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WEEK 09: TRANSMISSION: COMPONENT SPECIFICATIONS Sections: Power-Factor Correction | Classifications | Applications | Protection Principles
Sections: Power-Factor Correction | Classifications | Applications | Protection Principles Power Capacitors Classifications Classifications Capacitor installations are usually shunt-connected across the power lines and are either energized continuously or switched on and off during load cycles. There are two type of capacitors: secondary (low voltage) and primary (high voltage), the more common type. Secondary Capacitors, or low voltage capacitors, are generally available in voltage ratings from 240 to 600 V over the range of 2.5 to 100 kilovar three-phase. When low-voltage capacitors are connected to secondary lines, they are usually physically located nearer to the lagging reactive loads. This reduces the kVA requirements of the immediate lines and transformers or, conversely, allows a larger kilowatt load with the same-size lines and transformers. Most low-voltage capacitors use metalized polypropylene dielectric materials. Primary Power-Factor-Correction Capacitors are connected to high-voltage lines and are generally available in voltage ratings from 2.4 to 25kV over the range of 50 to 400 kilovar. Higher voltage and kvar ratings are achieved by connecting capacitor units in series and parallel arrangements. The cost of high-voltage capacitors is lower per kvar than low-voltage capacitors because of the basic difference in dielectric materials which allows high-voltage capacitors to be operated more efficiently. Also, modern high-voltage capacitors operate at lower watts loss per kvar than low-voltage capacitors. Most high-voltage capacitors use all-film dielectric materials. Sections: Power-Factor Correction | Classifications | Applications | Protection Principles Mounting Capacitors for overhead distribution systems can be pole-mounted in banks of 300 to 3600 kvar at nearly any primary voltage up to 34.5 kV phase-to-phase. Pad-mounted capacitor equipments are used for underground distribution systems in the same range of sizes and voltage ratings. They could either be housed or stack-rack capacitors, which improve phase relation between line voltage and line current at substation. Connections There are four most common capacitor connections; 3-phase grounded wye, 3-phase ungrounded wye, 3-phase delta and single-phase. Grounded or ungrounded wye connections are usually made on primary circuits whereas delta and single-phase connections are usually made on low-voltage circuits. With the grounded-wye connection, switch tanks and frames are at ground potential. This provides increased personnel safety, faster operation of the series fuse in case of a capacitor failure, the ability to bypass some line surges to ground, exhibits a certain degree of self-protection from transient voltages and lightning surges; and provides a low-impedance path for harmonics. If the capacitors are electrically connected ungrounded-wye, the maximum fault current would be limited to three times line current. If too much fault current is available, generally above 5000 A, the use of current limiting fuses must be considered. Sections: Power-Factor Correction | Classifications | Applications | Protection Principles Safety of all personnel who are required to work near or with the equipment should be of prime importance. Several fundamental principles must be observed in the selection of fuses for capacitor applications. These are: Tank Rupture will occur if the total energy applied to the capacitor under failure conditions is greater than the ability of the capacitor tank to withstand such energy. Tank-rupture curves are essential to the correct selection of fuse links for overcurrent protection of any capacitor installation. Fuse selections should be based upon the coordination of the fuse link maximum-clearing curve (time vs. current, minimum clearing time [0.8 cycle] for safe coordination) and the high-voltage capacitor tank-rupture curve (time vs. available short circuit current, rms amperes). Ventilation The normal free-air operating temperature for power capacitors is approximately -40 to +50 degrees Centigrade. Capacitor applications must be designed for adequate voltage and corona capabilities. |
