ABB RELION 670 Series Technical Manual page 495

Generator protection

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1MRK 502 066-UUS B

The Under-Time logic always uses the pickup signal from the step 4. The Under-Time logic will

normally be set to operate for a lower current level than the SOTF function. The Under-Time logic

can also be blocked by the 2

power transformer inrush currents can occur at breaker closing. This logic is typically used to

detect asymmetry of CB poles immediately after switching of the circuit breaker. The Under-Time

logic is activated either from change in circuit breaker position or from circuit breaker close and

open command pulses. This selection is done by setting parameter

pickup from step 4 this logic will give a trip after a set delay

to a relatively short time (default 300 ms). Practically the Under-Time logic acts as circuit breaker

pole-discrepancy protection, but it is only active immediately after breaker switching. The Under-

Time logic can only be used in solidly or low impedance grounded systems.

Close command

ANSI06000643 V6 EN-US

Figure 258: Simplified logic diagram for SOTF and Under-Time features

EF4PTOC (51N/67N) Logic Diagram Simplified logic diagram for the complete EF4PTOC (51N/67N)

function is shown in figure 259:

Technical manual

harmonic restraint feature. This enables high sensitivity even if

ActivationSOTF

Close command

2nd Harmonic

ActUnderTime

ActUnderTime . In case of a

tUnderTime . This delay is normally set

ANSI06000643-6-en.vsdx

Current protection

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