10.5
Advanced islanding detection
Due to decentralized generation, there is the possibility that a deactivated part of the grid will remain live in an uninten-
ded island due to the balance of load and generation in this part of the grid. The detection of unintended island formation
is an important function of decentralized generating units and is related to the prevention of damage to equipment as
well as safety of personnel.
Depending on the structure and the operation of the distribution grid several dangers exist:
In case of maintenance work in a distribution grid, personnel may be placed in danger if the deactivated part of the
grid remains live as an island. This is especially the case if not all safety rules are followed.
If fast auto-reclosure is used in a distribution grid and the deactivated part of the grid remains live as an island, reclo-
sure will likely happen during phase displacement which might cause damage to rotating machinery on the grid.
In the event of a fault in a medium voltage grid, the faulty part of the grid is disconnected. If the fault has a significant
resistance, the deactivated part of a medium-voltage grid remains live as an island. Depending on the type of fault, but
explicitly in case of a fault in the transformer, dangerous medium voltage might be accessible or even present in low-
voltage appliances.
Especially for the last example very fast disconnection of the generating units to cause collapse of the forming island is
necessary. At the same time any island formation detection method may cause false tripping. The industry is therefore in
constant research to develop methods that are fast and reliable and at the same time reliably prevent false tripping.
Enhanced island detection method
The enhanced island detection of KACO new energy, employs a strategy to reliably detect island formation that is based
on the characteristic differences between an interconnected grid and an islanded grid, thus ensuring reliable fast detec-
tion and prevention of false tripping.
An interconnected grid is dominated by rotating machinery, as a consequence frequency is proportional to active power
balance and voltage is proportional to reactive power balance. In contrast an islanded grid behaves like a resonant circuit,
as a consequence frequency is proportional to reactive power balance and voltage is proportional to active power balan-
ce. The active enhanced island detection method detects this difference by monitoring the behaviour of the grid. The
enhanced island detection is monitoring the natural fluctuation of the grid frequency and injects a minimal reactive po-
wer proportional to the rate of change of frequency. In the moment of formation of an island the connected power sys-
tems is closing a positive feedback loop what allows the inverter to detect the changed situation and to disconnect. In ca-
se of formation of an island, the inverter disconnects within some 100 ms, well below 1000 ms.
The number of parallel inverters does not affect the reliability of this function.
This method also ensures that the impact on the distribution grid is kept to a minimum.
In normal operation no effects on harmonic content, flicker and grid stability are detected.
This detection method is combined with a two stage passive rate of change of frequency (ROCOF) observation. If the RO-
COF of the grid exceeds the configured disconnection threshold (stage 1) for the configured disconnection time, the devi-
ce switches to zero current mode. If the ROCOF of the grid exceeds the configured shutdown threshold (stage 2) for the
configured shutdown time, the device shuts down. In case of an island, this will shut down the island instantaneously. If
the grid stabilizes, what might be the case if the ROCOF event was due to a short disturbance in the power grid, the inver-
ter will resume normal operation. In the event of active stage 1, the device switches to zero current mode, and re-starts
the infeed after a few 100ms. At stage 2, the device has shut down and the set reconnection conditions apply.
Kaco blueplanet 29.0TL3
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