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2 Functions
2.14
Impedance Protection (ANSI 21)
Machine impedance protection is used as a selective time graded protection to
provide shortest possible tripping times for short-circuits in the synchronous machine,
on the terminal leads as well as in the unit transformer. It thus also provides backup
protection functions to the main protection of a power plant or protection equipment
connected in series like generator, transformer differential and system protection de-
vices.
2.14.1 Functional Description
Pickup
General
Fault detection is required to detect a faulty condition in the power system and to ini-
tiate all the necessary procedures for selective clarification of the fault:
• Start the time delays for the final stage t3,
• Determination of the faulty measuring loop
• Enabling of impedance calculation,
• Enabling of tripping command,
• Indication/output of the faulty conductor(s).
Pickup is implemented as overcurrent pickup and can be optionally supplemented by
an undervoltage seal-in circuit. After numeric filtering, the currents are monitored for
over-shooting of a set value. A signal is output for each phase where the set threshold
has been exceeded. These pickup signals are considered for choosing the measured
values. The pickup is reset when 95% of the pick-up threshold is undershot, unless
maintained by the undervoltage seal-in feature.
Undervoltage Seal-In
With excitation systems powered from the network, excitation voltage can drop during
Feature
a local short circuit, resulting in decreasing short-circuit current which, in spite of the
remaining fault, can undershoot the pickup value. In such cases pick-up is maintained
for a settable time period by means of the undervoltage controlled seal-in circuit using
the positive sequence voltage U1. Pickup drops off when this holding time has expired
or when the restored voltage reaches 105% of the set undervoltage seal-in value.
The seal-in logic operates separate for each phase. The first pickup starts the timer T-
SEAL-IN.
Figure 2-27 shows the logic diagram of the pickup stage of the impedance protection.
Determination of the Short–Circuit Impedance
For calculating impedance only the currents and voltages of the faulty (shorted) phase
loop are decisive. Accordingly the protection, controlled by the pickup, evaluates these
measurement values (see also Table 2-5).
92
7UM61 Manual
C53000-G1176-C127-3
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