Get in Step with Synchronization

Daniel L. Ransom, PE
Senior Member IEEE
Basler Electric Company
12570 S tate Route 143
Highland, Illinois 62249 US A
danielransom@basler.com

ABSTRACT-This paper presents a review of power­ connecting together two power systems. At first it was
system synchronization. When two sources are considered too complex; it was too difficult to match the
paralleled, it is crucial to close the interconnecting circuit frequency of two ac (alternating current) power systems
breaker when both sources are in voltage, frequency, so that these systems could be paralleled to share
and phase coincidence. growing loads. During the War of Currents George
Operators can synchronize manually, or use the latest, Westinghouse and Thomas Edison became adversaries,
state-of-the-art autosynchronizers (ANSI/IEEE device with Edison promoting direct current (dc) and
25A) [1] and sync-check relays (ANSI/IEEE device 25) to Westinghouse endorsing alternating current (ac) as the
automate closing. standard for electric-power distribution.
Generator and bus synchronization share most Three-phase ac power distribution "won" the war
principles, with some important differences for each type because of the ease of converting one ac voltage level
of synchronization. For generation plants, closing to another to distribute electric power. However, the
manually or applying an automatic synchronizer problem of paralleling ac systems was more difficult with
depends a lot on the plant configuration and operating ac than with dc. In dc systems only the amplitude needs
mode. For bus-line applications, synchronizing depends to be matched. However, in ac systems the magnitude,
on power-system stiffness, motor loads, and whether a frequency, and angle of the two systems must be
wye-delta transformer is between the line and bus. matched to connect the power systems (from different
Methods for attaining proper synch-check and fast generator sources) in parallel with minimal system
synchronization are discussed. disturbance.
Early attempts at paralleling power-system sources
Index terms-synchronization, sync check, synchronizer, were aided by Nicola Tesla's work on three-phase ac
closing, protective relays, voltage, frequency, phase power systems. Rudimentary investigation showed the
possibility of paralleling sources. Soon it was apparent
that sources must match in voltage amplitude,
I. INTRODUCTION frequency, and especially in phase angle, to eliminate
disturbances in the power system upon paralleling.
Synchronization is the process of matching the
voltage, frequency, and phase angle of a source (a
generator) to an existing power system, making it III. CONSIDERATIONS FOR
possible to operate these systems in parallel. When
paralleled, the synchronized power systems can SYNCHRONIZATION
exchange power and load flows. Sources must have Certain conditions must be met to reach
(nearly) identical voltage magnitude, frequency, and synchronization, where two ac systems can be
phase-angle relationships to parallel two systems safely. connected with no harm to both systems and to the
Proper synchronization provides the following outcomes: connected loads. The oncoming source must match
• Minimum disturbance to the two paralleled these power-system quantities to those of the existing
systems system:
• Minimum shock to an oncoming generator • Phase sequence
(mechanical and electrical) • Voltage amplitude
• Equipment lasts longer; saving money • Frequency
• Rapid loading of the oncoming generator • Phase angle
provides power to loads quickly
When two segments of a grid are disconnected, these
segments cannot exchange power and load again until
the systems are brought back into synchronization. A. Phase Sequence

Phase sequence is very important for proper

II. HISTORY synchronization. The phase sequence (for example, A­
B-C or A-C-B) of the oncoming system must match the
One of the arguments against alternating current order of the phase sequence of the existing power
began during the War of Currents in the late 19th system. The process of matching the sequence is
century [2]. Early inventors were concerned about

978-1-4799-4739-3/14/$31.00 ©2014 IEEE 401 ProReJay 2014

referred to as "phasing," and the sources are said to be instantaneously; the shaded portion of Fig. 1 represents
"in phase." the fast change in the VL phasor from the instantaneous
slip frequency.
B. Amplitude, Frequency, and Phase As the oncoming generator output increases, the
Angle generator voltage phasor (VL) amplitude increases into
an acceptable voltage level dictated by the setting for
The voltage amplitude, frequency and phase angle !:lV. The generator speed control advances and retards
must be controlled each time a generator is connected to the angle, denoted bye, of the difference between VL
a power system. In practice, reaching the exact point and VB, until the angle difference falls within the
where voltage amplitude, frequency, and phase angle acceptable range. Further, a phase-angle/slip-frequency
match perfectly, and then closing an intertie circuit sync check permits or denies the close command by
breaker at this precise moment is not possible. Instead, comparing the instantaneous phase relationship of the
systems are paralleled within an acceptable tolerance two phasors.
window for mismatches of these three important An older, less-precise method employed a phase­
quantities. angle/time approach where a sync-check relay (25)
The term "window" describes the acceptable limits of issued a close to the intertie circuit breaker within the
the real-world mismatch of synchronizing quantities. If an window, but without matching phases. This method did
oncoming generator output is within the window for not take into account the dynamic, variable nature of the
voltage amplitude, frequency, and phase angle, then two slip frequency (the difference in generator speed, and
power systems can be paralleled with little disturbance. thus the difference in frequency) which is set by the
Defining this synchronization window is essential for generator controller. The phase-angle/time approach
steering (via manual methods or an automatic was valid at one set slip frequency, and intertie circuit­
synchronizer [25A]) the voltage amplitude, frequency, breaker close commands were early or late depending
and phase angle of an oncoming generator, and for on the rate of change in the slip frequency.
setting a protective relay to monitor for conditions inside
this window (25, sync check) to allow connection of the
two systems. See the synchronization window in Fig. 1. C. Live/Live Sync-Check and Dead Closing

o· The sync-check function (25) in the purest definition

9-- refers only to live-line/live-bus closing (when two sources
Phase (Slip) Angle
are active). For any other combination of dead-bus and
t..f-Slip Frequency (Hz)
dead-line closing, a switch is included for the supervisory
tN-Voltage Difference 25 sync-check relay that bypasses the sync-check
function by shorting across the sync-check relay output
Dead (Line/Bus) contacts. This bypass switch closes the sync-check
output contacts when there is a need to close the circuit
breaker for combinations of dead-bus and dead-line
conditions. The switch can be a physical switch, and it
can be logical circuitry in the sync-check relay. Often this
switch capability for dead closing is called the "voltage
monitor." Settings parameters for voltage levels control
the definitions of live and dead, bus and line

IV. SYNCHRONIZATION METHODS

Figure 1. Synchronization Window Potential transformers (pts) on the bus side and on the
oncoming source (generator) side of the paralleling
The synchronization window shows the circuit breaker connect to circuitry (for example: meters,
synchronization target, bus voltage VB (bus), on the lamps, switches, a protective relay) that indicates and
vertical axis. The oncoming generator is VL (line), a compares the voltage and phase angle relationships of
phasor sweeping clockwise. Settings determine !:lV, each source. Manual switching, assisted manual
which is the acceptable difference in voltage amplitude. switching, and automatic switching are synchronization
Settings determine the voltage levels for Dead (DB­ methods for closing the paralleling (intertie) circuit
dead bus, and DL-dead line) as well as Live (LL-live breaker at the correct moment. Each switching method
line, and LB-live bus). Settings also determine the has related equipment and indicators.
acceptable phase-angle (slip-angle) window, e, in A typical synchronization event begins with applying
degrees, and the acceptable slip frequency, /:If, in Hz. sufficient excitation to a generator to lock it into
Slip frequency is the difference between the synchronous operation. However, the generator is not
instantaneous frequencies of both power systems, ready to parallel the power bus to which it will be
measured in Hz. A usual setting range for slip frequency connected until the criteria for the synchronization
is 0.05 Hz to 0.5 Hz. The slip frequency varies window are met. The voltage amplitude and phase angle

402
of the oncoming generator need to be matched to the
Existing -....,---- A
existing source.
Source (Bus)_-+ .,....
____ B ____

Before switching occurs, operators adjust manual

controls, or a synchronizing relay steers the voltage -+----+---,.... C
magnitude and the frequency phase angle to place the
oncoming generator source within the synchronization
window. Then the intertie circuit breaker is closed in the
window and the generator is connected in parallel to the
bus. There are three methods for synchronizing the
source with the bus: manual, assisted-manual, and
automatic control.
Oncoming -....----....
. ----+-
. A
Source ( Line )'-------+- B
A. Manual Synchronization --------.... C

Manual synchronizing is performed by power-plant Fig. 2a Dark Lamp

operating personnel. These personnel control excitation Existing -.....,---- A
and speed switches to adjust the voltage and frequency Source (Bus)_-+ _----- B
____

of the generator. When the phasors are within the

synchronization window an operator closes the intertie --�--��-C
circuit breaker to connect the generator to the load bus.
This type of synchronizing scheme is simple and
economical. Synchronizing meter panels provide
information to operators for manual synchronization.
Typically, the metering devices include the following:
• Synchroscope
• Indicating lamps (see Fig. 2) ----+-----1- A
Separate bus-frequency and generator­ Oncoming -.....
•
-----+--- . B
....-
frequency meters for matching frequency Source (Line)-
----....--
... -C
• Separate bus and generator ac voltmeters for
matching voltage Fig. 2b Two-Bright, One-Dark Lamp
The synchroscope displays multiple parameters. It
shows the slip rate (or slip-frequency rate), revealing Frequency meters and voltmeters provide a numeric
whether the generator frequency is running slower or representation of the state of synchronization. When the
faster than the bus frequency. A dial pointer rotates indicators are connected correctly and are within an
depending upon the frequency mismatch. The acceptable range, then the sources are synchronized.
instantaneous position of the pointer indicates the In practice, for manual synchronization, an operator
phase-angle difference between the bus and generator creates a very slow positive-slip rate by adjusting the
voltages. The twelve o'clock position indicates a-degrees generator speed slightly faster than the bus frequency.
phase-angle coincidence. The goal of synchronizing is to This positive rate causes the generator to pick up kW
close the generatorlintertie circuit breaker at a a-degree load immediately rather than have the generator
phase angle to minimize power-flow transients and operating in a motoring condition when the intertie circuit
generator damage when the breaker is closed. breaker is closed. Typically, generators are not operated
A basic scheme for synchronizing consists of in the underexcited condition (to avoid having the
incandescent lamps connected to the same phases on generator consume valuable system VARs, and to
either side of the generator breaker as shown in Figure prevent pulling out of synchronism). Therefore, an
2a and 2b.[3)If both the generator and bus voltages are operator adjusts the generator voltage slightly greater
"in phase" there is a volts potential difference; the lamps than the bus voltage, so that a small amount of reactive
will not be illuminated. This method is known as "dark­ power is exported from the generator when the circuit
lamp" synchronizing. breaker is closed.
Another method uses illuminated lamps (called
"bright-Iamp") along with a dark lamp. By making slight
adjustments in the speed of an oncoming generator, the B. Assisted-Manual Synchronization
frequency can be equalized so that the synchronizing
lamps will light and go out at the slowest possible rate. Adding a supervisory relay, known as a sync-check
When two lamps are bright and one light is out, then relay (25), to the manual synchronization process assists
synchronization is achieved. with proper synchronization. Manual synchronization
Although simple, these are a reliable method of with a supervisory relay still requires the operator to
synchronization verification. For both methods be sure to control generator voltage and frequency. The
have a lamp-test switch to confirm that the lamps are supervisory relay enforces a synchronization window for
working. safe conditions that must be in place before the circuit
breaker can be closed to parallel the oncoming

403
generator source. The supervisory sync-check relay (25) switching, one of the generators can pick up the dead
compares the voltage difference, slip frequency, and bus and start the synchronizing process for the
phase-angle (slip) differences between the oncoming remaining generators. An anticipatory synchronizer is
generator and the station bus. These parameters and best for this application (see subsection below).
some typical ranges are listed in Table 1. (Caution: As the prime mover brings the oncoming generator to
some relays use an actual-difference-voltage setting, speed, the generator voltage is applied to the
and some relays use a percentage-voltage-difference synchronizer. Fig. 4 shows a typical synchronizer block
setting.) The supervisory 25 relay does not allow a circuit diagram. When the generator input voltage reaches a
breaker close until all of these parameters are satisfied. minimum threshold, the synchronizer begins to sense
both the oncoming generator and the existing bus for
Table 1. Typical Sync-Check Parameters voltage, frequency and phase angle. The order of
Parameter Typical Value operation is the following:
• Compare voltages
Voltage Difference 4-8 V (secondary) • Compare frequency
Phase (Slip) Angle 0°-30° • Change voltage to match bus
• Change frequency to match bus
Slip Frequency 0.10 Hz • Compare phase angle
• Issue a close command to the intertie circuit
The sync-check relay output contacts are in series breaker (52)
with the operator control switch. Circuit breaker closing
occurs only when the operator manually closes the
Main Bus
switch, and the supervisory relay contacts are closed.
Fig. 3 shows the timing of the operator-commanded
close and the sync-check-relay close as the voltage Bus
phasor approaches 0 degrees. Sensing
o·
25A
Synchronizer

t----I 52a/b
52
)----1 Out
Close

Phasor
Rotation

Fig. 3 Assisted-Manual Synchronization with Sync

Check

c. Automatic Synchronization
Frequency
Prime-Mover Correction
Manual and assisted-manual paralleling schemes
Governor
have a shortcoming. These methods require skilled
operators at the controls to adjust voltage and frequency
to avoid costly damage to equipment caused by
Fig. 4 Synchronizer Connection Diagram
improper synchronization. With automatic
synchronization, the automatic synchronizer (25A)
monitors voltage, frequency, and phase angle. The When first applied, the synchronizer senses a large
synchronizer outputs correction signals to a generator difference between the sources for voltage and
governor to achieve voltage matching and frequency frequency. The synchronizer begins to output corrective
matching, and provides the circuit-breaker-close output amplitude voltage signals and corrective frequency
contact. signals to the oncoming generator to match it with the
Because of the importance of restoring electrical bus. The process occurs recursively until the oncoming
power following an emergency outage, a dedicated generator is synchronized with the bus and the
automatic synchronizer should be used for each synchronizer commands the intertie circuit breaker to
generator, allowing the generators to parallel to each close.
other and to the main bus as quickly as possible. If the The circuit breaker cannot close instantaneously. To
automatic synchronizing equipment includes dead-bus achieve circuit-breaker closure exactly at zero degrees,
the synchronizer must initiate the breaker-close signal in

404
advance. The circuit-breaker blades close at minimal
phase difference. [3] V. THE AUTOMATIC SYNCHRONIZING
The autosynchronizer gives the close command while PROCESS
the slip-frequency rate is moving slowly, approaching the
zero-degree phase angle. The synchronizer calculates In the automatic synchronizing process the generator
the advance angle, issuing the close command early to starts, and the synchronizer starts as the generator
compensate for circuit-breaker-closing time. This comes up to speed. As the generator accelerates and
capability minimizes system transients by closing at zero approaches the system frequency, the synchronizer
degrees (at "midnight" on a clock face). commands governor controls for raising and lowering
The anticipatory autosynchronizer compensates for voltage and frequency.
the actual breaker closing time plus the output-relay The raise and lower outputs can be set to continuous
contact-travel time (the time for moving the armature of mode or proportional mode. Continuous mode turns the
the physical output relay [6 to 8 ms]). The synchronizer required logic output on until it is in the synchronization
"anticipates" the actual point of synchronism. window or overshoots it. Proportional mode toggles the
The anticipatory synchronizer calculates the advanced voltage and frequency outputs based upon the
angle (AA) that is required to compensate for the circuit­ calculated error, and on the respective pulse-width and
breaker closing time by monitoring the slip-frequency pulse-interval settings. [4]
rate and the preset slip-rate value for breaker closing. It The voltage monitoring portion of the automatic
also adds the constant of the physical relay contact synchronizer adjusts the voltage regulator to bring the
movement time to complete the calculation. The generator terminal voltage within the synchronization
mathematical relationship is the following: window. Next, the phase-monitoring circuitry calculates
AA=360 (TcB+TR) Fs (1) the advance angle required to close the circuit breaker at
Where: a zero-degree phase difference based on the actual slip
AA is the advance angle, which is the electrical phase frequency and on the (preset) circuit-breaker-closing
angle of the generator with respect to the system time.
bus when the synchronizer initiates a close Fig. 5 illustrates the relationship among slip frequency,
command circuit-breaker-closing time, and the advance angle
T CB is the circuit-breaker close time. This is the time required prior to closing the circuit breaker at zero
between the initial application of the close degrees.
command and the actual contact of the circuit­
breaker poles
TR is the response time of the output relay (6-8 ms) ADVANCE
Fs is the slip frequency ANGLE

30·1---+---+-��;.L-, ""--I�=-----4
Reducing the advance angle AA also reduces the
absolute value of the slip frequency FS (which is the
maximum permissible speed difference for which a
generator is allowed to close onto the bus). Lesser slip 20·1---+-----,l.��, �t---+---�
frequencies produce less system disturbance and
machine damage (known as "softer").
Larger slip frequencies allow synchronization to be 10·1--�¥-.,.c.-+_-
accomplished faster, but there is more system
disturbance ("harder"). These considerations should be
weighed:
• How fast must the generator be on line? 0.1 0.2 0.3 0.4 0.5
• How critical is the generator? CIRCUIT BREAKER CLOSING TIME (SECONDS)
• How expensive is the repairing/replacing the
Fig. 5 Sync-Check Closing Quantities
generator versus the cost of possible outage
(down) times?
A proper synchronizer application accounts for these A sync-check relay (25) should be used with the
considerations, as well as others that are unique to the automatic synchronizer (25A) to safeguard the generator
system. circuit breaker from closing out of phase; see Fig. 5. The
sync-relay does not allow closing if there is a problem
with the synchronization point provided by the automatic
synchronizer. [5]

405
 DC BUS + auxiliary transformer changes phase connection and
s= Interlocks
voltage level. With some sync-check relays this might be
necessary when the pt inputs are separated by a
wye/delta transformer, as shown at PSEC. Again, a
Enable 43M i i Enable 43A

$
modern, numeric relay can offer phase and voltage
matching for this situation.
Manual Close 25A Close
T T Today, many generators are impedance grounded to
- - - - ---'
limit damage caused by ground faults. Connecting the
�
L..----t- 25Sync Check pts from phase to ground gives an unreliable voltage
level to the sync-check relay because the neutral is
offset by the grounding impedance. Phase-to-phase pts
DC BUS- should be used for sync-check inputs instead of phase­
to-ground signals.[61
A fast sync-check function makes possible critical
Fig. 5 Supervise Autosynchronizer (25A) with Sync
applications such as motor-bus transfers. Modern,
Check (25)
numeric relays have response time less than one power­
system cycle. [71
VI. ApPLICATIONS
Applications for a sync-check relay (25) and an B. Synchronizer Applications
automatic synchronizer (25A) are straightforward.
However, experience in the field shows that these Applying an autosynchronizer depends on the plant
applications have circumstances that should be and on the operation mode. A standby system requires
considered. that the generators are at speed and are closed online
quickly. Some applications will tolerate a hard sync in
exchange for a fast close (little down time). However, a
A. Sync-Check Applications prime power plant with natural gas generators and/or
heat recovery will require a longer time to come online.
The basic sync-check (25) application is shown in In the past an anticipatory automatic synchronizer was
Figure 6. A potential transformer (pt) P on the system expensive to apply to a number of machines on a
bus provides bus data to the sync relay. The pt at X dedicated, one-to-one basis. A sequencing circuit was
provides the line-voltage data on the generator side of used to switch the anticipatory synchronizer from one
the circuit breaker. The sync-check relay (25) monitors generator to another. Sequencing a synchronizer adds
these potentials when comparing sources for time to system restoration, as well as complexity to the
synchronism check. overall control circuitry.
Main Bus Today, a dedicated autosynchronizer (25A) is not
expensive. By applying a modern protective relay with a
built-in synchronizer on a per-machine basis the need for
sequencing logic is eliminated. Each
synchronizer/governor/generator combination (together
with the voltage-regulating equipment) can be optimized
for performance and synchronizing speed.
25
Transformer
Sync
(if needed) C. Breaker-Failure Protection
Check
A flashover can occur when a source is synchronizing
to an existing power system. As the source voltage on
one side of the circuit breaker slips past the bus voltage
Fig. 6 Typical Sync-Check Connections on the other side of the circuit breaker, there are points
where the voltage is 180 degrees out of phase. It is at
this point that the voltage difference across the circuit
It is important to have the same pt connections, or use breaker is twice nominal. A destructive flashover can
a method to adjust the relay sync-check inputs. If there strike if the circuit breaker is not rated suffiCiently, or is
is a phase-to-phase connected pt at P, then the pt at X compromised (there might be a mechanical failure such
should be a phase-to-phase connection as well. Some as an insulating gas leak).
modern, numeric (digital) relays offer phase matching as A flashover can damage a generator and the
well as voltage matching. Another benefit of modern connected generator step-up (GSU) transformer.[81 This
relays is the enhanced isolation between the sync-check situation calls for breaker-failure protection (50BF) to trip
input circuits. the surrounding breakers to remove the flashover.
If phase matching and voltage matching are not (Because it is arcing, there is no reason to issue a trip to
available in the sync-check relay, then an interposing the already open generator circuit breaker.)

406
 [5] Hartmann, G.; "Automatic Synchronizing for
Generation and Tie Lines," Western Protective
VII. CONCLUSIONS Relay Conference (WPRC), Oct. 1991
[6] Thompson, M.J., "Fundamentals and advancements
Power-system operation is improved by applying a
in generator synchronizing systems," Protective
thorough understanding of synchronized closing and
. Relay Engineers, 2012 65th Annual Conference
synchron!zat!on. Proper application of the many
TAMU, vol., no., pp.203,214, 2-5 April 2012
synchronization methods helps to prevent expensive
doi: 10.1109/CPRE.2012.6201234
outages and equipment damage.
[7] Ransom, D.L.; Chelmecki, C., "Using GOOSE
M�dern sync-check relays (25) match voltage
messages in a main-tie-main scheme," Industry
amplitude, frequency, and instantaneous phase angle.
Applications Society Annual Meeting (lAS), 2012
Fast sync check makes possible critical main-tie-main
IEEE , vol., no., pp.1,8, 7-11 Oct. 2012
applications (such as fast motor bus transfer).
doi: 10.1109/IAS.2012.6374119
Oncoming generators support power-system loads
[8] Wieck, H.; Gutman, I.; Ohnstad, T., "Investigation of
faster and with less connection disturbance when
Flashover Performance of Snow-Covered
pr�perly synchronized. These results are achieved by
Breakers," Dielectrics and Electrical Insulation, IEEE
uSing an anticipatory, automatic synchronizer (25A). This
Transactions on, vo1.14, no.6, pp.1339,1346,
synchronizer issues the close command in advance of
December 2007
the precise synchronization point, adjusting for the
doi: 10.1109ITDEI.2007.4401216
circuit-breaker delay, for contact-closure delay ' and for
varying slip frequency.
Use phase-to-phase pt connections on high­ IX. VITA
impedance-grounded power systems.
Contingencies should be considered for a failed sync­ Daniel (Dan) Ransom, P.E., has 40 years of industrial
close attempt. Apply circuit-breaker-failure protection and utility electronics experience, including many years
(50BF) when needed to prevent flashover. In motor-protection development and application
support. He has been a consulting engineer for power­
system protection and for communications systems. Dan
VIII. REFERENCES IS an electrical engineering graduate (BSEE) of Gonzaga
University, Spokane, Washington, USA; he also holds a
liberal arts degree from Washington State University in
[1] G. ANSI/IEEE C37.90, Standard for Relays and
Pullman, WA. He is a Senior Member of the IEEE ' with
Relay Systems Associated with Electric Power
membership in the lAS (Industry Applications), PES
Apparatus, IEEE, 2005
(Power Engineering), Communications societies and he
[2] Jonnes, J., "Empires of Light," Random House, 2003,
is a voting member of the SA (Standards Ass�ciation).
ISBN-10: 0375507396
To date, he has one US patent. He is a Professional
[3] Horak ' J., "Introduction to Synchronizing," Basler
. Electrical Engineer in numerous USA states. Dan joined
Electnc
Bas�er Electri� in 2010 and is Principal Application
Technical Resource Library, 2005
Engineer. He IS the application engineer for the West
[4] Beckwith, T.; "Automatic Synchronizing
. region of the United States and Canada.
ConSiderations and Methods," Western Protective
Relay Conference (WPRC), Oct. 1985

407