Overvoltage protection in photovoltaic systems

Ljubljana (ots) - Surge Protective Devices (SPDs) are non-linear
resistive elements connected between an active (live) conductor and
the neutral or ground connection of the network

The primary function of an SPD is to limit a voltage transient by
moving into a highly conductive state, thereby conducting current to
earth and limiting the voltage transient to the downstream system to
be protected. The SPD moves back into a high impedance state once
the over-voltage event has passed.

The fundamental difference between photovoltaic and other systems
is a relatively high voltage at a limited short-circuits current.
This requires the adaptation of surge arresters with regard to type
of connection and the use of the component elements.

Typical connections of SPDs in photovoltaic systems:
1. V connection
2. L connection
3. delta connection
4. Y connection

The Metal Oxide Varistor (MOV) and the gas discharge tube (GDT)
are two commonly used components in the design of a SPDs. GDTs must
not be used independently as surge arresters since they cannot quench
the arc when in a state of conductivity. That is why they are always
connected in series with MOVs, primarily as galvanic protection for
MOVs by not letting leakage current pass through them.

In SPD applications, particularly in DC applications, certain
faults started to appear during the SPD service life, previously
unknown in AC applications. In DC applications, problems due to
leakage current typical for MOVs began to appear because leakage
current degrades the parameters of MOVs much faster. This is mainly
reflected as a lowering of MOV's operating voltage point which
eventually causes increased leakage current. Relatively extreme
operating temperatures and moisture will only accelerate the MOV's
degradation process.

Although solutions employing a thermal circuit breaker integrated
in an SPD have been known for some time as protection against
degradation and destruction of the SPD, this is not the most reliable
solution in DC applications. The diagram below illustrating the
functional relationship between current, voltage and distance between
electrodes clearly shows why the existing solutions are not suitable.
Disconnecting distances that would reliably prevent the generation of
an electric arc between the thermal breaker and the disconnected
electrode are simply not long enough to provide safe disconnection.

Challenges and the need for new technologies

All the above stated problems and some of them known from previous
applications are now being solved in an innovative way with the
technology known as SAFETEC TC and with its upgraded technology
SAFETEC TC-G which eliminates the leakage current completely.

The new technology is facing the following challenges as
summarized below

1. How to provide a safer transition of an MOV into a low-ohmic
state when a surge occurs which exceeds the nominal voltage of
the MOV. Known solutions utilize the transition of the MOV into the
short- circuit (low-ohmic) state. This results in the destruction of
the MOV, and the SPD needs to be replaced. When the device reaches
this state, over current protection is activated, which can be either
external or integrated in the SPD. As is generally known, all these
over current protection solutions further increase basic overvoltage
protection costs, and SPD failures result in expensive service
repairs. This problem is typical for SPDs used in AC applications and
some other DC applications, where short-circuit current of the source
is not limited (as e.g. in traction).

2. How to prevent or at least slow down the deterioration of an
MOV due to leakage current and thus prolong its service life.
The simplest method and standard solution is a series
connection of GDT and MOV. Its disadvantage is, however, a
decrease in surge arrester protection levels.

Safetec TC-G

The new technology TC-G actually creates two independent circuits:
1. A circuit, enclosed by GDT and MOV, functions as overvoltage
protection in transient overvoltages or voltage surges,
characterized by a swift increase of current and voltage in a
relatively short period of time. Typical transient
overvoltages are caused by e.g. switching operations, direct
and indirect atmospheric discharges.

2. The second circuit consists of the advanced TC-G technology
The elements are activated in the event of increased voltage, if the
voltage exceeds the declared rated voltage between the SPD's
terminals.

Functions of the elements involved in this second circuit:

a. enables and creates a dynamic voltage drop, which is
needed for the activation of GDT in the branch taking part in
transient voltage events.

b. a function to galvanically separate the MOV from the
supply voltage. As already mentioned, it is the leakage
current that causes the more intense ageing and
degradation of the basic varistor parameters. By fitting
this element in the process of serial production it can be achieved
that MOV is actually activated solely in transient overvoltage
events.

c. limit the current through the MOV, if the voltage is
increased above the arrester's declared rated voltage.The value of
the is carefully selected, ensuring that the maximum current through
the MOV in the initial state of conductivity is about 1A, whereas
after approx. 40 seconds, a current balance is established at the
level of about 10 mA. The above mentioned currents do not exceed the
MOV's energy capacity, and this is precisely the solution ensuring
that after the voltage surge the MOV is still functioning.

3. Another important feature is the mechanical design of the
product realized by a special rotating breaker used to quench the
electric arc between the MOV's surface and the thermal disconnection
mechanism. The essential feature of the design is the insulated
enclosure covering the conductive surface that could trigger an
electric arc.

4. An additional feature, the so-called SPD overload protection,
simply disconnects the SPD from the supply voltage when the surge
current value (Imax) is increased above the declared rated value.

ISKRA ZASCITE d.o.o., Stegne 35, 1521 Ljubljana, Slovenia,

www.iskrazascite.si

BE ON THE SAFE SIDE

The company Iskra Zascite was founded in 1989 in order to develop
and produce surge protection devices for various applications of
protecting electrical and electronic appliances in industry, business
facilities and people's homes. Despite a number of different
technologies which have been appearing on the market in the field of
surge protection devices, Iskra Zascite has remained loyal to
varistor technology and thus contributed its part to the development
of this particular technology. The company also provided guidelines
for the development of varistors which represent the main basic
components of modern surge protection devices. Despite constant
changes which have been appearing in the field of surge protection
devices regarding technology as well as different approaches towards
protecting electrical and electronic appliances against devastating
consequences of lightning and other industrial or switch boosting
voltage, the company has nevertheless managed to keep its position
among the leading companies in this field due mainly to its constant
development and innovative approaches. The development of the company
has been greatly influenced by the development in electronics since
parallel to introduction of new technologies in the field of
electronics also new demands regarding construction and technological
characteristics of surge protection devices appeared. By keeping in
touch with the novelties in electronics as well as constantly
searching for more efficient ways of protection the company has
managed to hold its position in the field to the present day.

Contact:
Jörg Damschen
Senior Executive Manager/Direktionsbevollmächtigter
T: +49 521 60481 M: +49 151 54617156
E: joerg.damschen@iskrazascite.si Skype: damschen.joerg