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OPERATION, MAINTENANCE AND REPAIR OF AUXILIARY GENERATORS Episode 2

Tham khảo tài liệu 'operation, maintenance and repair of auxiliary generators episode 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | TM 5-685 NAVFAC MO-912 RATED FOR 2 TO 6A.I C. Figure 2-2. Types of system grounding. A UNGROUNDED GENERATOR B SOLIDLY GROUNDED C LOW RESISTANCE GROUNDING D HIGH RESISTANCE GROUNDING be close to ground potentials because of the capacitance between each phase conductor and ground. When a line-to-ground fault occurs on an ungrounded system the total ground fault current is relatively small but the voltage to ground potential on the unfaulted phases can reach an unprecedented value. If the fault is sustained the normal line-to-neutral voltage on the unfaulted phases is increased to the system line-to- line voltage i.e. square root of three 3 times the normal line-to-neutral value . Over a period of time this breaks down the line-to-neutral insulation and results in insulation failure. Ungrounded system operation is not recommended because of the high probability of failures due to transient over-voltages especially in medium voltage i.e. 1 kilovolt Kv -15 Kv caused by restriking ground faults. 2-5 TM 5-685 NAVFAC MO-912 2 Overvoltage limitation is particularly important in systems over 1 Kv because equipment in these voltage classes are designed with less margin between 50 60 Hz test and operating voltages than low voltage equipment. The remaining various grounding methods can be applied on system grounding protection depending on technical and economic factors. The one advantage of an ungrounded system that needs to be mentioned is that it generally can continue to operate under a single line-to-ground fault without significant damage to electrical equipment and without an interruption of power to the loads. g. A solidly grounded system refers to a system in which the neutral or occasionally one phase is connected to ground without an intentional intervening impedance. On a solidly grounded system in contrast to an ungrounded system a ground fault on one phase will result in a large magnitude of ground current flow but there will be no increase in voltage on the .