Diversification of technologies and, as a result, seeking new solutions particularly in terms of how to provide greater safety in cases of surge arrester failures, is becoming the basic principle and mission of all internationally recognised manufacturers. These have been and will be great challenges also in the next development stages with the greatest emphasis laid on safe operation, higher level of protection for users, a lower-priced design and greater operating reliability.

Temporary overvoltages followed by line and earth-fault currents, earth faults in the primary distribution network, neutral conductor failure and heavy voltage fluctuations as a consequence of unstable networks most frequently contribute to a destruction of the varistor element. Temporary overvoltages on low-voltage side normally don’t reach values above 2x Un (in the case of neutral conductor failure), although they may last multiple hours and in the event of a primary distribution network faults they reach values as high as 1200 V with maximum duration of up to 200 ms.

New developments

The recently conducted analysis shows that surge arrester manufacturers are mainly trying to implement the solutions known primarily from the technology of contactors and residual-current circuit breakers. The solutions can thus be divided into the following groups:

1. Solutions to provide an ideal short circuit (lowest possible transient resistance) through the inside of surge arrester. This approach demands robust metal housings, as only robust design can provide a permanent and stable discharge of fault current through the inside of the arrester without the occurrence of flame or explosion of the arrester.

2. By installing residual-current circuit breakers and integrating them with surge arresters. Although various designs differ one from the other, it is essential that they all detect currents already within the range of some mA which enables disconnection of a faulty surge arrester already at low currents. Robust design of housings is therefore not necessary.

3. By integrating overcurrent protection in the very surge arrester. These solutions have certain limitations; there must be a fuse installed in a ceramic enclosure which requires additional space. The varistor must therefore be smaller and the discharge capacity of the arrester will be accordingly lower.

4. By limiting the short-circuit current discharged through the surge arrester; although this solution is relatively new in the market, it is ideal in the event of increased voltages in the power distribution system. The principle of operation is quite simple and robust. A special unit is used to limit the current. When the current runs through the resistor, its resistance is raised, and the available current is thus reduced to the value above the energy capacity of the installed varistor. When the power system is back to normal, there is no need to replace the surge arrester.

A comparison of solutions described above reveals two guidelines. According to the first one the manufacturers actually take advantage of the varistor’s low-ohmic state to activate some other additional protection, which in fact makes the use of surge arrester less cost-effective. Overvoltage protection or residual-current circuit breaker is then activated and the arrester is safely disconnected from the network. There are, however, two major disadvantages. Each time when this happens, a service action has to be carried out, which represents considerable additional costs, depending on location (e.g. wind turbine, photovoltaic power plant). The fact that the faulty arrester and the back-up fuse need to be replaced means additional maintenance costs. Even the clients themselves often lack sufficient awareness of these extra costs. Furthermore, it is clear that the devices are not protected against voltage surges during the time until the electrician arrives, which poses an additional risk of damage to the protected devices and may lead to even higher costs. One should be aware that the downtime costs of the wind generator or a larger photovoltaic plant may be as high as €10,000 per day.

As stated above, the basic condition for the technologies described under points 1 to 3 above is based on the transition of the varistor element into a low impedance state. This means that two conditions have to be met to provide proper operation: low impedance state of the short-circuit loop and of the varistor element.

In principle, this condition can be easily achieved in AC circuits or AC power supply systems. The solutions in this area are well-known and the connections of surge arresters are defined by the standard IEC 60364-5-543.

As already mentioned above, PEN or PE conductor is used in TN systems as a return conductor, whereas with TT networks a neutral conductor is used as a return conductor and connection of surge arresters differs from other configurations.

New challenges

The use of renewable energy sources based on utilization of the energy of the wind and the sun is rapidly growing all over the world. It is a characteristic of these sources that DC currents are generated in photovoltaic panels, DC currents in smaller wind power plants and DC or AC currents in larger ones. All those systems can be independent power generating units or they can be connected to power distribution systems. The ever increasing diversity of the systems and their connection brings potential problems and also requires new approaches.

To provide proper protection against harmful effects of electrical current it is understandable that all these systems must be earthed to prevent step touch voltage or eliminate the possibility of touch voltage by touching metal constructions.

By providing common earthing systems hazardous voltage levels can be transferred from one system to another.

As surge arresters are always connected between line conductors and earthing and as they pass into a low-ohmic state if a fault occurs, this means that the line conductor potential will be present on earthing or earthed elements.

In such conditions earth fault can occur in wind power plants, however, due to high impedance values in the short-circuit loop, the resulting current needed for the fuse protecting the surge arrester to respond, will not be high enough. The values of these currents reach only some amperes or tens of amperes, which is by far not enough. Such currents may generate harmful sparking which only causes accelerated ageing of insulation and shortens the service life of equipment installed in the wind power plant. As transformers are also installed in a wind power plant, such currents may also cause ignition of transformer oil. In addition, these currents may also be hazardous to human life due to the above mentioned touch and step voltages.

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 technology is facing the following challenges as summarized below:

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).

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.

The new technology TC-G actually creates two independent circuits:

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.

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.

Another important feature is the mechanical design of the product realized by a special rotating disc 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. 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.

Prepared by: Igor Juričev