Early Days of the Power Grid 3

[Mbrooke]

How did the power grid function in early 1900s by not being interconnected? How well did generation, such as a hydroelectric facility survive faults? Was step distance relaying or differential used on transmission feeders?

 

[waross]

Cuky, your graph shows the thermally limited power density as proportional to the number of phases.
eg: The thermally limited power density for six phases is twice the thermally limited power density of three phases.
Six conductors is carries twice as much as 3 conductors or 200%.
But the surge impedance limit is only 173%
Not quite as good as two three phase circuits.
They converted two circuits and got 173% of the capacity of one three phase circuit.
200% as much conductor and more expensive towers for a 173% gain?
I admit that there may be an advantage in restricted right of ways, but why use special expensive towers for a 173% gain when you can use twin circuit towers for a 200% gain?
I admit that there may be lower impedance when the conductors are in a hexagonal arrangement but that will be offset by the added impedance of the delta wye transformers.
In a relatively short time the industry gained a 173% advantage by changing from delta to wye distribution.
Now I am told that industry is turning their back on another 173% gain?
I saw a note that I can't find again. It seemed to indicate that this project was a pilot project for a planned 400 kV line.
The 400 kV six phase line was never built.
I have spent several hours searching for any information as to whether the original circuits were delta or wye.
Can anyone help with this?
Corona: one part of the information claims less corona with six phase.
In another place it indicates that the voltage was lowered because of corona concerns.
I repeat (with a slight edit)
Energized in 1992, de-energized in 1995. Never done again. That says it all.
You just spent over $10 million and did not get the dramatic results that you hoped for.
Now write the report and try to make yourself look as good as you can.

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

 

[Bill West]

Ah, now I see where the 493 pages are obscuring things. The utility did not have a need to upgrade this particular line, it was just offered as a convenient guinea pig for trying the concept. The concept's starting point was where someone already had twin circuit 115kv, needed more capacity and the ROW was too narrow to add a third line or increase the voltage. Running the two lines as one and adopting the 6 phase scheme enabled shifting the phase angles so that the voltage between adjacent conductors was reduced. Then the line to line voltage could be raised to resume using that insulation capacity and deliver more power. All while staying within the existing ROW and without all new towers, insulators and conductors. The 173% would be the increase over the previous two circuit capacity. I'm thinking they might have gone as far as 199kv if the corona would have permitted it but they used well known 161kv anyway so that the transformers could be resold after the experiment. I too would have to read the whole 493 pages to be sure.

Bill

 

[waross]

After 493 pages there is too much information not given.
Too much apples and oranges comparisons.
Too many inconsistencies.
Overall, there may be some advantages to a six phase system.
There are also many disadvantages.

By the way, is a two phase transformer a misleading name for a center tapped transformer?
I want one. I will be able to run an antique two phase motor without a Scott transformer.


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

 

[Mbrooke]

Great replies, and I love this debate for far!

 

[Bill West]

Something recently pointed out to me that people whose knowledge has just reached 3 wire 120/240v in homes and light commercial sometimes refer to the L1 and L2 terminals in a panel board as phases and thus a 2 phase system. Those of us coming from the other end of the scale understand 3 phase and of course call home 3 wire systems single phase. So I think all you are seeing with center tapped transformers is mis-description. I Googled two phase transformer and the first hit was exactly the misunderstanding you are questioning. When real 2 phase was around in 1890-1910 pretty well all transformers were single phase so I have not seen a real 2 phase transformer.

I also have not seen a trick transformer connection for creating 2 or 3 phase from a single phase source, only the other direction. If you are trying to run an old 2 phase motor on your home single phase supply you might buy or home build a 1 phase to 3 phase rotary converter and then settle for a Scott connection to cut it back to true 2 phase.

Bill

 

[waross]

Hi Bill
That was a rhetorical statement for those who insist that a center tap is two phase. grin


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

 

[Bill West]

Oh crap, I wrote all that just because I was being too serious?

Bill

 

[Mbrooke]

In the IEC world its frequently called two phase.

 

[cuky2000]

Hi Bill,
Thanks for your post challenging the viability of the High Phase Order (HPO) benefits particularly with the economics associated with this type of conversion. Although no very popular, HPO is one of the only few options available to increase the line power transfer capability without changing the line’ This could be valuable in any part of the congested power grids where permitting of new lines are virtually impossible to obtain without count how expensive can be a project. Here is an attempt to answer some of your inquiries in the following posts:
Let's clarify: If the phase-to-phase voltage remains the same for the 3 and 6 phase system, the power capacity of both system remain identical. However, if the phase-to-ground voltage and the rated current is the same in both systems, the phase-to-phase voltage on the 6-phase system could be increased up to 173% allowing the line to be up-rated up to the same ratio. (see sketch below for illustration)
[sub]The characteristic of the line needs to be considered. Short lines will operate in the thermal rating while longer lines will be derated and will be driven by the Power Delivery capacity dictated by the Surge Impedance Loading –SIL (MW). Upgrading a line from 100% capacity to 173% capacity without changing conductor, towers, obtain permitting, ROW acquisition, with similar or better performance for me, this is a great deal particularly in areas with a congested transmission corridor. The additional cost of equipment in the substations easily can be compensated by the extra delivery capacity.[/sub]
>>>>>>>

 

[waross]

Thank you for taking the time to respond to me Cuky.
I will be very embarrassed if I am wrong.
Let's look at six phase on the basis of raised voltage.

Let's consider a wye connected 7600/13200 Volt distribution circuit.
This will have 7600 Volt insulators. Converting this system to six phase will stress the 7600 Volt insulators with 13,200 Volts.
A line generally has more than enough clearance between phases for increased voltage.
The voltage to ground is generally the limiting factor.

Now if the line is 13200 Volts delta, or 13200 Volts resistance grounded wye than the insulators must be rated for 13200 Volts to ground.
With 13200 Volt insulators installed the line may be converted to six phase without over stressing the insulators and gain the 173% advantage.
Or it may be as easily be converted to two grounded wye 13200/23000 Volt lines, for the same 173% advantage.
The delta to wye conversion from 2400 Volts to 4160 Volts was done hundreds and possibly thousands of times for the same 173% advantage.
Phase to ground flash over is more likely than phase to phase flash over.
Show me a six phase conversion with existing plant that does not increase the phase to ground voltage.
A six phase system must be solidly grounded, else a line to ground fault will cause an over voltage to ground on three of the six phases.
An ungrounded or resistance grounded system will be insulated for phase to phase voltage rather than phase to ground voltage.
With a six phase system one of the disadvantages will be the loss of the ability to limit earth fault current with a neutral impedance.


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

 

[waross]

In the IEC world its frequently called two phase.

But didn't New York once had real two phase? grin

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

 

[Mbrooke]

They did, as well is Philadelphia and Hartford Electric Light Co. In fact in the past you could drive through these cities and see 4 prong potheads that were converted to 3 phase operation.

 

[waross]

I have a question that may cut through to an answer.
I have have found hints but not been able to find a definitive statement in the 493 page report, and frankly I have spent enough time with apples and oranges comparisons.
To gain the 173% advantage of the six phase system, is the voltage to ground raised by a factor of 1.73%?
You don't need six phases to gain that advantage.

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

 

[cuky2000]

Hi Bill,

Yes, the main advantage of the HPO is to allow raising the line-to-ground voltage by a factor of 1.73% as well increasing the phase-to-phase voltage resulting in overall gain up to 73% the power delivery capacity of the line without investing a dime. All the changes will be in the substations.

Will be great if you can share your thoughts on how to do that in an existing line without major modifications.

Regarding your early concerns of overvoltage do to the ground fault, here is an excerpt from the AESO in Canada regarding the overvoltage amplitude factors indicating that SLG is not the most severe stress on the line. Lightning and switching are the driving factors selecting the number of insulators for a power line. Below is a sketch with additional information.

Hope this helps.

 

[waross]

Hi Cuky
Looking at your post of 14 Feb and the graph of the Surge Impedances for higher orders;
Looking at the thermal capacity line it is linear and directly related to the number of conductors.
A 36 phase has 36 conductors and 12 times the thermal capacity of the three conductors of the three phase line.
The graph shows the surge impedance capacity of a three phase line to be basically the same as the thermal capacity.
The six phase line with twice as many conductors has only 1.73 of the surge impedance capacity of the three phase line.
That is twice as many conductors and only a 1.73 increase in the surge impedance limit.
It is an exercise in diminishing returns.
The 21 phase line with 7 times as many conductors has only 3 times the surge capacity limit as the three phase line compared to 7 times the thermal capacity.
The increase in the 36 phase line thermal capacity limit is 12 times the thermal capacity limit of a 3 phase line but the surge impedance limit is slightly over three times the three phase capacity.
High Phase Order systems are great for reducing ripple in rectifier circuits but an expensive futility for transmission lines.
The MBAs who commissioned and paid for the six phase experiment may be unable to sort through the bafflegab of that report.
It is in reality a detailed analysis of all the factors involved in a ten million dollar fiasco.
Two, three, or six transformers at each end is not cheap.
They have basically raised the voltage to ground by 1.73 pu by going from delta to star.
Move along folks, nothing new here.
If the system will withstand 1.73 pu voltage to ground on six phase, it will withstand 1.73 pu voltage to ground on three phase.
They have introduced more complicated switching, more complicated protection, more complicated physical arrangement of the conductors to get from the transformers to the towers, and more failure modes.
I note that they studiously avoided a true comparison.
With a very small part of the ten million they could have converted the connections at each end to two conventional wye circuits at the same voltage to ground and given us a true apples to apples comparison.
That would also have the redundancy of two independent circuits.
Converting from delta to wye plus counter wye is about the same capacity as converting to delta to wye, two times.
The root of the advantage is the conversion from delta to wye, not the conversion from three phase to six phase.
The more time I waste on that report the more I admire it as a masterful bit of obfuscation.
Ten million dollars worth of obfuscation, it should be good.

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

 

[cuky2000]

Still waiting for your Ofer In how to increase the power delivery capacity of the line.

In the thermal region, P[sub]3ph[/sub]= 2Circs.(3 V[sub]ln[/sub].I[sub]rated[/sub])= P[sub]6ph[/sub]= 6V[sub]ln[/sub].I[sub]rated[/sub]
Without increasing the voltage and/or current rating of the line, there is no gain in power capacity rating.

 

[cuky2000]

Bill question for you;

In an exiting double circuit T. Line rated for 115 kV line-to-line, will be possible to operate it at 230 kV line-to-line?

 

[waross]

Back at you.
In a 115 kV line to line circuit the line to ground voltage will be 66 kV.
In a six phase line this will be increased to 115 kV to ground.
The line to line voltage will not be 230 kV in either the six phase conversion nor the three phase conversion.
The line to line voltage will be 200 kV (198.95 kV)
If on a particular line it is not possible to raise the line to ground voltage by a factor of 1.73, then it will not be suitable for a six phase conversion.
In the six phase conversion much is made of the point that the line to line voltage between adjacent lines is less than in a three phase line.
While true, this is more misdirection and relatively unimportant.
Look at an existing double circuit tower, the conductor arrangements and the clearances.
Between each phase conductor on the tower there is a ground point.
The phase to phase clearance is twice or more of the phase to ground clearance.
The phase to phase voltage is 1.73 times the line to ground voltage, not twice.
I say again. The increase in capacity of a short six phase line is directly related to the increase in the line to ground voltage.
Historically, and this thread is a historical discussion, almost all distribution circuits gained an economical 1.73 pu increase in capacity by simply reconnecting the supply transformers from delta to grounded wye.
That is no capital cost either in the substation or out on the line.
The history is there and proven.
When the voltage to ground is increased by a factor of 1.73 the thermal capacity is increased by a factor of 1.73.
Another point of obfuscation due to no clear comparisons is pointed out clearly in the graph you posted on 14 Feb 20 20:37
The thermal capacity of a high order line is related to the number of conductors.
So the thermal capacity and the surge impedance limited capacity of two three phase lines is 200% of the capacity of one three phase line.
Your graph clearly shows that the surge impedance limited capacity of a six phase line is not 200% but only 1.73% of the capacity of two conventional circuits before conversion.
Your graph shows that the surge impedance limited capacity of a six phase line is 1.73/2 or 86.5% of the surge capacity of two conventional circuits left unconverted.
The surge impedance limited capacity of high order line is shown to further decrease as the phase order is increased.
Smoke and mirrors, obfuscation, apples and oranges.




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

 

[cuky2000]

Still waiting for your offer on how to increase the power delivery capacity of the line.
I am about to give up.

 

[Mbrooke]

Don't give up- I like this debate- I'm learning a lot. I remember an EE telling me years ago that 6 phase would be more efficient but not worth the added complexity for the small(er) gain.