ABB Relion 670 Series Technical Reference Manual page 536

Section 8
Voltage protection
530
The relative excitation M is therefore according to equation 136.
E f
( )
M p.u. =
( ) ( )
Vr fn
ANSIEQUATION2296 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 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 137.
E Vn
£ ×
1.1
f fn
EQUATION1630 V1 EN
or equivalently, with 1.1 · Vn = Pickup1 according to equation 138.
1MRK505222-UUS C
(Equation 136)
(Equation 137)
Technical reference manual