ABB Relion 670 Series Technical Reference Manual page 535

1MRK505222-UUS C
8.4
8.4.1
8.4.2
Technical reference manual
Overexcitation protection OEXPVPH (24)
Function description
Overexcitation protection
Introduction
When the laminated core of a power transformer or generator is subjected to a
magnetic flux density beyond its design limits, stray flux will flow into non-laminated
components not designed to carry flux and cause eddy currents to flow. The eddy
currents can cause excessive heating and severe damage to insulation and adjacent
parts in a relatively short time. The function has settable inverse operating curves and
independent alarm stages.
Principle of operation
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 on load,
but are more likely to arise when it is on open circuit, or at a loss of load occurrence.
Transformers directly connected to generators are in particular danger to experience
overexcitation condition. It follows from the fundamental transformer equation, see
equation 135, 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
IEC 61850 IEC 60617
identification identification
OEXPVPH
U/f >
SYMBOL-Q V1 EN
Section 8
Voltage protection
ANSI/IEEE C37.2
device number
24
(Equation 135)
529