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3-51
Protection Functions
In rigidly earthed 4-wire networks with loads between phase and
neutral overvoltage protection may be needed for phase-to-ground
voltages, too. In such applications the programmable stages can be
used. See "Programmable Stages".
Three independent stages
There are three separately adjustable stages: U>, U>> and U>>>. All
the stages can be configured for definite time (DT) operation
characteristic.
Configurable release delay
The U> stage has a settable release delay, which enables detecting
intermittent faults. This means that the time counter of the protection
function does not reset immediately after the fault is cleared, but
resets after the release delay has elapsed. If the fault appears again
before the release delay time has elapsed, the delay counter continues
from the previous value. This means that the function will eventually
trip if faults are occurring often enough.
Configurable hysteresis
The dead band is 3 % by default. It means that an overvoltage fault is
regarded as a fault until the voltage drops below 97 % of the pick up
setting. In a sensitive alarm application a smaller hysteresis is
needed. For example if the pick up setting is about only 2 % above
the normal voltage level, hysteresis must be less than 2 %.
Otherwise the stage will not release after fault.
Setting groups
There are two settings groups available for each stage. Switching
between setting groups can be controlled by digital inputs, virtual
inputs (mimic display, communication, logic) and manually.
Figure 3.26 shows the functional block diagram of the overvoltage
function stages U>, U>> and U>>>.
Figure 3.26 – Block diagram of the three-phase overvoltage stages U>, U>> and U>>>
857-UM001A-EN-P – July 2009
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