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SINGLE-PHASE METALLIC RETURN TECHNIQUE

Presented by

Moncef AISSASenior Engineer

STEG-TunisiaMarch 7, 2006

contributors- E. Cecelsky- A. Ounalli- J. Dunkerly

RURAL ELECTRIFICATION IN TUNISIA :

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Situation before RE program -1975

Rural electrification rate: 6% Overall electrification rate 35% MV grid length : 4000 km 3-phase 30 kV Rural population: 53% Real GDP per capita: $770 Government decided to develop RE

INSTITUTIONAL APPROACH Ministry of Economic Development (MDE)

- Policy-plan-funding Ministry of Industry and Energy (MIE)

- Energy policy - supervisory authority over STEG STEG (Public Utility)

- appointed as Operator

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STEG: STRATEGIC CHOICE STEG: STRATEGIC CHOICE

Rural area indicators income, load, density, high cost

STEG undertook tech-econ study / comparison

Main findings

- "Never has a technical recommendation raised as many debates and exchanges as STEG“

GRID LEVELGRID LEVEL SAVINGSSAVINGS

MV network 30 to 40 %

MV/LV substations 15 to 20 %

LV network 5 to 10 %

Overall 18 to 24 %

ADOPTION of MALT technique in January 1976 kept existent 30 kV voltage level

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Z

HV/MV Substation inverse time protection

Earthing coil

Grounded neutral

MV 4-Wire line

H Type Fused cutout

T Type Fused cutout

1-phase MV line

1-ph. MV/LV transformer

LV 1-ph lines

MALT CONFIGURATION

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Characteristics of Tunisian MALT system 148.1 mm² AAAC for 3-Φ overhead backbone lines

(Winter: 360A; 16.8 MW : Summer 250A, 12 MW) 54.6 mm² AAAC for rural 1-Φ overhead lines (150A; up

to 2.6 MW)

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Recent cost cuttings estimate (2001)

27% cost-cutting with suspension insulators 37% cost-cutting with pin insulator

Distribution of cost reductions in % Main savings come from

Conductors 7% Line accessories: 13% Poles: 7%(supension insul) / 17% (Pin Insulators) Single phase transformer: 44% for 25 kVA

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Further cost reductions

26-30% additional cost reduction compared to single-phase technique

1200km SWER lines- 425 villages

No need for “isolating transformer installation”

Additional Cost reduction 7-14% if houses are fairly

dispersed 31-33 % if houses are widely

dispersed

1. SWER technique

2. 4,16 kV single phase

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Non technical approach:Continuous efforts to optimize costs

1. Early computerization 2. Training 3. Demand Forecasting –

Networks Planning 4. Operational norms and

guidelines5. Private enterprise

participation in construction (Competition)

6. Private industry participation in grid material supply (Competition)

7. Inventory Management system

8. Technical and non technical losses management

9. Maintenance planning10. Live line Works11. Customer Management 12. Customer Service

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Funding Households STEG Government thru loans

Affordability Affordability

Easy terms for STEG consumersEasy terms for STEG consumers Agriculture Bank

Sustainability: thru TARIFF STRUCTURE

Lifeline tariff < 50kWh/month (= 600 kWh / year / Customer)

Special tariffs for Rural development – irrigation, olive oil plant, milling/grinding

Socio-Economic Impact– Education, health, security, economic opportunity

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Advantages of Single Phase Technology

Cheaper

Suited to rural area characteristics

Reliable and secure

Numerous opportunities to coordinate between protection devices

Good quality service

Upgradeability to duplex and triplex

Possibility to use converter

High permissible load current

Allows extension in SWER without isolating transformers

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Disadvantage of Single Phase Technology

Adaptation of existing conventional 3-phase network to MALT technique

–power cuts

–fixing neutral on every pole

–keeping requested conductor clearance for every span

–upfront cost

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-- OVERAL Electrification Rate 98.9%

-- National Rural ER 97%

-- Minimum REGIONAL Rural ER 96.6%

NORTH WEST96,6%

CENTER WEST96,8%

NORTH EAST97%

CENTER EAST97,9%

GRAND TUNIS97,5%

SOUTH WEST96,3%

HAMMAMET

SFAX

SOUSSE

GABES

GAFSA

TATAOUINE

KEBILLI

SOUTH EAST96,8%

BIZERTE

Recent situation: end 2004

% of single-phase lines and substations • 53% of overhead lines are single phase type

• 72% of public MV/LV substations are single phase type

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Conclusion After 30 years using single phase

technique we believe we made a good technical choice

The choice was made early enough to allow maximum benefits from low-cost rural electrification techniques

What To Do Differently More SWER lines / more 4.16 kV single phase lines Use more pin post and line post insulators Fix neutral conductor at top of pole

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Some Recommendations

Start Single Phase Technique as soon as possible

Adapt existent network to MALT system

Fix neutral conductor at top of pole to prevent cable theft

Use single phase lines for expandable network

Use SWER or 4 kV single phase technique for remote areas

Use small single phase transformers (1; 3; 5; 10; 15; 25 kVA)

– possibility of overload max 20%

Use conductors with small cross-section

Use ACSR conductors if terrain permits long span

Use pin post, line post insulators or pin insulator every time suspension insulator / crossarm can be avoided

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stork nest