ABB RELION 670 Series Technical Manual page 436

Section 7
Impedance protection
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10 FilterLength
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2 FilterLength
IEC11000002 V1 EN-US
Figure 231: Trip time characteristic as function of fault resistance
During run-up and shut down of the generator, i.e. when the rotational speed of the generator
changes, there will occur harmonic voltages with varying frequency at the injection equipment
connection point (for example see voltage generator V
with the injected frequency this might create an error in the fault resistance estimation. Such
situations are identified in the function and the function is automatically stabilized to prevent
unwanted operation of the protection.
In connection with this calibration the reference impedance is also derived.
It is possible to have up to five different reference impedances. The need to change reference
impedance is due to different operating conditions for the generator, for instance:
1. Generator stand still
2. Generator running, not synchronized to the power network
3. Generator in normal operation, synchronized to the power network
The following automatic choice for the actual reference impedance can for example be made:
• Generator voltage < set value and generator circuit breaker open: Reference impedance 1
• Generator voltage > set value and generator circuit breaker open: Reference impedance 2
• Generator voltage > set value and generator circuit breaker closed: Reference impedance 3
The analysis of the quantities that are available in the monitoring tab of ICT are a useful support in
order to detect the need of several reference impedances. If another reference impedance is
enabled for a particular operating condition of the generator, then it is necessary to identify the
events that lead to the operating condition. A user logic can be defined in order to use the events
to change the active reference impedance. In some applications, the user logic can be simply
based on the output URMSSTAT and related setting ULimRMS.
430
Trip time
R
Trip
Fault resistance
R
Alarm
IEC11000002-1-en.vsd
in Figure 225). If such frequencies interfere
n
1MRK 502 066-UUS B
Technical manual