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Elektroenergetyczne sieci rozdzielcze – SIECI 2004
V Konferencja Naukowo-Techniczna
Politechnika Wrocławska
Instytut Energoelektryki
Gerd BALZER
Bartosz RUSEK
Institute of Electrical Power Systems, D-64283 Darmstadt, Landgraf-Georg-Str. 4
e-mail: [email protected], [email protected]
OIL LEAKAGES OF THE HV CIRCUIT BREAKER HYDRAULIC DRIVE,
THEIR EFFECTS AND POSSIBILITIES OF THEIR DETECTION
Monitoring of the circuit breaker is a complex problem, which requires great knowledge about its
behaviour and the elements which can fail. In this paper the oil leakages as one of the most often failures
of the hydraulic drive are discussed. The possibilities of their detection and prevention are also presented.
The analysis and simulative investigations were performed using digital model of circuit breaker
developed in MATLAB / SIMULINK programming package. The simulated failures were specially
selected from the database in order to consider the real cases. It is shown that the leakages can be
detected with use of specially applied sensor technique.
1. INTRODUCTION
The newest generations of the high voltage circuit breakers (CB) has relatively long life cycle.
During this period CB should operate reliable and secure. To make that possible there should have
been carried out special maintenance procedures, which require financial expenses. The
liberalisation of the energy market requires minimization of the costs. For this reason the CB users
would like to expand the time between consecutive maintenance actions or/and fully eliminate them
to reduce the costs. Additionally, if the maintenance has to take place they would like to be
prepared for them. This means that the replacements can be earlier ordered and can wait for
maintenance term. Described procedures are not possible for carrying out without extend
knowledge about the breaker. As a help for the solution of these problems there have been
introduced variety of monitoring systems [1, 2]. However, they have limited possibilities or they
cannot fully analyse received data.
In this paper the innovative approach to the problem of the oil leakages is presented. The new
method for leakages detection and its location will be described and compared with old ones.
Additionally, extended use of energy storage behaviour for failure detection will be presented.
2. FAILURE RATE
The failure rate in SF6 und oil circuit breakers have been already discussed in [3, 4]. More
precise considerations of research group [4] show that failure rate of oil circuit breaker amounts
1.6 failure/100 years and breaker. For SF6 this value reaches 6.0 failure/100 years and breaker.
However, both sources locate about 40% percent of all failures in drive. Such results explain a great
interest in monitoring of the actual condition of drive. For this purpose, exacter look at failure
causes and faulted components has been made. The results of extensive examination of the
database, which is in possession of Institute of Electrical Power Systems of Darmstadt University of
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Technology, are presented on Figure 1. This database consists of over 2300 records about
irregularities in work of CB hydraulic drive. The first detailed search shows that the cause of almost
70% of failures was leakage. Almost 50% of the uptight components were unknown (Figure 1).
This means that it is easier to detect the leakage and change all the seals then to detect only uptight
seal. The 31% consists of the components with failure rate under 2%. The remainder uptight
components have been selected for simulations. Their location in drive has been marked on Figure
2. It is important to notice that all selected components are mobile. It means that the seals of moving
elements are more subjected for leakages.
Fig. 1. The failure causes of hydraulic drive and its uptight components.
3. DETECTION OF THE OIL LEAKAGES
The knowledge where the leakages occur allows the planning of the maintenance. It can be
said that in most cases the inner leakages (from high to low pressure container) are not critical
because the loss of oil can be rebuild by motor-pump system. The problem appears when there is no
more oil in storage container and the energy reserve can not be rebuild. Such situation can only
appear by extern leakages. Therefore, the constant control of the oil reserves has high importance.
Currently, the detection is performed with use of two methods:
• Counter of the motor start-ups – after every operation of the circuit breaker the motor
should be turned-on in order to restore the energy. In hydraulic drive with use of the
pump, the pressure of the oil is increased. However, if the circuit breaker was not
operating and the motor has started up, i.e. some leakages are present in a high-pressure
system. The time between consecutive motor start-up is an indicator about the leakages
magnitude. This method cannot directly detect the place of the leakage. Process of
counting the turns-on can be fulfilled remotely.
• Visual – this method allows locating only the extern leakages, i.e. the oil appears at the
housing and can be spotted by maintenance person. Such a visual inspection is carried out
not very often because an expert which knows the drive is needed.
The first idea is the continuous control of the energy storage position (ESP). Currently, only
some discrete points of ESP are used for turning-on the motor and for some protection purposes.
Continued monitoring allows to determinate the trend of leakages in relative short time. Variety of
sensors can be used for measuring [3]. The example sensor installation point (S4) is shown on
Figure 2.
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Fig. 2. Hydraulic drive of the high voltage circuit breaker in off
position (photo ABB)
The second of innovative
method is installation of three oil flow
sensors. Their location (S1, S2, S3) in
hydraulic drive is shown on Figure 2.
The basic work principles of different
producer of hydraulic drives are
similar. However, the mechanical
solutions and the energy storages
differ. Therefore, chosen sensor
location is adequate only for
considered drive. The combination of
signals received from sensors allows
allocation of the leakages (F1 – F5).
The sensors have to be very sensitive
for small oil flows.
4. VALIDATION AND TESTS
The previously selected failures were thoroughly tested in digital model (Figure 3) of
hydraulic drive (Figure 2). The main blocks of model can be subordinate to the real components of
the drive from Figure 2. The model has been created in MATLAB / SIMULINK programming
package. The good quality of movement characteristics can be achieved with use of “ode5 solver”
and 100kHz sampling frequency. However, to see exact oil flows the sampling frequency has to be
increased to 100MHz. Basically, model executes three types of the processes:
• Electrical – to calculate motor and coil current profile;
• Mechanical – to calculate positions, velocities, accelerations and forces;
• Hydraulic – to calculate the oil flows and oil pressures.
Fig. 3. Equivalent digital model of the circuit-breaker hydraulic drive.
One of the ways to model leakages is to define the gap in the seal. The amount of oil, which
will flow through the gap (oil current), is calculated from equation:
2 ⋅ ∆p
V& = αA
ρ
where:
α – coefficient,
A – area [m2],
(1)
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∆p – pressure difference (p1-p2) [Pa],
ρ – density [kg/m3].
The oil current depends on the pressure difference and the area of the gap. In considered
cases, the area was set to 1-nanometre circle which gives A=0,00628 [mm2]. In case of the valve
leakage, the gap can be set in already existed oil flow block. However, to simulate the leakages
from the pressure sensor joint, the additional oil flow block has to be added to the model. Figure 4
presents the energy storage position
characteristic with (the curve declines) and
without leakages. The curve gradient is
good indicator of the leakages magnitude.
Such behaviour can be observed for all
leakages aside from their occurrence
location. Knowing the gradient angle the
next turn-on of the motor can be defined and
therefore the time between consecutive
switching. That gives actual rate of the
leakages without need to wait for the next
turn-on of the motor. Therefore, this method
shows the actual state of the drive. In case of
motor start-ups counter observation, the
Fig. 4. Energy storage position with and without leakages
with assumption that circuit breaker is on
leakage rate has discrete values, which
change only after switching-on of the motor.
The interval between switching can grow to months. The ESP variable can be also used for leakages
detection in different circuit breaker status. For example, if during ON status the leakages are
observed and if after switching-off of the circuit breaker the leakages disappear or significantly
decrease then it means that the main valve on-seal is uptight. Such behaviour of ESP shows the
Figure 5.
Fig. 5. Energy storage position during simulations of the valve on-seal leakages
before and after switching of the circuit breaker.
After switching the ESP with leakages has lower value then without leakages. This depends
on the status of the ESP before switching. In case of leakages the switching will be performed from
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lower level. Assuming that during switching the outflow of oil is the same then after switching the
ESP has to lie below curve without leakage. However, what is more important the curve does not
fall and this means that system is tight.
The Figure 6 shows the characteristic of the coil current for above described case. The
leakages appear only before switching-off of the circuit breaker. Afterwards, the oil current falls to
zero which means that off-seal of main valve is tight. If opposite, situation exist in a system, i.e. the
leakages occurs only on off-seal of the main valve and the oil current after switching will be higher
then zero (Figure 7). The differences between the normal and faulty oil flows are well seen on the
figures 6 and 7.
Fig. 6. Oil current measured by the sensor located between
energy storage and main valve during the simulations of
the valve on-seal leakages compared with the undisturbed
characteristic.
Fig. 7. Oil current measured by the sensor located
between energy storage and main valve during the
simulations of the valve off-seal leakages compared
with the undisturbed characteristic.
The measurements of the oil flows should be performed only in steady state of circuit breaker
because during switching the oil flows reach much higher values and their flow direction changes.
The sensors should be able to detect very small oil currents.
The simulation results of leakages located in other places are presented in Table 1.
Table 1. Matrix with the rest of the simulation results.
main valve offseal uptight
S5
State of
Circuit-Breaker
F2
S4
Energy storage
position
main valve onseal uptight
S3
oil flow sensor
over piston –
pump
F1
S2
oil flow sensor
energy storage
– over piston
Failures
(leakages)
S1
oil flow sensor
energy storage
– main valve
Variables
S1>0
~
~
~
~
~
S1>0
~
~
~
~
~
falls
constant
falls
falls
constant
falls
on
on
off
off
on
off
on
off
on
off
S2>0
over piston
S3=0
falls
~
S2>0
seal uptight
under piston
S2=0
F4
~
S3=0
falls
seal uptight
S2>0
joint of the
F5
~
S2=S3
S2=S3
falls
pressure sensor
F6
security valve
~
S2=S3
S2=S3
falls
1) before and after switch the oil current has to have the same value
2) if oil current in ON-state of CB equals to S2=0 and after switching S2>0
F3
NOTES
look Fig.
5.
or F4
1)
2)
~
or F6
~
or F5
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It can be seen that in order to detect the leakages more then one variable is needed. For
example to detect leakages of the piston two oil currents and state of the circuit breaker is required.
In cases where the signals S1 - S3 are used the energy storage position is not required. The leakages
F5 and F6 are equiprobable and the sensor S4 placed at present location cannot differentiate those
two failures. Obviously, the leakages of the pressure sensor are well seen on the housing of the
drive. Therefore, if the conditions in Table 1 for F5 and F6 are fulfilled then the system should warn
CB user that seal F5 or F6 is uptight. User can visually investigate the drive. If the oil will not be
spotted on the housing then the security valve is uptight.
5. CONCLUSIONS
In this paper, the main failures of the hydraulic drives were pointed. The old and new
techniques of oil leakage detection were described. The following features of the new methods are
worth to be pointed out:
• Energy storage position determinate faster leakages and therefore shows actual status of the
drive;
• The new three oil flow sensor method allows location of the most often leakages of the
hydraulic drive.
The implementation of the proposed methods in monitoring systems should allow the
reduction of minimum 15% of the hydraulic drive failures. In the future, the simulation of different
failure arts with use of the digital model is planned to be done.
ACKNOWLEDGMENT
This project is sponsored by DFG (Deutsche Forschungs-Gemeinschaft).
LITERATURE
[1]
[2]
[3]
[4]
Gebhardt, D., Kirchesch, P., Schiemann, A.: “New Digital Control and Monitoring Devices for High
Voltage Circuit Breakers” Diagnostik elektrischer Betriebsmittel, ETG-Fachbericht, Berlin 2002,
s. 43-46.
Richter F.: “Verfahren zur Durchführung und Bewertung von Schaltgeräteprüfungen” Periodical ETZ,
book 15/2003.
Cigre - Working Group 13-09.: “User guide for the application of monitoring and diagnostic
techniques for switching equipment for rated voltages of 72.5kV and above”. CIGRE Report Nr. 167,
2000.
Balzer G., Drescher D., Heil F., Kirchesch P., Meister R., Neumann C.: “Evaluation of Failure Data of
HV Circuit-Breakers for Condition Based Maintenance”. CIGRE 2004, B3.