4.8.3.5
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.
4.8.3.6
Signals
Table 707: DQPTUV Input signals
Name
Type
Default
Description
U_AB
SIGNAL
0
Phase-to-phase volt-
age AB
U_BC
SIGNAL
0
Phase-to-phase volt-
age BC
U_CA
SIGNAL
0
Phase-to-phase volt-
age CA
U
1
SIGNAL
0
Positive phase se-
quence voltage
I
1
SIGNAL
0
Positive sequence
current
BLOCK
BOOLEAN
0=False
Block signal for acti-
vating the blocking
mode
1MRS757644 H
Protection functions
620 series
Technical Manual
747
