3 CHAPTER 3

SYNCHRONIZATION

3.1 Introduction to Synchronization

Today’s power grid is a huge and complex electrical network as human requires

big demand of electrical energy for different purpose. So, to fulfill huge power

requirement, power grids are forms. In power grid different types of generating sources

and load centers are connected to balance the power need at various locations.

Synchronizing a generator or alternator to the ac network is very important task and

should be done carefully. To connect additional generator to ac grid network, voltage

magnitude and frequency of machine have to match with respective grid and phase

angle of both generator and grid should be matched while connecting. In Electrical

power system synchronizing failure may introduces:

i. Synchronization failure may affect system stability by producing voltage

deviation, transients and unnecessary oscillations within network which

disturbs power system stability and reduces overall efficiency of system.

ii. Mechanical strains due to sudden speeding up and speeding down, may harm

respective generator and the prime mover.

iii. High currents flow through system which can damage the windings of power

transformer and respective generator permanently.

iv. Generator may get disconnected from taking load and system gets affected by

unbalanced parameters.

v. Due to increasing abnormalities within system total black out may occurs which

shut down all power system.

So, it is very important that all generators connected to power grid should proper

synchronized with grid parameters.

Traditionally, generator control systems include a synchronizing panel. The

synchronizing panel includes indications of voltage, angle, and slip that show what

adjustments the operator needs to make to the governor and exciter and when it is

acceptable for the operator to close the breaker. In many cases, the process is automated

26

using an automatic synchronizer with manual control available as a backup. In

generating facilities with more than a single generator or installations with multiple

synchronizing breakers, complicated synchronizing circuits with many contacts are

required to switch the VT and control signals between the operator and automatic

controls and the high-voltage equipment. Maintaining proper isolation and safety

grounding of sensing and control circuits often requires the use of problem-prone

auxiliary relays and VTs, which can reduce the reliability of the system.

Nowadays, protective-relay-grade microprocessor devices can significantly

improve manual and automatic synchronizing systems. This technology can simplify

synchronizing circuits to reduce cost, improve reliability, and easily accomplish

complete integration, automation, and remote control of the system. For example,

complete galvanic isolation between circuits is possible. Signals can be wired to

isolated VT and I/O terminals on the microprocessor-based devices and selected in

logic. Remote I/O modules with fiber-optic links can allow an automatic synchronizer

to be located near remote breakers eliminating long VT circuit runs. Enhanced

visualization of the synchronizing process using a computer-based soft synchroscope

that provides the operator with better information can make it easier to correctly

synchronize the unit every time.

3.2 Consequences of Faulty Synchronization

3.2.1 Potential Damage to Generator and Prime Mover

A synchronous generator is an electrical-mechanical system. Three-phase

voltages of the power system, when applied to the stator windings of a generator, create

a rotating magnetic field that rotates at synchronous speed. Synchronous speed is

determined by the number of poles wound on the stator. The rotor has a fixed magnetic

field that creates a rotating magnetic field when the rotor is turning, and a field is applied.

When the stator is connected to the power system, the rotor and stator are linked by the

rotating magnetic field, and the rotor must turn at synchronous speed. The rotor is also

connected to a prime mover by a mechanical shaft that supplies the mechanical energy

for conversion to electrical energy. The prime mover is typically a turbine (steam,

combustion, hydraulic, etc.) or reciprocating engine (diesel, natural gas, etc.). When the

generator is connected to the power system, the electrical and mechanical systems are

tied together.

27

Prior to closing the generator breaker during synchronizing, the angular velocity

of the rotating magnetic field and therefore the frequency of the voltage induced in the

stator is governed by the rotor speed. Once the generator breaker is closed, the angular

velocity of the rotating magnetic field is governed by the power system frequency. The

rotor and prime mover rotating masses have to change speed and position nearly

immediately to match the power system. If the speed and position of the rotor are

closely matched at the instant the generator is connected to the power system, the

transient torque required to bring the rotor and prime mover into synchronism is

acceptable. If the position, as measured by the angular difference between the incoming

and running voltages, is close but the angular velocity (frequency) is significantly off,

as measured by the slip between the incoming and running voltages, there will be a

large transient torque on the mechanical systems to accelerate or decelerate the rotating

masses to match the power system angular velocity. If the rotor position is also off

(voltage phase angle difference is large), there can be an even higher transient torque

required to snap the rotor and prime mover position into phase with the power system.

This transient torques can cause instantaneous and/or cumulative fatigue damage to the

generator and prime mover over the life of the system.

IEEE Standards C50.12 and C50.13 provide specifications for the construction

of cylindrical-rotor and salient-pole synchronous generators, respectively. They specify,

“Generators shall be designed to be fit for service without inspection or repair after

synchronizing that is within the limits listed…”. The limits for both types of generators

are:

i. Angle ±10 degrees.

ii. Voltage 0 to +5 percent.

iii. Slip ±0.067 Hz.

IEEE Standard 67, which also discusses synchronizing requirements,

specifically says that the standard does not apply to the prime mover. The author is

unaware of any standards that apply to the design of the prime mover, so the generator

limits are typically the ones used to design a synchronizing system. However, in

practice, wider limits are often applied because the prime mover control is not fine

enough to reliably achieve a 0.067 Hz slip.

Note that the generator standards allow ± slip. However, from the mechanical

perspective, it is desirable to limit synchronization from zero to positive slip to reduce

shock in the mechanical system because of drive-train lash. There can be clearances in

28

the mechanical drive train that cause a small amount of free play between forward and

reverse torques. When the prime mover is driving the generator prior to synchronization,

the entire drive-train lash is made up in the forward direction. If the generator is running

slightly faster than the system, the generator and prime mover will decelerate, and the

lash is made up in the correct direction. If the generator is running slower than the

system, it will have to accelerate, and the drive-train lash will now have to shift to the

opposite direction. A similar issue occurs if the moment of breaker closure is late

(synchronizing angle is more than 0 degrees). The free play in the drive train allows the

generator to move quickly until the lash is made up, which will cause a transient torque

in the mechanical system.

Finally, the instantaneous current associated with a severely faulty

synchronization can exceed the three-phase bolted fault duty that the generator and

transformer must be designed to withstand. Large forces in the generator and

transformer windings caused by the current surge can damage the windings and

associated blocking, leading to catastrophic failure or reduced life.

3.2.2 Problems in synchronization

Along with the transient torques to the mechanical system, there will be

electrical power oscillations. If the generator is synchronizing to a weak system, these

oscillations can be relatively large.

On the other hand, the generator constitutes a large dynamic source/sink for

reactive power. If the exciter is in manual mode during synchronization and the

generator voltage is lower than the system voltage, it can cause a voltage dip to the local

power system if the connected system cannot supply the VARs to hold the voltage up

until the exciter control mode can be switched to automatic. The situation can actually

be worse if the exciter is in voltage regulation mode during synchronization. As soon

as the unit is synchronized to the system, the voltage regulator could immediately back

off excitation to try to bring the voltage down to its set point, resulting in an extreme

underexcited condition. The weak magnetic field can result in the machine not pulling

into synchronism or pulling back out of synchronism shortly after synchronization.

3.2.3 Generator Protection Issues

The problems associated with faulty synchronization discussed in the previous

two sections can manifest themselves as generator protection operation. Generator

29

protection includes many elements to detect abnormal operating conditions so that the

generator and mechanical systems can be tripped before damage can occur. Two of the

abnormal operating condition protective elements associated with synchronizing

problems are reverse power/antimotoring and underexcitation/loss-of-field protection.

Reverse power is used to detect motoring of the generator. If the prime mover

stops producing mechanical energy (for example, loss of steam flow or fuel), the

generator will continue to turn at synchronous speed, drawing power from the system

as a motor. This can be damaging to the prime mover, which is typically not designed

to be driven by the generator. The reverse power protection setting is dependent upon

the type of prime mover. Some prime movers require extremely sensitive settings below

0.2 to 0.5 percent of unit rating. If the generator frequency is less than the system

frequency, the initial power flow direction is into the generator to accelerate it to

synchronous speed, which can cause the unit to immediately trip back out on reverse

power.

Underexcitation protection safeguards a generator from going out of step when

the field is too weak to keep it in synchronism with the power system. Round-rotor

generators have an end-iron heating limit in the underexcited operating region that will

damage the generator if the condition persists. Underexcitation is generally detected by

impedance relays that detect the apparent impedance when the generator is sinking

VARs. If the generator voltage is lower than the system voltage at the point of initial

synchronization, a very sensitive loss-of-field relay may cause the unit to immediately

trip back out.

Electrical engineers are often quite familiar with these modes of protection

operation during synchronization, but it is important to recognize that these problems,

while certainly a nuisance, are a symptom of faulty synchronization and not the real

issue.

3.3 Synchronizing Parameters

For proper synchronization of generator and ac power grid, basically four

parameters have to consider:

i. Sequence of phases of generator and grid.

ii. Voltage Magnitude of both.

iii. Frequency level.

iv. Phase Angle of each phase.

30

Phase Sequence

It is important that, the three phases of both generator and ac network grid

should be in same sequence for proper synchronizing. Generator alternator has three

120-degree apart phases which can be delta or star connected. This phase sequence

should be perfectly matched with grid phase sequence for correct synchronization.

Voltage Magnitude

Every generator is designed to give specific output voltage magnitude. While

synchronizing generator to respective grid, voltage level should satisfy essential

condition in which the voltage magnitude (sinusoidal) generated by must be equivalent

to the magnitude of the grid voltage.

Frequency

Generator produces electrical energy at certain specific frequency designed by

manufacturer as per requirement. The generator frequency should be equal to the

frequency on which grid operating. Frequency matching is very important in order to

reduce post synchronization transients on overall system. Generally, Synchro scope is

used to consider frequency parameter.

Phase Angle

Similar to phase sequence, the phase angle also very important parameter of

generator synchronization to grid. Phase angle is an angle between the voltage

generated by the generator and the voltage of grid. This phase angle difference must be

zero while synchronizing a generator to grid. From observation of peaks and zero

crossing incidence of the sinusoidal waveform (i.e. 0 to 360°), the phase angle can be

measured.

3.4 Synchronizing Conditions

For proper synchronizing operation to connecting a generator to existing power

grid, following conditions must be satisfied: Sequence of Phase should be same.

Voltage magnitude of generator and grid should be equal. Frequency of generator and

grid should be same.

31

Figure 3.1. Generator and grid synchronization.

The Phase Sequence

The sequence of three phases or rotation of the three phases of generator which

be connected in star or delta configuration must be the same as three phases of the

electrical system i.e. ac grid network is to be synchronized. There are only two possible

sources of in sequence. During maintenance, the generator or transformer terminals

might be interchanged, or the voltage transformer leads can be interchanged.

Voltage Magnitude

The voltage magnitude grid should be equal to the generator voltage magnitude.

If the two voltages are not the same and all other conditions are met even though,

synchronization may fail. Big MVAR flow causes if there is a difference between

generator and grid voltage magnitude. If the grid voltage is less than the generator

voltage still it linked to the ac grid, then generator gets overexcited and it flows more

MVAR though system. If there is voltage difference such that, the grid voltage

magnitude is more than the generator voltage magnitude and if under this condition,

generator connected with grid then the generator will behaves underexcited so it will

take up more MVAR from system.

Frequency

The generator frequency and voltage frequency ac grid network should be same

for proper synchronization.

32

Figure 3.2. Generator slower than grid.

The synchro scope rotates rapidly counterclockwise. In case of breaker of the

generator accidently closed it will cause the generator out of step with the ac grid system

is to be connected. After this situation generator will acts as motor and grid tries to take

it up to speed. Due this, it will harm the generator because slipping of stator and rotor

poles occurs. If generator were faster than the grid, then similar problem will be

observed with system.

Figure 3.3. Generator and grid matching speed.

The generator and grid are matching speed is shown in figure 3.3. At same speed

rate zero crossing and peak point of voltage sine wave occurs. In figure 3.2 it appears

as a non-rotating synchro scope because phase angle and the grid exist among them. At

this instance, if a generator breaker gets closed, the grid system will pull generator into

step. Under this situation stress on stator and rotor will increases, with high inrush

current flow through generator. Due to this generator may damage permanently. With

same forces, a leading generator will try to take power into grid network instantly. So,

it is very essential to match exactly waveform of generator and grid.

33

3.5 Synchronizing Methods

Now days followings three most used methods are used for generator

synchronizing:

i. Manual synchronizing method

ii. Manual with permissive relay (synch check) synchronizing method

iii. Automatic synchronizing method.

Manual Synchronizing

Manual synchronizing method is generally used on a variety of generating

stations. In manual synchronizing system basically, it includes component like

synchronizing lamps or lights, a synchro scope instrument, metering devices and a

breaker to close switch at right instant. In manual synchronizing method operator

Controls generators voltage magnitude and speed and operates the relays or breaker at

the exact synchronism instant. The main advantages of this system are simplicity and

cost effectiveness. Any type of generator can be synchronized by an operator is possible

and easy monitoring the power plant is possible in manual method.

Figure 3.4. Manual synchronizing.

Manual with Permissive Relay

This synchronizing method is similar as manual method, but it has additional

sync-check relay (i.e. ANSI/IEEE Device 25). Main purpose of the sync-check relay is

to give back up to close the generator breaker for synchronization which is decided by

operator. Closing of breaker takes place only after sync-check gives permission to close,

when synchronizing parameters i.e. voltage, frequency and phase angle and sequence

are satisfied for proper synchronization. With this additional facility the operator can

close the breaker with confidence. This method provides more efficiency and errorless

synchronization operation.

34

Figure 3.5. Manual with permissive relay.

Automatic Synchronizing

As name indicates, this synchronization method is an automatic synchronizing

process, The automatic synchronizer (ANSI/IEEE Device 25A) is capable to monitor

and control frequency, voltage and phase angle, and it also provides improvement in

respective signals for voltage matching and frequency matching with grid parameters

and after satisfying all conditions it will provides signal to the breaker to close contact

at correct instant.

Figure 3.6. Automatic synchronizing.

Phase Lock Type Automatic Synchronizers

This is one of automatic system for synchronization of generator with grid. The

synchronizer which is phase matching type is automated system in which it gives a

breaker closing angle window and voltage magnitude approval. In this system if a

generator is within specified operation window, the synchronizer system activates a

relay and initializes the breaker closing process at correct instant.

35

The phase lock type synchronizer uses the concept of improving the signals that

are useful to operate governor control system and automatic voltage regulation system.

These signals are provided by synchronization system until all synchronizing

parameters are perfectly matched with grid parameter. This type of synchronizer can

provide respective control signal to governor system to control the speed of generator

to match various parameters with grid.

As the control signal controls the prime mover output which controls speed of

generator. When the voltage reaches a correct magnitude with respect to grid system it

will initialize the synchronizing switch to connect generator to grid. The synchronizer

senses frequency, phase angle and bus voltage of both generator and the grid.

(a) Compare voltage (b) Compare frequency

(c) Change voltage to match bus (d) Change frequency to match bus

36

(e) Compare phase angle

Figure 3.7. Phase-lock synchronizer block diagram

The synchronizer measures a difference between the generator and grid for

voltage, frequency, and phase angle and if any difference found by system it will

generate the appropriate signals to correct that difference and to match all parameters

with grid. This automatic synchronizing system is designed with the help of using

various electronics component like comparator; microprocessor/microcontroller etc.

Advantages of Automatic Synchronizer

Frequency/Phase Correction Option

Automatic synchronizing system designed with electronic components which

will provide comparative values and appropriate corrective signals. For frequency

correction bipolar d-c correction output signals provide to the governor until the

generator frequency is corrected to within ±3 Hz of the grid bus frequency. After ±3 Hz

difference is achieved, the output signal of system is proportional to the frequency

difference. Then synchronization system contacts output correction signals and if the

phase angle is greater than the setting of front panel then the output contact will close.

The contact will open only after phase angle is less than the setting of front panel setting.

Voltage Correction Option

Automatic system provides voltage correction facility. Nowadays, generally generating

units have automatic voltage control system. By using this concept of AVR it will

provide the corrective signals to the system to match voltage magnitude of generator to

the grid voltage. Process of the voltage matching selection is same as frequency or

phase angle matching, but in this correction signal used to regulate generator voltage

magnitude. When the automatic synchronizer gets in touch with output voltage

37

matching and it will wait until voltage difference of the generator and the grid brings

within particular limit the switch contact remains closed.

Dead Bus Option

The automatic synchronization system has the dead bus option in which it allows

the generator to connect to the bus bar even if the bus may very low or inactive. This is

very useful characteristic of automatic synchronization system to provide emergency

reserve systems that need the first connect generator up to the dead grid bus.

3.6 Synchronizing Techniques

There are various techniques that are used for generator or alternator

synchronization, here explained various schemes to connect the generators to the grid.

Following are synchronization schemes:

i. Three-bulb technique

ii. Two bright, one dark technique

iii. The synchro scope technique

These techniques are most preferred in old days but due to manual operation and

less accuracy this technique needs a very skilled individual also these methods have

less reliability and security. So, now days generally Synchro scope technique and

microprocessor or microcontroller device based synchronizing systems are more used

for the automatic synchronization operation of the generators. Because using

microcontroller base systems generator synchronization becomes much more reliable

and easier to operate than conventional methods.

Three-bulb technique

In the three-bulb technique, a light bulb is placed across each pole of the three-

phase switch that connects the generator to the power system. The objective is to close

the switch when all of the lights are dark, as that would indicate that each pole of the

switch has nearly zero voltage across it.

If the generator and the bus are operating at significantly different voltage levels

or if they are separated by a constant phase angle, then all three poles of the switch will

have voltages across them and all three bulbs will be on all of the time.

If the generator and the bus have different frequencies, then the phasor voltages

will rotate at different speeds. At some point, the two sets of phasor voltages will come

38

into phase with each other and the bulbs will go dark, but then they will drift out of

phase due to the different rotating speeds. Thus, if the bus and generator have the same

phase sequence, as shown, the bulbs will go light and dark together.

On the other hand, if the phase sequences are different, then only one of the

phases can be in phase at any one time. Thus, the lights would go dark sequentially, one

at a time. Note that the bulbs will be dark for a period of time when the voltage is too

low to light them but not zero.

Figure 3.8. Three-bulb method for synchronizing a generator to an infinite bus.

To properly parallel a generator using the three-bulb method, the generator

should be brought up to a few RPM over synchronous speed and the excitation should

be adjusted so that the voltages on the two sides of the switch are approximately equal.

If the phase sequences are correct, then the bulbs should be glowing and be

darkening together. With the generator running only a few RPM above synchronous

speed, the cycle time for the bulbs to lighten and darken should be five to ten seconds.

If that is the case, then the switch can be closed during the middle of the dark cycle and

the generator will pull into synchronism with the infinite bus.

39

If the phase sequence is incorrect, then the bulbs will go dark one at a time. In

that case, the generator should be shut down and two of its connections to the switch

should be reversed. Then the process can be repeated.

It is important that the generator is operating slightly above synchronous speed.

That way, when the switch is closed, the generator must slow down slightly, which

means it will deliver power to the bus. If the machine were running below synchronous

speed, it would have to speed up when the switch was closed, which would require that

it draws power from the bus, making it a motor. The light bulbs must have a voltage

rating of twice the phase voltage since the voltages can be 180° out of phase. Obviously,

on higher voltage systems, that can be a problem. To solve that, transformers can be

used.

Two-bright, one-dark technique

The two outside phases are cross connected. When the system and the generator

are properly phased, the middle bulb is dark, and the outside ones are bright. If the

frequencies are different, the bulbs will flash sequentially, and the direction of the

flashing tells whether the generator speed is high or low.

This technique also offers the advantage of being able to close the switch when

the two outside bulbs are at their maximum (at equal brightness). Since it is easier to

determine the maximum brightness than the middle of the dark period, this allows the

operator to close the switch when the system and generator are more nearly in phase.

Figure 3.9. Two-bright, one-dark technique.

40

The synchro scope technique

For knowing exact instant to closing synchronizing switch a synchro scope

device is used. A pointer rotates on the dial in synchro scope. If the generator is running

slower then pointer rotation direction will be in anticlockwise and if generator is

running faster, then pointer rotation direction will be in clockwise. When the pointer of

synchro scope is exact straight upwards it is correct movement of closing the switch of

synchronization. In lamps method synchronization effectiveness generally depends on

operator experience, decision, and skill. The three-lamp technique is easy and cost

efficient but, in this method, it cannot provide information about frequency i.e. it is

higher or lower for synchronizing generators. So, for this purpose synchro scope device

used which informs about incoming generators frequency whether it is lower or higher.

For big voltage systems, lamp technique cannot be used so generally synchro scope

technique is preferred. In its construction it consists one moving vane and three coils.

The coils of synchro scope are connected to the generators and bus bar which is to be

synchronized and pointer connected to moving vane. The use of voltage transformer for

measuring the difference in voltage, the pointer will rotate in clockwise direction or anti

clockwise direction and after speed of incoming generators becomes equal the pointer,

it will stop at upward vertical direction and relays switched to connect the generator to

the grid bus bar. Figure 3.6 shows below, the pointer moves in anti-clockwise or

clockwise direction and after incoming generators speed becomes equal to that of bus,

relays will be closed for synchronizing.

Figure 3.10. Synchronization of alternator using synchroscope.

The pointer of synchro scope will continuously in motion if the two voltages

phase angle is different. If the incoming generator is slower then it will move to slower

41

point and if then incoming generator is faster than it will move towards the faster point

and after equal frequency of generator and grid bus bar pointer will stop moving. This

situation is for closing synchronization switch. A synchro scope is preferred for

synchronizing two generators because it informs very correctly the exact synchronism

instant. Connection of synchro scope is across only one phase due to this reason it

cannot be used safely. It can be connected only after checking proper phase rotation

and testing the generator. For determining proper phase rotation Synchronizing lamps

may be used. In various applications, the generator connections to the grid through a

paralleling switch are permanent. In these case phase rotation checking is not essential.

For synchronizing generator to the grid only syncroscope instrument is needed in these

cases. For double check system, a set of bulbs can be used for synchronization purpose.

Figure 3.11. Synchroscope meter and synchronizing indicator.

3.7 Simulink

Simulink, an add-on product to MATLAB, provides an interactive, graphical

environment for modeling, simulating, and analyzing of dynamic systems. It enables

rapid construction of virtual prototypes to explore design concepts at any level of detail

with minimal effort. For modeling, Simulink provides a graphical user interface (GUI)

for building models as block diagrams. It includes a comprehensive library of

predefined blocks to be used to construct graphical models of systems using drag-and-

drop mouse operations. The user is able to produce an “up-and-running” model that

would otherwise require hours to build in the laboratory environment. It supports linear

and nonlinear systems, modeled in continuous-time, sampled time, or hybrid of the two.

Since students learn efficiently with frequent feedback, the interactive nature of

Simulink encourages you to try things out, you can change parameters “on the fly” and

42

immediately see what happens, for “what if” exploration. Lastly, and not the least,

Simulink is integrated with MATLAB and data can be easily shared between the

programs.

There are two major categories of elements in Simulink:

i. Blocks

ii. Lines

Blocks are used to generate, modify, combine, output, and display signals. Lines,

on the other hand, are used to transfer signals from one block to another.

3.7.1 Blocks

There are several general classes of blocks, some of which are:

i. Sources: Used to generate various signals. Sources blocks have outputs but no

inputs. One may want to use a Constant input, a Sine Wave, a Step, a Ramp, a

Pulse Generator, or a Uniform Random number to simulate noise. The Clock

may be used to create a time index for plotting purposes.

ii. Sinks: Used to output or display signals. Sinks blocks have inputs but no outputs.

Examples are Scope, Display, To Workspace, Floating Scope, XY Graph, etc.

iii. Discrete: Discrete Filter, Discrete State-Space, Discrete Transfer Fcn, Discrete

ZeroPole, Unit Delay, etc.

iv. Continuous: Integrator, State-Space, Transfer Fcn, Zero-Pole, etc.

v. Signal routing: Mux, Demux, Switch, etc.

vi. Math Operations: Abs, Gain, Product, Slider Gain, Sign, Sum, etc.

3.7.2 Lines

Lines transmit signals in the direction indicated by the arrow. Lines must always

transmit signals from the output terminal of one block to the input terminal of another

block. One exception to this is that a line can tap off of another line. This sends the

original signal to two (or more) destination blocks. Lines can never inject a signal into

another line; lines must be combined through the use of block such as a summing

junction. A signal can be either a scalar signal or a vector signal.

43

Figure 3.12. Basic simulink model