Kohler EKOZD Series Service page 208

In addition, the controller possesses underfrequency
unload (Volts/Hz) so the bus voltage may be
significantly lower than rated voltage if the load on the
bus exceeds the generating capacity of the online
generators and the bus frequency has drooped (which is
a very important time for an additional generator to be
able to synchronize and connect to the bus), hence
matching frequency with the bus will cause the
synchronizing generator to follow the same volts/Hz
curve, resulting in a much closer initial voltage match.
The Frequency Match Control is only initiated if the
generator frequency differs by more than the Frequency
Window from the bus frequency (typically only occurs on
startup and when the bus is overloaded). The
Frequency Match Control adjusts the speed bias to
control generator frequency to within 10% of the
frequency window of the bus frequency before
Frequency Match Control is disabled and Phase and
Voltage Control are enabled. Phase Match Control and
Frequency Match Control are not active simultaneously.
Note: The Speed Bias has an effective range of 5% of
nominal speed. The controller can't synchronize
a generator to the paralleling bus if the frequency
is below 95% of nominal (57 Hz on a 60 Hz
system, 47.5 Hz on a 50 Hz system).
Voltage Match: The Voltage Match Control adjusts the
target voltage of the voltage regulator on the generator
to match the measured RMS voltage of the bus using the
voltage bias. The Voltage Matching Control is not
typically active in a standby system unless the bus is
heavily loaded or one or more of the generators in the
system has a different voltage regulator target, does not
have the trims enabled, or is not calibrated correctly.
Voltage matching is more common when synchronizing
to a utility source that may have up to 10% variation in
the voltage amplitude depending on load on the system.
Phase Match: The Phase Match Control adjusts the
target speed of the engine in the engine speed governor
using the speed bias. The engine speed is increased if
the generator voltage lags the bus voltage, decreased if
the generator voltage leads the bus voltage. This
technique is call bi-directional synchronization.
The Phase Match Control is active nearly every time that
a generator synchronizes to the paralleling bus, so it is
very important that it works correctly. In addition, the
generator may stabilize speed with the phase difference
between the output of the generator and the paralleling
bus within the entire possible range of phase angles
(-180_ to 180_), so the phase matching sequence may
be different with each start of the generator.
208 Section 12 Paralleling System Information
In Synchronism: After the voltage amplitude,
frequency, and phase of the generator matches that of
the bus, the controller considers the generator to be 'In
Synchronism' with the paralleling bus.
When in Synchronism, the controller considers it to be
safe to close the paralleling breaker. The Trip signal to
the generator circuit breaker is removed when the
breaker is ok to close, this allows the trip coil time to clear
so that the breaker will be ready to close when it is
signaled to do so.
Dwell Timer: The Dwell Timer becomes active as soon
as the generator is in synchronism with the paralleling
bus. It elapses time until it expires unless the generator
falls out of synchronism with the paralleling bus, when it
is reset.
Breaker Close Signal: When the Dwell Timer expires,
the controller will signal the paralleling circuit breaker to
close. The Contactor output from the controller is also
energized at the time that the controller signals the
breaker to close (some applications may use the
contactor output for breaker status annunciation).
The Breaker Close Signal is maintained until the
breaker status indicates closed, the close command is
removed, or the failure to close time delay expires.
Fault Handling: If the breaker status does not indicate
closed within the Failure to Close time delay, the
controller will issue a close attempt fault. After the Close
Retry Delay expires, the controller will try to close the
breaker again until the maximum number of close
attempts is reached, than the controller will issue the
Failure to Close Fault and start the CB Trip to Shutdown
delay.
The generator will continue to run until the CB trip to
Shutdown time delay expires, at which time it will stop.
The circuit breaker fault can be reset by sending a
remote reset command through Modbus or SiteTecht,
by stopping the controller and pressing OFF a second
time, or by pressing the AUTO button while the controller
is already in 'Auto' mode.
TP-6953 7/19