19/05/2011

Overvoltage protection in photovoltaic systems

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 are
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.

Source: ISKRA ZASCITE d.o.o.