ON THE PROCESSING OF THE RECORDED DATA FOR THE SF6
CIRCUIT BREAKERS FROM THE TRANSFORMATION SUBSTATION
110/20/6kV CRAIOVA SOUTH
Maria Brojboiu
Virginia Ivanov
University of Craiova
Facultaty of Electrical Engineering
107 Decebal Blv., 200440, Craiova, Romania
mbrojboiu@elth.ucv.ro, vivanov@elth.ucv.ro
KEYWORDS
Circuit breakers, maintenance, SF6, ablation.
ABSTRACT
The assurance of the reliability for the transformation
substations is one of primary goals of the manufacturers
and distributors of electricity. Therefore, a proper
monitoring and maintenance program is required. The
circuit breaker is a complex device, subject to the
thermal and mechanical stresses during the normal or
fault currents switching. The substations are frequently
equipped with SF6 circuit breaker. The circuit breaker
components subjected to the thermal stresses are the
main contacts which suffer electrical erosion and the
nozzle which is subjected to the ablation process. The
ablation process appears because of the energy radiation
which is transferred from the electric arc. As a result of
the ablation, the nozzle geometry, the gas pressure and
the electric withstand are changed. Based on the
recorded data from the transformation substation
110/20/6kV Craiova South, the mass loss from nozzle,
the admissible number of disconnections and the throat
nozzle diameter are computed.
The graphical
representations highlight the impact of the interrupted
current, of the arcing time and of the integral I2t over
the mentioned ones. Consequently, if the switching
process and the time arc value are controlled, the
thermal wear can be limited and the equipments users
may provide a maintenance program with minimal costs
and an increasing of the lifetime.
INTRODUCTION
The circuit breaker is one of the most important and
complex equipment from the medium and high voltage
electric substations having the switching functions of the
electrical circuit in the normal or fault conditions.
Depending on the thermal and dynamic stability of the
electrical equipment from stations, the commutation
must be carried out in a prescribed period of time.
Consequently, the failure or decommissioning of circuit
breaker has undesirable effects on the operation of the
power station, thereby providing a program of
monitoring, diagnosis and maintenance of the circuit
Proceedings 28th European Conference on Modelling and
Simulation ©ECMS Flaminio Squazzoni, Fabio Baronio,
Claudia Archetti, Marco Castellani (Editors)
ISBN: 978-0-9564944-8-1 / ISBN: 978-0-9564944-9-8 (CD)
Andrei Savescu
S.C. RELOC S.A.
109 Decebal Blv., 200746, Craiova, Romania
andreisavescu@hotmail.com
breaker it is absolutely necessary. The program of
monitoring, diagnosis and maintenance has the purpose
to increase the lifetime of the equipment and to reduce
the operation and maintenance costs. From the
maintenance costs of the electrical stations, a 40% are
dedicated to the circuit breaker maintenance, meanwhile
a 60% are dedicated to the general revisions ( Milthon
S. et all, 2005). Therefore, the predictive maintenance
systems based on the continuous monitoring of the
circuit breaker lead to the significantly reducing of the
costs. The predictive maintenance system has the
advantage to be carried out during the operation of the
equipment. A large number of references in the field are
dedicated to the analysis of the functioning and
monitoring of the circuit breakers (Milthon S. et all,
2003), (Richard, T., 2004), (Thanapong, S. 2006).
The power stations from Craiova South are equipped
with oil circuit breaker or, becoming frequently after
upgrading, with SF6 circuit breakers. The use of this gas
has reduced the frame sizes and increase performance s
of switching. The medium voltage SF6 circuit breakers
are designed of the self blast principle. This type of
circuit breaker generates a gas flow by means of a piston
and cylinder attached to the moving contact. When the
circuit breaker is in close position the gas pressure from
the puffer cylinder is equal to the pressure of filling gas.
During the disconnecting operation, the SF6 gas is
compressed in the cavity between puffer cylinder and
the piston. The switching arc occurs between the
stationary contact and the moving contact and it is
develops inside of the blowing convergent- divergent
nozzle from PTFE with a lower thermal conductivity. A
successful current disconnecting depends on the
interaction between the switching arc, the radiated
energy from arc, the ablation of the nozzle material and
the pressure of the gas flow. During the period of the
current disconnection, in normal or fault regime, occurs
the thermal wear of the circuit breaker components
which are in contact with the switching arc (Richard, T.,
2004), (Bang, H, 2012), (Bogatyreva, N, 2013),(
Muratovic, M, 2013), (Weizong W.I., 2013). The
components which are directly exposed to the radiative
or conductive energy transferred from the switching arc
are the electrical contacts and the blowing nozzle. As a
result of the thermal wear, after one operation time or
after a cumulative number of disconnected currents,
these components must be replaced. In the reference
(Brojboiu, M. et all, 2013) the aspects of the contact
electro erosion are presented and mass loss from the
contacts because of the thermal erosion is computed. It
is well known that there is an admissible limit of the
thermal wear beyond which the circuit breaker operation
cannot be assured and this fact requires the replacement
of the used components. Having in view the thermal
wear of the circuit breaker, the on line monitoring of the
disconnections number, the arcing time and the
disconnected current values allow the estimation of the
circuit breaker condition and therefore a maintenance
plan can be set out.
Concerning the nozzle wear, during the operation,
because of the electric arc presence, the ablation
phenomenon of the nozzle material occurs. The electric
energy arc is mainly absorbed by SF6 gas. An important
part of this energy is absorbed by the contacts and
nozzle. The energy absorption produces heating, melting
and material vaporization, this being the main cause of
the thermal wear. Following the ablation phenomenon
occurs the increasing of the nozzle throat diameter and
consequently the changing of the gas flow. In the same
time, the mixing of the PTFE vapors (C2F4) with the
SF6 gas appears. The influence of the vapors over the
dielectric breakdown of the hot was analyzed in the
reference (Weizong W.I., 2013) The PTFE vapors
modify the properties of the quenching arc medium.
Therefore, the quenching of the electric arc depends on
the ablation intensity of the nozzle material.
Consequently, the nozzle ablation has a significant
influence over the composition of the gas or residual
plasma between the main contacts, after the arc
quenching. The withstand voltage of the gas - C2F4
vapors mixture is reduced in comparison with the one of
the cold gas. The values of the critical electric field for
various percentages of the PTFE vapors mixed with gas
at a gas pressure of 0.40MPa were experimentally
determined. Due to this fact, the re ignition of the
electric arc can occurs and therefore a disconnection
failure can happens. At the same time, the controlled
ablation of the nozzle can produce the gas overpressure
associated with the movement of a mechanical piston
(Cae-Yoon B., et all, 2006). The severity of the thermal
wear or the nozzle ablation depend on the amplitude of
the disconnected current value, the electric arcing time,
the integral I2t, the constructive solution of the circuit
breaker and the recovery voltage amplitude arc
(Richard, T., 2004). There are a large number of
parameters that should be monitored to evaluate the
circuit breaker condition during the operation time.
Concerning the main contacts of the circuit breaker, the
monitoring assumes the evaluation of the thermal
erosion, electric arc duration, cumulative disconnected
currents and the contact resistance. The monitoring of
the nozzle implies the supervision of the inner diameter
of the nozzle throat, the increasing pressure in the puffer
chamber and the critical electric field value.
THE PROCESSING OF THE RECORDED DATA
The medium voltage SF6 circuit breaker, whose data
were recorded (Savescu A. 2013), is Fluarc FG3 type, is
installed in the transformer substation 110/20/6kV
Craiova South - Ghercesti. The main rated values of this
circuit breaker are: rated voltage 24kV, rated current
1250A, rated breaking capacity 25kA.
The data were recorded using the protection system
F650 General Electric and can be used for all equipment
from transformation substations (oil or SF6 circuit
breakers). Such protection system is able to store the last
20 events of the protected equipment. For circuit
breakers, a recording contains the collected data in a
range of time between one to two seconds. On this range
of time, the currents, the voltages and the operating state
of the circuit breaker (connected/disconnected/RAR) are
graphically represented on every moment of time. The
recorded data can be visualized by means the SIGRA4
program which is a part of the DIGSI software.
The recording of variation in time of the interrupted
current is shown in Figure 1. The current values are
acquired from the secondary of a current transformer
with transformation ratio 200/5. The arcing time is
measured from the moment of time when the current
falls below the value of 1A, at this value of arcing time,
the post-arc currents occur between the main contacts of
the circuit breaker. In the recording from figure 1, the
time duration of the arc is of 24,3ms.
Figure 1: The Recorded Interrupted Currents vs. Time –
L1,L2, L3 Phases of the SF6 Circuit Breaker
In the Figure 2 the recording of the variation in time of
the voltages on the three L1,L2, L3 phases is shown.
The recorded voltages are corresponding to the recorded
interrupted currents. In this case, the transformation
ratio of the voltage instrument transformer is of 1000/5.
That means the voltage value in the moment of
occurrence of the current interruption is about 22kV.
The recorded interrupted currents values on three phases
of the circuit breaker and the arcing time values are
shown in Table 1.
South has been processed in order to observe the
dependencies between the mass loss from PTFE nozzle
at the fault currents interruption and the electric arc
energy, the integrals Gidt and Gi2dt. The computation of
the electric arc energy was performed with formula:
Wa = U a I t a
Table 1: Recorded Values: Interrupted Currents, Arcing
Time / Computed Values: Mass Loss, Integral I2t
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
IL1
[A]
4200
4320
5000
1316
2228
896
832
592
1076
1088
728
2192
2160
2016
IL2
[A]
4560
4400
4480
6.8
2216
904
852
7.2
5.6
7.2
7.2
6.8
19.6
9.68
IL3
[A]
4360
4480
4320
1332
14
6.8
25.6
4.8
9.2
5.6
5.2
2200
2164
2020
ta
[ms]
27.3
27.3
27.4
22
32.8
25.7
49.2
27.3
31.3
30.3
26.3
33.3
27.5
27.3
m L1
[mg]
500.0933
514.3816
597.5299
178.5367
318.7338
146.8912
100.4339
70.4893
143.7840
146.8912
83.5076
318.3639
259.0750
240.0448
I2t 105
[A2s]
5.0943
5.3895
7.2471
0.3548
1.5506
0.2118
0.3459
0.1012
0.3490
0.3551
0.1455
1.5330
1.3573
1.1737
The recorded data using the SIGRA 4 program must be
combined and processed in order to make conclusions
on the operation system and the circuit breaker state.
That could help to increase the reliability of the
electrical station. Because the circuit breakers must
interrupt the fault currents throughout lifetime, their
aging is due to the thermal and mechanical stresses of
the blowing nozzle and the main contacts between the
electric arc occurs. The thermal wear or cumulative
electroerosion of the main contacts is used as criterion to
evaluate the electrical endurance of the circuit breaker.
Additionally, for the SF6 circuit breakers is very
important to assess the cumulative wear of the blowing
nozzle. Accordingly to (Thanapong, S. 2006), the total
admissible electroerosion value of the main contacts and
of the nozzle is depending on two parameters: the
maximum breaking current Iscmax and the admissible
number of disconnections of the fault currents Nadm. In
the works (Brojboiu, M. et all, 2013), (Savescu,
A.,2013) the computation of the mass loss from the main
contacts depending on the recorded data in the oil circuit
breaker from medium voltage substation Craiova is
presented. The recorded data on the SF6 circuit breaker
from the transformation substation 110/20/6kV Craiova
where, the interrupted currents values and the arcing
time values are presented in Table 1. The arc voltage
drop Ua has been computed based on the described
algorithm in (Hortopan, Gh., 1980). Knowing the
maximum value of the recovery voltage as a function of
maximum phase voltage, the amplitude factor ka, first
phase factor kf, the oscillation frequency f, the following
formula was deduced:
T/8 )
U a = 0.707 u r max e (
(2)
u r max = U n k a k f
(3)
where:
2/3
is the maximum value of the recovery voltage, rated
voltage Un=24kV, T=1/f, f is the oscillation frequency
of the recovery voltage, is the time constant. In the
work (Rong, M., 2005) it is estimated that 40% from
electric arc power is used for the nozzle ablation, the
ablation rate being around of kab=15…17mg/kJ. The
mass loss mab as a function of the interrupted current
value or the arc energy has been computed with formula:
m ab = k ab 0.4 Wa
(4)
The variation of mass loss through ablation from PTFE
nozzle depending on the interrupted currents values
from three phases is shown in Figure 3.
0.7
0.6
0.5
Mass loss [g]
Figure 2: The Recorded Voltages vs. Time – L1,L2, L3
Phases of the SF6 Circuit Breaker
(1)
0.4
0.3
0.2
0.1
0
0
1000
2000
3000
4000
Interrupted current [A]
5000
6000
Figure 3: The Mass Loss vs. Interrupted CurrentL1,L2, L3 Phases
The processing of the experimental results was
performed using a Matlab application. By applying a
least squares approximation method to the recorded
data, using a Matlab application, it was possible to plot a
continuous curve (solid line) that approximates the
variation of the mass loss from nozzle depending on the
interrupted current. This Matlab application has been
applied to all the recorded and computed data which
were processed in this work. Concerning the
computation of the integral I2t, one integration Matlab
procedure has been applied. A sinusoidal variation of
interrupted current was taken into account.
2006) allow the establishing of the one empirical
formula in order to estimate the limit mass loss
depending on the admissible number of disconnections.
M lim = 85.86 + 205.94 N
The admissible number of disconnections (the allowable
number of disconnecting operations) is computed as a
function of the ratio between the interrupted current and
the maximum breaking current of the circuit k=I/Iscmax,
Iscmax=25kA, using the following formula (Thanapong,
S. 2006):
ta
I2 t = ( 2 I sin( t )) 2 dt
(7)
Nadm = 4.4 k
(5)
1.03245
(8)
0
100
In the Figure 4 the variation of the loss mass depending
on the integral I2t is presented.
Inner Diameters Ratio [%]
0.7
0.6
0.5
Mass loss [g]
99
0.4
98
97
96
95
0.3
94
0
0.1
0.2
0.2
0.3
0.4
Mass loss [g]
0.5
0.6
0.7
Figure 5: Inner Radius Ratio [%] vs. Mass Loss [g]
0.1
0
0
1
2
3
4
5
Integral I2t [A2s]
6
7
In the Figure 6, the variation of the admissible number
of disconnections depending on the ratio k, for the
recorded current values of the L1 phase, is shown.
8
5
x 10
Figure 4: The Mass Loss vs. integral I2t
220
ra =
ri2
+ (m L1
/ / h)
(6)
where ri and h are the values of the internal radius and
the height of the throat nozzle respectively, measured
for the circuit breaker under measurements. For the
PTFE as the material of the nozzle, the density value is
taken as =2200kg/m3.
In the Figure 5 is graphically represented the variation
of the ratio of the inner diameter after ablation to the
initial inner diameter, depending on the amount of the
mass lost from the throat nozzle.
The large values of the ratio for reduced values of the
mass loss can be observed. The experimental
determinations carried out in reference (Thanapong, S.
200
Admissible disconnections number
From this figure, the increasing of the mass loss
according to the increasing of the integral I2t values can
be noticed, as it is expected. Using the values of nozzle
mass loss and for a known geometry of the nozzle, the
inner radius of the nozzle throat after ablation ra was
calculated using the following formula:
180
160
140
120
100
80
60
40
20
0.02
0.04
0.06
0.08
0.1
0.12 0.14
Ratio k=I/Iscmax
0.16
0.18
0.2
0.22
Figure 6: The Admissible Number of Disconnections vs.
k=I/Iscmax
From the graphical representation, the decreasing of the
admissible number of disconnections as the ratio k
increases can be noticed.
CONCLUSIONS
The lifetime and the SF6 circuit breaker performances
depend on the ablation intensity and the changes in the
nozzle throat geometry, which are produced by the
radiative and conductive energy of the electric arc. If the
arcing time duration can be controlled and restricted in
order to avoid the large values, the thermal wear of the
contacts or the nozzle ablation can be limited.
Consequently, the interval between maintenance
activities can be extended.
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AUTHOR BIOGRAPHIES
MARIA BROJBOIU is currently
working as Professor at the
University of Craiova, Electrical
Engineering Faculty, Department
of Electrical Energetic and
Aerospace Engineering. Before
that, she worked as design
engineer at the Electroputere holding the Research and
Development Center. She is Doctor in Science
Technique – Electrical Engineering. She teaches the
courses Electrical Equipment, Electrotechnologies and
Industrial Systems Engineering. She published 5 books
and 92 scientifical papers for different national and
international conferences and symposiums.
VIRGINIA IVANOV was born in
Vela, Dolj, Romania, 1963. She
was graduated in Electrical
Engineering at University of
Craiova, Romania, in 1986 and
Doctor in Electrical Engineering in
2004. From 1986 to 1998 she
worked as researcher with the Researching Institute for
Motors, Transformers and Electric Equipment Craiova.
In 1998 she joined the Faculty for Electrical
Engineering, Department of Electrical Equipment and
technologies. She is concerned with research activities
in monitoring and modeling of electrical equipments.
ANDREI SAVESCU. He graduated
bachelor studies in 2013 at the
Faculty of Electrical Engineering at
the University of Craiova and
currently he is attending the Master's
program "Energy quality and
electromagnetic compatibility in
electric systems". Since July 2013 he
works as design engineer at RELOC
SA company from Craiova, which has as business line
the maintenance and repair of locomotive as well as
designing and construction activities concerning the new
locomotive models.