Download SURGE ARRESTORS OR LIGHTNING ARRESTORS. Lightning

SURGE ARRESTORS OR LIGHTNING ARRESTORS.
Lightning Arrestors are used for protecting the switchyard equipment against traveling
surges like switching impulse, lightning impulse waves. These are connected between
the line and earth at the sub-station. This is the first equipment electrically connected
when a transmission line is terminated in the sub-station. Hence it is understood that LAs
are used as a protecting device and it should be noted that LA should provide a low
impedance path when a traveling surge reaches the LA and high impedance path during
the normal working condition. A good lightning arrestor or surge arrestor should possess
the following essential characteristics.
1. It should not absorb any current during normal operation but during the over
voltage surges it must provide an easy path to earth.
2. After the discharge is over it should restore back to normal functioning i.e.
providing high resistance path during normal operation.
3. It must be capable of taking the load of discharges of designed system conditions.
Types of Lightning Arrestors
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Rod gap arrestor
Sphere gap arrestor
Horn gap arrestor
SiC (Silicon Carbide) gap type arrestors.
Zinc oxide (Gap Less) arrestors
Rod Gap arrestor: It is one of the most simplest type of arrestor and consists of two
rods which are bent at right angles as shown in the figure. One rod is connected to line
circuit and other is connected to ground. When any surge comes the air gap between the
rods gets ionized and surge is passed to earth. The limitations of the arrestors are
 The arrestor is not capable of stopping to power frequency voltage to earth
because of ionization of air in the gap.
 The climatic variation also affects the performance of the arrestor.
 Due to large amount of heat produce at the time of surge, the rod may melt
and cause damage.
 The polarity of surge also affects the arrestor
Sphere gap arrestor: In this type of arrestor the air gap is provided between two
spheres. The distance between the two spheres can be adjusted with the help of guage.
The minimum air gap is adjusted such a way that the discharge does not take place at
normal working condition but at predominantly excessive voltages the arc is set in the
gap. The arc in such an arrestor is not self-extinguishing as a very small voltage will
maintain even in during normal operating condition when the surge is over. In such a
case the relays may operate and trip the circuit unwarrantedly.
Horn gap arrestor: This type of arrestor consists of two bent metal rods in the form of
horns and mounted in a vertical plane. The horns are separated by a small air gap and
horns are mounted on small porcelain insulators. The air gap is adjusted such a way that
the discharge does not take place at normal working condition but at predominantly
excessive voltages the arc is set in the gap. A choking coil is introduced in series to the
arrestor so that the high frequency waves do not enter the transformer or system and a
series resistor is also introduced to avoid the permanent short circuit during arcing
conditions. The disadvantages with this type of arrestor is that that the atmospheric
conditions effect the performance of arrestor and the corrosion and pitting of horns
change the characteristics of the arrestor. These type of arrestors are used in the
distribution lines.
Silicon Carbide arrestors: In such arrestors the SiC discs and vacuum gaps held in
series and housed in Nitrogen filled porcelain housing. The gap sparks over during
voltage surges above break down value. The SiC discs will have non-linear V/R
characteristics. The resistance is less for surge voltages and very high for power
frequency voltages. During the surge voltages the LA discharges the energy to earth and
maintains the voltage across the its terminals to “residual voltage”. After discharge the
vacuum gap recovers and the follow current is reduced by non-linear resistance and
interrupted by vacuum gap.
Ordinary rod gaps create short circuit during the surge passage and the spark over
voltages are not accurate. Where as for SiC arrestors, they do not create any shot circuit
and their characteristics are accurate and repetitive. A surge counter and Leakage
monitor (ammeter) is connected to count the number of surges passed through the LA
and to measure the leakage current respectively.
ZnO Arrestors: The Zn0 surge arrestors do not have any gap and the resistor discs are of
superior sintered material with metal oxides (zinc oxide). The voltage / current
characteristics on log scale is nearly flat hence protective levels can be precisely selected
and insulation coordination becomes precise. Energy dissipation capacity is very high.
Number of zinc oxide discs are arranged one over above and kept in a porcelain housing.
The Zn0 arrestor components are given below:
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Zinc oxide discs
Insulator housing
Pressure relief diaphragm
Surge counter & Leakage monitor
Ratings of Surge arrestors: The ratings of surge arrestor for a 390 kV LA used in 400
kV transmission system is given below:
1. Rated voltage – Maximum permissible operating voltage between its terminal at
which LA is designed to perform its duty cycle – 390KV
2. Follow current – The RMS value of power frequency current which flows from
source after passage of discharge current -
3. Nominal discharge current – The surge current which flows through LA after
spark over, expressed in peak value for a specified way – 10KA of 8/20 Micro
Sec wave.
4. Power frequency / impulse spark over voltage – The value of power frequency
/ impulse voltage which causes spark over.
5. Residual voltage (discharge voltage) – The voltage that appears between the line
terminals and earth during the passage of discharge current – For a 1KA
switching surge current the max. and min. values of residual voltages are 780KVp
and 730KVp. 900KVp is the residual voltage for 10KAnominal discharge current.
6. Rated current – The maximum impulse current at which peak discharge residual
voltage is determined – 10 KA of 8/20 microsecond wave.
7. Maximum Continuous Operating Voltage (MCOV) - It is the highest power
frequency voltage the arrestor can continuously withstand expressed in RMS
value – 303 kV (RMS).
Insulators:
Insulators are used in the transmission system for various applications. We have various
types of insulators which are classified based on their applications.
Following are the types of insulators used in sub-station applications
1. Insulator strings: Disc insulator string or Long rod insulator string.
2. Bushings: These are used for housing the auxiliary CTs in the transformer or
reactor. And also to make the connections of transformer winding to the line.
3. Hollow Insulators: These are used as housings for CBs, CTs, CVTs, LAs etc.
4. Support Insulators: These are used for supporting the rigid Bus-Bars, to support
the wave trap and also to support the metalics of Isolator.
Let us discuss briefly about the insulator strings.
Disc insulator strings: Disc insulators are made up of glazed porcelain. These insulator
discs are connected in series and are having ball and socket (Pin and Cap) connection.
These insulators are either suspension or tension springs. Depending upon the application
of use in polluted areas the insulator strings can be classified as Anti-Fog type and
Normal type also. The anti-fog insulators are used in heavily polluted areas.
Long rod insulators: Instead of disc insulators with ball and socket connection, long
rod insulators have been developed recently to take care of the sub-station and line
insulator string requirements. 2 or 3 long rod insulators are used instead of one disc
insulator string.
Important Components of Disc Insulator string:
Shell: The insulator shell is made up of glazed electro-porcelain of brown colour for
porcelain type of disc insulators and of toughened glass for glass type insulators. The
glass insulators have the advantage of easy of identification of hair line cracks in
insulators during the service.
Pin and Cap: The pin and cap are designed such a way that the mechanical stresses are
transmitted to the shell to develop uniform mechanical strength in the string.
Zinc Sleeve: (Used in anti–fog insulator only) The Zinc sleeve is fused on the pin. It is
used to protect the pin against corrosion.
Security Clip: Security clip is of R-shaped hume type and is used to provide the positive
locking of the coupling.
Technical Parameters of Discs:
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Type of insulator – Disc insulators/Long rod insulators
Size of insulators – 255X145 mm
Electro mechanical strength – 120 kN
Creepage distance of individual insulator disc – 430 mm
Power Frequency puncture withstand voltage – 1.3 times actual wet flashover
voltage.
Technical Parameters of Insulator strings:
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Power frequency withstand voltage – 680 kV RMS
Lightning impulse withstand voltage - +/- 1550 KVp
Switching surge withstand voltage - +/- 1050 KVp
Minimum corona extinction voltage – 320 KV RMS
Total creepage distance – 10500 mm
Total number of discs per string – 25
For tension application, double insulator string and for suspension application, single
suspension insulator string shall be used.
Insulator strings used in transmission lines :
Single-I suspension insulator string – These insulators strings are used for Suspension
type towers.23 number of discs are used in one string having a mechanical strength of
120 kN.
Single suspension pilot insulator string – These insulators strings are used in the angle
tower for connecting jumpers and also in transposition towers. 23 number of discs are
used in one string having a mechanical strength of 120 kN.
Double tension insulator string – These insulators strings are used in tension towers.
2X23 number of discs are used in one string having a mechanical strength of 160 kN.
Tripple tension insulator string – These insulators strings are used in river crossing
anchor towers. 3X24 number of discs are used in one string having a mechanical strength
of 160 kN.
Single tension insulator string – These insulators strings are used in transposition
towers. 24 number of discs are used in one string having a mechanical strength of 120
kN.
Wave Trap Or Line Trap.
The wave trap is a series equipment in the transmission line. It is used for blocking the
Power Line Carrier frequencies from entering the sub-station. It is an inductive element
and the inductive element provides high impedance to the Carrier Frequencies (High
frequencies), hence the all the carrier waves are blocked by wave trap at the entrance of
the sub-station it self. A CVT which is connected prior to the wave trap is tuned to the
carrier frequencies and provides a low impedance path to the high frequency waves.
Hence the carrier communication waves which are blocked by wave trap are filtered by
CVT and are sent to the PLCC equipment through coupling device and co-axial cable for
further processing.
The line trap shall be broad band tuned for the entire range of carrier frequency i.e. 40 to
500KHZ range. A surge arrestor is provided to line trap to protect the line trap from the
surges. The line trap is also equipped with the bird barriers. The line trap is either
pedestal mounted or suspension mounted. For pedestal mounting the line trap is mounted
on a tripod structure formed by three insulator stacks arranged in a tri angular form.
Bus-Bars
Generally two types of materials are used for the bus-bars in the 400KV bus system. Fir
strung bus the material used is Quad-Moose conductor consisting of four Moose
conductors and for the Rigid Bus system the material used is 4” IPS Aluminium Tube.
Rigid Bus-Bar (Tubular Bus Conductors):
Aluminium of grade 63401WP (Range2) conforming to IS:5082 is used for rigid bus. The
rigid aluminium tube is mounted on the bus post insulators.
Technical Parameters:
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Size
Outer Diameter
Thickness
Cross Sectional Area
Weight
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4” IPS (EH Type) of Aluminium.
114.2 mm.
8.51mm.
2825.61mm.
7.7 Kg/mtr.
The phase to phase spacing of rigid bus bars is 8trs for 400KV system and the height of
bus-bar is at 8mtrs from ground.
Strung Bus:
Strung bus is formed by quad-moose conductor consisting four moose Conductors . It is
strung through the sub-station gantries.
Technical Parameters of ACSR Moose conductor:
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Stranding and wire diameter
Overall Diameter
Approximate weight
Minimum UTS
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54/3.53 mm Al + 7/3.53mm Steel.
31.77 mm.
2004 kg/km.
161.2 KN.