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What is synchronizing? 

The overall operation of ensuring that the two

alternating current supplies to be paralleled are

within prescribed limits, in a state of equal

magnitude, equal frequency and phase coincidencefollowed by closure of the appropriate switch to

parallel the two supplies

- Modern Power Station Practice, BEI, London

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Requirements for synchronizing 

To achieve the synchronizing the following shall be satisfied

on either side of the circuit breaker 

• Voltage Magnitudes are same

• Voltages are in Phase

• Frequencies are equal

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Types of synchronizing 

System synchronising:

When two different parts of a system are connected together 

(i.e. both Voltage Magnitude, Phase and Frequency cannot

be controlled like generator circuits)

For example Closing of a Bus coupler CB fed by twoTransformer feeders, the voltage magnitude can only be

varied through Tap changing mechanism

Generator synchronizingGenerator is connected to a Grid. The voltage magnitude,

Phase and frequency of the generator supply can be varied.

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Synchronizing methods

Manual synchronisingIt is performed by the operator using following synchronizing

devices. This is generally used for both Generator 

synchronisation and System synchronisation

• Synchroscope

• Check Synchronising Relay• DV, DF Meters, Guard Relay

Automatic synchronisingIt is performed through “Automatic Synchronising Relay”. This

provides closed loop control for adjusting Speed & Voltage of 

the Generator during synchronization. This is used for 

Generator synchronisation

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Synchronizing Equipment 

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Slip frequency 

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Slip frequency / Frequency error 

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 Advance Angle

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Load pick-up after synchronising 

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Criteria for Generator Synchronising 

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Machine slower than system – Sub-synchronous

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Properly Synchronised 

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Permissible Synchronising Criteria for Generators

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Procedure for Manual Synchronizing of a Generator 

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Procedure for Auto Synchronizing of a Generator 

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Sources of errors in Generator Synchronizing 

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Mechanism of Wrong Synchonising of Generator 

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Possible out of Synchronising 

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Effect of Faulty Synchronising on Generator 

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Precommissioning checks for Synchronising system

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