ABB Relion REG670, Technical Manual, Page: 525 / 1298

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 conditions may occur when a transformer unit is
loaded, but are more likely to arise when the transformer is unloaded, or when loss of
load occurs. Transformers directly connected to generators are in particular danger to
experience overexcitation conditions. It follows from the fundamental transformer
equation, see equation
122 , that the peak flux density Bmax is directly proportional to
induced voltage E, and inversely proportional to frequency f and turns n.
E . f n Bmax A 4 44
= × × × ×
EQUATION898 V2 EN
(Equation 122)
The relative excitation M is therefore according to equation 123 .
( )
( ) ( )
E f
M p.u. =
Ur fr
IECEQUATION2296 V1 EN
(Equation 123)
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
1MRK502052-UEN B
Section 9
Voltage protection
519
Technical manual