SINGLE-PHASE METALLIC RETURN TECHNIQUE

1
SINGLE-PHASE METALLIC RETURN TECHNIQUE
RURAL ELECTRIFICATION IN TUNISIA

• Presented by
• Moncef AISSA
• Senior Engineer
• STEG-Tunisia
• March 7, 2006

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

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
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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MALT CONFIGURATION

• Rural area

1-ph. MV/LV transformer
H Type Fused cutout
HV/MV Substation inverse time protection
T Type Fused cutout
MV 4-Wire line
Grounded neutral
Earthing coil
Z
1-phase MV line
LV 1-ph lines

• Rural area

5
Characteristics of Tunisian MALT system

• 148.1 mm² AAAC for 3-F overhead backbone lines
  (Winter 360A 16.8 MW Summer 250A, 12 MW)
• 54.6 mm² AAAC for rural 1-F overhead lines (150A
  up to 2.6 MW)

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
Further cost reductions
1. SWER technique

• 26-30 additional cost reduction compared to
  single-phase technique
• 1200km SWER lines- 425 villages
• No need for isolating transformer installation

2. 4,16 kV single phase

• Additional Cost reduction
• 7-14 if houses are fairly dispersed
• 31-33 if houses are widely dispersed

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Non technical approachContinuous efforts to
optimize costs

• Early computerization
• Training
• Demand Forecasting Networks Planning
• Operational norms and guidelines
• Private enterprise participation in construction
  (Competition)
• Private industry participation in grid material
  supply (Competition)

1. Inventory Management system
2. Technical and non technical losses management
3. Maintenance planning
4. Live line Works
5. Customer Management
6. Customer Service

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Funding

• Households
• STEG
• Government thru loans

Affordability

• Easy terms for STEG consumers
• Agriculture Bank

Sustainability thru TARIFF STRUCTURE

• Lifeline tariff lt 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
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
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

• 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

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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

• neutral wire

• phase wire

• THANK YOU