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Abstract
The rapid growth of the world
market for telecommunications equipment will continue for the
next several years. Manufacturers of radio infrastructure and
switching equipment are based in the USA, Europe and Japan with
some minor exceptions. Yet, these manufacturers are often not
familiar with power quality in places far away from their home
countries. The needs for surge protective devices vary from
market to market and are especially challenging in the emerging
countries. Why? Surge protection in the USA and Europe is based
on the assumption that the ac power-line voltage is relatively
stable. Developing countries, however, often have inadequate
infrastructures for the generation and distribution of electric
power. The lack of stable ac power complicates the selection and
coordination of surge protective devices considerably. This
paper will review the new IEC surge protection standards and
discuss how to achieve coordination of surge protective devices
in accordance with IEC 1312 principles.
Introduction
IEC standards are used throughout
Europe and in many other parts of the world. CE marking is based
on such IEC standards. CE marking involves both equipment safety
and electromagnetic compatibility while UL only involves safety.
Thus, all electronic equipment that carry the CE label should
have been tested to withstand static electricity, electric fast
transients and surge voltages. This paper will focus on the
surge voltage aspects.
IEC 1000-4-5 is a combination
wave consisting of a voltage waveform of 1.2/50 µs with an
amplitude of 2000 V in normal mode and 4000 V in common mode,
and a current wave form of 8/20 µs at 2000 A. The combination wave is identical to the U.S. ANSI/IEEE C62.41-1991 Combination
Wave with the following exceptions: the ANSI/IEEE voltage is
6000 V and the current is 3000 A. In the IEC standard, the
equipment that is tested for surge immunity is referred to as
the "vulnerable equipment." Hence, the term
"vulnerable equipment" will be used in this paper. All
"vulnerable equipment" must be able to withstand IEC
1000-4-5 without upset, thus guaranteeing a certain level of
surge immunity, which will be sufficient for most office
environments. IEC has developed a series of standards that apply
to enhanced surge protection of equipment and structures in
cases where built-in equipment surge immunity is not sufficient.
Telecom installations in outdoor shelters and enclosures are
good examples of when enhanced surge immunity is required.
This paper will review key
aspects of the following IEC standards:
IEC 1024-1:
1990 Protection of Structures against Lightning—Part 1:
General Principles
IEC 1312-3:
Protection against Electromagnetic Impulse—Part 3:
Requirements of Surge Protective Devices.
Special
problems in developing countries
Fluctuating
power line voltage
Causes
Overloaded, inadequate power
generation and distribution systems, particularly when large
step load changes are present, result in large voltage sags and
surges. Voltage fluctuations between 155 to more than 310V have
been observed in the field. Differences between day- and
night-time voltage of 40V have also been observed.
Such conditions are common in
Africa, India, Pakistan and China, but can be found in many
other areas. Additional power plants and distribution
infrastructure is needed in many developing countries. Timely
power plant expansion is prevented by political and economic
issues, which means the problem is here to stay for the
foreseeable future.
Effects
Power supplies can fail for a
variety of reasons related to voltage fluctuations, the most
common ones are listed below:
-
Sags and
undervoltages
can cause component overheating or destruction, which
reduces the life and deteriorates the real reliability as
opposed to the estimated reliability, based on steady-state
conditions of the power supply.
-
Swells and
overvoltages
can cause component overheating and destruction of MOVs,
SCRs and input capacitors that are rated too close to the
line voltage. It is fairly common for power supply and
telecom rectifier manufacturers to rate input MOVs at 275
Vrms with the apparent objective of reducing the surge
remnant voltage, thus providing better surge immunity of the
power supply. The author of this paper has observed
still-functioning telecom rectifiers where the input MOVs
had been completely destroyed by high line voltage.
Boost converters can be destroyed by voltage swells that
cause increased energy storage in the input filter, which
charges the output capacitor to an unsafe level. The charge
level is dependent on the value of the output capacitor and
the load levels for the dc/dc converter connected to the
output of the boost converter.
Frequent
lightning conditions
Review of IEC
1024-1
This standard relates to the
protection of structures against lightning. A structure can be
anything from a high-rise building to an outdoor shelter
containing telecommunications equipment. IEC 1024 introduces the
concept of Lightning Protection Zones (LPZ). Figure 1 below
illustrates the basic zone concept.
The purpose of LPZs is to divert
the surge energy from lightning and other disturbances away from
the "vulnerable equipment." The majority of the energy
should be diverted at the lowest zone boundaries. The number of
required zones is determined by the physical properties of the
structure and the sensitivity of the "vulnerable
equipment." Sensitive equipment installed in outdoor
shelters, particularly when a poor physical ground is present,
require a higher number of zones. Basic lightning protection
zones and surge current waveforms are explained below:
LPZ 0A
This zone is subject to direct
lightning strokes and may have to carry the full lightning
current. The unattenuated electromagnetic field occurs here.
100 kA, 10/350 µs 200 A, 0.5 s
25 kA, 0.25/100 µs
Figure 2 shows a comparison
between the 10 x 350 µs test waveform and lower-energy
Combination Wave.
LPZ 0B
This zone is not subject to
direct lightning strokes. The unattenuated electromagnetic field
occurs here.
10 kV, 1.2/50 µs 5 kA, 8/20 µs
LPZ 1
This zone is not subject to
direct lightning strokes and currents on all conductive parts
within this zone are further reduced from previous zones. The
electromagnetic field may also be attenuated depending on the
screening measures.
LPZ 2
When a further reduction of
conducted currents and/or magnetic field is required, subsequent
zones can be introduced.
In general, the higher the number
of zones, the lower the electromagnetic environment parameters.
LPZ 3
According to the author’s
interpretation of IEC standards, this zone should contain the
protected equipment. The requirement for Zone 3 is that the
remnant surge voltage does not exceed the test levels specified
in IEC 1000-4-5:
Combination wave, 1.2/50 µs,
8/20 µs 2000 A
Normal-mode voltage: 2000 V
Common-mode voltage: 4000 V
Review of IEC
1312-3
Lightning surges contain
significant amounts of energy which needs to be diverted away
from the "vulnerable equipment." Electronic equipment,
if CE marked, is designed with a certain built-in surge voltage
immunity in accordance with IEC 1000-4-5—this level of
immunity will be sufficient for the majority of office buildings
when the "vulnerable equipment" is located far away
from the electric service entrance. Thus, equipment that carries
the CE mark will work well without surge protection in many
environments.
However, when the
"vulnerable equipment" is installed close to the
electric service entrance, or in an outdoor enclosure,
particularly in areas with a poor physical ground (dry sand,
rock, etc.), it is often necessary to add external surge
protective devices (SPDs). IEC recommends (IEC 1024) that
lightning protection zones (LPZ) be established to successively
reduce lightning currents, in stages, down to the built-in
immunity of the "vulnerable equipment." The number of
zones required depends on the structure containing the
installation and the "vulnerable equipment." IEC 1024
requires that an SPD be installed at each zone boundary. The
surge current and voltage ratings of individual SPDs must not be
exceeded.
Purpose of
coordination
To progressively reduce the
lightning threat, in stages (SPD 1, 2 & 3) down to the surge
withstand capability of the "vulnerable equipment,"
without exceeding surge current and voltage ratings of
individual SPDs.
Coordination
concepts
IEC 1312 defines four
coordination "variants." The first three utilize
individual single-port SPDs while the fourth variant is a
two-port hybrid design.
Variant I
Shown in figure 3, the rated
voltage of SPD's is identical and coordination is achieved by
separating each SPD by means of series impedance.
Variant I is not recommended by
the IEC, probably because it would require approximately 30’
(10 m) of wire or separate inductors between each SPD, which is
not practical when installing equipment close to the electric
service entrance or in outdoor enclosures.
Variant ll
Shown in figure 4, the rated
voltages of SPD's are stepped so that the SPD in the
"vulnerable equipment" has the highest rated voltage
and SPD 3, 2 and 1 have progressively lower rated voltages,
thus assuring that each upstream SPD would divert progressively
higher currents.
Variant II is difficult to
implement as most power supplies, uninterruptible power systems
and telecom rectifiers, rated for 230V ac, use input SPDs with a
275V rms rating. Thus, stepping down rated voltage, would cause
SPD 1 and 2 to be destroyed by normal line voltage
fluctuations-experience has shown that fluctuations of plus 10%
resulting in 250V is not unusual.
Variant III
Shown in figure 5, includes a
component with a non-linear current/voltage characteristic, such
as a spark gap. The SPD 1 spark gap would divert the majority of
the surge current and output a Combined Wave, similar to
ANSI/IEEE C62.41-1991, Category B3, Combination Wave, 3000A,
6000V, to downstream SPDs 2 and 3. The voltage ratings of SPD 2
and 3 are identical, but each device will handle considerably
less current due to the much higher energy handling capability
of spark gap SPD 1.
Variant III is a better choice
than I or II as the spark gap is insensitive to fluctuations in
line voltage. However, spark gap follow-on, short-circuit
current may be an issue, and MOVs will still be sensitive to
line voltage fluctuations.
Variant IV
Figure 6 shows a two-port hybrid
device that incorporates cascaded stages of SPDs internally
coordinated with series impedance.
A hybrid device can be designed
to maximize performance while reducing the undesirable
characteristics of spark gaps and SPDs based on varistor or
silicon-avalanche-diode technology. The use of a hybrid device
eliminates the need to coordinate surge protective devices, but
does not eliminate sensitivity to prolonged AC overvoltage.
Proposed
hybrid device in
accordance with Variant IV
The purpose of the circuit shown
in figure 7 below is to provide, in a condensed space, the
critical functions required for a universal ac power interface
used in outdoor shelters for cellular, fixed wireless and other
telecom equipment. The major functions required are outlined
below:
-
Surge protective device
coordination for successive reduction of surge currents,
-
High energy handling
capability,
-
Tolerance against high line
voltage,
-
Adjustment for local nominal
voltage variation,
-
Low dv/dt rise time,
-
Function not dependent on
performance of internal protection of "vulnerable
equipment."
Discussion of
circuit features
shown in Figure 7
The protection scheme is designed
to work with an outdoor equipment shelter even when
"vulnerable equipment" uses 275V MOVs as internal
protection. In this case it is impossible to provide
coordination in accordance with Variant II and Variant I is not
practical, because the required wiring distances of 30’
between SPD's are too short in an outdoor equipment shelter.
The spark gap is designed to
provide primary protection within LPZ 0 and ignites when the
surge voltage reaches approximately 2000V. The series
inductance provided introduces the necessary coordination and
reliable ignition of the spark gap. The output of the spark gap
is a Combination Wave and is easily handled by the circuits in LPZ 1 and 2. Spark gaps can handle considerably more energy than
MOV or SAD arrays and are not sensitive to line voltage
fluctuations and are also more cost effective. The major
disadvantage of a spark gap is the follow-on current which can
cause tripping of upstream circuit protection devices. However,
new-technology spark gaps based on the arc-chopping principle
provide significant performance improvements. The new spark gap
technology shown in figure 8 consists of two electrodes
positioned opposite each other, held in place by a barrier and
separated by a baffle. This arrangement and spacing of
electrodes is called "arc chopping" and provides
reliable ignition of the arc, which is then chopped by the
baffle into several smaller arcs. Special versions with reduced
follow-on current are available for branch circuits and services
up to 75 kVA.
The neutral-to-ground bond
provided at the service entrance or by a separate isolation
transformer eliminates all surge voltages between neutral and
ground and the requirement for SPD's between power carrying
conductors and ground.
The series inductor with primary
buck-boost windings provides static adjustment for different
nominal voltages that can range from 200 to 250V. The
buck-boost windings can also be controlled automatically if the
line voltage is chronically unstable. The series inductor also
forms a two-pole filter together with the shunt capacitor. The
filter reduces the surge voltage rise time (dv/dt) from 6kV/µs
to 10V/µs thus providing considerable relief for the
"vulnerable equipment." The filter which starts
conduction well before MOV 1 also provides protection against
electric fast transients and other line noise.
MOV 1, located within LPZ 2, is a
40-mm device rated for 275Vrms, which is made possible due to
the voltage adjustment provided by the buck-boost windings on
the primary of the series inductor. In other words, MOV 1 is
well-protected against premature failure caused by high nominal
voltages. Now available in attractive packaging, easy-to-replace
MOVs provide thermal disconnect and alarms in case of failure.
Summary
This paper has reviewed IEC 1024
and 1312 standards for protection of structures against
lightning and for coordination of surge protective devices
(SPDs).
Line voltage fluctuations outside of the typical regulation
window of most power supplies have been observed in many
developing countries. Methods for designing an economical power
interface which provides hybrid surge protection for demanding
international applications have been discussed. The hybrid
circuit provides full coordination of SPDs while being tolerant
to line voltage fluctuations.
Copyright held by
Intertec International, Inc. Originally published in the Power
Quality ‘97/Power Value ’97 Proceeding for the POWER SYSTEMS
WORLD ’97 Conference held in Baltimore, MD September 1997.
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