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Modeling of grounding systems of power lines under pulse influence

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Recent Advances in Energy, Environment and Materials
Modeling of grounding systems of power lines under pulse influence
Kirill I. Netreba
Ph.D. Student at Electrical
Engineering Department
SPbSPU, Russia
[email protected]
Nikolay V. Korovkin
Head of Electrical
Engineering Department
SPbSPU, Russia
[email protected]
m
Nikolay V. Silin
Head of Electrical
Engineering and
Electrotechnical Department
FEFU, Russia
[email protected]
Abstract: - The technique of determination of equivalent electromagnetic parameters of grounding systems of
power lines is offered and examined. The technique is based on use of the experimental data gained by means
of specially developed mobile experimental set. The result of identification is the linear circuit correctly
reproducing electromagnetic characteristics of the grounding systems of power lines under arbitrary influences.
Key-Words: - grounding system, pulse influence, RLC model, lightning overvoltage
account electromagnetic energy in reactive
elements, without usage of standard electrical circuit
synthesis methods is not substantiated and
unpromising.
Exception is [9] where oscillogram of transient
resistance was obtained as a result of pulse
experiment’s data processing. Based on this
oscillogram the simplest RL model was fitted. Time
constant was also found graphically. In [10]
parametric synthesis of the second order passive
impedor was done based on pulse experiment data.
However, in case of GS not only parameters of
equivalent circuit are unknown, but its topology as
well.
So, resistive model of grounding systems based
on “pulse” resistance does not fully reproduce real
processes result in GS under pulse influences.
1 Introduction
In present time in Russia [1] and in other countries
[2] so called resistive model of grounding system
(GS) is used. According to this model grounding
system is characterized by stationary resistance R on
main (low) frequency and by “pulse” resistance
Rimp=max(u)/max(i) or r(t)=u(t)/i(t) on pulse
influences. It should be noted that in theoretical
electrical engineering by pulse resistance it is
traditionally understood reaction of the circuit on
delta function [3], therefore term “pulse” resistance
used in grounding theory is in quotes.
Resistive model of GS is simple but theoretically
substantiated only for stationary regimes. In [4]
absence of correlation between “pulse” resistance
obtained under aperiodic and damped vibrational
influences is shown. In [5] dependence of “pulse”
resistance on frequency (or duration of pulse front)
is demonstrated. Therefor when the frequency is
changed correction coefficient has to be applied to
Rimp.
Despite imperfections of usage Rи as main
characteristic of GS experimental and theoretical
researches are conducted based on “pulse”
resistance. To be able to compare “pulse” resistance
of different GS without usage of correction
coefficient special pulse generator with reference
pulse characteristics 0.25/100 and 10/350 mcs is
created [6]. In [7, 8] an attempt to synthesize
equivalent RLC circuit of GS based on specially
defined “local pulse resistance” at time interval till
1mcs was made. Obviously, synthesis of equivalent
RLC circuits is the next step in GS model
development. However, synthesis of such circuits on
“pulse” resistance basis, which doesn’t take into
ISBN: 978-1-61804-250-7
2 Synthesis of RLC models of ground
grids
In this paper methodic of synthesis of GS RLC
models is presented. It allows to analyse pulse
regimes of GS work in lightning protection systems
on strict theoretical basis as opposed to methods
based on “pulse” resistance. Transient resistance z(t)
is used as primary parameter of grounding system.
Numerically it is equal to voltage on the GS under
current unit step and it is defined from voltage
oscillograms for any current test pulse. Realization
of z(t) by methods of electrical circuit synthesis
allows to define equal circuit and electromagnetic
parameters of GS.
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Recent Advances in Energy, Environment and Materials
of antenna will have frequency about 1 MHz. Wave
processes due to wave reflection in grounding ware
from power line towers have frequencies (vC/4l,
vC/8l) 0.5–1 MHz. Sparking and wave processes in
the grounding system itself have higher equal
frequencies because of small geometrical size of
areas where they occur. So, initial data can be
smoothed up to 500 kHz equivalent frequency.
With decrease of smoothing frequency
complexity of parametric synthesis task reduces and
accuracy of initial data approximation increases.
Therefor GS models for relatively short frequency
range (switching overvoltage) can be obtained quite
easily and significantly increase modeling accuracy.
2.1 Initial data processing
Experimental data (see Fig.1) i(t) and u(t) obtained
under standard lightning pulse influence include
information about different physical processes going
with process of current flow through GS. Sparking
in the soil, wave processes in grounding ware,
electromagnetic radiation by tower pole for aerial
lines are among these processes. They are quite
high-frequency, fast damped and not associated with
equivalent circuit of GS though its influence is
clearly seen in experimental data. For reduction of
this influence experimental data is smoothed and
then used for equivalent circuit synthesis.
u(t), i(t), o.e.
1
2.1.1 Usage of optimization algorithms for
synthesis of RLC models
In general parametric synthesis problem can be
definitely solved by exhaustive search among all
possible values of varying parameters. However,
huge amount of possible solutions makes such
approach unreasonable concerning time resources.
In case of continuous varying parameters which is
the case for this work such approach is found to be
impossible.
Application of different optimization algorithms
allows significantly reduce number of estimated
solutions. Gradient methods are widely used to
solve optimization problems with continuous
parameters and with possibility to calculate gradient
of functional. However in present work since
optimization problem is not unimodal soft compute
techniques which allow to find global extremum of
the problem are preferable [11, 12].
To do parametric synthesis of equivalent circuit
of GS next optimization algorithms were studied
and software realized: simulated annealing, genetic
algorithm and artificial bee colony algorithm (bee
algorithm). These algorithms are able to work with
large number of optimized parameters which allows
to synthesize circuits of any order to approximate
transient processes with high equivalent frequency.
Let’s consider application of artificial bee colony
algorithms to parametric synthesis problem.
Let’s equivalent circuit of the GS has topology as
it is shown in Fig.2, a. Voltage source e(t) is
experimentally obtained and smoothed voltage
oscillogram u(t). The task is to fit values of circuit’s
parameters so the error between circuit current icirc(t)
and experimental smoothed current iexp(t) was
minimized (Fig.2, b):
0.8
0.6
0.4
i(t)
0.2
u(t)
0
-0.2
-0.4
-0.6
-0.8
-1
t, мкc
0
10
20
30
40
50
60
70
80
90
100
Fig.1. Experimental data.
Experimental data smoothing process is not
formal because smoothing degree influence on
topology and parameters of equivalent circuit of GS.
Smoothing algorithm used in this work for typical
set of experimental data is shown below. Initial data
is obtained for rod-shaped GS of overhead line
tower.
Experimental data (about 104 points) obtained by
double-channel oscilloscope in 0-10-4 s time range is
interpolated on 215 points and then by Fast Fourier
Transform (FFT) converted into frequency domain
(214 of complex amplitudes). After this frequency
spectrum is cut off up to 211 harmonics. Therefore,
frequency of the highest harmonic is 20 MHz which
correspond to bandpass of used oscillograph. So, on
this stage only high frequency noises appearing due
to decimation of analog signal, high frequency noses
of measuring complex and electromagnetic
environment are removed.
For further smoothing equal frequencies of the
processes going with process of current flow
through GS have to be evaluated. For this purpose
let’s use next appropriate estimations. Supposing 25
meter tower pole as quarter-wave oscillator this kind
ISBN: 978-1-61804-250-7
t
I ( p) = ∫ | iexp (t ) − icirc (t ) |dt , I ( p ) 
→ min, (1)
p∈Π
0
109
Recent Advances in Energy, Environment and Materials
where I – minimizing function, Π – tolerance
range of parameter’s vector p, p=[R1 R2 R3 L C] –
vector of equivalent circuit parameters.
found for lower frequency oscillograms stay the
same for higher frequency oscillograms.
200
i, A
u, V/10
8,6
Iэiexp
I icirc
uexp
U/10
4,8e-5
150
38,4
2,8
3,8e-5
100
50
εave=2,7%
εmax=25,8%
0
-50
t, mcs
0
10
20
30
40
50
60
70
80
90
100
Fig.3. Voltage and current oscillogram smoothed
under 500 kHz equal frequency and equivalent
circuit.
140
i, A
u, V/10
120
uexp
iexp
icirc
8,6 4,8e-5
100
Fig.2. a – example equivalent circuit, b –
formulation of optimized function.
80
40
Next parameters were applied for bee algorithm:
number of scouts s = 20, number of best patches n =
4, number of perspective patches m = 20, number of
bees following best patches N = 15, number of bees
following perspective patches M = 6.
The result is shown in Fig.3. One may see that
current i(t) and voltage u(t) smoothed up to 500 kHz
are rapidly oscillating functions, however
approximation accuracy of obtained equivalent
circuit enough high. Thus, average error is
εave500=2.7 % and maximum error is εmax500=25.8 %.
In Fig.4 equivalent circuit for voltage and current
oscillograms smoothed up to 75 kHz is shown.
Comparison between Fig.3 and Fig.4 allows to
estimate influence of smoothing degree of initial
data on topology and parameters of equivalent
circuit of GS.
As one may see with increase of equivalent
frequency of experimental data circuit topology is
changed, however not in essence. New physical
processes are modeled by appearance of new
elements in the circuit. As one may see moving
from 75 kHz to 500 kHz one RC chain was added to
approximate high frequency processes. At the same
time common parameters of the equivalent circuit
ISBN: 978-1-61804-250-7
38,4 104
5,8e-11
εave=2,3%
εmax=3,8%
60
2,8
3,8e-5
20
0
-20
t, mcs
0
10
20
30
40
50
60
70
80
Fig.4. Voltage and current oscillogram smoothed
under 75 kHz equal frequency and equivalent
circuit.
3 Calculation of lightning surge
Let’s estimate influence of GS model on
overvoltage at substation and its protecting surge
arrester after lightning stroke at overhead line 150
kV. Fig.5 shows system model to calculate
overvoltage for different GS models.
Standard lightning impulse 1,2/50 mcs with 350
kV amplitude is appear at voltage maximum in
stationary regime. Overvoltage wave goes to
protected object PO (substation) and its surge
arrester SA through Line1. The length of Line1
enough large so wave processes between PO and SA
is damped before arrival of reflected from voltage
source wave on Line1.
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Recent Advances in Energy, Environment and Materials
Fig.5. Test model to calculate overvoltage dependence on GS model.
Protected object is the transformer working at
active-inductive load. Active resistance of the load
is equal to wave impedance (R = 300 Ohm). Surge
arrester is placed 450 m from PO and connected to
grounding system GS.
Let’s consider several GS models. The first one
(Fig.6, a) neglects active-inductive resistance of the
ground system. In this resistive model the value of
the R and calculated as stationary resistance of the
equivalent circuit.
The second one (Fig.6, b) GS is modeled simply
as part of full equivalent circuit. The third one
(Fig.6, c) is full equivalent circuit obtained by
method described above. Calculation results of the
overvoltage at surge arrester and protected object
are shown in Fig.6.
Let’s describe wave processes between SA and
PO in general on resistive model sample (curve 1, 5
in Fig.6). As it is seen in the figure protected object
voltage may significantly exceed SA voltage.
Overvoltage wave is reflected from transformer
without sign change which leads to voltage double
at protected object comparing to incoming wave (by
duration of lightning impulse about a few
microseconds transformer is equal to capacitor)
Going back to surge arrester reflected from
transformer wave goes though protected span almost
without distortion at the beginning while total stress
of incoming from Line1 wave and reflected from
ISBN: 978-1-61804-250-7
transformer wave is less than discharge voltage of
surge arrester (Udis). When total voltage of SA is
close to Udis reflected from transformer wave stats to
reflect from surge arrester with sign change.
Therefore after several reflections overvoltage at
transformer gradually becomes equal to discharge
voltage of surge arrester.
According to results shown in Fig.6 GS model
has significant influence on maximum overvoltage
which occurs at protected object and surge arrester.
Usage of RLC circuits as GS models leads to
appearance of maximums at each transitional
process. For resistive model with surge arrester
response its current increase instanter limiting its
voltage level. For RLC model current through surge
arrester cannot increase immediately because of
inductance (see Fig.7) which lead to transitional
process and thus higher voltage level. As one can
see maximum overvoltage at SA for resistive model
is 10,3 % less than for full model which is
considerable.
For protected object errors for the first
maximums are even larger because of voltage
doubling (see table in Fig.6). When used resistor as
a GS model maximum voltage level at protected
object is 13,8 % less comparing to maximum
voltage with full circuit as a GS model.
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Recent Advances in Energy, Environment and Materials
550
U, кB
R1
8
500
R1*10
RLсхема
model
RL
полная
схема
equivalent
7
450
circuit
6
5
400
350
300
4
3
2
1
250
200
150
100
t, мкс
50
860
862
864
866
868
870
Fig.6. Overvoltage at SA (1,2,3,4) and PO (5,6,7,8), table of maximum errors (relative to equivalent
circuit), variants of GS models.
To compensate active-inductive character of GS
it is suggested sometimes to increase simply the
value of active resistance. Such an approach as
results shows does not give qualitative and
quantitative correlation. Increase of active resistance
value ten times (from 1,28 Ohm to 12,8 Ohm)
remains error between maximum voltage values
considerable high (see curves 2, 3 and 6, 7 in Fig.
6). At the same time approximate accounting of GS
inductive characteristics leads to wrong results. For
concerned case the results are overrated (see curves
4, 8 in Fig. 6).
2.5
I, кA
From obtained results one may see that the
reason of overvoltage at protected objects depends
not only on surge arrester characteristics. Significant
error in real overvoltage definition is the error in
calculation of grounding system’s transient
resistance.
4 Conclusion
The technique of determination of equivalent
electromagnetic parameters of grounding systems of
power lines is offered and examined. The technique
is based on use of the experimental data gained by
means of specially developed mobile experimental
set. Calculations of the overvoltage occurring due to
lightning stroke show significant dependence of
maximum voltage levels at protected object on GS
models.
Ω
ЗУ: R1=1.28Ohm
R1=1,28
полная
схема ЗУ
full
circuit
2
1.5
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1
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860
862
864
866
868
870
872
874
876
878
Fig.7. Current through surge arrestor.
ISBN: 978-1-61804-250-7
112
Recent Advances in Energy, Environment and Materials
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