ABB Relion 670 Series Technical Reference Manual page 537

1MRK505222-UUS C
Technical reference manual
E Pickup1
£
f fn
ANSIEQUATION2297 V2 EN
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 139.
E f
( )
M p.u. =
Vn fn
ANSIEQUATION2299 V1 EN
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 /
Section 8
Voltage protection
(Equation 138)
(Equation 139)
531