EPN, Hyper Electronics, "TVSS (Transient voltage surge suppression) what is it and why all of a sudden do we need it?", 17 May 2001.
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Titre : EPN, Hyper Electronics, TVSS (Transient voltage surge suppression) what is it and why all of a sudden do we need it?, 17 May 2001.
Cité dans : [DIV039] Liste de revues diverses en électronique, avril 2016.
Cité dans : [PAP312] EPN, Hyper Electronics, Selecting The Optimum voltage transient suppressor to ensure suppressors can tolerate high energy levels for short periods, 1 June 2001.
Auteur : EPN
Auteur : Hyper Electronics
Site : http://www.hyperelectronics.com
Site : http://www.hyperelectronics.com/servlet/be.xm.ebi.website.servlets.CommunityServlet?comm=E1&id=117398&cmd=showArticle
Date : 17 May 2001
Group : Linear ICs
Introduction :
Transient voltage surge suppression has been essential ever since
the modern electricity distribution system was put in place. For
many years the effect of transient voltage surges was not recognised
even though equipment damage did occur in many cases. This is
especially the case over the last ten years, as mains powered
electronic equipment has become much more sophisticated and
susceptible to transients. In addition, although the number of power
outages is lessening, transient surges have increased, as have the
effects of harmonic distortion on the supply.
Auteur : M.E. Burgoyne - Chief Engineer - Advance-Galatek
Up to 80% of transients are generated from internal sources such as
inductive load switching and normal equipment operations. These can
cause cumulative damage, premature equipment failure, data losses,
system resets, and down time. Illustrating this statistic, tests
have shown that a twin four-foot fluorescent light fitting can
generate 24x1200volt transients when it is switched off. Secondly,
just imagine a high tech data storage company geographically close
to and on the same power distribution branch as Fred doing some arc
welding in his garage. The potential repercussions are obvious,
resulting in equipment faults that can range from temporary glitches
to the evaporation of components, contacts and PCB track. The
remaining 20% of transients are generated from external sources such
as lightning and power company grid switching. Effects range from
catastrophic equipment failure with immediate operation shutdown, to
long term disruption of business and expensive equipment repair and
replacement costs.
The impulse type has a large voltage and current over a short
duration i.e. lightning strike. The oscillating transient has lower
value voltage and current but over much longer time period (up to 50
times that of lightning strike), e.g. photocopiers, SCR. controlled
equipment, inductive load switching. Lightning strikes, or faults on
power distribution cables can create direct coupling. Inductive
coupling can be caused by electromagnetic field generated when
lightning hits tall objects such as trees. Capacitive coupling
occurs when a lightning strikes building lightning conductors. Also,
resistive coupling happens when ground strikes raise the ground
voltage which, in turn, causes large potential differences between
earth points. The most commonly discussed transient voltage surge is
a lightning strike, although a lightning strike often comprises up
to 20 strikes. Both cloud to cloud and nearby ground strikes produce
electrical field voltages of hundreds to thousands of volts per
metre. A cloud-to-cloud flash can induce a surge of 7,000 volts per
metre in power and/or telephone cables and 70volts per metre a mile
away. An incidence of two strikes per square mile per year would
seem to be an average figure. As a result of lightning activity, a
power surge of between 10,000 to 20,000 volts could reach your
building. However the maximum normally considered is 6000V, with
currents up to 3,000A appearing at your building main power
distribution board.
The impedance of the cabling installed on site limits the amount of
current that reaches your equipment. The main low impedance bus bar
could carry the full 3000A, but the 30A twin & earth feeding a spur
presents a much higher impedance that will limit the current to
around 200A for the same transient. Inevitably, insulation breakdown
in cabling will limit the level of transient surge voltage within
the building wiring. The effect of a transient surge on a circuit is
not only dependent on size but also where it hits on the AC cycle. A
200V transient appearing on the sine wave at zero crossover will
have little effect but the same transient appearing on the peak of
the sine wave will add 200V to the peak value of the sine wave
value. In this example, the load will be subjected to a voltage of
525Vac. And remember, transient surges can be positive or negative
going.
Calculating the actual effect of a transient over voltage can be
done by looking at the way a capacitor or inductor responds to it.
In a capacitor any rapid change across the capacitor will produce a
large current, which is dependent on the size of capacitor and rate
of voltage change. Its effect can be expressed by the following
formula.
I = C dv/dt
A rapid change in current in an inductor will cause a large
transient voltage to be generated, which can be calculated using the
following formula.
V = - L di/dt
The shape of a transient surge is normally expressed with two
numbers. For the impulse transient, the first number indicates the
rise time and the second the duration, for example 8x20
microseconds. For an oscillatory transient, the first number shows
the rise time and the second represents the frequency, i.e 5/100
microseconds/kHz.
In the real world your equipment will see far more longer duration
transient surges than the short ones caused by lightning, but of
course, testing must be done for both. For short duration testing,
up to a 6kV, 1.2/50 voltage waveform can typically be applied into
an open circuit with the TVSS device across it. Here, the voltage
reaches 90% of its peak in 1.2µs and then decays to 50% of its value
in 50µs. In essence, a TVSS device is a component that limits the
amount of energy from a transient. The three basic types are the gas
discharge tube (GDT), the metal oxide varistor (MOV) and the silicon
avalanche diode (SAD).
These devices modify an uncontrolled flashover by the use of
specially designed electrodes in a tube containing one or more
gasses under pressure, for different breakdown voltages ranging from
100V to several kV. Once the GDT fires it becomes a crowbar device
that can reduce the applied voltage down to a few tens of volts in
nanoseconds, and is able to handle surge currents of 20kA or more
for a single 8X20µs transient surge. The arc may not be extinguished
once the transient has passed if the normal line voltage exceeds
10-15volts, distorting the normal signal or even leading to the tube
destruction.
MOVs are made from zinc oxide fragments compressed under very high
pressure. Their resistance decreases as the applied voltage
increases, providing clamping of an applied transient surge.
Although the in circuit response time is 35-50ns, the resistance
characteristic is non linear, and the volt drop across the MOV will
increase dramatically as the current increases. A 20mm MOV
specification may claim 6500A peak surge current, but this will only
be for a single 8/20µs short circuit transient. As the peak current
of the 8/20µs transient surge decreases, the quantity of surges the
MOV can take will increase. However a single long duration current
waveform of much lower peak current value can cause the MOV to fail.
This could be as low as a 100-200A surge current pulse lasting 1ms.
These semi-conductors are similar to very large junction zener
diodes, responding very rapidly to transient surges with an in
circuit response time equal to or less than 5ns (dependant on
circuit inductance) and can therefore handle the rapid rise time of
a transient surge much better than an MOV. Clamping voltages range
from a few volts to several hundred volts. However, the clamping
voltage selected should be as close to the peak value of the sine
wave as possible without continually conducting. This subjects the
device to high levels of transient energy that single diodes cannot
dissipate, requiring numerous diodes so that the energy can be
dissipated without the devices sacrificing themselves. This means
that the suppressor will be larger and more expensive than using
MOVs. Unlike MOVs, silicon avalanche diodes can conduct their
maximum current without any increase in clamping voltage.
When designing an effective suppression unit, one could consider
using a combination of SADs and MOVs, because SADs alone would be
far too costly for a unit designed to take the same amount of
energy. The presence of SADs will prevent the clamping voltage
rising as the MOVs take more current. Because the SADs react so much
quicker, there is a danger that they will be subjected to over
current before the MOVs start to conduct. Experience dictates that
there must be enough SADs to take the current, without damage.
Putting a number of MOVs in parallel does not give a total current
figure equal to their sum. This is because of their typical
tolerance of +15%, giving a variation of 30%, plus the fact that
their voltage characteristics vary by +10% when they start to fail.
The current is not spread equally over all of the devices, an effect
that is compounded by the impedance of PCB track, component leads
and soldering.
Ideally, the level of protection provided by the suppression unit
should be dependent on where it will be installed within the
distribution system. A unit installed on the load side of the main
building isolation switch (zone C) will be subjected to much higher
voltage and current levels than one protecting individual rings or
distribution boxes (zone B) which, in turn, will see more than a
unit protecting individual spurs or equipment (zone A). A staged
approach should be preferred with units fitted to all three zones.
In diagrams 'A' and 'B' the devices are far less effective at
reducing a transient surges, because of the impedance of the
connecting wires. Connecting as 'C' or 'D' will give far better
results although in 'C' some losses will occur due to the
connections and PCB track. The TVSS devices are passive, lying
between the live and neutral, without posing a threat to the load
as, in the event of component failure, the wire and/or thermal fuses
will break the cross connection.
The 'C' TVSS unit as designed by Advance-Galatrek (TDC209J) uses
28x15KP 51V bi-directional SADs and 32x20mm 275L40 MOVs. The SADs
are arranged in four parallel strings of seven each with a surge
capability of 75kW for a 8x20µs surge and 15kW for a 10x1000µs
surge. The breakdown voltage is 396.9V which is close to the maximum
peak voltage of the mains, 373V (264Vac), with each string protected
by thermal and wire fuses. In the event of a single SAD going short
circuit, the breakdown voltage is reduced to 340V which, if
accompanied by high mains, means the string will conduct and heat up
causing the thermal fuse to blow. Under normal conditions, i.e.
nominal mains, one other SAD in the same string would need to fail.
The SADs used are capable of withstanding the standard test 6kV
surge indefinitely, and will receive a peak of 95kW for 10µs, and an
energy pulse of 1.5Joules over the 20µs which is within their rating
(75kW for 20µs =1.5Joules). The total rating for the SAD section is
42 Joules. The maximum Joule rating for the SADs is 15J (10x1000
µs), giving a maximum theoretical Joule rating of 420J. The MOV
section consists of 32 parallel devices, grouped in fours, with each
group protected by a wire fuse. This approach was taken because
parallel small diameter devices achieve larger current handling and
a lower clamping voltage than an equivalent single large diameter
device. In addition much greater redundancy is achieved by the
correct fusing of the parallel devices. Again, using a standard 6kV
2 Ohm surge the MOVs will clamp at around 700V, giving a total
current of 2650A i.e. 83A/MOV. This should give us a survival time
of 100,000 strikes and allowing for the failure of the weakest
string, the life expectancy should not be reduced with the remaining
devices taking 95A/MOV. The stated maximum Joule rating for the MOVs
is 140J (10x1000µs), which will give a maximum theoretical Joule
rating of 4,480J. If any of the fuses in the combined unit fail, an
alarm is raised, flagged by two bi-colour LEDs (Green ok/Red fail)
one for the SAD section and one for the MOVs, and a single pole
change over relay. Because the alarm circuitry and relay drive are
derived straight from the mains, a 8mm creepage and clearance
distance is maintained for the alarm contacts, including the relay,
a point that some manufacturers appear to overlook. The PCB layout
must provide the largest area of copper allowed by the board size to
conduct the transient surge, while keeping the current path as
straight as possible. This must be balanced against having enough
track-to-track clearance to prevent flash over, even with open
circuit fuses. Flash over can be further reduced by uniformly
coating the populated board. One last reminder for total protection,
the addition of TVSS devices to the supply line is not enough,
signal and communication lines should also be covered.
Source:
Advance-Galatrek
Advance Park
Wrexham LL14 3YR
UNITED KINGDOM
tel : +44-1978-821000
fax : +44-1978-810852
Url : http://www.ael.u-net.com/
Lien : "mailto:sales@aelgroup.co.uk"
Info :
- Experience with surge voltage protection devices
- ThinPak package for power modules and hybrids
- Transient Voltage Suppressors
- Selecting The Optimum voltage transient suppressor
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