Effects of plasma lamps on the power-line communications channel

Effects of Plasma Lamps on the Power-line Communications Channel Ashraf Emleh 1, Arnold de Beer 1, Hendrik Ferreira 1, Adrianus Han Vinck 2 1 Department of Electrical and Electronic Engineering Science, University of Johannesburg, P.O. Box 524, Auckland Park, 2006, South Africa 2 Institute for Experimental Mathematics, Duisburg-Essen University, Ellernstr. 29, D-45326, Essen, Germany aemleh@uj.ac.za Abstract— Plasma is one of the states of matter. The other states of matter are liquids, solids and gases. Plasma is used in television, neon signs and fluorescent lights. Stars, lightning, aurorae and most flames consist of plasma. Although energy effective, they inject conductive noise into the power-line system. This can have a detrimental effect on the power-line communications channel. This paper investigates these effects when plasma lamps are seen as noise sources on the power line. It is shown that in the CENELEC band: (3kHz – 150kHz) the interference level from Plasma lamps is significantly below the allowed maximum PLC signal levels. In the band 150kHz – 30MHz however, PLC signals compete with Electromagnetic Compatibility (EMC) levels and the SNR can be equal to zero, but only if the lamps have active power electronic converters. Keywords— Plasma, Lamp, Power-line Communications, PLC, Interference, EN 50065-1, EMC. I. INTRODUCTION The well-known plasma lamps design was invented during the 1970's by a MIT student named Bill Parker [1]; nevertheless, the original plasma lamps were first created by Nikola Tesla [2] while studying the effects of high frequency current discharged into low pressure gases contained by a glass tube. Plasma lamps come in different constructive design and shapes, globes, domes, orbs and other, but they all work on the same basic principle. They can be usually found in the shape of a clear glass orb, containing a mixture of low pressure gases such as xenon, krypton and neon, although the gaseous mix is not preferential. The other glass shell houses a much smaller glass orb that has the role of the electrode. High frequency, high voltage alternating current is being pumped into the electrode with the help of a high voltage transformer. A standard plasma lamp device uses an electric current having an oscillating frequency of 35kHz and a voltage ranging from 2 to 5 kilovolts. As the lamp is being powered, the gas mixture inside it is ionized and gives rise to multiple beams of colored light discharges extending from the inner glass orb to the outer glass container. The outer glass orb can heat up to dangerous temperatures, not enough to determine a failure of the device but sufficient to cause light burns. Plasma lamps are ideal sources of static charge that could determine a high voltage discharge even through the plastic protective casing. Because they use high-frequency electric currents to obtain the desired effects, plasma lamps present a series of potential hazards to the operators and other electrical devices widely used today. The high frequency of the current, respectively 35 kilohertz, produces parasite frequencies in the radio spectrum that could affect the operation of several house appliances such as the touch pad of a laptop or the correct functioning of several other digital devices. The procedure of the measurements was made to comply with CENELEC narrowband rules in one case, and to serve the broadband signals in the other case [3]. II. MEASUREMENT SET-UP The measurement set-up used is shown in Fig.1. A plasma lamp is supplied with 9000VAC through a 220VAC isolation transformer, Line Impedance Stabilization Network (LISN) and a step-up transformer. The isolation transformer is used as the LISN causes an earth-leakage current to flow that trips the supply. Floating the LISN rectifies this fault condition. The LISN as used in this set-up has two functions: x Firstly, it filters noise from the AC supply. The measurement side (current probe and plasma lamp in Fig. 1) is therefore clean from any noise on the powerline and an accurate assessment of the noise produced by the plasma lamp can therefore be made. A clean 50Hz 220VAC is supplied to the step-up transformer that supplies the plasma lamp with 9000VAC. x Secondly, it supplies a standardized noise load to the conducted interference created by plasma lamp. At higher frequencies (typically > 1MHz) the noise load impedance presented by the LISN (and seen by the plasma lamp) is 50Ω. Measurements and conclusions in this paper are made for two regions of the emission spectrum: x 3kHz – 150kHz: This is the frequency range of the so called CENELEC bands as defined by EN 50065-1 [4]. Measurements for these bands were made in the time domain and a Discrete Fourier Transform (DFT) performed to obtain harmonics in the frequency domain. A Tektronix DPO7254 oscilloscope and Tektronix TCP0030 current probe were used. It was assumed that the Common Mode (CM) currents are negligible in this band and that all interference is in Differential Mode (DM) – an assumption also used in EN 50065-1. Results were downloaded to a PC for processing. This is shown in Fig. 1. th 55 International Symposium ELMAR-2013, 25-27 September 2013, Zadar, Croatia 125 Figure 1. Set-up for measurements in the 3kHz – 150kHz range. 150kHz – 30MHz: This is the frequency range for Broadband PLC. It spans the range traditionally used to measure conducted emissions as per CISPR-22 [5]. In this case, measurements in the frequency domain were directly made using a Rhode & Schwarz FSH323 Spectrum Analyzer and ETS-Lindgren 94111-1L 1GHz bandwidth current probe. Since interference on a PLC system occurs in DM, a special arrangement of cables with respect to the current probe was used for measuring in this range. This special arrangement can be seen in Fig. 2. It cancels the CM current and measures the DM only. x Figure 2. Set-up for measurements in the 150kHz – 30MHz range. The circuit of the second type of plasma lamps is basically 220VAC in a 25.2V step down transformer, into a full wave bridge rectifier, and a large filter capacitor, which gives over 25.2V DC at just under 2 amperes in the primary side of the circuit with no load. The capacitor is necessary as it charges during the peak of the AC cycle, and releases during the trough. This voltage charges the primary coil and consequently the ferrite core of the transformer, which induces a charge in the "feedback" coil, which turns off the transistors. When the transistors have stopped conducting the power is rerouted through the resistors. The EM field in the ferrite core then collapses with no charge to support it, which induces a large high voltage spike in the secondary coil in the reverse direction (basic laws of inductors). The transistor then begins to conduct again because there is no longer any current in the feedback winding, which causes the DC input to oscillate AC, out from the secondary at a high frequency like 15-40 kHz, and around 10-25kV output. Diodes can be used as a safety device to keep high voltage induced in the primary coil from frying the transistors (the system works without them, but they keep the structure from heating up. This is shown in Fig. 4. Figure 4. Second class of plasma lamp driver. PLASMA DRIVER STRUCTURE There are two main classes of plasma lamp drivers. The first and the simplest form is the one that uses the step-up transformer, which consists of a 220VAC to 9000VAC transformer that powers the plasma lamp as shown in Fig. 3. Figure 3. First class of plasma lamp driver. th III. HARMONICS - CENELEC BANDS In order to determine what effect the harmonics of a plasma lamp has on the power-line communications channel, the current in Fig. 3 and 4 must be represented in the frequency domain. This is done by performing a Discrete Fourier Transform (DFT) on exactly one period of data from Fig. 3 and 4. Performing a DFT on one cycle of data yields the harmonics (or frequency domain components) that make up the time domain waveform. The current harmonics (magnitude) for the waveform in Fig. 5 is shown in Fig. 6. The DFT was performed for the waveform in Fig. 5. The harmonics (starting in tens of mA’s at the fundamental) roll off to the tens of µA’s around 20kHz which is the noise floor. In order to compare the current harmonics to typical powerline channel signal voltages, the current magnitudes in Fig.6 must be multiplied with typical power-line channel impedances 55 International Symposium ELMAR-2013, 25-27 September 2013, Zadar, Croatia 126 Figure 5. Time domain line current waveforms for the plasma lamp. Figure 6. Frequency domain line current harmonics (DFT) for the plasma lamp shown in Figure 5 for each of the harmonic frequencies. This power-line channel impedance can be approximated by using the values of the LISN that is used for measurement (Fig. 1 and 2). The LISN characteristics are specified in EN 50065-1. Fig. 7 gives the results when the harmonics (in voltage) are plotted against the CENELEC EN 50065-1 standards for maximum power-line communications signal and Electromagnetic Compatibility (EMC) levels. At 135dBµV (around 5V) the allowable wideband signal strength for a PLC signal is very high. It is around 60 - 70dB higher than the noise harmonics from the signal of the plasma lamp. The noise harmonics are also well below the allowable EMC limit as stated in EN 50065-1. It can therefore clearly be 150kHz plasma seen that in the CENELEC bands from 3kHz – 150kHz plasma lamp is unlikely to interfere with power-line communications channel. IV. BROADBAND SPECTRUM Fig. 8 shows the interference voltage from the plasma lamp versus the EMC average and peak/quasi-peak disturbance level limits in the band 150kHz – 30MHz. In the CENELEC bands from 3kHz – 150kHz there are dedicated maximum signal transmission levels. These do not exist in the 150kHz – 30MHz band and maximum signal transmission is assumed to be at the EMC limit levels. This can be expected as the manufacturer will only filter noise to the EMC limit in order to save on manufacturing costs. In stark contrast from the CENELEC bands, PLC signals in the 150kHz – 30MHz band have to directly compete with noise from devices with power electronic converters and may have a zero S/N ratio at certain frequencies. V. CONCLUSION This paper showed that there are two distinct classes of plasma lamps when considering noise generation in the power-line communications channel. The plasma lamps produce noise in the 150kHz -30MHz band as the noise level and PLC signal level is governed by the same EMC standard. Unless this standard is revised and PLC signals allowed to exceed the EMC limit, power-line communication signals will have to compete with zero signal to noise ratios.. They also produce noise in the 3kHz – 150KHz CENELEC bands, but this th 55 International Symposium ELMAR-2013, 25-27 September 2013, Zadar, Croatia 127 Figure 7. Comparison of the harmonics with the EMC standards and maximum allowable PLC signal strength in the CENELEC band 3k Hz – 150kHz. Figure 8. High Frequency, including the 150kHz – 30MHz band, spectrum results. interference level is 60dB to 70 dB lower than the allowable PLC signal level and therefore pose no risks for power-line communications. REFERENCES [1] B. Parker, “Plasma Luminescent Art,” The MIT Museum, MIT Institute, the MIT 150 Exhibition, Jan 2011. [2] N. Tesla, “Incandescent Electric Light” United States Patent Office, Patent No. 514170, February 06, 1894. th [3] H. C. Ferreira, L. Lampe, J. Newbury and T. G. Swart, Power Line Communications: Theory and Applications for Narrowband and Broadband Communications over Power Lines, Chichester, England: John Wiley & Sons, 2010. [4] EN 50065-1: Signaling on low-voltage electrical installations in the frequency range 3 kHz to 148,5 kHz - Part 1: General requirements, frequency bands and electromagnetic disturbances. European Standard, CENELEC, Ref. No. EN 50065-1:2011 E, Brussels, April 2011. [5] CISPR 22: Information technology equipment – Radio disturbance characteristics – Limits and methods of measurement, International Electrotechnical Commission (IEC). 55 International Symposium ELMAR-2013, 25-27 September 2013, Zadar, Croatia 128