DIgSILENT PowerFactory Application Guide
Cable Modelling Tutorial DIgSILENT Technical Documentation
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Copyright ©2013, DIgSILENT GmbH. Copyright of this document belongs to DIgSILENT GmbH. No part of this document may be reproduced, copied, or transmitted in any form, by any means electronic or mechanical, without the prior written permission of DIgSILENT GmbH. Cable Modelling Tutorial (DIgSILENT Technical Documentation)
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Contents
Contents 1 Introduction
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2 Cable Components
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2.1 Single Core Cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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2.2 Comparison between vendor and PowerFactory data . . . . . . . . . . . . . . . .
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2.3 Laying methods in three phase systems . . . . . . . . . . . . . . . . . . . . . . .
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2.4 Pipe type cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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2.5 Rating and Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3 Input of Cable Parameters in PowerFactory
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3.1 Cable Type Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3.2 Output Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3.3 Pipe Type Cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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4 Bonding of cables in PowerFactory
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4.1 Cable System definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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4.2 Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Cable Modelling Tutorial (DIgSILENT Technical Documentation)
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Cable Components
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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.
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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.
2.1
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 2.1.
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 conducting element, usually copper or aluminum, which is defined by means of the resistivity material and the section thickness. If the core were to be solid, without hollowness, 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.
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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. 2. 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, 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 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, thermal, chemical and electrical 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 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 2.1 Name in PowerFactory Sheath Oversheath Serving
Name in Datasheets Screen Armor bedding 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 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.
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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 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.
2.4
Pipe type cables
As mentioned in the last section, some cable installations, for example, submarine cables, 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. 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) is surrounded by the armour and serving (jacket), 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.
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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: Conductor, 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 is the filler, whereas the black area represents the armor. No graphical representation is available for 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 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, amongst other factors, 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.
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Input of Cable Parameters in PowerFactory
Figure 2.4: Cross bonding connection scheme
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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.
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-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 14.2 mm 9.0 mm 34.6 mm 35 mm2 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 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)
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• 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, number of circuits, disposition (Buried in Ground, in Pipe), type of cable per circuit, etc. Right click on the cell corresponding to the TypCab of the Circuit 1 row and click 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. 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 3.2: New Object button • The new dialog of the Basic Data tab page, 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.
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Input of Cable Parameters in PowerFactory
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 material of the conductor with “Copper”. • The next layer is the Insulation. As you can see from the vendor data, the value is 9.0 mm and can be directly inserted in PowerFactory . 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 1.2 mm. Thus, we should insert “10.2” mm as 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 material element, we have to manually define the Resistivity of the Sheath. Input the value of “1.7241” uOhm*cm in the corresponding cell. 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, being “0.32” mm approximately. 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, 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, as should be. Also, 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 46.0 mm, coincident with the vendor catalog data.
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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: Cable Modelling Tutorial (DIgSILENT Technical Documentation)
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Coordinate Value
X1 -0.116
X2 0
X3 0.116
Y1 1
Y2 1
Y3 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.
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Figure 3.7: Load flow results
3.2
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, press the button Calculate. • PowerFactory will automatically reproduce the matrixes defining the cable system and showing 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 parameters matrix. The following figures indicate in detail the internal sections of the matrixes, and how the parameters should be identified.
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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 a correspondence with the legend at the top, e.g. (1) is equivalent to the Phase 1, as defined in the 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, the real (conductance) and imaginary (capacitance) part is given in uS/km.
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Figure 3.10: Matrix of impedance parameters per sequence
Figure 3.10 refers to the impedance parameters per sequence. 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, 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 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:
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Resistance Reactance Susceptance
Pos.seq. 0.1096 Ohm/km 0.2057 Ohm/km 58.8723 uS/km
Neg.seq. 0.1096 Ohm/km 0.2057 Ohm/km 58.8723 uS/km
Zero seq. 0.2573 Ohm/km 1.8573 Ohm/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 values are 0.21 uF/km and 0.65 mH/km correspondingly, which is equivalent to a 65.97 uS/km susceptance and 0.20 Ohm/km reactance. 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, 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 the Relative Permitivitty. • 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.
3.3
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, usually made of steel. 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. Now, 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.
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Description Conductor Conductor Screen Insulation Insulation Screen Metallic Sheath Inner Jacket Bedding Armour Serving Outer Diameter
Thickness 15.2 mm 1 mm 17.0 mm 1.0 mm 2.9 mm 2.9 mm 2.0 mm 7.0 mm 4.0 mm 207 mm
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 3.4: 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 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, 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” and the 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 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 3.12.
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Input of Cable Parameters in PowerFactory
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 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”. Then, for a trefoil arrangement, 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
(1)
Where Ri is the radius of the trefoil arrangement and ri is the radius of the Single Core Cable.
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Figure 3.13: Ri and ri layout definition • Enter the following values in the Polar Coordinates of Line Circuits field: Magnitude 1 0.0462
Magnitude 2 0.0462
Magnitude 3 0.0462
Angle 1 90
Angle 2 210
Angle 3 330
Table 3.5: Polar coordinates • Now, for the bedding, armour and serving layers, we will include this information in the Thickness and Insulation Thickness of the pipe. Input 7.0 mm for the Thickness of the pipe, corresponding to the “Armour” and 6.0 mm for the Insulation Thickness, corresponding to the “Bedding” plus “Serving” layers. • Define the Depth to be 0.4 m. 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. Rel. Permittivity
Value 13.8 uOhm*cm 1 2.3 2.3
Table 3.6: Pipe parameters • Check the Reduced box and press Calculate. This will output the electrical values of our new cable. Leave it unchecked, press Calculate and press OK. 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)
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Parameter Conductor RDC at 20°C Cable Inductance Cable Capacitance
Vendor Data 0.0283 Ohm/km 0.0913 Ohm/km 60.63274 uS/km
PowerFactory Data 0.02375 Ohm/km 0.0962 Ohm/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 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 3.14: Access to the Single Core Cable Type dialog • In the Conducting Layers field, 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 0.0283 Ohm/km. Press OK.
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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.
4.1
Cable System definition
First, 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)
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• 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 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.
4.2
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.
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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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