Deif Multi-line 2 Designers Reference Handbook page 133

AGC
In the example above the synchronising gen-set is running at 1503 RPM ~ 50.1Hz. The generator
on load is running at 1500 RPM ~ 50.0Hz. This gives the synchronising gen-set a positive slip
frequency of 0.1Hz.
The intention of the synchronising is to decrease the phase angle difference between the two
rotating systems. These two systems are the three-phase system of the generator and the three-
phase system of the busbar. On the illustration above phase L1 of the busbar is always pointing at
12 o'clock, whereas phase L1 of the synchronising gen-set is pointing in different directions due to
the slip frequency.
Of course both three-phase systems are rotating, but for illustrative purposes
the vectors for the generator on load are not shown to be rotating. This is
because we are only interested in the slip frequency for calculating when to
release the synchronisation pulse.
When the generator is running with a positive slip frequency of 0.1Hz compared to the busbar, then
the two systems will be synchronised every 10 seconds.
In the illustration above the difference in the phase angle between the synchronising set and the
busbar gets smaller and will eventually be zero. Then the gen-set is synchronised to the busbar,
and the breaker will be closed.
Close signal
The unit always calculates when to close the breaker to get the most accurate synchronisation.
This means that the close breaker signal is actually issued before being synchronised (read L1
phases exactly at 12 o'clock).
The breaker close signal will be issued depending on the breaker closing time and the slip
frequency (response time of the circuit breaker is 250 ms, and the slip frequency is 0.1Hz):
The synchronisation pulse is always issued, so the closing of the breaker will
occur at the 12 o'clock position.
The length of the synchronisation pulse is the response time + 20 ms (2020 Dynamic
synchronisation
) .
Load picture after synchronising
When the incoming gen-set has closed its breaker, it will take a portion of the load depending on
the actual position of the fuel rack. Illustration 1 below indicates that at a given positive slip
frequency, the incoming gen-set will export power to the load. Illustration 2 below shows that at a
given negative slip frequency, the incoming gen-set will receive power from the original gen-set.
This phenomenon is called reverse power .
DEIF A/S
1
=
t
SYNC
−
50 1 . 50 0 .
=
deg 360 * t *
CLOSE CB
=
deg 360 * . 0 250
CLOSE
=
deg 9 deg
CLOSE
Designer's Reference Handbook
=
10 sec
f
SLIP
* 0 1 .
Page 133 of 168