Parallel transformer synchronization 1

[newelecteng]

Hi all

We have 2 identical step down 33/11kv power transformers fed from single source and feed radial lods with no generation at 11kv side.

Does we need to have synchronisation between them in 11kv side

Thanks in advance

 

[waross]

No.

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

 

[newelecteng]

So we just close the tie beacker without any synch.

 

[davidbeach]

Before closing it you should do a phase check across the open switch. But once you've proved that the two sides are in phase then you can switch to your heart's content without the need for any synch checking for each operation. But phasing could be checked across any newly constructed or revised open prior to allowing it to be closed first time.

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

 

[newelecteng]

So we don't need any synch check in case of identical power transformer closing because there is no phase shift in the secondary

 

[davidbeach]

No dynamic phase shift, but a 120 degree shift might have been built in and that possibility has to be eliminated first.

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

 

[wcaseyharman]

Wouldn’t hurt to double check that the taps are set the same too if so equipped. If these have LTCs then each time you would need to make sure they are on the same tap. But I don’t think sync check would be necessary unless the circuit could be energized by another substation with different phasing or phase shift.

 

[stevenal]

Always check phasing across the open switch prior to closing it the first time.

 

[cranky108]

Also good to check phasing with other close by substations.

 

[newelecteng]

Thanks all

 

[crshears]

wcaseyharman wrote:

Wouldn’t hurt to double check that the taps are set the same too if so equipped. If these have LTCs then each time you would need to make sure they are on the same tap.

All the directions stating it's only but absolutely necessary to confirm phasing the first time are spot on.

As to paralleling transformers, I was trained to either adjust taps so that both sources are at as near the same voltage as possible, or, depending on the prevailing conditions, to actually have the voltage on the transformer to be loaded just a little lower than on the already loaded unit; that way, if the in service bank is carrying a very heavy reactive current, due to either heavily inductive or extremely capacitive loads, there will be little if any change to the voltage supplying the load.

Having the incoming transformer secondary voltage a little lower will tend to cause a small reactive current to flow through that transformer back to the source upon loading, compensating either somewhat or completely for the rise in voltage that will normally accompany the halving of the impedance between the system and the load when the second trafo goes on load.

Then and only then are the taps adjusted to place equal loads on them so as to minimize circulating currents or, if swapping transformers, to zero off as closely as possible the reactive flow through the transformer to be removed from service. Doing this minimizes the amount of current flowing at "zero crossing" and maximizes breaker contact life.

Naturally the corollary for this is that adequate metering must be provided to give the operators the conditions visibility needed to not only continuously monitor but also to adjust flows as necessary to enable optimal switching.

An extra benefit that accrues to providing this metering is that if there is a loss of cooling on one of two transformers in parallel service it becomes highly desirable to skew the taps so that there is as little reactive load as possible on the transformer with the cooling trouble, mitigating the temperature rise of that transformer until the staff can get there to see what the problem is.

CR

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

 

[davidbeach]

For a close tie, as would be closing something in the substation, the transformer LTCs should always be set for the closest ratio match possible, regardless of how well the voltages match. For ties that aren't close ties voltage matching becomes more and more important relative to ratio matching as the intervening impedance increases.

Assuming a close tie, if you match voltage rather than ratio you will have standing circulating current from the time the tie is closed until the LTCs are returned to auto (or somebody does a fantastic job of changing taps) but if you have a perfect ratio match (as would happen in a new substation with matched transformers on the same DETC tap and LTC tap) the loads will balance out as soon as the tie is close.

In older substations, with mismatched equipment, "close" has to be good enough on ratio matching because exact may be impossible.

Once you've got that down pat you are ready to move on to a substation where there's one transformer serving 13.2 kV feeder and another transformer serving 11kV load. Plus a voltage regulator on one side of a NO tie switch between one of the 13kV feeders and one of the 11kV feeders. Transformer LTC step sizes are different than the regulator step sizes. Takes a bunch of calculations that eventually became a spreadsheet chart that lines out the best matches of both LTCs and the regulator for going in both directions.

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

 

[crshears]

Hello David,

I wasn't going to go as deep as you did right off the jump; baby steps first, in my view.

That being said, I know exactly where you're coming from; "mismatched equipment" is what I've encountered my entire career, much of it generally a deal older than I am, which is saying something.

A number of our transformer stations have paired trafos with differing ratios and degrees of voltage regulation per step, with operator diligence having to be applied along the lines of "with this transformer on ULTC from tap 1 < > 6, run its companion three taps higher; 7 < > 12, run its companion four taps higher; from tap 13 < > 18, run its companion five . . . " you get the idea.

Same can apply with quadrupled inter-utility phase shifters; with different discrete shift angles per tap, increasing spreads at higher tap counts are de rigeuer. These can also be affected by heavier flows, where above a certain flow companion shifters have to be run with a one-tap spread in order to balance their loads back up and optimize power transfer capability. Throw into the mix that one of the shifters controlled by my utility was installed the other way round, meaning that on two of them you raise taps to increase export while the third you lower to increase export, and an operator must really bring one's "A" game and stay on one's toes.

CR

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