8/12/2019 L7-Over Voltage Protection
1/79
Over Voltage Protection
8/12/2019 L7-Over Voltage Protection
2/79
Protection
There are two areas of protection which
power engineers deal with
Over Current
Over Voltage
8/12/2019 L7-Over Voltage Protection
3/79
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.
8/12/2019 L7-Over Voltage Protection
4/79
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
8/12/2019 L7-Over Voltage Protection
5/79
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.
8/12/2019 L7-Over Voltage Protection
6/79
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
8/12/2019 L7-Over Voltage Protection
7/79
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
8/12/2019 L7-Over Voltage Protection
8/79
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.
8/12/2019 L7-Over Voltage Protection
9/79
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.
8/12/2019 L7-Over Voltage Protection
10/79
Insulation curve
8/12/2019 L7-Over Voltage Protection
11/79
8/12/2019 L7-Over Voltage Protection
12/79
8/12/2019 L7-Over Voltage Protection
13/79
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.
8/12/2019 L7-Over Voltage Protection
14/79
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
8/12/2019 L7-Over Voltage Protection
15/79
Proper Overvoltage protection if Lightning strikes the bus
If you have an overhead shield wire
8/12/2019 L7-Over Voltage Protection
16/79
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.
8/12/2019 L7-Over Voltage Protection
17/79
8/12/2019 L7-Over Voltage Protection
18/79
8/12/2019 L7-Over Voltage Protection
19/79
8/12/2019 L7-Over Voltage Protection
20/79
8/12/2019 L7-Over Voltage Protection
21/79
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
8/12/2019 L7-Over Voltage Protection
22/79
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.
8/12/2019 L7-Over Voltage Protection
23/79
8/12/2019 L7-Over Voltage Protection
24/79
8/12/2019 L7-Over Voltage Protection
25/79
8/12/2019 L7-Over Voltage Protection
26/79
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.
8/12/2019 L7-Over Voltage Protection
27/79
8/12/2019 L7-Over Voltage Protection
28/79
8/12/2019 L7-Over Voltage Protection
29/79
8/12/2019 L7-Over Voltage Protection
30/79
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
8/12/2019 L7-Over Voltage Protection
31/79
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
8/12/2019 L7-Over Voltage Protection
32/79
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
8/12/2019 L7-Over Voltage Protection
33/79
8/12/2019 L7-Over Voltage Protection
34/79
8/12/2019 L7-Over Voltage Protection
35/79
8/12/2019 L7-Over Voltage Protection
36/79
8/12/2019 L7-Over Voltage Protection
37/79
8/12/2019 L7-Over Voltage Protection
38/79
8/12/2019 L7-Over Voltage Protection
39/79
8/12/2019 L7-Over Voltage Protection
40/79
8/12/2019 L7-Over Voltage Protection
41/79
8/12/2019 L7-Over Voltage Protection
42/79
8/12/2019 L7-Over Voltage Protection
43/79
8/12/2019 L7-Over Voltage Protection
44/79
8/12/2019 L7-Over Voltage Protection
45/79
8/12/2019 L7-Over Voltage Protection
46/79
8/12/2019 L7-Over Voltage Protection
47/79
8/12/2019 L7-Over Voltage Protection
48/79
8/12/2019 L7-Over Voltage Protection
49/79
8/12/2019 L7-Over Voltage Protection
50/79
8/12/2019 L7-Over Voltage Protection
51/79
8/12/2019 L7-Over Voltage Protection
52/79
8/12/2019 L7-Over Voltage Protection
53/79
8/12/2019 L7-Over Voltage Protection
54/79
8/12/2019 L7-Over Voltage Protection
55/79
8/12/2019 L7-Over Voltage Protection
56/79
8/12/2019 L7-Over Voltage Protection
57/79
8/12/2019 L7-Over Voltage Protection
58/79
8/12/2019 L7-Over Voltage Protection
59/79
8/12/2019 L7-Over Voltage Protection
60/79
8/12/2019 L7-Over Voltage Protection
61/79
8/12/2019 L7-Over Voltage Protection
62/79
8/12/2019 L7-Over Voltage Protection
63/79
8/12/2019 L7-Over Voltage Protection
64/79
8/12/2019 L7-Over Voltage Protection
65/79
8/12/2019 L7-Over Voltage Protection
66/79
8/12/2019 L7-Over Voltage Protection
67/79
8/12/2019 L7-Over Voltage Protection
68/79
8/12/2019 L7-Over Voltage Protection
69/79
8/12/2019 L7-Over Voltage Protection
70/79
8/12/2019 L7-Over Voltage Protection
71/79
8/12/2019 L7-Over Voltage Protection
72/79
60 Hz peak
8/12/2019 L7-Over Voltage Protection
73/79
8/12/2019 L7-Over Voltage Protection
74/79
We can see that for the same spacing a rod gap flashes over at a lower
8/12/2019 L7-Over Voltage Protection
75/79
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
8/12/2019 L7-Over Voltage Protection
76/79
8/12/2019 L7-Over Voltage Protection
77/79
8/12/2019 L7-Over Voltage Protection
78/79
8/12/2019 L7-Over Voltage Protection
79/79
Notes from John Paserba - Mitsubishi
Top Related