Technical description
2 Protection functions
2.14 Overvoltage protection U> (59)
VM259.EN007
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2.14.
Overvoltage protection U> (59)
The overvoltage function measures the fundamental frequency
component of the line-to-line voltages regardless of the voltage
measurement mode (chapter 4.7). By using line-to-line voltages
any phase-to-ground over-voltages during earth faults have no
effect. (The earth fault protection functions will take care of
earth faults.) Whenever any of these three line-to-line voltages
exceeds the user's pick-up setting of a particular stage, this
stage picks up and a start signal is issued. If the fault situation
remains on longer than the user's operation time delay setting,
a trip signal is issued.
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 chapter 2.22.
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.