Cable Systems_PeterWillis_DIGSILENT_Pacific.pdf

DIgSILENT Pacific User Conference 2007

DETAILED MODELLING OF HV SINGLE CORE CABLE SYSTEMS USING POWERFACTORY …..to determine armour and screen current magnitudes under varying conditions of installation Peter Willis, DIgSILENT Pacific Perth Office DIgSILENT Pacific PowerFactory Training

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SYSTEM MODELLING

Cable Systems – Frequency domain model • Traditionally represented by equivalent PI representation for Z1 and Z0 impedances. • Impedance data is typically taken from manufacturers’ catalogues

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CABLE SYSTEM MODELLING – Multi-Core Cables

For multi-core cables, manufacturer data is used. Here we see a non-SWA OLEX cable.

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CABLE SYSTEM MODELLING – Multi-Core Cables

..and here we see the same OLEX cable with SWA.

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CABLE SYSTEM MODELLING – Single-Core Cables For single core cables systems, the overall impedance is based on assumptions on the installation and connection of screens.

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CABLE SYSTEM MODELLING – Single-Core Cables

For single core cables systems, the overall impedance must be calculated from the geometric arrangement of the conductors, as is the requirement for overhead lines. These complexity of these calculations are also compounded by the multiple insulation and conducting layers of single core cable profiles, however the new PowerFactory cable system tools encapsulates this.

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CABLE SYSTEM MODELLING – Single-Core Cables

Unlike multi-core cables, the individual phase screens are not in electrical contact along the cable length, so there can be substantial induced currents flowing when screens are bonded together and earthed at the cable terminations. If the screens are not bonded at both ends, induced currents do not flow however screen voltages become elevated. All this requires engineering assessment, hence new tools were developed (first supported in Version 13.1) to model single phase cable systems.

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CABLE SYSTEM MODELLING – New PowerFactory Objects

There are three new PowerFactory objects in the model:
TypCab Single Phase cable profile
TypCabsys Geometric arrangement of cables making up the circuits in a cable system
ElmCabsys The link between cable system type and the line elements in the active network Overhead line equivalents:
TypCon Arial Conductor Type
TypTow Tower geometry Type
ElmTow Tower Element

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CABLE SYSTEM MODELLING – TypCab

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CABLE SYSTEM MODELLING – TypCabsys

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CABLE SYSTEM MODELLING – ElmCabsys

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CABLE SYSTEM MODELLING – ElmCabsys (EMT Tab)

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GRAPHICS

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GRAPHICS

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WHY DO IT ???

Why do we need this tool? • Customers asked for this • Screen and armour currents need to be quantified so connections & equipment can be correctly dimensioned. • Where screens or armours are lifted, the standing voltage can be determined under load and fault conditions. • Screen-cross bonding distances can be examined • Effect of cascading failures can be examined DIgSILENT Pacific PowerFactory Training

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CASE STUDY – Real Failure

Example of how the ‘Cabsys’ model was used to quantify screen currents following screen joint failure in 6.6kV generator incomer and how the geometric orientation of cable grouping in the field affects current imbalance across all conducting layers.

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CASE STUDY

Oil & Gas Installation - VESDA System detected fault but damage was already done - Generator outage - Very significant $AUD in production losses

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SINGLE CORE CABLE SYSTEMS

3 Main Areas of Concern - Orientation & configuration multiple cable runs, rotating phases within trefoil groups..

- Circulating currents armour and screen bonding and earthing, termination kits..

- Eddy Currents steel pipes, transit boxes, gland plates..

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SCREEN CURRENTS – Cable Damage due to Hot Joints

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SCREEN CURRENTS – Cable Damage due to Hot Joints (cont)

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Cable Profile

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Joint Instructions

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Constant Force Springs

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Final outer layer of joint

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What should have been done

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Thermography – Effect of screen connection

Screens Connected

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Screens Lifted on end

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Thermography – Effect of screen connection

PowerFactory Study cases: • Effect of parallel circuits • Lack of transposition • Phase positions in trefoil groups • Effect of high resistance joints leading to cascading failure • Lifting screens and isolating armour-clamping glands

Screens Connected

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Repaired Joints

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SUMMARY

Modelling Cable Systems• A tool that has a time and a place • Can also be used to offline to valid Z1/Z0 in conventional models. • Has associated line objects for each conducting path (sheath, armour, conductor) of each circuit. Going forward….. • Easier to visualise with composite branch line graphic (V14 feature) • Has some application for EPR calculation, whereby ground impedance is modelled from external tool (CDEGS). Feedback welcome…always

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