Functions
2.35 Sensitive Rotor Earth Fault Protection with 1 to 3 Hz Square Wave Voltage Injection (ANSI 64R - 1 to 3 Hz)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
246
Measurement Method
From the control voltage U
Ctrl
, the function determines the timing for the polarity reversals and triggers the mea-
surement. At the same time it calculates the voltage amplitude and converts it to the driving voltage U
g
. The
actual fault resistance is determined from the voltage U
Meas
, which is proportional to the current I
g
. Every time
the polarity of the control voltage is reversed, the DC component of the measurement voltage is determined by
a mean value filter. The frequency of the series device must be set low enough to ensure that during the mean-
value generation the rotor-earth capacitances are charged, so that only the steady-state portion is evaluated.
This allows detection of high-resistance faults (max. approx. 80 kΩ) without being influenced by the earth ca-
pacitance.
However, the measurement is distorted by two sources of interference. One of them is a DC voltage component
in the measurement circuit which depends on the intensity of the excitation voltage and on the location of the
earth fault in the excitation winding, and the other are considerable high-frequency AC voltage peaks that may
be superimposed on the DC excitation voltage. These peaks are attenuated by a numerical filter.
To eliminate interference from the superimposed DC voltage components, the polarity of the voltage U
g
is re-
versed (square-wave voltage). The measurement voltage calculation described above is performed for both
polarities. On formation of the difference between two subsequent measurement results for I
g
, namely I
g1
and
I
g2
, the DC component originating from the excitation circuit (I
offset
) is eliminated, whereas the DC components
originating from the injected voltage U
g
accumulate.
With the measured quantity thus obtained, and the calculated amount of the displacement voltage U
g
, the earth
resistance can be calculated, taking into account the series resistors R
s
(see Figure 2-111).
Figure 2-111 Curves of the Displacement Voltage Ug, Shunt Voltage U
Meas
and the Measurement Current I
g
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