1. 1 SINGLE-PHASE METALLIC RETURN TECHNIQUE Presented by Moncef AISSA Senior Engineer STEG-Tunisia March 7, 2006 contributors - E. Cecelsky - A. Ounalli - J. Dunkerly RURAL ELECTRIFICATION IN TUNISIA :

2. 2 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 n 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

3. 3 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 LEVEL SAVINGS SAVINGS MV network30 to 40 % MV/LV substations 15 to 20 % LV network5 to 10 % Overall18 to 24 % ADOPTION of MALT technique in January 1976 kept existent 30 kV voltage level

4. 4 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

5. 5 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)

6. 6 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

7. 7 Further cost reductions n 26-30% additional cost reduction compared to single-phase technique n 1200km SWER lines- 425 villages n No need for “isolating transformer installation” Additional Cost reduction n 7-14% if houses are fairly dispersed n 31-33 % if houses are widely dispersed 1. SWER technique 2. 4,16 kV single phase

8. 8 Non technical approach: Continuous efforts to optimize costs 1. Early computerization 2. Training 3. Demand Forecasting – Networks Planning 4. Operational norms and guidelines 5. 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 planning 10. Live line Works 11. Customer Management 12. Customer Service

9. 9 Funding  Households  STEG  Government thru loans Affordability Affordability  Easy terms for STEG consumers  Agriculture Bank Sustainability: thru TARIFF STRUCTURE n Lifeline tariff < 50kWh/month (= 600 kWh / year / Customer ) n Special tariffs for Rural development –irrigation, olive oil plant, milling/grinding Socio-Economic Impact –Education, health, security, economic opportunity

10. 10 Advantages of Single Phase Technology n Cheaper n Suited to rural area characteristics n Reliable and secure n Numerous opportunities to coordinate between protection devices n Good quality service n Upgradeability to duplex and triplex n Possibility to use converter n High permissible load current n Allows extension in SWER without isolating transformers

11. 11 Disadvantage of Single Phase Technology n 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

12. 12 -- OVERAL Electrification Rate 98.9% -- National Rural ER 97% -- Minimum REGIONAL Rural ER 96.6% 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

13. 13 Conclusion n After 30 years using single phase technique we believe we made a good technical choice n The choice was made early enough to allow maximum benefits from low-cost rural electrification techniques What To Do Differently n More SWER lines / more 4.16 kV single phase lines n Use more pin post and line post insulators n Fix neutral conductor at top of pole

14. 14 Some Recommendations n Start Single Phase Technique as soon as possible n Adapt existent network to MALT system n Fix neutral conductor at top of pole to prevent cable theft n Use single phase lines for expandable network n Use SWER or 4 kV single phase technique for remote areas n Use small single phase transformers (1; 3; 5; 10; 15; 25 kVA) –possibility of overload max 20% n Use conductors with small cross-section n Use ACSR conductors if terrain permits long span n Use pin post, line post insulators or pin insulator every time suspension insulator / crossarm can be avoided

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