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
186, 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 B max A4 44= × × × ×
EQUATION898 V2 EN-US (Equation 186)
The relative excitation M is therefore according to equation 187.
( )
( ) ( )
E f
M p.u. =
Vr fn
ANSIEQUATION2296 V1 EN-US (Equation 187)
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
188.
EQUATION1630 V1 EN-US
(Equation 188)
or equivalently, with 1.1 · Vn =
Pickup1
according to equation 189.
1MRK 502 066-UUS B Section 9
Voltage protection
639
Technical manual
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