English translation of publication in „eb - Elektrische Bahnen“ 100 (2002), Heft 8-9, Seite 321-328
Metal-Oxide Surge Arresters for Electric Railways
Kai Steinfeld, Reinhard Göhler, Berlin
Power systems of electric
railways may be protected against
overvoltages by metal-oxide (MO)
arresters. Basic designs of the
housing are porcelain, polymer
composite and directly moulded
polymer. With respect to safety, their
different short-circuit behaviour must
be considered. The permissible
bending moment of directly moulded
arresters is lower as compared to
arresters with porcelain or polymer
composite housing. Furthermore, the
MO varistors may be pre-damaged
by improper handling.
Metalloxidableiter für elektrische
Bahnen
Metalloxid-Ableiter können zum
Überspannungsschutz in Anlagen
elektrischer
Bahnen
verwendet
werden. Die Gehäuse sind aus
Porzellan, Kunststoff-Verbund oder
direkt vergossenem Kunststoff mit
unterschiedlichem
Kurzschlussverhalten. Die direkt vergossenen
Ableiter sind preiswerter, jedoch
geringer auf Biegung belastbar und
empfindlicher gegen unsachgemäße
mechanische Handhabung.
Parafoudres à oxyde métallique
pour réseaux de traction
Des déviateurs en oxyde métallique
peuvent servir à la protection contre
les surtensions dans les installation
des chemins de fer électriques. Leurs
carters consistent de porcelaine, de
matières synthétiques assemblés ou
des matières synthétiques coulées
directement,
présentant
des
comportements différentes en courtcircuit. Les déviateurs coulées en
direct sont plus avantageux, mais
moins résistant contre le maniement
mécanique maladroit.
1. Introduction
The power systems of electric
railways, like all electrical energy
systems, are at risk of damage from
voltage surges. This is because the
equipment insulation is invariably of
only limited dielectric strength,
which can be exceeded by the
overvoltage, leading to a breakdown
of
the
insulating
materials.
Breakdown of a solid or liquid
insulation generally results in
permanent damage to the equipment
because it allows a short-circuit of
varying degree of conductivity to
develop. An event of this nature
usually puts the affected item of
equipment or the surrounding power
system out of operation pending
replacement or repair. Apart from the
actual electrical systems, track-side
electronic
equipment
is
also
increasingly at risk as it becomes
more ubiquitous from overvoltages.
Factors which cause voltage surges
in the power systems of electric
railways
include
atmospheric
discharges, switching operations as
well as certain circuit and/or loading
conditions of the power system.
Atmospheric
discharges,
i.e.
lightning strikes actually on the
power system or close by, produce
surges lasting typically around 10 µs.
Switching operations in the power
system may produce overvoltages
with a typical duration of several
100 µs, depending on the impedance
switched. Certain circuit and loading
conditions in a power system cause
temporary
overvoltages
(TOV),
whose duration is highly dependent
upon the way the power system is
managed. The figure specified in IEC
60099-4 is 100 s, for example [1].
Lightning strikes pose the greatest
hazard for electric railway traction
systems in terms of overvoltages.
The incidence and current amplitude
of such strikes are a function of the
local topography and the structural
design of the power system. With the
resources currently available, full
protection is very costly and is
possible only in exceptional cases. A
lightning protection concept is
therefore often a compromise
between operational requirements
and economic criteria and is only
able to cope with a certain proportion
of
possible
strike
incidents.
Moreover, the overvoltage protection
concepts needed in traction system
applications must also protect against
the indirect consequences of a
lightning strike, for example induced
voltages in electronic equipment.
Metal-oxide (MO) surge arresters are
an
important
and
nowadays
indispensable element of modern
lightning protection concepts. These
protective devices limit the level of
overvoltages within a power system
to values below the withstand voltage
of the equipment used within that
system [2].
2. Metal-Oxide (MO)
Varistors for Surge Arresters
The operating principle of an MO
surge arrester is essentially based on
the strongly non-linear currentvoltage characteristic of specific
ceramic elements, so-called MO
varistors, as shown here in Fig. 1,
taking the example of an arrester for
a 2 kV DC traction system. These
MO varistors limit the voltage acting
on them, and fluctuating in a range
equivalent to several powers of ten of
the current, to a relatively constant
level and in this way protect parallelconnected equipment against an
impermissibly high voltage stress.
The most important parameter is the
surge arrester’s so-called protection
level, i.e. the voltage received by the
surge arrester under exposure to a
lightning impulse with waveform
8/20 µs and a current amplitude of
10 kA.
Fig. 1: Current-voltage characteristic of a surge arrester
for 2 kV DC traction systems.
The material used for MO varistors is
approximately 90% zinc oxide (ZnO)
with 5% bismuth oxide (Bi2 O3 ),
doped with 5% other metal oxides.
As can be seen in Fig. 2, the
microstructure of an MO varistor
consists of zinc oxide grains with a
diameter of around 10 to 50 µm,
which are separated by bismuthoxide-rich phases during production.
During this process, every boundary
between the essentially electrically
conductive zinc oxide and bismuth
oxide forms a potential barrier of
approximately 2.9 V, resulting in an
overall electrical performance that is
similar to a statistical series-parallel
circuit of many Zener diodes. For a
given grain size of the MO ceramic
material, the protection level
provided by the surge arrester is
determined by the height of the MO
varistor or stack thereof. A typical,
specific figure for the normalgradient varistors commonly used for
rail traction systems is approximately
300 V/mm height, i.e. the varistor
stack in the surge arrester described
in Fig. 1 is about 16 mm high.
MO surge arresters used in traction
system applications absorb large
amounts of energy when a surge
occurs and the varistors can undergo
a significant temperature rise of up to
50°K as a result. To prevent
overheating of the surge arrester, it
must contain sufficient material to
provide an adequate heat storage
capability. This is assured for a given
protection level by providing
elements of appropriate diameter. For
example, the surge arrester illustrated
Fig. 2: Microstructure of an MO varistor material.
in Fig. 1 has a diameter of
approximately 70 mm.
Previously, surge arresters which
contained silicon carbide (SiC) and
featured a series gap were used.
These are no longer manufactured,
but are still found in some power
systems.
3. Types of MO Surge
Arresters for Electric
Railway Systems
3.1. General
The electrical dimensioning of surge
arresters produces a stack of MO
varistors with a specific height and a
specific diameter. This stack is
enclosed in a housing which, for the
entire life of the arrester, must retain
the
required
electrical
and
mechanical properties for the specific
application as well as resistance to
the specified ambient conditions.
Previously, porcelain was the only
suitable material for insulated
housings, but polymer housings are
now also available, allowing for
certain design innovations.
3.2. MO Surge Arresters with a
Porcelain Housing
The classic insulation material for
surge arrester housings in the power
engineering sector and for railway
traction systems is porcelain. Fig. 3
shows a typical arrester with a
porcelain housing for a DC traction
system. The stack of MO varistors is
tensioned with a construction of
fibreglass-reinforced plastic (FRP)
Fig. 3: Surge arrester type 3EC3 with porcelain housing for DC traction systems
(Table 1). Total height 233mm
1 case
3 MO-Varistors
5 seal
2 compression spring
4 flange
6 pressure relief membrane
-2 -
bolts, contact and pressure springs
and retaining plates. It is contained
inside a porcelain housing. The
flange-like metal end fitting is
fastened to the porcelain housing
using sulphur cement. This has
significant advantages in terms of
production over Portland cement,
which can also be used, particularly
because it becomes stable under load
soon after pouring. The end fitting is
also equipped with a pressure relief
and sealing system, which are vital
for reliable surge arrester operation.
Although modern MO surge arresters
are exceptionally reliable items of
equipment, adverse conditions, e.g.
incorrect
dimensioning,
direct
lightning strike or voltage overspill
can nevertheless cause them to fail.
In such cases, a short-circuit occurs
inside the surge arrester and the
resultant arc produces a significant
pressure increase inside the housing.
To prevent the housing from
exploding,
which
would
be
hazardous
for
personnel
and
materials, the surge arrester flange is
sealed with a fine metallic membrane
only a few tenths of a millimetre
thick, which quickly ruptures in the
event of a short-circuit, so protecting
the
housing
against
internal
overpressure. This process, known as
blowout, also expels the arc from the
housing where it can continue to burn
until the power system has been shut
down.
While surge arresters with a
porcelain housing must never be
allowed to explode after shorting, a
so-called
thermal
secondary
breakdown as defined in EN 50123-5
or IEC 60099-4 [1] is permissible. In
this situation, the thermal stresses
produced by the action of the electric
arc on the porcelain cause the
housing to break down. This
disintegration is a depressurised
event, however; in other words,
fragments merely drop off the
housing.
The pressure relief system must be
reliably sealed for the entire service
life of the surge arrester, a criterion
which is met through meticulous yet
relatively uncomplicated design
principles and the selection of
suitable and, above all, correctly
coordinated materials.
3.3. MO Surge Arresters with a
Polymer Composite Housing
The availability of polymer materials
with the right properties for power
engineering applications has also led
to more surge arresters with polymer
housings being used in traction
systems for electric railways.
Initially, the previously described
design features of surge arresters
with porcelain housing were adopted
and only the material or design of the
housing was modified. A surge
arrester with a polymer composite
housing designed for DC or AC
traction systems is shown in Fig. 4.
As these diagrams show, the internal
configuration of MO varistors and
retaining device conforms with that
of the surge arrester with porcelain
housing. Where the design differs,
however, is with regard to the
insulating enclosure, which consists
of an FRP tube with integral
injection-moulded
shielding
of
silicone
elastomer
or
with
individually bonded shields. In this
design configuration, the FRP tube
gives the housing its mechanical
strength, while a coordinated system
of FRP tube and silicone elastomer
shielding safeguards the arrester’s
electrical properties. The FRP tube is
bonded to the flange, so permitting
maximum bending moments. The
polymer composite housing, like the
porcelain housing before it, is a
mechanically stable, hermetically
sealed system and therefore requires
a pressure relief system to prevent its
explosive disintegration in the event
of a short-circuit. However, it differs
from the porcelain housing in that the
material properties of the FRP tube
do not allow for any thermal
secondary breakdown and also
because the housing retains a
bending moment of at least 75% of
the original figure following a shortcircuit. This type of housing is
therefore the safest version from the
point of view of personnel and plant
protection, despite being the most
complex and expensive of the
various alternatives available.
3.4. MO Surge Arresters with a
Directly Moulded Polymer
Housing
Commercial demand for costoptimised surge arresters for electric
power systems has resulted in the
development of directly moulded
designs. With this design, a
mechanically stable, sealed housing
has been dispensed with in favour of
a very much simpler structure and
minimised use of materials. The disklike MO varistors and their
respective end fittings are fastened
together by means of FRP rods,
screws or loops to form a self-
Fig. 4: Surge arrester type 3EB1 for DC- or AC traction system with polymer
composite housing (Table 1) mounted on ICE 2 power car.
Total height 296 mm
1 compression spring
3 FRP tube
5 flange with seal and
2 MO-Varistors
4 HTV s ilicone sheds
pressure relief membrane
-3 -
supporting configuration around
which the silicone elastomer is then
moulded. This design is similar to
that of a reinforced concrete bridge in
that its mechanical stability relies on
a sufficiently high level of prestressing; in other words the MO
varistors are mechanically prestressed. The term “direct moulded“
indicates that the silicone elastomer
is applied directly to the surface of
the MO varistors. Directly moulded
surge arresters are now used almost
exclusively in medium-voltage threephase systems, and they will become
increasingly commonplace in highvoltage three-phase systems in
future. Surge arresters with directly
moulded polymer housings are also
available for the power systems of
electric railways, and appropriately
modified directly moulded surge
arresters are frequently used for
medium-voltage three-phase systems.
The design of such surge arresters is
remarkably
uncomplicated
and
essentially consists of metallic
terminal fittings with the MO
varistors fastened between them by
means of FRP tensioning elements.
The housing is then formed by
moulding a layer of silicone
elastomer directly onto these
elements, as shown in Fig. 5; this
design has no pressure relief or
sealing system.
The arc produced in the event of a
short-circuit exits to atmosphere
through the mechanically weak
silicone elastomer
layer,
and
continues to burn there without any
explosive destruction of the housing.
Even without a pressure relief
system, therefore, there is no highspeed ejection of housing fragments.
However, the arc accompanying a
short-circuit generally damages the
surge arrester’s mechanically selfsupporting
internal
structure
seriously enough to destroy its
mechanical stability, with the result
that it is no longer able to absorb
forces from bolted on busbars or
connecting cables, for example.
Another possible scenario, similar to
that
occurring
with
thermal
secondary breakdown of a surge
arrester with a porcelain housing, is
that internal fragments of the arrester
or of the MO varistors may be
expelled from the housing.
However, short-circuit tests on the
surge arrester performed in the
context of its type testing are
required to demonstrate that any
fragments ejected in this manner
merely drop away loosely from the
housing
and
that
there
is
consequently no risk to personnel or
adjacent plant components from
high-speed splinters.
The mechanical strength of directly
moulded surge arresters, particularly
their permitted bending moment, is
generally significantly lower than
that of alternative designs with a
porcelain or composite polymer
housing. Moreover, forces applied
through the connections are added to
the pre-tensioning force applied to
the MO varistors, the elements
responsible for the surge arrester’s
electric
response,
potentially
resulting in imperceptible damage to
the MO varistors through improper
handling. There is no chance of such
damage with surge arresters with a
porcelain or composite polymer
housing because mechanical forces
are absorbed by the housing and the
only mechanical load on the MO
varistors comes from their spring
loading which is necessary to ensure
perfect electrical contact. Table 1
compares the permitted bending
moments of Siemens surge arresters
of the various types described above.
4. Use of MO Surge
Arresters in DC Traction
Systems as per VDV 525
Publication No. 525 [3] of the
Association of German Transport
Undertakings (VDV) gives operators
of
DC
traction
systems
recommendations on effective surge
protection in the event of lightning
strikes. It first of all distinguishes
between external and internal
lightning
protection,
whereby
external lightning protection applies
directly to the overhead traction line
systems, and also the running rails in
the case of traction systems with
contact lines, while an overhead
contact line acts as a lightning
arrester device. Internal lightning
protection refers to measures
implemented in the substations, but
also encompasses the use of surge
arresters in low-voltage systems or
signalling
and
communications
equipment close to or in the tracks.
4.1. Earthing of Power Systems
for Electric Railways
The planning of lightning protection
concepts pays particular attention to
the earthing of power systems. If the
rails of DC traction systems are
insulated to earth to reduce stray
Table 1:Permissible bending moments of Siemens surge arresters with
porcelain housing, polymer composite housing and directly moulded
polymer housing as used for the power systems of electric railways
Type
Housing
3EC3
Porcelain
Max. system
voltage [kV]
DC
AC
Permissible
bending Moment
Fig. 5: Surge arrester type 3EB2 with
directly moulded polymer housing
(Table 1).
Application
kV
kV
4
Nm
2000
stationary
and on cars
-4 -
3EB1
Polymer
composite
4
37
3EB2
Directly
moulded
2
2600
1250
mainly
on cars
mainly
stationary
current corrosion, as is required for
new tracks, they are unsuitable as
earth
terminations.
In
such
applications, low-resistance tower
footings, driven poles, reinforced
concrete guideways or separate earth
rods must be used as earth
terminations, depending on local
track conditions. Where the rails
have been laid without any additional
insulating measures, however, they
generally exhibit only minimal
leakage resistance and can be used as
earth terminations. In such cases,
however, the discharge current is
conducted to earth via the rails,
which poses a risk for electrical and
electronic equipment close to or
actually in the track. An effective
means
of
protecting
against
overvoltages resulting from this
process is to provide additional surge
arresters in the actual equipment.
Substation earthing arrangements are
unproblematical
because
the
structure’s own earthing system
serves as an efficient, low-resistance
(≤ 2 Ω) earth electrode for lightning
protection measures.
Fig. 6: Protection of the overhead contact line by A1 surge arresters (from [3]).
4.2. Use of A1 Surge Arresters to
Protect Overhead Contact Lines
Fig. 6 shows that to provide
comprehensive
protection
for
overhead contact lines, “A1” type
outdoor surge arresters to VDV 525
designation should be installed at
every feed point, at the ends of feed
sections and dead-end feeders, at tie
points and at power extraction points.
Additional A1 surge arresters are
recommended for sections of track
that are particularly prone to
lightning strikes, for example bridges
or free overland stretches without
trees. A favoured method of
installation in order to keep the
electrical connection to the overhead
contact line as short and straight as
possible is to mount the A1 surge
arrester at the same height as the
overhead contact line. The earth
connection should be connected to
the earth electrodes by the most
direct route possible and the cable
used for this purpose must itself be
insulated against the overhead
contact line tower or the structure’s
earth, as shown in Fig. 7.
Fig. 7: Connection of A1 surge arresters for earthed and non-earthed rails (from [3])
4.3. A1 and A2 Surge Arresters
for Substation Protection
The wiring of supply and return lines
in the substations with surge arresters
is an important element of a lightning
protection concept for a railway
power system. Two differently
dimensioned surge arresters are used
in this case and their principal mode
of action is described in Fig. 8:
The A1 surge arresters are connected
between the line circuit-breakers or
cable terminals and the return line,
where
they
effectively
limit
overvoltages on line circuit-breakers
and power rectifiers when positive
lightning
strikes
occur
and
simultaneously protect the associated
control and measurement equipment.
The currents from negative lightning
strikes are conducted through the
-5 -
diodes of the power rectifier, which
is not itself damaged thanks to its
substantial current carrying capacity.
The unavoidable rise in potential due
to a lightning impulse current is
limited by the A2 surge arresters
between return line and the
structure’s earth. As in the case of the
A1 surge arresters, these connection
cables should also be as short as
possible and also short-circuit-proof.
References
[1] IEC 60099-4, Edition 1.2, 2001-12
Metal-oxide surge arresters without
gaps for a.c. systems
[2] Fien H., Hinrichsen V., Pelmer J
Hochspannungs-Metalloxid-Ableiter:
Auslegungs- und
Konstruktionskriterien.
Elektrizitätswirtschaft, Vol. 93
(1994), Issue 19, pp. 1148-1156
[3] VDV Schriften 525 12/01: Schutz der
Fahrstromversorgungs-anlagen von
Gleichstrombahnen bei
Blitzeinschlag , Verband Deutscher
Verkehrsunternehmen (VDV),
Karnekestraße 37-39, 50672 Cologne
Fig. 8: Protection of a substation using A1 and A2 surge arresters
EBS: equipotential bonding strip
4.4. Protection Following Failure
of an A1 Surge Arrester
Although MO surge arresters are
extraordinarily reliable items of
equipment with a failure rate
significantly below 0.1%/year, they
do sometimes break down under
adverse conditions. Such failures are
mostly due to non-specification
loadings, e.g. direct lightning strikes
of unusually high current intensity or
a voltage overspill, both of which
make the A1 surge arrester
permanently conductive. If the
running rails exhibit a low leakance
per unit length, the earth electrode
can absorb an inadmissibly high fault
voltage. However, connection of an
additional A2 surge arrester with a
low continuous voltage (120 V ≤ Uc
≤ 300 V) between the earth electrode
and the return conductor (as shown in
Fig. 9) limits this fault voltage and
trips the line circuit-breaker by backfeeding from the overhead contact
line system.
5. Conclusions
Metal-oxide surge arresters are an
indispensable element of lightning
protection concepts for the power
systems of electric railways. The live
electric element consisting of MO
varistors can be enclosed in a
porcelain, polymer composite or
directly moulded polymer housing.
With each of the various forms, it is
important to consider their different
mechanical properties and failure
performance, so as to optimise the
level of protection provided for
personnel and property. VDV
publication
No.
525
gives
recommendations on the use of
metal-oxide surge arresters for the
protection of railway power systems.
A concept using A1 and A2 surge
arresters
to
protect
against
inadmissibly high potential rises or
contact voltages - both in the
substations and in items of track-side
equipment - is presented for the first
time.
Fig. 9: Protection of a contact line
using A1 and A2 surge
arresters (from VDV 525)
-6 -
Dr.-Ing.
Kai Steinfeld (38)
studied
electrical
engineering at Berlin
Technical
University
and was awarded his
doctorate there from the
Institute of Electrical
Engineering
(HighVoltage Department).
He is Head of Development at the Surge
Arrester and Limiter Unit at Siemens AG.
Fon: +49(0) 30 386 2 46 55, Fax: -2 49 08
Email: kai.steinfeld@siemens.com
Dipl.-Ing.
Reinhard Göhler (47)
studied
electrical
engineering
at
Brunswick Technical
University. He is in
charge of the Test
Laboratory (and also
Sales Director for
Special
Surge
Arresters) in the Surge Arrester and Limiter
Unit of Siemens AG.
Fon: +49(0) 30 386 2 33 90, Fax: -2 67 21
Email: reinhard.goehler@siemens.com
Address: Siemens AG, PTD H 42,
Nonnendammallee 104, D-13629 Berlin