ACTA UNIVERSITATIS APULENSIS Special Issue [609757]

ACTA UNIVERSITATIS APULENSIS Special Issue
SIMULATION OF LIGHTNING OVERVOLTAGES WITH
ATP-EMTP AND PSCAD/EMTDC
Violeta Chis ¸, Cristina B ˘ala and Mihaela-Daciana Cr ˘aciun
Abstract. Currently, several offline digital simulation software tools are
available, with varying degrees of modeling and simulation capabilities. This
paper aims to compare the capabilities of two program packages for electric
network simulation: ATP-EMTP and PSCAD/EMTDC for modeling lightning
overvoltages.
2000 Mathematics Subject Classification : 68Uxx/Subject Classification for
Computer Science. 281, 242.2
1. Introduction
The use of computer simulation tools is essential in power system studies.
The number of tools suitable for transient analysis is increasing in the last few
years. This paper aims to compare the capabilities of two program packages for
electric network simulation: ATP-EMTP and PSCAD/EMTDC. The first one
ATP is a universal program system for digital simulation of transient phenom-
ena of electromagnetic as well as electromechanical nature. PSCAD/EMTDC
is another popular tool in Electromagnetic Transients Program category which
was developed at the Manitoba HVDC Research Centre.
Modeling of power components that take into account the frequency depen-
dence of parameters can be currently achieved through mathematical models
which are accurate enough for a specific range of frequencies. The simulation
of transient phenomena may require a representation of network components
valid for a frequency range that varies from DC to several MHz. Although an
accurate and wideband representation of a transmission line is not impossible,
it is more advisable to use and develop models appropriate for a specific range
of frequencies [1]. Each range of frequencies will correspond to a particular
transient phenomenon.
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V. Chi¸ s, C. B˘ ala, M. D. Cr˘ aciun – Simulation of lightning overvoltages…
2.Simulation softwares PSCAD/EMTDC and ATP/EMTP
PSCAD is a powerful graphical user interface that integrates seamlessly
with EMTDC, a general purpose time domain program for simulating power
system transients and controls. Together they provide a fast, flexible and accu-
rate solution for the simulation of virtually any electrical equipment or system.
PSCAD/EMTDC represents and solves the differential equations of the entire
power system and its controls in the time domain (both electromagnetic and
electro-mechanical systems) [2].
The Alternative Transients Program (ATP) is considered to be one of the
most widely used universal program system for digital simulation of transient
phenomena of electromagnetic as well as electromechanical nature in electric
power systems. With this digital program, complex networks and control sys-
tems of arbitrary structure can be simulated [3].
ATPDraw can be used as a simulation center, that provides an operating shell
for other ATP-EMTP components. To solve the differential equations of sys-
tem components in the time domain is used trapezoidal rule of integration
[3].
3.Lightning overvoltages
Lightning strikes to overhead transmission lines cause travelling waves
which propagate along the overhead line and enter substations where they
cause overvoltages which can pose a risk to any items of equipment connected.
The causes of overvoltages in power systems may be external (e.g., lightning) or
internal (e.g., switching maneuvers, faults, ferroresonance, and load rejection),
and may also occur as a combination of several events. Since their magnitude
can exceed the maximum permissible levels of some equipment insulation, it
is fundamental to either prevent or to reduce them.
Lightning surge voltages that arrive at a station, traveling along a transmis-
sion line, are caused by a lightning stroke terminating either on a shield wire,
a tower, or a phase conductor [4].
The voltage magnitude and shape of these voltages are functions of the mag-
nitude, polarity, and shape of the lightning stroke current, the line surge
impedance, the towersurge impedance and footing impedance, and the light-
ning impulse critical flashover voltage (CFO) of the line insulation.
For lines that are effectively shielded, the surge voltages caused by a backflash
are usually more severe than those caused by a shielding failure; that is, they
have greater steepness and greater crest voltage [5].
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V. Chi¸ s, C. B˘ ala, M. D. Cr˘ aciun – Simulation of lightning overvoltages…
4.Case study
Consider a transmission line (220 kV, 100 MVA) with towers that are 40
m tall and spaced 280 m apart. Assume that there is a single shield wire with
a characteristic impedance of 520 ohm and that the tower ground strap has
a characteristic impedance of 135 ohm. The tower has a footing resistance of
30 ohm. Assume a propagation velocity of the speed of light for the ground
wires and 0.85 times the speed of light for the tower ground strap. The phase
conductors are 75% of the way up the tower and the ground wire is segmented
and open at the top of each of the adjacent towers. For modeling lightning
overvoltages it was used ATP-EMTP and PSCAD/EMTDC.
Shield wires and phase conductors of the transmission line can be modeled by
several spans at each side of the point of impact. For lightning overvoltage
calculations, a constant-parameter line model can be accurate enough, and
parameters are usually calculated at 400-500 kHz [5]. A line termination at
each side of this model is needed to avoid reflections that could affect the
simulated overvoltages around the point of impact. This termination must be
represented accordingly to the model chosen for the line spans.
Several models have been proposed to represent transmission line towers (single-
phase vertical lossless line model, multiconductor vertical line model, mul-
tistory model); they have been developed using a theoretical approach or
based on an experimental work [6]. The simplest representation is a lossless
distributed-parameter transmission line, characterized by a surge impedance
and a travel time.
Travelling waves propagate in air or gaseous insulants at the velocity of light
c. In the case of exposed conductive parts with solid insulation (for example,
cables), the speed of propagation is and depends on the level of the relative
dielectric constant εr.
ν=c√εr(1)
The surge impedance Z describes the high-frequency behaviour of the items
of equipment and can be calculated with the relationship:
Z=1
µ·C/prime (2)
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V. Chi¸ s, C. B˘ ala, M. D. Cr˘ aciun – Simulation of lightning overvoltages…
where
Z=/radicalbigg
L/prime
C/prime (3)
L/prime=inductance per km
C/prime=capacitance per km
For overhead lines and overhead earth wires, the resultant surge impedance
can be calculated with the relationship:
Z= 60 ·ln2h
r(4)
where h describes the suspension height above ground, and r is the equiv-
alent radius of the conductor. Grounding modeling is a critical aspect. A
nonlinear frequency-dependent representation is required to obtain an accu-
rate simulation. Since the information needed to derive such a model is not
always available, a lumped circuit model is usually chosen for representing the
footing impedance, although it is recognized that this model is not always ad-
equate.
A lightning stroke is represented as an ideal current source whose parameters,
as well as its polarity and multiplicity, are randomly determined according to
the distribution density functions recommended in the literature [5]. It was
considered the case of a lightning strike where the current rises to 40 kA in 2
µs and falls to 20 kA after 40 µs (Figure 1).
Figure 1: Lightning current in PSCAD
Because PSCAD requires a terminating impedance at the end of a line, it
can’t be open circuited, a very large resistance has been placed at the end of
the line. The Bergeron model is based on a distributed LC parameter travelling
wave line model, with lumped resistance.
After simulation the peak voltage at the tower top, the bottom of the tower
and at the conductor height was obtained. (Figure 3, Figure 5).
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V. Chi¸ s, C. B˘ ala, M. D. Cr˘ aciun – Simulation of lightning overvoltages…
Figure 2: Electric diagram implemented in PSCAD
Figure 3: Voltages at tower top, cross arm and footing resistance in PSCAD
The results obtained with PSCAD/EMTDC and ATP were compared with
Mathcad (Figure 6, Figure 7, Figure 8).
After the simulations performed with PSCAD/EMTDC and ATP-EMTP
were obtained very close results.
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V. Chi¸ s, C. B˘ ala, M. D. Cr˘ aciun – Simulation of lightning overvoltages…
Figure 4: ATP Draw Circuit Diagram
Figure 5: Voltages at tower top, cross arm and footing resistance in ATP
Figure 6: Tower top voltages
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V. Chi¸ s, C. B˘ ala, M. D. Cr˘ aciun – Simulation of lightning overvoltages…
Figure 7: Voltages at the crossarm
Figure 8: Voltage at footing
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V. Chi¸ s, C. B˘ ala, M. D. Cr˘ aciun – Simulation of lightning overvoltages…
6.Conclusion
Calculation of lightning overvoltages necessitates simulation of the electrical
equipment, such as for example overhead transmission lines, cables, towers and
substations as travelling wave model.
In order to analyse wave processes within power lines due to lightning dis-
charges there are various computer techniques including software packages (eg
EMTP Electro-Magnetic Transients Program) or special processing programs.
Computer analysis of overvoltages in this paper has been carried out using
processing program ATP-EMTP and PSCAD/EMTDC.
The accuracy of results of simulation is extremely influenced by the accuracy
of the input data.
References
[1] J. A. Martinez-Velasco, Power System Transients Parameter Determi-
nation, CRC Press Taylor & Francis Group, 2010.
[2] PSCAD/EMTDCTM vs 4.2 User’Guide, Manitoba HVDC Research
Centre, 2005
[3] L. Prikler, H.K. Hoidalen, ATPDraw version 3.5 for Windows, User’s
Manual, 2002.
[4] CIGRE WG 33-02, Guidelines for representation of network elements
when calculating transients, 1990.
[5] J. A. Martinez-Velasco, F. Castro-Aranda, Modeling of overhead trans-
mission lines for lightning overvoltage calculations, Ingeniare. Revista chilena
de ingenieria, Vol. 18, No 1, (2010), pp. 120-131.
[6] C.F. Wagner, A.R. Hileman, A new approach to the calculation of the
lightning performance of transmission lines-Part III, AIEEE Trans. Part III,
Vol. 79, No 3, (1960), pp. 589-603.
Violeta Chi¸ s, Cristina B˘ ala, Mihaela-Daciana Cr˘ aciun
Department of Mathematics and Computer Science, Faculty of Exact Sciences
”Aurel Vlaicu” University of Arad
Address
email: viochis@yahoo.com, cristinabala@yahoo.com, mihaeladacianacraciun@yahoo.com
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