Dimmable LED Lamps: analysis of the harmonic content introduced in the power network

Transcription

1 Dimmable LED Lamps: analysis of the harmonic content introduced in the power network Salvatore Di Mauro, Salvatore Musumeci, Angelo Raciti, Gaetano Vasta Department of Electrical, Electronics and Computer Engineering, University of Catania Viale A. Doria, Catania, Italy Abstract In recent years, the world has experienced a new revolution in the lighting technology with the advent of LED lamps, which are based on the use of the light-emitting diodes (LEDs). The LED technology is a viable solution for lighting together to the compact fluorescent lamp (CFL) technology, and it is steadily in development. Accordingly, the quality of the LED lamps is going to improve since lately LED devices are more powerful and brighter. The two types of lamps in many fields of applications work with dimmer devices, which allow adjusting the flux. However, the CFL lamps are subject to many problems related to compatibility with the dimmers, which in turn considerably limit their flux control. The two technologies introduce a high harmonic content in the power line, which perhaps increases in case of the flux control with dimmers. In this paper, the harmonic content introduced by dimmable LED lamps of different brands is experimentally investigated and analyzed. The experimental tests aim to evaluate the variations of the performances, according to the IEEE Standard Keywords: LED lamp, dimmer, power quality. I. INTRODUCTION Today the production of lamps for home lighting applications mainly belongs onto two main technologies the compact fluorescent lamp (CFL) and LED (light-emitting diode) lamp. The continuous research on the LED technology has produced significant improvements in terms of luminous efficiency, intensity, lifetime, reliability, and cost. The state-ofthe-art of the LEDs already allows many applications for lighting []. LED lamps use a few of devices connected in series to produce the required light flux. Recent technological innovations in the high-brightness LEDs would lead a revolution in the field of lighting [], which exploits about two-thirds of the electricity for home application. Then it follows that noticeable savings may be achieved from the development of the LED device technology. LED lamps also have other advantages over the CFL lamps since do not contain toxic substances, like mercury, and therefore do not represent critic problems for environmental pollution. Moreover, in large-scale facilities (including industrial facilities) the remarkable reliability of LED lamps lowers the maintenance costs and reduces the number of critical faults [3]. The two lamp types introduce a high content of current harmonics in the power line, thus getting worse the quality of power to other users. Furthermore, often the dimmers control the lamp to obtain the desired level of luminous flux, thus resulting in a more comfortable condition for the user, both in home and commercial applications. In most power lines with lamps, having dimmer with voltage control, it is used a pulse width modulation (PWM) control of the mains voltage. The PWM technique allows controlling the light flux by varying the duty cycle. In case of systems with dimmable LED lamps, which use the dimmer with phase control of the voltage, the current flows only for a shorter time of the period. Moreover, since the LED are fed by their driver as series devices having the same current, the LED lamps can not be easily used with the commercial dimmers that adjust the rms value of the mains voltage. In this respect, they need dedicated control circuits able to control the current value while maintaining a ripple-free waveform, which allows a good dimmable function [4]. To limit the above problem many lamps have special driver design that are compatible with commercial dimmer [5]. Examples of this design is a flyback converter having a control scheme on the primary side compatible with the common dimmers [6]. The LED lamps, due to their power conditioning performed by the converter, introduce many harmonics in the power line. In addition, the flux regulation by dimmer can greatly increase the content of the current harmonics. A few of technical papers available in literature investigate the harmonics introduced into the mains by the unregulated LED lamps [3], [7]. These papers investigate and analyze the harmonic content produced by dimmable LED lamps into the mains current. Looking to investigate the features of commercial products, an experimental set-up was prepared to perform extensive measurements. For this purpose, two dimmable LED lamps of different manufacturers were investigated. The measurements

2 are carried out by varying the dimming angle, while the current and voltage waveforms are detected and recorded. In off-line conditions, the experimental data are analyzed in terms of harmonic content, total harmonic distortion (THD), active power and power factor. Finally, the experimental measurements are verified whether they comply or not with the applicable standard IEC [8]. II. BRIEF DESCRIPTION OF THE USED DIMMER Commercial dimmers, which are used to adjust the flux produced by the lamp (load), perform the control of the luminous flux by changing the shape of the feeding voltage to the load, and transferring to it only a part of the sine waveform of the supply voltage, (phase control cutting), or by PWM control. Commercial dimmers (Fig. ) that use the Triac device allows the partial cutting of the sine wave. In this case, a Diac device (two antiparallel Zener diodes) has the function of giving the appropriate pulse to the Triac gate when the voltage across its terminals exceeds a fixed threshold value. The threshold value of the Diac is reached in different times by changing the value of the variable resistance (Pot), which means by changing the time constant of the RC circuit. As long as the Diac goes into conduction, then it brings into conduction also the Triac, and the mains voltage is shaped similarly to the example depicted in Fig.. Actually, the LED devices require a dc current supplied from a low voltage source, therefore it is necessary to use a converter firstly to rectify the ac voltage, and then to regulate the voltage, and finally to control the current circulating into the LEDs. As a consequence, the drivers of the LED lamps should be compatible with the dimmer actions. To work properly with a dimmer, an LED lamp must sink a holding current and produce a latching current for the ignition. These two currents can vary greatly depending on existing products. An LED lamp used with a dimmer designed for a load of adequate power obliges to provide enough holding current. However, this decreases the efficiency, which is a parameter that designers highly take into consideration. The problem in dimmable applications design is to find a compromise between the efficiency and compatibility of the lamp with dimmer, because if we increase the holding current, this will increase the power losses. Thus, if we try to increase the compatibility with dimmers then the efficiency decreases [9]. III. MAIN ELECTRICAL QUANTITIES OF A NON-LINEAR LOAD A. Total Harmonic Distortion (THD) The overall deviation from its ac fundamental component of a distorted waveform is represented by the quantity called total harmonic distortion (THD). The total harmonic distortion of the voltage is defined by: THD V = V H = V V () V Similarly, the total harmonic distortion of the current is defined by: B. Power factor THD I= I H = I I () I Fig. Simplified scheme of a commercial dimmer with a Triac. The power factor is defined as: PF= P = P S VIP H = V P V PH (3) H I I H If V is negligible, and H P P, then: PF= V I P I H = V I cos = VI THD I cos =PFPFdist (4) THD I Fig.. Mains voltage shaped by the phase control of the dimmer. Two terms compose the power factor. The first term is called the fundamental power factor, or displacement power factor, the second is the distortion power factor [0].

3 IV. LIMITS FOR HARMONIC CURRENT EMISSIONS International standard IEC :006 [8] is applicable for electrical equipment that are supplied from the mains network with voltage not less than 0V and current up to 6A to limit the harmonic component emission. International Standard IEC : 006 gave a classification of equipment and covers: Class A: Three phase equipment Class B: Portable tools Class C: Lighting equipment including dimming devices Class D: The following types of equipment with power less than 600W Personal computers and similar Television receivers According to this Standard, therefore the LED lamps are in the class C. Conducted emission requirements covered by this Standard are up to 40 th harmonic. Table I shows the maxima of the permissible limits for the harmonics up to the eleventh harmonic (i. e. maxima of the permissible current harmonics). TABLE I. LIMITS FOR HARMONICS EMISSIONS CLASS C EQUIPMENT Number of the harmonic [h] Percentage of the fundamental [%] limited distortion, being the calculated THD V equal to about %. Thus, we considered the wave shape of the mains voltage as sinusoidal []. The tests were performed waiting 5 minutes after the powering of the lamps, so that the system was in steady-state conditions from both the electrical and thermal point of view. The recorded electrical quantities are the rms values of the first six harmonics, the waveform of the line currents, and the power factor for different values of the dimming angle. TABLE II. TECHNICAL SPECIFICATIONS OF LED LAMPS RATED VALUES A B V Nominal rms voltage [V] P Power [W] 7 8 E Class of Energy consumption A A Φ Luminous flux [lumen] T Color temperature [K ] I Nominal rms current [ma] PF Power factor NA NA Life Average lifetime [hours] Cost Market price medium high Firstly, the A bulb lamp was tested (7W). The waveform of the line current is shown in Fig. 3 in the case of a dimming angle equal to 0, which means uncontrolled mains voltage and maximum of the flux value. 3 30*PF where PF is the power factor as above defined. V. EXPERIMENTAL TESTS ON LED LAMPS The measurements were performed onto two LED lamps with rated power values of about eight watt that are produced by different manufacturers supplied by a dimmer. The lamps tested are named lamp A, the first one, and lamp B, the second one. Some relevant quantities extracted from the technical data supplied by the manufacturers are given in Table II. The experimental tests were performed at the rated voltage of 30V and the frequency of 50Hz. The mains voltage applied to the terminals of the lamps is almost sinusoidal with a very Fig. 3. Line current absorbed by the bulb lamp A at 0 dimming angle. The line current is characterized by the following relevant values: rms value of the current waveform I = 4.0 ma rms value of the current first harmonic I = 3.5 ma total harmonic distortion of the current THD I = 8% power factor PF =

4 Many measurements were carried out looking to record the main quantities. In Fig. 4 is depicted the voltage shaped by the dimmer that is applied to the lamp terminals in case of a dimming angle of 45. Figs. 5, 6, and 7 depict the waveforms of the mains current in case of dimming angles equal to 45, 90, and 0. From inspection of such waveforms, it is evident how the harmonic distortion of the line current increases as consequence of the increasing angle of the dimmer. In Table III the dominant harmonics of the current for dimming angles of 0, 30, 45, 60 and 90 are given. For a dimming angle equal to 60, the absorbed power is reduced to about 70% of the rated power. Fig. 7. Line current absorbed by the lamp A for a 0 dimming angle. Fig. 4. Shaped voltage across the bulb lamp A for a 45 dimming angle. Fig. 5. Line current absorbed by the lamp A for a 45 dimming angle. Angle [degree] TABLE III. DOMINANT HARMONICS OF THE LAMP A I I I3 I5 I7 I9 I THD I P [ma] [ma] [ma] [ma] [ma] [ma] [ma] [%] [W] % % % % % From inspection of the data given in Table III, it can be argued immediately as the lamp under test does not respect the limits imposed by the standard IEC , even without the dimming function. In fact, none of the harmonics falls below the limits imposed by the standard, for example, the third harmonic should not exceed the value of 6.5 ma, while it has the value of 0.0 ma. Moreover, the deviation from the limitations imposed by the standard IEC 6000 increases at increasing values of the dimming angle. For the lamp B (about 8 W of power), the line current and other electrical quantities were detected and stored. The waveform of the current on the power line is shown in Fig. 8, in case of a dimming angle of 0, which means uncontrolled shape of the mains voltage and maximum luminous flux. PF Fig. 6. Line current absorbed by the lamp A for a 90 dimming angle. Fig. 8. Line current absorbed by the lamp B for 0 dimming angle. 4

5 The line current is characterized by the following relevant values: rms value of the current waveform I = 47,5 ma rms value of the current first harmonic I = 45,5 ma total harmonic distortion of the current THD I = 30% power factor PF = 0.8 Even for the second lamp, many measurements were carried out looking to compare the respective performances. In Figs. 9, 0, and are shown the waveforms of the mains current in case of dimming angles equal to 45, 90, and 0. As we see from the waveforms shown in Figs 8- the lamp B better behaves in respect of the lamp A in terms of harmonic content of the current on the power line. In fact, apart the high frequency harmonics, from Fig. 8 (that refers to 0 dimming angle), it is evident as the departure from the sinusoidal waveform is quite small, circumstance that is confirmed by the reduced values of the THD I=30% and PF=0.8. However, as may be expected, the harmonic distortion increases at increasing dimming angle. In Table IV are given the dominant harmonics and the THD I of the current for dimming angles of 0, 30, 45, 60, and 90. For a dimming angle of 60, the absorbed power is reduced to about 50% of the rated one. From inspection of the data given in Table IV, relative to the B lamp, it is evident that in absence of the dimming control, the produced harmonics are well within the limits dictated by the standard IEC As an example, the third harmonic does not exceed the value of. ma, and in fact, it is 8.5mA. lamps are shown in Fig. 3, where we can see the equivalent active powers for the case of 90 dimming angle. Fig. 9. Line current absorbed by the lamp B for a 45 dimming angle. Fig. 0. Line current absorbed by the lamp B for a 90 dimming angle. Angle [degree] TABLE IV. DOMINANT HARMONICS OF THE LAMP B I I I3 I5 I7 I9 I THD I P [ma] [ma] [ma] [ma] [ma] [ma] [ma] [%] [W] % % % % % PF However, as long as the dimmer angle is increased, for a dimming angle of 30 the harmonic content introduced does not meet the standard IEC , although the deviation from the limits imposed by the standard is lower than that of the lamp A. By using the experimental data, the values of THD I, of the active power, and the power factor, for the different dimming angles, were calculated and Figs. -4 show such quantities as functions of the dimming angle. In particular, Fig. gives evidence to the best performance of the lamp B in terms of THD I in comparison to the lamp A. The active powers of the two Fig.. Line current absorbed by the lamp B for a 0 dimming angle. Perhaps, in such a condition was also verified the equal brightness of the two lamps. Fig. 4 shows how the values of the power factor of the lamp B are always better than the power factor performed by the lamp A. This happens into the entire field of variation of the dimming angle. In turn, the values of the dominant harmonics of lamp A are greatly exceeding the limits set by the IEC In addition, they exceed those of the lamp B into the entire range of variation of the dimming angle. 5

6 Fig.. THD i values of the two lamps as function of the dimming angle. supply network have considerable value, but what is even more surprising is that the values of the dominant harmonics greatly exceed the values dictated by the standard IEC This problem affects, in different extent, the investigated lamps. In fact, even the lamp B, which presents an excellent behavior in terms of harmonic content in absence of dimming, introduces a high harmonic content in the case of dimmer action. As it was verified, the harmonic contents are outside the limit values dictated by the standard IEC , although the deviation from these limits is less marked for the lamp B. Dimmable applications have the serious problem of the interface between the dimmer and the converter of the lamps, in particular with the driver circuit. Researchers and producers try to maximize the compatibility between the dimmer and the converter. In particular the driver circuit of the converter must allow a suitable holding current, which is also linked to the technical characteristics of the dimmer, and at the same time has to ensure a good efficiency. A careful design allows achieving a trade-off for the driver circuit and the dimmer looking to ensure both compatibility and good performance. The goal is to reduce the harmonic content injected into the power line. REFERENCES Fig. 3. Active power of the two lamps as function of the dimming angle. Fig. 4. Power factor of the two lamps as function of the dimming angle. VI. CONCLUSIONS The paper shows the main results of a comparison between two LED lamps. The lamps were investigated experimentally and analyses of the results were carried out. In particular, analysis focused on the harmonic content introduced by the LED lamps into the power line as consequence of the dimming function. The tests show that the harmonics introduced into the [] R. A. Pinto, M. R. Cosetin, T. B. Marchesan, M. F. Da Silva, G. W. Denardin, J. Fraytag, A. Campos, R. N. Do Prado, Design procedure for a compact lamp using high-intensity LEDs, in Proc. of the 35th Annual Conf. of IEEE Industrial Electronics, IECON 009, pp [] F. Xiansong, X. Wencui, N. Pingjuan, T. Haitao, D. Chenming, Design of a single-chip white light-emitting InGaN/GaN diode, International Conference on Mechanical and Electrical Technology, 00, pp [3] S. Di Mauro, A. Raciti, Analysis and comparison of CFLs and LED lamps, AEIT Annual Conference AEIT, 04, pp. -6. [4] D. Rand, B. Lehman, A. Shteynberg, Issues, models and solutions for Triac modulated phase dimming of LED lamps, in Proc. of IEEE Power Electronics Specialists Conf., PESC 007, pp [5] H. J. Chiu, Y. K. Lo, J. T. Chen, S. J. Cheng, C. Y. Lin, S. C. Mou, A high-efficiency dimmable LED driver for low-power lighting applications, IEEE Trans. on Industrial Electronics, vol. 57, Issue, 00, pp [6] L. Xu, H. Zeng, J. Zhang and Z. Qian, A primary side controlled WLED driver compatible with Triac dimmer, in Proc. 0 the 6 th Annual IEEE Applied Power Electronics Conf. and Exposition, pp [7] N. Kumar, G. Kumar, A. Kumar, A techno economic comparative analysis of energy efficient luminaries in the context of emerging domestic customer, Annual IEEE India Conference (INDICON) 0, pp. -6. [8] Limits for harmonic current emissions (equipment input current 6A per phase), IEC Standard , Ed. 3., 006. [9] F. Mercier, N. Hamza, C. Delcourt, P. Maugars, S. Bara, W. Langeslag, V. Zwanenberg, M. Sturkenboom, R. Grisel, A dimmable power supply unit for testing LED lamps built around a dedicated integrated circuit, Proc. of the IEEE International Symposium on Industrial Electronics, ISIE 0, pp [0] IEEE Standard, IEEE Standard definitions for the Measurements of Electric Power Quantities under Sinusoidal, Nonsinusoidal, Balanced or Unbalanced Conditions, IEEE, 00, pp 8-. [] A. E. Emanuel, On the assessment of harmonic pollution, IEEE Transactions on Power Delivery, vol. 0, no. 3, pp , July