By Tim Patel and Henry Yu, Littelfuse, USA
August 17, 2015: Growth in demand for LED lighting is accelerating as consumers and industrial users seek more energy-efficient illumination options. Expansion of LED lighting mechanism is also being driven by government actions designed to discourage continued usage of incandescent lamps. Because of this the high adoption rate of LED lighting is definitely a global phenomenon. Countries with the most advanced economies have well-established plans and programs to phase out legacy incandescent lighting. That shouldn?t be surprising because, the global lighting account is about 25% of the total energy consumption.
Though the initial cost of installing outdoor LED lighting can be substantial, this cost is justified and payback is established based on the lower wattage demand, lower maintenance cost, and longer lifetime it offers. In order to protect outdoor LED lighting from failing within an investment payback period of about five years, the lighting must offer high durability and reliability.
Threat to outdoor LED lighting
One significant threat to outdoor LED lighting is transient surge events in AC power lines that can damage lighting fixtures. An LED lamp contains power conversion electronics (AC/DC), driver IC for the LEDs, a heat sink for thermal management and optics to optimize light quality. An LED light directly connected to AC mains, for example, 120/220 V AC can be damaged by short circuit and overload conditions caused by component and/or circuit failures inside the bulb. In addition, lightning surges or load switching transients which originate outside the bulb can create voltage spikes or ring waves that can stress and ultimately damage components, and cause the bulb to collapse. Given that the value proposition for LED bulbs is not only lower energy usage, but longer lifetimes, it will be crucial that transient voltage protection is taken into account to eliminate field failures driven by the electrical environment.
Indirect lightning-induced surges
Over-voltage transient surges can occur in AC power lines as the result of a nearby electrical equipment being switched on/off. Nearby lightning strikes can also generate transient surges in AC power lines, especially in outdoor environments.
Lightning strikes are travelling-electrostatic discharges usually coming from cloud to the ground in the magnitude of millions of volts. Current carrying copper wires (for example, underground power cable for streetlight) can be induced up to thousands of volts from lightning strikes occurring up to a few miles away. These indirect strikes can be characterised with specific waveforms that often contain large amounts of energy in the magnitude of over 1000 A2s.
Approximately 70% of lightning occurs on land located in the tropics where the majority of thunderstorms occur. African countries have had the heaviest and most persistent lightning attacks from the beginning of time. The place where lightning occurs is most often near the small village of Kifuka in the mountains of eastern Democratic Republic of the Congo, where the elevation is around 975 m. On an average, this region receives 158 lightning strikes per 1 km2 a year. A large area around this place is also badly affected by the induced surge strikes which increases the chances of damage to outdoor LED lighting installations.
According to National Aeronautics and Space Administration?s (NASA) research on worldwide lightning strike frequency, Central and South America, Africa, Southern and South-Eastern Asia have similar lightning strike frequencies to the US, and therefore we suggest equivalent surge immunity level to the US at 5 kA to 10 kA. For other regions with fewer lightning strikes like Europe, Eastern Asia and Australia, lower surge immunity levels could be considered at 3 kA to 5 kA.
This type of indirect lightning energy from storms can adversely affect outdoor LED lighting installations. The luminaire is susceptible to damage both in the differential and common modes:
Differential mode: High voltage/current transient between the L-N or L-L terminals of the luminaire could damage components in the power supply unit or LED module board.
Common mode: High voltage/current transient between the L-G (earth) or N-G (earth) of the luminaire could break over safety insulation in the power supply unit or LED module board, including the LED to heat-sink insulation.
The test waveform is a combination of 1,2 x 50 ?s open circuit voltage and 8 x 20 ?s short circuit current waveform. To perform this test, the specified peak current is calibrated on the surge generator by shorting the output to ground prior to connection to the luminaire.
Technologies to handle induced surge events
The way to protect outdoor LED lighting from induced surge strike is to block high voltage/current transient interference from entering the lighting. A surge protective device (SPD) is therefore utilised in outdoor LED lighting to suppress surge energy and minimise surge impact to the lighting.
There are several overvoltage protection components available for SPD, including components like a metal oxide varistor (MOV), gas discharge tube (GDT), and transient voltage suppression (TVS) diode which are placed between AC power lines with normally high impedance and become low impedance when high voltage is detected. They divert surge energy back to AC power lines by low impedance and turn back to high impedance after the surge event. Among available technologies, MOV is preferred and widely implemented in SPD in power distribution panels for its high surge energy handling capability and fast response to transient voltage. Therefore MOV is the best suitable for surge protection devices in outdoor LED lighting applications.
Incorporating a robust surge suppression circuit in an outdoor luminaire can eliminate damage caused by surge energy, thereby enhancing reliability, minimising maintenance and enhancing the useful life of the lighting installation. A surge protection subassembly that can suppress excessive surges to lower voltage levels is an optimal way to protect the LED luminaire investment.
Thermally protected MOV for SPD safety
MOV technology is not only inexpensive but also a highly effective technology for suppressing transients in power supplies. It is also effective in many other applications, such as SPD modules that are often placed in front of an LED driver.
MOVs tend to degrade gradually after a large surge or multiple small surges. This degradation leads to increasing MOV leakage current, which in turn raises the MOV?s temperature, even under normal conditions like 120 VAC/240 V AC operating voltage. A thermal disconnect adjacent to the MOV can be used to sense the increase in MOV temperature while it continues to degrade to its end-of-life condition; at this point, the thermal disconnect will open the circuit, removing the degraded MOV from the circuit and preventing its catastrophic failure.
MOVs are designed to clamp fast over-voltage transients within microseconds. However, in addition to short duration transients, MOVs inside SPD modules can be subjected to temporary over-voltage conditions caused by loss of neutral or by incorrect wiring during installation. These conditions can severely stress a MOV, causing it to go into a thermal runaway condition; in turn, this will result in overheating, smoke, and the potential for fire. UL 1449 and IEC 61643-11, the safety standards for SPDs, define specific abnormal conditions under which devices must be tested to ensure SPD safety. Robust designs include thermal disconnects within the SPD to protect the MOVs from thermal runaway.
End-of-life/replacement indication for SPDs
When a MOV gets overheated due to temporary over-voltage or degradation end-of-life, thermal disconnect helps to cut the MOV from the AC circuit. The SPD therefore stops providing the surge suppression function. Proper indication should be considered so that maintenance personnel know the SPD is not working and needs a replacement.
Luminaire designers can choose from two main types of SPD module configurations based on their maintenance and warranty strategies. Those are parallel- and series-connected surge protection subassemblies.
Parallel connection: The SPD module is connected in parallel with the load. A SPD module that has reached end-of-life is disconnected from the power source while leaving the AC/DC power supply unit energised. The lighting still remains operational, but the protection against the next surge to which the power supply unit and LED module are exposed is lost. In a parallel-connected SPD module, replacement indication can be added through the use of a small
LED that indicates the SPD module status to the maintenance technician. Options for a green LED indicating an online SPD module or a red LED indicating an offline SPD module are available. Rather than an LED indication at each lighting fixture, the need for SPD module replacement could be indicated remotely to a light management center with SPD module end-of-life indication wires connected to a networked smart lighting system.
Series connection: The SPD module is connected in series with the load, where the end-of-life SPD module is disconnected from the power source, which turns the light off. The loss of power to the luminaire serves as indication for a maintenance call. The disconnected SPD module not only turns the lighting off to indicate the need for replacement but also isolates the AC/DC power supply unit from future surge strikes. General preference for this configuration is growing rapidly because the luminaire investment remains protected while the SPD module is awaiting replacement. It is far less expensive to replace a series-connected SPD module than the whole luminaire as in the case of a parallel-connected SPD module.
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