How important is synchronizing?

Synchronizing requirements
There are four conditions that need to be met when synchronizing a generator or generators to the grid, which are:

• Voltage - Voltage magnitude of the Bus and Line must be the same or within the allowable magnitude set in the synchronizing check relay. Typical settings are at 5% up to 10%.
• Frequency - Frequency must be the same or within the allowable slip frequency between the Bus (Generator) and the Line (Grid).
• Phase Sequence - Phase sequence must be the same sequence i.e. R, Y, B sequence for both Bus and Line.
• Phase Angle - Phase angle for each Bus and Line must be 120 deg apart respectively. The phase angle between e.g. R - Y of Line and R-Y of Bus must be within the maximum allowable angle difference. Typical settings are from 5 deg up to 15 deg. The charge man or competent electrical engineer would normally close the breaker when the synchroscope is close to 000 hrs, which means that the angle difference is closing to 0 deg.

Let us look at a typical settings of a synchronizing check relay. In this case, we will be using Siemens Reyrolle 7SR2244 as an example:

 In one of the plants, the setting used for this relay was as per following:

• Voltage reference: Vab, This is the voltage that it will compare between Bus (R,Y or Va, Vb) and Line (R, Y). The selection depends on the Bus phases selected at voltage input Vx.
• Check Sync Angle: 15 deg, Angle difference must be within this angle 0 to 15 deg. 
• Check Synch Slip: 250 mHz, the frequency difference between Bus and Line
• Check Sync Timer: 1 sec
• Line UV: 20 V (Secondary)
• Bus UV: 20 V (Secondary)
• Volt Diff: 6 %, Voltage difference of both sources must be within this setting.

What if we had closed the breaker under a borderline synchronizing conditions i.e. at an angle difference of 15 deg?

This can actually be calculated using the laws of Cosines.

Let us assume that the Line (Grid) is b and the Bus (Generator) is c. Thus we can calculate the Voltage Difference, c based on the laws of cosine. Shown below are the calculated V_difference for a Line and Bus which are are already at equal magnitude 132kV Phase to Phase. 

Here is the link to the excel file.

Even at border line of 15 deg angle difference, we are closing the breaker at pole to pole voltage difference of 34.5kV. Imagine, if the secondary settings were wrongly set, and you closed it at an angle difference of 150 deg, which is almost the 1 minute AC Power Frequency High Voltage Test of a 132kV system. It could cause your system to experience a high MW, MVar flow into or out of your system depending on whether the Bus is lagging or leading the Line at the instance of breaker closing.

So how important is synchronizing?