rural electrification with the shield wire scheme applications in

RURAL ELECTRIFICATION WITH THE SHIELD WIRE

SCHEME APPLICATIONS IN

DEVELOPING COUNTRIES

by F. ILICETO University of Rome “La Sapienza”

Rome, Italy

AEI – AFRICA ELECTRIFICATION INITIATIVE WORKSHOP IN DAKAR, NOV. 2011

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• Low cost power supply from the interconnected grid to villages, small towns, farms, factories, water pumping stations located near or at some distance from the route of the HV lines (110-330kV)

• The SWSs consist of: - Insulating for MV operation (20-34.5kV) the shield wire(s) from the towers of the HV line - Energising the shield wire(s) at MV from the HV/MV transformer station at one end of the HV line

1. AIM OF SHIELD WIRE SCHEME (SWS)


2

- Using the earth return of current - Supplying the loads by means of distribution transformers branched between the shield wire(s) and the ground. • The most used SWSs are shown in Figs. 1/A - 1/B. • If the HV line is protected by one shield wire, only

the Single-Phase Earth-Return SWS can be realised (Fig. 1/A)

• If the HV line is protected by 2 shield wires, by using the earth return as the 3rd phase conductor, a 3-phase MV line is realised (Fig. 1/B).

2. CONCEPT OF SWSs AEI – AFRICA ELECTRIFICATION INITIATIVE WORKSHOP IN DAKAR, NOV. 2011

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Fig.1/A - “Single-phase Earth-return” SWS applicable to HV lines provided with one shield wire

4

Fig.1/B - “3-Phase” SWS applicable to HV lines provided with two shield wires

5

• Scheme of Fig. 1/B shows how the SWS is balanced in the simplest manner, with a grounding resistor-reactor and with unsymmetrical power factor correction capacitors.

• Fig. 2 shows the typical circuit schematic of a “3 Phase” SWS in the villages and the independent multiple earthing system for the earth return of current and for safety of LV networks.

• In the HV/MV substations supplying the SWS, the station ground mat is used for earth return of current.


6

Fig. 2 – Circuit schematic of “3-Phase” SWS distribution in the villages, showing independent earthing of MV and LV networks 7

• The insulation of shield wires does not worsen the

lightning performance of the HV line. Fig. 3 shows the rigid insulator strings usually

applied for insulation of shield wires. • The voltage imbalance at supply points of all the

consumers of “3-Phase” SWSs (Fig.1-B) and at the busbars supplying Single-Phase Earth-Return SWSs (Fig. 1/A) is limited to a very small value (negative-sequence voltage ≤1%).

3. MAIN FEATURES OF SWSs AEI – AFRICA ELECTRIFICATION INITIATIVE WORKSHOP IN DAKAR, NOV. 2011

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Fig. 3 – Typical rigid toughened glass insulator string for 34.5 kV “3-Phase” and “Single-Phase Earth-Return” SWLs

Dimensions are in mm • Wet 50 Hz – 60 s withstand voltage 130kV rms +) • Dry 1.2/50 µs impulse withstand voltage 270kV peak +) • Creepage distance 1200 mm • Electromechanical failing load ≥50 kN

+) without arcing horns

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• In the new HV lines, the SWSs use ACSR shield wires, with cross section of 70÷125 sqmm. A suitable cable has 19 wires with 63% of aluminium in the cross-section.

Some SWSs have been implemented in existing HV lines, by insulating their steel or alumoweld shield wire(s).

• The reach of SWSs with rated voltage of 34,5kV is up to and also over 100 km.


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• The “3-Phase” SWSs have about the same loading capability of a normal MV overhead line with the same phase-to-phase rated MV and same conductors: capability is several MW at 34,5kV. Typical loading capabilities are shown in Fig. 4.

• Earth-return of current has been used for several decades in the single-wire rural electrification in some countries.


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+) load p.f. on LV side of MV/LV transformers; ––––– Distributed load; – – – Concentrated load Fig. 4 – Loading capability versus length of “3-Phase” SWLs operated at 34.5kV: a.1–a.2: ACSR, S=125.1 sqmm shield wires on a 230kV-60Hz line b.1–b.2: ACSR, S=76.9 sqmm shield wires on a 161kV-50Hz line

0

2

46

8

10

12

14

16

0 25 50 75 100 125 150d [km]

∆V=7.5%

P [MW]

cosΦ = 0.9

cos = 0.97Φ

60 Hz - ACSR- 125.1 sqmm

a.2)+)

+)

0

2

4

6

8

10

12

14

16

0 25 50 75 100 125 150d [km]

P [MW]

∆ V=10%

cos Φ

cosΦ

=0.9

=0.97

60 Hz - ACSR - 125.1 sqmm

a.1)

+)

+)

0 20 40 60 80 100 120 1400

2

4

6

8

10

12

∆ V=10%

cosΦ =0.97

cosΦ =0.9

P [MW]

d[km]

50 Hz - ACSR - 76.9 sqmm

b.1)+)

+)

0 20 40 60 80 100 120 1400

2

4

6

8

10

12

∆ V=7.5%

cos Φ =0.97

cosΦ=0.9

d [km]

P [MW]

50 Hz - ACSR - 76,9 sqmm

b.2)+)

+)

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• Earth is an ideal conductor in developing countries:

- It has a small cost (cost of grounding rods and conductors installed by local manpower, used in common for other purposes) - Losses are very small (at 50Hz it is equivalent to an aluminium cable of 570sqmm) - Unlike conventional insulated conductors, it is neither exposed to insulation failure nor to interruption (“broken wire”) - Maintenance is negligible.


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• Design criteria and constraints of “3-Phase”

SWSs are the same as the ones for conventional MV lines, with the additional requirement of limiting the negative-sequence voltage.

• The analysis of SWSs is somewhat complex, due to interaction with HV circuit, earth-return of current and voltage balancing needs.

Operation is however simple and reliable because only conventional distribution equipment are applied, devoid of power electronic devices and using ordinary operational methods.


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• Although in the “3-Phase” SWSs the phase-to-ground operating voltage is higher by a factor of √3=1.732 in comparison with the conventional lines, the required increase of equipment insulation is only 15-20% above the standard of the MV equipment.

• SWLs are part of the HV line and therefore do not require specific maintenance, since it is performed for the HV line.

• No permanent faults have occurred on the SWLs lines, part of which have been in operation for over 20 years.


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• No permanent faults have occurred on the SWLs lines,part of which have been in operation for over 20 years.

• The outage rate due to transient faults has been reported lower for SWLs than for equivalent overhead MV lines.

• The cost of making electricity available at MV with the SWSs to communities located along the HV lines is only 10-15% of the cost of conventional solutions.


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• If an optical ground wire (OPGW) is applied in the HV line for telecommunications, the SWS can be realised as well by insulating for MV a standard OPGW.

• SWLs are a deterrent to vandalism and theft of HV lines, because the communities along the line must protect the line to ensure power supply to themselves from the SWL.


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• Ghana: About 1000km of 161kV–50Hz lines with insulated shield wires, most of which have been in operation for over 20 years (Fig. 5).

• Brasil: “3-Phase” SWSs have been in operation since 1995 at 34.5kV in a long 230kV-60Hz line.

• Laos: “Single-Phase Earth-Return” SWSs are in operation since 1996 in 190 km of 115kV-50Hz lines. “3-Phase” 34.5kV SWSs are in operation since 2002-2003 on 335km of 115 kV lines (Fig. 6).

4. SWSs in operation


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Fig. 5 - Single-line diagram of some insulated SWSs in Ghana (year 1989)

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Fig. 6 - Single-line diagrams of 34.5 kV “3-Phase” SWSs in Laos (year 2002)

Total length of 34.5kV lateral lines:4.7km

Length of SWL: 25.6km Forecast load of SWL in year 2018: 790kW

Total length of 34.5kV lateral lines:3.3km

Forecast load of SWL in year 2018: 1500kW

[kW]Loads

23

67.1

511

3

Xiang Ngeun SS

Muang Cha

(Initial stage)+1x452kVAR

2x333kVAR

22kV ( Initial stage )

( Initial stage )

[kW]

Distance

Loads

[Km]N° of node

2x60kVAR

2

4.27

120

188

Length of SWL: 74.6km; total length of 34.5kV lateral lines: 21.6km

E34.5kV

0+j0

0

015

0

1

115kVBan Don SS

Non Hai SS115kV

D

0

34.5kV

22kV

0.00

1.58

0

22kV

0.0

40.6

12

66Future 34.5kV lateral line

Houay Deua

2x125kVAR


32.3

9

30.3

928

.81

22.2

9

17.7

320

.25

12.5

7

[Km]

[kW]

5

120

120

3 4

75

6

150

7

188

120

8 9 10

75

N° of node

Loads

120

11

75

12

23.2

120

32 41

+1x115kVAR

9.58

7.82

5.12

33 7648 76End of SWL

10.3

17.6

16.5

20.5

8 10680 40

Distance

48 22 38 34

31.1

32.1

25.6

528

.67

34.1

9

37.6

11

Length of SWL: 32.4km

(Initial stage)+1x145kVAR

13 14

62 56

Na Am

44.6

47.1

Length of SWL: 76km; total length of 34.5kV lateral lines:19.6km

Length of SWL: 104.4km; total length of 34.5kV lateral lines: 54.2km

4.00

75

+1x225kVAR

N° of node [Km]

[kW]

0.00

115kVXaignabouli SS

C

B

22kV

34.5kV

13.5

00

0.00

34.5kV

19

0

0

115kV

Xieng Khuang SS

115kVNam Leuk SS

A

0

2x170kVAR

22kV

13.6

22kV

34.5kV

0.00

0

[Km]Distance

2x170kVAR

2x200kVAR

Loads

N° of node

[kW]



Muang Cha SS

28.7

48.9

28.1

25.0

45.1

22.0

20.9

19.7

1

[kW]

75

3x333 kVAR

350

11.3

318

8

42 31

120

12075

7.27

9.70

9.26

65

120

17.5

7

Loads 6575

1 2

80

3 4

Distance

N° of node7

Loads

300

1200

[Km]

98

75

28.7

4

32.1

6

(Final stage)

5

75

6

75

7

( Initial stage )

27.1

6

11

12880 808040 119

160 40 80 4080

Distance

15.2

716

.98

10.0

46.

93

12.6

7

21 3 54

24.2

9

19.3

9

21.6

920

.51

876 9 10

128

24080 40 8080 128

37.9

1

17

31.7

4

29.1

528

.12

141213 1615

39.9

2

18

64.1

76.0

60.4

62.1

22kV

11

+1x334kVAR

12

41.6

0

1110

120 75 75

38.4

740

.40

151413

3875 7546

.57

45.3

0

48.4

3

150

10

15035

8 9

3950

19

59.6

738

1716

7575

53.9

1

51.2

8

2120 22

7538 120

61.3

560

.93

64.8

7

14+j4

( Final stage )

80 256 804040

+1x225kVAR

61.1

1

20

55.9

3

19

68.3

9

63.3

466

.41

2221 23

80280 80208

80

40128

83.2

9

25

77.3

9

24

89.1

291

.08

26 27

3x200kVAR( Final stage )

28

73.8

675

26272524

3876 387570

.43

69.1

1

73.7

172

.12

29

3874

.60

N°

of n

ode

121.3529

[K

m]

129.28

123.28

3130

Dis

tanc

e

104.

40

28

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• Sierra Leone: “3-Phase” SWSs (Un=34.5kV) are in operation in 150km of 161kV-50Hz lines (Fig. 7).

• Ethiopia: “Single-Phase Earth-Return” SWSs (Un=34.5 kV) are in operation on 200 km of 132kV-50Hz lines (Fig. 8).

• Togo: “3-Phase” SWSs (Un=34.5kV) are in operation in 265km of 161kV-50Hz lines. One of the shield wires is an insulated OPGW (Fig.9).

• Burkina Faso: “3-Phase” 34.5 kV SWSs are in operation in 330 km of 225kV-50Hz lines. One insulated shield wire is an OPGW (Fig.10).


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Fig. 7 - Single-line diagram of 34.5 kV “3-Phase” SWSs in Sierra Leone

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Fig. 8- Single-line diagrams of 34.5 kV “Single-Phase Earth-Return” SWSs in Ethiopia (year 2000)

132 kV-115.7 km132/15 kV

132 kV-84.7 km

GHIMBI S/S NEKEMPTE S/SGHEDO S/S

34.5 kV

Sire

230 kV230/132/15 kV22/22/5.5 MVAtwo units

132/15 kV25 MVA15 kV

25 MVA

IT ITIT: 15kV/34.5kV+2x3.75%-3 MVA

15 kV

15 kVIT

84.3

1299

35 32.5 31.4 8 km

487 133 1155 100Loads

kW

km 42.7

2180125250179

15 18 28

936 325

60 50

34.5 kV LoadskW

34.5 kV

300 kVAR 300 kVAR 300 kVAR

0 0 73 km 0

Sum of simultaneous peak loads of SWLs in year 2019 =2734 +1987=4721 kWTotal length of SWLs=42.7+73=115.7km

Sum of simultaneous peak

Total length of SWL=84.3 kmloads of SWL in year 2019 =3174 kW

726

132 kV

23

Fig. 9 – Single-line diagrams of 34.5kV “3-Phase” SWSs in Togo 24

Fig. 10 Single-line diagrams of 34.5kV “3-Phase” SWSs in Burkina Faso

25

rural electrification with the shield wire scheme applications in

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