto which the CT complies with composite error requirements. This is typically 5, 10 or 15, which means that the composite error of the CT has to be within specified limits at 5, 10 or 15 times the rated primary current. Short time rating: The value of primary current (in kA) that the CT should be able to withstand both thermally and dynamically without damage to the windings, with the secondary circuit being short-circuited. The time specified is usually 1 or 3 seconds. Instrument security factor (factor of security): This typically takes a value of less than 5 or less than 10 though it could be much higher if the ratio is very low. If the factor of security of the CT is 5, it means that the composite error of the metering CT at 5 times the rated primary current is equal to or greater than 10%. This means that heavy currents on the primary are not passed on to the secondary circuit and instruments are therefore protected. In the case of double ratio CT’s, FS is applicable for the lowest ratio only. Class PS/ X CT: In balance systems of protection, CT s with a high degree of similarity in their characteristics are required. These requirements are met by Class PS (X) CT s. Their performance is defined in terms of a knee-point voltage (KPV), the magnetizing current (Imag) at the knee point voltage or 1/2 or 1/4 the knee-point voltage, and the resistance of the CT secondary winding corrected to 75C. Accuracy is defined in terms of the turns ratio. Knee point voltage: That point on the magnetizing curve where an increase of 10% in the flux density (voltage) causes an increase of 50% in the magnetizing force (current). Summation CT: When the currents in a number of feeders need not be individually metered but summated to a single meter or instrument, a summation current transformer can be used. The summation CT consists of two or more primary windings which are connected to the feeders to be summated, and a single secondary winding, which feeds a current proportional to the summated primary current. A typical ratio would be 5+5+5/ 5A, which means that three primary feeders of 5 are to be summated to a single 5A meter. Core balance CT (CBCT): The CBCT, also known as a zero sequence CT, is used for earth leakage and earth fault protection. The concept is similar to the RVT. In the CBCT, the three core cable or three single cores of a three phase system pass through the inner diameter of the CT. When the system is fault free, no current flows in the secondary of the CBCT. When there is an earth fault, the residual current (zero phase sequence current) of the system flows through the secondary of the CBCT and this operates the relay. In order to design the CBCT, the inner diameter of the CT, the relay type, the relay setting and the primary operating current need to be furnished. Interposing CT’s (ICT’s) : Interposing CT’s are used when the ratio of transformation is very high. It is also used to correct for phase displacement for differential protection of transformers.

        احسان آرزومند

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