Operation Principle - ABB REG650 Technical Manual

Section 9
Voltage protection
9.4.7
274

Operation principle

The importance of Overexcitation protection (OEXPVPH, 24) function is growing as
the power transformers as well as other power system elements today operate most of
the time near their designated limits.
Modern design transformers are more sensitive to overexcitation than earlier types.
This is a result of the more efficient designs and designs which rely on the
improvement in the uniformity of the excitation level of modern systems. Thus, if
emergency that causes overexcitation does occur, transformers may be damaged unless
corrective action is promptly 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 81, that peak flux density Bmax is directly proportional to
induced voltage E, and inversely proportional to frequency f, and turns n.
= × × × ×
E 4 44 . f n Bmax A
EQUATION898 V2 EN
The relative excitation M is therefore according to equation 82.
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 calculation of the relative volt per hertz (V/
Hz) ratio. Protection might initiate a reduction of the generator excitation (in case of a
1MRK 502 043-UUS B
(Equation 81)
(Equation 82)
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