DC-PLC Modem design for PV module monitoring

Journal of International Council on Electrical Engineering ISSN: (Print) 2234-8972 (Online) Journal homepage: http://www.tandfonline.com/loi/tjee20 DC-PLC Modem design for PV module monitoring Seong-Duc Ma, Min-Su Park & Jae-Eon Kim To cite this article: Seong-Duc Ma, Min-Su Park & Jae-Eon Kim (2016) DC-PLC Modem design for PV module monitoring, Journal of International Council on Electrical Engineering, 6:1, 171-181, DOI: 10.1080/22348972.2016.1217817 To link to this article: https://doi.org/10.1080/22348972.2016.1217817 2016 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group Published online: 10 Aug 2016. Submit your article to this journal Article views: 489 View related articles View Crossmark data Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalinformation?journalcode=tjee20

Journal of International Council on Electrical Engineering, 2016 VOL. 6, NO. 1, 171 181 http://dx.doi.org/10.1080/22348972.2016.1217817 OPEN ACCESS DC-PLC Modem design for PV module monitoring Seong-Duc Ma a, Min-Su Park b and Jae-Eon Kim b a LSIS HVDC Solution Team, Anyang, South Korea; b School of Electrical Engineering, Chungbuk National University, Chungju, South Korea ABSTRACT Nowadays, power line communication (PLC) is a very efficient technique in cost aspect because it is communication system using power line without extra communication cables. In case of DC-PLC, it send a communication signal of the high frequency band to the monitoring system through the DC power line and is widely used for DC grid or the distributed generation system using DC source. Especially, the management system which evaluates the performance and efficiency of PV module by measuring PV power through monitoring system based on the DC-PLC has been increased. In this paper, apply the DC-PLC method for low cost PV module monitoring system and constitutes a measuring device and communication modem on a MCU. For the communication performance, we use the digital filter and apply the multi-carrier communication. The proposed DC-PLC Modem is verified through simulation and experiment. ARTICLE HISTORY Received 1 June 2016 Accepted 25 July 2016 KEYWORDS DC-PLC; ASK modulation; multi-carrier communication; PV module monitoring; PV system 1. Introduction The PV system as a most typical renewable energy source has been gradually increased and there has been a growing people s interest for large-scale PV generation plants. PV module in the PV system is comprised of a series- parallel and it will reach the PV inverter via a DC line. If the one of PV modules consisting of series-parallel is damaged or aging, it Degrades the overall power generation amount and the power generating efficiency. Therefore, the solar module unit monitoring in large-scale PV generation plants (or PV system) is very important. PV system is composed of the DC/DC converter for performing MPPT and DC/AC inverter for converting DC power to AC power. These power converter would be generated harmonics by being switched at a frequency of 10 100 khz. DC-PLC is a method of performing communication via the existing DC power lines, and does not require a separate communication line, so it is very economical communication method. Therefore, it is very efficient way to operate by applying a DC-PLC of the PV cell module monitoring system. In general, the low frequency DC-PLC has a frequency band of about 1 500 khz. Among them, communication is performed by a specific frequency to the carrier signal. [1,2] When applied the DC-PLC to the PV system, monitoring systems and measuring devices should be designed as a low cost. Because there are large amount of PV module and data-set. [3 6] In the following Chapter 2 describes the DC-PLC sheme of PV system. And Chapter 3 describes multi- carrier communication for a improvement of communication speed and performance, Chapter 4 shows the experiment and result. End of the paper, chapter 5 is conclusion. 2. DC-PLC in the PV system Using the DC-PLC in PV module monitoring system, we can get the various information (such as voltage, current, temperate of PV module), and could be more efficient the operation and maintenance of PV systems. We modeled the PV system using the PSCAD/EMTDC to confirm the effects and results when applied the carrier signal in the actual PV system. 2.1. Modeling of the PV system In Figure 1, the PV system model is composed of 3 kw single-phase grid connected inverter. This is the current controlled inverter and it outputs the power by following the grid voltage phase and controlling the current. The DC/AC inverter is using SPWM and switching frequency is 10 khz. DC/DC converter is a boost CONTACT Jae-Eon Kim sdma@lsis.com 2016 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

172 S.-D. Ma et al. Inductor is designed at 0.5 mh (about 100 times in 45 uh) to reduce the DC current ripple. 2.2. DC power line configuration Figure 1. Basic PV system model. converter, the switching frequency is 50 khz and performs MPPT. DC link voltage of the inverter is designed to maintain a 400 V. And the value of the decoupling cap as follows. [7] P dc C dc = = 2000 uf 2π f (1) grid V dc Δv dc The minimum value of the current controlled inductor of DC/DC boost converter for continuous conduction mode is as follows. L dc = T s V o D(1 D) = 45 uh 2 I (2) L Figure 2 is the overall structure of PV module monitoring system structure in PV system using DC-PLC. It is consists of the conventional PV system, measuring part of the voltage/current/temperature data, transmission and detection part using DC-PLC and data monitoring system. 2.3. Line filter design and simulation The capacitor component and switching frequency of the PV inverter is cause of the attenuation and distortion of the power line communication (PLC) signal. So it has an adverse effect on the communication. Figure 3 is the PSCAD/EMTDC circuit diagram of the PV generation system applying DC-PLC. The simulation result in Figure 4 shows the effect on the signal attenuation and distortion of the DC-PLC from the PV system. The low pass filter is configured by using Line Filter with decoupling capacitor Figure 2. DC-PLC PV module monitoring system structure in PV system. Figure 3. PSCAD/EMTDC circuit of PV system applied DC-PLC.

Journal of International Council on Electrical Engineering 173 (a) (b) Figure 4. Result of PLC carrier signal of PV system (PSCAD/EMTDC). (a) Line trap coil = 0 H, (b) Line trap coil = 220 uh. to reduce the effects of PV inverter. The transfer function of low-pass filter is as follows. H(s) = 1 1 + s 2( L 1 + L 2 ) + C (3) The C of transfer function is the decoupling capacitor of DC link, and L 1 is the Line Filter and L 2 is the current controlled inductor of the PV inverter. The value of the Line Filter is set to 220 uh in order to set the cut-off Table 1. Digitlal filter specification. Parameter Value A pass (db) 0.01 A stop (db) 70 ω pass (X rad sample) [0.4, 0.6] ω stop (X rad sample) [ 0.48, 0.52 ] frequency of the low-pass filter is about 100 Hz or less. So low-pass filter could supply the only DC power to the PV inverter.

174 S.-D. Ma et al. Figure 4(a) is the result of the simulation that Line Filter is 0 H and (b) is the result of simulation that Line Filter is 220 uh. Figure 4(a) shows that the 50 khz component of switching frequency is larger than 10 khz component of carrier signal. Figure 4(b) shows 10 khz component is increased, and 50 khz component is decreased. As a result, Line Filter serves to reduce the influence by the switching frequency from the PLC carrier signal detection. 3. DC-PLC Modem design Figure 5. Frequency response of digital filter. 3.1. Amplitude shift keying modulation/ demodulation The modulation method of digital communication is used the amplitude shift keying (ASK) modulation. (a) (b) (c) (d) Figure 6. ASK simulation result (MATLAB). (a) Digital message signal, (b) Signal of band-pass filtering, (c) Envelope of signal after low-pass filtering, (d) Demodulated DC-PLC signal.

Journal of International Council on Electrical Engineering 175 Figure 7. Communication flow of monitoring system applying single-carrier communication. Figure 8. Communication flow of monitoring system applying multi-carrier communication.

176 S.-D. Ma et al. Figure 9. Frequency allocation of each PV string and PV module. Figure 10. Frequency band of each PV string. Figure 11. Multi-carrier communication using variable sampling.

Journal of International Council on Electrical Engineering 177 (a) (b) (c) (d) (a) (b) (c) Figure 12. Simulation result of multi-carrier, (a) 1st string message, (b) 2nd string message, (c) 1st signal of filtering, (d) 2nd signal of filtering, (e) 1st string envelope, (f) 2nd string envelope, (g) 1st demodulated signal, (h) 2nd demodulated signal. (d)

178 S.-D. Ma et al. Figure 13. Block diagram of DC-PLC Modem. additional calculation and assistance circuit for detection of the frequency and phase of the carrier signal. So, in general, a separate modem is configured when such method of FSK or PSK are applied to the PLC. We implemented the PLC applying a cost-effective ASK modulation. We used the rectifier detector for the demodulation of the signal. Table 1 is the specification and Figure 5 is the frequency response of the digital filter for a signal detection. Parameter A pass, A stop, ω pass, ω stop are means each passband ripple, stop-band attenuation, pass-band cutoff frequency, stop-band cutoff frequency. ASK demodulation has the advantage of being easy to implement. In Figure 6, Simulation result shows the ASK modulation and demodulation. 3.2. Multi-carrier communication using variable sampling Figure 14. Simple structure of DC-PLC Modem H/W. ASK method is most basic digital modulation method for modulating the amplitude of the carrier signal. Each method of Frequency shift keying (FSK) and Phase shift keying (PSK) for modulating each phase and frequency of the carrier signal is also one of the basic method. A case of FSK and PSK, it needs the In the DC-PLC Modem using binary ASK method, the measuring device(slave mode) sends the data only by the request of master. In this case, the entire data(from the first module to the last module) takes a long time delay in collecting the whole data. In this section, apply the multi- carrier communication which carrier frequency allocation of each PV string as improvements for the communication rate. Figure 7 is request/response message flow of each slave modem applying the single-carrier communication. If Figure 15. Configuration of DC-PLC Modem experiment.

Journal of International Council on Electrical Engineering 179 each PV string was assigned each different carrier frequency, the communication rate will increase as the number of PV strings as shown in Figure 8. Each of Figures 9 and 10 show each configuration of frequency allocation and frequency band. The variable sampling that can use the designed filter without frequency tunning is applied for detecting the message of each string s carrier frequency. Figure 11 is variable sampling of multi-carrier communication of PV system. Figure 16. DC-PLC Modem test board. (a) (b) (c) (d) Figure 17. Experiment result of multi-carrier. (a) 1st string message, (b) 2nd string message, (c) 1st signal of filtering, (d) 2nd signal of filtering, (e) 1st string envelope, (f) 2nd string envelope, (g) 1st demodulated signal, (h) 2nd demodulated signal.

180 S.-D. Ma et al. (e) (f) (g) (h) Figure 17. (Continued) 3.3. Multi-carrier communication simulation For simulation of multi-carrier communication 1st string is using 2.4 khz and 2nd string using 1.2 khz which is down-sample value 2 times. Each string bit is as follows: 1st string bit: [1,1,1,1,0,0,1,1,0,1] 2nd string bit: [0,1,1,0,1,0,1,1,1,1] Figure 12 is Simulation result of each 2.4, 1.2 khz. 4. Experiment and result 4.1. DC-PLC Modem Figures 13 and 14 is block diagram of DC-PLC Modem and simple structure of hardware. And Figure 15 is configuration of experiment. In experiment, C2000 series processor of TI, TMS320F28335 was used in implement of digital filter and modulation and demodulation. DC-PLC Modem test board are shown in Figure 16. 4.2. Experiment result First string board and 2nd string board are used each frequency same as simulation, 2.4 and 1.2 khz. Each string bit as follows: 1st string bit: [1,1,1,1,0,0,1,1,0,1] 2nd string bit: [0,1,1,0,1,0,1,1,1,1] Experiment result is shown in Figure 17. Each data bit is constituted by dividing a total of 10 times the data bit period for 100 10 ms can be converted to the 10 binary signal. There are some errors per each bit interval but for the value of the bit period 10 ms, if it hold the interval of more than 90 110% demermines the data value of bit interval. 5. Conclusion In this paper, we proposed a cost-effective PV modem design using DC-PLC to communicate via the DC power lines of PV systems. Proposed DC-PLC Modem was used for ASK Modulation that economical and easy to impement. The Digital Filter and Line Filter as detection filter was designed. And for a improvement of communication rate, each PV string per carrier frequency, multi-carrier commnucation was proposed. This system was verified through the simulation and experiment. The proposed DC-PLC Modem is economical and easy to implement for the construction of low cost PV module monitoring system has advantages. Disclosure statement No potential conflict of interest was reported by the authors. Notes on contributors Seong-Duc Ma received M.S. degree in electrical engineering from Chungbuk National University, Korea, in 2016. He has been a researcher at LSIS HVDC Solution team since 2016. His research interests include operation and design of energy management system, power distribution systems with distributed generation and HVDC system.

Journal of International Council on Electrical Engineering 181 Min-Su Park received the B.S. degree in Electric Engineering from Chungbuk National University, Korea, in 2015. He is currently working toward his M.S. in Electric Engineering at Chungbuk National University. His research interests include operation and design of power distribution systems with distributed generation. Jae-Eon Kim received the B.S. and M.S. degrees from the University of Hanyang in 1982 and 1984, respectively. He was affiliated with KERI as a researcher from 1984 to 1989; a senior researcher form 1989 to 1996; and a team leader of advanced distribution systems and custom power lab from 1997 to 1998. He received his Ph.D. from Kyoto University, Japan in 1996. He has been a professor at Chungbuk National University since 1998. His current interests are analysis of power quality; operation and design of power distribution systems with distributed generation and advanced distribution systems, such as micro-grid or smart grid. References [1] IEEE Std 1901.2. Standard for low-frequency (less than 500 khz) narrowband power line communication for smart grid applications. 2013. [2] Wade ER. Design of a broadcasting modem for a DC PLC sheme. IEEE Trans. Mechatron. 2006;11:533 540. [3] Roman E. Intelligent PV module for grid-connected PV systems. IEEE Trans. Ind Electron. 2006;53:1066 1073. [4] Nosato H. A very low-cost low-frequency PLC sytem based on DS-CDMA for DC power lines. 2012 IEEE International Symposium on Power Line Communications and Its Applications; March 2012; p. 398 403. [5] Sanchez-Pacheco FJ. PLC-based PV plants smart monitoring system field measurements and uncertainty estimation. IEEE Trans. Instrum. Meas. 2014;63:2215 2222. [6] Han J. PLC-based photovoltaic system management for smart home energy management system. IEEE Trans. Consum. Electron. 2014;60:184 189. [7] Hu H. A preview of power decoupling techniques for microinverters with three different decoupling capacitor location in PV systems. IEEE Trans. Power Electron. 2013;28.