ABB ret650, Technical Manual

Page: 235 / 670

applied, with a 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 after approximately 5 times the 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.
9.4.7.4 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 during small or moderate overexcitation situations.
The relative excitation M, shown on the local HMI and in PCM600 has a monitored
data value VPERHZ and is calculated from the expression:
( )
E f
M p.u. =
Ur fr
IECEQUATION2299 V1 EN
(Equation 84)
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.
The 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. Also, if, during a low
degrees of overexcitation, the very long delay is limited by 30 minutes and the TRIP
output signal of OEXPVPH will be set to 1 and TMTOTRIP will reach 0 seconds
before THERMSTA reaches 100%. The TRIP output is provided as a pulse of 100 ms.
9.4.7.5 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 warning.
1MRK 504 135-UEN A
Section 9
Voltage protection
229
Technical manual