ArresterWorks Facts-001 Arrester Lead Length

8/8/2019 ArresterWorks Facts-001 Arrester Lead Length

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ArresterFacts-001ArresterWorks.com

Arrester Lead Length

The arrester leadlength issue is not anew one, but is stillroutinely

misunderstood. If astakeholder wants thehighest return ontheir arresterinvestment, then theleads connecting the

device should be asshort as possible.

This ArresterFactswill show you why!

This ArresterFacts applies to distribution arrester andmedium voltage arresters. For a similar effect onsubstation arresters, see ArresterFacts: SeparationDistance

Note: This is an excellent example of short lead lengths.

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Contents

Definition

Why does it matter?

How to Calculate LeadLength Effect

How to Identify RelevantLead Length

Zero Lead LengthConfiguration

Arrester lead length is of the utmostimportance when protecting underground

circuits also.



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Definition of Relevant Arrester LeadLength

Any section of the arrester leads that areelectrically in parallel with the protectedinsulation and conduct impulse current

during the lightning strike

Only relevant at impulse frequencies Only important for damageable insulation Can be inside or outside the tank The arrester rating has no effect on thelead length effect

Can be a ground lead or line lead

The voltage

across thedamageableinsulation isthe sum ofthe leadvoltages andarresterclampingvoltage

Arr

NotRelevant

DamageableTransformerInsulation

LighteningCurrentPath

Not Relevant


Not Relevant

Relevant

Relevant

Not Relevant


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Why Does it MatterThe purpose of an arrester is to limit the voltage stress on theinsulation electrically in parallel with it during a surge event.

The voltage stress on the insulation in parallel with the arresteris the sum of the arrester residual voltage and the inductive

voltage of the leads that are also in parallel with the protectedinsulation.

The longer the leads, the higher the voltage created by theleads during the surge.

The higher the stress on the insulation, the higher theprobability of equipment failure.

During a surge, the voltage stress onthe transformer insulation is

Vtran = VL1 + Varr + VL2

It should be obvious then that if VL1 and VL2 can be minimized,then the stress on the transformer insulation will be minimized.


Arrester

Varr Vtran

VL1

VL2


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How to Calculate the Effect

Varr is a function of the surge current magnitude

, surgecurrent wave shape and VI characteristics of the arrester

VL1 and VL2 are a function of the rate of rise of the surge

current, and the inductance of the lead.For an accurate value, the total voltages is not anarithmetic sum since the max values of each voltage donot occur at the same time.


Arrester

Varr Vtran

VL1

VL2


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Example: In this example, the line leads generate 29kV crest voltage and the arrestergenerates 32kV crest and the total voltage across the protected device is 54kV.

Note that theinductive voltageof the lead ismaximum wherethe rate of riseof the current isthe highest

Note that sincethe crestvoltages are at

different times,the total voltageis not a simplesum.



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Approximate Calculation for the Lead onlyThe approximate lead voltage (kVp) that can be addedto the arrester residual voltage is

VLead = L/unit * Length * (di/dt) * KVLead = kV crest

L/unit = .34uh/ft or .94uh/m (typical)Length = feet or meters (total length of lead)di/dt = crest surge current kA/ time to crest in sK =2.0 = compensation for inaccurate di/dt-------------------------------------------------------Example2.5 feet of lead , 31kA surge that crests in 2.5 s

VLead=.34 * 2.5* (31/2.5) * 2.0Vlead = 21.08 kV-------------------------------------------------------

The arrester voltage ( Varr)for the rating of arrester usedand the current level used comes directly from thecatalog section of the arrester supplier.


Arrester

Varr Vtran

VL1

VL2


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A moreaccurate

calculation

By downloading

the InteractiveLead LengthEffect CalculatorfromArresterWorks,one cangenerate agraphic output

similar to theone seen here.This excel basedcalculator usesthe exact di/dt.



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Basic Rule to Follow

The lead length that isin parallel with the

protected insulation andconducts the surgecurrent during a strikewill be the relevant leadlength.

The red lines

indicate where

the lead should

be counted in

the lead length



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Surge

ZERO LeadLength

Zero Lead Length ConfigurationIn the configuration below, two changes will turn this into a Zero Lead LengthConfiguration.1. Move the line lead from the top of the transformer to the top of the arrester.2. Add one more lead from the bottom of the arrester to the pole ground. (Do not

remove the lead between the arrester base and the transformer ground.

Move the linelead from the topof the Trans tothe top of thearrester

Add asecondgroundlead to thepoleground.

Keep thetwo leadsfrom thearrester tothetransformer



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Thank you for using ArresterFacts

This ArresterFacts is just one of many that make up the ArresterFacts TutorialSeries on Arresters.

All ArresterFacts are Copyrighted.

If you use any part of this presentation for training material, please giveArresterWorks proper reference.

Thank you for using ArresterWorks as your source of information on Arresters.

Jon Woodworth


Date post:	09-Apr-2018
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ArresterWorks Facts-001 Arrester Lead Length

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