24 Overexcitation Protection
Overview of Function
The
Overexcitation Protection
function (24) is used to detect high flux values in the transformer.
Excess flux can cause transformer core saturation and some of the flux will radiate as leakage flux through the
transformer tank. This leakage flux causes eddy currents. The I
2
R losses from these currents heat the trans-
former tank and can cause overheating.
The flux level is recorded indirectly by measuring voltage and frequency (V/f). Overvoltage leads to excessive
magnetizing currents, while under-frequency leads to higher losses when resetting the magnetization.
If the power system is disconnected and the voltage and frequency control functions do not react quickly to
the power unbalance introduced, there is a risk of over-fluxing.
Structure of the Function
The
Overexcitation Protection
function has group dependent settings. These settings can be different
in each settings group – group numbers (Gn) 1 to 4.
This function corresponds to the logical nodes
A24DTPVPH*
and
A24ITPVPH*
in IEC 61850.
The function requires VT inputs, it monitors the primary system using the 3 phase voltage inputs e.g. VT1/2/3.
The relay has selectable V/f protection operate characteristics:
•
User Definable Inverse curve 24IT
•
Independent Definite Time Lag (DTL) 24DT
24DT Method of Measurement
The stage comprises a threshold and a definite time-delay stage. If the V/f value is greater than the set
threshold value then the stage is tripped after an adjustable time delay has elapsed. The dropout ratio for the
threshold is fixed to 98 %.
The definite-time stage can be applied when fast tripping is required in the presence of high V/f values. In this
case, the stage acts superimposed on the thermal, dependent stage.
24IT V/f Method of Measurement
The input voltage and frequency values are the continuously measured. Both values form the V/f ratio. In order
to arrive at an absolute value, standardized data is applied.
Thus, the ratio is derived from:
[fo_function24methodofmeasurement, 1, en_US]
Where:
V = Measured voltage (maximum phase-to-phase voltage)
V
rated, obj.
= Adjusted rated voltage of the protected object
f = Measured frequency
f
rated
= Rated frequency
5.2
5.2.1
5.2.2
Protection and Automation Functions
5.2 24 Overexcitation Protection
150
Reyrolle 7SR5, Transformer Protection Device, Device Manual
C53000-G7040-C015-1, Edition 11.2019