In an alternating current electric power system, synchronization is the process of matching the speed and frequency of a generator or other source to a running network. An AC generator cannot deliver power to an electrical grid unless it is running at the same frequency as the network. If two segments of a grid are disconnected, they cannot exchange AC power again until they are brought back into exact synchronization.

A direct current (DC) generator can be connected to a power network by adjusting its open-circuit terminal voltage to match the network voltage, by either adjusting its speed or its field excitation. The exact engine speed is not critical. However, an AC generator must match both the amplitude and the timing of the network voltage, which requires both speed and excitation to be systematically controlled for synchronization. This extra complexity was one of the arguments against AC operation during the War of Currents in the 1880s. In modern systems, synchronization of generators is carried out by automatic systems.

ConditionsThere are five conditions that must be met before the synchronization process takes place. The source (generator or sub-network) must have equal line voltage,frequency, phase sequence, phase angle, and waveform to that of the system to which it is being synchronized.

Waveform and phase sequence are fixed by the construction of the generator and its connections to the system. During installation of a generator, careful checks are made to ensure the generator terminals and all control wiring are correct so that the order of phases (phase sequence) matches the system. Connecting a generator with the wrong phase sequence will result in a short circuit as the system voltages are opposite to those of the generator terminal voltages.[1]

The voltage, frequency and phase angle must be controlled each time a generator is to be connected to a grid.

Generating units for connection to a power grid have an inherent droop speed control that allows them to share load proportional to their rating. Some generator units, especially in isolated systems, operate with isochronous frequency control, maintaining constant system frequency independent of load.

ProcessThe sequence of events is similar for manual or automatic synchronization. The generator is brought up to approximate synchronous speed by supplying more energy to its shaft - for example, opening the valves on a steam turbine, opening the gates on a hydraulic turbine, or increasing the fuel rack setting on a diesel engine. The field of the generator is energized and the voltage at the terminals of the generator is observed and compared with the system. The voltage magnitude must be the same as the system voltage.

If one machine is slightly out of phase it will pull into step with the others but, if the phase difference is large, there will be heavy cross-currents which can cause voltage fluctuations and, in extreme cases, damage to the machines.

Synchronizing lamps

Formerly, three light bulbs were connected between the generator terminals and the system terminals (or more generally, to the terminals of instrument transformers connected to generator and system). As the generator speed changes, the lights will flicker at the beat frequency proportional to the difference between generator frequency and system frequency. When the voltage at the generator is opposite to the system voltage (either ahead or behind in phase), the lamps will be bright. When the voltage at the generator matches the system voltage, the lights will be dark. At that instant, the circuit breaker connecting the generator to the system may be closed and the generator will then stay in synchronism with the system.[2]

An alternative technique used a similar scheme to the above except that the connections of two of the lamps were swapped either at the generator terminals or the system terminals. In this scheme, when the generator was in synchronism with the system, one lamp would be dark, but the two with the swapped connections would be of equal brightness. Synchronizing on "dark" lamps was preferred over "bright" lamps because it was easier to discern the minimum brightness. However, a lamp burnout at the wrong time could cause synchronization errors[why?].

Synchroscope

Another manual method of synchronization relies on observing an instrument called a "synchroscope", which displays the relative frequencies of system and generator. The pointer of the synchroscope will indicate "fast" or "slow" speed of the generator with respect to the system. To minimize the transient current when the generator circuit breaker is closed, usual practice is to initiate the close as the needle slowly approaches the in-phase point. An error of a few electrical degrees between system and generator will result in a momentary inrush and abrupt speed change of the generator.

Synchronizing relays

Synchronizing relays allow unattended synchronization of a machine with a system. Today these are digital microprocessor instruments, but in the past electromechanical relay systems were applied. A synchronizing relay is useful to remove human reaction time from the process, or when a human is not available such as at a remote controlled generating plant. Synchroscopes or lamps are sometimes installed as a supplement to automatic relays, for possible manual use or for monitoring the generating unit.

Sometimes as a precaution against out-of-step connection of a machine to a system, a "synchro check" relay is installed that prevents closing the generator circuit breaker unless the machine is

within a few electrical degrees of being in-phase with the system. Synchro check relays are also applied in places where several sources of supply may be connected and where it is important that out-of-step sources are not accidentally paralleled.

Synchronous operationWhen the generator is synchronized, the frequency of the system will change depending on load and the average characteristics of all the generating units connected to the grid. Large changes in system frequency can cause the generator to fall out of synchronism with the system. Protective devices on the generator will operate to disconnect it automatically.

Synchronous speedsSynchronous speeds for synchronous motors and alternators depend on the number of poles on the machine and the frequency of the supply.

The relationship between the supply frequency, f, the number of poles, p, and the synchronous speed (speed of rotating field), ns is given by:

.

1. Synchronizing and Synchronizing Equipment

 1.1 Theory of Synchronizing

 When closing a circuit breaker between two energized parts of the power system, it is crucial to match voltages on both sides of the circuit breaker before closing. If this matching or "synchronizing" process is not done correctly, a power system disturbance will result and equipment (including generators) can be damaged. In order to synchronize properly, three different aspects of the voltage across the circuit breaker must be closely monitored. The three aspects of the voltage are called the synchronizing variables and are:

1. The voltage magnitudes

2. The frequency of the voltages

3. The phase angle difference between the voltages

1.1.1 Voltage Magnitude Synchronizing Variable

 If the voltage magnitudes are not closely matched, a sudden rise in Mvar flow will appear across the circuit breaker as it is closed. For example, if a 345 kV circuit breaker

were closed with a 20 kV difference in voltage across the open circuit breaker, a large Mvar flow would suddenly occur upon closing. The allowable voltage magnitude differences across the open circuit breaker are system specific. However, for general guidance, a difference of a few percent is unlikely to cause any serious problem.

 1.1.2 Frequency Synchronizing Variable

If the frequencies on either side of an open circuit breaker are not matched prior to closing, a sudden change in MW flow will appear across the circuit breaker as it is closed. The sudden MW flow change is in response to the initial frequency difference as the system seeks to establish a common frequency once the circuit breaker is closed. The allowable frequency difference is again system specific. However, a general guideline would be to have the frequencies within 0.1 Hz of each other prior to closing.

 1.1.3 Phase Angle Synchronizing Variable

 The third synchronizing variable - and likely the most important of the three - is the voltage phase angle difference. If the phase difference between the voltages on either side of the open circuit breaker is not reduced to a small value, a large MW flow increase will suddenly occur once the circuit breaker is closed. The voltage phase angle difference is the difference between the zero crossings of the voltages on either side of the open circuit breaker. Ideally, the voltage phase angle should be as close to zero degrees as possible before closing the circuit breaker.

 1.2 Synchronizing Examples

 The importance of synchronizing cannot be overstated. All system operators should understand the theory and practice of synchronizing. If two power systems are synchronized via an open circuit breaker, and the synchronizing process is not done correctly, generators can be severely damaged. Two scenarios for synchronizing follow to further describe the synchronizing process.

Synchronizing Equipment

 1.3.1 Synchroscope

 A synchroscope is a simple piece of equipment that is used to monitor the three synchronizing variables. A basic synchroscope (illustrated in Figure 3) inputs voltage waveforms from the two sides of the open circuit breaker. If the voltage waveforms are at the same frequency, the synchroscope does not rotate. If the voltage waveforms are at a different frequency, the synchroscope rotates in proportion to the frequency difference. The synchroscope needle always points to the voltage phase angle difference.

 A synchroscope is a manual device in that an operator must be watching the "scope" to ensure they close the circuit breaker at the correct time. The synchroscope is normally mounted above eye level on a "synch panel". The synch panel also contains two voltmeters so that the voltage magnitudes can be simultaneously compared.

 The synchroscope in Figure 3 reflects a slight voltage magnitude mismatch, and a stationary synchroscope with a phase angle of approximately 35°. The fact that the synchroscope needle is not rotating indicates frequency is the same on either side of the circuit breaker. 

 

Figure 3

Synchroscope in a Synch Panel

 1.3.2 Synchro-Check Relays

 A synchro-check or synch-check relay electrically determines if the difference in voltage magnitude, frequency and phase angle falls within allowable limits. The allowable limits will vary with the location on the power system. Typically, the further away from generation and load, the more phase angle difference can be tolerated. Synch-check relays typically do not provide indication of the voltage magnitude,

frequency or phase angle. A synch-check relay decides internally whether its conditions for closing are satisfied. The synch-check relay will either allow or prevent closing depending on its settings. A typical synch-check relay may allow closing if the voltage angle across the breaker is less than 30°.

 1.3.3 Application of Synchronizing Equipment

 At power plants, synchroscopes are routinely installed to permit manual closing of a circuit breaker. In addition, synch-check relays can be used to "supervise" the closing of the circuit breaker and prevent distracted or inexperienced operator from initiating a bad close.

 Modern power plants typically utilize automatic synchronizers. Automatic synchronizers send pulses to the generator exciter and governor to change the voltage and frequency of the unit. The synchronizer will automatically close the breaker when it is within an allowable window.

 Substations on the transmission system have traditionally had synchroscopes installed. However, few substations are now manned due to the availability of powerful SCADA systems. Because of this development, newer substations may or may not have a synch panel, depending on the transmission company procedures. Since most circuit breaker operations are done remotely, transmission companies often rely on synch-check relays to supervise closing of breakers.

 Figure 4 illustrates a possible synchronizing system for substation breakers. Note the use of a synch scope and a synch-check relay. Electrical contacts can be opened or closed to rearrange the synchronizing system as desired.

 

 

Figure 4

Synchronizing System for a Substation Breaker

 

Conditions

In order to synchronize a generator to the grid, four conditions must be met:1. Phase Sequence2. Voltage Magnitude3. Frequency4. Phase Angle

Figure 1 - Synchronizing a Generator to the Grid

1. Phase Sequence

The phase sequence (or phase rotation) of the three phases of the generator  must be the same as the phase sequence of the three phases of the electrical system   ( Grid ) .The only time that the phase sequence could be wrong is at initial installation or after maintenance. There are two possible problem sources.

The generator or transformer power leads could actually be interchanged during maintenance orthe potential transformer leads could be interchanged during maintenance.

2. Voltage Magnitude

The magnitude of the sinusoidal voltage produced by the generator must be equal to the magnitude of the sinusoidal voltage of the grid.

If all other conditions are met but the two voltages are not the same, that is there is a voltage differential, closing of the AC generator output breaker will cause a potentially large MVAR flow.Recall that before a generator is synchronized to the grid, there is no current flow, no armature reaction and therefore the internal voltage of the generator is the same as the terminal voltage of the generator.

If the generator voltage is higher than the grid voltage, this means that the internal voltage of the generator is higher than the grid voltage. When it is connected to the grid the generator will be overexcited and it will put out MVAR.If the generator voltage is less than the grid voltage, this means that the internal voltage of the generator is lower than the grid voltage. When it is connected to the grid the generator will be under-excited and it will absorb MVAR.

3. Frequency

The frequency of the sinusoidal voltage produced by the generator must be equal to the frequency of the sinusoidal voltage produced by the grid.

Figure 2 - Generator Slower than Grid

In Figure 2 above the generator is slower than the grid.The synchroscope would be rotating rapidly counter clockwise. If the generator breaker were to be accidentally closed, the generator would be out of step with the external electrical system. It would behave like motor and the grid would try to bring it up to speed.

In doing so, the rotor and stator would be slipping poles and damage (possibly destroy) the generator as described previously. The same problem would occur if the generator were faster than the grid.The grid would try to slow it down, again resulting in slipping of poles.

Figure 3 - Generator at Same Speed asGrid but not in Phase

Figure 3 shows the condition where the generator and grid have matching speed. The high points and zero crossings of the sinusoidal voltages occur at the same rate of speed.However, if you notice in 2 with the grid and a phase angle exists between them. This would appear as a non-rotating synchroscope (both generator and grid at same frequency), where the pointer would appear stuck at about 9:00 o’clock (generator lagging grid).

If the generator breaker were to be closed at this time, the grid would pull the generator into step.However, this again would cause a large current in-rush to the generator and high stresses on the rotor/stator with subsequent damage   to the generator . If the generator were leading the grid, it would try to immediately push power into the grid with the same destructive forces as mentioned.

Hence the generator must be brought to a point where the grid voltage waveform exactly matches what it is producing.

4. Phase Angle

As previously mentioned, the phase angle between the voltage produced by the generator and the voltage produced by the grid must be zero.The phase angle (0 to 360°) can be readily observed by comparing the simultaneous occurrence of the peaks or zero crossings of the sinusoidal waveforms.

If the generator breaker is closed when they match exactly, the connection will appear smooth and seamless.

At that instance (Figure 4 below), the pointer on the synchroscope would indicate 12:00 oíclock.The worst case occurs if the generator is exactly out-of phase, with a phase angle of 180° and the synchroscope pointing at 6:00 o’clock.

Figure 4 - Generator in Phase with Grid

What is Synchronization?Synchronization is the process of comparing the two source parameters like voltage, frequency and phase angle and connecting them together to operate in parallel. I.e. it is desired to assure that the two AC power sources are in synchronism

before connecting together. The device used to measure the degree of synchronization between the two AC power systems is called synchronization unit which consists of Dual voltage meter, Dual Frequency meter, Synchroscope and Synchronization check relay. Figure shows the basic arrangement of synchronization unit between the two AC power sources. The existing power source is called running power source and the power source to be connected is called incoming power source.

 Here the power system can be a single power source if only two power sources to be operated in parallel or it is a group of running generators in case of incoming generator has to be connected with grid power supply. The bus voltages are sensed by the potential transformers and provided

as input to the synchronizing unit. The output of the synchronization unit is connected to the circuit breaker to close it after the permission of synch check relay. The synchronization operation can be done in two ways based on the selection by the operator; one is AUTO and another one MAN. Manual operation is done by the operator and AUTO operation is done by the micro controller. In both the Manual and Automatic synchronization the process will be done carefully to unnecessary damage to the equipment and power system because of poor synchronization.Need of Synchronization

To avoid the damage to the prime mover due to heave acceleration forces

To ensure the perfect connection of two AC power sources without any disturbance

To avoid the damage to the Generator and Generator transformer windings due to high currents

To ensure the safety of the equipment and operating personal

Perfect synchronization connection reduces the power and voltage variations in the plant and outside the plant.

Generator synchronization procedures in the power plantGenerator synchronization is the process of the closing circuit breaker after matching the generator frequency, phase angle & voltage magnitude with grid voltage frequency, phase angle &

magnitude respectively. Simply it is used to close the circuit breaker after synchronization check.

Synchronization tasks in power plant: Controls the Governor to control the speed of the turbine. Controls the Generator exciter to change the voltage of the generator. Closing the circuit breaker after the synchronization checkSynchronization Unit in the power plant:

Synchronization unit cosists of synchroscope, Double voltage meter and double frequency meter which is mounted on the power plant generator control panel and genertor local control panel.

Synchroscope:  This is the clock type of instrument with a needle which rotates according to the difference of phase angle and frequency. It consists of FAST and SLOW notations which indicates the incoming machine frequency difference with the existing machine. I.e. if the generator is running with high frequency compared to grid frequency then the needle rotates in FAST direction or clock wise direction. The same way for lower generator frequency it rotates in SLOW direction or anti-clock wise direction. If the both incoming and existing pahse angles matches then it will stop at 12 o clock position.

Double Voltage Meter: The dual voltage meter is mounted on the panel to indicate the incoming and running voltages which are to be synchronized. The Voltages of incoming and running machines are measured and step down to lower voltage signals and connected to dual voltage meters to indicate them for the operator. This meters are generally analog meters which are positioned oppositely for better view.

Double   Frequency  Meter: The double frequency meters are positioned on the control panel to indicate the frequencies of incoming and running frequencies. These meters are normally have the range of 45-55 Hz which shows the marked colors corresponding to frequency. These are to provide the frequencies to operators for adjusting them during synchronization. 

Synchronization Procedure:

Synchronization of generator with the grid power supply should be done with proper care to avoid the severe damage to the generator. They are two types of synchronization procedures which manual synchronization done by the operator and automatic synchronization done by the control system.

Manual synchronization procedure:  The Generator synchronization of grid power supply is done by the operator by adjusting the frequency and voltages to match

with the grid frequency and voltage. The follwoing steps shows the synchronization procedure sequentially.

Manual synchronization procedure will be initiated when the turbine speed crosses 98% of rated speed by selecting synchronization MAN button from the control room panel. Operator will select the circuit breaker to be synchronized from the control room. Corresponding voltages and frequencies will be selected by the synchronization logic and displays on the double voltage and frequency meters. The operator observes the incoming frequency and adjusts it by controlling the governor to control the speed of the turbine and hence the generator frequency. To adjust the voltage of the generator the operator controls the excitation by the Automatic Voltage Regulator. The Operator observes the needle position in the synchroscpoe, and when reaches exactly 12 o clock position then he initiates the closing of circuit breaker. The best operator estimates the closing time of the circuit breaker and how fast the phase angle is increasing, then according to that he closes the circuit breaker.In this method of synchronization because of the operator intervention there may be human mistakes while initiating the closing of the breaker which can damage the system. But the Operator can visualize the other system failures like any cooling

system of generator failure and properly initiates the closing of circuit breaker. Automatic Synchronization Procedure:In the recent advancements of control systems enhanced the use of automatic synchronization in synchronizing the generator. This removes the human error for closing the closing the circuit breaker at exact time. This control logic calculates the closing time of circuit breaker and the phage angle increment rate and initiates the closing of circuit breaker. Microprocessor based automatic synchronization has became popular which verifies the incoming and running machine frequency, voltage & phase angles and adjusts them automatically to equalize them.

This automatic synchronization will be enabled by choosing AUTO synchronization selection from the control room. All the incoming and running machine parameters are wired to the micro-controller and this checks the differences between incoming and running machine parameters, according to the difference it gives an output signal to the governor and exciter to control the speed and voltage of the incoming machine.

When the two machine parameters are matching then it initiates the closing of the circuit breaker after estimating the closing time of the circuit breaker. The main problem of this

automatic procedure is it cannot sense the total system for any failure before closing the circuit breaker.   Pole slipping typically occurs under severe fault conditions which cause a transient torque on the generator which exceeds the ability of the field to hold the generator rotor synchronised to the stator. A generator is most susceptible to this problem when it has a low excitation, as this produces a weak magnetic field. For this reason, capability diagrams show the stability limit for the machine when in an under-excited state. Outside of this line, pole slipping becomes a real possibility in the event of a system fault.

It is a condition that can only occur when an alternator is synchronized to other alternators, or an AC grid. The synchronizing torque weakens to the point that synchronism can no longer be maintained at that load, and the alternator "falls out of step". Protective relays will disconnect it from the bus. 

The synchronizing torque is a function of the excitation field, which in turn affects the power factor the alternator will carry. Therefore, alternators should run all at the same power factor.  

If the overall load power factor is capacitive, self excitation begins and the synchronizing fields start to weaken making the system increasingly unstable. In such a condition any disturbance in the network may cause one or more alternators to trip. This, in turn, may cause a cascading overload effect, bringing the whole network down. This is what, more or less, seemed to have happened during the major US Northeast blackouts.