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The relay characteristic angle setting is applied to compensate for lag of the fault current.
Generally accepted angle settings are -45° for solidly earthed distribution systems and -60° for
transmission systems.
Due to system imbalances and measuring tolerances, small levels of residual voltage can be
present during normal operating conditions. Therefore, GRD150 provides a voltage threshold
which must be exceeded before the directional protection will operate. Although this threshold is
user programmable, most applications will be satisfied by the default setting of 3V.
2. Unearthed (insulated) systems
An insulated system has no intentional connection to earth, although all systems are in fact
earthed by natural capacitive coupling. Fault current is very low, being made up of capacitive
charging currents, thus limiting damage to plant. However, high steady-state and transient
overvoltages are produced, and selective isolation of faults is difficult.
An earth fault on an ungrounded system causes a voltage shift between the neutral point and
earth, and the fault can be detected by measuring this shift. So called neutral voltage
displacement protection is commonly applied but, unfortunately, the shift in voltage is
essentially the same throughout the system and so this method cannot selectively isolate a
faulted section.
The method of directional earth fault protection described previously for solidly earthed systems
cannot be used in the case of insulated systems because of the absence of real fault current.
However, an alternative method can be applied, using GRD150 directional sensitive earth fault
protection. The relay must be connected using a core balance CT, to measure the flow of
capacitive charging currents, which become unbalanced in the event of a fault.
A phase to earth fault effectively short circuits that phase's capacitance to earth for the whole
system, thus creating an unbalance in the charging currents for all feeders connected to the
system. The resulting fault current is made up of the sum of the combined residual charging
currents for both the faulty and healthy feeders.
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