Parallel Feeders with 30 Degrees Phase Shift 2

[protoslash]

A theoretical question. If I have 2 feeders supplied from 2 different substations with different vector group. These 2 feeders are 30 degrees apart, what happens if I close a tie switch between the 2 feeders?

The instantaneous voltage is different so there should be fault at the tie switch? However I know if the phase shift is small, the system can sort itself out and have no problem paralleling. So question is how big of the phase shift is too much?

 

[jghrist]

If the two substations have a 30° phase difference, closing a switch between the two circuits would be like a Ø-Ø fault. If the phase difference at the open point is because of voltage drop along the line or in the substation transformer (high sides in phase), then the current in the switch can be determined by making a load flow analysis with the loads modeled and the switch closed.

 

[che12345]

Dear Mr protoslash (Electrical)(OP)16 Mar 23 19:27
"...#1. If I have 2 feeders supplied from 2 different substations with different vector group. These 2 feeders are 30 degrees apart, what happens if I close a tie switch between the 2 feeders? #2. The instantaneous voltage is different so there should be fault at the tie switch? However I know if the phase shift is small, the system can sort itself out and have no problem paralleling. So question is how big of the phase shift is too much?..."
I look at it in two different setups:
1. The two feeders are from different utility substations with different vector group. This can happen even from the same single utility supply but with two transformers with different vector groups.
Note: The supply is from the utility. with different in vector group which you have no control.
In this case you have no chance to sym the two out of phase voltages
Caution: Do NOT make any attempt to close the tie breaker. A short would result.
2. This may not be your case, where the two sources are from your own generators, where you have full control, (i.e. same phase sequent, about equal Voltage but may be slight difference in Frequency)
Note: with two sources are of the same phase sequent but different frequency, there would be a phase displacement, between the same phases.
2.1 In this case, the higher frequency gen will pull in the slower frequency gen when you tried to sym.
Caution: Use syn scope + syn lamps to syn them with as small angle difference as possible. Do NOT attempt to syn them at 30 deg apart! High current and damage to Gen or may cause trip out of breakers!
Che Kuan Yau (Singapore)

 

[wayne440]

A theoretical question. If I have 2 feeders...30 degrees apart, what happens if I close a tie switch between the 2 feeders?

Practically- do not do that.

Theoretically, there could be a bright flash, loud BOOM and immediately after the sound of someone shouting "what did you do?"

 

[waross]

30 degrees is not a small difference.
At 30 degrees you will be closing in on about 50% of line to line voltage.
I know of several instances where a 600 kW diesel generator sheared the key in the coupling when closed in at a 30 degree error.
In addition, many breaker trips trying to synchronize.
After a successful closure, the synchro-scope would jump to 1 o-clock.
The synchro-scope was incorrectly wired.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[davidbeach]

Stick the details into your modeling software. Maybe it's a hard fault, maybe it's a moderate overload. It depends. We've got one open on our system with a 30 degree shift across it and I modeled it to see what would happen if somebody closed it hot-hot. Not great, but it would take several seconds for the overcurrent to trip. But to get to that 30 degree shift there'd be the impedance of three transformers, one of which wasn't very big.

But it would also be easy to have a situation where the 30 degree shift could result in 10s of kA of fault current. Back to "it depends".

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[wcaseyharman]

IEEE limits the phase angle during generator synchronizing to plus or minus 10 degrees per C50.12 and C50.13, believe it’s also in the generator protection standard c37.102 . We’ve used that value of 10 degrees elsewhere on the system to set the maximum sync check angle. It’s interesting where the company has parallel lines sometimes the phase angle across the line can be quite high, 15 degrees or more, which makes setting sync check relays interesting.
A full 30 degrees would definitely be exciting, especially if the sources had low impedance.

 

[cranky108]

So how do you get a 30 degree shift, except one with a wye-wye transformer, and the other with a delta-wye transformer?
That said, I know where you can find a Wye-Wye 60 degree transformer shift (34 to 2.4). I doubt anyone needs one of those.

 

[protoslash]

thanks for everyone's reply.

modelled a sample system in CYME using two transformers, one is DY11, the other one is Yg Yg. (44/12kV 5MVA). closing the tie switch cause 1000A circulating current in the loop. Not the biggest fault, but certainly would trip all the feeder protection.

 

[crshears]

The possibility for such arose within my utility over the course of many years . . .

44 kV systems quite some physical distance apart and using dissimilar transformers were first constructed and then expanded over the course of decades, eventually creating circumstances essentially identical to that shown in the plot above.

I would agree completely with wayne440's first statement: "Practically - do not do that," then I would modify wayne440's second statement somewhat, and continue to say "because there will likely be a bright flash, a loud BOOM, and a possibly accompanying shower of molten copper. I would deem it an absolute certainty that immediately after the sound of some hockey player shouting 'Just WHAT THE PUCK did you do?!?!?' would be heard."

The simplest and by far the most common way to transfer loads between two sources 30° apart is break before make, viz., take a short interruption to the loads while transferring from one source to the other.

Another is used where a load has displacement or emergency generation; the latter is placed in service and carries - all? - essential? - load while the supply transformer is unloaded, removed from source "A" potential, placed on source "B", then reloaded using either ATS or synchronizing facilities where permitted by the AHJ.

Another much more complex but very elegant approach that does not involve any customer interruptions is to perform such switching as needed to bring the open point to a facility with both generation and synchronizing capability such that generation can carry the load while it is transferred between sources; can be quite convoluted but is very slick, earning some attaboys and LPSs.


CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[bacon4life]

Simulation is the way to know how much current will flow for your situation. We have some cases where distribution circuits come from substation fed by different transmission lines. Even though phase angles on the transmission can be 10s of degrees different, the loop flow on very long distribution ties is just a couple hundred amps.