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
  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
  dont 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.

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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 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

• 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.

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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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Lets 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
• Normal system voltage continuously
• 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 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

• 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.

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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 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

• 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 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 sphere gap
This could look like a rod gap
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Notes from John Paserba - Mitsubishi