RURAL ELECTRIFICATION WITH THE SHIELD WIRE SCHEME APPLICATIONS IN DEVELOPING COUNTRIES

Transcription

1 AEI AFRICA ELECTRIFICATION INITIATIVE WORKSHOP IN DAKAR, NOV RURAL ELECTRIFICATION WITH THE SHIELD WIRE SCHEME APPLICATIONS IN DEVELOPING COUNTRIES by F. ILICETO University of Rome La Sapienza Rome, Italy 1

2 1. AIM OF SHIELD WIRE SCHEME (SWS) 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 ( kV) The SWSs consist of: - Insulating for MV operation ( kV) 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 2

3 2. CONCEPT OF SWSs - 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 3 rd phase conductor, a 3-phase MV line is realised (Fig. 1/B). 3

4 Fig.1/A - Single-phase Earth-return SWS applicable to HV lines provided with one shield wire 4

5 Fig.1/B - 3-Phase SWS applicable to HV lines provided with two shield wires 5

6 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

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

8 3. MAIN FEATURES OF SWSs 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%). 8

9 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 Fig. 3 Typical rigid toughened glass insulator string for 34.5 kv 3-Phase and Single-Phase Earth-Return SWLs 9

10 In the new HV lines, the SWSs use ACSR shield wires, with cross section of 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. 10

11 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. 11

12 P [MW] cos Φ =0.9 +) cos Φ =0.97 +) 60 Hz - ACSR sqmm V=10% a.1) d [km] 150 P [MW] ) cosφ = Hz - ACSR sqmm V=7.5% +) cos Φ = 0.97 a.2) d [km] P [MW] cos Φ =0.9 +) 50 Hz - ACSR sqmm V=10% +) cos Φ =0.97 b.1) P [MW] 50 Hz - ACSR - 76,9 sqmm V=7.5% cos Φ =0.97 +) b.2) +) cosφ = d[km] d [km] +) 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 12

13 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. 13

14 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. 14

15 Although in the 3-Phase SWSs the phase-toground 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. 15

16 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. 16

17 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. 17

18 4. SWSs in operation 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 on 335km of 115 kv lines (Fig. 6). 18

19 Fig. 5 - Single-line diagram of some insulated SWSs in Ghana (year 1989) 19

20 Xieng Khuang SS 115kV A 115kV B Xaignabouli SS 115kV 22kV C Ban Don SS 115kV 22kV D Non Hai SS 115kV 22kV E j0 22kV 34.5kV Nam Leuk SS 22kV 34.5kV 34.5kV 34.5kV 34.5kV x170kVAR +1x225kVAR ( Initial stage ) x333 kvar (Final stage) 22kV Forecast load of SWL in year 2018: 50kW Length of SWL: 76km; total length of 34.5kV lateral lines:19.6km Loads [kw] 2x200kVAR +1x334kVAR 22kV ( Initial stage ) Forecast load of SWL in year 2018: 3605kW Length of SWL: 74.6km; total length of 34.5kV lateral lines: 21.6km End of SWL Distance [Km] 0 N of node 1 0 Loads [kw] 2x60kVAR +1x115kVAR ( Initial stage ) Forecast load of SWL in year 2018: 3100kW Length of SWL: 104.4km; total length of 34.5kV lateral lines: 54.2km Distance [Km] N of node Loads [kw] Distance [Km] 7 N of node Future 34.5kV lateral line Distance [Km] Loads [kw] N of node Distance [Km] N of node 19 Loads [kw] 2x170kVAR +1x225kVAR ( Final stage ) Muang Cha SS Na Am Houay Deua j Muang Cha Loads [kw] x333kVAR +1x452kVAR (Initial stage) Xiang Ngeun SS Forecast load of SWL in year 2018: 790kW Length of SWL: 25.6km Total length of 34.5kV lateral lines:3.3km Forecast load of SWL in year 2018: 1500kW Length of SWL: 32.4km Total length of 34.5kV lateral lines:4.7km 2x125kVAR +1x145kVAR (Initial stage) N of node Distance [Km] x200kVAR ( Final stage ) Fig. 6 - Single-line diagrams of 34.5 kv 3-Phase SWSs in Laos (year 2002) 20

21 Sierra Leone: 3-Phase SWSs (U n =34.5kV) are in operation in 150km of 161kV-50Hz lines (Fig. 7). Ethiopia: Single-Phase Earth-Return SWSs (U n =34.5 kv) are in operation on 200 km of 132kV- 50Hz lines (Fig. 8). Togo: 3-Phase SWSs (U n =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). 21

22 Fig. 7 - Single-line diagram of 34.5 kv 3-Phase SWSs in Sierra Leone 22

23 GHIMBI S/S 132 kv 132 kv-84.7 km NEKEMPTE S/S 132/15 kv 25 MVA Sire 132 kv km GHEDO S/S 230 kv 230/132/15 kv 22/22/5.5 MVA two units 15 kv km 0 km km kv Loads 34.5 kv Loads 34.5 kv kw kw 132/15 kv 300 kvar IT 300 kvar 300 kvar 25 MVA IT IT Sum of simultaneous peak IT: 15kV/34.5kV+2x3.%-3 MVA loads of SWL in year 2019 =3174 kw 15 kv Sum of simultaneous peak loads of SWLs in year 2019 = =4721 kw Total length of SWL=84.3 km Total length of SWLs= =115.7km 15 kv Fig. 8- Single-line diagrams of 34.5 kv Single-Phase Earth-Return SWSs in Ethiopia (year 2000) 23

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

25 Fig. 10 Single-line diagrams of 34.5kV 3-Phase SWSs in Burkina Faso 25