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WHITE PAPER...

GROUND FAULT...

Lowering the Limits for Ground Fault Detection...

Lowering the Limits for Ground-Fault Detection...

Current flowing to ground has only two paths--it can flow to ground through a ground fault, or it can flow to ground through distributed capacitance. Current flowing to ground through distributed capacitance can cause sympathetic tripping during a ground fault and it can cause nuisance tripping during normal operation. If the ground-fault trip level is high enough to eliminate sympathetic tripping, nuisance tripping due to unbalanced and harmonic capacitive current is usually not a problem. However, if sympathetic tripping is not a concern and ground-fault trip levels are lowered, nuisance tripping can become a problem that worsens with the increased use of adjustable-speed drives. This paper discusses the sources of current flowing to ground that are not the result of a ground fault, and shows how a digital filter tuned to the fundamental component of ground-fault current can provide lower trip levels without nuisance tripping. The limit to practical low-level ground-fault protection in industrial electrical systems is a function of physical parameters. Current sensing is the best method to detect and locate ground faults however, system capacitance, unbalanced loads, current-sensor limitations, and harmonics affect current measurement and limit the lower level of practical ground-fault detection. This paper discusses limiting factors and solutions that can be implemented to mitigate them. Grounded systems are presumed. Figure 1 illustrates that each phase in a distribution system has capacitance to ground. Phase-to-ground capacitance is distributed throughout the electrical system, but it can be modeled as a "lumped" value, as shown in the figure. When...

the system is energized, a capacitive current will flow from each phase to ground. The extent to which this capacitive current affects ground-fault detection will be discussed in some detail. Charging current is defined as the current that flows to ground when one phase of an ungrounded system is faulted to ground, as measured by the ammeter causing the fault in Figure 2. Charging current can cause sympathetic ground-fault tripping in the unfaulted feeders of a multiplefeeder system. As Figure 3 shows, core-balance current transformers (CTs) on the unfaulted feeders detect the charging current of the respective feeder. The CT on the faulted feeder detects the vector sum of the charg...

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Littelfuse - Protection - Relay - Lowering - the - limits - for - ground - fault - detection.pdf (507, 540 bytes) data en white papers littelfuse protection relay lowering the limits for ground fault detection

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