Introduction To Synchronizing PDF
Introduction To Synchronizing PDF
Introduction To Synchronizing PDF
AUTOMATIC SYNCHRONIZING
CONSIDERATIONS AND APPLICATIONS
I. INTRODUCTION
A. Definition
B. Necessity for Synchronizing
V. SUMMARY
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INTRODUCTION TO SYNCHRONIZING
AUTOMATIC SYNCHRONIZING
CONSIDERATIONS AND APPLICATIONS
INTRODUCTION
It is the intention of this presentation to provide an explanation of the automatic synchroniz-
ing process, to explore the considerations involved and to look at some synchronizing
applications for selection of the proper synchronizer.
Definition
Synchronizing, in its simplest form, is the process of electrically connecting additional
generators to an existing bus.
These additional generators will be connected to operate in parallel with each other and
supply power to the same load. The additional oncoming generators must be synchro-
nized properly to ensure:
To better understand the synchronizing process, let’s examine a typical facility with on-site
power generation shown in the example of Figure 1.
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Figure 1: Typical industrial facility with its own on-site generators
Let’s assume that this facility has a critical manufacturing process that cannot tolerate a
power failure, or the consequence would be an expensive and lengthy clean-up process.
Due to an open circuit breaker as a result of a fault on Utility Feed B, the plant manager has
decided to start the facility’s own generators and supply the majority of its own power
demands in the event that Utility Feed A would also trip and would cause their facility to
become islanded.
The loads connected to the Station Bus (the common power conductors for the facility) are
increasing beyond the capacity of Generators #1 and #2 that are already on-line, so it is
necessary to parallel another generator to provide the power for the increased load. See
Figure 2 for a more detailed electrical diagram of the generator to be synchronized.
As seen in Figure 4, synchronizing meter panels are used to provide information to opera-
tors for manual synchronization. The metering devices typically include individual bus
and generator frequency meters for matching frequency, individual bus and generator a-c
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voltmeters for matching voltage, a synchroscope, and two indicating lamps. A voltage is
provided from step-down potential transformers (in high voltage applications) for the input
signal to these devices. Note that single phase, line to line voltages from the same phases
are used. In most cases, single phase sensing for synchronizing equipment is adequate,
because the mechanical design of the generator dictates that the three phases of the
generator are displaced 120 electrical degrees apart. Before the generator is synchronized
the first time, it must be confirmed that the phase rotation (a.k.a. phase sequence) of the
generator matches the same sequence as the station bus. Matching the phase sequence
can be accomplished by the appropriate physical connections at the generator terminals
or other suitable locations.
The synchroscope is a multiple parameter information source. It tells you if there is a slip
rate (a frequency difference between generator and bus) and if the generator frequency is
running slower or faster than the bus frequency by causing the pointer to rotate in a coun-
terclockwise or clockwise direction. As seen in Figure 5, the twelve o’clock position indi-
cates 0 degrees phase angle difference. Any instantaneous position of the pointer indi-
cates the phase angle difference between the bus and generator voltage. Of course, the
object of the synchronizing process is to close the generator breaker at a 0 degree phase
angle to minimize power flow transients when the breaker is closed. Figure 6 illustrates
phase angle displacements of the voltage sine wave.
Figure 5: Synchroscope
In practice, for manual synchronization, an operator creates a very slow slip rate by adjust-
ing the prime mover speed slightly faster than the bus frequency. This allows the generator
to pick up kW load immediately rather than have the genset operate in a motoring condi-
tion when the breaker is closed. Generators typically aren’t operated in the underexcited
condition so as not to risk having the generator pull out of synchronism. Therefore, it is
preferred that an operator adjust the generator voltage slightly greater than the bus voltage
before closing the breaker, so that a small amount of reactive power will be exported from
the generator when the breaker is closed.
TYPES OF SYNCHRONIZING
For the purpose of this presentation, we will consider three methods of synchronizing: 1)
Manual, 2) Manual with permissive relay (synch check), and 3) Automatic synchronizing.
Manual Synchronizing
Manual synchronizing is widely used on a variety of machines. The basic manual synchro-
nizing system includes synchronizing lights, a synchroscope, metering, and a breaker
control switch.
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With manual synchronizing, the operator controls the speed and voltage of the oncoming
generator and closes the breaker at the proper time.
The chief advantages of manual synchronizing are system simplicity and low cost. This
method may be used with any type of generator where an operator is available to monitor
the power plant.
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Automatic Synchronizing
With automatic synchronizing, the automatic synchronizer (ANSI/IEEE Device 25A) monitors
frequency, voltage and phase angle, provides correction signals for voltage matching and
frequency matching, and provides the breaker closing output contact.
The phase lock type synchronizer operates on the principle of providing correction signals
to the governor and voltage regulator until the two waveforms are matched in phase and
magnitude and then initiating breaker closure. Until recently, this type of synchronizer was
capable of operating only with electronic governors. Today, it is also compatible with other
types of governors that require contact inputs.
Phase lock type synchronizers are intended primarily to be used one per generator.
As the prime mover brings the oncoming generator up to speed, the generated voltage is
applied to the synchronizer. When the voltage reaches a minimum threshold, the synchro-
nizer begins to sense both the oncoming generator and the existing bus for frequency,
phase angle, and voltage. Figure 11a-e shows a block diagram for a phase lock type
synchronizer.
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a. Compare Voltages b. Compare Frequency
When the synchronizer has contact output correction signals, the output contact is closed
for any phase angle greater than the front panel setting. The contact will openwhen the
hase angle is less than the front panel setting.
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Figure 12: Frequency/phase matching with bipolar output
Voltage Correction Option
Operation of the voltage matching option is similar to that of frequency/phase angle match-
ing, except that the correction signal controls the regulation set point of the voltage regula-
tor. When the synchronizer has the summing point output, the synchronizer outputs a bipo-
lar corrective signal that is proportional to the voltage difference between the generator and
the bus. If the reset jumper is installed, the synchronizer stops sending correction signals
when the generator voltage is within the front panel setting. If external reset contacts are
utilized (i.e., auxiliary contact from the breaker), the corrective signals continue until the
external reset contact closes.
When the synchronizer has contact output voltage matching, the contact remains closed
until the voltage difference between the generator and the bus is within the preset limit.
When the voltage difference is within the setting, the contact opens.
The breaker blades cannot close instantaneously; therefore, the synchronizer must have a
way to compensate for the actual breaker closing time as well as for the time spent in
moving the armature of the output relay (0.018 seconds). In order to close the breaker
blades at or close to zero degrees, the synchronizer must, therefore, initiate the breaker
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close signal in advance of the synchronism point. In other words, it must “anticipate” the
actual point of synchronism.
The anticipatory type synchronizer calculates the advanced angle that is required to com-
pensate for the breaker closure time by monitoring the slip frequency (frequency difference
between the oncoming generator and the bus) and the set in value for breaker closing. It
also factors in the constant of the armature movement (0.018 seconds) to complete the
calculation. The calculation relationship is:
= 360 (T + T ) F
A B R S
where
A
= the advance angle, which is the electrical phase angle of the generator with
respect to the system bus when the synchronizer initiates closure of the
controlled circuit breaker.
TB = the circuit breaker closing time. This is the time between the initial application of
the electrical stimulus to the closing circuitry and the actual contact of the breaker
poles. This is considered to be a constant by the automatic synchronizer.
TR = the response time of the output relay, which is approximately 0.018 seconds.
FS = the slip frequency, i.e., the difference between the oncoming generator frequency
and the system bus frequency.
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Figure 13: Slip Frequency Advance Angle Characteristic
Figure 13 illustrates the relationship among slip frequency, breaker closure time, and the
advance angle required prior to initiation of closure for a zero phase difference across the
blades at the instant of contact.
Modern synchronizers have the capability to match precisely or to control both speed and
voltage as well as to operate for very slow slip rates.
Units furnished with voltage matching and frequency or speed matching circuits will auto-
matically adjust the voltage and frequency to within limits acceptable to the synchronizer.
Both voltage matching and frequency matching corrections are through relay contacts.
Frequency Matching
The frequency of the machine relative to the bus is important, because if the machine
frequency is significantly less than the bus, the system must supply the power necessary to
accelerate the machine to synchronous speed. This power flow may result in tripping by
the reverse power relay or damage to the machine itself. On the other hand, if the machine
is rotating faster than the system, the machine attempts to supply the power required to
accelerate the system. If the frequency difference (FS) is too great, the transient power flow
is reflected into the prime mover shaft, and this may result in excessive shaft or coupling
stress.
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Figure 14: Proportional frequency matching
A significant advancement has been made regarding the problem of hypersensitive gover-
nor control on generators by utilizing proportional speed matching correction pulses.
Several factors inherent to hydro generators such as length of penstock and machine
inertia can cause a synchronizer with fixed correction pulses to repeatedly overshoot tar-
geted slip frequencies.
A proportional speed matching function, such as the F5 option in the BE1-25A, will allow
maximum correction pulse width trains at large slip frequencies. Correction pulse width is
then proportionally decreased when slip frequencies become smaller. It eliminates over-
shoots and hunting by responding instantaneously to changes in slip frequency.
In the event that the generator speed is very closely matched but the phase angle between
the generator and bus voltages is excessive, a bump pulse can gently increase the gen-
erator speed and reduce the phase angle.
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a. Change Voltage to match bus b. Change Frequency to match bus
Voltage Matching
Another consideration in the synchronizing process is the terminal voltage of the machine.
If it is not matched to the bus voltage, reactive power will flow either into or out of the sys-
tem at the instant of breaker closure. If the machine voltage is less than the bus voltage,
reactive power will be drawn by the machine from the system and excite the generator to
the voltage level of the system. Similarly, if the machine voltage is higher than the bus
voltage, reactive power will flow from the machine into the system. If this voltage difference
is too great, the reactive power flow may result in high transient stresses that could dam-
age the windings of the machine. Various voltage matching options, such as continuous
contact closure, fixed pulse and proportional pulse contact closure, are available.
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SYNCHRONIZING CONSIDERATIONS
Generator Size
For power to flow out of the machine and into the system at the time the breaker contacts
close, it is desirable for larger machines’ speed to be slightly greater than the system prior
to synchronizing. Therefore, the synchronizer must be capable of determining that the
machine frequency is greater than the system frequency (i.e., that the slip rate is positive).
However, with small machines, it may be acceptable to initiate closure of the generator
breaker while the machine is slightly slower than the system, providing that the synchro-
nizer parameters are within the preset limits and the machine is accelerating and capable
of accepting load.
For this paper’s intent, we will refer to small machines as those machines used for emer-
gency and standby operations and to large machines as those used solely for stationary
power plants.
Small Machines
The need for generator sets as standby power is crucial for the operation of many facilities.
For example, an airport facility requires several engine generator sets to maintain continu-
ity of service during emergency conditions or to supply specific load requirements during
peak demand periods. The load demands expected at an airport complex exceed the
generating capability of one generator and require additional generators to be connected
to the station bus.
Manual synchronizing could be performed by power plant operating personnel. The oper-
ating personnel would manually adjust the frequency and voltage of the generator to be
paralleled and would ultimately close the circuit breaker to tie the generator to the load
bus. This type of synchronizing scheme is quite simple and most economical. However,
the one drawback is that it requires skilled operators at the controls to avoid costly damage
to equipment due to improper synchronizing.
The addition of a supervisory relay to the manual synchronization process assists with
proper synchronization. Manual synchronization with a supervisory relay still requires the
operator to manually control voltage and frequency, but the supervisory relay sets up an
operating tolerance that must be equaled before the circuit breaker can be closed to paral-
lel the alternator.
The supervisory relay compares the slip frequency, phase angle, and voltage differences
between the oncoming generator and the station bus. These parameters and some typical
ranges are listed below. The supervisory relay does not close its output contacts until all
system parameters are satisfied.
Parameters Range
Slip Frequency 0.1 Hertz
Phase Angle 0° to 30° (adjustment)
Voltage 4 volts
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The relay’s output contacts are placed in series with the operator’s control switch. Closure of
the circuit breaker only occurs when 1) the operator manually attempts to close the circuit
breaker, and 2) the supervisory relay contacts are closed. This is illustrated in Figure 16.
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For this application, we could use the anticipatory type synchronizer discussed earlier.
However, this type of device is expensive to apply to a number of machines on a dedi-
cated basis. A sequencing circuit could be used to switch the anticipatory device from one
machine to another, but this adds time to the restoration of system power and complexity to
the overall control circuitry which might not be desirable in this application. So for this
particular job, we would use the phase lock type automatic synchronizer.
By applying the phase lock type synchronizer on a per machine basis, the need for se-
quencing logic is eliminated and each synchronizer/governor/engine combination, to-
gether with the voltage regulating equipment, can be optimized for performance and
synchronizing speed.
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Large Machines
Some typical applications where the larger generators are used include hydroelectric, gas
turbine, and steam turbine power plants. These facilities usually provide power for sale to
the utility. Typical facilities consist of multiple generators operated in parallel.
In these applications, a single automatic synchronizer can be used and shared by all
machines within the installation (See Figure 18).
In a hydro installation, the time for the generator to respond to a speed change signal
depends on several factors, including 1) the inertia of the machine, 2) the type of turbine,
3) the head, 4) length of penstock, and 5) location of the gates. These installations, there-
fore, require precise control and typically are synchronized by an anticipating device that
predicts when actual phase coincidences will occur. In installations, it is desirable that the
prime mover is accelerating so that the generator can pick up and supply the load immedi-
ately. In other words, a slip frequency is desired.
In restored hydro installations, it is conceivable that each breaker within the installation may
have a different operating time. The synchronizer must, therefore, be capable of compensa-
tion for these times. Modules are available in today’s synchronizer to provide this compen-
sation.
Because of the time and precise control requirements of the larger generating systems,
more control adjustment capability is required within the synchronizer.
In critical installations where precise speed matching is required, there are several factors
to be considered in applying an anticipatory type of synchronizer.
First, because of the precise speed matching requirement, very low slip frequencies will be
encountered. The synchronizer must be capable of measuring these small frequency
differences and calculating the required advance angle. This type of synchronizer also is
desirable from the point of view of the recommendation that the generator be running
slightly faster than the system to allow the generator to pick up load quickly.
Another part of the synchronizing problem is the precise control of the generator’s speed.
This is accomplished by supplying a correction pulse once per slip cycle. As the slip fre-
quency decreases, the interval between correction pulses increases.
Therefore, by being able to adjust the duration of the correction pulse, extremely sensitive
speed control can be achieved.
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SUMMARY
We have looked at the automatic synchronizing process and explored some of the consid-
erations involved. We have also evaluated some applications for automatic synchronizing
and have seen that there are many different factors that make up the application. Through
this process, we have tried to establish some guidelines for the selection of the proper
synchronizing system for the application.
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