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Surge protection for low voltage power supplies in case of direct lightning
strikes - testing of complete systems
Conference Paper · July 2005
DOI: 10.1109/PTC.2005.4524816 · Source: IEEE Xplore
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1
Surge protection for low voltage power supplies
in case of direct lightning strikes testing of complete systems
Josef Birkl, and Dr. Peter Zahlmann, DEHN + SÖHNE GMBH + Co. KG - Germany
Abstract-- The latest generation of surge protective devices
protects sensitive electronic equipment even in case of impulse
currents and impulse voltages resulting from direct lightning
strokes. Modern arresters should influence the power supply of
downstream consumers the least possible. Laboratory tests at
complete installations or parts of installations are a possibility to
prove the efficiency of such protection devices. Also the
performance of installation parts flown through by partial
lightning currents can be examined in laboratory tests. The
following paper will introduce test procedures by means of
application examples in telecommunications and railway
engineering, which allow to prove the lightning current carrying
capability of system components, the continuity of supply for the
system and equipment protection, even at direct lightning
currents. These engineering and test services performed in the
laboratories of the manufacturer of the SPDs provide a valuable
contribution to proving the efficiency of the lightning and surge
protective measures.
Index Terms-- Lightning protection, surge protection, system
testing.
L
I. INTRODUCTION
aboratory testing is an effective way to simulate the
effects of lightning and to prove the efficiency of surge
protection systems. Surge protective devices (SPDs) have
to withstand both the impact of high-energy lightning currents
and the interaction with the mains power supply. The
derivation of the relevant testing parameters by means of
computer simulations are explained. The main focus of this
paper is on SPDs for low- AC Power Circuits.. Specific
examples have been selected to illustrate requirements for a
reproducible and practice-oriented testing of surge protection
systems.
A. Necessity of system tests
The basic proof that surge protective devices (SPDs)
correspond to the requirements for use in low voltage
installations has been provided for protective devices which
comply with the valid product standards, such as IEC 6164311, UL 1449 and , IEEE C62.45-2002. [1]-[3]. The device
tests described in the product standards refer almost entirely to
the SPD itself. These tests, however, cannot always prove that
the full protective effect from the SPDs is also provided in the
actual installation environment. For a system test, the
conditions must therefore be adapted as closely as possible to
the actual system conditions. Such a test system installed in a
laboratory includes therefore
•
•
•
•
the required SPDs
further protective devices installed in the system, like
overcurrent protective devices and RCDs
actual length and the type of connecting cables and their
actual cabling,
equipment to be protected
These system tests aim at the most realistic replication
possible of the actual installation conditions, where the SPDs
are supposed to apply their protective effects.
B. Choosing the right system test
Laboratory examinations at complex lightning and surge
protection systems should always be carried out in close
cooperation between all parties involved , such as the user of
the SPDs and their manufacturer. A general rule defining
which system tests are required and sensible for each
individual application is not possible due to the high number
of the different parameters. Numerous system tests have been
performed and the principal processing sequence, as shown in
Fig. 1, turned out to be favorable:
Fig. 1. System testing for a DC- power supply of a telecommunication system
2
The test procedures should be based on standardised test
procedures as defined in the product and installation
standards: The laboratory tests thus provide comparable and
reproducible results.
II. SELECTED LABORATORY TESTS
Based on different application examples, the following will
introduce test procedures which have turned out to be
successful individually or in a complete test sequence within
the scope of system tests.
A. Railway Engineering – Determining the load parameters
of SPDs
A basic requirement for effective surge protection is the
correct choice of SPDs in correspondence with the necessary
impulse current capability on the respective installation site.
Computer simulations provide a possibility to analyze the
distribution of lightning currents within a complex electrical
installation. Fig. 2 shows the complex installation
environment of a lightning current arrester in railway systems.
This analysis requires a description of all electrical parameters
of the entire system as exactly as possible. For example,
• the earthing resistances
• the cable impedances and
• the electrical parameters as well as the dynamic
performance of the used SPDs
are transferred into an electrical equivalent circuit diagram.
Casel 2:
Direct stroke into a distant mast
itot = 100 kA (10/350)
Assuming a defined total lightning current load of 100 kA
10/350 in the illustrated example, the lightning current
distribution can now be determined within the entire system
to be examined. In this case, the risk factors were thus
determined for a switch-gear cabinet with integrated electronic
system and lightning and surge protection, which is installed
close to an electrified railway system. The maximum lightning
current load for this specific cabinet was calculated
considering different threat scenarios:
• Direct stroke into the mast of the contact wire, adjacent to
the distribution board
• Direct stroke into a distant mast of the contact wire
• Direct stroke into the overhead line of the low voltage
power supply.
The result of such a simulation then provides a matrix of
the calculated load parameters of the SPD for the different
threat scenarios. These calculated load values have to be
aligned with the nominal performance parameters of the SPD
provided for this application. This allows already a
preliminary estimation, whether the SPDs have been chosen
correctly for the actual installation site regarding their
discharge capacity. Not only the peak currents loading the
SPDs have to be taken into account but also the energy load,
i.e. the load or specific energy.
These calculated values form then the basis for the
following lightning current tests in the laboratory – possibly
including a certain safety margin.
Case 3:
Direct stroke into the low
voltage overhead line
itot = 100 kA (10/350)
Case 1:
Direct stroke into an adjacent mast
itot = 100 kA (10/350)
Low voltage
400 V / 50 Hz
Low voltage
overhead line
Transformer station
Station with electrical equipment
and surge protection
Contact wire
15 kV / 16 2/3 Hz
Earthed rail
Insulated rail
Fig. 2. Installation environment of a Surge protection in a railway system
Earthing system
of the low voltage installation
3
B. Construction of switch-gear cabinets – Proof of lightning
current carrying capability
For most applications multipole SPDs are used. Therefore,
the following will explain especially the decisive parameters
of a lightning current test for multipole SPDs.
The lightning current distribution among the individual
protection paths of the SPD, e.g. Phase to Neutral or neutral to
Earth has considerable importance. Therefore in the latest
amendment of the IEC 61643-1 a total discharge current test
has been introduced. This test is used to check for the
cumulative effects that occur when multiple modes of
protection of a multi-pole SPD conduct at the same time. The
distribution of the impulse currents and it's characteristic
parameters, such as peak current Ipeak, total charge Q and
specific energy W/R are shown in table 1.
TABLE 1 – TOLERANCES FOR PROPORTIONAL SURGE CURRENTS [1]
Test
classifi
cation
Proportional currents and tolerances
± 10 %
I peak(1) = I peak(2) = I peak(N) = I peak / N
Test
class I
± 20 %
Q ( 1) = Q (2) = Q ( N) = Q(I Total ) / N
W/R ( 1) = W/R (2) = W/R (N) = W/R(I Total ) / N ± 35 %
2
Test
class II
I 8/20(1) = I 8/20(2) = I 8/20 (N) = I T otal / N
± 10%
In the laboratory, this balanced distribution of the impulse
currents is ensured by series inductances and resistances.
These impedances consist typically of a resistance of 30 mΩ
and an inductance of 25 µH. These values correspond to
typical cable lengths of approximately 50 meters. For
unbalanced protective circuits, an additional external balanced
SPD is included into the test circuit, where required.
Assuming a balanced surge current distribution amongst
the phase lines and the neutral line represents a "worst-case"
analysis. Surge-protection systems tested under these
conditions can be applied in all application regardless the
specific earthing conditions at the individual site.
The test construction in the laboratory for proving the
lightning current carrying capability of e.g. a
telecommunication cabinet with integrated lightning current
arrester, is shown in Fig.3. In addition, the equipment to be
protected, which is in this example a dc power supply, is
connected additionally to the output terminals of the surge
protective unit.
This test thus exceeds the standardised requirements
considerably. However, it offers the user of the SPDs the most
realistic proof possible about the actual lightning current
carrying capability and the protection of downstream
equipment. Following this total discharge current test the
correction function both of the SPD and the connected power
supply is verified.
Fig. 3. Testing the lightning current carrying capability of a telecommunication power supply
4
C. Proof of SPD energy coordination
One further important test is the proof of energy
coordination between all SPDs installed into an electrical
installation. Basically, it is supposed to be proven that none of
the SPDs is overloaded at an assumed primary threat with
values higher than their max. load parameters (e.g. max. peak
current or max. energy absorption capacity). This has to be
proven for the entire impulse current range (O-Imax).
Different methods for coordinating SPDs are described in as
IEC TR 62066 and , IEEE C62.41.1-2002. [4], [5]. In
practice, coordination of external SPDs is no problem any
more. Today, manufacturers offer coordinated SPD ranges
(furnished with decoupling elements, if required) [6].
In contrast, energy coordination of external SPDs and
surge protective elements already included in the equipment
device to be protected, is often not provided. Especially the
growing trend of installing lightning current arresters flown
through by direct partial lightning currents and highly
sensitive electronic devices next to each other within very
small spaces, makes high requirements on the protective effect
of such SPDs. In either case, the surge protective elements
integrated into the terminal device have to be prevented from
“skipping” the protection of the more powerful upstream
SPDs.
If the design of the SPD integrated into the terminal device
can be influenced before, computer simulations can be an
effective tool and help to test the aforementioned “blind spot”
case. In all other cases, energy coordination can be proven by
so-called “coordination tests” in laboratories. The most critical
case during a coordination test is often not the maximum
impulse current load to be expected. For high impulse currents
even the low cable impedance of the internal wiring has often
a sufficient de-coupling effect due to its high current steepness
di/dt to activate upstream lightning current arresters before the
integrated surge protection device is overloaded. Therefore,
impulse current tests beginning with small impulse currents
are required. The proof of energy coordination must be
provided for each individual mode of protection. Therefore,
quite comprehensive measurements have to be made to
determine the partial currents for each protective element.
To obtain comparable and reproducible results of such
coordination measurements, a lightning current generator is
required with a high “fictive” internal resistance. Only such a
generator can ensure that the wave form and especially the
steepness of the test current is not influenced by the changing
impedance of the test units due to the response of the
protective elements. In practice, a lightning current generator
with a “fictive” internal resistance ≥ 10 Ω has proven to be
sufficient. Figure 4 shows the current oscillograms of such a
coordination test among several SPDs for lightning current
generators with different “fictive” internal resistances [7].
Fig. 4. Lightning current generators with different “fictive” internal
resistances
D. Proof of continuity of supply
The above test sequences B and C focussed on the
lightning current load of the systems. Therefore the SPDs and
the connected downstream equipment was tested with partial
lightning currents but not connected to the low-voltage power
supply during the tests.. But also the performance of the SPDs
connected to low-voltage AC Power Circuits – and thus the
reliability of power supply – are becoming more and more
important for the user. Therefore, the next test to be
introduced will include a test for the selectivity of backup
fuses and SPDs of the “50 Hz world“.
Especially "low-energy" surges have to be considered such
as switching overvoltages or coupling by distant lightning
strokes. Such low-energy transients arise much more
frequently than the especially high-energy interfering currents
of close or direct lightning strokes. If an SPD is activated by a
low-energy disturbance, no interfering influences on the low
voltage supply systems caused by e.g. voltage drops, mains
follow currents, false tripping of fuses, should ideally arise
neither during nor after the discharge. The mains performance
of an SPD is examined during a standard-conform test
according to IEC 61643-1 with the so-called "duty cycle test".
Following the test procedures of this standard, the SPD is
tested in combination with the maximum overcurrent
protective device assigned by the manufacturer and at a
maximum prospective short-circuit current permitted for the
SPD. In real installations, however, there are often overcurrent
protective elements installed with a much lower nominal
current for reasons of overcurrent protection. In such cases,
the frequency of follow currents and the follow current
limitation of a lightning current arrester are the decisive
parameters for a reliable power supply of a system. The proof
of reliable power supply for the respective system
configuration, is done by laboratory testing using the basic test
procedures in the standards but selecting the actual the
overcurrent protective element and the prospective shortcircuit current for the specific application. A complete test
series with different prospective short-circuit currents results
in a selectivity characteristic as shown in Figure 5.
A statement about the reliability of supply at low-energy
5
transients can then be provided for an “SPD” and a
“combination of surge and overcurrent protective devices”
without requiring further tests. Applying modern lightning
current arresters on a spark gap basis, it is ensured that lowenergy overvoltages are suppressed to a low protection level
without leading to any 50Hz-mains follow currents. Should
impulse currents arise with higher energies and possibly lead
to follow currents, these should be limited to ensure that an
upstream overcurrent protective element will not respond [8].
For example for tested the lightning current arrester (LCA),
described by below diagram , the let-through integral (I2 x t) of
the spark gap remains lower than the minimal melting integral
of a 32A NH-fuse even at a prospective short-circuit current
of 25kA (RMS). Due to the follow current limiting
characteristic of uses spark gap, the actually flowing current
will be limited to about 1,5% (500A peak) of the prospective
value (40kA peak).
Fig. 5. Selectivity-limit currents for Lightning Current Arresters (LCAs) in
different technologies in comparison with different back-up-fuses
That means in a real installation a tripping selectivity is given
with all overcurrent protective devices upstream this SPD
which have a nominal rating of ≥ 32 A. In order to achieve
this power current limiting characteristic the arc voltage of
this type of SPDs during the quenching phase hardly differs
from the mains voltages. The above described parameters are
derived from a complete test series performed for the lighting
protection concept of the low-voltage power supply of mobile
phone stations. Numerous applications have shown, that a
follow current limitation of a SPD, as described above,
increases the safety of power supply for the complete system,
protected by such an arrester.
E. Telecommunications - Proof of equipment protection from
direct lightning currents
For proving the resistance of electrical and electronic
devices against interference or destruction in case of
conducted impulse voltages or impulse currents IEC 61000-45 has stipulated defined test levels [6]. However, the standard
already points out in the application section, that a proof of
immunity of equipment does not always include the immunity
against surges of a complete system. Furthermore it is clearly
stated in the scope of this standard that direct lightning strokes
are not considered.
Therefore in the latest draft of IEC 61000-4-5 basic
requirements for a system level immunity test are listed:
• "In order to ensure system level immunity, a test at the
system level is recommended to simulate the real
installation. This simulated installation shall be
comprised not only of individual EUT’s, but must shall
also include protective devices (Surge Protective Devices
– SPD’s) and the real length and type of the
interconnection lines normally used, all of which can
affect the overall system protection level."
• "The simple addition of an external SPD that is not
coordinated with other internal SPD’s, might have no
effect, might reduce the effect on the overall system
protection, or might improve overall system protection."
• "In a real installation, higher voltage levels can be
applied, but the surge energy will be limited by the
installed protective devices in accordance with their
current-limiting characteristics."
The following described proposal of a test procedure for
complete systems, fulfills all these requirements. This new
test, called ”lightning current test under service conditions”
combines the basic test procedures of EMC-testing with the
lightning current tests levels, described in the SPD standards
for direct lightning strokes. The equipment to be protected , an
the externals SPDs, were tested in a common system test
under most real service conditions. In contrast to the lightning
current test as described in chapter B and C, the equipment to
be protected is tested here under service conditions, i.e. the
device is activated and connected to its nominal supply
voltage and stressed with partial lightning currents. Figure 6
shows the complete test set-up of this “lightning current test
under real service conditions”, including the switchboard,
which contains the external SPDs and the downstream dcpower-supply to be protected.
In this example, the proof of reliable power supply is
provided by measuring the dc output voltage of the rectifier.
Figure 7 shows clearly that the supply voltage of about 60 V
dc is also given for the duration of a flowing 30 kA 10/350
impulse current (high-frequency voltage spikes superimposed
on the dc supply voltage can be traced back to induction
effects within the measuring loop).
6
Fig. 6. Lightning current test under real service conditions – Example: dc power supply
Fig 7. Proof of continuity supply of a dc-rectifier even in case of partial lightning currents
These proposed test scheme for complete systems has been
successfully performed on various applications, such as:
• Power supply of mobile phone base stations
• AC-power supplies in railway applications
• Pitch drive control systems of wind turbines
• Water pumps and its control units
• Central DC-AC-inverters of solar power-plants
• Electronic power meters
In these wide range of applications field experience has
confirmed, that the test described in this paragraph simulates
the installation conditions as realistic as possible, in which the
system is intended to function later on.
7
F. Documentation of the tests
The final step of such a customer-specific test program is a
detailed and informative test documentation. Such a test report
certifies and confirms the successful test of the respective test
sequences performed. For comprehensive test constructions,
as inevitably required for complex system tests, also the
conditions of test constructions like
• equipment connected
• overcurrent protective elements
• and especially the cabling
must be documented and described as detailed as possible.
III. SUMMARY
For technical and economically optimal use of SPDs for
protection of complex installations even against direct
lightning strokes, both computer simulations and laboratory
tests are helpful.
• Computer simulations allow to determine the lightning
current distribution within the installation to be protected
and to analyse the dynamic performance of the SPDs to
be used. The suitable SPDs are chosen accordingly and
their installation sites are defined.
• Equipment testing does not ensure the immunity of a
system in case direct lightning strokes in all applications.
For that reason a lightning current test under real service
conditions on system level is proposed, which simulates
the real installation and its environment as close as
possible.
• Ideally, such engineering and test services should be
offered by the manufacturer of the SPDs.
IV. REFERENCES
Standards:
[1]
[2]
[3]
[4]
[5]
[6]
IEC 61643-1 Ed. 2.: 2005-03 “Low-voltage surge protective devices Part 11: Surge protective devices connected to low-voltage power
systems; Requirements and tests“.
UL 1449 2nd edition, August 15, 1996: "Standard for transient voltage
surge suppressors”.
IEEE Recommended Practice on Surge Testing for Equipment
Connected to Low-voltage (1000V and Less) AC Power Circuits, IEEE
Standard C62.45-2002.
IEC TR 62066: 2002 “Surge overvoltages and surge protection in lowvoltage a.c. power systems“.
IEEE Guide on the Surge Environment in Low-voltage (1000V and Less)
AC Power Circuits, IEEE Standard C62.41.1-2002.
IEC 61000-4-5 Ed. 1.: 2001-12; Electromagnetic compatibility (EMC)Part 4-5: Testing and measurement techniques - Surge immunity test
Papers from Conference Proceedings (Published):
[7]
[8]
J. Birkl, P. Hasse, P. Zahlmann, "Test procedures for Surge protection
systems to be installed in low-voltage power systems" in Proc.
International conference on lightning protection, Rhodos 2000, pp. 854859
P. Hasse, P. Zahlmann, "Lightning currents and overvoltages - one
arrester for universal use" in Proc. International conference on lightning
protection, Cracow 2002, pp. 516-520.
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V. BIOGRAPHIES
Dipl. Ing. (FH) Josef Birkl (37) has worked for
DEHN + SÖHNE since 1993. He studied Electrical
Engineering at the University of Applied Sciences
Regensburg. He is now head of the R & D
laboratories of DEHN + SÖHNE. Furthermore, he is
in charge of the elaboration and assessment of
lightning and surge protection concepts via computer
simulations and laboratory analysis as well as the
design and realisation of complete customer-specific
solutions.
Dr-Ing Peter Zahlmann (50) studied electrical
engineering at the Technical University of Ilmenau,
where he was awarded his doctorate in 1983. Since 1
July 2004 he has been a CEO of DEHN + SÖHNE .
Numerous patents for lightning protection
components, surge protective devices and safety
equipment for working at electrical installations
resulted from his work.
Dr Zahlmann contributes to national and
international standardization of lightning and surge
protection within the scope of technical-scientific
associations and institutions like ABB, DKE/VDE and IEC.