ABB Relion 670 series, Technical Reference Manual

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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. =
Vn fn
ANSIEQUATION2299 V1 EN
(Equation 145)
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.
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%.
8.4.2.5 Overexcitation alarm
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 abnormal voltages warning.
Section 8
1MRK505222-UUS C
Voltage protection
538
Technical reference manual