# How many customers can be connected to a Single Wire Earth Return node/pole in a power grid

I'm new to Electrical Engineering, and have a course in Technical communication, we will deliver a suggestions about a power grid. I'm afraid my current knowledge reaches little more then node analysis.

The course revolves around a project in which each group will deliver a report on different technology in different contexts. My group is tasked with designing a power grid with available technologies, we included 3-phase, SWER and direct DC current. This report if it's worth a wile will be part of the discussion making in the real world implementation in a village in Tanzania.

Me myself has taken on the task of "designing" a SWER power grid. Doing this I will estimate a cost per km. Since I plan to argue that poles and lines can be locally acquired the cost will be the cost of the hardware, which now seemingly to depends on how many costumers that can be connected to the sam pole or node in the grid.

I can't seem to understand or get a grips on how many customers that can connect to the same SWER or Single Wire Earth Return pole.

My hope that many customers can connect to the same transformer.

The SWER 19kV line will connect about 200-300 customers in a rural area of 38km2. And is one of 3 technologies presented.

So my question is, how many customers/households/consumers can be connected to a pole in a SWER power grid?

• I think around 25 homes per D.T. on our street <1m high on the lot front ground level. Each home has 200A panels with 240V 60Hz split phase to 120Vac. But this is urban not rural. Commented Oct 8, 2016 at 21:01
• That sir, is good news. Commented Oct 8, 2016 at 21:06
• Homes = SWER kVA / average home kVA? Commented Oct 8, 2016 at 21:19
• Yeah.. but does that necessitate that each home is connected to a pole dedicated to that home, or is it possible to have more then one home connected to the same pole. I realize it may seem basic, but I don't know how to judge these things yet. Commented Oct 8, 2016 at 21:27
• @TonyStewart.EEsince'75 Does that same limit apply to SWER systems? I don't know why they should be different but it's conceivable that they are. Commented Oct 9, 2016 at 0:06

SWER power and customer capacity depends on a number of factors.

What country is this in? - important due to regulatory aspects.
Are the 200-300 customers served by a single SWER line with a single feed point or are there multiple lines and feeds. If there are multiple lines, how many customers maximum are liable to be connected to a single line.

Capacity per 19 kV SWER line:

In some administrations maximum current is limited by regulatory restrictions amed at limiting interference with telecommunications systems (due to the purposeful 'injection' of all line current into the ground causing vastly more impact on telecom systems than do balanced multiwire power circuits.) In New Zealand maximum current was at one stage limited to 8A (may be different now).
An 8A,19 kV system can nominally accept 150 kVA - with output being constrained by losses - which vary with line length and soil resistivity, and system power factor.

SWER line customer capacity

Tony suggested a 25 residence capacity per distribution transformer in typical residential use. In this application if the power input was 150 kVA then assuming say 125 kVa delivered power (probably high), that would allow 5 kVa simultaneous peak load per user. While that level of usage is conceivable for users who understood the nature of the system and the need to minimise load, especially during peak periods, it is well below what might be drawn in some domestic situations.

In SWER systems per residence demand is usually restricted to far lower levels than in urban multi phase systems. A mean diversified load of 3 kVa / dwelling is typical in western use and in developing country applications with new electrification mean maximum loads of 1 kVa may be reasonable.

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An extremely good site for SWER related information is Rural Power who are highly SWER focused. The site is fronted by a NZ based engineer, Conrad Holland who notes - " ... qualified in power engineering and has been active in rural electrification in Africa, Asia, Australia and New Zealand for 25 years."

From their SWER FAQs 3

To calculate the single phase transformer size you must multiply the amount of customers within about a 600 to 1000 metre radius of the transformer by 600 to 1000 VA for newly electrified customers in rural areas e.g. if there are 60 households in a small village that has never had electricity before, then you would multiply 1000 VA x 60 houses = 60,000 VA or 60 kVA. The standard transformer sizes are approximately 50 kVA, 63 kVA and 100 kVA therefore you would probably install a 63 kVA transformer. In New Zealand the average household has an ADMD (after diversity maximum demand) of about 3000 VA, however a newly electrified village would have a lower demand as each household would probably only have lighting, TV etc for the first 5-10 years after this and as the economy improves they would buy freezers, air conditioners, washing machines etc and the load would increase. When the load increased you would install either a larger transformer or another transformer.

From their site - excellent SWER technical comments

What is SWER?
SWER, How does it work?
Low Voltage Service Drop
Low Voltage Pole Top Configuration for Rural Electrification
Distribution Transformer for Rural Electrification using SWER
SWER Inline Strain Pole for Rural Electrification
SWER Large Angle Pole for Rural Electrification
Small Angle Pole for Rural Electrification Using SWER
SWER Isolating Transformer
SWER Inline Pole

Here is an extremely good SWER overview - when one wire is enough. This contains substantial technical information re typical line lengths, customer density, allowed loads, distribution and feed transformer sizing, earth impedances and line loses and more. They note:

• Studies allowed 2% for low-voltage regulation and assumed the use of 16- and 25-kVA, 19,000-kV/500-250-V transformers with an approximate 4% impedance and taps of ± -5%, 2.5% and 0%.)

• Load densities. Load densities for a SWER distribution system typically are less than 0.5 kVA per kilometer (0.31 kVA per mile) of line with a maximum demand per customer of 3.5 kVA. A large system may supply up to 80 distribution transformers with unit ratings of 5 kVA, 10 kVA and 25 kVA. The load patterns and demands vary greatly from customer to customer and from one season to another; thus, as load growth continues, SWER systems are reaching their technical capability. And with customers keenly aware of supply quality, customer complaints are increasing.

Slideshow - 16 slides Practical SWER design - excellent (pdf)

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Superb 427 page World Bank pdf - 2006:
Sub-Saharan Africa - Introducing Low-cost Methods in Electricity Distribution Networks - ESMAP Technical Paper 104~06 October 2006

Sounds relevant :-)

• The level of electrification in sub-Saharan Africa is low, with less than 10 percent of the rural households having access to electricity. One of the key barriers to accelerating access is the high cost of connections, arising, inter alia, from the use of outdated, unsuitable, high-cost methods in electricity networks. A second key barrier is the small and dispersed nature of electricity demand, arising from low density of population and low income levels, which lead to high average costs of providing electricity service. The objective of this report is to help in reducing the high costs of electrification by documenting proven, low-cost methods and techniques in electricity networks that have not yet become widely used in sub-Saharan Africa.

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"Alternatives" - Papers on a wide range of non national-grid energy sources and applications in developing countries - Biomass, solar, charcoal, micro-hydro, biogas, sisal residues, Tanzania - solar home systems, wind, hydrothermal, ... PLUS regulatory, modelling, finance, access, sustainability, ... .

MICRO PERSPECTIVES FOR DECENTRALIZED ENERGY SUPPLY - Proceedings of the International Conference Technische Universität Berlin, 7th-8th of April 2011. 309 page pdf.

For interest only - URL of the above is: http://d-nb.info/1010874136/34

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Related:

Abstract - Extending SWER line capacity

MSc thesis 1972 - looks good AN ANALYSIS OF SINGLE WIRE EARTH RETURN(SWER) SYSTEM FOR RURAL ELECTRIFICATION

• This consulting project concerns a small village i Tanzania, with one transmission line from a 19kV grid to a mountain village. The rest is for us to decide, and in great detail deliver. With a estimated consumption of 2500$kWh/month$ ±10% within 1year. I see the picture much more clearly now. A very good answer and especially helpful links and report. Commented Oct 9, 2016 at 0:57
• Good 1972 Thesis .This was before " Degrees for Everybody" Commented Jan 8, 2017 at 1:29