| 
|
|
|
| Data
analysis |
|
| |
|
Data
Analysis is the final step of the timing test.
The
professional in charge has to have good knowledge of both
the circuit breaker being timed and the network requirements.
Since timing machines, nowadays, incorporate powerful
analysis tools, a good knowledge of these tools is very
helpful and time saver.
He
has to have also a developed analytical sense and to be
able to distinguish between the importance of the requested
results and the consequences of non-conformity.
In
addition to the entire above, one of the important factors
in good data analysis is always experience. |
| |
|
4.1
Timing Chart
|
Back
to top
The
circuit breaker has to comply with user requirements,
in addition to the international standards specifications.
The
designer takes into account these requirements when designing
a particular circuit breaker. The operation time references
and tolerances are established, based on tests and a reference
table, called a timing chart, is drawn up.
The
timing chart contains time references for all the operation
cycles the breaker is meant to accomplish. In addition
to these time references, the designer may consider it
useful to note other times to insure proper function of
the breaker or any of its subassemblies. |
| |
|
4.2
Priority
|
Back
to top
Following
the data analysis important decisions are to be taken:
1. Put the breaker into service
2. Suspend commissioning operations and take further actions
in order to correct any faulty condition.
In
the first case, putting an unsuitable breaker in service
would have disastrous consequences, either to the equipment
or to the maintenance personnel.
In
the second case, the cost would be enormous if it turned
out to be unnecessary as a result of an incorrect analysis.
Good
analysis and distinction of priorities are crucial. |
| |
|
4.2.1
Priority levels of the operation times
|
Back
to top
There
remains the question of determining the priorities in
order to perform a successful analysis. The priority levels
are described from high to low as follows: |
| |
|
4.2.1.1
"TRIP" Time
|
Reducing
the duration of short circuit currents on power transmission
lines is a permanent objective.
The
main advantage is higher transmitted power, since the
power transmission stability threshold is higher when
the short circuit duration is shorter.
The
user determines the current interruption duration, For
example 2 cycles.
The
duration of the current interruption is counted starting
from the instant the main mechanism coil energizes until
the final interruption of the current in the main contact,
including the arc duration.
The
interrupting time is then equal to the mechanical time
(operation time) plus the arc duration.
For
a 60 Hz network, 2 cycles are equivalent to 2/60 = 0.03334
s = 33.34 ms (ms = milliseconds)
Since
the timing test is done with the breaker out of circuit
(no load), the arc duration is not measurable.
The
arc duration depends on several factors: voltage level,
interrupting medium, interrupting techniques, etc. It
is determined during the design testing. The "TRIP"
time is then adjusted to obtain the interrupting time.
The
"TRIP" time is then, first and foremost, a
user requirement the designer has accepted to respect.
Nevertheless, an inappropriate "TRIP" time
may cause important risks of different natures for a longer
time or shorter one. |
| |
|
4.2.1.2
Longer "TRIP" time
|
Many
risks can be caused by a too long "TRIP" time.
It can be anything from a simple anomaly in the tripping
control circuit to a major failure in the main interrupting
circuit.
The
analysis should take care to examine all details to reach
a precise conclusion. Breakers' characteristics
play a fundamental role. Listing all probable cases is
impossible.
In
general, and independently of breaker's type, a
longer "TRIP" time can be caused by a slower
transition speed, the arc duration may be longer and premature
contact wear may take place.
For
small capacitive currents, the voltage spikes are strong
and may cause a consecutive fault.
Consecutive
faults are line to ground short circuits, consecutive
to an interrupting of small capacitive or inductive currents.
The
breaker that is interrupting a small current sees its
current growing instantly to full short circuit current.
Some breaker types have difficulty in correcting this
situation.
A
known method to overcome the consecutive fault is to interrupt
with high speed breakers. Lower speed can be crucial in
this case.
Conclusion:
Reduced
tripping speed can compromise the operation of the breaker
and possibly that of the power network itself, not to
mention the risk of consecutive faults. |
| |
|
4.2.1.3
Shorter "TRIP" time
|
Following
a short circuit, the nominal alternating current flowing
in the circuit grows instantly to a huge value, of the
same nature and frequency called the symmetrical short
circuit current.
Figure 4.2.1.3a - Short circuit current
Due
to the network's inductive nature, a temporary DC
component adds itself to the symmetrical short circuit
value. The result is called an asymmetrical short circuit.
The
initial value equals the instantaneous value of the symmetrical
short circuit at the point of the short circuit with a
negative sign. It decreases afterward, following a damped
exponential curve, with a speed determined by the time
constant of the circuit.
The
breaking capacity of a particular breaker, is the highest
value of current the breaker is capable of interrupting.
The
breaker is supposed to interrupt successfully every current
equal or less to its breaking capacity, whether the current
is symmetrical or asymmetrical.
Considering
the curve in figure 4.2.1.3b, one notes that the asymmetrical
value is function of the interrupting time.
If
it is higher, if the interrupting time is shorter. As
a result, if the breaker is too fast the asymmetrical
value can exceed its breaking capacity, and breaking is
no longer ensured.
The
curve of figure 4.2.1.3b shows the nominal value of the
aperiodic component as a function of the opening time
of the breaker. This curve uses a damping time of 20%
per hundredth of a second.
Conclusion
:
The
"TRIP" time should never be less than the
reference value, otherwise the asymmetrical short circuit
value can exceed the breaker's breaking capacity. |
| |
|
4.2.2
Contacts discrepancy
|
Back
to top
High
voltage breakers are three-phase apparatus. It contains
at least one contact per phase, and in some cases, multiple
contacts in series per phase, up to 12 per phase for certain
air blast breakers at 765 kV.
It
is crucial for the proper operation of the breaker and
of the network to limit the time discrepancy between the
contacts.
The
types of discrepancies can be divided into 2 groups:
1. Contact discrepancy between poles,
2. Contact discrepancy in the same pole. |
| |
|
4.2.2.1
Contacts discrepancy between poles
|
On
Trip:
According
to IEC 56 (parag.3.3.1) the phase is considered open when
the first contact of the pole is open.
The
biggest discrepancy measured should not exceed a maximum
value set by the designer, the user, or by an agreement
between them.
If
not conforming:
Poles'
contacts separation has to be simultaneous to prevent
high voltage transients, otherwise it would attain double
the rated value on the first parting pole. The maximum
discrepancy allowed is 1/6th of a cycle.
On
Close:
According
to IEC 56 (parag.3.3.2) the phase is considered closed
when the last contact of the pole is closed.
The
biggest discrepancy measured should not exceed a maximum
value set by the designer, the user or by an agreement
between them.
If
not conforming:
The
sudden energizing of circuits is always followed by a
moderate voltage increase, with the exception of long,
unloaded transmission lines, where the voltage rise can
be critically dangerous.
When
a line is connected to an energized network, a voltage
wave is forced on the line. This wave is reflected back
at the end of an open line, and returns with double the
amplitude.
Even
higher voltages may be encountered when the line has a
load before being reenergized, and if the breaker closes
at the moment that the polarity of the network is opposite
to that which was present on the line.
The
voltage may then be three times the network voltage, after
reflection of the wave. This situation may be produced
with a rapid reclose of a line.
Still
higher voltages may be encountered on three-phase lines,
when the three poles of the breaker do not close simultaneously.
A
wave on one phase will produce induced waves in the other
phases, and under unfavorable conditions, will increase
the voltage on another phase.
Higher
transition voltage rises can be encountered if the discrepancy
on closing is too high.
Note
that on networks where the nominal voltage is 500 kV and
higher, the isolation of the lines is determined by the
operation voltage spikes. |
| |
|
| 4.2.2.2
Discrepancy between contacts of the same pole
|
For
multiple-contact-per-pole breakers, grading capacitors
are installed in parallel with each contact to equalize
the voltage when contacts part.
In
general, the fastest contact has the longest arc duration
and higher contact wear.
In
case of excessive discrepancy, the fastest contact on
close and slowest on trip would cause higher voltage shocks
to their grading capacitors, thus reducing their life
expectancy and that of the contacts. |
| |
|
4.2.3
"CLOSE" time
|
Back
to top
During
closing, especially on short circuits, opposite forces
are considerable. In case of slow moving contacts the
pre-arc has a longer duration thus causing more contact
deterioration.
If
the closing time is not respected, this would compromise
the relative guarantee to the closing capacity.
This
time is usually supplied by the designer of the circuit
breaker. |
| |
|
4.2.4
Operation cycles
|
Back
to top
An
operation cycle is a sequence of basic "CLOSE"
and "TRIP" operations in specified time intervals.
The
most common sequences are conform to the following formula:
O
-> T -> CO -> T' -> CO
O:
Trip operation
CO:
Trip-Free cycle
T:
Time delay of 0.3 seconds or 3 minutes
T':
Time delay of 3 minutes |
| |
|
4.2.4.1
Trip-Free (CO) cycle, short circuit time
|
CO
cycles simulate closing on a short circuit. In the actual
event, the breaker closes first, then the protection relay
system detects the short circuit and trips the breaker.
In
the test event the timing machine can be programmed to
launch a trip command as soon as the contacts close. This
gives the fastest short circuit time the breaker is capable
of doing.
This
value is compared with the designer's references. |
| |
|
4.2.4.2
Reclose-Open (OCO) cycle, isolation
time
|
Experience
shows that a great number of short circuits are temporary.
It means they are caused by an event that disappears shortly
after the breaker opens. A few examples are: short circuits
caused by lightning, a bird, fallen trees or branches,
etc.
The
purpose of fast Reclose is to reduce the duration of power
interruption.
It
is important to reduce this duration to a minimum. For
the out of service circuit, it is important to give it
enough time to clear the fault.
In
effect, temporary faults create arcs; once the power feeding
this arc is cut, enough time needs to pass for the arc
plasma to deionize before reconnecting power, or another
trip may occur.
Statistics
show that a 0.3 sec duration between the contacts opening
and the contacts closing is enough to achieve this goal.
If
on closing another trip occurs, the breaker will have
to interrupt the short-circuit a second time. There will
have to be a sufficient delay between the interruptions
for the interrupting medium to regenerate, so the second
interruption will be performed correctly. If the breaker
trips a second time, it should remain open.
High-voltage
transport and interconnecting networks
Automatic
fast reclosing avoids tripping between two interconnected
sources.
In
effect, when breakers on an interconnecting line between
two networks trip, there is a rapid loss of phase synchronization
if this line is the only one interconnecting them. If
there is another line running in parallel, it may trip
in turn by overloading, which causes a loss of synchronization.
This
de-synchronization may be avoided, when the faults are
temporary, by quickly reclosing the breakers before the
phase shift becomes too great.
If
the tripped circuit is three-phase, the reclosing time
must be very short, on the order of 0.3 seconds. If the
reclose time is longer, there is a risk of closing on
a phase discordance.
During
timing tests, the O-0.3s-CO test verifies the behavior
of the circuit breaker in this particular type of operation.
On
Reclose-Open timing test event, the timing machine is
programmed to delay the closing command until the isolation
time of 0.3 seconds is obtained. This delay must not be
confused with that of certain type of delaying devices
installed on some breakers, even if these times are similar. |
| |
|
| 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 judgement
of the testing personnel. |
| |
|
Bibliography |
|
- BRESSON
( C ) Appareillage électrique
- HAMMERLAND
( P ) Tension transitoire de rétablissement
- POUARD
( M ) Appareillage électrique. Fonction de l'appareillage.
Problèmes généraux.
- POUARD
( M ) et COUVREUX ( J.P. ) Appareillage électrique.
Étude pratique des divers types d'appareillage
à moyenne et haute tension.
- TRENCHAM
( H ) Coupure des circuits.
- CONFÉRENCE
INTERNATIONALE DES GRANDS RÉSEAUX ÉLECTRIQUES
À HAUTE-TENSION (CIGRE) Comptes rendus des travaux
de sessions, 1re Section Groupe 13.
- CASSIE
( A.M. ) Une nouvelle théorie de l'arc et
de la coupure. CIGRE 1939 Rapport no 102.
- SKEATS
( W.F. ), TITUS ( C.H. ) et WILSON (W.R.) Taux de rétablissement
de la tension dans le cas de défauts sur les lignes
de transmission.
- POUARD
( M ) Nouvelles notions sur les vitesses de rétablissement
de la tension aux bornes des disjoncteurs à haute
tension
- TESZNER
( s ), MAURY ( E ) et PEROLINI( M ) Surtension de coupure,
moyens de les atténuer et possibilités d'une
réglementation des interrupteurs. CIGRE 1954 Rapport
no 147.
- MAURY
( E ) Problèmes apparaissant aux tensions les plus
élevées lors de la manœuvre des disjoncteurs.
CIGRE 1964 Rapport no 138.
- DUBANTON
( C ) ET GERVAIS ( G ) Surtensions de manœuvre à
l'enclenchement des lignes à vide. Influence
de la puissance et de la configuration du réseau.
Répartition statistique . CIGRE 1972 Rapport no
33-05.
- BRESSON
( C ) Phénomènes électrodynamiques
dus aux courants intenses dans l'appareillage
- MOUTON
( M ) Les disjoncteurs-sectionneurs associés à
des fusibles et les interrupteurs-sectionneurs.
- FERNIER
( B ) Les interrupteurs à coupure en charge en
moyenne et haute tension .
-
MAYO ( P.C. ) Distances d'ouverture en série
dans les sectionneurs dans l'air. I.E.E.E Trans
P.A.S . 1967
- HOFFMAN
( D ) Interrupteur de sectionnement C.I.R.E.D. 1975 Rapport
no 25.
- SWIFT-HOOK
( D.T. ) La fiabilité des disjoncteurs 1968
- MAURY
( E ) Évolution des disjoncteurs des réseaux
de transport 1971.
- PEROLINI
( M ) Disjoncteurs à air comprimé pour des
réseaux à haute tension 1955.
- ORGERET
( l ) et RENAUX ( J ) Le disjoncteur pneumatique à
haute tension et les vitesses de rétablissement
de tensions élevées. Lev défaut kilométrique.
1959
- FRIEDRICH
( R.E. ) et BATES ( G ) L'utilisation du SF6 pour
l'interruption haute tension) 1958.
- ARTHUR
( A ) Préparations, propriétés physiques
et chimiques et applications générales de
l'hexafluorure de soufre 1962.
- LEEDS
( W.M. ), FRIEDRICH ( R.E. ), WAGNER ( C.L. ) ET BROWNE
( t.e. ) Application des études sur les surtensions
de manœuvre, les arcs et l'écoulement
des gaz, à la définition des disjoncteurs
au SF6 CIGRE 1970 Rapport 13.11
- PRIGENT
( H ) Mécanisme de l'enclenchement et de
la coupure des batteries de condensateurs raccordés
en dérivation dans les réseaux moyenne tension.
1954.
- FINK
(D) et BEATY ( H.W. ) Standard Handbook for Electrical
Engineers MacGraw-Hill Edition 11
- WILDI
( T ) Électrotechnique Première Édition |
|
