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Reply To: معمای حفاظت شماره 90 – محاسبه TMS در رله هاي جرياني


    همانطور که بیان شده TMS ضریبی است که برای تعیین زمان عملکرد رله های با مشخصات عملکرد IDMT تعریف می شود. این ضریب معمولاً بین 0.1 تا 1 تنظیم می شود. وقتی این مقدار 1 تنظیم می شود رله در زمان تأخیری ذاتی خود عمل می کند و اگر مقدار دیگری مثل 0.5 را دارا باشد در 0.5 برابر زمان ذاتی با توجه به جریان تنظیمی عمل می کند. و اما برای هماهنگی زمانی تجهیزات حفاظتی سری با یکدیگر، برخی دستورالعملهای حفاظتی وجود دارد که در زیر نمونه ای از آن آمده است. Coordination time intervals. When plotting coordination curves, certain time intervals must be maintained between the curves of various protective devices in order to ensure correct sequential operation of the devices.These intervals are required because relays have overtravel and curve tolerances, certain fuses have damage characteristics, and circuit breakers have certain speeds of operation. Sometimes these intervals are called margins. (1) Coordination can be easily achieved with low voltage current-limiting fuses that have fast response times. Manufacturer’s time current curves and selectivity ratio guides are used for both overload and short-circuit conditions, precluding the need for calculating time intervals. (2) When coordinating inverse time overcurrent relays, the time interval is usually 0.3-0.4 seconds.This interval is measured between relays in series either at the instantaneous setting of the load side feeder circuit breaker relay or the maximum shortcircuit current, which can flow through both devices simultaneously, whichever is the lower value of current. The interval consists of the following components: (a) Circuit breaker opening . . . . . . 0.08 seconds time (5 cycles). (b) Relay overtravel . . . . . . . 0.10 seconds (c) Safety factor for CT satu- 0.22 second ration, setting errors, contact gap,etc (3) This safety factor may be decreased by field testing relays to eliminate setting errors. This involves calibrating the relays to the coordination curves and adjusting time dials to achieve specific operating times. A 0.355 margin is widely used in field-tested systems employing very inverse and extremely inverse time overcurrent relays. (4) When solid-state relays are used, overtravel is eliminated and the time may be reduced by the amount included for overtravel. For systems using induction disk relays, a decrease of the time interval may be made by employing an overcurrent relay with a special high-dropout instantaneous element set at approximately the same pickup as the time element with its contact wired in series with the main relay contact. This eliminates over-travel in the relay so equipped. The time interval often used on carefully calibrated systems with high-dropout instantaneous relays is 0.25 seconds. (5) When coordinating relays with downstream fuses, the circuit opening time does not exist for the fuse and the interval may be reduced accordingly. The total clearing time of the fuse should be used for coordination purposes. The time margin between the fuse total clearing curve and the upstream relay curve could be as low as 0.1 second where clearing times below 1 second are involved. (6) When low-voltage circuit breakers equipped with direct-acting trip units are coordinated with relayed circuit breakers, the coordination time interval is usually regarded as 0.3 seconds. This in terval may be decreased to a shorter time as explained previously for relay-to-relay coordination. (7) When coordinating circuit breakers equipped with direct-acting trip units, the characteristics curves should not overlap. In general only a slight separation is planned between the different characteristics curves. This lack of a specified time margin is explained by the incorporation of all the variables plus the circuit breaker operating times for these devices within the band of the device characteristic curve. (8) Delta-wye transformers. When protecting a delta-wye transformer, an additional 16 percent current margin over margins mentioned previously should be used between the primary and secondary protective device characteristic curves. This helps maintain selectivity for secondary phase-to-phase faults since the per-unit primary current in one phase for this type of fault is 16 percent greater than the per-unit secondary current which flows for a secondary three-phase fault.