Single Phase Electrical Service 5

[axd38]

The local electrical company drew up site diagram for a new single phase service that is suppose to feed an existing single phase 1200A 240V MDP panel. His drawing indicates 100KW pad mounted transformer, with 3 sets of 350 AL kcmil wiring coming in. Is the 100KW size of pad mounted transformer size big enough? My calculations give me that the tranformer would have to be around 390KW?

 

[itsmoked]

Ralph: Thanks for the definition.. wow. Bet that comes with some trade offs.

 

[akselj]

itsmoked
Danish house installations are normally 3 Phase 400 V +N +Gnd. European standard voltage.

Light and outlets are 230V (Phase +N) divided into 1 phase 13 A groups.
Stoves and many washing mashines (mainly older types) are 3 phase (1 phase for motor, 2 for heating) often 16 A groups

The installation is normally protected by HPFI (a switch that breaks the mains by current leaks over 30 mA)

The local HV supply is 10 kV with 10/0,4 KV transformers each covering some 50 houses, both 10 and 0,4KV distribution nets are normally in underground cables.

Greetings Aksel

 

[itsmoked]

Very interesting akselj, sounds like a nice system. Around here ALL the local residential distribution is 21kV single phase. (Calif,USA) Often wish I had 3ph.

 

[RalphChristie]

itsmoked:

SWER-lines have it pros and cons. This technology consists of a single overhead HV conductor and the earth is used as the current return path. Thus, it is much cheaper than the conventional three-phase and single phase (2 wire) networks in use - ideal for the electrification of rural areas. Savings are possible due to the fact that less material and labour are required to construct the network. However, the feasibility of a SWER-scheme is dependent on the earthing costs involved (due to the fact that there is a continuous flow of current in the earth electrode compared with the conventional networks where current will only flow in the earth electrode under fault conditions) and this is mainly determined by the ground potential rise (GPR) of 20V (safety limit)

Advantages
Simplicity - simple design allows speed in the construction of the system. No equalising of sags is necessary as in the case of 3phase lines
Maintenance - Reduced maintenance cost, as the possibility of conductor-to-conductor fault on single-wire lines is removed
Low capital cost - Using only one conductor which results in longer span lengths, thus less structures and material
Metering - Load growth can be easily checked by inserting low-voltage instruments directly in the earth lead at the isolating transformer.
Voltage - A unique advantage of SWER is that there is a voltage rise of 1% to 2% at the receiving end (light loads only) instead of a voltage drop whereas voltage drop invariably occurs in three-phase systems.
Power factor - not less than 0.9 to 0.95, the reason given that inductive loads are run in conjunction with static-phase converters which make use of capacitors as one of the components.

Disadvantages
Single phase - supply only single phase
Conversion - The full advantage of the long design spans cannot be utilised if three-phase conversion is ever desirable. This is because span lengths are optimised for a single conductor
Interferences on telephone lines - The system, when operating under high density load conditions, increase the degree of interference with telephone and telegraph lines. If this necessitates conversion of telephone lines, considerable costs can be involved.
Earthing - System necessitates a periodic check on earth electrode resistance in order to ensure that no hazard exists from voltage gradients across the surface of the ground.
Isolating transformer - Unit introduces additional system losses, construction and material cost.
Load balance of primary line - In common with all single-phase systems, the efficiency of the three-phase primary distribution line is reduced when large loads are to be supplied. Max. load that can be supplied is largely dependent upon the ability of the three-phase primary distributor to supply the unbalanced single-phase loading. This factor is the greatest disadvantage of SWER (and other single phase) systems, but it can be overcome if arrangements are made to supply three SWER networks from a common point on the three-phase distributor.
protection - protection against high impedance faults are difficult

From: An earthing design guide for SWER-systems in the Northern Cape region by C.H.L Sander

Hope this extra information is helpful

Regards
Ralph

 

[itsmoked]

Must be odd seeing one line trapsing across land.

Poor worms.

Does this cause chickens to sprout extra eyes?


Seriously, thanks, interesting stuff there.

I take it the current travels kinda direct back to the generator. Does it travel near the surface? Would it be detectable say 100 miles from the point of use?

 

[RalphChristie]

Virtually you can use it anywhere where you have a ground return path. I think the way how it travels back to the source through the earth-mass would depend on the characteristics of the soil. (resistivity of soil, moisture content of soil, how deep the water-table is, etc.) In Australia is one SWER-system of 107 consumers with 1.37MW load, fed through a network of 2300km. The most remote consumer is 150km from the source.

The SWER-system were first used in New Zealand, but is also used in Australia (biggest networks), South Africa, India and Canada.

 

[k2panman]

Thanks for the very interesting information.... wow, I'm thinking like "itsmoked"... what is the voltage drop thru a cow standing near a transformer? Seems like you could have some tingling current if you were standing in the wrong place... Is it possible to stick a pair of electrodes in the ground some distance apart and measure a voltage, maybe even light a small lamp?

K2ofKeyLargo

 

[RalphChristie]

The entire SWER line current must be carried by the source electrode, resulting in a voltage across this electrode that is equal to the product of the line current and the electrode resistance. The maximum ground potential rise (GPR) of the earth electrode is limited to 20V so as to prevent dangerous step-and-touch potentials. SWER lines are unlikely to be constructed in areas where the soil resistivity is higher than 300ohm metre. The maximum SWER line current is limited in accordance with the economical achievable source electrode resistance.

The steady state voltage gradients at which stock experience discomfort were found by experiments to be as follows:
Cows - 45 volts per metre of distance
Lambs - 25 volts per metre of distance

This evidence points to the possibility of risk with earthing system voltage drop in excess of 40V. The application of a safety factor of 2 over the voltage gradients at which stock feel discomfort indicates that SWER earthing systems should be designed with a maximum permissible voltage rise on the earth system of 20V under normal operating conditions. With such a limitation, the risk and discomfort are negligible.

From: An earthing design guide for SWER-systems in the Northern Cape region by C.H.L Sander

My chance (forced?) to read through my own books....

Regards
Ralph

 

[itsmoked]

I still say wow!
Thanks for that info(& hitting the books) Ralph.

Gday.

 

[mc5w]

Back in the days of telegraphs that used ground returns the preferred grounding electrode was a 3 foot by 4 foot metal plate buried below the water table. If a line was more than about 14 miles using a ground return presented less loop resistance than using a return wire. However, as the number of circuits increased a certain amount of crosstalk was introduced by the common return path. At a certain point ground returns had to be abandoned because of interference from both alternating current power systems and DC streetcar power.

The Spiral4 telephone carrier set that were used during and after World War 2 could be configured to carry 4 voice circuits, 3 voice + 3 teletypwriter channels, or 12 teletypwriter channels. The usual military configuration was 3 voice + 3 teletypewriter channels. Teletypwriter channels were done with a 3 channel modem. The 12 teletypwriter channel configuration using base 6 ( rather than binary ) modems was used for the sigsaly voice encryption system.

Part of how Alexander Graham Bell invented the telephone was that he was trying to build a modem that would carry say a dozen telegraph signals using audio tones for frequency division multiplexing. Turns out that it worked better for carrying voice.

 

[jghrist]

If you subscribe to the belief that power-frequency magnetic fields are hazardous to your health, then stay away from SWER lines. I personally do not subscribe to this belief. See

http://www.mcw.edu/gcrc/cop/powerlines-cancer-FAQ/toc.html

 

[EEJaime]

I would just like to thank all of you for this amazing range of information. I would never have known anything about SWER or it's history except for this forum. A star for Eng-tips in general.

 

[itsmoked]

EEJaime; I agree.

Man, mc5w, where do you dig this stuff up?
I get the feeling that *today* you are a 90 year old guy driving a 34 pickup, with replacement insulators rolling around on the dash.

 

[EEJaime]

Itsmoked,
That is a pretty good image, I couldn't have put it better. I've often wondered, what did it smoke?
Just kidding, good day all.

 

[PowerfulStuff]

Just to extend the thread a bit more!

The link from jghrist regarding safety of Powerlines was very thorough. One suggestion that has come up recently to explain possible slight increase in health problems when living around HV powerlines is not related to field exposure. The proposal is that the high electric fields around the line cause the conglomeration and then precipitation of existing air pollutants, so if you live next to a line you have a higher exposure to them. Unfortunately I'm not sure of any testing to verify it but it shows a different path to look at rather than focussing entirely on the direct effects of E and M fields on health.

 

[mc5w]

Part of my knowhow was reading old technology electrical books just as a matter of understanding some of the ways that people licked difficult problems. Turns out that most of the circuits that are used with transistors were originally used with VACUUM TUBES. Quadruplex telegraph essentially worked on the pulse amplitude modulation principle - Thomas Edison realized that by using 4 voltage levels instead of 2 he could transmit 2 signals in the same direction over the same circuit. Adding in hybrids at each end allowed 1 circuit to carry 2 or 4 signals in opposite directions at the same time.

Also, I have been subscribing to Mike Holt's National Electrical Code newsletters and some of the more recent ones were about how some NEC grounding rules date back to telephone practice before the U.S. Civil War. A 25 Ohm grounding electrode gives resonably good loop efficiency on say a 100 mile long circuit that uses a ground return.

It also turned out that telegraph systems eventually standardized to 60 volt batteries and this was perhaps why Thomas Edison used 120 volts for lighting.

If you really think about it the only big difference between fiber optic and an old style telegraph line is speed. You still have issues such as how to encode information, signal amplifiers, blah, blah blah.

The automatic district telegraph loops for fire alarms were in 1 sense a low speed version of Ethernet. There had to be protocol as to which sending machines would have priority if 2 senders tried to use the same loop at the same time.

 

[wrong100]

axd38
To get a concept.
In the US 60 years ago (about, no AC), I think 40%
was the num for homes.
Around 1965 to 1975 a decision was made
(no more "Y"), after ac.
Many things went into the decissions.
Center tap the 220 delta to get 115.
Rotation of the phases is easy at 2500, 3 wire
transformers.
Less pf, gnd, phase problems, and, 3/4 wires
for current in an ideal system.

THINK MONEY.

WRONG100

 

[mc5w]

If you look in the General Electric Blue Book you will see a wiring diagram for using a 3 stator meter to meter house current that is 120 volts 2 wire single phase plus 240 volts corner grounded 3-phase. By interposing current transformers you could use a form 9s meter as both a 3s ( for 3-wire single phase ) and a 5s meter ( for the 3-phase ) in the same box.

Eventually, the 120/240 volts 4-wire delta 3-phase idea won out as well as 120Y208 volts and 277Y480 volts.

The original reason why buildings had seperate single phase lighting and 3 phase power services was that design A motors had to be on their own transformer bank so that the motors would not dim the lights when starting. Design B motors fixed that problem which was partly a lamination varnish that made it possible to die cast aluminum conductors of any shape in th rotor slots.

 

[wrong100]

mc5w

I can only repeat what was given at the time.
Spoke to a person later, same same.
The last GE book I have is 1950 and is a quick ref.

In about 1972 I tried to get 3Phase "y" installed, at 2000A.
Houston lighting and power would only install "delta".
We had existing 2000amps 3 phase (to us) about 1000 in lghts, and aux and the other 1000 in motors (AC,etc).

I do not know about the first part of your reply.

I wish you could have told TI, FAirchild, Sig, Ray,
and a few others about the second part, in 72 to 82.
I could have spent about 3yrs home instead of on travel.

In a mfg complex. I have dimmed lights in a 32 km (20 miles) sq area, (made it dark). Seperation ???
One transformer one out put.
In the building I am at now, we have three, adding two.
Each is 300a 3 phase. I could shut the gen down.
if the subs were too slow.

I cannot understand In the second part about seperation or about insulation.
Sleep By

wrong100

 

[mc5w]

Up until the late 1990s Cleveland Electric Illuminating Company would only install 277Y480 volts for large commercial buildings. For some reason, factories and machine shops could only get 240 ungrounded, 480 ungrounded, and 120/240 volts single phase grounded.

When CEI mergered with Ohio Edison to form First Energy the dictate from Akron, Ohio was that all new 3-phase services had to be 4-wire solidly grounded or be convertible from corner grounded to 4-wire wye or delta. New 3-phase meter sockets have to be form 16s or 9s.

In Shaker Heights, Ohio CEI has separate 4,400 volt distribution circuits for motors and lights so that the power quality problems from apartment elevators will not mess up the lights. This is even for apartment buildings that are close to the substation that steps down from 34,500 volts to 4,400 volts.

There is also an air compressor testing shop in Akron, Ohio that knocks out all of the HID bulbs, streetlights, cash registers and computers every time they test a 50 HP or larger air compressor, even if the unit uses wye-delta starting. The wires on the primary circuit are too small and First Energy wants something like $15,000 to run a tap off of the 13,200Y23,000 industrial distribution that is 1 block away.

Up here in Cleveland we still have a lot of people who are 30 or 40 years behind on National Electrical Code. In 2004 somebody in Independence, Ohio put in a 277Y480 volt service with both the attachment insulators and the splices above the weatherheads. The requirement for the splices and point of attachment below the weatherheads was enacted in 1978 or 1981 ton prevent stranded conductor from acting as a poor excuse for a water hose. When I get the pictures developed I wil post them on my website and let you know.

Getting back to the original topic, the efficiency of modern capacitor start capacitor run motors is very much better than 30 years ago so the cost balance is in favor of single phase unless you use lots of juice or it is right out front on the street. You can also get static and rotary phase conveters that use a tapped autotransformer to match the amount of phase conversion to the load.

You can also get a combination static phase converter/ Harmonic filter from Mirus International that will run a 3-phase input vbariable frequency drive off of single phase power with harmonics equivalent to an 18 pulse rectifier.

Also, Marwell makes 2 different meter socket adapters that will allow you to build a service that is single phase upgradeable to 4-wire delta or 4-wire wye when your load grows. One of those adapters and well as 2 other can be used to build a service that is 3-wire open delta upgradable to 4-wire wye.