In the future, the structures for power generation, transmission and distribution in the high, medium and low-voltage range will be more complex and flexible than they are today.
New topics such as smart grid, smart metering or smart home require innovative solutions. But also the rapid rise of distributed, renewable energy sources, in combination with centralised power stations, energy storage systems and intelligent technologies, need a reliable and coordinated overall lightning and surge protection system.
The global trend towards the transition to sustainable energy is multi-faceted. To this end, it is important to keep the three objectives of energy policy (environmental compatibility, cost-effectiveness and supply reliability) in balance. Supply gaps quickly cause enormous economic damage, whilst the rapid developments in the energy sector inevitably result in higher demands on technology. This not only affects power generation and transmission networks, but also distribution network structures, where 90 percent of the transition to sustainable energy takes place.
The IEC/EN 62305 standard includes four distinct parts: general principles, risk management, physical damage to structures and life hazard, and electronic systems protection. It was fully adopted by SANS (SANS/IEC/EN 62305-1-4) and for the sake of this article shall be referred to as the standard.
Sources of damage and protection standards
There are various sources of damage for surges. According to part two of the standard, the causes of surges in the case of lightning discharges can be sub-divided into four groups, depending on the point of strike:
• Direct lightning strike to the structure.
• Lightning strike next to the structure.
• Direct lightning strike to the incoming supply line.
• Lightning strike next to/near to the incoming supply line.
Today, the radius of destruction around the point of strike is considered to be more than two kilometres due to highly networked power grids and data networks. In addition, surges are also caused by switching operations, earth faults and short-circuits, or tripping fuses (SEMP/Switching Electromagnetic Pulse).
To minimise the damage caused by the effects of lightning, the following solutions are outlined in the relevant protection standards:
• Material damage and life hazard in case of direct lightning strikes to a structure can be minimised by a conventional lightning protection system (LPS) according to part three of the standard.
• To ensure protection of structures with electrical and electronic systems, particularly if reliable operation and supply are essential, these systems must be additionally protected from conducted and radiated interference resulting from the lightning electromagnetic pulse (LEMP) in case of direct and indirect lightning strikes. This can be achieved by a LEMP protection system according to part four of the standard.
Possible solution approaches and criteria for selecting arresters
A detailed risk assessment of the local threat potential (both for power supply and information technology and communication systems) is required to protect the technologies used for modern grid expansion such as intelligent transformer substations, monitoring and telecontrol systems, adjustable regulated distribution transformers or longitudinal voltage controllers from the sources of damage. This involves certain challenges; for example lightning and surge protection measures for the electronic components lacking ease of maintenance and the frequently compact design of the systems.
According to part two of the standard, the total risk of lightning damage consists of the frequency of a lightning strike, the probability of damage, and the loss factor. If the technologies mentioned above are assessed according to these criteria in conjunction with practical experiences, you will get different individual results depending on the local thunderstorm activity, design and place of installation.
To prevent galvanic coupling to the 20 kV medium voltage overhead line network or outgoing low-voltage lines as a result of a direct lightning strike, a protective device must be installed in the main low voltage distribution board. This protective device must be selected in such a way that it meets the requirements concerning the lightning current carrying capability, short-circuit strength, follow current extinguishing capability and temporary overvoltages (TOV characteristic). A spark-gap-based type 1 combined arrester with integrated backup fuse is ideally suited for this purpose. This integrated backup fuse significantly saves space and installation work compared to a separate arrester backup fuse and is adapted to the discharge capacity of the spark gap. This ensures maximum performance and incorrect installation is avoided.
If only indirect lightning effects such as inductive / capacitive coupling, conducted partial lightning currents or SEMP are to be expected for the secondary technology according to a risk analysis as per part two of the standard, type 2 (sub-distribution board) and type 3 (protection of terminal devices) surge arresters are sufficient. Type 2 arresters with the compact circuit interruption technology are also available for restricted space conditions.
A surge arrester with integrated lifetime indication can also be used to implement a preventive maintenance concept. This lifetime indication detects pre-damage and indicates this status at an early stage before the surge protective device fails. The arrester can therefore be integrated in condition monitoring systems. This version has a higher discharge capacity than conventional type 2 arresters, thus increasing the protective effect.
In case of wired signal interfaces, injection is to be expected and therefore these interfaces must also be protected. A direct lightning strike to the relevant conductor system or a nearby lightning strike close to the relevant conductor system is possible. Therefore, a risk analysis must be performed and the components must be protected accordingly. The same applies to the transmission systems with external antennas, which are only exposed to surges resulting from the field of the lightning channel.
A practical solution for the direct installation of protective devices into intelligent transformer substations, which considers the possible threat potential, is, for example, a complete system for measuring, control and telecontrol systems in a single enclosure. This application includes network analysis, integration of electronic meters, short-circuit indicators and communication devices. To ensure the required availability, the system in a compact enclosure is protected from surges by adequate arresters.
Since the energy and data landscape is becoming increasingly complex and highly networked, the probability of damage to electronic equipment caused by electromagnetic interference significantly increases. This is due to the broad introduction of electronic devices and systems and their decreasing signal levels (and thus increasing sensitivity).
Even though destruction of electronic components is often not spectacular, it frequently leads to long operational interruptions. Consequential damage and the costs for clarifying liability issues are sometimes considerably higher than the actual hardware damage.
Numerous different lightning and surge protection components are available for preventing such damage in smart grids depending on the relevant requirements. In this context, it is important to consider all potential points of injection, namely both power supply and information technology and communication systems. Space-saving and powerful arresters with CI technology and lifetime indication can offer additional benefits. To achieve a consistent and functioning surge protection concept, energy coordination between the arrester types according to part four of the standard must be ensured.
To complement surge protection and to ensure a complete and comprehensive protection system, an external lightning protection system (air-termination system, down conductor and particularly earth-termination system) should be additionally installed and safety equipment should be worn in the intelligent transformer substation. An important topic is, for example, the correct dimensioning of earth-termination systems for transformer stations with respect to the current carrying capability and corrosion. Such an overall protection system meets the increasing demands that the industrial society places on a stable and reliable power supply.
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