Closing in two circuits maximum Phase Angle

[Palletjack]

What would be the maximum phase angle you would close a breaker to parallel circuits together?
I know a 60 degree phase displacement won’t go together, here I’m talking about angles due to line length and or Z.
Would this angle be larger or smaller depending on voltage? I think it would, but how much?
25kV line?
13kV?

 

[waross]

I encountered a generating plant with two 600 kW generators. The synchroscope had a hard wired error of 30 degrees.
The slower operators could generally close in as the error would be diminishing with time.
The "On the ball" operators who close the instant that the scope hit 12 o-clock would often have the breaker trip.
About once a year a drive coupling would shear off.
On generators up to about 1 MW I would like to see less than 15 degrees phase angle difference, less for larger sets. You would still be closing on 26% of peak voltage.
I realize that parallel generators is not the same as phase shifts on a transmission line.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Hmmm...

The short answer is, "as small as possible."

Expanding on that...

In my view, the major concern is what happens during the closing and synchronizing transients; since the two circuits are already connected elsewhere, placing them in parallel will cause changes in power flow directly proportional to the standing phase angle across them prior to breaker closure, and inversely proportional to the impedance around the path being closed.

Also pertinent is whether the impedance path around the loop is one of straight galvanic connection, or whether there are intervening step-up and step-down trafos along the way...

Also pertinent is where along the loop the sources of generation tie in, if any, the impedance of their trafos, and the amount of real and reactive power emanating from / being absorbed by the respective site or sites....ditto loads.

I would not have the impertinence to site a "maximum acceptable closing angle" for paralleling circuits, ever; perhaps others with greater knowledge, qualification and experience than me can speak to the matter.

And Bill: in the instance you cite it would make a fundamental difference in synchronizing success whether the incoming generator is at a higher or lower frequency than the system, viz., a "fast" or "slow" scope; I'm ASSuming the units were always synchronized FAST...

CR

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

 

[cuky2000]

Most large generator manufacturers recommend to synchronize no more than 10o to avoid winding over stresses and cumulative mechanical fatigue.
Usually synchronization is donde with less than the recommended value and protected with a synchro-check relay (device 25) set properly to avoid abort or miss operations.
Modern units are equip with auto synchronization. Manual synch. is a little more challenger for the operator to meet the ISO requirement.

 

[stevenal]

I don't see any mention of synchronizing a generator in the OP. I think crshears has the proper interpretation and answer. Think of one short line closing into a long one wrapping around so that it covers a significant part of a wavelength. In general, I don't think you'll find lines long enough at 25 and 13 kV for it to be a problem.

 

[waross]

In general, I don't think you'll find lines long enough at 25 and 13 kV for it to be a problem.

And any such problem may be greater at 60 Hz than at 50 Hz.
However unequal series reactance between the parallel lines may be a more common issue.

I'm ASSuming the units were always synchronized FAST...

Scope running clockwise. True synchronism at 1:00 o-clock.
(One 'scope input from a line to line PT, the other 'scope input from a line to neutral PT.)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Palletjack]

This is a distribution system, tying the circuits together at or near the end of the circuit.
We could be 10 miles from one Substation, and 7 miles from another Substation. One could have 1/0 AL, the other could be 336 ACSR.

Same transmission line.

I’ve got anywhere from a 3 to 13 degree shift on my tie switch.

 

[davidbeach]

1:00? Is that Canadian for 12:00? ;-)

 

[waross]

Hi David.
Comparing the synchroscope to a clock face, the normal synchronism point is straight up at the 12:00 o-clock position.
In this set-up, the scope was at 1:00 o-clock when the sets were in synchronism.
Once the breaker was closed the "scope would jump to 1:00 o-clock. eg. synchronized.
But the operators didn't understand this and closed at the 12:00 o-clock position on the 'scope.
It had been that way for years. Lots of breaker trips and the occasional sheared drive key.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DM61850]

I have seen 20-30 degrees on the Transmission system. A 30 degree close in I think could be a pretty hard transient. I suppose it depends on your application and equipment. If you make it too small, you might not be allowed to close in when you would like, not that should dictate how you protect your system. MISO had a situation awhile back during their summer peak where a line tripped and they couldn't close it back in until that night when the load decreased and the phase angles came closer together.

You may never get close to 30 degrees if you have a strong system. In the Houston area, the difference between 60 miles north of Houston and down to Galvenston Island (about 50 miles south) during normal conditions was normally only 11-13 degrees. Angular differences develop as the angle * angle difference. (B*(theta1 - theta2) = theta - for small angles. Angular differences will develop if you are moving a lot of power and/or the system is weak. If the system is strong and you are not moving relatively large amounts of power. That is how strong and tight the system in Houston is. That doesn't mean that you can't have weak radials that will have more separation but normally in a tight system, the angular difference does not prevent a tie breaker from closing.

 

[davidbeach]

Bill - gotcha. Thought you were taking about a correct install, reading too fast.

 

[bacon4life]

Don't be so quick to assume 60 degrees is too big. Per the Arizona-Sothern California Outages Report pg 31, a setting of 60 degrees was too small and thus blocked restoration efforts.

Palletjack asked about phase angles due to line impedance, which is a totally different issue from synchronizing generators.

When transferring long 15 kV circuits, the phase angle difference prior to closing can be more than 20 degrees. The impedance of the long MV feeder typically limits the loop flow to a few hundreds of amps. On our 115 kV system, angles of close to 20 degrees are possible during certain configurations. On your system, ensure that the loop flow the switching device is within it's inductive switching rating.

 

[Palletjack]

We have a load break gang operated air break switch there now. I’m thinking about installing a tie breaker with a 651R and using a 25 element. Trying to decide what angle to set the 25 element.

 

[stevenal]

I fail to see a need to synch check if there is only one source. Use phasing sticks prior to the first closure after the breaker is installed to ensure proper construction.

 

[Palletjack]

Stevenal, I also fail to see a need to sync check if coming from source. Phasing sticks would be all that’s needed.
Go back up about 7 posts.. it’s different substations...

 

[stevenal]

Different substations fed from the same transmission line; same system/source. (assuming substation transformers have the same voltage and vector group)

 

[Palletjack]

Impedance, loading, and line length all play a part.
Their phase displacement is the same.

 

[stevenal]

Phase displacement as in different vector groups? Best fix that first.

So you need to make the tie, and your sync check relay says no. Now what? Wait for the loads to shift? Do you disconnect the loads most likely to be causing the phase angle difference? Adjust or block the sync check? In my experience, the need for tying circuits together is generally immediate.

 

[Palletjack]

The sync relay wordbit on a 651R is adjustable...
I know what the angles are with the 651R.
I can have a couple of degrees just in length alone.
A couple of more on Z.

I’m asking for experience based answers on the maximum angle you would make a tie at. 8 degrees? 10 degrees? 13.5 degrees?

Transmission can be closed in on upwards of 40 degrees. What about distribution 12.5kV or 25 kV distribution?

 

[stevenal]

In my experience, disable it. If the load shift is great, preset and lock off LTCs or regulators to minimize.