The Y1936 Problem 7

[GregLocock]

"All power transfers between the three grids takes place over DC links."

can you explain the reasons for that in terms that a knuckle-dragging mechie could understand? Is it just to simplify synchronization, or are the distances sufficiently large that the skin effect becomes important? How is the DC generated at one end and then transmogrified into AC at the other end?

Maybe you could just point me at an article!

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[davidbeach]

The three grids are not synchronized, so there is no means of making an AC connection. The DC is created using high power rectifiers and then inverted back to AC using high power inverters. These are now power electronics but the original high voltage DC links used mercury arc valves. The inter-grid ties are generally short, with both ends of the DC link at the same facility. Long DC links are used for bulk transmission over long distances with no intermediate tap points, such as the Pacific Intertie that runs from the Columbia River in Oregon to the northern outskirts of Los Angeles.

 

[BJC]

Greg
A DC conversion can be likened to a control valve. You can requlate the power you let through. A straight AC intertie will let power flow without check. A big load on one section of the system will suck all the power the system has.

 

[zeusfaber]

Two other reasons for avoiding long ac interconnect links:

1. The capacitance (to earth) of a long line starts to absorb too much current. This is a particular problem for buried (or submarine) cables. The current may be out of phase with the voltage, so nominally wattless - but this doesn't stop it warming the cable up. At 50 Hz, you don't have to go too many km before a buried cable absorbs its own rated current.

2. To keep two networks locked in sychronism, you need to be able to transfer significant amounts of current between them. This becomaes difficult when you have large networks fighting one another over a relatively weak ac link. The loser is generally the link. A dc link gets round the problem by simply ignoring it (at which point it gives up and goes away).

A.

 

[JerryH2]

Gentlemen:

There are really two different questions in voiced in this:

1) Why the US (and most of the Americas) uses 60 Hz and Europe (and the rest of the world) uses 50 Hz?
2) Why does the US uses 110 V (now set at 120 V) and Europe uses 220 V (now set to 230 V)?

It does seem to be a conglomeration of historical reasons, including state of the art back in 1890’s, which company had a head start, and standardization. Some history:

George Westinghouse did his original engineering using 133 1/3 Hz. Westinghouse had an steam engine driven alternator set running at 2000 rpm (By 1886 mechanical engineers liked to have steam engines in integral numbers of rpm) and with 8 poles the set produced 8000 cycles per minute or 133 1/3 Hz. This was good for lighting as there was no flicker but it turned out it was too high for motors later developed.

The earliest experiments (1886 and 1887) used belt driven generators and tended toward high frequencies like 133 1/3 Hz. This suited illumination, which was practically all that alternating current was used for at that time. By 1889 and 1890 direct driven generators were coming on line. They were more robust but with lower rotation speeds they encouraged lower frequencies.

In the early years of ac there were many frequencies: each engineering team seemed to pick their own. Early frequencies in the US were 133 1/3, 125, 83 1/3, 66 2/3, 60, 50, 40, 30, 25 Hz. When Tesla joined Westinghouse, it was using 133 1/3 Hz. Tesla insisted upon 60 Hz because his ac induction motor was designed for 60 Hz and apparently wouldn’t work at 133 1/3 Hz.

On the Westinghouse Museum website it says that G. Westinghouse assigned his engineers Stillwell, Shallenberger, Schmid, and Scott to find a good frequency. Practical considerations of connecting alternating generators to reciprocating engines then in use demanded a lower frequency than 133 Hz.
Before the end of 1892 they chose 2 frequencies: 60 Hz for lighting and 30 Hz where power was to be converted to DC.

Why did Tesla/ Westinghouse engineering team choose 60 Hz? If it was Tesla that was the driving force, various biographies of Tesla declare different theories ranging from Tesla “thought it was the fundamental frequency of the universe” to “… considered the natural earth had a frequency of 10 Hz and doing experiments with 8 to 20 Hz and 20 to 40 Hz and finally 40 to 100 Hz; he decided that 60 Hz was safe.” It doesn’t seem to have been a desire to do accurate clocks because Henry Warren didn’t patent the synchronized clock until 1916 long after the frequency was chosen. Although Warren was diligent in getting utilities to have tight specs on frequency this didn’t happen until into the 1920’s.

Back in the early 1890’s Westinghouse was involved in bidding electrical equipment for the Niagara Falls power project. However the Cataract Company (in charge of the Niagara Falls project) had already selected hydraulic turbines running at 250 rpm. So if a 16-pole generator were chosen the frequency would be 33 1/3 Hz and if a 12-pole machine were chosen then the frequency would be 25 Hz. The project consultant proposed an 8-pole generator or 16 2/3 Hz. The compromise was 25 Hz. At the time lower frequencies were easier to handle on transmission lines. Another reason is that the Steel industry liked 25 Hz because of huge slow speed induction rollers, which had a low power factor for 60 Hz and worked better at 25 Hz. Niagara Falls generated 25 Hz way into the 20th century. The website says that the Westinghouse Company later wished it had forced through 30 Hz.

By 1910 it looked there would be two frequencies in North America, 25Hz for transmission and heavy industry that needed dc or slow moving heavy machinery and 60 Hz for lighting (less flicker) and general use.

There was an effort by GE to introduce 40 Hz as a compromise between 25 Hz and 60 Hz in the 1890’s but it was too late to overtake the 60 Hz and 25 Hz infrastructures already in place although there were some 40 Hz installations. Even so most installations in the US were done in 60 Hz after Westinghouse and GE cross licensed their patents.

Development of high-speed turbines instead of slow reciprocating machinery and later developments of the rotary converter that worked well at 60 Hz made it easy to shift everything to 60 Hz. By 1920 most of the problems associated with 60 Hz transmission had been solved so that there was no longer any advantage of transmitting 25 Hz over 60 Hz. That seems to be why the US is 60 Hz.

Germany took the lead in Europe of developing electrical power (primarily Emil Rathenau of AEG) and AEG seems to have used 50 Hz from day one. In 1891 AEG had demonstrated power delivery over long distances using 50 Hz. I don’t know why AEG chose 50 Hz. Did the penchant for integer rpm help influence AEG for 3000 rpm and 50 Hz as opposed to 3600 rpm and 60 Hz? Did the preference for preferred numbers influence the choice of 50 Hz over 60 Hz? Did Tesla’s influence pull Westinghouse to choose 60 Hz and resultant 3600 rpm over 50 Hz and 3000 rpm? Europe was even more fragmented in the early days than the US. In 1918 in London alone there were 70 electric authorities with 50 different types of systems and 10 different frequencies and 24 different voltages. But by the 1920’s and 1930’s more and more of Europe was changing to or working with 50 Hz.

As for voltages both Europe and the US seemed to have begun with about 100 to 110 Volts DC because of Edison’s success with replacing gas lights with electric lamps. Although many inventors worked on electric lights, generators and electrical systems, Edison was one of the first and was successful in putting together whole systems not just the pieces. Edison picked 110 VDC because that was the voltage he needed to get enough light out of his bulbs to compete with common gas lamps of the time and yet not blow the filaments in his bulbs too soon.

The Berlin Electric Works (utility owned by AEG) changed from 110 V to 220 V starting in about 1899 to enlarge the capacity of their distribution system since the city (Berlin) was already wired 2 wires. They were probably changing from dc to ac at the time also. They paid for their customers to change their lighting and motors to 220 V and saved on the cost of copper by avoiding having to add more wiring. This spread throughout Germany and later Europe but didn’t take hold in the US.

I wonder if the residue from the bitter conflict between Edison and Westinghouse about the safety of AC vs. DC spilled over into not going above 110 volts for residential users even after Edison’s forces conceded the need for AC.

A lot of this information comes from Thomas Hughes Networks of Power : Electrification in Western Society, 1880-1930 and Benjamin Lamme Technical Story of Frequencies IEEE transactions 37 (1918) 60. Benjamin Lamme was chief engineer for Westinghouse in the early 1900’s.

 

[waross]

Thanks for the information Jerry. I enjoyed it.
Respectfully

 

[KiwiMace]

A fairly informative read on the Tesla point of view -

Tesla, Man out of time.
Margaret Cheney
It's a bit of a promo for the Tesla vs Edison and later vs Marconi priority battles, but not bad reading. This is a 1982 biography reprinted in '93 and readily available at Barnes and Noble.

Unusual/brilliant sort of man, probably lacking due credit for his achievements. Notably he originated the 3-phase system and induction motor, had an agreement with Westinghouse "to earn $2.50 per horsepower of electricity sold" (holy crap!) then gave his patent rights to Westinghouse when asked nicely later on. That would have been worth 12.2 trillion dollars in 2003...provided he worked the same deal with everyone.