Hydro-Québec
TransÉnergie explores alternative diagnostic methods in its
high-voltage circuit breaker maintenance program
By Olivier Turcotte and Robert Gauthier, Hydro-Québec
TransÉnergie
The philosophy behind maintaining circuit breakers focuses on
preventive maintenance to maximize the longevity of the
electrical apparatus. The transmission division of Hydro-Québec
TransÉnergie (Québec, Canada) divides activities in this field
into three categories: corrective, conditional and systematic.
Moreover, maintenance is an integrated part of a continuous
improvement process of practices, methods and tools done in
collaboration with the manufacturers of circuit breakers and
test instruments.
Maintenance descriptions
Corrective and conditional maintenance are related to tasks
performed to repair an apparatus following a fault or a defect.
In the first case, the primary function of the apparatus cannot
be maintained; while in the other case, it is still possible to
operate the equipment. Whether it is corrective or conditional
maintenance, the impact of these interventions is significant as
they concern the safety of personnel and the operation of the
power network.
Systematic maintenance is based on a specific predefined time
frame or number of breaker operations, depending on the first
occurrence. The content and frequency of these inspections are
continuously adapted to the technical aspects, such as the
reliability of the technology and the position of the equipment
in the substation (lines, power plants, capacitor banks and
inductors), as well as problems previously experienced with the
same type of equipment.
The preventive inspection of high-voltage circuit breakers
includes traditional tests such as alternating-current
insulation, static contact-resistance measurement and timing
tests with contact travel curves. In addition, one of the
interrupting chambers must be inspected, based on the results of
the previously mentioned tests. Because of the specific
procedures regarding handling and recovery of SF6 gas, these
interventions a thorough examination can become quite complex
and demanding. Thus, the idea is to limit the number of
intrusive interventions and, in turn, minimize their cost.
Dynamic
contact-resistance Measurement
The original approach for dynamic contact resistance was to
measure the resistance during a slow open operation of the
breaker. Tests were done at low speed, so the partial contact
separation was not present, making it easy to measure the
resistance with a direct current of 100 A. However, during a
field test, a minor incident occurred on the spring operating
mechanism due to the slow operation of the breaker. Moreover,
extremely high resistance values were measured due to the
accretion of SF6 byproducts on the contacts. Although this
method was interesting because of the simplicity of its
implementation, the measurement at 100 A could not be
universally applied to all types of breakers.
A complementary study was launched to validate the measurement
at nominal speed and high current. On one hand, tests on the
electrical network confirmed that a continuous current source of
2800 A could burn the SF6 powders, verifying the actual
condition of the breaker contacts. On the other hand, validation
tests at nominal opening speed were performed on various types
of SF6 breakers. A direct current of 700 A was determined to be
sufficient to avoid the partial contact separation and produce a
clean dynamic-resistance curve. The dynamic-resistance
measurement allows a precise evaluation of the wear of both the
arcing and main contacts without having to open the breaker. The
static-resistance measurement gives information on the status of
the permanent contacts, but the wear mainly occurs on the arcing
contacts, which are subjected to the heat and energy produced by
arcing during each breaker operation. This measurement is
applicable to high-voltage circuit breakers (69 kV to 735 kV)
insulated with SF6 and fitted with two sets of parallel contacts
(main and arcing). For dead tank breakers, this method is only
applicable if necessary precautions are taken to avoid the
magnetization of the current transformers. Figure 1 shows a
typical dynamic contact-resistance curve.
Fig. 1. A typical dynamic-resistance curve
Fig. 2. An abnormal dynamic-resistance curve
Arcing contact Misalignment
A series of tests allowed Hydro-Québec TransÉnergie to establish
threshold levels that are easily interpretable by the users of
the equipment. For example, the utility was able to detect an
anomaly on a 120-kV circuit breaker on a capacitor bank. The
results of the dynamic contact-resistance measurement are
presented in Fig. 2. An internal inspection of this breaker
confirmed the diagnosis determined during the test. In fact, one
of the arcing contact fingers on the moving side was misaligned,
while the fixed side clearly showed abnormal wear due to
excessive arcing (Fig. 3).
Fig. 3. The arcing contact on the moving part is misaligned
(a)
and the fixed side is worn (b)
Tungsten tip of arcing contact unscrewed
In another case, a 120-kV circuit breaker on a capacitor bank
cumulated more than 4000 operations. The measurement indicated a
partial separation of the arcing contact prior to the other
phase (Fig. 4). An internal inspection showed that the tungsten
arcing tip had begun to unscrew as illustrated in Fig. 5. In the
two previous cases, an internal diagnosis of the interrupting
chambers allowed a necessary and well-timed intervention, which
avoided an eventual breaker failure.
Fig. 4. Partial separation of Phase C’s arcing contact
Fig. 5. Unscrewing of tungsten tip
Vibration analysis
According to a recent CIGRÉ survey on high-voltage circuit
breakers, 44% of major failures and 39% of minor failures are of
mechanical origin. These figures reflect the actual situation
prevailing at Hydro-Québec TransÉnergie. In fact, new market
regulations result in additional operations on specific
applications of circuit breakers (power plants, capacitor banks
and shunt inductors).
The improvement of mechanical reliability can be achieved by two
separate means. For the products in the certification process,
mechanical endurance requirements can be upgraded. As for the
existing equipment, the strategy relies on having tools with
better detection capabilities to find problems before major
defects can occur.
The initial strategy consisted of analyzing the market to know
what was available. In this case, the products offered did not
fulfill Hydro-Québec TransÉnergie’s needs. In fact, the sampling
frequency, resolution and vibration analysis software were
inadequate for diagnosing the mechanical condition of the
various types of SF6 circuit breakers. For these reasons, IREQ,
Hydro-Québec’s research center, was given the man- Hydro-Québec’
Québec’s research center, was given the mandate to develop a
measuring device prototype. The method and algorithms were
validated through real cases simulated in a laboratory as well
as field-testing. The latter was often followed by internal
inspection in order to validate the diagnosis. Once the
efficiency of the device was proven, Hydro-Québec TransÉnergie’s
industrial partner Zensol Automation Inc. (Québec) developed an
industrial version of the vibration analyzer. Once again, the
validation took place through field-testing.
Vibration analysis allows the detection of mechanical anomalies
on the mechanism or interrupting chamber of high-voltage SF6
circuit breakers. The measuring system includes accelerometers,
conditioning modules and a data-recording system, including the
corresponding Zensol software. In addition to vibration signals,
the system can record any analog signal relevant to the analysis
of the circuit breaker’s condition, such as contact displacement
and position. Two parameters are used to assess the breaker’s
condition: amplitude deviation measured in decibels and time
deviation measured in milliseconds. Universal threshold values
were determined based on research works of other authors in that
field and were validated through lab and field-testing.
Fig. 6. Vibration signals measured detecting a deformed gear in
a spring-type mechanism
Abnormal end
of travel impact
In the following case, accelerometers were positioned at the top
end of each interrupting chamber and inside the operating
mechanism (Fig. 7) of a gang-operated, staggered-pole, 120-kV
SF6 circuit breaker cumulating more than 4000 operations.
The comparison between data recorded two years earlier led
Hydro-Québec TransÉnergie to identify the presence of an
abnormal end of travel impact on phase C. The internal
inspection demonstrated traces of an impact on the crank located
at the bottom end of the support insulator (Figs. 7 and 8). Even
if the accelerometers were located far from the defect,
detection was possible. The circuit breaker was then readjusted
to avoid further degradation of the mechanical linkage.
Fig. 7. Location of accelerometers and impact traces
Fig. 8. Traces of impact in the bottom-end crank
Maintenance
tool development strategy
The need for tools to give the actual condition of the
interrupting chamber components emerged with the arrival of SF6
breakers whose internal inspections are complex because of gas
handling and recovery. The search for potential solutions must
go through an analysis of the products already offered on the
market. When the available products do not fit the utility’s
needs, the development of new diagnosis tools becomes necessary.
After the development phase, Hydro-Québec TransÉnergie will
determine the need to be associated with an industrial partner.
The partner is in charge of the evolution of the instrumentation
by taking into account the needs of and feedback from different
users, thus enhancing the reliability and the precision of the
diagnosis. This also facilitates the smooth integration of
adding the new methods to the traditional maintenance plan.
Traditional maintenance tests remain helpful when the aging of
circuit breakers is progressive. However, in cases aging of
circuit breakers is progressive. However, in cases where
anomalies appear suddenly on a family of breakers, new where
anomalies appear suddenly on a family of breakers, new diagnosis
methods such as the dynamic resistance and the vi diagnosis
methods such as the dynamic resistance and the vibration
analysis can definitely help to precisely assess the condition
of the apparatus and to better determine the priority of the
apparatus and to better determine the priority level of a given
intervention. In all, it is by clearly defining its needs and by
developing tools that fulfill them that Hydro-Québec
TransÉnergie can remain at the edge of technology while assuring
the longevity of its electrical equipment.
Olivier Turcotte
received his BEEE degree from Sherbrooke University in 2003.
Since 2004, he has been working for Hydro-Québec’s transmission
division, Hydro-Québec TransÉnergie, in the Electrical Apparatus
group, where he is involved in the certification and maintenance
of circuit breakers and surge certification and maintenance of
circuit breakers and surge arresters and in charge of R&D
projects on new diagnosis arresters and in charge of R&D
projects on new diagnosis methods for SF6 circuit breakers. He
is a Canadian member of IEC Technical Committee 37 (surge
arresters) and a registered professional engineer in Québec.
Turcotte.Olivier@hydro.qc.ca
Robert Gauthier
received his BEEE degree from Ecole received his BEEE degree
from Ecole Polytechnique in 1980. He started his career at
Hydro-Québec in 1981 and has worked in the Commercial department
of Hydro-Québec’s transmission division, Hydro-Québec
TransÉnergie, since 1999.
Gauthier.Robert@hydro.qc.ca
