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Functions

6-2117SA6 Manual

C53000-G1176-C133-1

6.14.1.2 Undervoltage Protection

Undervoltage

Phase–Earth

Figure 6-108 depicts the logic diagram of the phase voltage stages. The fundamental

frequency is numerically filtered from each of the three measuring voltages so that

harmonics or transient voltage peaks are largely harmless. Two threshold stages

8SKH and 8SKHare compared with the voltages. If phase voltage falls below

a threshold it is indicated phase-segregated. Furthermore, a general pick-up indication

“8SKH3LFNXS” or “8SKH3LFNXS” is given. The drop-off to pick-up ratio is

1.05.

Each stage starts a time delay common to all phases. Expiry of the respective time

delay

78SKHor 78SKH is signalled and results in the trip command “8SKH

75,3”.

Depending on the configuration of the substations the voltage transformers are

located on the busbar side or on the outgoing feeder side. This results in a different

behaviour of the undervoltage protection when the line is deenergized. While the

voltage remains present or reappears at the busbar side after a trip command and

opening of the circuit breaker, it is switched on at the outgoing side. For the

undervoltage protection this results in a pick-up state being present if the voltage

transformers are on the outgoing side. If this pick-up must be reset, the current can be

used as an additional criterion (current supervision &855683) to achieve this result.

Undervoltage will then only be detected if, together with the undervoltage condition,

the minimum current 3ROH2SHQ&XUUHQW (address ) of the corresponding phase

is also exceeded. This condition is communicated by the central function control of the

device.

The undervoltage protection phase–earth can be blocked via the binary input

“!8SKH%/.”. The stages of the undervoltage protection are then automatically

blocked if a voltage failure is detected (“Fuse–Failure–Monitor”, also see Section

6.19.1.3) or if the trip of the mcb of the voltage transformers is indicated (internal

blocking) via the binary input “!)$,/)HHGHU97”.

Also during a single-pole automatic reclose dead time (using the internal

autoreclosure function) the stages of the undervoltage protection are automatically

blocked in the pole open state. If necessary, the current criterion will be considered,

so that they do not respond to the undervoltage of the disconnected phase when

voltage transformers are located on the outgoing side.

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