Overexcitation Protection Function Measurands; Overexcitation Alarm - ABB REG650 Technical Manual

Section 8
Voltage protection
8.4.7.4
8.4.7.5
244
applied, with a default time constant of 20 minutes. This means that if the voltage
and frequency return to normal values (no more overexcitation), the normal
temperature is assumed to be reached not before approximately 5 times the default
time constant of 20 minutes. If an overexcitation condition would return before
that, the time to trip will be shorter than it would be otherwise.

Overexcitation protection function measurands

A monitored data value, TMTOTRIP, is available on the local HMI and in
PCM600. This value is an estimation of the remaining time to trip (in seconds), if
the overexcitation remained on the level it had when the estimation was done. This
information can be useful with small or moderate overexcitations.
The relative excitation M, shown on the local HMI and in PCM600 has a
monitored data value VPERHZ, is calculated from the expression:
E f
( )
M p.u. =
Ur fr
IECEQUATION2299 V1 EN
If VPERHZ value is less than setting V/Hz> (in %), the power transformer is
underexcited. If VPERHZ is equal to V/Hz> (in %), the excitation is exactly equal
to the power transformer continuous capability. If VPERHZ is higher than V/Hz>,
the protected power transformer is overexcited. For example, if VPERHZ = 1.100,
while V/Hz> = 110 %, then the power transformer is exactly on its maximum
continuous excitation limit.
Monitored data value THERMSTA shows the thermal status of the protected
power transformer iron core. THERMSTA gives the thermal status in % of the trip
value which corresponds to 100%. THERMSTA should reach 100% at the same
time, as TMTOTRIP reaches 0 seconds. If the protected power transformer is then
for some reason not switched off, THERMSTA shall go over 100%.
If the delay as per IEEE law, is limited by tMin, then THERMSTA will generally
not reach 100% at the same time, as TMTOTRIP reaches 0 seconds. For example,
if, at low degrees of overexcitation, the very long delay is limited by 30 minutes,
then the TRIP output signal of OEXPVPH will be set to 1 before THERMSTA
reaches 100%.

Overexcitation alarm

A separate step, AlarmLevel, is provided for alarming purpose. It is normally set
2% lower than (V/Hz>) and has a definite time delay, tAlarm. This will give the
operator an early abnormal voltages warning.
1MRK 502 034-UEN -
(Equation 83)
Technical Manual