Presented at the 2015 PIESA Conference – 16 September 2015

Background of SWER in Zim

Pilot Description

The SWER Technology

Some Comparisons and

Perceived Benefits

General learning points

SWER going forward

The SWER pilot project was agreed at a 2009 PIESA

meeting with the following objectives

To elevate the experience sharing within PIESA from theory

to practice

To demonstrate the SWER technology as a practical model

for enhancing electrification efforts.

To push for technologies that provide greater electrification

rates without exhausting the budget (stretching the dollar)

In May 2010, a SWER workshop was held in Harare

and Mudzi (pilot site).

Attendants were drawn from PIESA member

countries and engineers from ZETDC and REA.

The workshop was for discussion on SWER

principles and preparing for the actual pilot project.

PIESA supported by footing the costs of availing

consultants from ESKOM (SA)

PIESA

•Technical

support

ZETDC

•Operation &

maintenance

REA

• Installation

SWER

The Mudzi SWER pilot project was a 30.4km line of

19.1kV SWER line and 5km of 240V single phase and

dual phase lines

The project serves 7 localities comprising of 13

institutions which are clinics, primary and

secondary schools, business centres and an

orphanage.

The area is semi arid and (by Zimbabwean

standards) receives low rainfall. Farming is basically

subsistence and the road infrastructure is gravel

road.

Before the project, the main source of energy was

firewood for both villagers and institutional staff.

Average individuals (sometimes households) live

below a US dollar a day.

The project realisation was split into three main

deliverables:

◦ The planning and design stage

◦ The procurement stage

◦ The implementation stage.

The planning and design stage

◦ Became the most critical process

◦ Took the second longest time due to several

consultations and learning curve issues

◦ This stage included the experimentation stages

(conductor)

◦ The substation positioning and the earthing

resistivity readings etc

The procurement stage

◦ Proved to be the longest for the pilot project

◦ Conductor manufacturer had never made such a

construction before then.

◦ New items [1Ø recloser, transformers, conductor)

◦ Transformers had to be imported (shipment

damages and insurance arguments)

◦ Some delays were up to 9 months long

The Implementation stage

◦ Turned out to be the shortest stage for the pilot.

◦ The SWER line part took 7 weeks for 30km

(compared to our 1 week/km for 33kV)

◦ This was however not continuous as there were

learning or procurement stops, (one stop socio-

political)

◦ Some reworks and extensions had to be done

sometime (esp. on the earthings)

SWER – Single Wire Earth Return is a technology

where not three wires (three Phase), and not two

wires (common single Phase), but one wire is used

to transport electric power over a distance with the

earth used as the neutral or return path for

completing the circuit.

The objective is to achieve longer distances of

electrification on less money.

As electricity supply is extended to remote regions,

the cost of supply using the conventional three-

phase technology increases to levels that do not

often justify a utility’s investment into constructing

such a line. One main reason is due to the sparsity

of load centres with light/small loads.

SWER answers just that: long distances, light loads

and sparse settlement patterns.

1. SWER Site selection:

SWER is very ideal for terminal ends or radial

networks.

Where there is very low likelihood of new extensions

after the end point.

High level of dispersion in settlement patterns

Also recognising that the earth’s conductivity is

enhanced by salts, soil resistivity measurements

become the next indicative filter for site suitability.

2. SWER Planning:

◦ Load estimation is done first by assessments of actual

expected load centres

◦ Then forecast with consideration on the high case and

base case as well as envisaged consumption patterns

◦ Then load flow simulations (all similar to 3 Phase

considerations

◦ Then compare with 475kVA total load capacity of

typical SWER isolation transformers and decide for or

against SWER

2. SWER Planning:

The following should be kept in mind as guiding

constraint

◦ 475kVA upper limit

◦ 19.1kV operating voltage (applying +/- tolerance of the grid

code)

◦ Total load is 25Amps

◦ >2% voltage unbalance or deviation

3. SWER Network Attributes :

◦ 1. ISOLATION TRANSFORMER which is a main

element of the network

The isolation transformer provides for the

separation (Isolates) from the three phase

network into the SWER network often at 33kV or

22kV or even 11kV to a 19.1kv as the case may

be.

It also is the sink for the return currents from the

various distribution transformers

SWER Network Attributes :

ISOLATION TRANSFORMER is always at the head

of the network and is seen as the source.

The Mudzi isolation transformer can cater for up to

475kVA hence the limit of load on the SWER

network is 25Amps

◦ An ISOLATION TRANSFORMER

BACK

SWER Network Attributes:

DISTRIBUTION TRANSFORMER serves the function

of the normal customer end transformer

Common sizes in the Zimbabwe experience are

16kVA, 32kVA and 64kVA, though sizes like 128kVA

can be obtained.

All are typically pole mounted on either single pole

or double pole structures

A typical 32kVA

transformer

A typical 32kVA

transformer

SWER Network Attributes :

THE SWER LINE – this relates mainly to:

Choice of conductor

Span lengths

Construction and structure

Other line accessories

The Conductor :

The conductor used in Mudzi and and the rest of the

new projects (save for one) is Magpie.

An ultra high tensile conductor (4 steel/3 Al)

6.35mm diameter and 10.58sqmm effective cross-

sectional area

Current rating is 92Amps

UTS is 18573 Newtons

The Line structures and construction:

The SWER line had 12.6m pole just like for 33kV

Spans used were 300m max to cater for both

materials savings and upgrade provisions

The span caters for 4 poles per km compared to the

8 for standard 33kV lines (without mention of discs,

cross-arms etc)

A typical intermediate structure

A Swing angle

Vertical strain

SINGLE pole strain

A terminal point

A terminal point

Typical 3 Way

SWER EARTHING.

Because all the return currents have to pass through

the ground, the earth becomes a key attribute of a

SWER network.

The measurements of soil resistivity, electrode

selection and the quality of installation are

paramount to the success of SWER reticulations

The size of the electrode depends on the resistivity

of the ground and the transformer size being earthed

REMOTE SWER EARTHING.

In some cases, the ground around a load centre has

poor quality resistivities and the earths have to be

situated at a remote location

An under-strung earth is often strung under the

SWER line to such a location with suitable ground

resistivities.

Under-strung

earth

The electrodes used in Mudzi were 16sqmm

bare copper wire

The configuration chosen was strip buried

and twin strip buried ahead of vertical

electrodes which requires drilling.

Sometimes soil resistivity enhancement

agents like gypsum can be used when

resistivities are too poor (not used in Mudzi)

SWER Protection

◦ A recloser is used just after the isolation

transformer for the whole line

◦ In the Mudzi case, fuses then formed the rest of

the protection on the SWER size

◦ Normal LV side breakers and protection practices

apply.

SWER networks 33kV Network Perceived benefit

Use of 1 conductor Uses 3 conductors Spares supplier from a would be waste

Uses 10sqmm conductor Uses rabbit (or even 100sqmm)

Opportunity for making conductor choice with demand rather than norm

Magpie costs $0.47/m Rabbit costs $1.07, Dog costs - $1.98

Assuming 3Ø was used •Rabbit would cost 6 X •Dog would cost over 12X

No cross-arm used Needs cross-arms Costs of cross-arms saved

300m spans achieved 150m spans used Saves up to half the cost of poles per km

General line hardware greatly reduced (Insulators, bolts, nuts)

Standard 3Ø requirements apply

Costs of extra hardware reduced

SWER networks 33kV Network Perceived benefit

Required 2days per km Requires 7 days per km 5 days saving

Manpower morale (conductor much easier to strain)

Standard 3Ø conditions apply

Much lighter physical work demand on the conductor straining

Opportunity to redesign wind stay ratings

Standard 3Ø conditions apply

Opportunity to save on staying costs

$4 800/km (Mat North experience)

$16 000/km (assuming transferred capacity is same) opportunity to reduce cost to less than a third

More copper used for electrode

Comparatively less copper if same network were electrified with 3Ø

Copper extras are significantly compensated for by conductor and man day gains

SWER networks 33kV Network Perceived benefit

Need converters to run some 3Ø equipment like grinding mills

Ready for both 1Ø and 3Ø equipment

Cost benefit analysis and load patterns to be done at planning to ensure that the supply choice is made appropriately

Need to train installation and maintenance teams

Same routines and Eng Instruction

Opportunity for CPD

Give time to understanding the technology (Mudzi

plinth)

Keep the objective clearly in mind throughout

◦ To make a robust effective network without spending an extra

$ than necessary (engineer’s sometimes forget the $$$)

Attend to planning exhaustively

◦ Route surveys,

◦ substation sites to remote earths cost balance

Train the users and handlers

Watch out for procurement process

◦ For projects since the pilot, procurement has improved

greatly

◦ Earth Resistivity testing equipment bought early on, (quality

and calibration/reference)

Work with supplier

◦ ZENT now making SWER transformers

◦ CAFCA now making Magpie and Shrike

Work with fresh minds willing to learn!!!

Strengthen customer liaison (chief Nyamukoho)

Chapatarongo-Sabvure Project – 14 Institutions ◦ Scope: 0.15km of 33kV line, 39km of 19.1kV line, 3.5km

of MV line, 4 x 16kVA, 4 x 32kVA, 2 x 64kVA and 1 x 475kVA.

Chikafa Project – 10 Institutions ◦ Scope: 35.1km of 19.1kV line; 1.51km of MV line, 4 x

32kVA, 1 x 64kVA, and 1 x 475kVA.

Mabalauta Malipati Project – 7 Institutions ◦ Scope: 0.15km of 33kV line, 30km of 19.1kV line, 1.2km

of MV line, 1 x32kVA 2x64kVA and 1x475kVA.

Lubimbi Project – 8 Institutions ◦ Scope: 0.15km of 33kV line, 43km of 19.1kV line, 2.98

of MV line, 1x16kVA, 1x 32kVA, 3x64kVA and 1x 475KVA.

Swereki Project – 14 Institutions ◦ Scope: 0.3km of 33kV line, 50.5km of 19.1kV line;

4.2km of MV line, 2 x 16kVA, 3 x 32kVA; 3 x 64kVAand 1 x 475kVA.

Msala Project– 11 Institutions ◦ Scope: 0.216km of 33kVline, 45km of 19.1kV line, 1.53

of MV line,2 x 16kVA, 8 x 32kVA and 1 x 475kVA.

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