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Implement overvoltage protection for safe operation of LED street lighting
2017-03-06 13:44:18

 Outdoor SSL products operate in a decidedly harsh environment with frequent overvoltage spikes, explains PIOTR DUDEK, yet he provides a framework for driver specification that can allow reliable operation even in areas with frequent lightning strikes.

Energy-efficient LED light sources and associated electronic control gear or drivers offer numerous benefits such as long life, reduced maintenance, and controllable beam patterns in outdoor area-lighting applications such as street lights, but the outdoor application is environmentally harsh. In contrast to the situation with indoor lighting, the problem of possible overvoltages or surge voltages has to be addressed in the case of street lighting. A surge protection concept tailored to combat 10-kV spikes between L/N (line/neutral) and earth and 6 kV between L and N is therefore absolutely essential.

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The move to LED-based solid-state lighting (SSL) offers tremendous potential benefits to municipalities. Operating and maintenance costs for public lighting account for a large proportion of the energy costs of municipal and local authorities, but there is enormous potential for savings to be made in lighting applications by replacing conventional light sources with LED light sources. But LEDs are different from traditional light sources. The LEDs that are operated at low voltages and the electronic control gear that drives the LEDs are more susceptible to overvoltage conditions than are HID lamps and ballasts. Therefore, a retrofit to outdoor SSL products can only succeed if sufficient attention is given to protecting LED street lights against overvoltage conditions.

There are a variety of potential causes of overvoltage conditions:

• Switching operations in the power supply system or in nearby industrial facilities

• Electrostatic discharges, during maintenance work, for example

• Lightning strikes on the LED street light or the power supply cable

• Lightning strikes in the vicinity, leading to galvanic or inductive coupling

Small overvoltage conditions have little effect even on unprotected LED modules but frequent overvoltage events may have an adverse effect on the life of the LED light sources. Large overvoltages, such as those produced by a lightning strike, may well instantly destroy the LED modules or electronic control gear of several LED street lights.

Lightning remains primary risk

Overvoltages due to switching operations in power-supply systems often occur between phase and neutral - in other words, between L and N or differential-mode interference. They reach peak values of 6 kV and affect only the control gear. Standard control gear therefore has built-in surge protection of 4-6 kV so these overvoltage events are absorbed with no impact to longevity and reliability.

FIG. 1. The graph illustrates that overvoltage events related to lightning strikes are a far more significant risk for reliable street-light operation compared to other sources of surge voltage.

FIG. 1. The graph illustrates that overvoltage events related to lightning strikes are a far more significant risk for reliable street-light operation compared to other sources of surge voltage.

Surge voltages due to lightning strikes are much more difficult to calculate. Fig. 1 generalizes the surge voltage risks. From lightning, the risks primarily occur between the power lines (L/N) and earth (PE) - differential-mode interference - and can quickly reach several tens of kilovolts. The result is an induced voltage that can destroy the lights in entire street runs. The risk of lightning strikes is not the same everywhere, however. There are strong regional differences. The lightning ground flash density Ng, which defines the number of lightning strikes per square kilometer and year, is 1 in Belgium and 150 in South Africa, for example. Such regional differences therefore have to be taken into consideration.

Focus on protection classes

Due to environmental variability, the industry has developed different protection classes to which LED luminaires can be designed. And before we consider these classes, remember that an LED lighting system consists of the luminaire housing (also known as the lamp/luminaire head on street lights), the LED module with an optical system for directing the light (lenses, reflectors), and the electronic control gear for supplying the LED module with appropriate power. The classes are illustrated in Fig. 2.

FIG. 2. The lighting industry has long used protection classes to define how outdoor light fixtures are protected against overvoltage events.

FIG. 2. The lighting industry has long used protection classes to define how outdoor light fixtures are protected against overvoltage events.

Luminaires in Protection Class I are designed so that all the conductive parts have a defined connection to protective earth. A good surge-protection concept combats 10 kV between L/N and earth and 6 kV between L and N. This level of protection is tested to IEC61000-4-5 and will withstand even multiple overvoltage events. It is therefore recommended that, wherever possible, luminaires in Protection Class I should be used because in accordance with relevant standards high voltages should be compensated only with a protective earth.

For historical reasons, however, most of the street lighting in Europe falls in Protection Class II. In luminaires in Protection Class II, all of the live parts have protective insulation but there is no defined connection to the protective earth. Surge protection devices (surge arresters) must not compromise the protective insulation in accordance with IEC61643-11 even for the very short timespans of a lightning strike. Optimum surge protection in the form of a conductor connected to the metal housing or to earth is therefore not possible in a Protection Class II luminaire.

In Protection Class II, a distinction is made between designs featuring a luminaire head made of metal and ones in which the luminaire head is made from a non-conductive material. In the case of a luminaire head made of metal, it is best to provide equipotential bonding between the control gear and the LED module to prevent potential drag and therefore increased surge voltages. Even if the luminaire head is connected to earth via the mast, this connection may have a high or undefined impedance and the system has to be provided with appropriate insulation in accordance with Protection Class II.

If the luminaire head is made of non-conductive material, all the exposed parts must also be made of non-conductive material or be insulated in accordance with Protection Class II. Equipotential bonding is not necessary here. The weakest link here is the LED module and it depends on the insulation material and its thickness as to whether surge protection of 10 kV can be achieved. Without reliable surge protection, the light sources may be subject to premature aging and even complete failure.

Recommendations for reliable surge protection

According to the ZVEI, the Central Association for the German Electrical and Electronics Industry, the requirement for surge protection up to 10 kV for luminaires in Protection Class II is being included in more and more tenders for street lighting projects. If there is an option of converting the lighting to Protection Class I then this would be the best solution.

Otherwise, there are high-quality drivers for LED luminaires such as the Premium Outdoor Drivers from Tridonic. The LCA one4all C PRE OTD is a dimmable constant-current LED driver including surge protection against fluctuations in voltage that result from switching operations in the power supply system and that occur between L and N. The drivers also offer surge protection up to 10 kV between the power lines (L and N) and earth. This level of protection is tested to IEC61000-4-5 and will withstand even multiple overvoltage events.

There are a number of approaches to implementing surge protection in a driver. Tridonic has developed a special voltage-splitting arrangement in the driver based on different capacitors to ensure that even in the case of high input transients a maximum of only 500V reaches the output side of the driver. Most of the voltage is dissipated from the power side to the output side. For regions with only low to average frequency of lightning strikes, control gear with 10 kV offers high surge protection, which previously could only be achieved with an additional surge module in the luminaire head. This is no longer permitted in Protection Class II luminaires, however.

FIG. 3. The graph shows how the impact of a lightning surge decreases with distance from a strike, but one strike can potentially damage multiple street lights.

FIG. 3. The graph shows how the impact of a lightning surge decreases with distance from a strike, but one strike can potentially damage multiple street lights.

Lightning strikes that occur within a radius of 150m from an LED street light can no longer affect the reliable operation of the LED module. If lightning strikes a luminaire directly, and assuming an LED luminaire spacing of 30m, the first five luminaires would fail but not the luminaires in an entire street run. Fig. 3 depicts how the impact of a lightning strike varies relative to the distance from the strike.

A tailored concept for greater safety

For regions with a high incidence of lightning strikes, a surge arrester in the main power distributor for the lighting system is recommended in addition to a robust driver design or, if the distributor is too far away, in the cable junction box. These external overvoltage devices must be tested in accordance with EN 61643-11 and matched to the integrated surge protection in the luminaire - in other words, in the control gear.

As high-voltage tests on LED street lights in accordance with ÖVE/ÖNORM EN 61547 at the certified AIT (Austrian Institute of Technology) have shown, compliance with the usual testing standards is not sufficient to guarantee the operational reliability of outdoor lighting installations. Actual practice shows that lamp failures occur even if the requirements of the standard are met, in some cases at overvoltage levels of only 2-4 kV.

In contrast to the standard tests, the tests carried out by the AIT involved increasing the voltage until the devices were destroyed. The results generally match the experiences from actual practice. Surge protection equipment only functions reliably if all the elements in the entire system are matched to one another. The higher the dielectric strength (surge and ESD) between L and N and also between L/N and earth (PE), the better the LED light sources will be able to withstand overvoltage events during actual operation.

Luminaire developers or even specifiers in municipalities and utilities should ensure that the drivers installed in a street light project are a match for the local environment. There are drivers that cross the application spectrum in terms of power and therefore lumen output. For example, the aforementioned Tridonic Premium Outdoor line includes products at six power levels ranging from 30W to 150W. All are tested in accordance with IEC-916000-4-5, feature asymmetrical surge protection to 10 kV, are rated for 100,000 hours of life, can be dimmed to 10%, and can optionally be combined with networked controls. Municipalities that do their overvoltage homework should reap the benefits of SSL - including low energy usage and long life with low maintenance requirements that can deliver tremendous lifecycle cost benefits relative to legacy sources.

Defining common terms for overvoltage conditions

Protective earth: Protective earth is defined as earthing of one or more points in a system or installation or component for the purposes of electrical safety. This is generally understood to be the electrical connection of all the metal parts, which are easily accessible to touch and which do not belong to the operating current circuit (i.e., inactive metal parts) to ground potential to prevent high contact voltages at the conductive components (e.g., the housing) in the event of a fault.

Functional earth/equipotential: In contrast to the protective earth, the functional earth or operational earth is provided not for the sake of safety or to protect people but to ensure trouble-free operation of electrical installations. The functional earth can reliably protect against interference currents. The functional earth also provides common reference potentials between electrical devices.

Protection Class I: Electrical devices in Protection Class I have a protective earth. Connecting the protective earth to the device housing ensures that in the event of a fault, fault current is routed to ground potential via the protective earth conductor.

Protection Class II: Equipment in Protection Class II has reinforced or double insulation between the mains circuit and the output voltage or metal casing. Even if the equipment has conductive surfaces, they are protected by good insulation against contact with other live parts.