Synchronization Check in AC systems

Introduction: In AC power system synchronization check is used when we are connecting two different systems. Two points are worth mentioning in this case:

1. The connection is always made through circuit breaker (CB)
2. We are not sure both the systems being connected through CB are different or same.

Therefore, synchronization check is used when closing circuit breaker to ensure we do not interconnect out of synchronism systems.

Implementation: There may be four cases when closing CB:

1.  Dead line Dead Bus
2.  Dead Line Live Bus
3.  Live Line Dead Bus
4.  Live Line Live Bus

In the first three cases, there is voltage only one side of CB, there no option of synchronization check. Therefore, CB is to be closed without any check. For radial feeders synchnization check is not used because we always close for Dead Line Live bus cases. In fourth case both side voltage is available. We have to check synchronization before closing the CB.

Parameters to be checked: For interconnecting two systems, we have to ensure the following three parameters are within limits: 

1. Phase angle difference - It will lead to active power flow after CB closing. If it is more (typical value  +30 deg), large current will flow depending on system impedance. This may lead to damage to equipment or grid (or infinite bus). As shown in image below phase angle difference is δ. (Blue line is Bus voltage Green is Feeder voltage, Red is difference between Bus and Feeder voltage)

2. Voltage difference - It will lead to reactive power flow after CB closing. If it is more (typical value  +10%), large current will flow depending on system impedance. This may lead to damage to equipment or grid (or infinite bus). As shown in image below voltage difference is V1 - V2 after taking r.m.s. value (Blue line is Bus voltage Green is Feeder voltage, Red is difference between Bus and Feeder voltage)

3. Frequence difference - In interconnected AC system all synchronus machines (Mostly generators) run at same frequency. All the machines have thier rotors magnetically locked with stator field which is rotating at grid frequency. If there are two different networks at different frequency, they have to run at same frequency after interconnection, otherwise interconnection will not work. Normally synchronization check is done that incoming feeder frequency is with permissible range typical value for frequency difference is +0.2% to +0.5 %. If any generator is rotating at higher or lower speed (prior to connection to grid), which means it has different frequency, is connected to grid, it will feel mechniacal shock upon CB close. Because if it is running slow (i.e. lower frequency), after CB close it has to run faster to match grid. But its prime mover input is low. It will start to run as motor, taking power from grid. In other case if it is running faster (i.e. higher frequency), after CB close it has to run slower. Due to more input to prime mover it will give more power than anticipated. As shown in image below frequency difference is f1 - f2.

Click here for detail about variation in these paramaters.

In addition when first time interconnecting two systems, phase sequence is also to be checked. Subsequently, it is not required to be checked because phase sequence can not change without physical change in connections. As shown in image below both bus and incomer voltages have sequence R - Y - B.

In the image shown below, let us assume we are connecting generator-1 to Bus bar by closing CB-1. Before closing we have to check synchronization by comparing any one phse voltage (Normally R-phase) of Bus VT to Feeder-1 VT. We do not compare voltage of all phases, becuase phase sequence for both sides have been already been matched during first time charging. 

Synchronization check is more significant at generating stations when connecting any generator to grid. During synchronization operator is also having control of generator i.e. he can control frequency / voltage of the generator. 

Synchronization check is used at substations also before closing any CB, so that two different systems are not interconnected without checking synchronization. As shown in image below Before closing feeder CB at Station-B Synchronisation is is required.

In substation already connected through alternate route, frequency will be same on both sides of CB, but phase angle and voltage needs to be check by synchronization check. As shown below, Station-A and Station-B are alraedy connected through Station-C, But before closing feeder CB at Station-B, voiltage difference and phase angle difference needs to be checked.