Sychronization of Sources from different Utilities

[NickParker]

I understand incase of Generator synchronization with Utility/grid, the generator parameters are controlled from the inputs of Synchronizer. How this is done if we have to synchronise two different sources from Utility?

 

[waross]

Power from two utilities in the same system will be inherently synchronized.
You may need a phase shifting transformer to control the direction of power flow.
If you are connecting two separate systems, then rectify and then invert.
You may be able to use a motor driving a doubly fed induction generator.

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

 

[Fischstabchen]

Waross,

Power from two separate utilities in the same grid won't necessarily inherently be synchronized.

 

[cranky108]

Normally they would be synchronized, but, sometimes life is not normal. Assume they are not synchronized, and be expecting to explain, later, why you assumed that.

 

[waross]

The frequency will be the same.
When long distances and distributed generation are involved there may be phase angle errors that may be accommodated with a phase shifting transformer.
If one utility includes both generation and distribution (Some do not) and becomes islanded, you will have a problem but, by definition, if the utility is islanded, it is no longer part of the grid.
In that case your protection would disconnect you from one of the utilities.
You will not be able to synchronize safely until the utility re-establishes connection with the grid.
An extreme example.
You have a direct, dedicated transmission line, hundreds of miles long, to your plant. The line bypasses a lot of distributed generation on the grid.
You may need a wide range of adjustment on a phase shifting transformer to bring the phase relationships within a usable range.
To put it into perspective, The Eastern Interconnect stretches from Central Canada to Florida. There was a thread on this site that mentioned the phase shift across the length of the grid, but phase shift should be slight to none between adjacent utilities.
You asked about synchronization so I won't go into DC links other than to say that a DC link typically transfers power without regard to relative frequencys or phase angles.

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

 

[crshears]

How this is done if we have to synchronize two different sources from Utility?

Hmmm; the replies so far have been interesting, to say the least . . .

Power from two utilities in the same system will be inherently synchronized.

I must agree with Fischstabchen here; it ain't necessarily so.

You may need a phase shifting transformer to control the direction of power flow.

This would work, but it would have to be equipped with under load tap changers, and these types of transformers are a truly expensive piece of kit.

The OP did not specifically mention interconnecting two utilities, so the way I initially read the question was to parallel two different sources from the same utility . . . but I may have been wrong.

If in fact interconnecting sources from two utilities or grids was actually intended, Bill's suggestion of using some manner of asychronous tie would work very well and pose no major technical issues; all AHJs would have to be on board with this, though, and getting the required approvals could be a tough sell. The same proviso would apply to use of a phase shifting transformer.

As to directly connecting such sources, I would answer that other than in extremely exceptional circumstances, this is something you should never, not ever, not even in your wildest dreams, ever think of doing, for the following reasons:

If in fact there is already some other path of interconnection between the two utilities in question, there may well be a standing connection of some fixed phase angle across the device you wish to close.

If those two standing-angle sources are in fact already in phase, when this connection is made there will be two parallel paths by which grid power will flow; the division of power flow between them will be inversely proportional to the impedances of the two paths, and generation dispatch on the grids will have a material effect on such flows as well.

As a result, you site may well serve as a transfer path for very high system power flows, in other words your equipment may be heavily overloaded with a power flow that dose not benefit you at all as a customer.

If the standing angle sources are not in phase, a phase shifting transformer may be required to get them into phase; see above.

If there are two unsynchronized utilities and you parallel these via your system, any generation or load mismatch between the two systems will attempt to equalize itself through your switchgear; not a pretty prospect.

CR

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

 

[FacEngrPE]

The fnet angle contour map shows how the angle changes across the eastern grid. There are not many places where angle alone would trip the protection under normal conditions.

Even when in phase the voltage differences in different paths can create large circulating currents. Always call utility power dispatch, so they can close their substation tie first.

crshears explains the results of not paying attention well.

 

[crshears]

Good points; but there may not always be a "substation tie" . . .

When we as power system operators have to perform load transfers, the vast majority of the time we are able to do the transfers on a make before break basis, ensuring that there will be no customer interruption.

Since earlier this year when the transmission and distribution functions at my utility were separated, it has become necessary for our distribution operators to confirm the presence of a reasonably short high-voltage path from one side of the device to be closed to the other. There are some occasions when the length of the loop is measure in several hundreds of kilometres, and particular care must be taken beforehand to calculate 'maximum safe break" values for the devices to be used to break the parallel - or, to be more precise, to open the loop. Manipulation/re-dispatch of generation in real time while monitoring flows is occasionally required; I once posted in these fora about a high-voltage path that formed a loop around Great Lakes Superior, Michigan, and Huron.

The "push" of generation from the Bruce Nuclear Power Development down to the Greater Toronto Area used to wreak havoc on a portion of our 44 kV sub-transmission/distribution system; form any years certain customers had to be interrupted during transfers from North supply to South and vice versa; unfortunately this was discovered by brutal experience, when the two sources were put into parallel, the currents on the two feeders in question immediately rose to intolerable values - and when in a panic an attempt was made to break the parallel using a standard air break switch, the switch literally melted to destruction, since the arc did not break, eventually tripping both feeders out of service on overcurrent. Heavy duty load break switches were eventually installed when the customers began complaining of numerous and "unnecessary" interruptions.

CR

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

 

[Palletjack]

Not sure if your talking about transmission level or distribution level.
From the distribution level ties from different circuits coming from the same substation is easy. Check the phasing at the tie pole and tie them together. The lengths generally aren’t so great as to cause problems.
When tying two circuits from different substations care must be taken to make sure displacement is correct.
It’s easy on initial connect to rotate through a transformer the wrong way and end up with 60 degree displacement which makes hot ties impossible.
Get the displacement correct the ties between stations are again, easy enough.
Angular differences should be at a minimum here, generally out due to line length and power factor.
From the transmission level it’s possible to have some small angular differences due to different generation, and these can be closed in at larger angles than distribution.
Transmission generally 8-10 degrees is max, distribution is generally 3-5 degrees max.

 

[cuky2000]

I do believe that a single generator can be able to synchronize to one source at a given time.

Assuming that the generator has a transfer scheme with a set of permissive synchro check relay (device 25) that will verify that voltage, phase angle, frequency, and phase rotation across two sides of a breaker are the same prior to closing the breaker.

A disconnect from one source will be able to connect to the second source following the synchronization procedure similar to the one shown in the enclosed Link

 

[itsmoked]

Such a cool site FacEngrPE!!

Freq movie of a SW blackout. Note the right side frequency bar.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Without meaning to contradict other valuable information may I clarify my posts.
I made an assumption that in order to consider connecting to two utilities, both utilities must be close by, on the same grid and are fed from the same sources.
I assumed that a delta/wye transformer may be added if needed to line up the phases.
That leaves possible minor phase errors with different length feeders with different PF loads. A phase shifting transformer or similar device may be needed.
In the event of an outage by one utility, your paralleling gear may be overloaded.
Will a private customer ever do this.
Probably never.
On the other hand if one utility finds itself with more load than capacity in an area close to another utility, they may go parallel with the other utility as part of a power wheeling arrangement.
Protection and power sharing will be issues to be resolved on a site by site basis.
Yes, I know. Never assume.

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

 

[crshears]

Ah.

CR

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

 

[cranky108]

It is rare to be connected to two different utilities at the same time and at the same plant.
The one time I have seen it was at a large waste water plant that was in two different certified utility areas.
The two different substations were both connected to the same subtransmission line.
That said, the two sources would have been in phase, but because they required a delta/wye transformation, the two sources may have been at 60 degree angles, to each other. I never look at this from the customer perspective, so I am not sure what the customer saw.
Frankly, I was not involved with the design of either of the substations. Just the protection of the subtransmission line, and one of the substations.

 

[waross]

Thanks for that cranky.
That is what I visualized when I made my original post. I was lazy and assumed that there would be offsetting delta/wye transformations to avoid the 60 degree offset.
That mental picture was the basis of my statement that the utilities would be inherently synchronized.

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

 

[cranky108]

That said, my present company does, for a fee, provide two feeds to some customers. But because we do from time to time tie feeders together, we do ensure the feeders are in phase.
From different companies, that may not be true.

My old company did intertie with self generating small towns, so it is possible that the two sources from different companies would also be at different frequencies.
That was because many of the small town would purchase on an interruptible rate, and we would interrupt them from time to time to keep the REA's from asking for the interruptible rate.
Yes the games we play.