ABB Relion REG670, Technical Manual

Page: 527 / 1298

between the primary and the secondary winding: Xleak = Xleak1 = Xleak2 = Xsc / 2
= 0.075 pu.
OEXPVPH calculates the internal induced voltage E if Xleak (meaning the leakage
reactance of the winding where OEXPVPH is connected) is known to the user. The
assumption taken for two-winding power transformers that
Xleak = Xsc / 2 is
unfortunately most often not true. For a two-winding power transformer the leakage
reactances of the two windings depend on how the windings are located on the core
with respect to each other. In the case of three-winding power transformers the
situation is still more complex. If a user has the knowledge on the leakage reactance,
then it should applied. If a user has no idea about it,
Xleak can be set to X
c/2
.
OEXPVPH protection will then take the given measured voltage U, as the induced
voltage E.
It is assumed that overexcitation is a symmetrical phenomenon, caused by events such
as loss-of-load, etc. A high phase-to-earth voltage does not mean overexcitation. For
example, in an unearthed power system, a single phase-to-earth fault means high
voltages of the “healthy” two phases-to-earth, but no overexcitation on any winding.
The phase-to-phase voltages will remain essentially unchanged. The important
voltage is the voltage between the two ends of each winding.
9.4.7.1 Measured voltage
If one phase-to-phase voltage is available from the side where overexcitation
protection is applied, then Overexcitation protection OEXPVPH shall be set to
measure this voltage,
MeasuredU . The particular voltage which is used determines the
two currents that must be used. This must be chosen with the setting MeasuredI .
It is extremely important that MeasuredU and MeasuredI are set to
same value.
If, for example, voltage UL1L2 is fed to OEXPVPH, then currents IL1, and IL2 must
be applied. From these two input currents, current IL1L2 = IL1 - IL2 is calculated
internally by the OEXPVPH algorithm. The phase-to-phase voltage must be higher
than 70% of the rated value, otherwise the protection algorithm exits without
calculating the excitation. ERROR output is set to 1, and the displayed value of
relative excitation V/Hz shows 0.000.
If three phase-to-earth voltages are available from the side where overexcitation is
connected, then OEXPVPH shall be set to measure positive sequence voltage and
current. In this case the positive sequence voltage and the positive sequence current
are used by OEXPVPH. A check is made if the positive sequence voltage is higher
than 70% of rated phase-to-earth voltage, when below this value, OEXPVPH exits
immediately, and no excitation is calculated. ERROR output is set to 1, and the
displayed value of relative excitation V/Hz shows 0.000.
1MRK502052-UEN B
Section 9
Voltage protection
521
Technical manual