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Description

DIgSILENT PowerFactory Application Guide

Cable Modelling Tutorial DIgSILENT Technical Documentation

DIgSILENT GmbH Heinrich-Hertz-Str

9 72810

Copyright ©2013,

DIgSILENT GmbH

Copyright of this document belongs to DIgSILENT GmbH

No part of this document may be reproduced,

by any means electronic or mechanical,

without the prior written permission of DIgSILENT GmbH

Cable Modelling Tutorial (DIgSILENT Technical Documentation)

Contents

Contents 1 Introduction

2 Bonding

Cable Modelling Tutorial (DIgSILENT Technical Documentation)

Cable Components

Introduction

This tutorial has been prepared with the objective to give an understanding to the PowerFactory user about the cable modelling

First we start giving a description of the general aspects for the different types of cables,

then some examples are used in order to show the capabilities of PowerFactory for cable modelling

Cable Components

In this chapter,

a brief description of the components found in a cable will be pointed out

Additionally,

a comparison between the main parts constituting the cable geometry and the modelling capabilities of PowerFactory will be defined

Single Core Cable

Every electric power cable is composed of at least two components: an electrical conductor and the conductor insulation which prevents direct contact or unsafe proximity between conductor and other objects [1]

Regarding modelling purposes in PowerFactory ,

a cable is divided mainly in two categories: single core cables and three core cables

For simplicity reasons,

we will start describing the components of a single core cable type and then expand its definition for the application of three phase cable systems

The geometry of a single core cable type in PowerFactory is as depicted in figure 2

Figure 2

• Conductor: the hollow red portion of the cable consists in a section with a conducting element,

which is defined by means of the resistivity material and the section thickness

If the core were to be solid,

the core diameter 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 minimize electrostatic stresses

It has a very thin section and is located between the conductor and the insulation

Cable Modelling Tutorial (DIgSILENT Technical Documentation)

Cable Components

• Insulation: this section is intended to prevent the flow of electricity from the energized conductors to the ground or to an adjacent conductor

Typically,

the material of this section is of the thermoplastic (PVC) or thermosetting (EPR,

XLPE) type

• Insulation Screen: A semi-conducting material having a similar function as the conductor screen

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 the system

To be used as a shield to keep electromagnetic radiation in and,

in the case of paperinsulated cables,

to exclude water from the insulation

Usually,

when a solid sheath is used in the cable construction,

it is made of lead or aluminum

In some special cases,

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 metallic sheath 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,

chemical and electrical protection to the cable

Comparison between vendor and PowerFactory data

The first task an engineer must confront with,

in order to model a cable and for being the most representative as well of the real condition,

is to identify the constitutive parts of it and bring them to a simulation software

PowerFactory defines each part of the cable modelling tool with universally accepted names,

however this process of identification still can be a difficult task as it may lead to errors of interpretation

In order to help with this identification process,

a comparison between the PowerFactory and the most common used names in cable datasheets is shown in Table 2

Name in Datasheets Screen Armor bedding Armor serving

- Jacket

Table 2

Laying methods in three phase systems

Laying methods for three phase systems with single core cables are usually found in two categories: trefoil and flat

Trefoil is used to minimize the sheath circulating currents due to the magnetic flux linking the cable conductors and metallic sheath

Flat formation is appropriate for heat dissipation and consequently to increase cable rating

Cable Modelling Tutorial (DIgSILENT Technical Documentation)

Cable Components

Figure 2

PowerFactory is able to define a widespread of laying arrangements for single core cables,

For cables installed in pipes,

a slightly different configuration must be defined,

in order to cope with all the internal arrangement for these types of cables

Nonetheless,

both arrangements are to be defined in a cable system element

The definition of the common geometry and the differences with the single core cable type are described in the next section

Pipe type cables

As mentioned in the last section,

are constructed in a pipe type arrangement

The distribution of the internal components is slightly different in comparison with the single core cable type

The pipe type cables are related to three phase cables,

where each one of the phases is included inside the pipe

In PowerFactory ,

the definition of a pipe type cable is done by means of using a single core cable type,

and then using 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 pipe have layers ending at the oversheath

No armor is included individually for each phase,

but an overall armor surrounding the pipe is used

a filler is used to define the bundle geometry,

usually of a soft polymer material

The arrangement (similar as the trefoil described in 2

the latter being as the outmost layer of the pipe cable

This implies that the modelling of a pipe type cable will need the definition of a single core cable without armour and serving,

and the definition of these components have to be made inside the cable system

Below is a representation in PowerFactory of the single core cable type without armor and serving layers and the cable system used for a pipe type cable with serving and armour layers

Cable Modelling Tutorial (DIgSILENT Technical Documentation)

Cable Components

Figure 2

In figure 2

the layers from the center to the outermost layer are in the following order: Conductor,

Conductor screen,

Insulation,

Insulation screen,

Sheath and Oversheath

In figure 2

the white area between the single core cable trefoil arrangement and the armor is the filler,

whereas the black area represents the armor

No graphical representation is available for the serving in pipe type cables

Rating and Bonding

For three phase systems composed of single core cables with metallic sheaths,

the bonding arrangement and the thermal resistivity of the trench fill are the most important factors influencing cable 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 and cross bonded bonding types can be found

Since the electric power losses in a cable are dependent,

on the currents flowing in the metallic sheaths,

by reducing the current flows in these layers,

the ampacity of the cable can be increased

A double bonded configuration will reduce induced voltages,

but will provide a path for the circulating current through the sheaths,

thus reducing the current-carrying capacity of the cable

In the cross bonded configuration,

the sum of the induced voltages in the shielding of the phases will be zero and thus the current flowing through the shielding will be minimized,

improving the available cable rating

The use of armor wires on cables with lead sheaths,

installed in three phase systems with close spacing,

causes additional sheath losses because the presence of armor wires reduces sheath resistance,

since both armor and sheath are connected in parallel,

and the losses are largest when the sheath circuit resistance is equal to its reactance

Without armor wires,

the reactance of the sheath is always very much smaller than the resistance

To minimize this increase in losses,

armor wires made of high resistance material such as copper-silicon-manganese alloy are sometimes used

Please note that the losses in the sheath and armor combination could be several times the conductor losses,

depending on the bonding arrangements of the sheaths and armor [1]

PowerFactory is capable of modelling a simple,

double or cross bonded configuration,

for cable systems using single core cables

For a pipe cable,

and for the single core cable systems,

a bond between the sheath and the armour is also available

A schematic example of a crossbonded configuration is shown in the figure below

The dashed lines represent the sheath and/or armor

Cable Modelling Tutorial (DIgSILENT Technical Documentation)

Input of Cable Parameters in PowerFactory

Figure 2

Input of Cable Parameters in PowerFactory

For the modelling of a cable,

we will describe the steps needed to achieve a proper representation in PowerFactory

For this,

a model example has been prepared for use

The system consists in two independent cables of 1 km length each,

connected between two 66 kV busbars to feed a 40 MW load

The external grid is connected at the Busbar A,

providing the electric power to the system

Note: Please import the project file ”Cable Tutorial 0

pfd” to PowerFactory and activate it

Cable Type Definition

The first step is to define the cable type

We will model the cable as to be representing a 3-phase single core cable,

disposed in a flat arrangement

The representative values to be entered in PowerFactory are extracted from a vendor catalog [2]

All the data related to the cable as seen in the catalog is summarized in the table below

Description Cable Type Nominal Voltage Cross Section of Conductor Diameter of Conductor Insulation Thickness Diameter Over Insulation Cross Section of Screen Outer Diameter of Cable

Value XLPE Single Core 66 kV 150 mm2 14

0 mm 34

Table 3

Please note that the cable has no armour nor shield in its structure

It can be seen from the information presented in the table above that first of all we need to identify the values which PowerFactory need in order to make a proper modelling,

and then insert them on our model

To create a new cable type,

please follow these steps: • Double click on the Single Core Cable A line element

Cable Modelling Tutorial (DIgSILENT Technical Documentation)

Input of Cable Parameters in PowerFactory

• Click on the down arrow in the Type field and choose New Project Type

→ Cable Definition (TypCabys)

A new dialog appears

• Inside the Cable Definition dialog,

we can define all the relevant parameters of the cable geometry,

disposition (Buried in Ground,

Right click on the cell corresponding to the TypCab of the Circuit 1 row and click Select Element/Type

Figure 3

This can be done directly from the actual dialog,

clicking on the New Object icon shown in the figure below

A new dialog appears again

Figure 3

contains all the geometrical data for the Single Core Cable at the Conducting,

Semiconducting and Insulation layers

We have to determine which parameters must be entered to properly define the Single Core Cable

Cable Modelling Tutorial (DIgSILENT Technical Documentation)

Input of Cable Parameters in PowerFactory

Figure 3

from the vendor information the diameter of the conductor is 14

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

define the material of the conductor with “Copper”

• The next layer is the Insulation

As you can see from the vendor data,

However,

if you compare this thickness with the thickness resulting from the Diameter Over Insulation minus the Diameter of Conductor,

you will notice a slight difference of about 1

This thickness is decisive in the equivalent capacity of the cable

Define the Material as to be of “XLPE (>18/30(36)kV cab

• The Sheath is made of copper,

but since there is no copper category inside the material element,

we have to manually define the Resistivity of the Sheath

Input the value of “1

For the thickness,

note that the vendor data is in terms of a cross section

By means of a geometric calculation,

we can determine the thickness of this layer,

Input this value in the corresponding cell

• The final layer is the Oversheath

Define the material as to be of the same type of the Insulation layer,

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,

note the Overall Cable Diameter value at the lowest part of the dialog

If the values were entered correctly,

the cable should show an overall diameter of 46

coincident with the vendor catalog data

Cable Modelling Tutorial (DIgSILENT Technical Documentation)

Input of Cable Parameters in PowerFactory

Figure 3

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

Input the following coordinates for the line circuits: Cable Modelling Tutorial (DIgSILENT Technical Documentation)

Input of Cable Parameters in PowerFactory

Coordinate Value

Table 3

The final arrangement should see as in the figure below

Figure 3

Assign the same cable system for the Single Core Cable B element and perform a load flow

The system should show the following results

Cable Modelling Tutorial (DIgSILENT Technical Documentation)

Input of Cable Parameters in PowerFactory

Figure 3

Output Matrix

In order to check if the parameters entered will be representing the cable electrical parameters in reality,

we can compare the capacitance and inductance values given from the vendor catalog and 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 Type field of the dialog

• From the Cable System element,

• PowerFactory will automatically reproduce the matrixes defining the cable system and showing all the data in the output window

For the output information,

two sections can be identified: a phase and a sequence parameters matrix

The following figures indicate in detail the internal sections of the matrixes,

and how the parameters should be identified

Cable Modelling Tutorial (DIgSILENT Technical Documentation)

Input of Cable Parameters in PowerFactory

Figure 3

Figure 3

For each row,

the real (resistance) and imaginary (reactance) part is given in Ohm/km

The index indicated in parenthesis has a correspondence with the legend at the top,

(1) is equivalent to the Phase 1,

as defined in the Cable System

Figure 3

Similar description applies for phase admittance matrix shown in Figure 3

For each row,

the real (conductance) and imaginary (capacitance) part is given in uS/km

Cable Modelling Tutorial (DIgSILENT Technical Documentation)

Input of Cable Parameters in PowerFactory

Figure 3

Figure 3

For each row,

the real (resistance) and imaginary (reactance) part is given in Ohm/km

The index indicated in parenthesis has a correspondence with the legend at the top,

(1) is equivalent to the positive sequence

Figure 3

Similar description applies for sequence admittance matrix shown in Figure 3

For each row,

the real (conductance) and imaginary (capacitance) part is given in uS/km

Note that the conductance values are zero,

since there is no dielectric losses defined for the insulation material in the Single Core Cable type

The comparison must be performed with the sequence parameters

For the resistance,

reactance and susceptance values,

we extract the following information from the figures presented above:

Cable Modelling Tutorial (DIgSILENT Technical Documentation)

Input of Cable Parameters in PowerFactory

Resistance Reactance Susceptance

8723 uS/km

8723 uS/km

Zero seq

8723 uS/km

Table 3

it can be seen that the capacitance and inductance values are 0

Comparing these values,

we see that the reactances have very close values,

however the susceptance has a difference of about 11% with respect to the vendor data

We can adjust the value by changing the Relative Permitivitty of the insulation layers,

without affecting the geometry of the cable

This adjustment is a valid approach since we don´t have the specific data of the insulation layers,

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

Search for the Single Core Cable Type and click on the icon

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

• Click OK and close the Object Filter window

• Perform a calculation of the electrical parameters of the cable system,

in order to display the new susceptance values

• Check that the positive susceptance has now changed to 65

74 uS/km

Pipe Type Cable

Pipe type cables are used normally as submarine cables

They are constructed in such manner that the three phases are inside a pipe,

The modelling for these type of cables in PowerFactory is similar to the single core cables

Note: Please import the project file ”Cable Tutorial 1

pfd” to PowerFactory and activate it

The system is now in 132 kV,

feeding a load of 40 MW through a line element of 50 km

we have to define the cable type and cable system for a pipe type cable

The relevant data for the selected cable is summarized below

Cable Modelling Tutorial (DIgSILENT Technical Documentation)

Input of Cable Parameters in PowerFactory

Description Conductor Conductor Screen Insulation Insulation Screen Metallic Sheath Inner Jacket Bedding Armour Serving Outer Diameter

Thickness 15

Material Copper Extruded semi-conducting compound XLPE compound Extruded semi-conducting compound Lead Semi-conduction polyethylene Polypropylene strings Galvanised steel wires Single layer of polypropylene strings

Table 3

we begin defining the single core cable,

which will represent each one of the phases inside the 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,

• Note that a new field has appeared,

which contains all the geometrical data for the pipe

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” and the Rated Voltage to 132 kV

• Input the parameters in the dialog,

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

• Leave the Serving and Armour layers unchecked,

since we are going to define them in the Cable System

• The dialog should see as shown in Figure 3

Cable Modelling Tutorial (DIgSILENT Technical Documentation)

Input of Cable Parameters in PowerFactory

Figure 3

This will send you back to the Cable System dialog

• First input the polar coordinates (Magnitude / Angle) for each phase

In order to calculate this value,

we must first know the overall cable diameter per phase,

which is shown at the bottom of the Single Core Cable Type dialog

In our example,

this value corresponds to “80 mm”

it can be proven that the distance from the center of the trefoil arrangement to the center of each single core cable is as follows: ri Ri = √ 3

Where Ri is the radius of the trefoil arrangement and ri is the radius of the Single Core Cable

Cable Modelling Tutorial (DIgSILENT Technical Documentation)

Input of Cable Parameters in PowerFactory

Figure 3

Magnitude 2 0

Magnitude 3 0

Angle 1 90

Angle 2 210

Angle 3 330

Table 3

we will include this information in the Thickness and Insulation Thickness of the pipe

Input 7

corresponding to the “Armour” and 6

corresponding to the “Bedding” plus “Serving” layers

The Outer Radius should be defined as 0

1035 m,

since the 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 Resistivity Rel

Permeability Fill: Rel

Permittivity Ins

Permittivity

Value 13

Table 3

This will output the electrical values of our new cable

Leave it unchecked,

Note: Keep in mind that the Reduced option actually bond the sheaths and armours of the cable,

resulting on a reduced element matrix

Now we have to compare the nominal values of the cable from the vendor data with the matrix output values from PowerFactory

The vendor electrical parameters of the cable are as follows: Cable Modelling Tutorial (DIgSILENT Technical Documentation)

Bonding of cables in PowerFactory

Parameter Conductor RDC at 20°C Cable Inductance Cable Capacitance

Vendor Data 0

PowerFactory Data 0

4676 uS/km

Table 3

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 the Single 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 the line element,

or accessing the filter tool at the icon toolbar in PowerFactory

See figure below

Figure 3

click on the black right pointing arrow and select DCResistance in Ohm/km

Press OK

• Change the value of the DC-Resistance of the conductor to 0

Press OK

Bonding of cables in PowerFactory

This section will describe how to define an independent modelling of the cable between sheaths and cores and the subsequent bonding between them

This is useful for monitoring the current that is flowing through the sheaths against different types of bonding

Note: Please import the project file ”Cable Tutorial 2

pfd” to PowerFactory and activate it

Cable System definition

we begin defining our Cable System that will take into account the coupling between the core and the sheath of our cable

• Define the default voltage level fo new elements to be 66 kV

This has to be changed in the toolbar

• Insert two new terminals parallel to the existing ones

Name them “Sheath A” and “Sheath B” 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

Cable Modelling Tutorial (DIgSILENT Technical Documentation)

Bonding of cables in PowerFactory

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

See figure

Figure 4

Select the highlighted Cable Definition and press OK

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

• The Cable System dialog will pop-up

This element assigns the coupling between the core and 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 flowing through the sheath is zero

• Observe that the terminals now have a voltage value

This represents the induced voltage in the sheaths of our cable due to the definition of the coupling system

Bonding

As you can see,

no bonding has been defined yet

By means of earthing one or both busbars for the sheath element,

we can reduce the voltage against an increased flow of current through the sheaths

The process of earthing one or both busbars is known as the single bonding or double bonding configuration respectively

Normally,

for high loaded cables a cross-bonding is used

This will reduce furthermore the current flowing through the sheaths whilst keeping the voltages at both sides of the sheaths equal to ground potential

To define a cross-bonding configuration,

please do the following: • Edit the Cable Definition

You can quickly access to this object by means of using the Edit Relevant Objects for Calculation button in the PowerFactory toolbar and clicking in the 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 flowing through the sheaths has been reduced

Cable Modelling Tutorial (DIgSILENT Technical Documentation)

Bonding of cables in PowerFactory

References [1] Anders,

“Rating of Electrical Power Cables: Ampacity Computations for Transmission,

Distribution,

and Industrial Applications”,

IEEE Press,

[2] “XLPE Land Cable Systems User´s Guide”,

Cable Modelling Tutorial (DIgSILENT Technical Documentation)