l7-over voltage protection

8/12/2019 L7-Over Voltage Protection

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Over Voltage Protection


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Protection

There are two areas of protection which

power engineers deal with

Over Current

Over Voltage


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We all have had dealings with over current

protection if fact we have a whole group that dealswith it.

But very few of us deal with over voltageprotection. Why well simplistically currents are afunction of load and impedance of the system andthat can be pretty much anything

But even though there appears to a lot of voltagesin reality there are only about 20 to 30 whichmeans once you have provided protection forthese voltages you can use it again and again anddont have to re engineer. Example for 12kv weuse insulation rated 110kV BIL and an arresterrated 10kV. This means the 12kV at State Collegeuses the same over voltage protection as the 12kVat Parkersburg or Colorado for that matter.


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Over voltage Protection verses

Insulation coordination

This course was titled insulation

coordination not over voltage protection

That is true but insulation coordination is asubset of over voltage protection. So what

we are going to learn is the entire area of

over voltage protection.

What is insulation coordination


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Insulation Coordination

Insulation Coordination developed before

arresters were developed.

Insulation coordination basically is that youwant the cheaper insulation to fail before

the more expensive insulation and in doing

so its failure shorts out the over voltage and

thereby protects the more expensive

insulation.


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Example

Air insulation is cheaper than paper and oil insulation usedin transformers, not only that but after a failure of airinsulation and the over voltage and power follow current isremoved, fresh air re establishes the insulation level so thecircuit can be re energized. If an insulator flashes over it is

the air that broke down. If this insulator is next to atransformer, the air insulation broke down (creating a faultand shorting the over voltage to ground ) before theinsulation in the transformer failed. Therefore the failureof the insulator protected the more expensive transformer.

And when the fault was cleared fresh air went around theinsulator and everything could be returned back to normal


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In the early days this was all they had so proper insulationcoordination was necessary

But when arrester technology started to appear, arresterscould protect both the insulator and the transformer so itwas not as important to remember the method of insulationcoordination.

In fact it became just a standard that on a given voltage

you place a given arrester and things will work. Asarresters became better even manufactures tended to loosesite of the insulation coordination principles and justdesigned their equipment to be protected by arresters.

However there are problems with that. For example you

want the phase to ground insulation on a switch or breakerto fail before the phase to phase insulation. As this

provides protection to workers working past an openswitch or breaker


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So for us to understand completely we need

to know proper insulation coordination and

proper application of surge arresters. Thus

we need to know over voltage protection.


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Lets look at insulation

Insulation whether air, oil, paper, varnish,

vacuum, silicon used in electronics all has

the same type of pattern. That is shorter the

time of the over voltage is applied to the

insulation the greater the value of that over

voltage the insulation can withstand.


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Insulation curve


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So our insulation system has to be able to withstand

1. Normal system voltage continuously

2. Over Voltages produced by transients or the system. These include:

A. Lightning

B. Switching

1. Cap switching

2. Faults

3. Long Line Switching

C. Ferroresonance

D. High system voltage

Many of these overvoltages can be controlled in magnitude by

grounding, switching resistors, synchronous close,etc. Or the

insulation must be designed to handle the maximum overvoltage or a

surge arrester applied to remove the overvoltage.


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Proper Insulation Coordination

If lightning strikes the phase conductor and it

generates enough voltage to flash the insulation I

want it flashing over the cheapest, self restoring

insulation first and I want it to go to ground


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Proper Overvoltage protection if Lightning strikes the bus

If you have an overhead shield wire


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If Lighting strikes you want it to hit the shield wire.

For a switching surge you want it to be removed by the surge Arrester

Otherwise you want your insulation to be coordinated so that it flashes

the cheapest self restoring insulation first.


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Types of Insulation

Insulation can be described as a dielectric

with the job to preserve the electrical

integrity of the system.

Insulation can be in

A. Internal

B. External


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Internal Insulation

Is the internal solid, liquid, or gas elementsof the insulation of the equipment, whichare protected from the effects of

atmospheric and other external conditionssuch as contamination, humidity, andanimals.

Transformer insulation, cable insulation,gas insulated substation, dielectric fluid incapacitors, oil, etc.


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External Insulation

Air insulation and the exposed surfaces of solid

insulation equipment, which are both subjected to

dielectric stresses and to the effects of atmospheric

and other external conditions such ascontamination, humidity, and animals.

Examples are Bushings, bus support insulators,

switches, air, etc.

Can be affected by the environment by such things

as rain, altitude, winds, dirt, etc.


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Characteristics of Insulation

Strength

Non Self Restoring- An insulation that

losses its insulating properties or does not

recover them completely, after a disruptive

discharge. Paper such as on a transformer

winding. Under ground cable insulation

Self Restoring- Insulation that completely

recovers its insulating properties after adisruptive discharge. Air


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Internal insulation is typically non selfrestoring insulation and is usually defined in

terms of Convention withstand

External insulation is typically self restoring

insulation and is usually defined in terms of

Statistical withstand


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How do you define insulation

strength

Conventional- The strength of the insulation

described in terms of the voltage it is able to

withstand without failure or disruptive discharge

under specified test conditions. Statistical- The strength of the insulation described

in terms of the voltage it is able to withstand with

a given probability of failure or disruptive

discharge under specified test conditions


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60 Hz peak


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We can see that for the same spacing a rod gap flashes over at a lower


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We can see that for the same spacing a rod gap flashes over at a lower

voltage than a sphere gap. So geometry makes a difference.

So just because the spacing to ground is less than the spacing line to load

on an air switch does not necessarily mean that the switch whenimpulsed will flash to ground first(we want it to), it depends on how the

switch components look. If line to load looks more like a rod gap and

line to ground looks more like a sphere gap then it is possible even

though the switch as a greater line to load spacing than line to ground it

will flash line to load first. The only way to be sure is to test it.

Spacing here is greater than distance here

So you would think it

will flash here first, but

it depends on how the

switch looks.

This could look like a sphere

gapThis could look like a rod gap


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Notes from John Paserba - Mitsubishi

l7-over voltage protection

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