Yes, HID lighting is affected by poor power quality. External occurrences can cause HID lighting distribution voltage to either interrupt the HID lamp's arc stream or to sag below the minimum voltage required to sustain the lamp's arc.

Even minor power disturbances can extinguish HID lamps. For example, a power interruption of 1/2 cycle (1/120th of a second) or more can cause HID lamps to extinguish. Furthermore, they won't restart when the power is reapplied. Besides the obvious safety and liability problems, there's the major concern for productivity losses, which are estimated at billions of dollars each year.

If you're working with HID systems, you need to account for these operating characteristics and the effects of voltage sags and power outages. Lightning and power line problems during storms or accidents can take HID systems off line. Even automatic fault clearing on utility distribution lines can result in darkness. Internally, voltage sags can originate from the normal operation of large electric loads such as motors, compressors, induction furnaces, and elevators.

The selection of metal-halide (MH) system ballasts influences susceptibility to voltage sags and the restrike time. (See sidebar).

Constant wattage autotransformer type (CWA). These ballasts have fairly good regulation, and the lamp wattage will change only 10% with 10% changes in the input voltage. They can tolerate a 30% voltage sag without extinguishing the lamp. This is the most common type of ballast used with standard MH lamps. Here, you don't need an ignitor, since standard MH lamps have starter electrodes. The CWA ballast transforms to a higher voltage, so you can use lower line voltages (120VAC). CWA ballasts use a capacitor and have a higher power factor (PF): greater than 90%.

CWA with ignitor (Super CWA). This type ballast operates the newer pulse-start MH lamps. These lamps, which have no starting electrode, require a nominal 4kV (3kV to 5kV) pulse to start. By separating the starting function in an ignitor and the operating function to the ballast, the Super CWA provides better lamp performance. The ballast is no longer required to deliver the high open circuit voltage needed to start standard MH lamps that contain starting electrodes. Faster starts (strike) and restarts (restrike) improves lamp life and lumen maintenance. Strike time is rated at 2 min (compared to 4 min for standard) and there is less time in the dark, with 3 min to 4 min restrike time (compared to 15 min to 20 min for standard systems).

Reactor ballast. This type ballast is the simplest and least expensive MH ballast, but lamp regulation is very poor with variations in line voltage. It's often referred to as a lag ballast or choke. It's used when the line voltage is high enough to start the lamp (240VAC or 277VAC). Reactor ballasts normally have a low PF, but can be corrected to 90% with a capacitor.

Autotransformer (HX) ballast. This high reactance ballast is the most common type used for low wattage (less than 100W) MH lamps. In these cases, an ignitor is used. The HX ballast has a low PF unless a capacitor is used. Like the CWA ballast, this ballast can transform lower input voltages to a voltage high enough to start the lamps.

Regulated lag (Reg/Lag) ballast. These ballasts can tolerate line voltage sags of 50%, without extinguishing the lamps. You can now use this type ballast to operate specially designed MH lamps, as well as ordinary high pressure sodium (HPS) lamps.

This is a premium ballast, with excellent lamp regulation. For example, a 10% change in line voltage will only change the lamp wattage 4% to 5%. Here, you need an ignitor. Pulse-start MH lamps operating with this ballast usually have extended life and higher lumen output.

Linear reactor ballast. This is the newest ballast for MH lamps. Using a reactor circuitry, which can reduce the normal ballast losses by 50%, it eliminates the need for voltagetransformation. Combined with lamps designed to operate with it, the linear reactor ballast can have system energy savings of 25%, with little or no light loss.

It's designed to operate 150W to 400W pulse-start lamps on an input voltage of 277VAC. It uses an ignitor to start the lamp. Run up (strike) time is less (2 min) compared to 4 min with standard systems. Restrike time also is less: 3 min to 4 min, compared with 15 min to 20 min with standard.

Electronic ballast. This is an integrated unit that replaces the traditional ballast, capacitor, and ignitor components of magnetic ballast systems. Electronic units operate MH lamps at savings up to 30% over conventional magnetic ballasts. The main advantages of electronic ballasts are:

• The ability to keep the lamp lighted over line voltage fluctuations,

• Lower lumen depreciation and longer lamp life,

• The stability of color and light output, and

• The elimination of flicker.

Most of the electronic MH ballasts are available for the lower wattage lamps. However, at least one manufacturer has a unit for the ubiquitous 400W MH lamp. This electronic unit, tested by the EPRI Power Electronics Application Center (PEAC), shows exceptional ride-through capability of 2 1/2 cycles at 0V and low susceptibility to sustained sags up to 25%. EPRI predicted this ballast should reduce by half the nuisance outages due to system voltage fluctuations.

There are HPS ballast system alternatives having different voltage sag and restrike capabilities.

Reactor ballast. The reactor or lag ballast (which derives from the mercury lamp reactor ballast) is similar in nature to some used for MH lamps. It's the least expensive and has the lowest power loss among HPS ballasts. But, it has very poor lamp regulation.

Lead circuit ballast. The lead circuit ballast operates with a combination of capacitance and inductance in series with the lamp. This design differs from the CWA ballast, which maintains a constant current as the lamp voltage increases. This keeps the lamp operating wattage within the trapezoidal limits. This ballast can maintain lamp wattage within the trapezoid for line voltage variations of 10%. (See sidebar "HPS lamps behave differently throughout their lives.")

Constant-wattage (Magnetic Regulator) ballast. This design has a voltage-regulating section that connects to a current-limiting reactor and a pulse starting circuit. Included is a PF correction capacitor. This type ballast has good wattage regulation for changes in line voltage and lamp voltage. It differs from the MH CW ballast. It is the most expensive HPS ballast and has the highest losses.

Here are some products and options you can use to cope with power quality problems.

• Using a double arc-tube lamp in place of a standard HPS lamp eliminates the normal long restrike time of HPS systems (time in minutes). Built with two arc tubes mounted in parallel, this type lamp can relight after a momentary power interruption or voltage sag. Only one arc tube is lighted at a time. When power comes back on after an interruption, the second arc tube lights immediately and produces 3% to 5% of full light output in less than 2 min.

• A new hot restrike ignitor, available for low-wattage (50W to 150W) HPS lamps, restarts the lamp even when the lamp has extinguished due to a voltage dip. But, it will not restrike the lamp if it extinguishes due to age, preventing lamp cycling. All you have to do is change out the existing ignitor with the new hot restrike unit.

• Instant restrike and quick restart systems eliminate the delay in restarting MH lamps after a sag or interruption. They use specially wired CWA ballasts and high-voltage ignitors that produce a high voltage (8kV to 40kV) to restart special lamps. Instant restrike lamps come back on immediately after reapplying power. Quick restart lamps take about 1 min. Good applications are public meeting places such as sports arenas, coliseums, and convention centers. Correctional facilities and manufacturing facilities also apply.

Quartz restrike option is another answer. Some lighting loads are dedicated backup (emergency) lighting systems' circuits. You would arrange these circuits to transfer to an emergency generator upon loss of power (automatically or manually). In either case, there is a brief power outage during the transfer. Any HID lighting on these circuits will take time to restrike. Except in closed-transition transfer, transferring back to the normal lighting circuit will extinguish the HID lamps.

To provide light during restrike times, you can select HID fixtures with a quartz restrike control wired into them. The control senses the state of the HID lamp and energizes a secondary lamp, which is normally a quartz or incandescent lamp located within the reflector area of the fixture.

Some controls are sophisticated and provide desirable features such as hot start and time delay. The hot start feature activates the auxiliary lamp only if the HID lamp has been operating and is hot when the power fails. When power is reapplied, the auxiliary lamp illuminates while the HID lamp cools then restrikes, at which time the auxiliary lamp is extinguished.

The time delay feature keeps the auxiliary lamp on during the HID lamp warm up period prior to automatically turning off the auxiliary lamp. The quartz or incandescent lamp operates from a 120VAC supply, which may be a separate circuit or provided from a 120V tap on the ballast.

Other ride-through options. Ride-through sag correction solutions for an entire facility are usually for critical applications only. Here, you're protecting the HID system and the more critical process machinery.

These solutions include the dynamic voltage restorer (DVR), the battery energy storage system (BESS), and the static series voltage regulator (SSVR). If the application is super-critical and the owner/end user has deep pockets, you can use a superconducting magnetic energy system (SMES), with ratings from 2MW to 6MW, costing several million dollars.

A more cost-effective approach is the Dynamic Sag Corrector (DSC). It's a new concept in power conditioning, compensating for voltage sags down to 50% of nominal voltage and interruptions up to 9 cycles (0.15 sec). The DSC addresses the type of sags prevalent at industrial and commercial facilities fed from distribution level utility feeders.