1. health and fitness
2. disease
3. autism and pdd

IX International Symposium on
Lightning Protection
26th-30th November 2007 – Foz do Iguaçu, Brazil
HOW TO VERIFY LIGHTNING PROTECTION EFFICIENCY FOR
ELECTRICAL SYSTEMS?
TESTING PROCEDURES AND PRACTICAL APPLICATIONS
Birkl Josef
Zahlmann Peter
DEHN + SOEHNE
DEHN + SOEHNE
[email protected]
[email protected]
Hans-Dehn-Strasse 1, D-92318 Neumarkt, Germany
Abstract – There are increasing numbers of applications,
installing Surge Protective Devices (SPDs), through which
partial lightning currents flow, and highly sensitive,
electronic devices to be protected closely next to each other
due to the design of electric distribution systems and
switchgear installations which is getting more and more
compact. In these cases, the protective function of the SPDs
has to be co-ordinated with the individual immunity of the
equipment against energetic, conductive impulse voltages
and impulse currents. In order to verify the immunity
against partial lightning currents of the complete system
laboratory tests on a system level are a suitable approach.
The proposed test schemes for complete systems have been
successfully performed on various applications. Examples
will be presented.
Testing of a SPD
according to IEC 61643-1
In general, the protective performance of a SPD is
described by stating the “voltage protection level UP”,
which is determined according to the test procedures
specified in IEC 61643.
The immunity of equipment from voltage and current
surges is described by the magnitude of the applied
voltage test level (installation class). Considering the
protection of equipment by means of an upstream SPD,
the following basic criteria applies:
-
i
CWG
1.2/50 // 8/20
SPD
CWG
1.2/50 // 8/20
Umax
Voltage protection level Up
<
Coupling
network
Imax
DUT
Umax
W EUT
Test level
Combined system test:
Testing the SPD and equipment to be protected
Imax*
CWG
1 INTRODUCTION
The protective effect of SPDs is verified in tests
according to the relevant product standard such as IEC
61643-1 and IEC 61643-21 [1], [2]. The verification of
immunity or surge immunity against conductive impulse
voltages or impulse currents of electrical and electronic
devices is carried out according to IEC 61000-4-5 [3].
However, the tests described in these two standards
mainly refer to the SPD itself or the equipment to be
protected. The necessity to combine both test levels
within one system level test is stated by both parties.
Figure 1 shows the basic protective criterion for the coordination of the individual surge immunity of a
equipment and the "protective performance of SPDs".
Testing the surge immunity of equipment
according to IEC 61000-4-5
SPD
DUT
< Imax*
Imax
U max < Umax*
WEUT < W EUT*
Umax*
WEUT*
Fig. 1 - Protective criterion for the co-ordination SPD and
equipment to be protected.
The voltage protection level UP, of an upstream SPD has
to be below the verified immunity level of the equipment
to be protected. UP should be also co-ordinated with the
withstand voltage of the equipment Uw according to IEC
60664-1 [4]. However, when applying this basic
protective principle, not only the maximum voltage level
Umax to be expected across the terminals of the SPD has to
be compared as generally assumed. Furthermore, a
number of additional parameters might be relevant for
ensuring effective protection of equipment by means of
external SPDs:
• Maximum impulse current Imax flow into equipment
• Maximum energy Wmax transfered into equipment
• Maximum voltage-time integral u dt at equipment,
•
∫
Maximum voltage change du
dt
.
In order to evaluate the above parameters following basic
questions are to be taken into account and will be
discussed further on:
• Different protection performance of SPDs depending
on the functional principle of individual SPDs
• Comparability of different test parameters according
to IEC 61000-4-5 and IEC 61643
2
LIMITATION PERFORMANCE OF SPDs
Depending on their design SPDs are subdivided
according IEC 61643-1 into following three functional
principles:
• Voltage switching type SPD: "can have a sudden
change in impedance to a low value in response to a
voltage surge". Typical examples of such
components are spark-gap-based SPDs.
• Voltage limiting type SPD: " will reduce impedance
continuously with increased surge current and surge
voltage". Typical examples are MOVs or diodes.
• Combination type SPD: " incorporates both voltage
switching type and voltage limiting type components"
The protection performance of SPDs may significantly
differ depending on the design features described. In
Figure 2.1 and 2.2, this matter is outlined based on two
basic examples. In both examples it is assumed, that the
equipment to be protected contains a MOV directly at the
equipment input terminal. Furthermore it is supposed that
the equipment is protected by an upstream SPD at a
voltage protection level of UP = 1,5kV. In example 2.1,
the SPD is designed as spark gap, in figure 2.2 a MOV
type SPD is assumed. Based on a total load of 1kA
10/350µs, the two pictures show the impulse current,
which flows into the equipment, and the voltage across
equipment. In this simplified examples the primary values
Imax and Umax have almost the same value. However, when
comparing the integral values Wmax and u dt , it is
∫
obvious that the let-through energy generated in the
equipment to be protected and the voltage-time integral at
the equipment may differ considerable in value when
using two different SPDs which have the same voltage
protection level Up, but different functional principles. A
varistor-based SPD did result in a higher let-through
engery, especially at impulse currents with a long time to
half-value, such as the 10/350µs current waveshape.
However at a spark-gap-based SPD it can be expected
that the load on the equipment to be protected is reduced
due to the switching performance particularly if impulse
currents with a long time to half-value occur [6].
Impedance
Spark gap
Impulse current
generator
-
External
Voltage-limiting componet in
voltage-switching type SPD equipment to be protected
i[kA]
1.2
MOV
800
total current
u[V]
voltage across SPD
current into SPD
0.6
0
400
current into equipment
0
40
80
120 t[µs]
0
voltage at equipment
0
40
80
120 t[µs]
Figure 2.1 - Protection of equipment by an external spark gap
]
Iimpedance
MOV
Impulse current
generator
External
voltage-limiting SPD
i[kA]
1.2
Voltage-limiting component
in equipment to be protected
U [V]
800
total current
current into SPD
0.6
MOV
voltage across equipment
voltage across SPD
400
current into equipment
0
0
40
80
120 t[µs]
0
0
40
80
120 t[µs]
Figure 2.2 - Protection of equipment by an external MOV
When assessing whether equipment can be protected by
an upstream SPD, the protection performance,
considering also let-through energy and voltage-time
integral, which depends o the functional principle of the
SPD, may be decisive factors.
Table 1: Comparison of load parameters for the calculation
examples in figures 2
Load Parameter at
equipment
Maximum voltage
Umax
Maximum current
Imax
Voltage-time
integral u dt
∫
Voltageswitching SPD
< 800V
Voltage-limiting
SPD
< 800V
< 200A
< 200A
≈ 350mVs
≈ 1000mVs
Maximum energy ≈ 2 Ws
Wmax transfer
≈ 70 Ws
3 COMPARISON OF TEST PROCEDURES
ACCORDING TO IEC 61000-4-5 AND IEC 61643
In the scope of IEC 61000-4-5 it is clearly stated, that
“direct lightning strikes, are not considered”.
Furthermore the difference of “equipment level immunity”
and “system level immunity” is pointed out: "The
manufacturer should test his equipment to confirm the
equipment level immunity" and “those responsible for the
installation should then apply measures necessary to
ensure that the interference voltage caused by lightning
strokes does not exceed the chosen immunity level”.
Comparability of the parameters determined in various
tests is a basic requirement that equipment with a specific
immunity according to IEC 61000-4-5 can be protected
by a SPD with specific voltage protection level according
to IEC 61643. Comparability is in some cases difficult
due to different test philosophies and test methods, on
which the two standards are based. Some of the basic
differences between these two standards are described
below.
3.1 Different source impedances
3.2 Different waveforms and threat values
Equipment immunity according to IEC 61000-4-5 is
determined with a Combination Wave Generator (CWG)
with an internal impedance of 2 Ω, a short circuit current
of wave form 8/20µs and an open-circuit voltage of wave
form 1.2/50µs. When testing SPDs Type III, which are
mainly used for the protection of equipment, such a CWG
is also required according to IEC 61643-1. However,
when testing the immunity of equipment, different
coupling elements are used. For example, when testing
low-voltage power supply lines against earth, an
additional resistor of 10 Ω is connected in series.
IEC 61000-4-5 specifies different installation classes
based on the installation conditions. Vlass 4 installations
are defined as “power installation which can be subjected
to inteference voltages generated by the installation itself
or by lightning”. The AC-power supply input of
equipment in installation class 4 will be tested with a
hybrid impulse of the waveshape 1.2/50µs (8/20µs).
Whereas partial lightning currents are simulated in a
laboratory by means of an energetic current impulse with
the waveform 10/350µs according to the primary threat
values, which are part of the lightning current standards
Therefore, SPDs ClassI, often called lightning current
arresters, are used for this purpose. These are installed
where cables enter buildings for protection against
lightning currents. These SPDs, are tested according to
IEC 61643-1 with this energetic 10/350µs test impulse. A
comparison of the different waveshapes, as given in
figure 4, shows that a general statement whether
equipment can be protected by an external SPD also in
case of lightning currents with a long current wave is
sometimes difficult, due to the different current
waveforms. As pointed out above, spark-gap-based SPDs
with a “wave-breaking” performance reduce the actual
stress parameters for the equipment, due to it’s switching
characteristics. The remaining stress for the equipment
then corresponds to the current waveform used to
evaluate the test level of the EMC-surge immunity test.
The example in Figure 3 shows that different parameters
might be obtained from both tests due to this additional
series impedance. In the assumed example a MOV based
SPD is tested according to IEC 61643-1 with an opencircuit voltage UOC =10kV. In this example, a voltage
protection level of UP=1kV is achieved. An equipment
immunity of 2kV was determined according to IEC
61000-4-5. In order to ensure this surge withstand it is
assumed that varistor is integrated inside the equipment.
This MOV is charged with a total energy of 1 Joule when
testing immunity according the EMC-standard. However,
if the SPD and equipment to be protected are tested in
combination with a 2Ω-CWG of 10kV 1.2/50µs, the
energy produced in the internal MOV of the equipment is
20 times higher due to the different source impedance.
This may result in an energetic overload of this
component although the basic criterion, i.e. the immunity
level of equipment has to be greater than the maximum
voltage protection level of the SPD, is met in this
example.
U [kV]
1.0
0.5
Case 1: Test of a single SPD according to EN 61643-1
2.0
0
current through SPD
0
U [kV]
0.8
10
20
30
Case 2: Test of a single SPD according to EN 61000-4-5
I [A]
100
50
voltage above terminal equipment
current in terminal equipment
0
U [kV]
1.0
10
20
30
0
SPD
u
10Ω 9µF
test voltage
2 kV
i
DUT
Partial lightning current 10/350
20
Short circuit current
of a CWG 8/20
200
1000 t [µs]
u
Umax = 600 V, Imax = 100 A
Wmax = 1 J
t [µs] 40
Figure 4 - Comparison of different wavevforms
for 10/350 partial lightning current and 8/20 surge current
Case 3: System test of an SPD in combination with the terminal equipment
I [kA]
1
0.5
voltage above terminal equipment
0
current in terminal equipment
0
i
test voltage
10 kV
Umax = 1 kV, Imax = 4 kA
Wmax = 60 J
t [µs] 40
0.4
0
50
I [kA]
4.0
voltage above SPD
0
i [kA]
100
10
20
30
t [µs] 40
0
i*
DUT U*
SPD
test
voltage
10 kV
Umax* = 850 V, Imax* = 1.1 kA
Wmax* = 20 J
Figure 3 - Comparison of the threat values when co-ordinating
SPD and equipment to be protected
3.3 Adoption of the various test philosophies
For several years, experts of the IEC 77B and IEC 37A
committees responsible have been trying to adapt the
various test philosophies, on which the installation and
device regulations are based [5]. A description of this
problem of different test procedures was added to the
latest standard version of IEC 61000-4-5 Ed. 2.0 in an
additional informative annexe. In this article, possible test
procedures for such a cross-standard system test are
outlined based on practical examples.
4 SYSTEM TEST
In general the following statement of IEC 61000-4-5
applies: "In order to ensure system level immunity, a test
at the system level is recommended to simulate the real
installation”. Therefore, when carrying out a system test,
the actual installation conditions have to be simulated as
realistically as possible. A test installation set up in the
laboratory includes for example the following:
• SPDs required
• Additional protective equipment installed, such as
overcurrent protective devices and RCDs
• Actual length and type of the connecting cables
between the individual system components
• Equipment and terminal equipment which have to be
protected against surges
The response of upstream SPDs result in secondary
effects such as change of wave form. Carrying out a
system test also verifies that these effects do not have
impermissible effects on the function of the equipment.
4.1 Energetic coordination
It has to be observed that immunity of the total system
cannot be increased by means of uncoordinated adding of
SPDs. Moreover, it has to be ensured in all cases that the
Surge protective components installed in the terminal
equipment do not make the high-performance, upstream
SPDs ineffective. This "Blind Spot" can be verified in the
laboratory by carrying out tests, which are referred to as
"coordination tests". The test current has to be increased
gradually for the most critical case of this coordination
test is often not the maximum impulse current load to be
expected. In case of maximum test current, low line
impedance of the connecting cable often results in
sufficient coupling. The high current steepness di/dt
ensures that the upstream SPD is activated before the
surge protection integrated in the terminal equipment to
be protected is overloaded. For reproducable coordination
tests special requirements for the test generators apply.
Impulse current generators with a "fictive" internal
impedance ≥ 10 Ω have proven to be sufficient. Thus, the
requirement that lightning currents are to be regarded as
"ideal" power source is met with adequate accuracy [7].
4.2 Lightning current test of SPD and equipment
under real service conditions
The protection of equipment in the case of a direct
lightning strike can be verified by means of carrying out a
“Lightning current test under real operating conditions“.
Equipment and SPDs are tested in a combined system test
under operating conditions, which have to be as real as
possible. The basic idea of such a lightning current test
under operating conditions is to combine the standard test
philosophy of an immunity test according to IEC 610004-5 with the increased requirements of an impulse current
test or lighting current test according to IEC 61643-1.
4.3 Example I: Protection of a central inverter for
solar power plants
Figure 5 shows the basic circuit diagram of the individual
surge protection measures for a solar power plant. There
are two possible ways how lightning currents and surges
can couple into the central DC-AC-converter in the
operation building: The collectors and their connecting
cables with the DC-input of the DC-AC-converter form a
wide conductor loop. In this conductor loop high impulse
currents will be induced even at distant strikes. If,
however, the DC-conductors are run in a steel conduit
only a low energy impulse current loading have to be
taken into account. In such cases the installation of Class
II SPDs according IEC 61643-1 directly at the DC-input
of the DC-AC-converter and close to the solar generator
is necessary and sufficient [8]. In the operation building,
which includes the DC-AC-converter all metal systems
shall be connected directly and all systems under
operating voltage shall be connected indirectly via Class I
SPDs to the lightning equipotential bonding.
Central building with
DC-AC-converter
Partial lightning
currents in AC-lowvoltage-power-supply
Induced surge into
DC-input of converter
Earthing
Figure 5 Basic protection scheme of the surge protection
for a solar power plant
Testing procedure:
In the following an example of such a system level test,
called "lightning current test under service conditions"
will be presented. The DC-AC-converter to be protected
is tested under operating conditions, i.e. the device will be
loaded with lightning partial currents at live state while
being connected with a DC supplying voltage. Figure 6
shows the basic circuit diagram of this system level test.
This circuit diagram shows how 8/20µs impulse currents
are coupled into the DC input of the DC-AC-converter
under operating conditions. It has also been verified that
during and after the feeding of lightning partial currents
of wave form 10/350 into the AC connection of the DCAC-converter via an AC supply transformer, electrical
energy has been supplied into the general low-voltage
mains.
S2
DC - AC-converter
~
~ 25 A-DC
~
+
L1
L1
L2
L2
L3
L3
IAC
PEN
+
AC-power-transformer
230 / 400 V 50 Hz
N
A
IImpuls
S1
IImpuls
-
V
-
A
-
IDC
A
A
DC-Power supply 600 V
Wh
Uprotect
Impuls current
generator 8/20
A
Itotal
Multipole surge arrester
- Class II SPD- for PV system
N
L1
L2
L3
External Lightning current arrester
- Class I SPD - for AC-power supply
Figure 6 Circuit diagram lightning current test of a central DC-AC-converter under real operating conditions
4.4 Example II: Lightning and surge protection of
electrical systems in nacelle and hub of a windturbine
The verification of the effectiveness of lightning and
surge protection for electrical and electronic systems in
the nacelle and hub of wind turbines by laboratory
testing, will be described on the example of a pitch drive
control system. Pitch systems in the rotor hub are used for
adjusting the rotor blades. If the wind exceeds a critical
value, the turbine will be moved out of the wind.
Description of pitch drive control system
The examined pitch drive system did include several ACmotors, AC converter, for communication a serial link
(Profibus DP) and several multi turn position sensors. All
power-supply lines were protected by Class 2 SPDs
according IEC 61643-1. The data lines were protected by
multi-stage data line protectors tested according IEC
61643-21. These type of arresters include in the first stage
a powerful gapped arrester and downstream of the
decoupling element a diode element, ensuring the low
protection level.
Lightning current parameters
It is assumed that the pitch drive control system is located
with Lightning Protection Zone 1, that means no direct
lightning currents but surge currents are flowing in the
electrical lines within the system. Furthermore the
complete pitch drive system, consisting of a control unit,
AC-motors, position sensors and the complete cabling
between these different components is also stressed by
the magnetic field, which is generated by direct lightning
currents flowing in the surrounding metallic hub.
A) Induction effects due to lightning
Impulse currents up to 100kA (10/350) were injected into
a defined metal, mounting plate, in order to examine the
behavior of the complete system within an
electromagnetic field generated by lightning currents. The
resulting induced impulse currents within the cabling of
the complete system were monitored. The characteristic
values Imax, QStroke and W/R of the lightning currents were
determined for every test impulse. The functional
endurance of the pitch drive system during the injection
of direct lightning currents into the mounting plate was
monitored in order to verify any influence of conducted
interference to the control unit caused by effects of closeby lightning currents. Figure 7 shows the laboratory test
set-up for this test series.
Measurement of
impulse currents
Impulse current generator
200 kA 10/350
Distribution board
including control system
Injection of impulse
current into metal frame
Figure 7 System test of a wind turbine pitch control system
•
B) Impulse test 8/20 of low voltage power supply and
Surge immunity test of data line
Impulse currents 8/20 were injected directly both into the
power supply and into the data line conductors in order to
check the surge withstand of the connected equipment.
During this test also the co-ordination of the installed
external SPDs both for data lines and power lines and
surge protection components, installed already inside the
connected equipment was verified. In the described
example the test of the surge current carrying capability
onto the power supply of the pitch drive control unit
under service conditions was performed with discharge
currents up to 40 kA 8/20. The impulse current was
injected in this case in the line conductor to PENconductor, while the system was connected with 255 V
mains voltage.
Additionally a surge immunity test of the data line type
Profibus DP under service conditions has been
performed. It was the aim to examine the interference on
the connection lines of the Profibus system as they are
caused by effects of lightning. Therefore 8/20 impulse
currents up to 5kA have been directly injected into the
data bus. The injection of the 8/20 discharge currents has
been done in two different coupling modes:
• Line-to earth-coupling
• Screen-to-earth-coupling
During the test the pitch drive system was running in test
mode. The data transfer between the pitch controller and
an external computer has been monitored. The correct
function of the complete pitch control system could be
successfully verified during and following the complete
test series. No data interruption or any damage to
connected the pitch drive control system has been
observed.
C) Lightning current withstand of pre-wired connection
unit for low voltage power supply
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 basic test procedure
of the total discharge current test for multiple SPDs
according IEC 61643-1 is applied. 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 monitored during the test, as IEC 61643-1 assumes a
balanced impulse current distribution. In the laboratory,
this balanced current distribution is ensured by series
inductances and resistances. Assuming a balanced surge
current distribution amongst the phase lines and the
neutral line represents a "worst-case" analysis. Different
earthing practices in different parts of the world have a
very huge influence onto the actual lightning current
distribution. However surge-protection systems tested
under these conditions can be applied in all applications
regardless the specific earthing conditions at the
individual site. Additionally the equipment to be
protected has been connected to the output terminals of
the surge protective unit. So this test combines again the
stress parameter of the lightning protection standard with
the immunity verification of equipment and therefore
exceeds the standardized requirements considerably.
However, it offers the user of the SPDs the most realistic
proof about the actual lightning current carrying
capability and the protection of downstream equipment.
D) Proof of continuity of supply
The above test sequences mainly focussed on the
lightning current behaviour of the systems. But also the
the reliability of the low-voltage AC power supply are
becoming more important for the user. Therefore an
additional test for the selectivity of backup fuses and
SPDs was included. This was done by laboratory testing
using the basic test procedure of a “duty-cycle test”,
described in the SPD-standard, but selecting the real
overcurrent protective element, prospective short-circuit
current and system voltage for the specific application.
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.
It was ensured that low-energy 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.
5
SUMMARY
It is not possible in every case to compare all parameters
determined due to different test philosophies specified in
standards concerning immunity tests of terminal
equipment and test requirements specified in the product
standards for SPDs.
Therefore, the system test presented in this article is a
method for verifying immunity on system level, which
has been tried and tested in various applications.
6 REFERENCES
[1] 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“.
[2] IEC 61643-21 Ed. 1.0: 2000-09 “Low voltage surge
protective devices - Part 21: Surge protective devices
connected to telecommunications and signalling networks Performance requirements and testing methods”
[3] IEC 61000-4-5 Ed. 2.0: 2005-11
Electromagnetic
Compatibility (EMC)- Part 4-5: Testing and measurement
techniques - Surge immunity test
[4] IEC 60664-1: 2002 Insulation coordination for
equipment within low-voltage systems- Part 1:
Principles requirements and tests
[5] H. Bachl "Überspannungsschutz Koordination Geräte SPDs; IEC/EN 61000-4-5 versus IEC/EN 61643-11" [Surge
protection coordination devices; IEC/EN 61000-4-5 versus
IEC/EN 61643-11] on the occasion of the D-A-CH
conference 08/2004, Rostock Wannemünde, Germany
[6] IEC 62305-4: 2006: Protection against lightning - Part 4:
Electrical and elctronic systems within structures
[7] J.Birkl,
P.
Hasse
"EMV-Testverfahren
zur
Ableiterkoordination"
[EMC
test
procedures
for
coordination of SPDs], in EMC Kompendium 1998
[8] H. Pusch., B. Schulz "Blitz- und Überspannungsschutz für
Solarkraftwerke" in TAB 7-8/2003, S. 79-83