IEEE C37.99-2000 pdf download IEEE Guide for the Protection of Shunt Capacitor Banks
4.2.2 Internally fused shunt capacitor banks Figure 2(b) illustrates a typical capacitor bank utilizing internally fused capacitor units. In general, banks employing internally fused capacitor units are configured with fewer capacitor units in parallel and more series groups of units than are used in banks employing externally fused capacitor units. The capacitor units are normally large because a complete unit is not expected to fail. An internal fuse is connected in series with each capacitor element. Each internally fused capacitor unit is constructed with a large number of elements connected in parallel to form a group and with only a few groups connected in series.
This construction is the opposite to that found in externally fused capacitors, which normally employ a large number of series groups made up of parallel connected elements, with corre- spondingly fewer elements connected in parallel per series group. With internally fused capacitors, when a capacitor element fails, the current through its individual fusible link will be considerably higher than the normal current. This higher current will blow the fusible link, thereby isolating the failed element.
4.2.3 Fuseless shunt capacitor banks Fuseless shunt capacitor banks are normally used for applications at or above 34.5 kV. The capacitor units are normally designed with two bushings with the elements insulated from the case. The capacitor units are connected in series strings between phase and neutral (or between line terminals for delta-connected or single-phase installations). The protection is based on the capacitor element’s failing in a shorted mode. The discharge energy is small because no capacitor units are connected directly in parallel. Another advantage is that the unbalance protection does not have to be delayed to coordinate with the fuses. See Figure 2(c).
4.2.4 Unfused shunt capacitor banks The unfused shunt capacitor approach uses a series/parallel connection of the capacitor units. The voltage across the remaining elements will increase more than in the fuseless design. The unfused approach would normally be used on banks below 34.5 kV (where series strings are not practical) or on higher voltage banks with modest parallel energy. This design does not require as many capacitor units in parallel as an externally fused bank. See Figure 2(d).
4.3 Capacitor bank design
4.3.1 Externally fused An externally fused shunt capacitor bank of a given size and voltage rating may be made up of a number of series and parallel groups. Use of capacitors with the highest possible voltage rating will result in a capacitor bank with the fewest number of series groups. This arrangement generally provides the simplest rack struc- ture and the greatest sensitivity for unbalance detection schemes. The available unbalance signal level decreases significantly as the number of series groups of capacitors is increased or as the number of capaci- tor units in parallel per series group is increased.
IEEE C37.99-2000 pdf download
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