ABB Relion Technical Manual page 309

1MRK 502 048-UUS A
Technical manual
overexcitation ALARM signal. If this is not possible, the power transformer can be
disconnected from the source, after a delay, by the TRIP signal.
The IEC 60076 - 1 standard requires that transformers operate continuously at not more
than 10% above rated voltage at no load, and rated frequency. At no load, the 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 86.
E Vn
£ ×
1.1
f fn
EQUATION1630 V1 EN
or equivalently, with 1.1 · Vn = Pickup1 according to equation 87.
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 88.
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.
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 remain essentially unchanged. The important voltage is
the voltage between the two ends of each winding.
Section 9
Voltage protection
(Equation 86)
(Equation 87)
(Equation 88)
303