Functions
2.19 Impedance Protection (ANSI 21)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
161
Figure 2-65 Time Grading for Machine Impedance Protection – Example
Z1B Overreach Zone
The Z1B overreach zone (address 3308 ZONE Z1B) is an externally controlled stage. It does not influence the
Z1 zone normal stage. Consequently there is no changeover, but the overreach zone is enabled or disabled
depending on the position of the high-voltage side circuit breaker.
The Z1B zone is usually enabled by an opened high-voltage circuit breaker. In this case every impedance pro-
tection pickup can only be due to a fault in the protection zone of the block, since the power system is discon-
nected from the block. Consequently the fast tripping zone can be extended to between 100 % and 120 % of
the protection zone without any loss of selectivity.
The Z1B zone is activated via a binary input controlled by the circuit breaker auxiliary contact (see Figure 2-
65). An individual 3309 T-Z1B time delay is allocated to the overreach zone.
Final Stage
For short circuits outside the Z1 and Z2 zones, the device functions as a time-delayed overcurrent protection.
Its nondirectional final time T END is selected so that its time value overreaches the second or third stage of
the series-connected network distance protection.
Power Swing Blocking
The power swing blocking is only effective if address 3313 POWER SWING has been set to ON.
For the distance between the power swing polygon and trip polygon (parameters: P/SPOL-TPOL (address
3314)) and rate of change (parameter: dZ/dt (address 3315)) an appropriate compromise must be found. It
must be taken into account that the rate of change is not constant. The closer to the coordinates origin, the
smaller it becomes. Furthermore the power system conditions such as impedance between swinging systems
and the swinging frequency, determine the rate of change (see also Section 2.20 Out-of-Step Protection).
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