Instruction manual
–
AQ G3x7 Generator protection IED
147 (211
)
3.2.24
O
VER EXCITATION
V/H
Z
(24)
The over excitation protection function is applied to protect generators and unit transformers
against high flux values causing saturation of the iron cores and consequently high
magnetizing currents.
The problem to be solved is as follows: The flux is the integrated value of the voltage:
In steady state, this integral can be high if the area under the sinusoidal voltage-time
function is large. Mathematically this means that in steady state the flux, as the integral of
the sinusoidal voltage function, can be expressed as
The peak value of the flux increases if the magnitude of the voltage increases, and/or the
flux can be high if the duration of a period increases; this means that the frequency of the
voltage decreases. That is, the flux is proportional to the peak value (or to the RMS value)
of the voltage and inversely proportional to the frequency.
Note: the overexcitation protection function is intended to be applied near the generator,
where the voltage is expected to be pure sinusoidal, without any distortion. Therefore, a
continuous integration of the voltage and a simple peak detection algorithm can be applied.
The effect of high flux values is the symmetrical saturation of the iron core of the generator or
that of the unit transformer. During saturation, the magnetizing current is high and distorted;
high current peaks can be detected. The odd harmonic components of the current are of high
magnitude and the RMS value of the current also increases. The high peak values of the
currents generate high dynamic forces, the high RMS value causes overheating. During
saturation, the flux leaves the iron core and high eddy currents are generated in the metallic
part of the generator or transformer in which normally no current flows, and which is not
designed to withstand overheating.
The frequency can deviate from the rated network frequency during start-up of the generator
or at an unwanted disconnection of the load. In this case the generator is not connected to the
