ABB REG670 - Page 319

• The influence from the impedance Z
mT
will be different when the generator is
stand-still and when it is in operation
• The capacitance to earth will vary if the generator breaker is open or closed
• The capacitance will vary if the generator is energized or not depending on stator
end winding corona protection
• Non-linearity of used injection transformer, different properties at low and high
total voltage and temperature changes
• Impedance to ground is affected by auxiliary loads connected between generator
and unit transformer. If these loads vary, the estimated earth fault resistance will
be affected.
The difference from the first reference impedance is identified by monitoring
impedance (RAVE + jXRAVE) during different operation modes. This impedance is
available as a service value from the stator function both on built-in HMI and in ICT
tool. If the difference is significant more than one reference impedance is required. In
the ICT up to five different sets of reference impedances can be derived during
commissioning for different states of operation and downloaded to the protection
function as different values of the reference impedance; RefRn and RefXn where n =
1, 2, and up to 5. Switching of reference impedance can be made automatically.
During commissioning ICT also makes “cross-calculations” between acquired
references, giving basically the calculated fault resistance between each existing
reference combination. This would be the fault resistance measured by the function
when the reference change occurs from one reference to another if the real generator
impedance stays the same, in other words a worst case scenario.
RMS voltage (rmsVolt) value at the injection point can be used for detecting when a
reference needs to be changed and logical outputs can be set to reflect whether the
RMS voltage is higher or lower than a prescribed value. There is one such output for
the voltage signal and one for the current signal. It is advantageous to use RMS to
determine a change of machine condition because the RMS makes a distinction
between the measured values and the total amplitude of the signal. The standstill
condition only contains the injected frequency, while the full load condition and full
speed condition contains other frequencies, which amplitudes may change under
varying machine conditions.
In case of a stator earth fault with fault impedance Z
f
the measured admittance will be:
1 1 1
ref f
Z Z Z
= +
EQUATION2513 V1 EN
and this gives:
1 1 1
reff
Z Z Z
= -
EQUATION2514 V1 EN
1MRK502052-UEN B Section 7
Impedance protection
313
Technical manual