Application - ABB RELION REX640 Technical Manual

1MRS759142 C
4.8.3.6
REX640
Technical Manual
The Blocking mode setting has three blocking methods. In the "Freeze timers" mode,
the operation timer is frozen to the prevailing value. In the "Block all" mode, the whole
function is blocked and the timers are reset. In the "Block OPERATE output" mode,
the function operates normally but the OPERATE output is not activated.

Application

Use of distributed power generating units (PGU) is rapidly increasing due to
liberalized markets (deregulation) and the global trend to use more renewable sources
of energy. As the capacity of these generating units increase, they are connected
directly to medium voltage networks. Until recent years it had been a practice by grid
operators to disconnect the distributed power generator from the network in case of
fault in the network.
If there is a considerable loss in the power generation, it may affect the system's ability
to recover. To ensure power system stability, various grid codes have revised their
requirements and therefore require that the distributed PGUs have to make a
contribution to network support. In case of network faults, the distributed power
generator should not be immediately disconnected from the network. Instead, as a
matter of principle, generating plants connected to the medium-voltage network must
be capable of participating in steady-state voltage control and dynamic network
support. However, if the generators stay connected, it must be ensured that they do not
take reactive power from the network because this may lead to collapse of the grid.
DQPTUV is used for detecting such situations, that is, simultaneous undervoltage and
reactive power (under excited generators) and trip the generators.
The protection function DQPTUV is developed considering various grid codes. For
example, in the BDEW Technical Guideline "Generating Plants Connected to the
Medium-Voltage Network" (June 2008 issue, Germany), it is stated that if all three
phase-to-phase voltages at the grid connection point decrease and remain at and below
a value of 85% of the rated and if reactive power is simultaneously consumed at the
grid connection point (under-excited operation), the generating facility must be
disconnected from the network with a time delay of 0.5 s.
Section 4
Protection functions
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