The protection element also provides a non-sensitive area to avoid instability due to small asymmetries and unbalances that can generally be present in the network system. This condition is represented by the characteristic reproduced below, where the hatched area is the tripping zone.
Setting Guidelines
The relay with this type of protection element must be co-ordinated with any other overvoltage relay at other locations on the system. This should be carried out in a similar manner
to that used for grading current operated protection devices.
The protection element over-voltage has two programmable thresholds and two delay timers,
tU>, tU>>.
After a trip command the thresholds will be reset when all phase voltages have fallen below
95% of setting values.
The overvoltage protection element, which can be set as OR or AND logic, operates by
comparing each UAB, UBC and UCA voltage input with the U>, U>> thresholds.
NOTE: If the relay is connected in the mode 3Vpn to control the phase to
neutral voltages, the thresholds must be multiplied by 3 .
When OR operating logic is set and one or more voltage has risen above the threshold
value, the tripping command is sent after the tripping timer has reached the set overtime
condition.
When AND operating logic is set and all of the voltage values have risen above the threshold
value, the tripping command is sent after the tripping timer has reached the set overtime
condition.
Information as to the thresholds setting range can be found in the FT and TD parts of the
technical guide.
3.6 Negative sequence overvoltage protection
Where an incoming feeder is supplying a switchboard which is feeding rotating plant (e.g.
induction motors), correct phasing and balance of the ac supply is essential. Incorrect phase
rotation will result in any connected motors rotating in the wrong direction. For directionally
sensitive applications, such as elevators and conveyor belts, it may be unacceptable to allow
this to happen
Any unbalanced condition occurring on the incoming supply will result in the presence of
negative phase sequence (nps) components of voltage. In the event of incorrect phase
rotation, the supply voltage would effectively consist of 100% negative phase sequence
voltage only
3.6.1 Setting guideline
The negative sequence overvoltage protection has two independent thresholds: V2> and
tV2>.
Any unbalanced condition occurring on the incoming supply will result in the presence of
negative phase sequence (nps) components of voltage. In the event of incorrect phase
rotation, the supply voltage would effectively consist of 100% negative phase sequence
typical.
The operation time of the element will be highly dependent on the application. A typical
setting would be in the region of 5s.
The relay will trip according to an inverse characteristic or a definite time characteristic for
the first stage and according to a definite time characteristic for the second stage.
The inverse characteristic is given by the following formula:
t = K / (M - 1)
Where: K = Time Multiplier Setting
t = operating time in seconds
M = Applied input voltage / Relay setting voltage (Vs).
