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breaker timing tests |
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3.1
Definition
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The
timing of a breaker is the time measurement of the mechanical
operations in order to verify its integrity and good working
order.
Mechanical
operation means all operation or cycle of operation the
breaker is intended to do without being connected to the
power grid. |
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3.2
Importance
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It is a necessary to time each breaker, in order to have its functional signature. Incorrect operation can have disastrous consequences on the equipment or the substation personnel. Not to mention the out of service losses of revenue and the repair costs.
Timing
tests are done first in the factory during the routine
tests and after installation, during the commissioning
tests.
They
have to be done periodically in order to validate the
good working order and reliability of the breaker.
Timing
tests are also a powerful tool for troubleshooting faulty
breakers. |
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3.3
Description
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In
order to measure the operation times of a breaker, we
need a device capable of detecting the instant of contact's
state changing starting from the point the order was launched.
This
device sends electric signals via cables connected to
each contact, in the case of several contacts in series
each signal has its own source to eliminate interference.
The
signal has two possible states. The first is when the
contact is closed, the second when the contact is open.
All is recorded for consultation and analysis.
These
special devices are called timing machines or circuit
breaker analyzers. They are designed to generate all the
needed signals and incorporate a data acquisition system.
Four
steps constitute the main activities to accomplish a timing
test:
1. Cable installation and connection
2.
Data Acquisition
3.
Data interpretation
4.
Data analysis
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3.3.1
Installation and connection
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The
connection, mainly between the timing machine and the
breaker, has to be done correctly taking into account
certain external factors, for instance magnetic induction
by nearby high voltage transmission lines.
Another
connection has to take place between the timing machine
and the mechanism coils responsible for launching the
operation order.
Some
other connections may also be required as travel transducer,
pressure transducer, auxiliary contacts, etc. |
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| 3.3.1.1
General rules |
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are no special rules applied as long as the wanted information
is obtained. But in order to avoid any unpleasant surprises,
some precautions should be observed. |
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3.3.1.2
Connection description |
3.3.1.2.1 Connection to main contacts
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Each
breaker contact has to be verified separately. For multi-contacts
per phase, each contact has to have its own verification
circuit.
Each
verification circuit includes a voltage source that injects
current on contact making; a detection circuit, to detect
the current and determine if the contact is closed; two-wire
shielded cable to carry the signal.
The
timing machine supplies the verification circuits. Each
verification circuit is called a channel.
The
following figure shows en example of how the main contacts
are connected to the channels:
For
multiple contacts per phase, special care has to be observed
to avoid the mirror effect (treated later in this article),
where the data for some contacts will be corrupted by
the data from other contacts.
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3.3.1.2.2
Connection to mechanism (command)
coils :
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The
circuit breakers control circuit has various safeguards
against malfunctions, for instance against phase discrepancy,
or against pumping.
The
following figure illustrates a command circuit, with some
protection circuits. It is important not to bypass any
of these protection circuits
A
connection that bypasses the auxiliary contacts may cause
the destruction of the coils.
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3.3.1.3
Mirror effect
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No
intervention on high voltage equipment should take place
without proper protection to the intervening personnel
first and to the intervened equipment. One of the most
important protection measures is to install ground cables.
Ground cables connect the line to the ground. In case
of accidental energy supply to the line the current will
pass through the ground cable and avoid the working personnel.
Ground cables should be installed on each side of the
breaker.
In
the case of timing a multi-contact phase the presence
of ground cables on both sides of the contacts may mask
the signal of a faulty contact by the signal of a good
contact, making it undetectable.
The
following illustration shows this phenomenon.
If
contact 2 stays open, while contact 1 closes, the channel
2 circuit will detect the current passing through the
ground cables and will show contact 2 as being closed.
The
solution would be to break the return circuit of the ground
by disconnecting the breaker from the line, between the
extremity of the breaker and the grounding cable next
to it. By no means should the grounding cable be removed,
under the risk of injury. |
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3.3.2
Data acquisition
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In
addition to the signal generation, timing machines are
in charge of contact transition detection and recording.
The
earliest timing machines used light reflected on moving
mirrors driven by the current signal coming from the contacts.
The light is projected on paper film, thus creating a
visible trace on the paper.
Today,
in the computer age, timing machines have undergone a
tremendous evolution. They use electronics and computer
technology for data acquisition. Powerful software serves
to analyze and conserve data for future studies. Data
transmission has never been easier. Decisions can be made
quickly and accurately. |
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3.3.3
Data interpretation
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Many
external agents can influence greatly the collection of
information. It is important to distinguish in the collected
information, between the useful data and the external
noise.
Good
interpretation is based on three main principals:
1. Know the breaker and the surrounding environment:
Induction
due to the proximity of overhead power transmission lines,
or a bad cable connection could alter the collected signal.
2. Know the timing machine:
Incorrect
programming or a failed circuit can cause unnoticed signal
alteration as well.
3. Know the significance of the values looked for:
When
timing a breaker, we are looking for certain values, such
as: contact switching times, coil energizing times, etc.
that can allow quick identification of a problem, and
if a value seems excessive, we can look to it in time
and repeat the test before taking all the cables down. |
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3.4
Breaker's operation times
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In
its description, the circuit breaker is a device that
conducts or interrupts currents to the circuit. This is
done by mechanically joining or parting two sets of contacts.
These are the basic operations of the circuit breaker
and are called "CLOSE" when joining, and "TRIP"
when parting, the contacts.
As
we mentioned before, these are not the only operations
that the breaker is meant to do. In most of the cases
it has to execute on demand a combination of the basic
operations, called cycles. The most popular are the following:
1. "Trip Free", (C-O): simulates a trip on
short circuit after a "CLOSE". The breaker
must open instantly.
2. "Reclose", (O-C): Simulates a fast close
after short circuit trip to reestablish the current.
3. "Reclose-open", (O-0.3s-C-O): Simulates
a "RECLOSE" on short circuit. The breaker
should clear the fault successfully.
4. (C-O)-15 sec-(C-O)-15-sec-(C-O) : Simulates a multiple
close after short circuit trips in purpose of reestablishing
the current, hoping that the short circuit is disappeared.
This cycle is mostly used in the Medium voltage applications.
The
times measured, for the different cycles starting from
the coil energizing until the switching of the main contacts
are called the Breaker's mechanical times of operations. |
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3.5
General definitions
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Before
proceeding with the operation times, here are some of
the most used terms in the breaker timing field. |
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3.5.1
Interrupting element (or unit) :
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Also
called arcing chamber, it is a closed volume containing
the main contacts and in which current interrupting occurs. |
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3.5.2
Pole :
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A
pole is a part of the breaker that is installed on a line
phase. The breaker is installed on three-phase line. It
necessarily has three poles. Each pole includes at least
one interrupting element. For high voltage levels, multi-element
poles are most common. |
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3.5.3
Main contact :
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These
are the contacts in charge of establishing or interrupting
the current flow in power circuits. They include a fixed
and a moving contact.
The
contact's material has to be chosen to have minimum
resistance when closed, to minimize the losses by joule's
effect when transiting nominal currents.
The
best-suited material is silver plated copper. |
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3.5.4
Auxiliary contact :
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As
mentioned earlier, insertion resistors are usually used
on high voltage circuit breakers, for closing, opening,
or both. These resistors minimize the transition voltage
before closing or after opening by engaging or disengaging
auxiliary contacts in two or three steps. |
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3.5.5
Arcing contact :
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For
breakers, the arc can be a powerful generator of heat
energy. This heat can cause fast deterioration of main
contact's material. To extend life of the main contacts,
breaker designers tend to separate the permanent current
carriers, called "Main Contacts", from those
subjected to the arc effect, and called "Arcing
Contacts".
The
most common material for arcing contacts are, in general,
tungsten alloys, reputed to have high arc resistant quality,
but are less conductive than silver-plated copper. |
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3.5.6
Indicating contact:
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In
order to control the breaker on low voltage side, two
types of contacts were created, (a) contact and (b) contact.
They are both driven by the operation of the breaker and
switch states with main contacts.
They
are mainly used to indicate the breaker's position
and to electrically interlock between basic operations. |
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"a" contact : |
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has the same state of the main contact. It closes when the
main contact closes, and opens when the main contact opens. |
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| 3.5.6.2
"b" contact : |
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has the opposite state of the main contact. It closes when
the main contact opens, and opens when the main contact
closes. |
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3.6
Time reference
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The
basis of timing tests is to collect information in order
to compare it with reference values. The comparison should
be done on comparable parameters. So the collected values
should follow the same rules as the reference values,
and be independent of individual interpretations.
Since
we are talking time, we should determine the same reference
points in reading the information.
Earlier,
the designers determined reference values. The users had
also their reference values. This created confusion between
the designer's reference values and the user's
interpretation to the collected values.
In
order to solve this problem, professionals tend to use
the definitions as stated by international standards.
The most widely used is the IEC 56 international standard. |
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3.7
Timing definitions according to IEC 56
international standard
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The
IEC 56 international standard defines these times as follows: |
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3.7.1
Opening Time (IEC 56 3.105.32)
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For
a circuit breaker tripped by any form of auxiliary power,
the opening time is the interval of time between the instant
of energizing the opening release, the circuit breaker
being in the closed position, and the instant when the
arcing contacts have separated in all poles.
Notes
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1. The opening time may vary significantly with the breaking
current.
2. For circuit breakers with more than one interrupting
unit per pole the instant when the arcing contacts have
separated in all poles is determined as the instant of
contact separation in the first unit of the last pole.
3. The opening time includes the operating time of any
auxiliary equipment necessary to open the circuit breaker
and forming an integral part of the circuit breaker. |
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3.7.2
Closing Time (IEC 56 3.105.35)
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The
interval of time between energizing the closing circuit,
the circuit breaker being in the open position, and the
instant when the contacts touch in all poles.
Note
: The closing time includes the operating time of any
auxiliary equipment necessary to close the circuit breaker
forming an integral part of the circuit breaker.. |
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3.7.3
Open-Close Time, O-C or Isolation time
(IEC 56 3.105.38)
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The
interval of time between the instant when the arcing contacts
have separated in all poles and the instant when the contacts
touch in the first pole during a reclosing operation. |
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3.7.4
Close-Open Time, or short-circuit time
(IEC 56 3.105.42)
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The
interval of time between the instant when the contacts
touch in the first pole during a closing operation and
the instant when the arcing contacts have separated in
all poles during the subsequent opening operation.
Note
: Unless otherwise stated, it is assumed that the opening
release incorporated in the circuit breaker is energized
at the instant when the contacts touch in the first pole
during closing. This represents the minimum close-open
time. |
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3.7.5
Minimum Trip Duration (IEC 56 3.105.44)
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The
minimum time the auxiliary power has to be applied to
the opening release to ensure complete opening of the
circuit breaker. |
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3.7.6
Minimum close duration (IEC 56 3.105.45)
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The
minimum time the auxiliary power has to be applied to
the closing device to ensure complete closing of the circuit
breaker.
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| Conclusion |
An accurate analysis makes it possible to make decisions that are profitable to the breaker, the network and to the maintenance personnel. In order to achieve this, knowing the timing machine and the significance of the operating times is important but not enough.
Knowing well the breaker itself, the reference values (timing chart) and the network characteristics is necessary. All of this backed with the experience and sense of
judgment of the testing personnel. |
