Underground cable is commonly used in situation where power need to be transmitted across river or sea or through heavily populate areas. Even though underground cables are not directly exposed to hazard, lightning can induce a potential on the insulation of the underground cable. 80% of the lightning strikes in Malaysia produce current in excess of 20 kA. This analysis was done to determine the possibility of having insulation breakdown to the underground cable either by direct stroke or by induction. The simulation was performed using ATP-EMTP software to determine whether the induced voltage due to lightning can cause any insulation breakdown.
Underground or submarine cables are used to transmit power across crowded areas or body of water such as river or sea. For some cases, it is impossible to accommodate for distribution using the overhead line system approach and as an option the underground cable become necessary to replace the overhead line system for transmission and distribution.
Lightning is the transient discharge of a static electricity generated in parts, cells of storm clouds. Even though underground cables are not directly exposed to natural hazard such as lightning, it is such a way that lightning can induced current and voltage into the cable. The effects of electric fields due to direct lightning strikes on ground to underground cable need to be considered.
1.2 Objectives
The objectives of this study are
To investigate the effects of lightning strike to ground on underground cable system over various condition due to its current and induced voltage.
To verify any possibility of having insulation breakdown or damage due to lightning induced voltage and current.
To compare the analysis (ATP-EMTP) with the previous research (CDEGS).
1.3 Problem formulation
The analysis was carried out on a 132kV Cu/XLPE/SCW/MDPE underground cable having a span of 150 meters. The single phase circuit has its sheath grounded with both-end-bonding method. Along the cable span, two straight through joints were installed.
There are undoubtedly many possible factors that can cause failure to the system but this analysis particularly intended to prove that lightning currents and its induced voltages on the conductor are the main reasons for the recently observed and reported insulation failure.
1.4 Scopes
This project emphasizes on the voltage induced when lightning current injected into the system designed in aforesaid system. The 132kV rated underground cable system was modeled by taking into consideration all cable and soil parameters and conditions
CHAPTER 2
LITERATURE REVIEWS
2.1 Introduction
Overvoltage is a condition where the voltage raised higher than it's rated. A transient overvoltage is a high voltage which has a rapid rise to the peak value and slowly decays to zero value [6].
A typical natural source of transient overvoltage events is lightning. Lightning is natural phenomena that accomplished by thunder which is very intense and unpredictable that can induce overvoltage. The current diffusion in the ground may also affect underground networks.
When lightning strikes the ground, the discharge current diffuses uniformly into the surrounding soil. The electric field strength in soils at a radius of r meters is given by the following equation, by determining of lightning current distributed in radius around lightning strike point in hemisphere [3,5].
2.1
Ïs soil resistivity (Ωm)
J(r) current density at radius r (A/m2)
I lightning current amplitude (A)
2.2 Standard Lightning Wave Shape
The Basic Lightning Impulse Insulation Level (BIL) are specified for the standard lightning impulse wave shape. The general lightning impulse wave shape is illustrated in Figure 2.1 below.
tf
tt
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Figure 2.1: Lightning impulse wave shape
BIL is implies the limits up to which the insulator could withstand impulse due to lightning stoke. The front time and the tail time of the impulse is represent by tf and tt respectively. Front time is the interval between t=0 to the peak voltage or current. The standard lightning impulse wave shape is 1/50 µs which means 1µs for the front time and 50 µs for the tail time.
2.3 Underground cable parameters [4]
The line equations are the same for underground or submarine cables and overhead lines because the parameters R', L', G', C' per unit length are distributed along a cable in the same way as on an overhead lines.
2.2
2.3
Overhead lines are simple in geometry. There are more variations in underground and submarine cable geometries. Shunt conductance G' is negligible on overhead lines but in underground cable it is much larger and represents dielectric losses.
2.4
The shunt capacitance C' is much larger than on overhead lines because the conductors (core conductor, sheath etc) are very close together. The value for inductance L' is small which typically of L'overhead. While the value for C' is large which typically 20 times of C'overhead. The parameter L' and C' can be converted to surge impedance Z and wave speed c by the following equation.
2.5
2.6
Give typical values for underground cable
Z 30 to 70 Ω ( of overhead line)
c 160 000 km/s ( of overhead line)
The shunt capacitance for insulation between core conductor and sheath, or sheath and amour, or sheath and soil can be calculated using equation (2.7).
2.7
εo permittivity of vacuum
εr relative permittivity
rout outside radius of insulation
rin inside radius of insulation
Since C' of an underground cable is very large, it may be good enough to represent a "short" cable as a lumped capacitance, if the frequencies are not high.
Zseries
½ Yshunt
½ Yshunt
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Figure 2.1: Underground cable circuit model
CHAPTER 3
METHODOLOGY
3.1 Introduction
With the support of many computer simulations software package, analysis of the transient overvoltage becomes more accurate, efficient and easy. Transient overvoltage studies on underground cable due to lightning strike are very important to determine any possibility of having insulation failure or breakdown. The selection of suitable simulation software according to the supporting modal and analysis of the project will facilitate the work for designing the model, running the simulation and analysis of the result.
Running and executing this simulation only take small amount of time but deciding on the parameters of the components and circuit models representation for source, cable and soil are the actual challenge when performing the analysis. The correct and accurate model design is essential to ensure reliability of the analysis. The parameters setting for models used in the simulation are very important since the simulation result depends on the data and circuit model.
3.2 Digital Simulation Program
The Alternative Transient Program (ATP) and Electromagnetic Transient Program (EMTP) are one of the most widely used software by electric power industry for digital simulation of electrical system transient phenomena of electromagnetic as well as electromechanical nature in electric power systems. ATP program is a powerful tool for modeling power system transients.
The Alternative Transient Program version of the Electromagnetic Transients Program (ATP-EMTP) is an integrated engineering software tools that have been used world-wide for switching and lightning surge analysis, insulation coordination studies and etc.
ATPDraw is a graphical preprocessor to the ATP version of the EMTP. ATPDraw has a standard Windows layout and offers a large Windows help file system. User can build up the electric circuit in the program by selecting predefined components from an extensive palette.
3.2.1 Operating Windows
Circuit window is the container of circuit objects and the circuit is built up in this window. User can load the circuit objects from disk or simply create an empty window to start building a new circuit from file menu.
3.2.2 ATP Setting
Before user run the simulation, several option for the active circuit window must be specified. Figure shows an example of dialog box for the simulation setting. Under simulation type user can switch between Time domain, Frequency scan and Harmonic frequency scan (HFS). Tmax is the end time of simulation in seconds and delta T is the time step of simulation in seconds.
3.2.3 Data Setting
After selecting a component user must specify the value for all parameters used in the simulation. The component dialog box will appears after double click on that component and user must keying in the required data in the columns provided.
3.2.4 PlotXY
PlotXY is a plotting program to generate scientific line plots using data collected from *.pl4 files created with the program ATP. A *.pl4 file will be automatically created after user has run the simulation.
3.3 System Configuration
The system consists of a 132kV Cu/XLPE/SCW/MDPE rated cable with a length of 150 meters buried underground at a depth of 1.5 meters. The cable dimensions are illustrated in the Figure 3.1 below.
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Figure 3.1: 132kV cable dimensions layer
To enable the injection process and to allow the injected current to penetrate into the soil accordingly, a steel conductor that acts as a conductor with a length of 0.5 meter and diameter of 0.01 meter is added into the system with half of its length buried in the ground. Figure 3.2 illustrate the cable layout configuration network in the system into the Cartesian plane.
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Figure 3.2: Cable layout configuration network
3.3.1 Lightning Source
The lightning stokes represent by surge function of Heidler type 15 forms. The type of source can be set to be either current or voltage. Amp is the multiplicative number in Ampere or Volt and it does not represent peak value of the surge. T_f is the front duration time in seconds which is the interval between t=0 to the function peak. The stroke duration which is the interval between t=0 to the point on the tail where the function amplitude has fallen 37% of its peak value is represented by tau in seconds. Tsta is the starting time in seconds, Tsto is the ending time also in seconds and n is the factor influencing the rate of rise of the function. The maximum steepness will be increased if the value of n increase.
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Figure 3.3: The Heidler type source
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Figure 3.4: Dialog box for Heidler source
The lightning surge current used in this study is defined by the following double exponential type function:
3.1
where Im = 40 kA, α = 1.4x104 s-1 and β = 6x106 s-1. The lightning surge waveform is characterized by a rise time of 1 μs and a half-value time of 50 μs, which are typical values for lightning strikes. The lightning surge current wave shape as shown in Figure 3.4 below is injected at the ground above the cable.
Figure 3.5: Lightning wave shape injected into the system
3.3.2 Soil Model
Besides soil resistivity, the electric breakdown strength of soil was one of the important value to consider. Dielectric strength of soils is considered as the value of the electric field intensity, which causes breakdown under homogeneous field configuration. Figure 3.5 shows the soil model representation used for this simulation.
Figure 3.6: Soil model circuit representation
3.3.3 Underground Cable Model
From the previous chapter many parameters need to be considering in modeling the underground cable circuit. Figure 3.6 shows the underground cable model distribution in this analysis.
½ Yshunt
½ Yshunt
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Figure 3.7: Underground cable model circuit representation
3.4 System Data
The characteristics of air and soil used in the analysis are shown in the table below. For the purpose of simplifying the computation, a uniform soil type was chosen. The permeability and permittivity are relative to the free space values of 1.2566x10-6 Henries/meter and 8.854x10-12 Farads/meter, respectively.
Table 3.1: Air and soil characteristics
Layer
Resistivity (Ωm)
Relative Permeability (p.u)
Relative Permittivity (p.u)
Air
1x1018
1.0
1.0
Soil
328
10.0
25.0
The dimensions and properties of the intermediate components are specified as in Table 3.2. The thickness and properties of intermediate component's insulation are specified as in Table 3.3.
Table 3.2: Dimensions and properties of intermediate conductor components
Components
Inner Radius (m)
Outer Radius (m)
Relative Resistivity
(p.u)
Relative Permeability (p.u)
Core
0
0.01025
1
1
Sheath
0.02625
0.02855
1.635636
1.000022
Amour
0.04055
0.04395
30.160664
696.323412
Table 3.3: Thickness and properties of intermediate component's insulation
Components
Thickness (m)
Resistivity (Ω)
Relative Permittivity (p.u)
Relative Permeability (p.u)
Core
0.016
8.98x1013
2.35
1.000023
Sheath
0.012
8.98x1013
2.35
1.000023
Amour
0
8.98x1013
2.35
1.000023
The dielectric strength of cross linked polyethylene (XLPE) is around 20 - 160 MV/m. The typical value of relative permittivity, εr for XLPE insulation is 2.4 and based from equation (2.6) and (2.7), the following data were obtained.
Table 3.4: Insulation data
Components
Inner radius (m)
Outer radius (m)
Shunt capacitance C' (pF)
Inductance L'
(mH)
Inner insulation
0.01025
0.02625
141.9818
275.12
Outer insulation
0.02855
0.04055
380.5245
102.65
3.5 Observation profile
To study the effect of different insulation level and thickness against the lightning induced voltage, the observation profile as shown in Figure 3.7 below are located. The induced voltage was compare between inner insulation and outer insulation.
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Figure 3.8: Observation profile for different insulation level
To study the effect of different distance from strike point on the induced voltage due to lightning current on the cable insulation (inner insulation), the voltages were measured at different points. The value of induced voltage will be compared between two different points to verify the effects for different distance from strike point. Figure 3.9 below show the overall circuit that has been modeled in the simulation.
Figure 3.9: Circuit model to determine the effect of different distance from strike point
The cable depth was varied in this analysis to investigate the effect of different depth of cable buried by changing the value of the soil parameters for different depth level. Figure 3.8 shows the observation profile for different depth of the buried cable.
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Figure 3.9: observation profile for different cable depth
CHAPTER 4
RESULTS AND DISCUSSIONS
4.1 Introduction
The simulation has been carried out based on the configuration and observation profile. The data were collected for desired response using ATP-EMTP digital simulation software and the induced voltage wave shape were analyzed to determine the effects of lightning to the underground cable system. The results also will be compared with the previous research which used a different simulation program. The previous research was performed the analysis and simulation using Current Distribution, Electromagnetic Fields, Grounding and Soil Structure (CDEGS) simulation program.
4.2 Induced voltage on different XLPE insulation cross-sectional layer
Figure 4.1 and 4.2 shows the induced voltage at the inner and outer insulation of the underground cable when lightning current injected into the system. The induced voltage for inner insulation approximately 2 GV is much higher than the outer insulation which is approximately 800 MV.
Figure 4.1: Induced voltage at the inner insulation
Figure 4.2: Induced voltage at the outer insulation
In figure 4.2 and 4.2 it can be seen that the lightning current induced a large potential to the cable at the inner insulation compared to the outer insulation. The induced voltage at the outer insulation should be larger than the inner insulation since it is closer to ground surface but smaller value was recorded and this might be due to the thickness factor.
Figure 4.3 and 4.4 shows the induced voltage at the inner and outer insulation from CDEGS simulation from previous study.
Figure 4.3: Induced voltage at the inner insulation (CDEGS)
Figure 4.4: Induced voltage at the outer insulation (CDEGS)
From previous study using CDEGS simulation program, the potential rose to a value of 726 kV during the current peak for inner insulation and 1032 kV for outer insulation.
Different results were obtained based from this two different software simulation program.
4.2 Induced voltage on XLPE insulation at different distance from strike point
To determine the effect of different distance from strike point, the following waveforms were obtained. Figure 4.5 shows the induced voltage near to the strike point while Figure 4.6 shows the induced voltage far from the strike point. The result should shows that the further distances from strike point the less voltage will induced in the insulation.
Figure 4.5: Induced voltage near from the strike point
Figure 4.6: Induced voltage far from strike point
Figure 4.6 and 4.7 were obtained from previous research using CDEGS program and the induced voltage across the insulation layer decreased with the distance.
Figure 4.7: Induced voltage near from the strike point (CDEGS)
Figure 4.8: Induced voltage far from strike point (CDEGS)
4.3 Induced voltage on XLPE insulation at various depth from strike point
To determine the effect of different depth of the buried cable, the value of the soil parameters were change. Figure 4.9 shows the cable buried near to the earth surface and Figure 4.10 shows the cable buried deeper. The result obtain shows the same value even the parameter of the soil have been changed, this may be due to the configuration of the soil circuit model is not accurate.
Figure 4.9: Induced voltage near the earth surface
Figure 4.10: Induced voltage when the cable buried deeper
Figure 4.11 and 4.12 shows the result obtained from CDEGS simulation program. As the depth of the cable is further increased, the induced voltage was found to be decreased
Figure 4.11: induced voltage near the earth surface (CDEGS)
Figure 4.12: Induced voltage when the cable buried deeper (CDEGS)
4.4 Discussion
The analysis has been carried out by modeling the soil and 132kV underground cable based on the parameter into the ATP-EMTP simulation program. The voltage induced in the cable insulation layer has been observed. Various conditions have been considered in the simulation.
Based from the results, there will be no possibility of having insulation failure or breakdown to the cable. Since the dielectric strength of cross linked polyethylene (XLPE) is around 20 - 160 MV/m.
CHAPTER 5
CONCLUSIONS AND RECOMMENDATIONS
5.1 Conclusions
From the simulation study conducted, a series of conclusion can be made based from the observation profile and analysis on the 132kV Cu/XLPE/SCW/MDPE rated underground cable system. The conclusion that can be made in case of lightning, it may not damage cable insulator. It depends on amplitude of impact current, electric breakdown strength of insulators, grounding system configuration and cable length [5].
5.2 Recommendations
After completing this analysis, these are several recommendations:
The circuit model of the cable and soil need to be improving in term of adding other parameter.
The configuration of the circuit need to modify to get the better result.
[1] An Electro-Magnetic Transient (Emt) Analysis On A 132 kV Rated Cu/XLPE/SCW/MDPE Cable System And Its Related Networks
[2] ATPDraw for Windows 3.1 User's Manual; Lázló Prikler, Hans Kr. Høidalen; 1998
[3] Complete assessment of impact of lightning strikes on buried cables,
[4] Underground and Submarine Cable Parameters
[5] Effects of electric fields generated by direct lightning strikes on ground to underground cables, Klairuang
[6] High Voltage Engineering, 3rd edition M S Naidu
