In the future, DC sources will play an increasingly important role in the power supply of many electronic devices. The new as well as the old direct voltage systems require new surge protective devices that comply with up-to-date safety requirements.
Starting from 1880, the first DC networks were developed by Thomas Alva Edison, and operated in the form of microgrids. To this day, DC networks are still used for historical reasons in certain applications, such as subways and tramways. Numerous other applications either run mainly on DC, or are switched over to DC in the event of a power failure. These include the automation systems of power stations and process technology systems, emergency modes of operation of power stations as well as numerous emergency lighting systems in a variety of installations.
Thanks to the advancement in power electronics, high outputs at a high degree of efficiency can be generated today. This does not only result in the photovoltaic systems becoming more cost-effective and efficient. Inverters, rectifiers, and DC/DC converters are becoming more compact because less components are needed on the printed-circuit board. This opened up the way for numerous new applications – from the generation of energy from regenerative sources through to power stations established in a decentralised manner. Telecommunications and computer centre operators also benefit from the advantages of DC power supplies and develop new concepts. Battery storage systems and electric vehicles are other examples of new applications. To protect sensitive power electronics against damage from overvoltages in order to ensure system availability, DC applications require an effective protection concept.
Interfering pulses and their consequences
Inverters, rectifiers, and DC/DC converters usually consist of electronic components designed to convert voltage with losses that are as low as possible. Due to their low noise immunity, these components are sensitive to overvoltage events. As the unexpected interfering pulses that occur often far exceed the noise immunity level of the components, the components are damaged or even destroyed as a result of this stress. Interfering pulses can be caused by direct or indirect lightning striking the ground or the electric installation, and by transient overvoltages resulting from switching operations in the electrical grid.
Usually, lightning strikes are highly energetic events, however, they may occur rather infrequently depending on the region. Lightning energy can destroy electrical and electronic systems or cause fire. In addition, lightning strikes generate voltage increases of several thousand volts at the point of strike. For example, if lightning strikes a building with external lightning protection, or even a tree in the vicinity, the ground potential is increased. This results in a potential difference between the power line and the grounded parts, which can run into several thousand volts. Often, this exceeds the dielectric strength of devices and leads to flashovers in the installation or built-in electronic components.
In the event of direct lightning strikes to peripherals, the lightning current flowing to ground always travels along the path of least resistance. Due to galvanic coupling occurring in the electrical installation, the lightning impulse can follow a protective conductor to the power electronics which it then destroys.
Another physical phenomenon is the coupling in of surge voltages via an external lightning protection system. For example, the current flow in the down conductors from the external lightning protection system generates an electromagnetic field around itself, which in turn induces surge voltages in the power supply line that runs parallel.
Switching operations in the distribution network are another common cause of overvoltages. These can be up to several thousand volts in strength and are caused, for example, by the switch-on and switch-off of nearby electrical equipment, by ground faults and short circuits, and by a fuse tripping. Overvoltages can cause early damage to working power electronics, thereby significantly reducing the service life of the components.
For those who want to avoid this type of failure caused by interfering pulses and wish to protect their investments in expensive power electronics, a comprehensive surge protection concept is crucial.
Protective mechanisms for DC applications
Fig. 1: Charging stations contain a number of sensitive electronic components. If they are to maintain a high degree of availability, they must be protected against transient overvoltages and the effects of lightning.
With the number of DC systems increasing, DC protective devices now need to fulfil new safety requirements due to the different physical properties of direct and alternating voltage. As opposed to alternating voltage, direct voltage has no zero-crossing point – electric arcs occurring during the switching operations are not extinguished automatically and can cause fire. Thus, the aim for the event of a fault must be to extinguish the resulting electric arc in the DC surge protective device by means of a suitable disconnect or backup fuse. To help users to optimally protect their systems, the new VALVETRAB SEC DC product family has been developed (Fig. 1).
The surge protective device has been designed to fulfil new requirements for safety and reliability alongside all present-day safety-relevant requirements of the IEC 61643-11, EN 50539-11, UL 1449, and future IEC 61643-41 standards. Thanks to its compact design – with just 12 mm per channel – it can be easily installed in a control cabinet. Along with the compact design, the new surge protective device for DC circuits also features a powerful and innovative DC disconnect device. It is designed such that the switch arc is securely extinguished in the event of an overload. Potential fire damage is thus avoided. The high self-extinguishing capability of the surge protective device enables its use with up to 200 A of prospective short-circuit current without a backup fuse.
Thanks to the high nominal discharge current of 20 kA, the new components ensure ideal protection for the plant or system. They are available in 48, 120, 220 und 380 V versions. The new product family includes free-of-leakage-current products for insulated networks. They don’t interfere with the insulation monitoring in the application. These products additionally comply with EN 50539-11, the latest photovoltaics standard, and can be used in smaller PV systems.
Moreover, the new product family offers a high degree of protection against mismatching, thanks to the base element being keyed to each individual plug. The plugs can be rotated by 180° before being inserted, thus, the markings on the installed products are easier to read (Fig. 2).
Fig. 2: 180° rotatable and especially narrow plugs.
By means of the optical-mechanical indicator in each plug, and an integrated potential-free remote indication contact in the base element, the current status of the surge protective device can be checked visually and electronically at any time. For documentation purposes, Phoenix Contact offers the Checkmaster 2 test device for checking the individual plugs for possible pre-existing damage. It can be used to check pluggable surge protective devices conveniently and automatically – faulty devices and those with pre-existing damage are reliably detected and can be replaced during preventive maintenance. In addition, all test results are documented in line with standards.
The new direct voltage systems are coming more and more into focus in numerous applications – first of all because of the high energy efficiency demands of the market. Today’s power electronics allow inexpensive and space-saving
Contact Tony Rayner, Phoenix Contact, 011 801-8200, tonyr@phoenixcontact.co.za
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