7/1/91

Wisconsin PSC Advance Plan 6, 1991 Testimony on Underground HVDC

I introduced the concept of extruded aluminum electric pipelines as an alternative to overhead power-lines in a report and testimony I presented to the Wisconsin Public Service Commission (Advance Plan 6 planning docket, 1991/92). On the subject of proposed transmission options for linking western Wisconsin to eastern Wisconsin more strongly, I noted that:

All the utility proposals involve overhead transmission lines. Overhead transmission lines have always been controversial, and have become more so because of increased public concern about EMF (electromagnetic fields). Overhead transmission lines also have numerous technical problems which have tended to be accepted as unavoidable consequences of transmitting bulk power."

This report went on to develop the concept of DC coaxial electric pipelines for Wisconsin's specific case. My testimony was politely received, but had little effect on that proceeding. I am greatly indebted to Professor Willis Long of UW Madison, who served as the author's expert witness in the AP6 Docket, and Paul Nonn of the UW Nuclear physics support staff, who aided in the preparation of my AP6 testimony.)

The following excerpt is taken from the testimony of Professor Bill (Willis) Long of UW-Madison in the Wisconsin Public Service Commission's Advance Plan 6 (AP6) hearings in 1991; he was my expert witness in this matter. It summarizes several important features of HVDC grids succinctly and with authority.  (The support document cited in Professor Long’s testimony, AP7 Exhibit 175, is essentially a similar document to the Appendix in my NYSERDA PON 1208 application:

NYSERDA_Transmission_Proposal_2009

but was written from a state utility planning perspective rather than the regional/continental scale of the proposals above.) Following is Bill Long's 1991 testimony:

Yes, particularly under contingency conditions such as when a major transmission line or generating station has been taken out of service for maintenance, an HVDC link can improve overall system stability in case of an unplanned system disturbance, such as may occur due to lightning, for example.

A:Probably the major negative factor is the relatively high cost of the AC/DC converter stations. These converter stations have been coming down in price for decades, but still cost much more than comparable AC transformers. The cost of such converter stations would probably come down significantly in price if a larger market developed. At present, only a few large AC/DC converter stations are built each year.

Q: Please define the concept of an AC synchronous area.

A: AC grids normally operate as a large area in synchronization. This does not imply that all generating plants are in exactly the same phase at a given time, but rather that the various generating plants have a stable phase relationship. An AC grid of infinite expanse could be kept in synchronization, but only if it were not disturbed. Real electrical grids must withstand various kinds of disturbances, and this requirement limits the maximum size of an AC grid. The North American Continent is too large to operate effectively as a phase-linked AC grid. There are in fact three large synchronous AC areas in the continental US, consisting of the East (includes Wisconsin), the West, and Texas AC grids.

Q: Can these synchronous areas be interconnected by AC lines?

A: Under stable conditions, it would be possible to interconnect the various AC grids in the US. Under disturbance conditions, however, this interconnection will prove unstable and unreliable.

Q: Can different AC synchronous areas be interconnected by DC linkages?

A: Yes, they can, and improved reliability of both systems will generally result from such an interconnection. The degree to which reliability is improved for such an HVDC intertie between to asynchronous areas depends on how powerfully the end-points of the HVDC link are tied into the AC grid. Tying the HVDC link in at two or more key points in each of the asynchronous AC grids will improve the stability enhancement due to the HVDC intertie considerably over a two-terminal design in which each of the AC grids is only connected to the HVDC link at a single point. The value of DC interties is well recognized by system planners; in fact, such DC interties are often referred to as "asynchronous links."

Q: Can you cite a few examples of AC/DC converters being used as an asynchronous link between two synchronous AC systems?

A: Yes; for example in Japan there are different areas that operate at 50 Hertz (the European standard frequency) and at 60 Hertz (the North American standard). These two areas are tied together through back to back AC/DC converters. Another example occurs near the Austria/Czechoslovakia border across the former iron curtain, where an asynchronous link is used to connect the two synchronous AC areas.

Q: Would it be fair to say that the reliability of both the Eastern and Western USA grids would be improved by a strong asynchronous DC intertie between these grids?

A: Yes. There would also be a decrease of spinning reserve and reserve margin requirements, though it is impossible to quantify these effects without detailed modeling which I have not performed. I should note, however, that there was a DOE-sponsored study of this issue in approximately 1978.

Q: What is the largest DC multi-terminal system that has been designed to date?

A: The question of multi-terminal DC lines and the control aspects thereof, is one that's gotten a lot of attention in the literature in the last five years or so. The system being built now between Quebec and New England has five terminals, one of which will not be operated under normal conditions. This system starts at James Bay, drops part of its power at Montreal through two stations, one a little north and one a little south of Montreal, then continues to a terminus west of Boston. The fifth terminal was added after the system was initially engineered because of political considerations, and is not expected to operate much, if at all.

Q: Would you say that a five terminal HVDC transmission project is a proven technological option at this point?

A: Yes; it's been studied extensively by computer simulation. The people who are studying it are people who used to work for me, and are extremely good at what they are doing. The James Bay to Boston system is designed to operate either with full communication through a central computer, or in the absence of communication. When communication is lost, the system remains stable and functional, though without communication the operation is somewhat less optimal. The specific problem encountered with the fifth terminal in the Hydro Quebec system came about because of a capacity mismatch with the other terminals, and because the system was originally designed as a four terminal system.

Q: What is the status of DC breaker development?

A: DC breakers have been developed for 400 KV service, and have been tested at field currents of 2000 amps and laboratory currents of 4000 amps. The people that developed this device would claim it is commercially available. No one has bought it and installed it on a commercial power system yet, however, so most power system engineers remain skeptical about the reliability of these breakers, as they have not seen large scale commercial installation of such breakers yet.

Q: Would the DC breaker problem prevent the construction of any of the project proposals cited in AP6 Exhibit 175?

A: No.

Q: Do you believe that two electric pipelines in the same service corridor can be considered for reliability purposes as two independent circuits?

A: With several caveats, the answer is yes. However, the corridor system must be so designed that a worst-case short circuit in one electric pipeline cannot damage the other pipeline. Also, the two pipelines would have to be separated at high-risk crossings, such as some highway crossings and any crossings over navigable rivers.

Q: Do you think that AP6 Exhibit 175 presents a reasonable case for HVDC electric pipelines as an appropriate means to interconnect the three US AC synchronous regions?

A: Yes. I would like to amplify on the notion which is contained in AP6 Exhibit 175 but not really emphasized, that conventional aluminum electric pipelines installed in an accessible service corridor would make the future upgrade of that transmission line to superconductive transmission both easier and less costly.   Superconductive cable will almost surely be installed in a service corridor, since refrigeration equipment will be required to keep the line cold. In the future eventuality that a superconductive line would be installed in the same corridor as one or a pair of conventional aluminum electric pipelines, this would not necessarily imply that the aluminum electric pipelines would lose their value, since in that case the electric pipeline backups would significantly improve the safety and reliability of the linkage since loss of cooling for the superconductive line will always be a possible fault mode.

Q: Does this conclude your direct testimony?

A: Yes, it does.

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