using single-phase transformer in distribution feeder 1

[power209]

dear friends, In our network we have some 20Kv feeders that are long and customers are very distributed.
could somebody please explain about using single-phase transformers for some remote customers? what are the advantages and disadvantages?
with taking into account that the primary side of single-phase transformer can be connected phase to ground or phase to phase.

 

[jghrist]

Advantages are lower cost, both to the utility and the customer if the customer does not need three phases. The system must be effectively grounded to connect the single phase transformers phase to ground. If the system is effectively grounded and the protection does not include sensitive earth fault protection, then less expensive transformers can be used by connecting them phase to ground. Phase to ground connection is the most common connection in North America, but phase to phase connection is most common elsewhere (I don't know about Japan).

 

[power209]

thanks jghrist,
I want to study more about this subject.
Do you know where I can find more information?

 

[jghrist]

Sorry, I don't have any sources for general distribution design outside of North America, which I assume you are. Differences in protection and grounding make a lot of the US literature on distribution systems not very applicable to the rest of the world.

 

[RalphChristie]

Do a search for SWER (single wire earth return) on this site or on Google.
Also check

http://www.ruralpower.org/

I mention SWER specific because it is used (many parts of the world) in rural areas with a low population and density. Some advantages and disadvantages of SWER-systems:

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

Regards
Ralph

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

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[magoo2]

I really don't think you should promote the SWER system unless you have some practical experience with it. It is very limited in the amount of load current that can transported and doesn't have a real good means of regulating the voltage. Grounding has to be quite good to have a reliable system that depends on earth return.

Sure, it is presently operating in remote parts of Australia, but I think you'd be better served by understanding the differences between the single-phase North American approach and the three-phase European design.

I know a number of countries that have European style three phase distribution with 3-wire medium voltage systems. Because of low load density in rural areas, they are picking off some single-phase load with phase-to-phase connected transformers or are converting some of this to 4-wire circuits and then following North American practice of predominately phase-to-neutral connected transformers.

 

[RalphChristie]

I really don't think you should promote the SWER system[/quote magoo2]

magoo2:

Like any other system, SWER does has its disadvantages, but also has its advantages. And one advantage, which is really the biggest, is cost. You can compare no other system with SWER costwise. That is a very important factor, especially in big areas with a low population.
Also, SWER is actually much more used in the world than just Australia. Other countries include New Zealand, South Africa, India, and Canada.

Yes, I am involved in SWER-systems. In the area I am living, (the Kalahari-dessert, South Africa) it is quite a common sight.

Regards
Ralph

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

Make the best use of Eng-Tips.com
Read the Site Policies at FAQ731-376

 

[magoo2]

There's a link that provides a little more incite on the SWER system.

http://tdworld.com/mag/power_one_wire_enough/

The link makes a point about circuit losses. I would assume voltage drop goes hand in hand with circuit losses. With a 3- or 4-wire distribution, I feel confident that I can design and control voltage within +/-5%. How well can you hold the voltage on a SWER system?

I'd be more concerned with the performance over time. How good is your earth resistance in a desert region? How often are ground readings checked?

With just one conductor and the variability of the earth return path, I don't see how you can achieve reasonable performance compared to the more conventional systems. Sure it's cheap, but at what performance level?

I once heard of a rural cooperative in the US who sought funds from the Rural Electrication Administration to build a 50 mile single-phase line. When the REA approved his project, he wound up building a 100-mile single-conductor line (SWER wasn't named at this time) with all transformers connected from the conductor to ground. The system had all sorts of problems with voltage regulation, particularly in the drier months of the year. This was a cheaply designed system, but its performance was not acceptable.

 

[Jonturner]

In Ergon Energy, Queensland we have around 4,000km 11kV SWER, 16,000km 12.7kV and 44,000km of 19.1kV SWER. All up enough to streach 1.5 times around the world. Amoungst the SWER technologies we have unisolated, isolated, duplex and triplex operating. The isolated SWER is our prefered choice of course. Australia has around 191,000km SWER I believe. Our largest schemes are around 400km with 2x200kVA isolating transformers, SWER voltage regulators, shunt reactors, 25kVA customer subs. We supply some small towns with SWER - even a town on a beach. The largest single SWER load is 100kVA. We balance the SWER loading back on the 3 phase network and use 3xsingle phase regulating transformer banks to keep any unbalanced feeder voltages fine tuned. Now using new low voltage regulators as well! SWER system technology advancing quickly with help of local universities. SWER is now well accepted world wide.
Jon Turner