Voltage sags may cause lamps to extinguish. Light bulbs will just twinkle; that will likely not be considered to be a serious effect. High pressure lamps may extinguish; it takes several minutes for them to re-ignite.
All lamps, except incandescent lamps, require high voltage across the lamp electrodes during starting. This voltage is essential to initiate the arc. Traditionally, a choke coil is employed across the electrodes to produce high voltage pulses. The lamp starting voltage is affected to a large extent by the ambient temperature and humidity levels as well as the supply voltage. Fluorescent lamps reach their full emission level immediately after ignition. High-pressure lamps need a few minutes to reach their full light output, while low-pressure lamps take up to 15 minutes for the same.
The types of industrial lights are described below.
INCANDESCENT LAMPS
This is the oldest and therefore, the most basic technology used in lighting systems. Current passed through a filament (typically tungsten) produces infrared radiation initially. At temperatures greater than 500°C, emitted radiation falls in the range of visible light. Tungsten has a high melting point and is ideal for such applications. The filament is usually coiled to reduce thermal losses. It also helps in fitting the entire length of the filament within the glass bulb. The level of the nominal voltage dictates the length of filament required.While the immediate discernible effect of a sudden sag in the line voltage is the lessening of visible light emitted by the lamp, there is no documented evidence on its effect on the overall life of the bulb. Research conducted by Phillips in 1975 found a working relationship between prolonged operation at reduced voltage and the life of the lamp. The lamp life is found to be inversely proportional to the nth power of the voltage. Value of n is 13 for vacuum lamps and 14 for general lighting service lamps. Thus, prolonged operation at 5% increased voltage would reduce the lamp life by half. The current varies proportionally with the square root of the voltage. The efficacy of the bulb is proportional to the square of the voltage while the luminous flux is proportional to the operating voltage raised to the power 3.6.
FLUORESCENT LAMPS
Fluorescent lamps have two tungsten electrodes on either ends of a sealed glass tube filled with mercury and argon gas. When voltage is applied to the electrodes, thermionic emission takes place from the surface of the electrodes. In a cascading effect, the mercury and argon gases inside the tube emit radiation in the ultraviolet range. This radiation stimulates the phosphor coating on the inside of the glass tube to emit visible light. To start the lamp, a high voltage is required at the electrodes. This high voltage is generated using special starter circuits that are typically associated with some thermal inertia. There are also rapid starters available for fluorescent lamps.Fluorescent lamps are more resilient to variations in line voltage. Usually, manufacturers recommend operation within 10% variation of line voltage. Unlike incandescent lamps, fluorescent lamps have proportional variation of luminous flux, current, and power with the variation in line voltage. If the voltage sag is severe, the lamp may go off, and according to its starter characteristics, take time to light up again. The starter may also have a minimum voltage below which it is unable to start the tube light. Manufacturers typically provide the minimum voltage values.
SODIUM VAPOR LAMPS
The natural wavelength of sodium metal is corresponds to the most visually sensitive wavelength region. This makes it one of the most efficient lamps currently available. In sodium vapor lamps, the gas inside the glass tube is sodium vapor, which has a higher melting point than mercury. Therefore, it operates at a higher temperature level, thus requiring special insulating mechanisms. Sodium vapor lamps, like all discharge lamps, require special ballast circuits to enable their starting. They are slow to start, with starting time as high as 5 minutes.Due to the inherent principle of operation, when there is a minor sag in the line voltage, the arc temperature falls leading to a rise in the arc resistance. This lowers the current through the lamp, and thus stabilizes the effect of the sag. This happens in the case of a low-pressure sodium vapor lamp. It must be remembered that if the sag is very severe, then the lamp may turn off. On reapplication of nominal voltage, the lamp will take time to start up (normally couple of minutes). It takes about 10-15 minutes to reach full light output condition. The high-pressure sodium vapor lamp operates at a low power factor as a result of which, it is considerably more vulnerable to voltage sags. High-pressure sodium vapor lamps require ballasts that are typically of an inductive type. If the lamp goes off due to a sag event, on voltage recovery, the ballast takes about 30s to reignite the lamp. The lamp is most vulnerable to a sag event during the time of run up because the light output and the power developed by the lamp are directly proportional to the line voltage.
MERCURY VAPOR LAMPS
Mercury vapor lamps are high-pressure mercury vapor filled lamps that emit light that is a combination of blue, green, and yellow. The resultant color of the light is white and is very soothing to the eyes. The construction is similar with two electrodes separated inside a glass tube filled with mercury vapor that reaches a minimum vapor pressure of 5atm during operation.Mercury lamps have high resistance initially, which falls as the arc establishes itself within the tube. A series choke (sometimes along with a parallel capacitor) is used to limit the current flowing into the lamp. In the event of sag, the current through the lamp will be marginally reduced, according to the ballast characteristics. If the lamp is in its normal operating region, marginal changes in current will not lead to any condensation of mercury within the tube. Hence, mercury is added to the lamp in limited amounts; otherwise, small changes in the current would lead to rapid condensation of mercury. Since the operating pressures are very high, instant re-ignition in the event of a sag is almost impossible. It takes 3-4 minutes before the arc can re-strike within the tube.
METAL HALIDE LAMPS
Metal halide lamps consist of the halide (such as fluorine, chlorine, and bromine) salts of metals mixed with small amounts of mercury. These salts have a high vapor pressure at the arc temperature and are extremely stable compounds. Initially, the lamp light is due to the mercury vaporizing. Subsequently, when the arc temperature rises above a certain level (800°C), the metal halide salt vaporizes and its natural wavelength of emission improves the color of the lamp. Metal halide lamps require electrical (or electronic) ballasts to limit the current flowing through them as well as for starting purposes. Compared to mercury vapor lamps, these lamps require a higher voltage pulse in the range of 10kV to get started.In general, the materials inside the metal halide lamps exceed the minimum amounts require to effectively sustain the arc. As a result, metal halide lamps are more immune to minor voltage variations than most other lamps. Typically, voltage sag of 10% for duration of 5 cycles is easily tolerated without extinction.
BALLASTS
Most discharge lamps require a current limiter, as the arc has negative V-I characteristics. These current limiters, also called ballasts, are conventionally series inductor type. Sometimes the choke inductor has a capacitor connected in parallel to increase the ballast tolerance to voltage disturbances. Electronic ballasts are a great improvement on electromagnetic ballasts. For understanding purposes, discharge lamps are modeled by a resistor and a non-linear inductor is series. The result of the non-linearity is that the impedance of the lamp is a function of the frequency of the supply voltage and the generation of harmonics.Compared to incandescent lamps, discharge lamps are less sensitive to voltage sag, but this variation is due to the effect of the ballast more than anything else. The variation of the supply voltage appears across the choke primarily. The choke operating in the linear region shows minimum change in current, and consequently, the arc within the lamp is unaffected by the sag event. The power output is also held steady by this phenomenon. The stability of operation is characterized by the ability of the lamp current and light output to remain immune to sudden changes in supply voltage. Minimizing the voltage across the lamp electrodes and maximizing the voltage across the series ballast element helps achieve this stability. For instance, when the ratio of the supply voltage to the voltage across the terminals of a mercury vapor lamp is 1.667, the maximum sag it can tolerate before extinguishing is 20%. However, if this ratio is 2.0, the maximum sag tolerated is 28%.
In summary, in this section, effects of voltage sags on lighting loads have been discussed. This has helped in understanding the behavior of lamps and other illumination components during sags.