AC Mains Distribution energized from multiple AC voltage sources

[arkaddas]

Even though its possible to keep a bus charged from more than one AC voltage sources, what can be the consequences of doing so?
My thoughts: - Assuming thst both the sources have similar (but not equal) phase and magnitude, we can expect small circulating currents. Which brings into question the nature of these currents. I think the vector difference of the voltages will give us a good idea about it.
Now, it will also be possible for current to flow into the bus from transformer or even motor feeders depending on the voltage level of the bus...

 

[scottdave]

Are you talking about a panel inside a building, or something like at the power grid level? Yes there would be some current flow if the voltages are not exactly the same.
There could be safety concerns, as well - unless care is taken. Suppose you tie 2 sources together, as described. Call them A and B. Now suppose that somebody needs to do work on a circuit, and disconnects the circuit between them and Source-B. Depending on the configuration, their circuit may still be hot, if it is now being fed from Source A, through the bus tie.

 

[arkaddas]

Are you talking about a panel inside a building, or something like at the power grid level? Yes there would be some current flow if the voltages are not exactly the same.
There could be safety concerns, as well - unless care is taken. Suppose you tie 2 sources together, as described. Call them A and B. Now suppose that somebody needs to do work on a circuit, and disconnects the circuit between them and Source-B. Depending on the configuration, their circuit may still be hot, if it is now being fed from Source A, through the bus tie.

I'm talking about substation level, say 415V or 6.6kV switchboards.
I'm more interested in how the system would behave electrically and be affected by such a configuration. I do understand the safety concerns which you mentioned. It is certainly a good point.

 

[Windadct]

• IMO - for AC it is best to use the term Energized,vs Charged.

OK -so an AC bus energized by 2 or more sources. The sources need to be very well matched, in both voltage and inductance to prevent any pass through current. AC bus is very low inductance, so small voltage difference will yield high currents.

Then - probably the biggest issues is available fault current and back feeding of one source to the other through the bus..

Relay / protection coordination needs to account for all possible system configurations.

 

[Svein]

I'm talking about substation level, say 415V or 6.6kV switchboards.
I'm more interested in how the system would behave electrically and be affected by such a configuration. I do understand the safety concerns which you mentioned. It is certainly a good point.

You will also need to consider the phase of the two sources.

 

[sophiecentaur]

You will also need to consider the phase of the two sources.

You also need to be aware of the LOAD on the bus and the fact that the two alternators will be voltage stabilised to some degree. From what I know about alternators, if one is dishing out more volts than the other one then the lower source can be driven by the higher. The devil is in the detail and the initial description of the situation is incomplete.
If the two generators are 'ideal' then you have a smoke making scenario, in which the connecting cables must carry an infinite amount of current (two pure Voltage Sources will fight to the death). There is no more (useful) to be said about that situation. In a real scenario there can be many different conditions. A properly managed system (with the alternators properly synchronised) will involve sharing of the current into the load(s) between the two generators. It's all down to the regulators on the alternators (how many buckets of coal per hour into the boiler has to be controlled for each power station).

 

[Svein]

That brings me back...

Some years ago I did some research into using Ethernet in substations and as a side effect I had to learn the necessary response tolerances for the measurements. Just scribbling some numbers showed me that a 500kV line (at 50Hz) rises (or falls) 157V/μs at zero-crossing time(!).

 

[arkaddas]

• IMO - for AC it is best to use the term Energized,vs Charged.

OK -so an AC bus energized by 2 or more sources. The sources need to be very well matched, in both voltage and inductance to prevent any pass through current. AC bus is very low inductance, so small voltage difference will yield high currents.

Then - probably the biggest issues is available fault current and back feeding of one source to the other through the bus..

Relay / protection coordination needs to account for all possible system configurations.

Oh thanks for the update.
So if two different sources energise the bus, the source at a lower voltage will draw current instead of actually acting as a source, though the amount of current would be proportional to the voltage difference.

I think this will be the correct representation of the system.
Let's consider a situation in which the sum of feeder current is say 100A. If the voltage sources were identical, then this load would have been shared as 50A for each source. In our system, what will be the distribution?
Will it be entirely supplied by source 1 (including the current that source 2 draws) from the bus?

 

[arkaddas]

You will also need to consider the phase of the two sources.

Yes of course, considering both phase and magnitude.

 

[arkaddas]

You also need to be aware of the LOAD on the bus and the fact that the two alternators will be voltage stabilised to some degree. From what I know about alternators, if one is dishing out more volts than the other one then the lower source can be driven by the higher. The devil is in the detail and the initial description of the situation is incomplete.
If the two generators are 'ideal' then you have a smoke making scenario, in which the connecting cables must carry an infinite amount of current (two pure Voltage Sources will fight to the death). There is no more (useful) to be said about that situation. In a real scenario there can be many different conditions. A properly managed system (with the alternators properly synchronised) will involve sharing of the current into the load(s) between the two generators. It's all down to the regulators on the alternators (how many buckets of coal per hour into the boiler has to be controlled for each power station).

Only considering practical situations here. You've partially answered another of my doubts in this reply. The load sharing will be as you say, dependent on the alternator regulators. What about transformers though?

 

[sophiecentaur]

What about transformers though?

I think all a transformer between alternator and bus will do is to transform the impedance of the alternator that the bus sees. Assuming it's a 1:1 transformer, there will be no difference. I would think there would be a momentary change of alternator speed to re-establish the appropriate phase. If there is some step up /down involved then the regulators would come into play again.
I imagine @jim hardy Hardy could help here but I expect that in a real system, one alternator would be chosen as Master or there could be endless oscillation as one chases the other and back again. I think he has probably even done this with a pair of Nuclear Power ~Stations .

 

[jim hardy]

I imagine @jim hardy Hardy could help here but I expect that in a real system, one alternator would be chosen as Master or there could be endless oscillation as one chases the other and back again. I think he has probably even done this with a pair of Nuclear Power ~Stations

Briefly,
There are impedances in the feeds to your bus. Current flow will be in strict accordance with the laws of Kirchoff and Ohm. You should be familiar with phasor notation to really enjoy them though.

A transformer has impedance so its output voltage drops (usually) a bit when current increases.
Impedance is usually given not in ohms but in "Per Unit" expressed as % . Let's just say 10%, it'll be given on the transformer nameplate.
All that means is that in changing current from no load to full load, the voltage drop across the transformer's impedance will reduce its output voltage by 10%.

So your bus will receive current from its sources according to their voltages and respective impedances.

10% is a not atypical number for a transformer. We had big ones ranging from 3% to 17 %
For a generator there's "Synchronous Impedance" and 150% is a not atypical number. The voltage regulator is usually adjusted to correct for that and reduce effective impedance to less than 10%. We set ours for 6%.
Transmission lines i don't know for sure, i think 3% is a typical number @anorlunda would know better than i. So things tend to self balance.
Indeed currents can and do circulate.
In our two unit control room raising excitation on one machine changes current in the other one by circulating current out through the switchyard.
Same thing applies to the whole system, changing excitation in Miami affects generators in West Palm Beach. So the system dispatchers request plant operators to adjust voltage so as to keep the system in harmonious balance.

This chart shows real time phase angle between regions of the country. No it's not perfect but it demonstrates the grid is a living breathing dynamic thing.
http://fnetpublic.utk.edu/anglecontour.html

@anorlunda wrote an insight
and this might be of help
http://education.curent.utk.edu/wp-content/uploads/2014/06/Fundamentals-of-the-Electric-Grid.pdf

Yes, we once used a nuke unit as a source to search for resonant frequency of our grid. It was 2/3 hz. Study of harmonic motion tells us you can get sustained even divergent oscillations anyplace there's inertia and energy involved. Mother Nature just loves a sine wave.

old jim

 

[sophiecentaur]

Mother Nature just loves a sine wave.

And to make us fall over!
Cheers for that Jim.

 

[jim hardy]

And to make us fall over!

Thank you Mr Sophie

When i was in school power was rather looked down upon as 'low tech'. With space age dominating the news communications and computers were the glorious majors. Of course when we think of 'The Grid" we imagine stationary power poles and gray dull looking substations. We do not realize that those wires are the electrical equivalent of rotating shafts and the substations the analog of gearboxes.
"The Grid" is a single interconnected machine spanning the continent . As with any machine so huge it must be maneuvered gently. I had the great fortune to look over the shoulders of a couple of industry giants for a glimpse into their world. It is far from low tech or boring.
But as you'd expect with such huge machinery it takes a decade to become competent .

So i encourage people who have good math skills , patience, and inquisitiveness to take courses in power system engineering and feedback controls.
It's going to be an interesting decade in that field as "The Grid" changes from high inertia steam turbine generators to electronic converters for photovoltaic and wind generation neither of which seems to me capable of mimicking the inertia that keeps things stable.

I was just a tiny cog in the big machine, lucky enough to catch an occasional glimpse of its grandeur.

There's plenty of room at the top.

old jim

 

[arkaddas]

A transformer has impedance so its output voltage drops (usually) a bit when current increases.
Impedance is usually given not in ohms but in "Per Unit" expressed as % . Let's just say 10%, it'll be given on the transformer nameplate.
All that means is that in changing current from no load to full load, the voltage drop across the transformer's impedance will reduce its output voltage by 10%.

So your bus will receive current from its sources according to their voltages and respective impedances.

old jim

I see. That makes things a lot clearer to me. I'm not getting into the calculations but on a rough approximation, I understand that the currents in the secondaries of both transformers will adjust themselves in such a manner (assuming the phases are in sync, for now) that the voltage at the output terminals of the secondaries will match. Is that correct?

...I could never grasp the essence of the per unit system because it's just hard to imagine it. What I know is that the pu impedance is like a relative representation of the original impedance with respect to a base power and voltage. According to what you're saying, the percentage impedance of a transformer is the pu of the transformer expressed in %. We can find the impedance of a transformer by adding the primary impedance with the secondary impedance transformed to the primary. After that, we can find the Z_pu by using the Z_base. However, I don't see how %Z is connected to Z_pu.

 

[arkaddas]

@sophiecentaur @jim hardy I have a lot of respect for the power industry and I see an enormous wave of change coming as renewables gain ground. I am curious about how the transition will bring new challenges to it. What could be the emerging areas of research influenced by the foray of renewables?

 

[jim hardy]

We can find the impedance of a transformer by adding the primary impedance with the secondary impedance transformed to the primary. After that, we can find the Zpu by using the Zbase. However, I don't see how %Z is connected to Zpu.

Per Unit is confusing at first. But it SOOO simplifies calculations that it's universally used.

It really is as easy as i said - a 5% transformer at 100% load will experience a 5% voltage drop. You don't have to convert to amps and ohms and volts .
Similarly a 5% transformer with a short circuited secondary will deliver 20X full load fault current .

It's handy enough you should work some examples.
I oversimplified a little, you have to choose a VA base. By agreement utilities use 100MVA base.

 

[jim hardy]

What could be the emerging areas of research influenced by the foray of renewables?

The obvious one is power system control and stability. I'm not very good at predicting the future, though .

 

[anorlunda]

@Arkansas, that would be a fun topic. Start a new thread if you want to discuss it

 

[jim hardy]

...I could never grasp the essence of the per unit system because it's just hard to imagine it.

Ooops i intended to post this link

http://www.powersystem.org/media/articles/pse-fall15-web.pdf

it shows how the common base makes absolute value of ohms unnecessary.
If something is too small to pass base MVA its per unit impedance will calculate out too high to allow it.
But it's the sort of thing i have tp practice in order to gain confidence it works. It's very tedious and counterintuitive at first; well for me anyway.

old jim

 

[jim hardy]

@Arkansas, that would be a fun topic. Start a new thread if you want to discuss it

@arkaddas i fixed @anorlunda 's tag for ya