9.4.7.3
Cooling
The overexcitation protection function (OEXPVPH, 24) is basically a thermal protection,
therefore a cooling process has been introduced. An exponential cooling process is
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. =
Vn fn
ANSIEQUATION2299 V1 EN
(Equation 92)
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, is limited by
t_MinTripDelay
, 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 (24) 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.
1MRK 502 048-UUS A
Section 9
Voltage protection
307
Technical manual
