shield wire

8/3/2019 Shield Wire

1/36

THE SHIELD WIRE POWER SCHEME(SWS)

EXPECTATIONS


2/36


Learning Objectives:By the end of this Program, theparticipants should be able to;

1. Describe accurately using schematics,the 4 Shield Wire Power DistributionSchemes (SWS).

2. Identify specifically the type of schemeadopted for various SW communities inthe NED operational areas.


3/36

Learning Objectives, contd:3. Identify all major components used in

the SWS and their functions.

4. Identify all potential dangers associatedwith the operation of the scheme andhow to mitigate them during operations

and maintenance.5. Adopt Shield Wire Equipment on

Conventional Schemes and vice versa.

6. Operate and maintain Safely the SWSs.



4/36

What is the Shield Wire Power Scheme?

The technique of using the Sky/ShieldWire to transmit power for distribution of3-phase and single-phase power tosmaller communities along the High

Voltage Transmission Lines in VRA.



5/36

THE SKY/SHIELD WIRE

What is a Shield Wire and how is ashield wire normally connected?

What are the uses of Shield Wires?

Protection of Power Lines againstLightning discharges

Mechanical Protection for Fibre OpticCables for Communication and SCADASystems.

Insulated at the Towers to carr Power
http://hyperlink%20pictures/SkyWire%20Presentation.ppthttp://hyperlink%20pictures/SkyWire%20Presentation.ppt


6/36

THE CONCEPT OF SWSSky/Shield wires are insulated at Towers and

energised at the substations nearest to thecommunities to be served. They are:

Insulated for medium voltage operation.

Energised at 20-34kV from HV/MV sub-station. Supplies loads using earth return current.

Supply loads by means of distribution transformersbranched between the shield wires or wire(s) and

ground. Still performs power line protection with appropriate

Arcing Horns.

With no additional environmental impact.
http://hyperlink%20pictures/Insulated%20Skywire%20Presentation.ppthttp://hyperlink%20pictures/Arcing%20Horns%20Presentation.ppthttp://hyperlink%20pictures/Arcing%20Horns%20Presentation.ppthttp://hyperlink%20pictures/Arcing%20Horns%20Presentation.ppthttp://hyperlink%20pictures/Insulated%20Skywire%20Presentation.ppt


7/36

The History The citizens of small communities living near HV

transmission lines who had no supply made it a

Corporate Social Responsibility and PublicRelations issue for VRA.

VRA invited Prof. Iliceto in early 1980s to makeinnovative proposals for a Low Cost solution to

serve the settlements near its 161kVtransmission lines.

BACKGROUND AND MOTIVATIONOF SWS
http://cost%20consideration.ppt/http://cost%20consideration.ppt/


8/36

Shield Wire Scheme (SWS) is proposed &implemented on the Cape Coast TakoradiTransmission Line

The cost of SWS is only a fraction of standard MVline.

The operational performance of the SWS in termsof outage frequency and time is better than normal.

BACKGROUND AND MOTIVATIONOF SWS


9/36

Savings and Benefits

Installation Costs are lower because: Common usage of conductors & grounding rods.

Does not add to the power line right-of-way.

Operational Performance is better as: Outage rates are low & permanent faults are rare.

Low medium voltage losses due to conductor size.

Maintenance burden is extremely low.

Other Benefits.

Community protection of the transmission line.


10/36

Cost Comparison:MV vrs SWS

Insulators Insulators

Steel

Lattice

Towers

Survey &

Civil Works

Conductors

Accessories

0

5,000

10,000

15,000

20,000

25,000

30,000

MV Line SWS

US$perkm


11/36

MV Line SWS Comment

Acquisition of Right-of-Way Variable - Depends on local conditions.

Survey &

Civil Works4,500 -

Steel Lattice Towers 7,500 750 Increase in erection only

Conductors 10,000 -

Insulators 2,500 3,000

Accessories 500 -

Total Cost per kilometer 25,000 3,750 Saving = 85%

Typical Construction Cost of 34.5 kV line in US $ per km)


12/36

The massive extension of the HV grid to thenorth in the late 80s presented anopportunity to deploy SWS on the new lines.

SWS cost is only 15% of equivalent MV line.Yet its operational performance is superior. SWS is now proven technology which

supplies settlements within a 20km corridor

of the transmission line at very reasonablecost. Adopted in many African countries and

beyond.

TRANSFER OF SUCCESS


13/36

THE SWS DEVELOPED

Currently four schemes developed are asshown A, B, C and D in Fig 1 below which are:

SWSA - Single-Phase Earth-Return Scheme

SWSB - Single-Phase Metallic-Return Scheme

SWSC - V (Open Delta) Scheme

SWSD - 3-Phase () Scheme.


14/36


15/36

SWS A (in Fig. 1) Single-Phase Earth-Return Schemefeasible for HV line protected by one shield wire.

I61/34.5/11kV

Tm

I61/34.5/11kV

Tm
http://hyperlink%20pictures/Towers%20Presentation.ppthttp://hyperlink%20pictures/Towers%20Presentation.ppt


16/36

SWS B (in Fig. 1) Single-Phase Metallic-ReturnScheme feasible for HV line protected by Two shield

wires.

I61/34.5/11kV

Tm
http://hyperlink%20pictures/Towers%20Presentation.ppthttp://hyperlink%20pictures/Towers%20Presentation.ppthttp://hyperlink%20pictures/Towers%20Presentation.ppthttp://hyperlink%20pictures/Towers%20Presentation.ppthttp://hyperlink%20pictures/Towers%20Presentation.ppthttp://hyperlink%20pictures/Towers%20Presentation.ppt


17/36

SWS C (in Fig. 1) The V (Open-Delta) Schemefeasible for HV line protected by Two shield wires.

They can supply limited amount of 3-phase loads.

161/34.5 /11kV

Tm
http://hyperlink%20pictures/V%20Scheme%20Trafo%20Presentation.ppthttp://hyperlink%20pictures/V%20Scheme%20Trafo%20Presentation.ppthttp://hyperlink%20pictures/V%20Scheme%20Trafo%20Presentation.ppthttp://hyperlink%20pictures/V%20Scheme%20Trafo%20Presentation.ppthttp://hyperlink%20pictures/V%20Scheme%20Trafo%20Presentation.ppt


18/36

SWS D (in Fig. 1) The (Delta) Scheme feasible forHV line protected by Two shield wires.

They can supply 100% 3-phase loads.

I61/34.5/11kV

Tm
http://hyperlink%20pictures/D%20Scheme%20Trafo%20Presentation.ppthttp://hyperlink%20pictures/D%20Scheme%20Trafo%20Presentation.ppthttp://hyperlink%20pictures/D%20Scheme%20Trafo%20Presentation.ppthttp://hyperlink%20pictures/D%20Scheme%20Trafo%20Presentation.ppthttp://hyperlink%20pictures/D%20Scheme%20Trafo%20Presentation.ppthttp://hyperlink%20pictures/D%20Scheme%20Trafo%20Presentation.ppt


19/36

THE APPLICATION

SWS in NED AreasFig. 6Table 1
http://hyperlink%20pictures/FIG%206.dochttp://hyperlink%20pictures/Table%201.dochttp://hyperlink%20pictures/Table%201.dochttp://hyperlink%20pictures/FIG%206.doc


20/36

Single Phase Earth Return (In Fig. 1 A).

3-phase Earth Return (In Fig. 1 C & D) by usingthe earth return as the 3rd phase conductor, a 3-

phase MV line is realised (D in Fig. 1)

Fig. 2 shows the typical independent multipleearthing systems for the earth return of current and

for safety of MV and LV networks.

In the HV/MV substations supplying the SWS, thestation ground mat is used for earth return of

current.

THE EARTH RETURN OF CURRENT


21/36

EARTHING OF MV AND LV NETWORKS

Fig 2


22/36


Fig. 2 shows the typical independent multiple

earthing system for the earth return of current andfor safety of MV and LV networks.MV Earth Return is achieved by burying andconnecting a bare conductor between the 161kV

steel pylon and the 34.5kV take off steel pylon orthe 161kV pylon and a ground/earth electrodeunder the wooden pole serve as the ground for thesystem.The Earth-Return wire is connected to thisground and multiple grounded to the general massof earth at every 2 wooden poles interval for the

20kV/30kV transmission.


23/36


At the MV/LV Transformer Station 3

independent Grounding systems areprovided (Ref. Distribution NetworkEarthing Arrangement Drawing No

A/EAR/01 07) which are:

1.MV Grounding System2.Surge/Lightning Arrestor Grounding System

3.LV Neutral/Grounding System


24/36


1.MV Grounding System: This comprises ofMV Multiple grounded Earth return

Conductor

The Transformer MV Ground TerminalThe Metalworks (Non Current carrying parts)

of the Substation

Earth Electrode


25/36


2. Surge/Lightning Arrestors Grounding

System. This comprises of:

The Ground Terminal of the Surge Arrestors

Ground /Earthing Lead

Ground Electrode.

The Two electrodes are buried at the MV/LVtransformer station three meters deep, and atleast four meters away from each other.


26/36


3. LV Neutral/Grounding System:Consists of:

The Neutral/Grounding Feeder Wire

The Transformer Neutral/Ground TerminalMultiple rods along the LV feeders at 4 pole

intervals.

Transformer LV neutral electrode buried onepole span away from the MV/LV station.


27/36

Scheme A - Single Phase Earth Return

The Shield Wire Line (SWL) = 34.5kV/3=19.92kV or 20kV.The Transformer HV Side = 34.5kV/3=19.92kV or 20kV.The Transformer LV Side:=240V phase toGround, 480V phase to phase

Scheme B - Single Phase Metal Return.The Shield Wire Line (SWL) = 34.5kV.The Transformer HV Side: = 34.5kV.The Transformer LV Side:= 240V phase to

Ground, 480V phase to phase.

VOLTAGE LEVELS OF THE SWS


28/36

Scheme C The V Scheme (3-phase Open Delta)The Shield Wire Line (SWL) = 34.5kV.The Transformer HV Side: = 34.5kV, phase to phase,20kV phase to ground

The Transformer LV Side: 240V phase to Ground, 415Vphase to phase

Scheme D - 3-phase Close Delta SWL (In Fig. 1 D).The Shield Wire Line (SWL). = 30kV

The Transformer HV Side: 30kV, phase to phase, 30kVphase to groundThe Transformer LV Side: 433V, phase to phase, 240Vphase to ground



29/36

Questions:1.Explain the differences between the SWS

Transformers and Conventional MV/LV

Transformers.

2.Can a Shield Wire Transformer be used ona Conventional MV Network? Explain

3.Can a Conventional MV/LV Transformerbe used on a Shield Wire MV/LV Network?

Explain.



30/36

Insulators used on SWLs are either3-String for 20kV Lines and 4-Stringfor 30kV Lines.

Surge/ Lightning Arrestors 48kVand 60kV


MTCE & TESTING OF SWS


31/36

Inspection,Maintenance,Testing procedure and,

Calculation of results forConventional Transformers apply toSWS Transformers as well. (check

Nameplate Information)

MTCE & TESTING OF SWSTRANSFORMERS


32/36

Energised StateVoltage LevelsLimit of Approach

Isolated StateCapacitorsInduced Voltages

SAFETY CONSIDERATIONS

COMPONENTS OF THE SWS AND THEIR


33/36

The Main Transformer (HV/MV)Feeder Circuit BreakersInterposing TransformerCompensating Resistor-Reactor

Grounding SwitchThe Shield/Sky WiresInsulatorsInsulator Arcing HornsCapacitorsSurge/ Lightning ArrestorsThe Distribution Transformers (MV/LV)

Grounded Neutral

COMPONENTS OF THE SWS AND THEIRFUNCTIONS


34/36


35/36

TAP

POSITIONPRIMARY

VOLTAGE SECONDARY VOLTAGE1. 21,0002. 20,5003. 20,000 250 0- 2504. 19,5005.

19,000

TRANSFORMER TAP CHANGING


36/36

Points for ConsiderationThere is only one Voltage available at terminals

of Transformer.(Check Incoming Voltage to knowTap to be connected to.)Tap Voltages are to be considered as number ofTurns (V is proportional T ie connect more voltageto where more turns are.)

Primary Voltage Per Turn is equal to SecondaryVoltage per Turn (V/Tp = V/Ts or N1/N2 = V1/V2i.e. Secondary Voltage is inversely proportional to

P i T )

TRANSFORMER TAP CHANGING

shield wire

Documents