The benefits of establishing and

Transcription

1 Comparison Between Ballasting Alternatives for Medium-Pressure UV Discharge Lamps By László Laskai The benefits of establishing and maintaining a UV-process window, such as optimizing production and reducing waste, are well known. 1 It is less well known how this can be accomplished and what trade-offs UV-equipment manufacturers can make. To obtain any UV output from a gaseous discharge, medium-pressure UV lamps in this case, one would need a ballast. A ballast is required to adapt the gaseous discharge lamp, in this instance the UV source, to the mains. The ballast ignites and drives the lamp at some power level which, unless the ballast is regulated, will vary due to mains voltage variations and lamp variations, manufacturing and burn-time related. For optimal operation and long lamp life, it is important that the ballast provides adequate open circuit ignition voltage, quick glow-to-arc transition, and low lamp current crest factor (crest factor defined as a ratio between the peak and the RMS value), and regulates power into the lamps. 2 There are two types of ballasts: electromagnetic (EM) ballasts, essentially line-frequency transformers, with or without external components or controller, and electronic ballasts normally referred to as switching power circuits. Both types of ballasts can regulate lamp power, what is essential to establishing and maintaining a UV-process window. However, the regulation type (step or continuous), range, accuracy and repeatability, and the consequences to the regulation, if any, on efficiency, reliability, lamp lifetime, etc., vary significantly from one type to another. 3 Over time EM ballasts have been replaced by electronic ballasts in the highest volume of all ballasting applications fluorescent lighting. EM ballasts are still used in another high-volume lighting application, metal-halide lamp ballasting, in most but high-end, indoor applications due to a phenomenon known as acoustic resonance in the discharge. 4,5 The situation in UV applications is similar. Electronic ballasts power low-power, low-pressure UV lamps (essentially fluorescent lamps without phosphor coatings) while EM ballasts power medium-pressure lamps in most but high-end systems, today dominated by radio-frequency (RF) or electrodless UV systems (systems referring to integrated lamp-ballastfixture combo ). However, the situation in high-power, mediumpressure UV applications is gradually changing. In addition to traditional EM ballasts, customers can select between silicon-controlled rectifier SEPTEMBER/OCTOBER 2005 RADTECH REPORT 41

2 Figure 1 Series capacitor EM ballast a b Series capacitor EM ballast a) schematic and b) accompanying 4kW medium-pressure UV lamp, voltage (channel 1, 2,000V/div) and current (channel 2, 5A/div) waveforms (SCR) regulated EM ballasts, 6 UV systems with electronic ballasts, and stand-alone (one that can be matched with any manufacturers lamp and fixture) electronic ballasts. 7 These changes will lead to high-end systems at lower costs. It is interesting to note that the previously mentioned acoustic resonance problem, an obstacle to wider use of low-power metal-halide lamp electronic ballasts, occurs in some medium-pressure UV lamps (38" lamps for instance) when operated on line-frequency EM ballast. Unlike low-power metal-halide lamp electronic ballasting, electronic ballasting of medium-pressure lamps can assure resonance-free operation by raising the lamp operating frequency. This can be explained with significant differences in lamp geometry (arc vessels approximately 1" long or shorter in the case of the low-power metal halide lamps vs " long medium-pressure UV lamps) and mercury-vapor pressure, factors that determine acoustic resonance frequencies. 5 This paper will review the operation, performance, and pros and cons of EM ballasts traditional and SCRregulated and electronic ballasts, all commercially available in the U.S. market. The second part of the paper repackages the information presented in the first part with the intention to get a clearer cost/benefit picture of owning and operating one of these ballasts. Overview of Ballasting Alternatives Traditional Electromagnetic Ballasts Today nearly all high-power UV systems in the North American market are relying on electromagnetic ballasts. The pros of EM ballasts are well known. They are inexpensive even until recently when sharp rising raw materials (steel and copper) costs drove EM ballast cost up as well. And, they are reliable and efficient. The cons of EM ballasts are also identified poor regulation of the line and load variations, the cumbersome size and weight, and costly transportation and handling. When it comes to lamp power regulation, which is instrumental in establishing and maintaining a UV process window, there are several ways that an EM ballast can be regulated. One method involves saturating the magnetic core, a ferroresonant approach, which significantly degrades ballast efficiency. Another method involves switching capacitors, as illustrated in Figure 1. This approach can cause extremely high currents in the capacitors and mercury switches, having the effect of reduced reliability. 6 Figure 1 shows a schematic of a) a traditional ballast with a series capacitor and b) lamp voltage and current waveforms. Note: the typical reignition voltage peaks and non-sinusoidal lamp current. Current crest factor (crest factor defined as a ratio between the peak and the rms value) is approximately 1.7. Figure 2 shows typical waveforms of lamp voltage and current in series capacitor ballast for lamp running at a) full power and b) at about 20% power. Note the additional current reversals, ones beyond the expected mains frequency related reversals, an artifact of this typical ballast circuit arrangement, leading to an additional 42 RADTECH REPORT SEPTEMBER/OCTOBER 2005

3 Figure 2 Typical waveforms a b Series capacitor EM ballast with 5.4 kw medium-pressure UV lamp voltage (channel 2, 200V/div) and current (channel 1, 5A/div) waveforms for a) full power and b) about 20% nominal power increase in plasma cooling, which in turn limits power regulation range. SCR-Regulated EM Ballasts A block diagram of a SCR-regulated magnetic ballast is shown in Figure 3. 6 In this arrangement, the mains voltage is connected to the appropriate primary tap of a transformer through an SCR module (anti-parallel combination of SCRs). A SCR module is simply added to an EM ballast to continuously control lamp power by means of controlling ballast input voltage. Operation is similar to incandescent dimmer. Applying phase-control to an EM ballast does have its limitations. From the side of the power mains, the control process produces lagging currents and considerable harmonics in a single-phase power circuit. This lowers power factor, increases the power system kva requirements, causes harmonics to flow in the unprotected neutral line, creates potentially unbalanced load conditions and degrades power quality. Adding power factor capacitors will improve power factor, but will not help to reduce harmonic content. Power factor correction capacitors in these types of applications are common source to reliability problems since they can sink unknown system harmonics. From the load side, phase-control increases current crest factor, leading to shorter lamp life and to increased lamp reignition voltage, a factor reducing ballast efficiency. Similar to conventional EM ballasts, the SCR-controlled ballasts are designed to address the power-conditioning portion, ignition and normal operation. To interface with external world and the mains in the first place, an external on/ off switch or contactor is needed to meet safety codes and additional power factor correction capacitors to mitigate the low-power factor of a phasecontrolled SCR circuitry. In terms of size and weight, SCR-controlled EM ballasts are nearly the same or somewhat smaller than conventional EM ballasts. For applications where regulation is needed, the SCR-controlled approach improves the power conversion efficiency when compared to ferroresonant solutions and improves reliability when compared to switched capacitor solutions. Electronic Ballasts One approach to realizing a cost-effective high-power electronic ballast, block diagram shown in Figure 4, is based upon a current-fed, threestage design: 1) three-phase power from the mains is converted to a dc voltage with a three-phase rectifier, 2) the dc bus is converted to a controlled dc current with a polyphase chopper, and 3) the dc current is inverted and isolated with a current fed inverter and transformer. The lamp is connected directly to the mid-frequency, 400 Hz square-wave transformer. For safety purposes, the mid-point of the output transformer is connected to ground. Ballast and power supplies in general with three-phase rectifier SEPTEMBER/OCTOBER 2005 RADTECH REPORT 43

4 input represent a symmetrical load to the mains with more than 90% power factor. Hence, no power factor capacitors are needed nor does the mains kva rating require oversizing. When fitted with integrated step-start dual mechanical contactor arrangement the inrush current is limited below nominal and, with additional fuses, complies with applicable safety regulations. When this arrangement is compared to EM ballasts, with or without SCR controller, it means that there is no need for external contactor, fuses, mercury relays, wiring and cabinets to hold this entire accessory. Figure 5 shows lamp voltage and current. Due to fast current reversal (di/dt) and 400 Hz operation, the gaseous discharge lamp operates without cycle-by-cycle reignition. As long as the lamp is operated at an operating point like this and the arc is not extinguishing at current reversals, the operating frequency has no effect on the UV output. 8 For these frequencies, the UV output will be constant and dependent on lamp average power only. In the example shown in Figure 5, one would also have to note some high-frequency ripple terms, producing small, highfrequency UV power terms superimposed on the constant output. Note, in comparison, EM ballasts have a variable, pulsating at twice the linefrequency UV output. Figure 4 Figure 3 Block diagram of the electronic ballast SCR-controlled EM ballast Square-wave current operation also means a unity current crest factor that is ideal for minimizing electrode wear during warm-up, normal operation and dimming. For comparison, typical conventional electromagnetic ballast is considered adequate if its current crest factor is less than Note: the lamp manufacturers data implies operation on conventional EM ballast hence a lamp power factor (power factor defined as a ratio of average power and apparent power) around 0.9. However, when the lamp is operated on electronic Rectifier DC to DC Low Frequency Converter Inverter ballast, the power factor is one; therefore, less current is required to run at the same power level than required while operating on EM ballasts. Cost/Benefit Analysis The information presented in the first part is reorganized with the intention to get a clearer look at the cost/benefit picture of buying, integrating, owning and operating one of these ballasts. Consider the following costs: 1. Initial acquisition and integration in terms of dollars-per-uv watt. 2. Lifetime operation. 3. Lamp replacement. In addition to acquisition costs, integration cost include the system engineering, materials (enclosure, circuit breakers, relays, cooling, etc.), labor, procurement, etc. Ballast efficiency, reliability, functionality, and modern process management features are key factors in lifetime operation costs RADTECH REPORT SEPTEMBER/OCTOBER 2005

5 Figure 5 13 kw medium-pressure UV lamp voltage (channel 1, 1,000V/div) and current (channel 2, 5A/div) waveforms while operated on electronic ballast main cause of UV output decrease is the sputtered electrode material deposition onto the lamp walls. The actual sputtering occurs during ignition, warm-up and dimming, and at lower rates during normal operation. 9 In terms of ignition, ballasts rely on open circuit voltage and/or an igniter to start a lamp. When adequately engineered, neither design should cause more harm or benefit in terms of lamp life. 2 Unity current crest factor, an attribute of square-wave electronic ballasts, is ideal for minimizing sputtering under all conditions, hence minimizing electrode wear and extending lamp life. Sputtering is more of an issue with EM ballasts, in particular during low-power operation or under circumstances when the current crest factor exceeds 1.7. Lamp life, a function of the type of selected ballast, is the sole driver of the third cost. This cost includes lamp replacement and maintenance. Initial Acquisition and Integration Costs Electronic ballasts purchasing costs are somewhat higher than their EM counterparts. However, as discussed, when all additional elements are taken into account the integration costs are significantly lower for electronic ballasts. First, there are major differences between how electronic ballasts and EM ballasts, conventional and SCR controlled, integrate and interface with the remaining parts of the equipment. These differences translate into specific costs. In terms of mechanical packaging, electronic ballasts are self-contained and rack-mountable. In terms of mains connection, they are designed to be permanently connected to the power source and to require a readily accessible disconnect device incorporated into the fixed wiring. Equipped with all the necessary interfaces (contactor, fuses, etc.), these ballasts can be simply connected to the mains and lamps and are ready to go. There are no additional costs. Line-frequency magnetics at multi-kilowatt power levels are very bulky. Hence, mechanical packaging, thermal management, transportation and installation of EM ballasts are considerably more involved and costly. The fact that EM ballasts are designed to address the power conditioning portion (ignition and normal operation) only means that additional external components switches, relays, power factor correction capacitors, internal wiring, etc. need to be custom engineered for each system. The additional components, assembly and engineering weigh into the overall cost. Lamp Replacement Costs One of the key differences between electronic ballasts and EM ballast, SCR-controlled unit or otherwise, is the electrical conditions under which discharge lamps are operated. The Lifetime Operating Costs Reliability The advantage of the current-fed design-based electronic ballasts over voltage-fed EM ballasts design is robustness. In the case of current-fed design, the isolation transformer is driven by a controlled current source; whereas for a voltage-fed design, the transformer is driven by a voltage source, or in the case of the SCR-controlled EM ballasts, the mains. While this may be a subtle point, if for some reason there is a slight offset in volts seconds between switching operations of a voltage-fed transformer, the transformer core quickly saturates causing the switching devices to draw excessive currents. To prevent this condition a special circuitry is required. 6 This situation cannot exist in current-fed designs. Conventional and SCR-controlled electro-magnetic ballasts with highvoltage capacitor banks, series or power factor capacitors, and mercury filled relays are known for reliability issues. In the switching capacitor arrangement, the extremely high currents in the SEPTEMBER/OCTOBER 2005 RADTECH REPORT 45

6 capacitors and switches have a negative effect on reliability. 6 The SCR-controlled EM ballasts do not have power factor correction capacitors; however, in most applications the less than 0.7 power factor requires power factor correction. Adding power factor capacitors in these types of applications is a common source to reliability issues since they can sink unknown system harmonics. Efficiency EM ballasts are an efficient solution in applications where power regulation is not important. This is not the case when power regulation is required. With 92% and higher efficiency, the electronic ballast and the SCR-controlled ballasts are a significant improvement over inefficient ferroresonant ballasts. Either choice will result in significant lifetime operating cost savings. Functionality Historically, adding diagnostics as a component of process management to conventional ballasts has been a part of the system design. For mainly cost reasons, this has been limited to basic status indicators: lamp-out, warm-up, etc. Electronic ballasts lend themselves to all process control interfaces found today in the modern manufacturing process environment. This opens the door to real-time online access to all process parameters, lamp status, and ballast condition information necessary to achieve improved process control and, if leveraged appropriately, reduce maintenance costs. 1 Conclusions Establishing and maintaining a UV-process window in manufacturing is an economical necessity. Today, there are more choices than RF or electrodless systems and transformer powered medium-pressure lamp systems. Electronic ballast driven medium-pressure lamp systems are an economical alternative for controllable high-end UV equipment. Acknowledgments The author acknowledges Dr. I.J. Pitel for his contribution to this paper. References 1. Jim Raymont, Establishing and Maintaining a UV Process Window, RadTech Report, May/June 2002, pp J. F. Waymouth, Electric Discharge Lamps, MIT Press, L. Laskai and I.J. Pitel, Discharge Lamp Ballasting, IEEE Power Electronics Specialist Conference Tutorial, Atlanta, GA, L. Laskai, P. Enjeti and I.J. Pitel, White- Noise Modulation of High-Frequency High-Intensity Discharge Lamp Ballasts, IEEE Transactions on Industry Applications, v37 n4, July 1998 (presented at 94 IEEE-IAS Annual Meeting). 5. L. Laskai, High-Frequency Ballasting Techniques for High-Intensity Discharge Lamps, Dissertation, Texas A&M University, College Station, E. Persson and D. Kuusisto, A Performance Comparison of Electronic vs. Magnetic Ballast for Powering Gas- Discharge UV Lamps, RadTech 98 Chicago, 1998, pp L. Laskai and I.J. Pitel, White paper ( 8. O. Paleka et al, Effect of the Driving Waveform on the UVC Output of a Medium-Pressure Mercury-Vapour Lamp, 2nd International Ultraviolet Association (IUVA) Conference, Karlsruhe, J. Olsen and W. Moskowitz, Tungsten Sputtering and Transport Measurements in High Intensity Discharge Lamps During Starting, IEEE International Conference on Plasma Science Proceedings, 1996, pp László Laskai, Ph.D., is president of UV Electric, Jersey City, NJ. 46 RADTECH REPORT SEPTEMBER/OCTOBER 2005