Background of the invention
The invention relates to a system for lightning and surge protection
In particular, the invention may be used as a system for lightning
and surge protection for an object set up on a limited area, on the ground or on
a building. Example of such an object comprise an installation provided with an
antenna, e.g. a GSM base station. Surge protection devices for electrical power
supplies are generally known.
The protection device according to the application may be used to
protect against surge with a high energy content, such as surges caused by lightning
or electromagnetic pulse (EMP). More in particular, the application relates to protection
against surge caused by lightning strikes in a power supply for electrical equipment
set up in objects, such as transmitter/receiver stations for radio traffic.
For such a protection device, in addition to a number of specific
components and measures, one or more (preferably at least two) earth electrodes
are employed for the purpose of deflecting the charge which is inherent in the surge
and distributing it over the greatest possible area. It goes without saying that
these earth electrodes must have the least possible resistance to the zero potential.
It is, moreover, important that the ground area over which the charge of the lightning
strike is to be distributed is at least of a minimum magnitude.
E.g, in the case of electrical power supplies for base stations for
mobile telecommunication, such a minimum area is often not available because, for
economic reasons, the area is preferably chosen to be no larger than necessary for
the dimensions of the foot of the antenna mast. In the case that the object is positioned
on top of a building, usually only a limited number of conductors with earth electrodes
If the charge of a lightning or of EMP strikes the cabinet in which
the power supply is housed or the metal frame to which the cabinet is attached,
there is a danger of parasitic flash-over of the charge to the electrical conductors
of the power supply. Since this charge is dissipated relatively poorly, the voltage
in the power supply can rise to such an extent that flash-over can damage the components
of the power supply, such as switches or cause failures of the power supply. Also,
other equipment of the object, such as the equipment being supplied with power may
That this voltage can be substantial can be understood from the fact
that from a direct lightning strike a peak current of as much as 150 kA may arise,
which must be deflected via an earth electrode having an impedance of 2.5 Ohm (this
value being a standard value for earth electrodes, in practice this value may be
higher or lower).
Momentarily, this may lead to voltages of over 100 kV. For such a
peak voltage, a power supply for low voltage applications is not equipped.
Such a parasitic flash-over is prevented according to the state of
the art by connecting surge protective devices, such as varistors or spark gaps,
between the frame and each of the phases and between the frame and each of the neutral
conductor of the power supply. The frame is connected directly to an earth connection,
such as one or more earth electrodes. This way, parasitic flash-over from the part
on which the strike occurs to one of the conductors is prevented.
In this known solution, however, it can not be prevented that in the
power supply substantial damage occurs when a direct lightning strike occurs on
the frame, which will be further explained in the detailed description. Here, it
suffices to mention that this damage may comprise the burning of one or more components
of the power supply caused by the very large currents. Moreover, mechanical damage
may arise in the power supply as the large currents flowing through the conductors
of each of the phase conductors and of the neutral conductor cause the connecting
conductors to be pulled from the connection points, through the electromagnetic
fields caused by the large currents, as a result of which an interruption in the
current flow occurs.
It need not be mentioned that, also because of the earlier mentioned
periphery arrangement of the power supply and the less proper accessibility thereof,
repair of the damage will take a lot of time. As a result, the installation powered
by the power supply will be out of service for a longer period of time, which leads
to a higher risk of operational damage.
European patent application EP-A-0 128 344 describes an arrangement
for surge arresters in a high voltage transformer. In this arrangement, surge arresters
are connected between each phase conductor and the neutral conductor, and also between
the neutral conductor and an earth electrode. The surge arresters are all of the
same type. Additionally, a capacitor is connected between the neutral conductor
and a second earth electrode. This results in a protection of the transformer against
too high voltage peaks, in which the capacitor control the dynamic behavior of the
surge arresters. The surge arresters are usually chosen to be spark gap elements,
as these can be used in high voltage applications. The arrangement described is
meant specifically for protection of the high voltage transformer.
A disadvantage of the use of spark gap elements or spark gaps connected
between the phase conductors and the neutral conductor is that a rest voltage results
which is poorly defined and usually too large. Furthermore, spark gaps will keep
an undefined rest voltage, dependent on the rise time of the lightning pulse, which
may be 2.5 to 4 kV, which is too high for low voltage equipment. Also, the spark
gap elements cause a short circuit and thus a net following current, which almost
certainly results in breakdown of the fuses (of the electricity provider). Breakdown
of the fuses results in operational down time of the equipment supplied by the transformer.
In a conference paper "Bliksem Seminar 1999". 1999, page 49, figure
17, from which the claims are delimited, a lightning protective device is shown
that comprises a first protective device that is the same as the lightning protection
device as shown in EP-A-0 128 344. However, this figure 17 also shows an additional
protective device that comprises varistors between the three phase conductors and
the neutral conductor, and a spark gap between the neutral conductor and an earth
electrode. This additional protective device is arranged downstream from the first
protective device, seen in a direction from the mains. The first protective device
is disclosed to comprise components of a lightning current capability of a class
B, corresponding to a lightning protection class of > 50 kA (10/350 µs), >
75 kA (10/350 µs), or > 100 kA (10/350 µs). The additional protective device
is disclosed to be an overvoltage protection device with class C components. Class
C components have a surge current capacity of > 20 kA (8/20 µs).
The present invention seeks to provide a system for surge protection
for use in low voltage applications, which does not have the disadvantages of the
known systems described above. The present invention also seeks to provide a solution
to the problem that the power supply defined in the preamble above, has such a limited
deflection path in earth in order to deflect the charge of the strike, that the
peak voltage occuring is relatively high and decreases relatively slowly.
Summary of the invention
According to the present invention, a system for surge protection
is provided as claimed in claim 1.
The surge protective device of the first type is primarily meant to
provide a well defined safety level (maximum voltage over ist connection leads)
and the surge protection device of the second type is primarily meant to arrest
or deflect high currents.
By using different types of surge arresters between the phase conductors
and the neutral conductor and between the neutral conductor and the earth electrode,
the system provides a very efficient surge protection, e.g. caused by lightning
strike on an object. The solution of the present invention has as one insight whereon
the invention is based that the frontal edge of the lightning current flows through
the earth electrodes and the other components of the lightning current flow through
the connected conductors (i.e. the supply conductors and other conductors). It is
believed that this phenomenon occurs because of the limited ground surface to which
the earth electrode is connected. The neutral conductor between the power supply
unit and transformer to which the power supply unit is connected, is not connected
to a self-induction while the phase conductors are connected to a self-induction.
The self-induction may be a transformer coil or winding, or a coil of a kilo-watt
hour meter. This causes that the current through the neutral conductor will be larger
than the current through the phase conductors.
The surge protective device of the first type is a voltage dependent
resistor, or varistor. The resistance value of such an element abruptly decreases
when the voltage over the element passes a preset voltage value. The surge protective
device of the second type is a spark gap element, or spark gap. These elements cause
a discharge to occur when the voltage across its terminals increases above a preset
value, and are usually applied when high voltages are to be expected.
The surge protective devices of the first type ascertain that smaller
currents flow through the phase conductors while also ascertaining that too high
a voltage on the phase conductors is limited to a well defined value. Furthermore,
the surge protective device of the second type ascertains that the large lightning
current flows via the element into the central conductor which is not provided with
In an embodiment of the present invention, the surge protective device
of the first type and surge protective device of the second type are included in
front of a switch provided in the supply unit, seen in the direction of power flow
from the external transformer. This arrangement assures that the currents caused
by a lightning strike or EMP do not flow through the switch of the system, resulting
in a better protection of one of the elements of the power supply. In former actual
cases of damage caused by lightning strike, the switch was completely burnt.
In a further embodiment of the present system, the switch may be switched
off by means of an earth leakage circuit breaker. The earth leakage circuit breaker
is also protected by the present system. Earth leakage circuit breakers are applied
in general for high impedance earth circuit. In a normal arrangement (surge arresters
between phase and earth) a defect in one of the surge arresters can lead to two
high voltage of the high impedance earth, and thus also for the connected equipment.
The normal protection system can thus only be used after the earth leakage circuit
breaker, in order to disconnect such an unwanted situation, and as a result, the
earth leakage circuit breaker may still be damaged when a lightning strikes. The
present invention, however, may be positioned in front of the earth leakage circuit
breaker, as by using a spark gap, no galvanic connection as present between the
neutral conductor and earth. As a result, the earth (and all connected equipment)
cannot be put on too high a voltage when one of the surge arresters fails.
In a further embodiment, the earth leakage circuit breaker is of a
self-resetting type. Such an earth leakage circuit breaker will reset after a predetermined
period of time, thus reconnecting the power supply automatically. When there still
is an earth failure, the eath leakage circuit breaker will switch off again.
In an even further embodiment, the surge protective device of the
second type is of the non blowing-off type. The surge protective device of the first
type is a voltage dependent resistor or varistor and the surge protective device
of the second type is a spark gap element. This arrangement assures that no hot
gasses or high pressure occur, which are typical for state of the art spark gap
elements which are blowing off.
The elements of the power supply unit are positioned inside a closed
cabinet. This allows to build a small and reliable cabinet comprising the elements
of the power supply (i.e. power supply connections and the protection circuity),
which is moreover cost-effective and easy to assembly. By using surge protective
device of the non blowing-off type, the cabinet will not be exposed to high internal
pressures or hot gasses. This also has the added advantage that the connection between
neutral conductor and surge protective device may be a short connection, which results
in less mechanical forces on the connections cansed by strong electromagnetic fields.
The surge protective device of the second type has a rating of at
least 40 kA, more preferably at least 5 kA and even more preferably at least 100kA.
This will allow an effective surge protection system offering protection to currents
which have been encountered in practise after lightning strikes on objects with
a small foot print.
The surge protective device of the first type has a rating of at lest
4 kA, more preferably at least 8 kA. This will suffice for the current flowing through
these elements occuring after a lightning strike.
The neutral conductors of the system and the interconnections between
the neutral conductors have a cross section of at least 8 mm2, more preferably
at least 16 mm2. Also the conductors connected to the earth electrodes
and all interconnecting items have a cross section of at least 8 mm2,
more preferably at least 16 mm2. This should include all connection through
which current flows, including interconnection of clamps to which the neutral conductor
or earth conductors are connected. The highest currents will flow through the neutral
conductors and to the earth electrodes, and as a result the complete path through
which these currents flow should have a predetermined minimal diameter. Preferably,
at least part of the conductors connected to the earth electrode is formed by a
metal plate. This allows are effective flow of the current over the neutral and
earth conductors leading to lower electromechanical forces.
Short description of the drawing
The invention will now be explained in further detail referring to
the drawing, in which:
Detailed description of an embodiment
- Fig. 1 shows a schematic diagram of a system for protection of low voltage equipment
according to the prior art;
- Fig. 2 shows a schematic diagram of a first embodiment of a system according
to the invention;
- Fig. 3 shows a schematic diagram of a second embodiment of a system according
to the invention.
In Fig 1, which shows the state of the art, I indicates the part which
is positionned in the space of the electricity supplies. Three windings of a transformer
are referenced by numeral 1, the secondary winding of the transformer being in a
star configuration of which the star point is earthed. The impedance existing between
the star point and the point of the zero potential is indicated by Ra, which usually
has a very low value, e.g. a 0.5 Ohm and a self inductance of e.g. 5 µH. The secondary
windings of the transformer usually have a resistance value of about 0.01 Ω
and a self inductance of 50 µH. In this space, also the fuses 2 are positioned.
The equipment of the user, in the part indicated with II, comprises
a three phase switch 3. Each of the conductors (phase and neutral) is connected
behind the switch 3 to a connection 5 via a surge arrester 4, such as a voltage
dependent resistor or varistor. The connection 5 is connected to the frame of the
switch closet (or object). This connection 5 is being earthed by means of a earth
conductor, the impedance of which is Rb. The conductors leading away from the switch
3, which supply the further equipment with energy, are indicated with reference
numeral 6 for the phase conductors and reference numeral 7 for the neutral conductor.
The further equipment is provided with possible own protection and has a relatively
high input impedance.
When a lightning strike hits the frame of the object, the charge must
be deflected via the earth conductor 5 to earth. In an ideal case, the impedance
Rb, via which the lightning energy is deflected, has a zero value. Standardisation
norms for lightning protection require a value of maximum 2.5 Ohm.
A limiting factor for the deflection of the energy caused by the lightning
strike is the small ground surface on which the object (or equipment) is usually
positioned. The small dimensions of the ground surface prevent a quick deflection
of a large amount of charge within a short time period. This substantially enhances
the chance of heating of the equipment in the object II and also the risk of voltage
In a lightning strike in which a momentary value of the current may
be as high as 150 kA, a voltage on the earth conductor 5 may be as high as 75 kV.
This voltage may easily flash over to the switch 3, which in normal operation is
closed. In almost all situations, such a flash-over will cause severe damage of
the switch 3 and often to melting of the fuses 2.
In order to keep the peak voltage as low as possible and to limit
the time period as much as possible, surge arresters 4, such as voltage dependent
resistors are provided. The largest current will usually flow through the neutral
conductor 7, as this has the lowest impedance value.
At a lightning strike on the frame, the resistors 4 will decrease
the danger of flash surge to the fuses 2 and switch 3, but the large currents that
will flow from part II to part I (of the electricity supplier) will still have values
which may be substantially higher than 40 kA, as a result of which still burning
phenomena will occur on the fuses 2 and switch 3.
As a result of the arrangement of the object II, this may lead to
a prolonged period of time in which the object II is not operational, which disturbs
the service supplied by the equipment in an economically non-attractive fashion.
The circuit according to Fig. 2 provides a solution to this problem
in accordance with the present invention. The voltage dependent resistors 4, which
are on one side connected with the phase conductors 6, are not connected on the
other side to the earth conductor 5, but with the neutral conductor 7. The neutral
conductor 7 now connects the lightning current arrester 9 with the earth conductor
Such a lightning current arrester, which at flash-over causes a short
circuit situation of limited time duration, is known in the art (see e.g. German
patent applications DE-A-19 74 2302 and DE-A-19 75 5082 and European patent application
EP-A-0 128 344 mentioned in the introduction).
When a lightning strikes the frame of the object II, the peak voltage
on the phase conductors 6 will now be largely suppressed by the voltage dependent
resistors 4. The lightning current arrester 9 will cause an almost complets short
circuit between the neutral conductor 7 and the earth conductor 8, resulting in
that the current caused by the strike is only partially deflected from the object
II to the tranformer in part I via the phase conductors 6.
The peak current to be deflected, will now be deflected via the parallel
circuit of the earth resistance Ra and Rb. Of course, the resistance of the phase
conductors 6 and the neutral conductor 7 between the object II and the transformer
in part I still plays a role, but in pratical situations this connection will not
result in problems because of the low impedance.
Figs 3 shows a further embodiment of the system according to the present
invention, in which the switch 3, viewed in the direction of power flow to the equipment
in part II, is positioned behind the components for current and voltage suppression.
This further reduces the risk of overland of the switch 3 by large currents. The
only component in the system upward of the protection system are the fuses 2 of
the electricity provider. Although test have shown that the lightning induced currents
through the phase conductor 6 are relatively small, it may still occur that the
fuses 2 break down. To assure that down time due to blown fuses 2 is minimised,
it is preferred that the fuses 2 are of the automatic type, as these can better
withstand the lightning induced current than fuses 2 of the melting type. Moreover,
the fuses 2 of the automatic type can be reset manually, or form a remote location.
The surge protective device 9 of the second type has a rating of at
least 40 kA, more preferably at least 50 KA and even more preferably at least 100
kA. This will allow an effective surge protection system offering protection to
currents which have been encountered in practise when lightning strikes on objects
with a small foot print. The surge protective devices 4 of the first type have a
rating of at least 4 kA, more preferably at least 8 kA. This will suffice for the
currents flowing through these elements occurring after a lightning strike.
All elements of the embodiments described above are integrated into
a single cabinet. Using a surge protection device 9 of a non blowing-off type will
allow to also integrate this element in the cabinet, as no hot gasses or high pressures
can occur. To be able to withstand the high currents flowing through it, the neutral
conductor 7 and/or earth conductor 5 of the system and the interconnections between
them (such as clamps, etc.) are all made of a material having a cross section of
at least 8 mm2, and more preferably at least 16 mm2. This
should include all connections through which current flows, including interconnections
of clamps to which the neutral conductor 7 and/or earth conductor 5 are connected.
The highest currents will flow through the neutral conductor 7 and earth conductor
5, and as a result the complete path through which these currents flow should have
a predetermined minimal cross section. Preferably, at least part of the earth conductor
5 is formed by a metal plate. This allows an effective flow of the current over
the earth conductor 5 leading to lower electromechanical forces. Also, the connections
to the earth electrodes (towards Ra and Rb in Figures 2, 3) should have a minimum
diameter. In the closed cabinet, special attention should be given to the mounting
of the lightning current arrester 9, as the highest current will flow through this
element when a lightning strikes. These currents may cause large electromagnetic
forces, which may damage the mounting of the element 9.
It is evident that the effect according to the present invention will
also occur when the incoming conductors are connected to a kilowatt-hour meter present
in the space indicated by roman numeral I.
It will also be clear that the solution according to the present invention
is also usable for a single phase power supply.