ABB REG650 Technical Manual page 245

1MRK 502 034-UEN -
Technical Manual
The relative excitation M is therefore according to equation 77.
E f
( )
M p.u. =
( ) ( )
Ur fr
IECEQUATION2296 V1 EN
Disproportional variations in quantities E and f may give rise to core overfluxing.
If the core flux density Bmax increases to a point above saturation level (typically
1.9 Tesla), the flux will no longer be contained within the core, but will extend into
other (non-laminated) parts of the power transformer and give rise to eddy current
circulations.
Overexcitation will result in:
• overheating of the non-laminated metal parts
• a large increase in magnetizing currents
• an increase in core and winding temperature
• an increase in transformer vibration and noise
Protection against overexcitation is based on calculation of the relative volt per
hertz (V/Hz) ratio. Protection initiates a reduction of excitation, and if this fails, or
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 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 78.
E Ur
£ ×
--- - 1.1 ----- -
f fr
EQUATION900 V1 EN
or equivalently, with 1.1 · Ur = V/Hz> according to equation 79.
Section 8
Voltage protection
(Equation 77)
(Equation 78)
239