2008 International Conference on High Voltage Engineering and Application, Chongqing, China, November 9-13, 2008
Single-phase auto-reclosure studies: secondary arc model
research including a 500kV line experimental circuit
Câmara Alessandra S.B.1, Goncalves, Ricardo A. A.1, Rodrigues Marcelo G.2, Oliveira F. Orsino2, Portela Carlos M.3,
Tavares Maria Cristina(portelac@ism.com.br) 4
1
Furnas Centrais Eletricas R. Real Grandeza 219 – Botafogo Rio de Janeiro RJ 22281-900, Brazil
CEPEL Electric Power Research Center Av. Olinda s/n – Adrianópolis Nova Iguaçu RJ 26053-121, Brazil
3
COPPE/UFRJ-Federal University of RJ, Brazil
4
DSCE-FEE/UNICAMP - SP, Brazil
2
Abstract - Brazilian electrical transmission system has radial
sub-systems and not much meshed long EHV/HV lines, a profile
that suggests single-phase auto-reclosure (SPAR) as usual
procedure. Since year 2000, the national electrical energy
regulatory agency (ANEEL) has required the use of SPAR in the
design, construction and operation of new transmission lines.
Analysis of SPAR success is basically related to secondary arc
extinction in a reasonably short time. So, secondary arc modeling
became an important issue on defining optimized solutions. In
2003, FURNAS, UFRJ and CEPEL − transmission utility,
university and research center, respectively − started a research
project related to secondary arc modeling. A test infra-structure
was set up at CEPEL´s high power laboratory, including an out
door real 500 kV line piece, with three towers and two spans and
necessary measuring devices (electrical parameters, visual – films
- and wind records), where arcs have been generated and
monitored. The main purpose of this research project is to obtain
and validate a robust model of the primary and secondary arcs in
a transmission line, allowing to investigate the interaction between
the arc and the “network”, at arc terminals, and the evaluation of
SPAR procedure success (or not). This work presents: the basic
aspects of the laboratorial work conception; some difficulties
related to the accuracy quality of arc parameters evaluation, of
some usual measurement and data processing procedures; the
adopted solutions to overcome such difficulties. The results
already obtained support the basic conception of a gray type arc
model, developed previously, allowed the establishment of
adequate research type procedures for measurements and data
processing, supplied a huge amount of data for arc modeling,
giving basis to obtain arc parameters in an important range of
conditions. New results are still expected to complete the range of
conditions covered by the research project.
I.
INTRODUCTION
Although the Brazilian national power system represents a
natural scenario for single-phase auto-reclosure (SPAR) usage,
only after year 2000, when the national electrical energy
regulatory agency (ANEEL) imposed the SPAR procedure for
every new line project, specific studies became a national
concern and the most used studies/analysis philosophies were
questioned.
While line projects must be optimized, such optimization is
restricted by conservative (or even mistaken) criteria of
analysis for SPAR main phenomenon, which is the secondary
arc extinction. Such criteria are used to indicate whether and
which kind of strategy can be used to assure successful SPAR
switching, or, in other words, secondary arc extinction during
“dead time”. Inaccurate calculation can lead not only to overdimensioned solutions, but even to wrong or inappropriate
ones.
FURNAS, UFRJ and CEPEL - utility, university and
research center, respectively − started a research project aiming
to produce a robust and reliable model of electric arc in air.
Special installations were designed and built at CEPEL’s high
power laboratory, including two 500 kV line spans (Fig.1).
II. THE ELECTRIC ARC MODEL ADOPTED
The electric arc behavior analysis, specifically secondary arc
extinction, is the main concern in single-pole switching studies,
since secondary arc extinction dictates whether the reclosure
will be a successful one or not.
The arc starts by a single-phase fault, at any transmission
line point. By the faulted phase circuit-breakers opening, the
initial (fault) current is reduced from kA levels to the so-called
secondary arc, which is sustained by electromagnetic interphases coupling, and rarely exceed 102A (rms) magnitude
order. In fact, higher values of secondary arc may be observed
in very long lines, with no intermediate substations and low
compensations levels (or none) – a very rare configuration, but
only if adequate procedures to reduce secondary arc current are
not taken.
At initial arc transitory period, it can be observed also an
unidirectional component decay. Such initial transitory
duration depends on system and arc characteristics, besides
fault occurrence and phase opening instants.
Secondary arc extinction analysis takes place under three
basic phenomena related to arc behavior: thermic extinction,
dielectric reiginition and arc-grid interaction.
The arc model determination shall consider criteria with
physics consistency, namely arc parameters independent of
values and shape of small disturbances, and a dynamic model
that satisfies the static characteristic.
For a particular operation point, many of the most used
models are, in fact, “equivalent”, with chosen parameters’
adjustment, and it is useless try to define a “black-box” model,
according to experimental data limitations.
A black-box model can not have enough information to
consider simultaneous relevant phenomena, which, by the way,
The authors thank the financial support received from CNPq - National Council of Scientific and Technological Development
(Brazil) and FAPESP – The State of São Paulo Research in part of research work in which this paper is based.
978-1-4244-2810-6/08/$25.00 ©2008 IEEE
490
Figure 1 - 500kV experimental spans
still are difficult to be detailed even by more complex models,
such :
- Cathodic and anodic regions are affected by many
phenomena that are completely different from central region of
the arc channel.
- Arc channel can be divided in two very unlike regions, a
central one, with high temperature and small thermic inertia,
and an external one, with lower temperatures and higher
thermic inertia.
The best approach is to adopt a “gray-box” model, searching
for models that report the main physics phenomena, with a
minimum of independent parameters, avoiding parameters
strongly dependent on operation point, and using experimental
data covering a large range of disturbances.
III. THE DEVELOPMENT OF THE PROJECT
Brazilian policy, electric arc modeling state-of-art, and a
promissory arc model concept motivated a research in basis
mentioned bellow.
A. Experimental activities conception
The model under development follows a Gray-Box Model
philosophy [1], where transfer functions relating electrical
entities are joined with parameters that report some basic
physics phenomena and/or empirical characteristics observed.
In outlines, the research works under the following strategy:
(1) Analysis of the stationary and dynamic arc behavior by
experimental data information; (2) Definition and adjustment
of model parameters and (3) Validation of the proposed model
by system conditions reproduction.
The steps (1) and (2) mentioned above are defined as
Phase I, where different power frequency arc values and
impulses with different amplitudes, waveforms and polarities
are produced.
Besides arc voltage and current measurements, wind
conditions and visual recordings are processed. The impedance
frequency response of experimental circuit is also measured.
The aim of Phase I is to establish a robust and reliable arc
model.
At Phase II the model generated by Phase I is validated
considering also system conditions (arc-grid interaction).
At this very moment the research is in its Phase I.
In the next sections, it will be mentioned some measurement
procedures, experimental structure adaptations and data preprocessing cares.
B. Laboratorial structure
The necessary experimental data to develop and validate a
robust and consistent model of the primary and secondary arcs
shall be obtained under conditions as close as possible to the
on site conditions, as far as the transmission line components
and geometry are concerned. To meet this requirement, a
testing set up including two 500 kV line spans was installed at
CEPEL’s high power laboratory, as shown in Figs. 1 and 2,
representing part of a typical FURNAS 500kV transmission
line. Four arc sites have been considered to produce the arc:: at
a diagonal insulator string at Tower 1, at the horizontal
insulator string of Tower 2 and across the clearance between
one phase-conductor and ground-wire, as indicated in Fig.2.
Figure 2 – Arc generation sites at the laboratory tests arrangement
The diagram of the electric circuit used to carry out the tests
is shown in Fig. 3 and its main components are: a) two single
phase test power transformers (T1) in series, connected to the
138 kV substation bus, providing 50kV open circuit voltage; b)
circuit breaker CB to switch the test current; c) switch S; d)
making synchronous switch MS to establish the test current at
a chosen phase angle; e) Reactors XL and resistors R to limit
test current; f) One auxiliary test power transformer T2, used
for test current up to 300A, to provide 250kV open circuit
voltage. Such voltage level was found to be the minimum
necessary to sustain the arc under acceptable stability
conditions; g) Transmission line TL; h) Voltage dividers VD1
and VD2 to measure the voltage at the live and at the earthed
arc sides related to the ground potential; i) Current
transformers CT1 and CT2 to measure the arc power frequency
current and impulse current injected to the arc; j) Impulse
generator.
Figure 3 – Circuit for generation of arc and impulse currents.
C. Tests Characteristics
The tests consist of generating and monitoring electric arcs
with different amplitudes of power frequency electric current
ranging from 10 to 10 000 A(rms) (60Hz). To evaluate the arc
dynamic behavior and investigating the associated time
constants, current impulses with different waveforms,
polarities and amplitudes have been injected into the sustained
arc at different specific time within the period of the arc power
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frequency current. Impulse current front times of 1 µs, 5 µs
and 10 µs, peak values of 5, 10 and 20% of 60 Hz current
component amplitude, and of positive and negative polarity,
have been used.
The data considered useful for the arc modeling process,
recorded during each test, are:
ƒ Voltages and currents: measurements of arc terminal
voltages (Varc), power frequency arc current (I60Hz) and
impulse current (Iimp) with sampling time ≤ 100 ns and time
window of 1s;
ƒ Wind: measurements of the wind speed and direction close
to the arc region with sampling time ≤ 1 s;
ƒ Dynamic images: records of arc movies from two view
points with rate ≥ 1000 frames/s;
ƒ Environmental conditions: Atmospheric pressure, relative
humidity and temperature.
ƒ Circuit parameters: Impedance of test circuit components
in frequency domain up to 10 MHz.
D. Critical Experiments Aspects
Taking into account the quite complex experimental
work and the great number of tests required for the project,
some critical aspects and challenges have been faced at the
laboratory since the beginning. Some of them are listed
bellow together with the action to overcome them:
ƒ Arc ignition across the 500 kV insulator string that is
about 25 m above the ground: Before each test, a crane
lifts a person to connect a fuse wire in parallel to the string.
The arc ignition is provided applying 250 kV(rms) (for test
current up to 300 A(rms) or 50 kV(rms) (for test current
above 300 A) between the string terminals, during 1 s. The
test transformer used to apply the voltage is connected to
FURNAS power grid that feeds CEPEL’s high power
laboratory. Based on analysis of the arc current and arc
voltage arguments and magnitudes, it was concluded that
after about 500 ms the metallic vapor due to the fuse wire
burning has completely disappeared. After this time, the arc
can be considered “clean”. The adequate fuse wire
diameter and material were selected based on this analysis.
ƒ Generation of current impulses with suitable shapes and
amplitudes: An impulse generator was implemented to
provide impulses with different front times, peak values
and polarities, injecting a sequence of five impulses into the
sustained arc at each test, during the last 500 ms of current
circulation. A multi-terminal instant switch, based on
compressed air actuated components, was developed to
provide the conditions to inject five controlled current
impulses into the sustained arc. A digital time programmer
device automatically controls the operation of this switch,
as well other test circuit switches. The switching
mechanism is such that each branch of the impulse
generator is put in contact with the arc terminals just during
each current impulse injection. A double loop circuit was
used to generate very low impulse currents under high
voltage.
ƒ Measurement of electrical magnitudes, including low and
high frequency components: To minimize electromagnetic
interferences, optical fibers for signal transmission systems
were used and the optical transmitters located on site were
adequately shielded. A non-ordinary four-channel data
acquisition system capable of acquiring, storing and
processing more than 10 million points per second for each
measurement was developed, based on commercial
available hardware. This system is capable of acquiring,
storing and processing more than 10 millions of samples
per second for each measurement. The four channels are
independent and each of them can be set to acquire 10
million samples at each test. One test consists of
generating a sustained arc at a given power frequency
current amplitude during one second and injecting into it a
sequence of five current impulses of a given waveform,
peak value and polarity.
ƒ Arc movies for high current values: A power arc is a
strong light source that creates difficulties to make movies
focusing its nucleus with ordinary cameras. A high-speed
camera with maximum rate of 10.000 frames/s was used to
record very clear dynamic images showing the arc behavior
during all its duration. Such movies have been very useful
to study the arc elongation and are well correlated to arc
voltages and current variations.
E. Data analysis processes
Experimental data reflects not only the phenomena under
study but also other effects, including the measurement
apparatus influence. To identify noise, or other undesirable
effects, and to distinguish among available data the
information to be really considered (according to the kind of
analysis in course), it is necessary to have in mind not only a
previous theory basis knowledge but also a perspective of
facing some unpredicted facts. Working under such ideas and
considering experimental limitations on tests’ uncertainties, the
number of tests must be enough to, by statistical and other
types validity analysis, support conclusions. Besides, in order
to produce a robust, general and reliable model it is necessary
to have a large spectrum of data.
So, despite the great number of data, it is necessary to
implement some kind of pre-processing, in order to identify the
quality of the measurements, to filter data, and to discriminate
the desired information. For example, for steady-state analysis
(arc stationary characteristic), impulses region and effects are
not so important, and data filtration can be more comfortably
applied than when it concerns dynamic arc response.
Basically, when data measurement is available, the whole
data pre-processing involves:
1. To verify if data registers are complete and properly
synchronized (voltage at insulator string top, voltage at
tower, current impulse, total current (power frequency
+ impulse), wind and visual records (from two
cameras)).
2. To identify, according to data quality available for all
variables, whether it is appropriate or not to perform
stationary and/or dynamic arc analysis.
492
3.
To establish a reliable data filtering, taking into account
the domain under analysis (steady-state or transient).
After these three basic steps, data are analyzed and
manipulated considering the gray-box adopted model, with
parameters’ identification and adjustment.
IV. SOME RESEARCH RESULTS
The results already available are related to tests developed
over “I” insulators string, from 10A to 3kA. They lead to some
partial conclusions:
A.
Steady-State Arc behavior
Steady-state arc analysis is basically related to two main arc
behavior characteristics: arc elongation and stationary
characteristic.
A.1 Arc elongation
Arc length variation is a slow phenomenon (and visible),
related directly to arc extinction process. It reflects the
influence of many macroscopic environmental factors like
wind, thermal convection, air humidity, pressure and etc, in
ionization and recombination processes of the arc plasma
particles, becoming an indirect and simple way to consider
together environment global effects.
Arc elongation information is very useful for dynamic
response as well for transient arc processes analyses. It allows
making use of some simplifications/approximations, according
to the model adopted, for analyses with incremental variation
of arc conductance, or current, or voltage or electric field.
From arc voltage and current measurements, it is possible to
estimate a time function for arc elongation (l(t)). Visual and
wind records are important tools to support the l(t) definition.
Figure 5 – Stationary Arc Characteristic for a 150 A arc current test
Some interesting statements could be confirmed by visual
recordings, such as: (1) arc elongation easily reaches many
times the initial arc length during the arc process, with a nonmonotonic time variation, with loops formation (Fig.4); (2) a
kind of arc anomalous contraction-seeming behavior during
some milliseconds after arc initiation was observed; it is due to
insulators-arc contact cooling.
B.
Dynamic Electric Arc behavior
The dynamic electric arc behavior analysis is the culminant
point of SPAR electrical studies, since at this stage secondary
arc extinction is directly avaliated facing the “dead time”
question.
The large variety of impulses applied is motivated by
parameters adjusting that are strongly related to that domain of
analysis.
The arc is treated like a conductance vayring in time g(t)
associated to arc elongation l(t). The function g(t) x l(t) = gu(t),
which is less irregular than g(t) due to to l(t) effect, is related to
the fact that calculations are developed at one point surround or
a small arc fraction as function of electric field (e(t)):
g(t) = i(t)/u(t) = i(t) / (e(t) x l(t)) => g(t) x l(t) = i(t) / e(t) = gu(t).
Partial results point out at least two ranges of time constants
very different from each other (100 µs and between 101µs and
102 µs order of magnitude).
V. CONCLUSIONS
The partial results reached up to now suggest that a robust
arc model will be available as soon as the Phase II of this
project is completed.
ACKNOWLEDGEMENT
Figure 4 – Arc Elongation of a 150 A current test (1s)
As l(t) is a slow phenomenon, associated only to steady-state
variables, an easy way found to overcome noise problems was
to work with the “pseudo-harmonics” of the first order of arc
voltage and current. The expression “pseudo” translates the
fact that current and voltage are not exactly periodic functions
at ∞<t<∞. Fig. 4 presents an example of arc elongation, during
test of a 150A arc current (1 second), where arc elongation
reached five times the reference/initial arc length.
A.2. Static Arc Characteristic
Static arc characteristic is the stationary relation of arc
voltage and current (Fig.5), being an important tool for studies
in steady-state domain (arc stability analysis, in incremental
basis), or, as starting point for arc dynamic behavior analysis,
in transient basis.
The authors recognize the efforts of CEPEL’s laboratory
staff and their contribution to the success of a crucial part of
this project.
REFERENCE
[1]
[2]
[3]
[4]
[5]
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Portela, C. – “Some methods of behavior analysis of electric arcs in air.
Application examples” - VIII SNPTEE, São Paulo, 1986, art. SP/GSP/47,
18 p. , 1986 (in Portuguese);
Portela, C. et al – “Practical Applications of Arc Physics in Circuit
Breakers. Survey of Calculation Methods and Application Guide” CIGRÉ , ELECTRA n° 118, pp. 63 a 79, Maio de 1988;
Portela, C. ; Santiago, N. ; Oliveira, O. ; Dupont, C. – “Modelling of Arc
Extinction in Air Insulation” - IEEE Transactions on Electrical
Insulation, vol. 27, fasc. 3, pp. 457 a 463 , 1992 ;
Portela, C.; Dupont, C.; Meirelles, M. – “Deterministic and Statistic Arc
Modeling” - CIGRE Proceedings, art. 13-107, pp. 1-6, Agosto de 1994;
Câmara, A. – “Secondary Arc Extinction with Sigle-Phase Switching.
Fundamental Concepts and Analysis Criteria” – M. Sc. Thesis - 129 p. COPPE / UFRJ , Março de 2003 (in Portuguese).