ABB Relion Technical Manual page 308

Section 9
Voltage protection
302
uniformity of the excitation level of modern systems. If an emergency that causes
overexcitation does occur, transformers may be damaged unless corrective action is taken.
Transformer manufacturers recommend an overexcitation protection as a part of the
transformer protection system.
Overexcitation results from excessive applied voltage, possibly in combination with
below-normal frequency. Such condition may occur when a transformer unit is loaded,
but are more likely to arise when the transformer is unloaded, or when a loss of load
occurs. Transformers directly connected to generators are in particular danger to
experience overexcitation condition. It follows from the fundamental transformer
equation, see equation 84, that peak flux density Bmax is directly proportional to the
induced voltage E, inversely proportional to frequency f, and turns n.
= × × × ×
E 4 44 . f n B max A
EQUATION898 V2 EN
The relative excitation M is therefore according to equation 85.
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
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
1MRK 502 048-UUS A
(Equation 84)
(Equation 85)
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