Cable Tutorial

DIgSILENT PowerFactoryApplication Guide

Cable Modelling TutorialDIgSILENT Technical Documentation

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Cable Modelling Tutorial (DIgSILENT Technical Documentation) 1

http://www.digsilent.de

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Contents

Contents

1 Introduction 3

2 Cable Components 3

2.1 Single Core Cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2 Comparison between vendor and PowerFactory data . . . . . . . . . . . . . . . . 4

2.3 Laying methods in three phase systems . . . . . . . . . . . . . . . . . . . . . . . 4

2.4 Pipe type cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.5 Rating and Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Input of Cable Parameters in PowerFactory 7

3.1 Cable Type Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.2 Output Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.3 Pipe Type Cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4 Bonding of cables in PowerFactory 19

4.1 Cable System definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.2 Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20


2 Cable Components

1 Introduction

This tutorial has been prepared with the objective to give an understanding to the PowerFactoryuser about the cable modelling. First we start giving a description of the general aspects forthe different types of cables, then some examples are used in order to show the capabilities ofPowerFactory for cable modelling.

2 Cable Components

In this chapter, a brief description of the components found in a cable will be pointed out. Addi-tionally, a comparison between the main parts constituting the cable geometry and the modellingcapabilities of PowerFactory will be defined.

2.1 Single Core Cable

Every electric power cable is composed of at least two components: an electrical conductor andthe conductor insulation which prevents direct contact or unsafe proximity between conductorand other objects [1]. Regarding modelling purposes in PowerFactory , a cable is divided mainlyin two categories: single core cables and three core cables. For simplicity reasons, we willstart describing the components of a single core cable type and then expand its definition forthe application of three phase cable systems. The geometry of a single core cable type inPowerFactory is as depicted in figure 2.1.

Figure 2.1: Layers of a single core cable in PowerFactory

• Conductor: the hollow red portion of the cable consists in a section with a conductingelement, usually copper or aluminum, which is defined by means of the resistivity materialand the section thickness. If the core were to be solid, without hollowness, the corediameter shall be defined simply by only the overall section of the core.

• Conductor Screen: A semi-conducting tape to maintain a uniform electric field and mini-mize electrostatic stresses. It has a very thin section and is located between the conductorand the insulation.


2 Cable Components

• Insulation: this section is intended to prevent the flow of electricity from the energizedconductors to the ground or to an adjacent conductor. Typically, the material of this sectionis of the thermoplastic (PVC) or thermosetting (EPR, XLPE) type.

• Insulation Screen: A semi-conducting material having a similar function as the conductorscreen. It is located between the insulation and the sheath.

• Sheath: This layer has two main objectives:

1. To carry the neutral and/or fault current to earth in the event of an earth fault in thesystem.

2. To be used as a shield to keep electromagnetic radiation in and, in the case of paper-insulated cables, to exclude water from the insulation.

Usually, when a solid sheath is used in the cable construction, it is made of lead or alu-minum. In some special cases, copper may be used. Because of safety considerations,metallic sheaths are always grounded in at least one place.

• Oversheath: This layer covers the metallic sheath and acts as a filler between the metallicsheath and the armor. Most commonly used materials are PVC and PE.

• Armour: This layer is intended to provide mechanical protection of the conductor bundle.Usually, it is used for submarine and special purpose cables.

• Serving: This is usually a plastic cover and provides mechanical, thermal, chemical andelectrical protection to the cable.

2.2 Comparison between vendor and PowerFactory data

The first task an engineer must confront with, in order to model a cable and for being the mostrepresentative as well of the real condition, is to identify the constitutive parts of it and bringthem to a simulation software. PowerFactory defines each part of the cable modelling tool withuniversally accepted names, however this process of identification still can be a difficult task asit may lead to errors of interpretation.

In order to help with this identification process, a comparison between the PowerFactory andthe most common used names in cable datasheets is shown in Table 2.1

Name in PowerFactory Name in DatasheetsSheath Screen

Oversheath Armor beddingServing Armor serving - Jacket

Table 2.1: Comparison of cable component names for a single core cable type

2.3 Laying methods in three phase systems

Laying methods for three phase systems with single core cables are usually found in two cat-egories: trefoil and flat. Trefoil is used to minimize the sheath circulating currents due to themagnetic flux linking the cable conductors and metallic sheath. Flat formation is appropriate forheat dissipation and consequently to increase cable rating.


2 Cable Components

(a) Trefoil (b) Flat

Figure 2.2: Laying arrangements in three phase systems with single core cables

PowerFactory is able to define a widespread of laying arrangements for single core cables,using a coordinate system. For cables installed in pipes, such as submarine cables, a slightlydifferent configuration must be defined, in order to cope with all the internal arrangement forthese types of cables. Nonetheless, both arrangements are to be defined in a cable systemelement. The definition of the common geometry and the differences with the single core cabletype are described in the next section.

2.4 Pipe type cables

As mentioned in the last section, some cable installations, for example, submarine cables, areconstructed in a pipe type arrangement. The distribution of the internal components is slightlydifferent in comparison with the single core cable type. The pipe type cables are related to threephase cables, where each one of the phases is included inside the pipe. In PowerFactory , thedefinition of a pipe type cable is done by means of using a single core cable type, and thenusing these characteristics on a pipe arrangement in a cable system.

What the user must be aware of, is that normally each of the single core cables inside a pipehave layers ending at the oversheath. No armor is included individually for each phase, but anoverall armor surrounding the pipe is used. Then, a filler is used to define the bundle geometry,usually of a soft polymer material. The arrangement (similar as the trefoil described in 2.2a) issurrounded by the armour and serving (jacket), the latter being as the outmost layer of the pipecable. This implies that the modelling of a pipe type cable will need the definition of a singlecore cable without armour and serving, and the definition of these components have to be madeinside the cable system. Below is a representation in PowerFactory of the single core cable typewithout armor and serving layers and the cable system used for a pipe type cable with servingand armour layers.


2 Cable Components

(a) Single core cable (b) Pipe cable

Figure 2.3: Cable system representation for a pipe laying arrangement

In figure 2.3a, the layers from the center to the outermost layer are in the following order: Con-ductor, Conductor screen, Insulation, Insulation screen, Sheath and Oversheath.

In figure 2.3b, the white area between the single core cable trefoil arrangement and the armor isthe filler, whereas the black area represents the armor. No graphical representation is availablefor the serving in pipe type cables.

2.5 Rating and Bonding

For three phase systems composed of single core cables with metallic sheaths, the bonding ar-rangement and the thermal resistivity of the trench fill are the most important factors influencingcable rating.

The bonding of the cable to earth is the process where the metallic shield (sheath and/or armor)is grounded at one or both ends. Different variations exist, where the double bonded andcross bonded bonding types can be found. Since the electric power losses in a cable aredependent, amongst other factors, on the currents flowing in the metallic sheaths, by reducingthe current flows in these layers, the ampacity of the cable can be increased. A double bondedconfiguration will reduce induced voltages, but will provide a path for the circulating currentthrough the sheaths, thus reducing the current-carrying capacity of the cable. In the crossbonded configuration, the sum of the induced voltages in the shielding of the phases will bezero and thus the current flowing through the shielding will be minimized, improving the availablecable rating.

The use of armor wires on cables with lead sheaths, installed in three phase systems with closespacing, causes additional sheath losses because the presence of armor wires reduces sheathresistance, since both armor and sheath are connected in parallel, and the losses are largestwhen the sheath circuit resistance is equal to its reactance. Without armor wires, the reactanceof the sheath is always very much smaller than the resistance. To minimize this increase inlosses, armor wires made of high resistance material such as copper-silicon-manganese alloyare sometimes used. Please note that the losses in the sheath and armor combination couldbe several times the conductor losses, depending on the bonding arrangements of the sheathsand armor [1].

PowerFactory is capable of modelling a simple, double or cross bonded configuration, for cablesystems using single core cables. For a pipe cable, and for the single core cable systems, abond between the sheath and the armour is also available. A schematic example of a cross-bonded configuration is shown in the figure below. The dashed lines represent the sheath and/orarmor.


3 Input of Cable Parameters in PowerFactory

Figure 2.4: Cross bonding connection scheme


For the modelling of a cable, we will describe the steps needed to achieve a proper represen-tation in PowerFactory . For this, a model example has been prepared for use. The systemconsists in two independent cables of 1 km length each, connected between two 66 kV busbarsto feed a 40 MW load. The external grid is connected at the Busbar A, providing the electricpower to the system.

Note: Please import the project file ”Cable Tutorial 0.pfd” to PowerFactory and activate it.

3.1 Cable Type Definition

The first step is to define the cable type. We will model the cable as to be representing a 3-phasesingle core cable, disposed in a flat arrangement. The representative values to be entered inPowerFactory are extracted from a vendor catalog [2]. All the data related to the cable as seenin the catalog is summarized in the table below.

Description ValueCable Type XLPE Single Core

Nominal Voltage 66 kVCross Section of Conductor 150 mm2

Diameter of Conductor 14.2 mmInsulation Thickness 9.0 mm

Diameter Over Insulation 34.6 mmCross Section of Screen 35 mm2Outer Diameter of Cable 46.0 mm

Table 3.1: Datasheet values for a XLPE 66 kV 150 mm2 cable

Please note that the cable has no armour nor shield in its structure. It can be seen from theinformation presented in the table above that first of all we need to identify the values whichPowerFactory need in order to make a proper modelling, and then insert them on our model. Tocreate a new cable type, please follow these steps:

• Double click on the Single Core Cable A line element.



• Click on the down arrow in the Type field and choose New Project Type . . . → CableDefinition (TypCabys). A new dialog appears.

• Inside the Cable Definition dialog, we can define all the relevant parameters of the cablegeometry, number of circuits, disposition (Buried in Ground, in Pipe), type of cable percircuit, etc. Right click on the cell corresponding to the TypCab of the Circuit 1 row andclick Select Element/Type . . . , as shown in figure.

Figure 3.1: Cable Definition dialog

• Since there isn´t any Single Core Cable created yet, we have to create one first. Thiscan be done directly from the actual dialog, clicking on the New Object icon shown in thefigure below. A new dialog appears again.

Figure 3.2: New Object button

• The new dialog of the Basic Data tab page, contains all the geometrical data for theSingle Core Cable at the Conducting, Semiconducting and Insulation layers. We have todetermine which parameters must be entered to properly define the Single Core Cable.



Figure 3.3: Singe Core Cable dialog

• Define the rated voltage with a value of 66 kV.

• For the conductor, from the vendor information the diameter of the conductor is 14.2 mm.This will give a Thickness of the Conducting Layer section in PowerFactory of “7.1” mm.Insert this value in the cell of the Thickness of the Conductor row. Also, define the materialof the conductor with “Copper”.

• The next layer is the Insulation. As you can see from the vendor data, the value is 9.0 mmand can be directly inserted in PowerFactory . However, if you compare this thickness withthe thickness resulting from the Diameter Over Insulation minus the Diameter of Conduc-tor, you will notice a slight difference of about 1.2 mm. Thus, we should insert “10.2” mmas Insulation thickness. This thickness is decisive in the equivalent capacity of the cable.Define the Material as to be of “XLPE (>18/30(36)kV cab.(fil.))” type.

• The Sheath is made of copper, but since there is no copper category inside the mate-rial element, we have to manually define the Resistivity of the Sheath. Input the valueof “1.7241” uOhm*cm in the corresponding cell. For the thickness, note that the vendordata is in terms of a cross section. By means of a geometric calculation, we can deter-mine the thickness of this layer, being “0.32” mm approximately. Input this value in thecorresponding cell.

• The final layer is the Oversheath. Define the material as to be of the same type of theInsulation layer, and input “5.38” mm as the thickness value.

• Finally, change the name of the Single Core Cable to “Cable XLPE 66 kV”.

• Note that the Outer Diameter field of the Core is the double of the Conductor thickness, asshould be. Also, note the Overall Cable Diameter value at the lowest part of the dialog. Ifthe values were entered correctly, the cable should show an overall diameter of 46.0 mm,coincident with the vendor catalog data.



Figure 3.4: Singe Core Cable XLPE 66 kV data

• Press OK.

Now the Cable System dialog is automatically updated with the new geometrical information.Since all the coordinates are set to zero, the phases show as they are at one point only.

Figure 3.5: Cable System dialog

Input the following coordinates for the line circuits:



X1 X2 X3 Y1 Y2 Y3Coordinate Value -0.116 0 0.116 1 1 1

Table 3.2: Coordinates for the Single Core Three Phase System

The final arrangement should see as in the figure below.

Figure 3.6: Cable System with Coordinates

Assign the same cable system for the Single Core Cable B element and perform a load flow.The system should show the following results.



Figure 3.7: Load flow results

3.2 Output Matrix

In order to check if the parameters entered will be representing the cable electrical parametersin reality, we can compare the capacitance and inductance values given from the vendor catalogand the values determined in PowerFactory . Please follow the following steps:

• Edit any of the lines by double clicking on them, then click on the left arrow of the Typefield of the dialog.

• From the Cable System element, press the button Calculate.

• PowerFactory will automatically reproduce the matrixes defining the cable system andshowing all the data in the output window.

• Press OK and OK.

For the output information, two sections can be identified: a phase and a sequence parametersmatrix. The following figures indicate in detail the internal sections of the matrixes, and how theparameters should be identified.



Figure 3.8: Matrix of impedance parameters per phase

Figure 3.8 refers to the impedance parameters per phase. For each row, the real (resistance)and imaginary (reactance) part is given in Ohm/km. The index indicated in parenthesis has acorrespondence with the legend at the top, e.g. (1) is equivalent to the Phase 1, as defined inthe Cable System.

Figure 3.9: Matrix of admittance parameters per phase

Similar description applies for phase admittance matrix shown in Figure 3.9. For each row, thereal (conductance) and imaginary (capacitance) part is given in uS/km.



Figure 3.10: Matrix of impedance parameters per sequence

Figure 3.10 refers to the impedance parameters per sequence. For each row, the real (resis-tance) and imaginary (reactance) part is given in Ohm/km. The index indicated in parenthesishas a correspondence with the legend at the top, e.g. (1) is equivalent to the positive sequence.

Figure 3.11: Matrix of admittance parameters per phase

Similar description applies for sequence admittance matrix shown in Figure 3.11. For eachrow, the real (conductance) and imaginary (capacitance) part is given in uS/km. Note thatthe conductance values are zero, since there is no dielectric losses defined for the insulationmaterial in the Single Core Cable type.

The comparison must be performed with the sequence parameters. For the resistance, reac-tance and susceptance values, we extract the following information from the figures presentedabove:



Pos.seq. Neg.seq. Zero seq.Resistance 0.1096 Ohm/km 0.1096 Ohm/km 0.2573 Ohm/kmReactance 0.2057 Ohm/km 0.2057 Ohm/km 1.8573 Ohm/km

Susceptance 58.8723 uS/km 58.8723 uS/km 58.8723 uS/km

Table 3.3: Cable sequence parameters

Now, from the vendor catalog data, it can be seen that the capacitance and inductance valuesare 0.21 uF/km and 0.65 mH/km correspondingly, which is equivalent to a 65.97 uS/km sus-ceptance and 0.20 Ohm/km reactance. Comparing these values, we see that the reactanceshave very close values, however the susceptance has a difference of about 11% with respect tothe vendor data. We can adjust the value by changing the Relative Permitivitty of the insulationlayers, without affecting the geometry of the cable. This adjustment is a valid approach sincewe don´t have the specific data of the insulation layers, as mentioned in section 3.1.

• Edit the cable element by clicking on the Object Filter button.

• An icon list is displayed. Search for the Single Core Cable Type and click on the icon.

• From the new dialog, right click on the Single Core Cable element and select Edit.

• Change the Material of the insulation layer to Unknown and input the value of 3.35 for theRelative Permitivitty.

• Click OK and close the Object Filter window.

• Perform a calculation of the electrical parameters of the cable system, in order to displaythe new susceptance values.

• Check that the positive susceptance has now changed to 65.74 uS/km.

3.3 Pipe Type Cable

Pipe type cables are used normally as submarine cables. They are constructed in such mannerthat the three phases are inside a pipe, usually made of steel. The modelling for these type ofcables in PowerFactory is similar to the single core cables.


The system is now in 132 kV, feeding a load of 40 MW through a line element of 50 km. Now,we have to define the cable type and cable system for a pipe type cable. The relevant data forthe selected cable is summarized below.



Description Thickness MaterialConductor 15.2 mm Copper

Conductor Screen 1 mm Extruded semi-conducting compoundInsulation 17.0 mm XLPE compound

Insulation Screen 1.0 mm Extruded semi-conducting compoundMetallic Sheath 2.9 mm Lead

Inner Jacket 2.9 mm Semi-conduction polyethyleneBedding 2.0 mm Polypropylene stringsArmour 7.0 mm Galvanised steel wiresServing 4.0 mm Single layer of polypropylene strings

Outer Diameter 207 mm -

Table 3.4: Datasheet values for a three-core XLPE 132 kV 630 mm2 cable

First, we begin defining the single core cable, which will represent each one of the phases insidethe pipe arrangement.

• Edit the line and assign a new Cable Definition. Name it to “Pipe system”.

• Select from the drop-down menu in the Buried field, the option “in Pipe”.

• Note that a new field has appeared, which contains all the geometrical data for the pipe.Also, the coordinates system has changed to polar values.

• Create and assign a new Single Core Cable Type to the pipe system.

• In the Single Core Cable Type dialog, change the Name to “Three core XLPE 132 kV” andthe Rated Voltage to 132 kV.

• Input the parameters in the dialog, as in Table 3.4.

• Note that you can select the type of material by double-clicking on the corresponding cellof the Material column.

• Leave the Serving and Armour layers unchecked, since we are going to define them inthe Cable System.

• The dialog should see as shown in Figure 3.12.



Figure 3.12: Parameters for Pipe Single Core Cable Type

• Once all the values have been entered on the dialog, press OK. This will send you backto the Cable System dialog.

• First input the polar coordinates (Magnitude / Angle) for each phase. In order to calculatethis value, we must first know the overall cable diameter per phase, which is shown atthe bottom of the Single Core Cable Type dialog. In our example, this value correspondsto “80 mm”. Then, for a trefoil arrangement, it can be proven that the distance from thecenter of the trefoil arrangement to the center of each single core cable is as follows:

Ri =ri√3

(1)

Where Ri is the radius of the trefoil arrangement and ri is the radius of the Single CoreCable.



Figure 3.13: Ri and ri layout definition

• Enter the following values in the Polar Coordinates of Line Circuits field:

Magnitude 1 Magnitude 2 Magnitude 3 Angle 1 Angle 2 Angle 30.0462 0.0462 0.0462 90 210 330

Table 3.5: Polar coordinates

• Now, for the bedding, armour and serving layers, we will include this information in theThickness and Insulation Thickness of the pipe. Input 7.0 mm for the Thickness of thepipe, corresponding to the “Armour” and 6.0 mm for the Insulation Thickness, correspond-ing to the “Bedding” plus “Serving” layers.

• Define the Depth to be 0.4 m. The Outer Radius should be defined as 0.1035 m, sincethe outer diameter of the cable is 207 mm from the vendor data.

• For the rest of the parameters to the right, please input the following:

Parameter ValueResistivity 13.8 uOhm*cm

Rel. Permeability 1Fill: Rel. Permittivity 2.3Ins. Rel. Permittivity 2.3

Table 3.6: Pipe parameters

• Check the Reduced box and press Calculate. This will output the electrical values of ournew cable. Leave it unchecked, press Calculate and press OK.

Note: Keep in mind that the Reduced option actually bond the sheaths and armours of thecable, resulting on a reduced element matrix.

Now we have to compare the nominal values of the cable from the vendor data with the matrixoutput values from PowerFactory . The vendor electrical parameters of the cable are as follows:


4 Bonding of cables in PowerFactory

Parameter Vendor Data PowerFactory DataConductor RDC at 20°C 0.0283 Ohm/km 0.02375 Ohm/km

Cable Inductance 0.0913 Ohm/km 0.0962 Ohm/kmCable Capacitance 60.63274 uS/km 62.4676 uS/km

Table 3.7: Pipe parameters

We can see that the inductance and capacitance values are consistent with the vendor data.For the resistance value, the difference is due to the definition of the resistance per length in theSingle Core Cable Type dialog. To change this value do the following:

• Access to the Single Core Cable Type dialog. You can do this by double clicking on theline element, or accessing the filter tool at the icon toolbar in PowerFactory . See figurebelow.

Figure 3.14: Access to the Single Core Cable Type dialog

• In the Conducting Layers field, click on the black right pointing arrow and select DC-Resistance in Ohm/km. Press OK.

• Change the value of the DC-Resistance of the conductor to 0.0283 Ohm/km. Press OK.


This section will describe how to define an independent modelling of the cable between sheathsand cores and the subsequent bonding between them. This is useful for monitoring the currentthat is flowing through the sheaths against different types of bonding.


4.1 Cable System definition

First, we begin defining our Cable System that will take into account the coupling between thecore and the sheath of our cable.

• Define the default voltage level fo new elements to be 66 kV. This has to be changed inthe toolbar.

• Insert two new terminals parallel to the existing ones. Name them “Sheath A” and “SheathB” respectively.

• Connect a line element between both terminals. Name it “Sheath”.

• Hold the Ctrl key and select the predefined cable and the new created line element.



• Right click on the selection and select Define → Cable System. . . . See figure.

Figure 4.1: Cable System definition

• A new dialog appears. Select the highlighted Cable Definition and press OK.

• Now select the line element which will represent the core of our cable (Single Core CableA) and press OK.

• The Cable System dialog will pop-up. This element assigns the coupling between the coreand the sheath of our cable. Press OK.

• Change the length of both line elements (core and sheath) to be 30 km.

• Run a Unbalanced Load Flow calculation and check if the value for the current flowingthrough the sheath is zero.

• Observe that the terminals now have a voltage value. This represents the induced voltagein the sheaths of our cable due to the definition of the coupling system.

4.2 Bonding

As you can see, no bonding has been defined yet. By means of earthing one or both busbarsfor the sheath element, we can reduce the voltage against an increased flow of current throughthe sheaths.

The process of earthing one or both busbars is known as the single bonding or double bondingconfiguration respectively. Normally, for high loaded cables a cross-bonding is used. This willreduce furthermore the current flowing through the sheaths whilst keeping the voltages at bothsides of the sheaths equal to ground potential. To define a cross-bonding configuration, pleasedo the following:

• Edit the Cable Definition. You can quickly access to this object by means of using theEdit Relevant Objects for Calculation button in the PowerFactory toolbar and clicking inthe Cable Definition icon.

• Edit the highlighted Cable Definition from the Object Filter.

• Check the box Cross Bonded of the Circuit 1 row. Click OK.

• Now run again a Unbalanced Load Flow calculation and observe that the current flowingthrough the sheaths has been reduced.



References

[1] Anders, G., “Rating of Electrical Power Cables: Ampacity Computations for Transmission,Distribution, and Industrial Applications”, IEEE Press, 1997.

[2] “XLPE Land Cable Systems User´s Guide”, ABB, rev. 5.


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