Evaluation of methods for breaker flashover protection Reasons Conditions which cause breakers to lose their di-electric strength & allow arcing between their open contacts are àInternal & External Contamination àLow Di-electric Pressure àHumidity Risk of flashover increases if àoverload on transmission networks àlow-cost breakers with reduced security margins are more porn to flashover (taking equipment out of service for maintenance is very difficult) NTPC have reported an increase in the number of breaker flashovers. Dedicated protection is required to prevent/reduce damage resulting from breaker flashover This project evaluates à Different breaker flashover protection schemes with particular emphasis on reliability and on the equipment required We develop and use The fault tree analysis method to make numerical reliability calculations for comparison purposes. 1
Transcript
Evaluation of methods for breaker flashover protection
Reasons
Conditions which cause breakers to lose
their di-electric strength & allow arcing
between their open contacts are
àInternal & External Contamination
àLow Di-electric Pressure
àHumidity
Risk of flashover increases if
àoverload on transmission networks
àlow-cost breakers with reduced security margins are more porn to flashover
(taking equipment out of service for maintenance is very difficult)
NTPC have reported an increase in the number of breaker flashovers.
Dedicated protection is required to prevent/reduce damage resulting from breaker flashover
This project evaluates
à Different breaker flashover protection schemes with particular emphasis on reliability and on the
equipment required
We develop and use
The fault tree analysis method to make numerical reliability calculations for comparison
purposes.
An ATP (Alternate Transient Programming) Simulation model to get a better understanding
of this kind of failure (breaker flashover)
What is a FLASHOVER
From the power system point of view
à A flashover is a series fault.
à Not a ground or phase-to-phase fault.
But
à A condition that resembles one phase of a breaker closed, with a residual current much
lower than a phase to ground fault.
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A flash over can lead to a power oscillation.
Line, transformer, and generator protection are not effective in this situation because they
either do not detect flashover failure or do not detect it quickly enough.
Neither is traditional or standard breaker-failure protection effective at detecting flashover
failure, because these require an external trip signal from another protection device to
initiate the breaker failure
Causes For Flashover
Flashover can occur on any breaker in the network where an over voltage condition is present, but
the probability is higher on breakers used to synchronize two isolated power systems or on
generator breakers.
Cause1
During the synchronization process, the out of phase angle between breaker contacts
changes from 0 to 360° continuously.
The voltage between breaker contacts reaches its maximum instantaneous value when the
angle difference between the voltages is 180°, with a magnitude equal to double the
nominal phase to ground peak voltage.
Ex: Breaker that synchronizes a generator on a 500kV system
The voltage continuously changes between
à 0 & 577.3 kV rms (or)
0 & 816 kV Peak Instantaneous voltage
∆Vrms = 500/√3 l 0° - 500/√3 l 180°
= 577.3 kV.
∆Vpeak = 816 kV.
Voltage waves on both sides of an open breaker when the angle is 1800
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Cause2
When a long line (H.V) without line reactors, is energized.
When the local breaker is closed, the capacitive effect of the line will cause an over voltage
at the remote end. This over voltage could cause the remote end breaker to experience a
flashover.
Cause3
If the dielectric strength on any of the breaker phases is lower than normal, a flashover can
occur when the voltage across the open breaker contacts increase. The highest probability
that this will happen is when the voltage angle is near 180°. Besides damaging breaker, this
out of phase and unbalanced condition affects system stability and can lead to abnormally
high stresses on electrical equipment near the breaker, such as a generator or transformer.
Real Case Analysis
Our case study is of a system where a real flashover happened during the synchronization
process in a generator-transformer group connected to a 400 kV power system.
The group included a generator & a generator-transformer.
The substation arrangement is breaker and a half; the flashover has occurred in the main
breaker. The half breaker was open.
There was no oscillographic records for the 400 kV breaker where the flashover occurred, but there
were oscillographic records for the generator and adjacent 400 kV line
Case Study Data And Oscillographic Recorder Location
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During the fault a key record was obtained at the 20kv generator terminals.
We can see voltages & currents at the generator & calculate the same variables at the 400kv level
where the breaker flashed.
Voltages on the adjacent 400kv line were recorded at line capacitive voltage transformers.
Order of occurrence (flashover)
à 400Kv breaker – phase A – flashed over during synchronization
à Approx
after 1 sec(58 cycles) – power plant protection
tripped
after 9 cycles – breaker failure scheme sent a trip
signal to the breaker failure
auxiliary relay (86BF) & cleared
the bus.
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Therefore after 67 cycles flashover occurred. And
after 4 cycles 86BF tripped.
à Currents in A & B phases reappeared because of a winding failure in unit transformer
(400Kv phase A winding) from the high electro mechanical stress caused by the flashover.
à Analysis of current & voltage phase angles shows that at the beginning of flashover àBoth
phases are in phase.
à As the generator begins to deliver active power to the system, the phase angle between the
voltage changes & voltage & current begins to oscillate.
During flashover ( values of voltage & currents at the generator & 400Kv bus)
Failure Consequences
The high electromechanical stresses during the out of phase, unbalanced energization
caused SEVERE TRANSFORMER DAMAGE.
The failure to isolate the generator from feeding the damaged H.V transformer windings for
several seconds resulted in HIGH TRANSFORMER REPAIR COSTS.
CUMULATIVE DAMAGE & LOSS OF LIFE OF NEIGHBOPURING EQUIPMENT.
BASE GENERATION OUT OF DISPATCH FOR SOME DAYS until a replacement
transformer was installed & tested. HIGH COST OF REPLACING LOST ENERGY with
more expensive remote sources.
POWER SYSTEM OSCILLATIONS OCCURRED.
Even when the probability of a breaker flashover is low, the high costs of a failure
justify using dedicated flashover protection that isolates the failed breaker as soon as
possible, there by avoiding damage to primary equipment. Implementation costs depend on
the protection methods selected. But with present digital multifunctional relays this can be
done without additional equipment costs.
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Closing circuit of a 6.6Kv circuit breaker
S.No. Name of the part Nature of action proposed
1 F41,F42,F43,F44 Fuses – self explanatory
2 Emerg. Trip
(emergency push button
This will be at remote
A light will glow in the remote and this
shows that an abnormality is there in the
circuit. then noticing this the emergency
push button is operated manually.
3 K 24 RELAY This relay is also called as master trip relay.
This relay will trip when the following
relays are actuated. They are : over load
relay, earth fault relay, short circuit relay,
differential short circuit relays etc.
4 UCB(UNIT
CONTROLBOARD)
The close signal is resieved from UCB
only.
5 SETTINGS OF THE
BREAKER
(3 TYPES)
TRIP, NORMAL, CLOSE
SWGR, NORMAL,TRIAL
TEST, SERVICE
During the operation (or) service. The
normal condition is present and then the
tripping & closing will be the commands
obtained according to the requirement.
& two more settings was there .
Test/rack out position, Service/rack in
position
6 Mechanical interlocks
(switches, closing coils, anti
pumping relay,k1
Through anti pumping relay supply is
extended to closing coil.
Anti pumping relay is used to prevent the
burning and failure due to continuous
supply to the closing coil
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Tripping circuit of a 6.6Kv circuit breaker
Simulation of different flashover conditions
S.No. Name of the part Nature of action proposed
1 F45,F46 Fuses – self explanatory
2 Emerg. Trip
(emergency push button
This will be at remote
A light will glow in the remote and this shows
that an abnormality is there in the circuit. then
noticing this the emergency push button is
operated manually.
3 Process trip
(C & I)
If there is any abnormality in the process of the
system i,e any control systems (or) instruments
failure is present the C & I department (control
& instrumentation) will give an indication to
trip the circuit..
4 UNDER VOLTAGE TRIP This is self explanatory.
5 Electrical protection trip This tripping is initiated when ever there is any
tripping signal present from the following
relays.
Earth fault relay, over current relay, over
voltage relay, locked rotor relay, short circuit
relay, differential short circuit relay and over
load relay etc.
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Normal trip from USB
VAJC-K25
(voltage actuated relay)
This is for safety purpose.In USB we are having
three lights R,G,&W.
R-Red-indicates that the C.B.is ON
G-Green-indicates that the C.B.is OFF
W-Wight-indicates that the C.B.is under
AUTOTRIP.
Energizes when breaker is in ON condition.
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To obtain a better understanding for flashover conditions, a model of the actual power
system was created in ATP (Alternate Transient Program) comparing the simulation results
from the modeled power system to the actual recorded results validated the power system
model.
From graphs it can be seen that the actual & simulated results match closely, which
confirms the accuracy of the simulated model.
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Methods For Flashover Protection
Electrical utilities use several different schemes for flashover protection. These methods can
use information from any of the following.
Phase currents
Residual currents
Voltages from one or both sides of the breaker
Breaker position auxiliary contacts (52a or 52b)
Close signal monitoring or timers
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Circuit breaker flashover protection may be realized in a separate protection relay (or) in a
multifunctional breaker, line, transformer, or generator relay.
Separate flashover protection relays are available, but their functionality can be replicated in
multifunctional programmable protective relays.
Once the flashover is detected, all the breakers in the bus must be tripped, as in a
conventional breaker failure scheme. Security considerations are very important to avoid
mal operations
There is a very little literature available about breaker flashover protection
IEEE standard C37.102 – 1987 [5] describes a simple method to detect flashover in
generator breakers that has both LOW SECURITY & LOW DEPENDABILITY. In
addition this method cannot be directly applied in double breaker substation arrangements
(ring bus, double breaker or breaker and a half) or in single pole trip-and-reclose breakers
for transmission lines. It also fails to detect three phase flashovers.
Engineers at NTPC have had to look for other methods to resolve these problems. This
project tries to serve as a guide in selecting and comparing those different methods, from
the point of view of equipment needed and reliability. Most examples are based on
generator breakers, but may be used for any breaker.
