( )
E f
M p.u. =
Vn fn
ANSIEQUATION2299 V1 EN-US
(Equation 196)
If VPERHZ value is less than setting
Pickup1 (in %), the power transformer is underexcited. If
VPERHZ is equal to
Pickup1 (in %), the excitation is exactly equal to the power transformer
continuous capability. If VPERHZ is higher than
Pickup1, the protected power transformer is
overexcited. For example, if VPERHZ = 1.100, while
Pickup1 = 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
t_MaxTripDelay, and/or
t_MinTripDelay, 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
t_MaxTripDelay, then the OEXPVPH (24) TRIP output signal will be set to 1 before the
Thermal status reaches 100%.
9.4.7.5
Overexcitation alarm
M5854-123 v6
A separate step,
AlarmPickup, is provided for alarming purpose. It is normally set 2% lower than
(
Pickup1) and has a definite time delay, tAlarm. This will give the operator an early warning.
9.4.7.6
Logic diagram
M13333-9 v4
ANSI05000162-3-en.vsd
OR
&
&
BLOCK
Xleakage
Ei
M=
(Ei / f)
(Vn / fn)
M = relative Pickup as service value
Pickup2
M>Pickup2
Pickup1
M>Pickup1
IEEE law
Tailor-made law
Calculation
of internal
induced
voltage Ei
TD
M
M
M
ALARM
TRIP
AlarmPickup
0
0
0-tMax
0-tMax
t>tAlarm
t>tMin
t_MaxTripDelay
t_MinTripDelay
tAlarm
V3P
I3P
&
0
0-tMin
ANSI05000162 V3 EN-US
Figure 350: A simplified logic diagram of the Overexcitation protection OEXPVPH (24)
Section 9
1MRK 502 066-UUS B
Voltage protection
646
Technical manual
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