Power generation and ties to the utility grid

[bdn2004]

I work at a power plant that supplies power to a large industrial plant. It's generator's are also tied to the utility grid.

I was told that without the ties to the utility grid, the power plant could not function. So these ties are essential. Someone mentioned that it has something to do with the synchronicity of the generators. Why is this?

 

[tem1234]

hi

If your generator is asynchronous, it's true.

If it is synchronous, you can function without the grid.

Asynchronous aren't excited by themself, so they need the grid to excite them.

 

[jraef]

You probably have induction generators, not synchronous. Any induction machine can become a generator if it has two things:
1) mechanical power to drive it at super-synchronous speed
2) excitation of the windings

So f you have only AC induction generators, you must have a source of that excitation current for the windings in order to make them generate. When you hook up to the grid, the reactive element of the grid power supplies that current to the windings, so then all you need do s over-drive the machines and they start pumping power back to the grid.

If you had synchronous generators, you could have a source of DC power to put into the field windings and start generating on your own in "island mode" meaning that you supply your own excitation power and would not need to connect to the grid.


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[bdn2004]

Thanks. Attached is an abbreviated sketch of the generator circuit.

The exciting transformer is connected to the output of the generator, it's not connected to the grid. Can you shed anymore light on what we have here? Thanks.

 

[rcw retired EE]

Many reasons maybe.

The generator prime mover (gas turbine, steam turbine) may not have the governor controls capable of matching ouput to the MW required by the plant. Usually the utility tie takes care of the variations between generated power and actual load.

Or the plant load may be larger than the generator rating with no easy means to shed load when the utility is lost.

Maybe your plant scheme is too complicated for the generators to be able to figure out if they are islanded with the load or running parallel with the utility. When the utility is on line, the generators have to run in droop mode. When the utility tie trips, the turbine governor has to instantly switch to isochronous (contant output frequency) mode. Wihtout a fast signal, the generator may trip out on overspeed (if generation > remaining load) or under frequency ( if load > generation).

Maybe there is a fuel supply system that is fed only from the utility. (For example is the generator powered by excess steam from a process that shuts down on loss of utility?

 

[rmw]

Maybe the plant needs more vars that the generator(s) can provide so they have to be imported from the grid.

Maybe the plant has large starting loads that the generators can't sustain so the grid has to maintain the frequency during starting large loads or load upsets.

rmw

 

[waross]

I like droop mode. It is quite forgiving and an islanded plant may run in droop mode. There will be a little frequency change with load change but it is often not noticed by the users. Older isochronous sets responded to load changes with droop characteristics and then corrected the frequency to 60 Hz.
If the plant is unable to run islanded, it is probably a plant specific issue. There may be issues synchronizing with the utility after an outage. Your synchroscope and/or syncro check relay will be across the breaker used to place the generator online. I expect that the generator is placed online with a breaker between the generator and the load bus.
In order to run the plant and then recover from a utility outage I would expect to use a breaker on the line from the 138kV utility feed and the 15 kV load bus. This breaker would have to have synchronizing gear.
Add to this the issues mentioned by others.
The plant load may exceed the generator capability.
The plant load may be less than the generator can supply in a stable manner.
There may be plant specific issues with load shedding and block loading.
I am sure that islanding was considered and rejected by the design team for some good reason.
You may be limited by a lack of black start capability.
The simple answer is "Yes, your generator may run islanded" The rest of the story is probably that there are issues with islanding that were considered and a decision was made to not resolve the issues to allow islanding.
If synchronism is mentioned, I suspect that you lack the gear to recover from a utility outage.


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

 

[benita22]

When synchronising a generator what are the factors to be considered

 

[byrdj]

Matching;

-voltage (adjust excitation so gen output voltage matches across breaker)

-frequency (adjust driver speed to match grid)

-phase (verify the wave output timing matches the other side of the breaker. ie the wave peaks occur at same time and thus nill instantanoius voltage differtial accross the breaker)

 

[waross]

What byrdj said.
A a basic synchroscope just checks phase relationships on one phase. During initial commissioning the phase rotation must be verified as correct. In some installations a phase reversal relay is required.
Phase angle is more important than voltage. If the voltage is off it will first affect the production and/or sharing of reactive current. If the phase angle is off it will cause physical stress as the generator is abruptly pulled into sync by the system or the other generators. This will also entail high currents, much more so than voltage disparities cause.

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