ABB REG650 ANSI, Technical Manual

Page: 285 / 728

E
f fr
V Hz >
£
IECEQUATION2297 V2 EN
(Equation 87)
where:
V/Hz> is the maximum continuously allowed voltage at no load, and rated frequency.
V/Hz> 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 V/Hz> = 110 % given by the IEC
60076-1 standard shall be used.
In OEXPVPH, the relative excitation M is expressed according to equation 88 .
( )
E f
M p.u. =
Ur fr
IECEQUATION2299 V1 EN
(Equation 88)
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 Ur/fr. A power transformer is not
overexcited as long as the relative excitation is M ≤
V/Hz> , V/Hz> expressed in % of
Ur/fr.
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
A check is made if the Selected voltage signal is higher than 70% of the rated phase-to-
earth voltage. When below this value, OEXPVPH exits immediately and no excitation
is calculated.
The frequency value is received from the pre-processing block. The function operates
for frequencies within the range of 33-60 Hz and of 42-75 Hz for 50 Hz and 60 Hz
respectively.
• OEXPVPH can be connected to any power transformer side, independent from
the power flow.
• The side with a load tap changer must not be used, since the tap changer can
change the relative excitation (M)
1MRK 502 048-UEN A
Section 9
Voltage protection
279
Technical manual