[حمید]

قبل از هر چیز بهتر است ستینگ رله حفاظت نامتعادلی را چک نمائید که بیش از میزان توصیه شده حساس نباشد. در این ارتباط مراعات موارد زیر مفید است: 1- The unbalance relay should provide an alarm on 5% or less overvoltage and trip the bank for over voltages in excess of 10% of the rated voltage. 2- The unbalance relay should have time delay to minimize the damage due to arcing fault between capacitor units. Also, the time delay should be short enough to avoid damage to sensors such as a voltage transformer or current transformer. 3- The unbalance relay should have time delay to avoid false operations due to inrush, ground faults, lightning, and switching of equipment nearby. A 0.5 second delay should be adequate for most applications. 1-یک روش مرسوم جهت آشکار سازی نامتعادلی در مورد بانکهای خازنی زمین شده روش آشکارسازی جریان نوترال است که علاوه بر نامتعادلیهای با منشأ داخلی در مقابل ولتاژهای نامتعادل خارجی نیز حساس است. دیگر مزایا و معایب روش فوق به شرح زیر است: The advantages of this scheme are: 1. The capacitor bank contains twice as many parallel units per series group compared to the double wye bank for a given kVAR size which reduces the overvoltage seen by the remaining units in a group in event of a fuse operation. 2. This bank may require less substation area and connections than a double wye bank. 3. Relatively inexpensive protection scheme. The disadvantages of this scheme are: 1. Sensitive to system unbalance, which is a significant factor for large banks. 2. Sensitive to triple harmonics and will generally require a filter circuit. 3. Will not act when there is similar failure in all the phases. 4. It is not possible to identify the phase of the failed capacitor unit. 2-روش دیگر که به روش دیفرانسیل جریانهای نوترال معروف است، از حساسیت کمتری نسبت به نامتعادلیهای خارجی برخوردار می باشد. بطور کلی مزایای این روش به شرح زیر قابل ذکر است: The advantages of this scheme are: 1. The scheme is not sensitive to system unbalance and it is sensitive in detecting capacitor unit outages even on very large capacitor banks. 2. Harmonic currents do not affect this scheme. 3. For very large banks with more than one series group the amount of energy in the capacitors will decrease. This will lower the fuse interrupting duty and may reduce the cost of fuses. 3-روش آشکار سازی دیفرانسیل ولتاژی که دوگان روش 2 است نیز در برابر نامتعادلی خارجی از حساسیت کمتری برخوردار است. The advantages of this scheme are: 1. The capacitor bank contains twice as many parallel units per series group compared to a split-wye bank. The overvoltages seen by the remaining units in a group in the event of a fuse operation will be less. 2. This capacitor bank may require less substation area. 3. This scheme is less sensitive to system unbalance. It is sensitive to failure detection in the series capacitors. The main limitation of this scheme is that the number of PTs required is six and extensive connections are also required. 4-در بانکهای خازنی زمین نشده نیز می توان روشی به شرح شکل زیر انتخاب نمود. The advantages of this scheme are: 1. The capacitor bank contains twice as many parallel units per series group compared to a split-wye bank.The over voltages seen by the remaining units in a group in the event of a fuse operation will be less. 2. This capacitor bank may require less substation area and connection in the power circuit. 3. This scheme is less sensitive to system unbalance. به هر حال روشهای مختلفی جهت آشکار سازی نامتعادلی داخلی بانکهای خازنی وجود دارد با توجه به اتصالات آنها قابل تصور است که می باید متناسب با شرایط بهترین گزینه انتخاب شود.