An interconnected grid is dominated by rotating machinery, as a consequence frequency is proportional to act-ive 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 bal-
ance. 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
power proportional to the rate of change of frequency. In the moment an island is formed, the connected power grid
closes a positive feedback loop which allows the inverter to detect the changed situation and to disconnect. In case 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 guarantees the minimisation of effects on the distribution grid.
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 of stage 1 for the configured discon-nection time, the
device switches to zero current mode. If the ROCOF of the grid exceeds the configured dis-connection threshold of stage
2 for the configured disconnection time, the device switches off. In case of an is-land, this will shut down the island in-
stantaneously. If the grid stabilizes, what might be the case if the ROCOF event was due to a short disturbance in the
power grid, the inverter will resume normal operation. With stage 1 active, the device has switched to zero current mode
and will recommence feed-in after only 100 ms. With stage 2 active, the device has switched off and the set reconnection
conditions shall apply.
Powador 30.0-72.0 TL3
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