9440300990 6-1
DECS-250 Synchronizer
6 • Synchronizer
DECS-250 controllers with a style number of xxxxAxx are equipped with an automatic synchronizer that
acts to align the voltage, phase angle, and frequency of the generator with the bus. The synchronizer
function includes compensation settings for the generator breaker and bias control settings for the
generator governor. Related synchronizer features include voltage matching and bus condition detection.
Generator Synchronization
BESTCOMSPlus Navigation Path: Settings Explorer, Synchronizer/Voltage Matching, Synchronizer
HMI Navigation Path: Settings, Sync/Voltage Matching, Synchronizer
Two modes of automatic generator synchronization are available: phase lock loop (PLL) and anticipatory.
In PLL mode, the DECS-250 matches the voltage, phase angle, and frequency of the generator with the
bus and then connects the generator to the bus by closing the generator breaker. In anticipatory mode,
the DECS-250 drives the generator frequency close to the bus frequency and closes the breaker at a
phase angle of zero by compensating for the breaker closing time. (Breaker closing time is the delay
between the issuance of a breaker close command and closure of the breaker contacts.) The DECS-250
compensates for the breaker closing time by monitoring the slip frequency between the generator and
bus and calculating the advance phase angle required to close the breaker at a zero-degree phase angle.
Frequency Correction
The Slip Frequency setting establishes the maximum slip allowed for breaker closure. When the
measured slip frequency is less than the value of the Min Slip Control Limit setting, the error output is set
to zero. When the measured slip frequency is between the values of the Max Slip Control Limit and Min
Slip Control Limit settings, the error output is proportional to the difference between the measured slip
frequency and the value of the Min Slip Control Limit setting with the opposite polarity. When the
measured slip frequency is greater than the value of the Max Slip Control Limit setting, the error output is
set to maximum with the opposite polarity.
To minimize the impact on the bus during synchronization, the generator frequency can be forced to
exceed the bus frequency at the moment of breaker closure. If this is the case, the DECS-250 will drive
the generator frequency higher than the bus frequency before closing the breaker. The breaker closing
angle setting defines the maximum allowable phase angle difference between the generator and bus. To
close the breaker, the slip angle must remain less than the value of this setting for the duration of the
sync activation delay.
The Min Slip Control Limit, Max Slip Control Limit, and Breaker Closing Angle settings are used only in
the PLL mode.
Voltage Correction
Voltage correction is initiated when the generator voltage is outside the defined voltage window. The
voltage window setting is expressed as a percentage of the bus voltage and determines the band of
generator voltage surrounding the bus voltage where breaker closure will be considered. Enabling the
Vgen>Vbus setting causes the DECS-250 to drive the generator voltage higher than the bus voltage prior
to synchronizing. A generator to bus PT matching level setting is provided to compensate for step-up or
step-down transformers in the system. The DECS-250 adjusts the sensed generator voltage by this
percentage. This setting also appears on the Voltage Matching screen, below. When the value is
changed, it is reflected in both places.
Angle Compensation
An angle compensation setting is provided to offset phase shift caused by transformers in the system.
The angle compensation value is added only to the bus angle. For example, it is given that the generator
and bus are synchronized but the DECS-250 metered slip angle reads –30°. Equation 6-1, below,
illustrates the DECS-250 slip angle calculation. This means that the generator angle is lagging behind the
bus angle by 30° due to transformer phase shift. To compensate for this phase shift, the angle
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