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?

 

[davidbeach]

I have not delved into the minutia of this, but I'm not seeing how the phase-phase voltage of the 6-phase line could equal the phase-phase voltage of the 3-phase line without increasing the phase-ground voltage. Once you have to reinsulate, and respace the supports, of the existing structures I'm not seeing an advantage of doing this rather than just running two circuits at a higher voltage. Besides, with two 3-phase circuits I can trip and clear a ground fault at only a 50% reduction in capacity. Put all 6 conductors into a single circuit and that one ground fault now removes 100% of the capacity.

 

[DM61850]

Cuky2000,

It increases the capacity of a transmission line by making it so you can pack each phase together. It is a spacing issue I believe and not a thermal limitation of the conductor. The voltage difference between each phase reduces as you include more so you can pack them tighter together. If you had infinite phases, you would have a cylinder that you would be sending a rotating field down. I suppose there are application for this to be useful but probably only a few would justify all the added equipment. There is a guy from I believe from AEP that has a short video explaining this. It never went any further because load growth never increased as they had expected in their projections in the 1970s.

It is sort of like superconducting conductors being used for transmission. Very limited application and the one in New York City that is only a few kilometers long, might be one of the few cases were keeping super conducting material cold is less of a headache than if you had to use conventional means to transmit power. When I worked for a utility, they would drop by to talk about it but no one wanted to bite on it. The big substation in west texas to combine all three interconnects was supposed to use superconductors between each bus for each interconnect. That substation has been in perpetual limbo.

 

[Mbrooke]

Here is some info assuming we're thinking of the same cable:

http://mydocs.epri.com/docs/PublicMeetingMaterials/MRNYPKPLTGV/18-MCCALL-ABOUYAALA-HYDRA_Update.pdf

I think there is another one in midtown Manhattan as well.


As I understand the super conducting cables, primarily the Dunwoodie cable, were installed to eliminate the need of adding an extra transformer at Rockview Substation. I'd guess its the same for Manhattan.

You're not likely to loose more than one, or even two transformers in the same geographic area so it makes sense to pull reserve capacity from neighboring substations instead of adding more trafos to each individual station.


Although the way its described is complete hype:

http://energycentral.fileburst.com/EnergyBizOnline/2007-4-jul-aug/Tech_Debuts.pdf

Its not the major resiliency they make it out be. 93MVA is not a lot considering the average network load is 200-250MVA. Second by "surges" they mean limiting fault current. The cable has to in that you are linking two stations that on average have 5 65MVA transformers running in parallel. Nothing to with quashing transients or somehow making the system as a whole more resilient.

 

[waross]

Take a look at this, Cuky;
I found a paper that does an excellent job of explaining all aspects of 6 phase transmission and touches on 12 phase transmission as well. Only 79 pages.
A must read for anyone interested in High Phase Order Transmission.
I noted this quote:
"As a result, there has been a generation of a plethora of information which is very beneficial on the one
hand, but on the other is sometimes contradictory and inconclusive. Thus, there is a formidable challenge
in compiling a survey of case studies, and careful thought must be given to the selection of cases and their
applicability."

Link
Transmission Line Compaction Using High Phase Order Transmission
Jacob Bortnik
A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Master of Science in Engineering Degree awarded with distinction on 8 December, I998
Johannesburg,1998

Mr. Bortnik examines and comments on six case studies of High Phase Order Transmission Lines.
Here are all the answers that I couldn't find in the other paper.
Some take-aways;
HPO, Less inductive reactance per unit length, less need for series capacitors.
HPO, Comparable construction costs to equivalent 3 Phase lines.

The breakeven point may be under 15 km, however some configurations never reach a breakeven point​

HPO, Generally less corona, less radio noise, less audible noise, lower electric and magnetic field strengths at a given distance from the line.
3 phase; Where same voltage systems are connected by a transmission line without transformers, the HPO option may never be feasible.
The future;
The main advantages of HPO will develop in the future with rising land costs for ROW, restricted ROW, and restriction on the radiation of electrical and magnetic field gradients.
A couple of notes on the previous discussion;
Using an averaging voltmeter on unloaded wye transformer circuits? I have been bit by that one a couple of times.
Well balanced, and the ability to operate with one phase missing. The receiving end is a pair of grounded wye-delta transformers.
That's what they do.
The ability to run with a missing phase and forcing balance on an unbalanced system?
They do that by way of creating zero sequence current on the neutral.
Those effects are grounded wye-delta effects, not HPO effects.
I fought the good fight with the ideosyncrasies of the grounded wye-delta for over 15 years.
Total costs as opposed to line construction costs;
Two winding transformers needed, cheaper auto-transformers are not suitable.
The addition of extra substation costs may push the breakeven point out to 200 to 250 km.
I anticipate bulk transmission from distant generating system by HVDC, and possibly HPO from HVDC terminals into cities on ROWs subject to high land cost and stringent environmental regulations.



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

 

[cuky2000]

The phase to ground voltage can be increased in the 6 phase system without reinsulating and re-spacing the support. The cost for the additional work in the substation will determine the breakeven distance in a similar fashion than the HVDC project but much lower cost.
For a SLG event, 3 out of 6 phase can be tripped an even reclosed in a similar manner than a double three-phase circuit. IPO circuit breaker could be used if necessary.

Below is a simple geometry exercise indicating that a constant voltage gradient can be achieved in an existing double-circuit configuration.

 

[cuky2000]

Hi Bill,
I do believe that the HPO is a viable option particularly to up-rate the power capacity of an existing double-circuit configuration.

HPO is not the only method of uprating line capacity. Transmission line compensation, FACTS, and HVDC are also available technologies but at a higher cost than HPO. The big difference is that those technologies are heavily advertised by the large power manufacturers and there is not much to sell upgrading existing transmission lines.

We should recognize that there are a lot of challenges implementing an HPO particularly with the O&M folks and the complexity of protection and control systems that will a major effort to convince the conventional mentality of many utility P&C Departments and ISOs. The reward is that there are great opportunities to solve congested transmission corridors without the need to build new lines or a least defer the investment for another 30 years.

I am glad that at least you are giving a chance to openly discuss the possible applications of the HPO.

MBrook:
Coincidentally I also did some work in the high-temperature superconducting HTS) project primarily associated with the grounding system interconnecting two area substations in the Con Ed system. We did a lot of modeling to address the concerns regarding potential voltages induced in high congested metropolitan areas.

 

[Mbrooke]

@Cuky2000: The HTS must have been a great learning experience. I'd have added another transformer at Rockview and built a new substation in lower Midtown- hence why I would never been involved in such a project. You posses out of the box thinking I don't.


And oh, I know I'm nit picking- but the line you used in your pictorial example looks like a 400kv line from the UK.

 

[waross]

My problem and my objections were more to do with the extremely poorly written report than with HPO transmission.
HPO reduces the voltage and the voltage gradients between adjacent phases. That reduces the voltage gradients between the adjacent phases and reduces corona losses.
Hence the corona limit is raised and the phase to phase voltage may be increased.

Hello David. It has been stated in the reports that a six phase system may be operated with one of the six phases open at a reduced capacity.
Again this is a faulty comparison of apples to oranges.
The test section used grounded wye transformers at the receiving end.
The ideosyncracies of the grounded-wye-delta system were well known in the 40s and 50s and then forgotten as the connection fell out of favour.
When the wye-delta connection is used on a distribution circuit the wye point must be floated, not grounded.
When you lose one phase the circuit continues to transmit power as an open delta.
When you lose one of six phases the system operates as a full circuit in parallel with an open delta.
Had a wye-wye transformer arrangement been chosen over a grounded-wye-delta, the results would have been much different.

As I understand one of the reports, a test line was energized as a three phase line and the corona losses were measured at about 2000 Watts.
The second circuit was then energized so as to operate as a six phase line and the corona losses dropped to about 500 Watts.

Interestingly
Interestingly, Mr. Bortnik sees one of the advantages of HPO as reduced right of way width in areas where land is very expensive.
However he was unable to do a cost comparison due to the wide fluctuation in land values.
Mr. Bortnik points out that in South Africa, the width of right of ways is set by environmental legislation based on the voltage to ground and Under the current legislation HPO transmission would not be able to take advantage of a reduced width ROW.

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