Over Voltage Protection
Protection
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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 deals
with it.
But very few of us deal with over voltage
protection. Why well simplistically currents are a
function of load and impedance of the system and
that can be pretty much anything
But even though there appears to a lot of voltages
in reality there are only about 20 to 30 which
means once you have provided protection for
these voltages you can use it again and again and
don’t have to re engineer. Example for 12kv we
use insulation rated 110kV BIL and an arrester
rated 10kV. This means the 12kV at State College
uses the same over voltage protection as the 12kV
at Parkersburg or Colorado for that matter.
Over voltage Protection verses
Insulation coordination
This course was titled insulation
coordination not over voltage protection
 That is true but insulation coordination is a
subset 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 you
want 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
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Air insulation is cheaper than paper and oil insulation used
in transformers, not only that but after a failure of air
insulation and the over voltage and power follow current is
removed, fresh air re establishes the insulation level so the
circuit can be re energized. If an insulator flashes over it is
the air that broke down. If this insulator is next to a
transformer, the air insulation broke down (creating a fault
and shorting the over voltage to ground ) before the
insulation in the transformer failed. Therefore the failure
of the insulator protected the more expensive transformer.
And when the fault was cleared fresh air went around the
insulator and everything could be returned back to normal
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In the early days this was all they had so proper insulation
coordination was necessary
But when arrester technology started to appear, arresters
could protect both the insulator and the transformer so it
was not as important to remember the method of insulation
coordination.
In fact it became just a standard that on a given voltage
you place a given arrester and things will work. As
arresters became better even manufactures tended to loose
site of the insulation coordination principles and just
designed 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 breaker
to fail before the phase to phase insulation. As this
provides protection to workers working past an open
switch 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.
Let’s look at insulation
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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.
Insulation curve
Let’s understand where
overvoltages come from
First you have normal voltages, which
means that the insulation system has to
withstand these stresses continuously.
 Then you have overvoltages that are caused
by a number of things such as transients, 60
Hertz overvoltages. Etc.
 We covered a lot of what causes
Overvoltages in the Transient class
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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.
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
Proper Overvoltage protection if Lightning strikes the bus
If you have an overhead shield wire
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.
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 elements
of the insulation of the equipment, which
are protected from the effects of
atmospheric and other external conditions
such as contamination, humidity, and
animals.
 Transformer insulation, cable insulation, gas
insulated substation, dielectric fluid in
capacitors, oil, etc.
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External Insulation
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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 as
contamination, 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.
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 a
disruptive discharge. Air
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Internal insulation is typically non self
restoring 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
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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
60 Hz peak
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 when
impulsed 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 rod gap
This could look like a sphere
gap
Notes from John Paserba - Mitsubishi