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TLA Relevant Aspects Proper Selection

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Relevant Aspects for the Proper Selection and
Application of Transmission Line Arresters for
Improvement of the Transmission Lines
Lightning Performance
Lightning Activity and Effects on Transmission Lines
• In many countries lightning has
been reported as the major cause
of non-scheduled outages on
overhead sub-transmission and
transmission lines with rated
voltage up to 230 kV.
• Lightning is the most frequent
cause of transmission outage and
service interruption in the United
States, accounting for about 30%
of all power outages, and
resulting in economic losses
approaching $1 billion annually.
http://www.epri.com
Power supply utilities and industrial
consumers have been working on
lightning analysis aiming lightning
performance improvement for lines.
Lightning Activity and Effects on Transmission Lines
Lightning phenomena on overhead lines have important
consequences in many safety and technical aspects. The problem
has special importance in countries or in regions with higher electrical
activities and / or higher soil resistivity:
 Load losses on power systems caused by voltage sags due to
lightning transitory activities on transmission lines.
 Permanent and transitory disturbances on important lines / whole
system due to outages.
 Critical for all modern industries now so reliant on sophisticated
electronic equipment and especially production process very
sensitive to momentary disturbances in the system.
Lightning Activity and Effects on Transmission Lines
Three main aspects are involved in the
lightning performance of transmission
lines:
1.
2.
3.
Discharge current characteristics: peak
value, time-to-crest and rate of rise.
The attachment process between
lightning channel and the transmission
line components.
The electromagnetic response of the
overhead line struck by the lightning.
Transmission lines may present several
different configurations for the towers,
overhead conductors and tower-footings.
Different
configurations
establish
different transient responses under
lightning stresses.
Unshielded Lines:
• Basically all lightning strikes on
structures or on the phase conductors
will produce flashovers along the
insulator strings.
Shielded Lines:
• Possibility of back flashover occurring
across insulator strings.
• Transmission line lightning performance
depends strongly
on the transient
grounding system behavior.
Shielding failure:
• Flashover occurrence as in case of the
unshielded lines.
Induced voltages:
• Critical for systems with rated voltage up
to 45 kV
Procedures to Improve Transmission Line Lightning
Performance
Which Method Should Be Used?
INPUT
OUTPUT
Current outage
number
Desired outage
number
Lightning studies and analysis using computational tools in which for the
desired transmission line configuration is estimated in number of transmission
line outages per 100 km/year.
• Current configuration and other configurations considering different
procedures to improve the line performance
Procedures to Improve Transmission Line Lightning
Performance
Which Method Should Be Used?
INPUT
OUTPUT
Current outage
number
Desired outage
number
Starting from the results obtained in the theoretical studies and knowing the
target outage index, methods and procedures to improve the line lightning
performance have been evaluated taking into account the cost – benefit
balance.
• From the methods currently available for lightning improvement, in most
of the cases Transmission Line Arresters (TLA) sometimes associated
with the improvement of transient grounding behavior are usually
considered the most effective.
Transmission Line Arresters - TLA
General information:
• TLA have been used primarily for
lightning protection but can also
be applied to control switching
overvoltages.
• Installed along critical sections of
transmission lines with poorer
lightning performance to reduce
unscheduled outages due to
lightning.
 reduce system interruption
 increase system reliability
• NGLA or EGLA configuration
Transmission Line Arrester – TLA
Which TLA design?
Non-Gapped Line Arresters (NGLA)
+ Indication of overloaded NGLA by disconnector
+ Reasonable current/energy sharing for kA currents
+ Can be used for all applications.
• May be longer and heavier than EGLA
• Mechanical stress on disconnector lead
Externally Gapped Line Arresters (EGLA)
+ No continuous voltage stress on ZnO unit
• Can only be used for tripping protection on shielded lines
• Tricky coordination with insulator withstand levels to avoid spark-over for
slow front overvoltages or TOV events
• Minimal current sharing and no indication of overloaded EGLA
Transmission Line Arrester – TLA
Externally Gapped Line Arresters (EGLA)
Source: Supressor de sobretensiones por descargas atmosféricas
(SSDA) para líneas de transmisión – CFE - 2004
Transmission Line Arrester – TLA
Non-Gapped Line Arresters (NGLA)
Transmission Line Arrester – TLA
Which Application?
• Minimize tripping of unshielded and shielded lines due to lightning
o New transmission lines with high theoretical outage indexes due to lightning
o Virtually eliminating interrupted power supply for sensitive industrial processes
o To protect double circuit lines, virtually eliminating double-circuit flashover
• Compact line with reduced insulation withstand levels
o Building a new line or upgrading an existing line.
• Control of switching overvoltages profile along the line
o Substitute for pre-insertion resistors (works well together with controlled
switching)
• Extended substation protection
o Reducing both amplitude and steepness severity of the incoming overvoltages
entering at air and GIS substations.
o Eliminating flashover risks near substations.
• Temporary line arresters for hot line work
Transmission Line Arrester – TLA
Non-Gapped Line Arresters (NGLA)
Transmission Line Arrester – TLA
Non-Gapped Line Arresters (NGLA)
Source: various transmission line arrester applications, installations, and
designs – MSA / ABB – CIGRÉ tutorial – Rio de Janeiro – 2005
Transmission Line Arrester – TLA
Non-Gapped Line Arresters (NGLA)
Source: limitation of switching overvoltages by line arresters in combination
with controlled switching - Carl E. Solver - CIGRÉ 2006 - Q1.12 – Paris
Transmission Line Arrester – TLA
Non-Gapped Line Arresters (NGLA)
Proper Selection for NGLA
The proper selection for NGLA in terms of electrical and mechanical
requirements, as well as the proper design are important to guarantee
the long term performance especially for critical environmental
conditions.
• Electrical Requirements
• Mechanical Requirements
• Automatic disconnector device – Proper coordination between the NGLA
and the disconnector characteristics.
• Proper design – constructive characteristics
Proper Selection for NGLA - Electrical Requirements
Totally different application than substation arresters:
• Used to protect self-restoring equipment
• Short protective distances
• Its only purpose is to prevent flashover of the insulators strings
 Rated voltage and MCOV:
• Highest voltage of the system & maximum temporary overvoltages and their
duration.
 Housing withstand voltages for lightning / power frequency (switching).
 Maximum protective levels required.
 Energy requirements for lightning & switching surges.
 TLA short-circuit capability.
Proper Selection for NGLA – Electrical Requirements
Which energy requirements shall be considered?
Substation arresters - energy requirements based on:
 Transmission line discharges during close and reclose of the lines
 Capacitor bank discharges and sometimes TOV requirements
Transmission Line Arresters – first need to know the application
 Protection against lightning strikes only or also protection against
switching events?
 In case of protection against lightning only:
•
•
Outage rate acceptable for the specific line – TLA number and location
Acceptable failure rate for TLA due to excessive lightning energies – this
defines the minimum energy requirements.
Proper Selection for NGLA – Electrical Requirements
Which energy requirements shall be considered?
Factors that affect the energy requirement for a NGLA
In case of protection against lightning only:
• If the overhead transmission line is unshielded or shielded:
o Unshielded lines - Energy equivalent to previous IEC LDC 3 or IEC
LDC 4, depending on the Ground Flash Density of the region
o Shielded lines - Energy equivalent to previous IEC LDC 1 – 3,
depending on the Ground Flash Density and the transient ground
system behavior
• The transient grounding system behaviour
o Shielded lines - Higher grounding transient impedance usually means
higher energy absorbed by TLA
• Numbers of TLA installed on the protected tower & along the line
Proper Selection for NGLA – Electrical Requirements
Which energy requirements shall be considered?
Factors that affect the energy requirement for a NGLA
Protection against switching events:
• The energy absorbed by TLA during switching surge on the
transmission lines depends on the length of the line as well as the
number of TLA installed along the sections of the line.
• TLA installed along the sections of the transmission lines share the
total energy among them reducing the energy absorbed by each
TLA.
• For switching surges, lower energy discharge class can be chosen
when many TLA are installed along the line.
Proper Selection for NGLA - Mechanical Requirements
Installation issues
I.
Types of installation: usually chosen by the customer
•
•
•
•
Suspended on the conductor
Mounted on the insulator
Suspended at the tower
Supported in the tower
II. Common installations issues (more critical for suspended on the
conductor):
•
•
•
•
•
Hot line clamp x suspension clamps – degrees of freedom and vibration
Break at lugs/braid – material quality, crimping and proper length
Materials applied (stainless, galvanized, etc.) – proper material selection for
long term performance taking into account the risks of corrosion depending on
how critical is the environmental condition
Chain deformation – proper plating selection avoiding corrosion and size
Disconnector failure – Proper installation and enough free movement for the
disconnector cable, avoiding higher mechanical stresses on the disconnector
III. Preferable installations: From mechanical aspects it is usually
recommended mounted on the insulator or supported in the tower
Proper Selection for NGLA - Mechanical Requirements
Effects of conductor vibration
Suspended on the conductor
Resonance conditions happens if natural pendulum frequency of the
line arresters is similar to span natural frequency
1 3𝑔
𝑓𝑝𝑒𝑛𝑑𝑢𝑙𝑢𝑚 =
2𝜋 2ℎ
Arrester
height (m)
Frequency
(Hz)
0.25
1.22
1
0.61
2
0.43
3
0.35
4
0.31
𝑓𝑠𝑝𝑎𝑛 =
Where:
g: gravity acceleration
h: arrester height
T: conductor tension
w: conductor weight
n: nr. of standing wave loops
S: span length
𝑇. 𝑔
𝑛
.
𝑤
2. 𝑆
Aeolian
Galloping
Sub-span
Frequency (Hz)
3-100
0.1-3
0.15-10
Amplitude
(conductor ∅)
0.01-1
5-300
0.5-20
Wind speed
(km/h)
3-25
25-65
15-65
Proper Selection for NGLA - Mechanical Requirements
Effect of the vibration - Interaction with dampers
•
•
•
•
Aeolian vibration dampers are
placed in specific locations to
dissipate energy.
When installed suspended on
the conductor, TLA changes
the standing waves because it
adds an additional weight on
the conductor.
Dampers configuration need
to be checked.
It is recommended to install
additional dampers at each
side of the TLA.
Install another damper
outside the arrester
Proper Selection for NGLA - Mechanical Requirements
Disconnector failure due to mechanical issues
Disconnection of some line arresters due to failures in the flexible
cable and in the links connection (eye bolt) caused by the incidence of
strong winds and / or vibration in the line
 Allow enough free movement of disconnector cable.
Proper Selection for NGLA - Disconnector
Important Disconnector Characteristics
 How fast it operates.
 The disconnector shall always continue its
opening operation once it is triggered to
operate even if the voltage is switched off.
 A disconnector is a simple device that
typically reacts on heating from power
frequency currents. It cannot distinguish
between TOV currents or slow oscillating
front currents passing through the NGLA or
real short-circuit currents.
• This shows the importance to always select a
high enough rated voltage so that the NGLA do
not see TOV stresses that can interfere with its
disconnector operation.
Proper Selection for NGLA - Disconnector
Matching Disconnector and NGLA Characteristics
Typical curve showing opening time versus current
TIME versus CURRENT CURVE for disconnector
10000
Time (ms)
1000
100
10
1
1
10
100
Current (A)
1000
Proper Selection for NGLA - Disconnector
Matching Disconnector and NGLA Characteristics
Typical TOV withstand curve voltage versus time
TOV withstand Curve in p.u. of Ur
With Prior Duty
without Prior Duty
1.2
1.15
1.1
Utov / Ur (p.u.)
1.05
1
0.95
0.9
0.85
0.8
1
10
100
Time (Seconds)
1000
10000
Proper Selection for NGLA - Disconnector
Matching operation with line protection scheme
Which protection scheme is used?
• How fast is it?
• How many reclosing may occur?
• What is the short-circuit currents at installation?
Electrical characteristics:
• When the disconnector shall operate
• When the disconnector shall not operate
Proper mechanical strength:
• Vibrations should be verified
Proper Selection for NGLA - Disconnector
Proper Coordination with NGLA and with line protection scheme
1. Disconnector operates before the line trips: depends how quick
the disconnector operates plus how quickly it can quench the arc
during falling out, which will be strongly weather dependent. This is
then a race between the line protection scheme and the
disconnector and may vary from incident to incident. No tripping
occurs.
2. Disconnector operates before fast reclosing of the line: means
that once triggered the disconnector shall continue to disconnect
even if the power supply is switched off. This should be a
repeatable operation depending on coordination of the line
protection scheme including fast reclosing time and the
disconnector opening time.
Proper Selection for NGLA - Disconnector
Proper Coordination with NGLA and with line protection scheme
3. Disconnector has not completed its operation when
reclosing occurs: should not happen as this leads to a system
disturbance and also leads to a second short-circuit stress on the
NGLA which significantly increase the risk of complete
disintegration of the arrester with larger pieces coming down. This
indicates a mismatch of disconnector opening times compared to
the line protection scheme of the system and may cause system
disturbance every time there is a NGLA failure.
4. Disconnector operates but the arrester is not overloaded or
failed: should not occur and indicates either a disconnector not
matching the NGLA characteristics or a mechanically too weak
disconnector design.
Proper Selection for NGLA - Disconnector
Proper Coordination with NGLA and with line protection scheme
• Reclosing time: 500 ms
• Decisive TOV of 150 kV during 1 s. substation arresters of 132 kV rating
TOV currents in A
TLPXX
/DDX
NGLA
TIME/
Voltage
Voltage
Voltage
1 s (A)
10 s
kV or p.u.
kV or p.u.
p.u.
prior
energy
no energy
Ur = 132
no energy
no
Ur = 138
energy
SYSTEM TOV (kV)
DDX Current
(A)
Voltage
40 s
kV or p.u.
1.14
8
1.11
6
1.05
0.4
1.18
15
1.14
12
1.09
1.6
155
15
150
12
144
1.6
162
15
157
12
150
1.6
(A)
150
9A
• Typically a higher rated voltage than the substation arresters
o Limit arresters stresses due TOV and possible reduction of the grading ring
Conclusions
•
In many countries lightning has been reported as the major cause of unscheduled
outages on overhead unshielded and shielded sub-transmission and transmission
lines with rated voltages up to 245 kV. This fact has been taken up by several power
supply utilities and industrial consumers which have lead them to invest in the
promotion of improvements along the critical sections of their overhead lines with
poor lightning performance, thereby increasing their reliability.
•
Among the methods used for improvement of the overhead lines lightning
performance, TLA installed along the critical sections of the lines with poorer
lightning performance have been usually considered as the most effective for long
term performance.
•
In order to get a good solution in the technical and economical point of view, studies
shall be done to: select the TLA properly in terms of electrical and mechanical
requirements as well as evaluate the proper design for longer term performance,
and optimize the quantity and location of the line arresters along the line.
THANK YOU
www.te.com/energy
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