Home » معماهای فارسی » حفاظت الکتریکی » معمای حفاظت شماره 109 – ترانس جریان کوربالانس » Reply To: معمای حفاظت شماره 109 – ترانس جریان کوربالانس

Reply To: معمای حفاظت شماره 109 – ترانس جریان کوربالانس


    ترانسهای جریان کور بالانس ترانسهای جریان حلقوی با طراحی خاص جهت آشکار سازی شار مغناطیسی ناشی از عدم تعادل جریانهای گذرنده از آن هستند. از آنجا که این جریان عدم تعادل می تواند در مقایسه با جریانهای فالت بسیار کوچک باشد این نوع ترانسهای جریان می باید از دقت بالایی برای کارکرد در این رنج جریان اولیه برحوردار باشند. The core-balance method is based on primary current phasor addition or flux summation. The remaining zero-sequence component, if any, is then transformed to the secondary. The core-balance CT or sensor is the basis of several low-voltage ground-fault protective systems. (The core-balance CT is frequently called a zero-sequence sensor or window CT, but the term core balance is preferable because it more specifically describes the function of the CT.) All load-carrying conductors pass through the same opening in the CT and are surrounded by the same magnetic core. Core-balance CTs are available in several convenient shapes and sizes, including rectangular designs for use over bus bars. This method can be more sensitive than the residual method because the sensor rating is large enough for the possible imbalance, not for the individual conductor load current. Under normal conditions , all current flows out and returns through the CT. The net flux produced in the CT core is zero, and no current flows in the ground relay. When a ground fault occurs, the ground-fault current returns through the equipment grounding circuit conductor (and possibly other ground paths) and bypasses the CT. The flux produced in the CT core is proportional to the ground-fault current, and a proportional current flows from the CT secondary to the relay circuit. Relays connected to core-balance CTs can be made quite sensitive. However, care is necessary to prevent false opening from unbalanced inrush currents that may saturate the CT core or through faults not involving ground. If only phase conductors are enclosed and neutral current is not zero, the transformed current is proportional to the load zero-sequence or neutral current. Systems with grounded conductors, such as cable shielding, should have the CT surround only the phase and neutral conductors, if applicable, and not the grounded conductor. The core-balance current transformers are generally employed to detect small amounts of ground leakage currents, such as in mines and other sensitive installations. They are also used to protect sensitive equipment against small ground leakages. Installations having isolated neutral or using ground resistance or impedance, to limit the ground leakage currents, may also require this type of fault detection. A core-balanced CT is in a toroidal (circular) or rectangular form, like a conventional protection CT, except that it is designed, with a large core opening to accommodate all the 3 or 4-core feeder cables passing through it. The basic difference between this and conventional protection CTs is the low unbalance or leakage current at which a CBCT operates. A normal protection CT would operate between its rated and accuracy limit currents. The important design parameter in a CBCT is its magnetizing current at the relay operating voltage, rather than the class of accuracy and accuracy limit factor. As these CTs have to detect small to extremely feeble ground leakage currents their operating region is required to be very low, near the ‘ankle point’ (almost 10% of the knee point voltage) on the magnetizing curve. The design criterion is thus the minimum exciting current required at the relay operating voltage (details available from the relay manufacturer) to actuate the relay.