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Long Flashover Arresters

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Long Flashover Arresters


What I’ll be presenting…
    

Introduction to lightning and lightning arresters What are Long Flashover Arresters? Why do we need them? How do they work? What are their types?


What is a lightning stroke?

It is an electric discharge between
◦ Cloud and earth ◦ Cloud and cloud ◦ Charge centers of the same cloud


Lightning stroke waveform


Types of lightning strokes
 

Direct Stroke Indirect Stroke


What does lightning do?

Strikes live power line jumps across insulator to reach grounded tower. Lightning channel across insulator acts as conductor causing a short circuit. Man-made power from live wire flowing through section of old lightning channel in an intensely bright arc.


Lightning arresters

A device used on power systems above 1000 V to protect other equipment from lightning and switching surges Under normal operation the lightning arrester is off the line On the occurrence of over voltage air insulation across gap breaks down and arc is formed providing a low resistance path for the surge to the ground


Metal Oxide Lightning Arrester

The metal oxide varistor (MOV) contains a material, typically granular zinc oxide, that conducts current (shorts) when presented with a voltage above its rated voltage They exhibit an extremely high resistance during normal operation and a very low resistance during overvoltages

What are LFAs?

A new simple, effective and inexpensive system for lightning protection of overhead distribution lines

Pole Top Arresters LFAs

Costly; Easily Destroyed Cheaper; Cannot be Destroyed


Need for LFAs

Prevent Damage to Power Lines by Lightning Strike

Enhance the Reliability Performance of power lines

Reduce Maintenance Costs of Power lines


How do LFAs Work?

Basic Principle

E Probability of Flashover
 

α Vnom α 1/L αE

(Nominal Voltage)
(Length of flashover path)

In short, Probability of Flashover

α Vnom/L

Mean Gradient of Operational Voltage = Vph / L (Vph = Vnom/√3)

As L ↑, Mean Gradient of Operational Voltage ↓ and hence

probability of Flashover↓

Types of LFA

There are three types of LFAs:
1. 2.

LFA-M or Modular LFA-L or Loop


LFA-IT or Insulation Tube


LFA - M Construction

Two cable like pieces with semiconductive core of resistance R Arranged to form three flashover modules 1, 2 and 3


LFA - M Construction

a) Block diagram of LFA-M

b) Electrical schematic of LFA-M


 

The total voltage is applied to each flashover module at the same moment All three flashover modules are assured conditions for simultaneous initiation of creeping discharges which develop into a single long flashover channel. As the line conductor is stressed by a lightning overvoltage impulse, flashover channels develop at different rates forming a rather long flashover channel along the LFA Flashover does not give rise to a power arc


LFA - M Testing
V = atb Where V - flashover voltage in kV t - time to crest in seconds a & b – experimentally determined empirical constants

Test Object INS 130 INS 130 INS 160 INS 160 LFA –M L = 1m LFA –M L = 2m LFA –M L = 0.8m LFA –M L = 0.8m

Impulse Polarity + + +/+/+ -

a 190 185 243 280 109 173 159 107

b -0.352 -0.285 -0.407 -0.28 -0.784 -1.05 -0.5 -1.64

  

A piece of a steel rod covered with insulation is bent in a loop and connected to the pole with a clamp Loop and tube is at the same potential as the pole An overvoltage which appears between the line conductor and the pole will be also applied between the metallic tube and the line conductor Due to the overvoltage a surface flashover develops from the metallic tube to one or both ends of the insulated loop and then to the pole thus completing the discharge circuit Flashover channel is broken into serially connected pieces of channels(intermediate electrodes have protrusions on opposite sides) and due to this reason arc quenching is facilitated


Long Flashover Arrester in a form of a loop installed on a distribution line cross-arm a) Distribution Line b) Long Flashover Arrester
1.Insulated steel loop 2.Clamp 3.Steel cross-arm 4.Metallic tube 5.Power line conductor 6.Flashover channel 7.Insulator 8.Structure


Long Flashover Arrester in a form of a loop installed on a distribution line cross-arm


Installation of LFA - L



Under overvoltage conditions first spark over gap S flashes over and then a discharge channel travels along the tube until it reaches the line conductor. The arrester consists only of an insulation tube and a spark over horn. Spark-over horns are necessary for insuring flashovers of LFA-Insulation Tubes placed at different supports.



Figure: Long Flashover Arrester in a form of an insulation sleeve
1.Power Line Conductor 2.Insulation Tube 3.Metal Tube Clamp 4.Insulator 5.Structure 6.Flashover Channel 7.Sparkover Horn


Figure: LFA – IT Cross Section 1.Conductor 2.Air Gap 3.Insulation Material

Technical Specifications of LFA


Advantages of LFA
      

Help avoid conductor burnouts and overhead power lines outages caused by lightning induced overvoltages. Eliminate the consequences of lightning flashovers without damaging lines and substations equipment. Extend the working life of high voltage circuit breakers. Protect electric networks against arc overvoltages. Can’t be destroyed by lightning currents and power follow currents as currents are flowing outside of LFAs. Not stressed by operational voltage and don’t require maintenance. Do not require reduced grounding resistance of poles on which they are mounted


Disadvantages of LFA

Efficiency can be improved only by increasing the number of flashover modules

It can withstand very high over voltages only on increasing module number


Future of LFA

Number of flashover modules can be increased and as a result  LFA can be used for lightning protection of very high voltage lines.  Total arrester stressing is distributed and it can withstand very high over voltages



Is a relatively new technology  It is cheap, reliable and highly effective  Used widely in power systems in Russia  It is now being implemented in Brazil  Field experience with 10 kV LFAs very successful  Work on developing 13.8 kV LFAs





Podporkin G.V, Pilshikov V.E, Sivaev A.D; “LIGHTNING

MODULAR LONG-FLASHOVER ARRESTERS”; IEEE Transaction on Power Delivery;Volume 18, Issue 3, Page(s): 781 – 787; July 2003

Orsino Oliveira Filho, Darcy Ramalho de Mello and Gloria Suzana Gomes de Oliveira; “TESTING OF LONG FLASHOVER ARRESTERS DESIGNED FOR DISTRIBUTION LINES”; Ninth International Symposium on Lightning Protection; Nov,2007

DISTRIBUTION LINES BY LONG FLASHOVER ARRESTERS”; IEEE Transactions on Power Delivery;Volume 3, Issue 3; pp: 814-823; July 1998

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