ABB ret650, Technical Manual

Page: 230 / 670

E . f n Bmax A 4 44
= × × × ×
EQUATION898 V2 EN
(Equation 76)
The relative excitation M is therefore according to equation 77 .
( )
( ) ( )
E f
M p.u. =
Ur fr
IECEQUATION2296 V1 EN
(Equation 77)
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
Potection against overexcitation is based on calculating the relative volt per hertz (V/
Hz) ratio. The protection might initiate a reduction of the generator excitation (in case
of a step-up transformer), and if this fails, or if this is not possible or implemented 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
78 .
E
f
----
1.1
£
Ur
fr
------
×
EQUATION900 V1 EN
(Equation 78)
or equivalently, with 1.1 · Ur = V/Hz> according to equation 79 .
Section 9
1MRK 504 135-UEN A
Voltage protection
224
Technical manual