ratio of the actual generator terminal voltage to the actual frequency should not exceed
1.1 times the ratio of transformer rated voltage to the rated frequency on a sustained
basis, see equation
.
E
f
----
1.1
£
Ur
fr
------
×
EQUATION900 V1 EN
(Equation 124)
or equivalently, with 1.1 · Ur =
V/Hz>
according to equation
E
f
fr
V Hz >
£
IECEQUATION2297 V2 EN
(Equation 125)
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
.
( )
E f
M p.u. =
Ur fr
IECEQUATION2299 V1 EN
(Equation 126)
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.
The overexcitation protection algorithm is fed with an input voltage U 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 U which is
measured and fed to OEXPVPH , depending on the direction of the power flow
through the power transformer, the power transformer side where OEXPVPH 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 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
Section 9
1MRK502052-UEN B
Voltage protection
520
Technical manual
Summary of Contents for Relion REG670
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