ABB Relion 670 series, Technical Manual

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E
f fn
Pickup1
£
ANSIEQUATION2297 V2 EN-US
(Equation 189)
where:
Pickup1 is the maximum continuously allowed voltage at no load, and rated frequency.
Pickup1 is a setting parameter. The setting range is 100% to 180%. If the user does not know
exactly what to set, then the default value for
Pickup1 = 110 % given by the IEC 60076-1 standard
shall be used.
In OEXPVPH (24), the relative excitation M is expressed according to equation 190 .
( )
E f
M p.u. =
Vn fn
ANSIEQUATION2299 V1 EN-US
(Equation 190)
It is clear from the above formula that, for an unloaded power transformer, M = 1 for any E and f,
where the ratio E/f is equal to Vn/fn. A power transformer is not overexcited as long as the
relative excitation is M ≤
Pickup1, Pickup1 expressed in % of Vn/fn.
The overexcitation protection algorithm is fed with an input voltage V which is in general not the
induced voltage E from the fundamental transformer equation. For no load condition, these two
voltages are the same, but for a loaded power transformer the internally induced voltage E may be
lower or higher than the voltage V which is measured and fed to OEXPVPH (24), depending on the
direction of the power flow through the power transformer, the power transformer side where
OEXPVPH (24) is applied, and the power transformer leakage reactance of the winding. It is
important to specify in the application configuration on which side of the power transformer
OEXPVPH (24) is placed.
As an example, at a transformer with a 15% short circuit impedance Xsc, the full load, 0.8 power
factor, 105% voltage on the load side, the actual flux level in the transformer core, will not be
significantly different from that at the 110% voltage, no load, rated frequency, provided that the
short circuit impedance X can be equally divided between the primary and the secondary winding:
XLeakage = XLeakage1 = XLeakage2 = Xsc / 2 = 0.075 pu.
OEXPVPH (24) calculates the internal induced voltage E if
XLeakage (meaning the leakage
reactance of the winding where OEXPVPH (24) is connected) is known to the user. The assumption
taken for two-winding power transformers that
XLeakage = 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,
XLeakage can be set
to X
c/2
. OEXPVPH (24) protection will then take the given measured voltage V, 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-ground voltage does not mean overexcitation. For example, in an
ungrounded power system, a single phase-to-ground fault means high voltages of the “healthy”
two phases-to-ground, but no overexcitation on any winding. The phase-to-phase voltages will
Section 9 1MRK 502 066-UUS B
Voltage protection
640
Technical manual