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1 THE
DISPLACEMENT CURVE
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1.1 Description
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During
circuit breaker timing tests, the measurement of the
operation times from the first appearance of the order
on the command coil to the switching of the main contact,
is recorded with a timing instrument, such as the
ZENSOL CBA32-P.
This measurement
offers precious information on the state of the circuit
breaker and allows, in most cases, the precise verification
of the presence or absence of anomalies. But this
information does not reveal all the secrets of the
circuit breaker. Other important information remains
hidden from view.
Whenever
it is possible, a point-by-point measurement of the
movement of the internal components of the circuit
breaker, from the beginning of the movement until
it comes to a complete stop, traces a curve called
a DISPLACEMENT CURVE.
While
the main contact timing curve gives the moment when
the movement starts and the moment when the contact
switches over, the information contained in the displacement
curve is interesting because it allows us to follow
the entire movement from beginning to end.
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| 1.2 Operation on opening
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An example of a displacement curve for an Open operation
is shown in Fig. 1.2, superimposed on an opening timing
curve for the main contact (in red).
Fig 1.2 Displacement curve on Open operation
Even of the general shape of the curve is the first characteristic
to check, three Zones (circled in Fig. 1.2) merit
particular attention.
1. Zone A : the beginning of the movement.
2. Zone B : contact separation.
3. Zone C : from the beginning of the deceleration to the
final resting position.
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1.2.1 Zone A : the beginning of the movement
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This is where the movement starts. It is extremely important
to know if the movement has begun at the right moment.
For example, a delay with respect to the reference
specification means an electrical problem exists,
if the coil is not excited on time, or a mechanical
problem may exist between the command mechanism where
the movement is triggered and the mobile contact of
the breaker itself. |
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1.2.2 Zone B : contact separation
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This is where the main contacts separate from each other.
A this exact instant, the arc begins to form and the
breaker implements its measures to extinguish it.
The separation speed becomes an important and primordial
factor in order to succeed in breaking the circuit.
The method for calculating the average velocity in this zone
depends on the breaker designer. Only the designer
may determine the calculation method and establish
the reference specification. |
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1.2.3 Zone C : deceleration
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This is where the movement decelerates until the circuit
breaker's mobile contact comes to a complete stop.
The amount of energy required in the breaking process is
proportional to the intensity of the current being
interrupted. Once the current has been interrupted
and the arc has been extinguished, the energy developed
is quite excessive.
Effective means of damping are put into action to absorb
this excess energy and thus reduce the risk of damaging
the internal components of the circuit breaker. The
analysis of this zone makes it possible to determine
if the damping is optimal, which means the movement
is stopped gradually.
Insufficient damping, or underdamping, allows the moving
parts to undergo shocks at the end of the travel,
which causes severe damage.
A sudden damping, where the kinetic energy developed by the
moving parts of the breaker is absorbed over very
little time, causes damage similar to underdamping.
This phenomenon is called overdamping. |
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1.3 Operation on closing
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An example of a displacement curve for a Close operation
is shown in Fig. 1.3, superimposed on a timing curve
for the main contact (in red).
Fig 1.3 Displacement curve for Close operation
Even of the general shape of the curve is the first characteristic
to check, three Zones (circled in Fig. 1.3 merit particular
attention.
Zone A : the beginning of the movement.
Zone B : contact closing
Zone C : from the beginning of the deceleration to the final
resting position. |
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1.3.1 Zone A : the beginning of the movement
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As is the case for the opening displacement curve, this is
where the movement starts, and it is very important
to know if the movement started at the right place. |
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1.3.2 Zone B : contact closing
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This is where the main contacts come into contact. In this
zone, also called the pre-arc zone, as the contacts
come closer to each other, the dielectric, as a function
of the separation distance, becomes insufficient and
a pre-arc current forms within an arc, the duration
of which is a function of the speed of the contacts.
Thus, contact velocity is an important factor in limiting
premature wear of the contacts. |
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As is the case for the opening operation, the method for
calculating the average speed in this zone is also
determined by the designer of the breaker. Only the
designer may determine this calculation method and
establish the reference specification. |
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1.3.3 Zone C : deceleration
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This is where the movement slows down to a complete stop
of the breaker's mobile contacts.
The energy involved in the closing process is less than that
developed in the breaking process, but it is nonetheless
quite considerable.
Excess energy is translated into an overtravel which, if
it exceeds tolerances, may cause severe damage to
the device. |
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1.4 Velocity curve
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A velocity curve is calculated by the derivative of the displacement
curve, using the CBA Win analysis software, for example.
The velocity curves gives the speed as a function
of time, which gains new information on the dynamic
behavior of the circuit breaker. |
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1.5 Acceleration curve
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In the same manner, an acceleration curve can be traced,
as the derivative of the velocity curve, again using
the CBA Win analysis software, which gives us more
useful data. |
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2 performing the measurement
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To record the displacement curve, some kind of assembly is
needed between the ZENSOL CBA-32P and the breaker.
A displacement transducer is used to sense the movement
of the mobile contact. The CBA Win software processes
the data, traces the displacement curve and performs
the various velocity calculations. To understand the
process, a summary description of the transducer,
of the breaker and the CBA-32P follows. |
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2.1 Displacement transducer
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2.1.1 Description
The displacement transducer, in its most elementary
form, consists of a fixed part and a mobile part.
The mobile part is attached to the mobile contact
of the breaker under test, and moves with the contacts,
while the fixed part acts as a reference.
Several types of transducers are available on the market.
Their differences reside in the method used to identify
the relative value against the reference value. Some
examples of transducers are :
· Magnetic transducer
· Optical transducer
· Resistive transducer
· Etc.
The resistive transducer, which is the most popular, will
be described in detail. This type of transducer is
composed of a resistor and a cursor that moves along
the length of this resistor. There are two types of
resistive transducers :
· Linear displacement transducer, or linear transducer (fig.
2.11a)
· Rotary displacement transducer, or rotary transducer (fig.2.11b)
The difference lies in the physical layout of the reference
resistor and in the way the cursor moves.
Fig
2.11a Linear displacement transducer
For the linear transducer, the displacement of the cursor
is linear, in a straight line, whereas for the rotary
transducer, the displacement is a rotation around
an axis.
Fig
2.11b - Rotary displacement transducer |
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2.1.2 Operation
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The transducer is solidly attached to the breaker
support, while the mobile cursor is solidly attached
to the control arm of the mobile contact.
A known, fixed voltage source (E) is connected to terminals
(1) and (3). When the breaker is in the CLOSED position,
the voltage read between terminals (2) and (3) is
(V1). As the mobile contact moves toward the OPEN
position, the measured voltage (Vt) between terminals
(2) and (3) decreased as a function of time, down
to value (V2), which is less than (V1), when the breaker
has completely stopped.
Fig. 2.12 Operation of a displacement transducer |
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2.2 Circuit Breaker
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2.2.1 Description
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A high-voltage circuit breaker consists of two main
parts :
1. Active section (electrical power)
2. Command section
The active section must make or break the power current in
the high-voltage circuit where the breaker is installed.
The command section must develop the necessary energy
to perform these operations.
The link between the command and active sections is usually
an insulating rod, shown in red in the schematic in
Fig. 2.21.
Fig. 2.21 Circuit breaker principle |
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2.2.2 Electrical power section
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This part is generally constituted of three phases of equal
size. Each phase in turn is composed of a fixed contact
assembly and a mobile contact assembly. When the two
assemblies are in contact, it is said that the breaker
is CLOSED, so current flows through the power circuit.
To interrupt the flow of current in the power circuit, the
mobile contact assembly is mechanically moved away
from the fixed contact assembly and stopped at a distance
that is sufficient to ensure electrical isolation.
Fig
2.22 - Power section of the circuit breaker |
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2.2.3 Command section
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This section has the task of creating the energy required
to perform the mechanical operation of the breaker,
for both Open and Close operations.
Three types of commands are often used in high-voltage circuit
breakers :
- Pneumatic
- Hydraulic
- Mechanical spring |
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2.2.3.1 Pneumatic command
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Usually found on compressed air circuit breakers.
This type uses compressed air as a dielectric medium
and uses piston-type mobile contacts.
A series of valves activated in a precise sequence allows
the application of air pressure on one side of the
piston, which causes the latter to move through the
action of the pressure difference between the opposing
sides of the piston. The movement of the mobile contacts
is generally not accessible in this type of circuit
breaker, making the use of conventional transducers
almost impossible. |
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2.2.3.2 Hydraulic command
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This type of command has an energy reservoir, or accumulator,
either compressed nitrogen or springs compressed by
hydraulic fluid and a pump. The moving contact of
the circuit breaker is attached to the piston by an
insulating rod.
A set of hydraulic valves allows the application of the previously
accumulated pressure to one side or the other of the
hydraulic ram, which moves it and the contact in the
desired direction.
Fig
2.2.32 - Hydraulic command |
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2.2.3.3 Spring-loaded mechanical command
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This type is most in demand because of its proven
reliability and also because of its very low frequency
of periodic maintenance.
It usually consists of two compressed springs, one used to
accumulate the energy necessary to close the breaker
and the other to store energy to open the breaker.
The closing spring (E) is compressed manually, using a crank,
or electrically, using a motor. A locking mechanism
holds and controls the energy accumulated in the closing
spring.
When this energy is released, by the release of the closing
spring, the mobile contact is moved toward the fixed
contact, through the connecting rods, while simultaneously
charging the opening spring which maintains the energy
accumulated with its own locking mechanism, ready
to be released on the next opening order.
Fig
2.2.33 - Mechanical spring command |
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2.3 Choice of transducers
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The goal is to reproduce the exact movement of the moving
contact in the breaking chamber of the circuit breaker,
which is usually inaccessible because the entire assembly
is inside a closed container. While the final displacement
of this mobile contact is always linear, the initial
movement generated by the command mechanism is not
always so.
The movement generated by the command mechanism may be rotary,
which is translated into a linear motion by a set
of levers and connecting rods.
When the linear movement of the contact is accessible indirectly,
for example through the connecting rod, a linear transducer
may accurately reproduce this movement by attaching
it to the rod in question. In figure 2.3a, you will
find an example of a linear motion sensed by a linear
transducer (in the case of an accessible motion).
Linear
transducer with breaker in open position
Linear
transducer with breaker in closed position
Fig 2.3a Linear transducer examples
In the case where only the command motion is accessible,
and if this motion is rotary, the final motion of
the mobile contact is measured by transforming this
rotary motion using various levers and connecting
rods of different sizes.
The sizes of the levers add multiplying factors that influence
the desired curve, whereas the angular motion of these
levers is always the same. In this case, using rotary
transducers will give better results.
One condition must be observed, however. A translation curve
must be drawn, point-by-point, to convert the angular
motion into a linear one.
An example of the use of a rotary transducer is shown in
figure 2.3b.
Fig. 2.3b Example using rotary transducers |
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2.4 Acquisition and display of results
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Fig. 2.4a Zensol CBA-32P instrument
The acquisition and the display of the results is
performed by the analog input channels of ZENSOL's
CBA32P (see Fig. 2.4a)
Each analog channel has three leads numbered (1), (2), and
(3) on the corresponding plug.
Between leads (1) and (3), a 10 volt DC signal is generated
during the test.. The signal collected from the displacement
transducer is measured between (2) and (3), and transmitted
to the CBA Win analysis software, which draws the
curve on the computer screen. Fig. 2.4b shows an example
of CBA Win's graphic report, as it is shown on the
computer screen.
Fig 2.4b CBA Win graphic report as seen on the computer screen |
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2.5 Displacement curve
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The displacement curve is generally drawn with the unit of
measurement (millimeters - mm) on the vertical axis
and the time in milliseconds on the horizontal axis.
Figure 2.5 shows examples of displacement curves for
Close and Open operations.
Fig. 2.5 Examples of displacement curves for Close and Open
operations |
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2.5.1 Velocity
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The speed of the contacts on opening, as on closing,
is an important parameter in the operation of high-voltage
circuit breakers. The displacement curve also serves
to the calculate the speed of the contacts at a specific
moment, or instantaneous velocity, or it may serve to
calculate the average speed for a predetermined time
interval, or average velocity |
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2.5.2 Instantaneous velocity curves
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The CBA Win software draws a curve of the instantaneous velocities,
calculated by deriving the data in the displacement
curve. Below is an example of the derivation of a
displacement signal (in green) that produced a curve
showing the evolution of the displacement velocity
(in orange).
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2.5.3 Average velocity
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The velocity usually sought is at the moment the breaker
contacts make (on Close) or break (on Open). However,
since it is difficult to obtain a consistent speed
for each operation, it is better to calculate an average
speed over a time interval extending before and after
this point.
The exact calculation method must be obtained from the circuit
breaker's manufacturer, so the measured values may
be compared to the reference specification of the
manufacturer. |
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2.5.4 Average velocity calculation example for OPEN operation
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To calculate the average speed on Opening, point A
on the displacement curve, which is the exact moment
the main contacts separate, must be determined. Point
B is determined by adding dT milliseconds (ms) to
the time of Point A.
Displacement axis value of point A = YA mm
Time axis value of point A = XA ms
Displacement axis value of point B = YB mm
Time axis value of point B = XB ms
The average velocity on Opening, in meters per second (m/s),
is calculated using the following formula :
Vo(m/s) = (YA-YB) (mm)/ (XB-XA)(ms)
In this case XB-XA = dT = 100 ms
YA-YB = dY = 223.71 mm, so :
Vo = 223.71 / 100 = 2.24 m/s
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2.5.5 Average velocity calculation example for CLOSE operation
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To
calculate the average velocity for a Close operation,
point A on the displacement curve, which is the exact
moment the main contacts make contact, must be determined.
Point B is found by subtracting dT milliseconds (ms)
from the time of point A.
Displacement axis value of Point A = YA mm
Time axis value of Point A = XA ms
Displacement axis value of Point B = YB mm
Time axis value of Point B = XB ms
The average velocity on Closing, in meters per second (m/s),
is calculated using the following formula :
Vo(m/s) = (YA-YB) (mm)/ (XA-XB)(ms)
In this case XA-XB = dT = 100 ms
YA-YB = dY = 240.5 mm, so :
Vo = 240.5 / 10 = 2.4 m/s |
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2.5.6 GENERAL PRECAUTIONS
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Certain precautions are to be observed when the transducer
is installed and the cables are connected : |
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2.5.6.1 Inverted curves
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In general, displacement curves show the CLOSED position
higher than the OPEN position. To observe this rule,
the wires must not be interchanged between terminals
2 and 3 of the transducer. otherwise the curve will
be draw upside down (see figures 2.5.6a and b)
Fig 2.5.6a Connections causing an inverted displacement curve
Fig 2.5.6b Inverted displacement curve example (Close)
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2.5.6.2 Transducer capacity
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When the transducer is installed, it must be ensured that
the motion measured does not exceed the capacity of
the transducer or it will be damaged, and the curve
shown will not represent the true motion of the circuit
breaker. In the following graphic is an example of
what happens when the transducer « bottoms out » before
the breaker attains the end of its movement, as seen
by the sharp angle at the bottom of the graphic.
Fig. 2.5.6.2 Example of a displacement curve exceeding a
transducer's range |
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