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Transient Protections
Part I
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Effective transient suppression requires
that the impulse energy is dissipated in the
added suppressor at a low enough voltageso the capabilities of the circuit or device
are not exceeded.
onsemi AN843
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When the fuse blows out,
voltage at load1 goes high due
to inductive kick due to di/dt.
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Due to capacitance across primary and secondary,
huge surge can appear at the secondary.
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If there is a break at the primary when thetransformer has reached max flux (at Zero
Cross condition), there can be a huge
transient.
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WAVESHAPES OF SURGE
VOLTAGES
Indoor Wave shapes:
Measurements in the field, laboratory, and theoretical
calculations indicate that the majority of surge
voltages in indoor low-voltage power systems havean oscillatory wave shape.
This is because the voltage surge excites the natural
resonant frequency of the indoor wiring system.
These surges are oscillatory, can have different
amplitudes and wave shapes in the various places of
the wiring system.
The resonant frequency can range from about 5 kHz to over
500 kHz. 100 kHz is most typical.
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0.5 s 100 kHz Ring Wave
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Outdoor Locations
Both oscillating and unidirectional
transients have been recorded in outdoor
environments (service entrances and other
places nearby).
Unidirectional Wave Shapes
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Peak Surge Voltage versus Surges per Year
EIA paper, P587.1/F, May, 1979, Page 10
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The low exposureportion of the set of
curves is data
collected from
systems with littleload-switching activity
that are located in
areas of
light lightningactivity.
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Medium exposuresystems are in
areas offrequent
lightning activity
with a severeswitching
transient problem.
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High exposure
systems are rare
systems supplied by
long overhead lines
which supply
installations that
have high spark
over clearances and
may be subject toreflections at power
line ends.
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When using graph in the figure in previous
slides, it is helpful to remember that peak
transient voltages will be limited to
approximately 6 kV in indoor locations due
to the spacing between conductors using
standard wiring practices.
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Where the transient energy will go?
The energy contained in a transient will be dividedbetween the transient suppressor and the sourceimpedance of the transient in a way that is determinedby the two impedances.
With a spark-gap type suppressor, the low impedance ofthe Arc after breakdown forces most of the transientsenergy to be dissipated elsewhere, e.g. in a currentlimiting resistor in series with the spark-gap and/or in thetransients source impedance.
Voltage clamping suppressors (e.g. zeners, MOVs,rectifiers operating in the breakdown region) on the otherhand absorb a large portion of the transients surgeenergy.
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Three categories of service locations
Category I: Outlets and circuits a long distance from
electrical service entrance. Outlets more
than 10 meters from Category II or more
than 20 meters from Category III (wire
gauge #14 #10)
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Three categories of service locations
Category II: Major bus lines and circuits a short
distance from electrical service entrance.
Bus system in industrial plants.
Outlets for heavy duty appliances that are
close to the service entrance.
Distribution panel devices.
Commercial building lighting systems.
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Three categories of service locations
Category III Electrical service entrance and outdoor
locations.
Power line between pole and electrical
service entrance. Power line between distribution panel and
meter.
Power line connection to additional near-by
buildings. Underground power lines leading to pumps,
filters, etc.
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Surge Voltages and Currents Deemed to
Represent the Indoor Environment
Depending Upon Location
Notes:
1. Open Circuit voltage
2. Discharge current of the surge (not the short circuit current of the
power system)
3. The energy a suppressor will dissipate varies in proportion with the
suppressors clamping voltage, which can be different with different system
voltages (assuming the same discharge current).
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LIGHTNING TRANSIENTS
several mechanisms There are several mechanisms in which
lightning can produce surge.
1. Direct lightning strike to a primary (before
the substation) circuit. When this high current,that is injected into the power line, flowsthrough ground resistance and the surgeimpedance of the conductors, very large
transient voltages will be produced. 2. The lightning misses the primary power line
but hits a nearby object.
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LIGHTNING TRANSIENTS
several mechanisms 3.When a primary circuit surge arrester operates and
limits the primary voltage, the rapid dv/dt producedwill effectively couple transients to the secondarycircuit through the capacitance of the substation
transformer windings in addition to those coupled intothe secondary circuit by normal transformer action.
4. If lightning struck the secondary circuit directly, veryhigh currents may be involved which would exceedthe capability of conventional surge suppressors.
5. Lightning ground current flow resulting from nearbydirect to ground discharges can couple onto thecommon ground impedance paths of the groundingnetworks also causing transients.
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TRANSIENT SUPPRESSOR
TYPES
Carbon Block Spark Gap Oldest and most commonly used transient suppressor
in power distribution and telecommunication systems.
The device consists o
ftwo carbon block electrodesseparated by an air gap, usually 3 to 4 mils apart.
One electrode is connected to the system ground andthe other to the signal cable conductor. When atransient over-voltage appears, its energy isdissipated in the arc that forms between the twoelectrodes, a resistor in series with the gap, and alsoin the transients source impedance, which dependson conductor length, material and other parameters.
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TRANSIENT SUPPRESSOR
TYPES
Carbon Block Spark Gap
The carbon block gap is a fairly inexpensive
suppressor but it has some serious problems.
It has a relatively short service life
There are large variations in its arcing voltage. The
nominal 3 mil gap will arc anywhere from 300 to 1000
volts. This arcing voltage variation limits its use
mainly to primary transient suppression with moreaccurate suppressors to keep transient voltages
below an acceptable level.
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TRANSIENT SUPPRESSOR
TYPES
Gas Tubes
It is made of two metallic conductors usuallyseparated by 10 to 15 mils encapsulated in a
glass envelope which is filled with severalgases at low pressure.
Gas tubes have a higher current carryingcapability and longer life than carbon block
gaps. The possibility of seal leakage and the
resultant of loss protection has limited the useof these devices.
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TRANSIENT SUPPRESSOR
TYPES
Selenium Rectifiers
These do not have the voltage clamping
capability of zener diodes.
This is causing their usage to become more
and more limited.
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TRANSIENT SUPPRESSOR
TYPES METAL OXIDE VARISTORS (MOVs)
An MOV is a non-linear resistor which is voltagedependent and has electrical characteristics similar toback-to-back zener diodes.
It is made up of metal oxides, mostly zinc oxide withother oxides added to control electricalcharacteristics.
When constructing MOVs, the metal oxides aresintered at high temperatures to produce a
polycrystalline structure of
conductive zinc oxideseparated by highly resistive inter-granularboundaries. These boundaries are the source of theMOVs non-linear electrical behavior.
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METAL OXIDE VARISTORS (MOVs)
The MOV polycrystalline body is usually formed into
the shape of a disc.
The energy rating is determined by the devicesvolume, voltage rating by its thickness, and current
handling capability by its area.
The energy dissipated in the device is spread
throughout its entire metal oxide volume, Hence they are well suited for single shot high power
transient suppression applications where sharp
clamping capability is not required.
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METAL OXIDE VARISTORS (MOVs) The major disadvantages are that they can only
dissipate relatively small amounts of average powerand are not suitable for many repetitive applications.
Another drawback with MOVs is that their voltageclamping capability is not as good as zeners.
Perhaps the major difficulty with MOVs is that theyhave a limited life time even when used below their
maximum ratings. For example, a particularMO
V witha peak current handling capability of 1000 A has alifetime of about 1 surge at 1000 A pk, 100 surges at100 A pk and approximately 1000 surges at 65 A pk.
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TRANSIENT SUPPRESSION
USING ZENERS
Zener diodes exhibit a very high impedance
below the zener voltage (VZ), and a very low
impedance above VZ.
Because of these excellent clippingcharacteristics, the zener diode is often used to
suppress transients.
Zeners are intolerant of excessive stress so it is
important to know the power handling capability
for short pulse durations.
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TRANSIENT SUPPRESSION
USING ZENERS
Zener transient suppressors are designed totake large, short duration power pulses.
This is accomplished by enlarging the chip and
the effective junction area to withstand the highenergy surges.
The package size is usually kept as small aspossible to provide space efficiency in the circuit
layout, and since the package does not differgreatly from other standard zener packages, the
steady state power dissipation does not differgreatly.
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ZENER versus MOV TRADEOFFS
The clamping characteristics of Zeners
and MOVs are given for the following
transient waveform.
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Better
clamping is
seen forthe Zener