ABB REG650 Technical Manual page 281

1MRK 502 043-UUS B
Technical Manual
step-up transformer), and if this fails, or if this is not possible, the TRIP signal will
disconnect the transformer from the source after a delay ranging from seconds to
minutes, typically 5-10 seconds.
Overexcitation protection may be of particular concern on directly connected generator
unit transformers. Directly connected generator-transformers are subjected to a wide
range of frequencies during the acceleration and deceleration of the turbine. In such
cases, OEXPVPH (24) may trip the field breaker during a start-up of a machine, by
means of the 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 83.
E Vn
£ ×
1.1
f fn
EQUATION1630 V1 EN
or equivalently, with 1.1 · Vn = Pickup1 according to equation 84.
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 85.
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
Section 9
Voltage protection
(Equation 83)
(Equation 84)
(Equation 85)
275