ABB Relion REG670, Technical Manual

Page: 532 / 1298

( )
E f
M p.u. =
Ur fr
IECEQUATION2299 V1 EN
(Equation 132)
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, or Tailor-made Law, is limited by tMax , and/or tMin , then
the Thermal status will generally not reach 100% at the same time, when tTRIP
reaches 0 seconds. For example, if, at low degrees of overexcitation, the very long
delay is limited by
tMax , then the OEXPVPH TRIP output signal will be set to 1 before
the Thermal status reaches 100%.
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.
Section 9
1MRK502052-UEN B
Voltage protection
526
Technical manual