Synchronization of an alternator to the grid

[cnh1995]

When an alternator is synchronized to the grid, its voltage and frequency are same as those of the grid. So, it shouldn't take any load at that instant. How is it brought into operation then? What is the procedure followed practically? My book says prime mover is given some extra energy so the alternator advances in angular position and then takes up load. No further explanation is provided. What happens to the excitation? Is it also increased? Also, why is terminal voltage of the alternators connected to a grid constant? Doesn' t armature reaction affect the terminal voltage? Please explain. I'm having a hard time relating the behavior of an individual alternator to the one connected to an infinite bus.( Sorry if the questions sound dumb...)

 

[anorlunda]

hen an alternator is synchronized to the grid, its voltage and frequency are same as those of the grid. So, it shouldn't take any load at that instant. How is it brought into operation then? What is the procedure followed practically?

Normally it is given a small positive power at the start, just to avoid being tripped by reverse power relays (if any). Thereafter, power ramps at a rate and to levels according to a plan that fits the grid needs.

What happens to the excitation? Is it also increased? Also, why is terminal voltage of the alternators connected to a grid constant?

The voltage regulator manipulates field voltage to hold terminal voltage constant. Changing terminal voltage changes VAR flow. That is done according to dispatch instructions, to serve the needs of the grid as a whole.

'm having a hard time relating the behavior of an individual alternator to the one connected to an infinite bus.

My recommendation is to forget the individual case. Forget the case where is serves only one load. The only interesting case is when it is connected to the grid, and all the basic principles are revealed when studying one machine to an infinite bus.

p.s. To toot my own horn, the PF Insights article AC Power Analysis: Part 1, Basics, and parts 2 and 3 which will follow in the next 2 weeks, give insight about grid operations from the grid's viewpoint.

 

[cnh1995]

Thanks a lot for the reply.

Normally it is given a small positive power at the start

Does that decide the value of load angle δ?

The voltage regulator manipulates field voltage to hold terminal voltage constant.

This is done automatically, right? So is it correct to say:without a voltage regulator, it wouldn't be possible to hold the terminal voltage constant?
How is "need of the grid" analysed? Using SCADA?

p.s. To toot my own horn, the PF Insights article AC Power Analysis: Part 1, Basics, and parts 2 and 3 which will follow in the next 2 weeks, give insight about grid operations from the grid's viewpoint.

That would be really great! I'm eagerly waiting..

 

[cnh1995]

In theory, we have studied alternators assuming balanced loading(equal load per phase). In practice, is it possible that the load is always balanced? What happens when the load is not balanced?

 

[anorlunda]

Does that decide the value of load angle δ?

Real power delivered to the grid is proportional to angle, and that relationship between power and angle is perhaps the most important thing to know. Read part 2 of the insights article next week.

What happens when the load is not balanced?

That's complicated. It gets you into positive/negative/zero sequence three-phase currents that are difficult to explain or understand.

Suffice it to say, in normal operation the phases are always nearly balanced. It is when we get a short circuit on one phase that they get very seriously out of balance.

 

[cnh1995]

Suffice it to say, in normal operation the phases are always nearly balanced

But balancing of phases depends on the load, right? So, is it right that the phases are "kept" balanced by some automatic system? I mean if phase R is feeding power to three loads A,B and C and if A is switched off, then some power of phase A is diverted to some other load such that the phases are balanced..something like that.?

 

[anorlunda]

o, is it right that the phases are "kept" balanced by some automatic system?

No, not actively. Distribution engineers try to assign houses to circuits in such a way as to hopefully balance the phases most of the time. We also transpose the conductors of very long lines every X miles to help keep balance.

It is far from perfect. There is no automatic system.

 

[anorlunda]

http://i.imgur.com/YbSVvRW.gif
Here is a dramatic way to get unbalanced phases.

 

[jim hardy]

Here's somereally old threads. Maybe they address some of your alternator questions.

https://www.physicsforums.com/threads/armature-reaction-drop.826513/#post-5191404

https://www.physicsforums.com/threads/synchronous-generator-excitation.829603/#post-5214090

https://www.physicsforums.com/threads/ac-generator-on-grid-governor-and-amp.759387/#post-4785338

 

[cnh1995]

Here's somereally old threads. Maybe they address some of your alternator questions.

Thanks a lot Jim! That was really helpful, especially the third thread.

This simple little diagram has helped me more than once. Your way of explaining things is really amazing!
Just a couple of questions..
When steam supply to the turbine is increased, mechanical torque on the rotor is also increased, which makes it advance in angular position and take up the load. But what makes it rotate at a constant speed after providing extra torque? Is it the "motor action" of injected reactive current that creates an opposing torque?
Also, when the grid injects reactive current into the machine, total flux remains constant and hence, terminal voltage remains constant. Then when exactly are AVRs used? The voltage is kept constant by the grid itself. I know AVRs change field excitation and changing the excitation changes the power factor but since the voltage is held constant by the grid itself(by injecting reactive currents), how are AVRs used practically?

 

[jim hardy]

But what makes it rotate at a constant speed after providing extra torque? Is it the "motor action" of injected reactive current that creates an opposing torque?

Motor action is exact right term but the current that produces torque is the real component not the reactive...
Note in that phasor sketch above armature and field fluxes are nearly perpendicular so torque results. Were they aligned, they simply aid or oppose instead of making torque.

Also, when the grid injects reactive current into the machine, total flux remains constant and hence, terminal voltage remains constant. Then when exactly are AVRs used?

AVR is on nearly all the time.
Have you studied "Armature Reaction" ?
If not it's okay.

AVR holds terminal voltage constant when load on the grid changes.
If you just hold constant excitation, terminal voltage will sag when customers get home and switch on their airconditioners.
In simplest terms, a utility generator is like any other voltage source it has internal impedance. In fact, quite a bit of it and it's mostly inductive.
Think of the generator as a Thevenin, an internal ideal voltage source in series with that internal impedance.
The AVR measures terminal volts and adjusts the internal source so as to cancel the drop across that internal impedance.
That keeps customers' voltage nearly constant.

The voltage is kept constant by the grid itself.

Hmmmm i sort of think the other way
the grid is held constant by the combined actions of the hundreds of AVR's working to that end.
Were a generator actually tied directly to an infinite bus you're right, it could not change terminal volts.

Go back to the thevenin idea for a moment

V_bus is fixed, so is Z_internal

If you change V_internal by changing excitation,
machine amps will change by ΔV_internal / Z_internal
and Z_internal being (mostly) inductive and fixed, Δmachine amps will be reactive. That's why voltage regulator controls Vars. AVR's keep each machine's Var contribution constant, too

All the AVR's acting in concert is what keeps V_bus nearly constant.and the system stable.

There are whole books on this subject
i hope this helps prepare you for Anorlunda's insight article. I'm no power engineer , just try to help folks over the basic stumbling blocks i crawled across.
You'll get to where you work the system in your head. Hang in there. Does your school have a machinery lab? It is very instructive to shine a stroboscope on a generator shaft and watch the angle change with load and excitation, then figure out why.. That was my favorite EE course - we had ~10hp machines and dynamometers...

old jim

 

[cnh1995]

Were a generator actually tied directly to an infinite bus you're right, it could not change terminal volts.

Right, I was confusing ideal infinite bus with a practical grid.
So, in armature reaction, component of armature mmf which aligns with the field mmf is responsible for the sag in terminal voltage(due to demagnetizing action). Increasing the excitation will generate a reactive current in the machine.

and Zinternal being (mostly) inductive and fixed, Δmachine amps will be reactive.

This reactive current will compensate for the demagnetization and keep the terminal voltage constant. Is this correct?

 

[cnh1995]

Does your school have a machinery lab? It is very instructive to shine a stroboscope on a generator shaft and watch the angle change with load and excitation, then figure out why.. That was my favorite EE course - we had ~10hp machines and dynamometers...

We have all the equipment in the lab but unfortunately, no one's curious enough to be interested in such experiments. Everyone's just satisfied with the lecture notes and routine practicals(which are nothing but rote "procedure", followed by blind copying of the readings and somehow completing and submitting the journals so that the term is granted..). Sometimes I think they're making lab sessions as useless as possible.

 

[jim hardy]

This reactive current will compensate for the demagnetization and keep the terminal voltage constant. Is this correct?

Chicken-egg ?

Talk it through
the thought experiment was machine tied to infinite bus with fixed voltage on bus
so terminal volts are fixed as an initial condition
let's say we raised excitation
for our mental model the Thevenin internal voltage is what we raised and that pushed vars into the infinite bus
but Thevenin voltage exists only in our mind (and on the phasor diagrams i used to dread).

I'd say it this way

Because the terminal voltage is fixed by the bus. , this reactive current will compensate for the cause a demagnetization or a remagnetization that offsets the change in field current.

Maybe magnetizing or demagnetizing effect, i don't know - i polish words for days afterward...

Anyhow if you grasp that concept you have a leg up on Armature Reaction. Most texts just give you formulas but when you can work it in your head the formulas seem almost intuitive.

Over for the night - TTFN

old jim

 

[cnh1995]

Over for the night - TTFN

Thanks a tonne Jim for putting up with me for so late! Google showed me it's almost 2:00 am in Arkansas.. Have a good night!

 

[sophiecentaur]

Sometimes I think they're making lab sessions as useless as possible.

Speed of the slowest ship, probably.
Look around you and you will often see teachers who just couldn't handle the really interesting experiments that can be done with receptive students. In the UK, 'Science' can still be taught by non-specialists who only know about Biology or Chemistry and virtually nothing about Physics. Hence, the programmes of study include only rock bottom practical exercises and there's virtually no time for an inspired teacher to fit anything else in.
(Glad I'm out of it, to be honest).