Identification of Customers Served by Distribution Transformer using Power Line Carrier Technology

Transcription

1 Identification of Customers Served by Distribution Transformer using Power Line Carrier Technology T. T. Ku C. S. Chen C. H. Lin M. S. Kang H. J. Chuang Dept. of Electrical Engineering Dept. of Electrical Engineering Dept. of Electrical Engineering Dept. of Electrical Engineering National Sun Yat-Sen University I-Shou University National Kaohsiung University Kao Yuan University of Applied Sciences Kaohsiung, Taiwan Kaohsiung, Taiwan Kaohsiung, Taiwan Kaohsiung, Taiwan Abstract In this paper, a narrow-band power line carrier (PLC) based identifier has been designed and developed to support the on line identification of customers served by each distribution transformer. To investigate the transmission characteristics of power line carrier signal over low voltage distribution lines, the mathematical model of distribution components such as transformers, low voltage conductors, customer appliances, etc. at PLC carrier frequency are derived and included in the computer simulation. The two port network is used to represent the low voltage distribution systems and Matlab/Simulink is applied for the analysis of PLC signal attenuation from the secondary side of transformer to the customer locations. After completing the development of PLC based identifier, the field test of a commercial building has been executed. The PLC signal strength and the noise level have been measured to verify the effectiveness of the identifier to determine the connectivity of distribution transformer and the customers served without requiring power service interruption. Index Terms Power line carrier, connectivity of transformer and customers, two port network. I. INTRODUCTION With the advancement of power line carrier communication technology and the advantages of data transmission over ubiquitous power lines, PLC has been used for various types of applications. For instance, home network technology is able to transmit data over existing power lines without requiring the expensive communication infrastructure[1]. The automatic meter reading (AMR) has been used to retrieve the customer meter data by using PLC. The load management such as remote control of water heaters has been performed by utility companies with PLC technology for peak loading shaving when the power system encounters the problem of spinning reserve shortage. For the implementation of smart grid to enhance system reliability and operation efficiency, many utility companies have executed the joint research projects with internet providers for the application of broad band power line carrier (BPLC) to support system control of smart grid as well as to provide information service for customers. To support various applications of distribution system planning, Taiwan Power Company (Taipower) has completed the installation of Outage Management System (OMS) in her district offices[2]. All of the distribution components such as line conductors, transformers, poles, etc., have been stored as the digitized mapping with the attributes and connectivity being stored as the facility information. Due to the difficult to identify the connectivity for the underground system, connectivity of distribution transformers and the customers served is highly inconsistent in the OMS database[3]. To solve the problem, Taipower engineers have to interrupt the customer power service by disconnecting the power fuse of distribution transformer to identify the customers connected, by which in the outaged customers, often file the complaint for service interruption. To identify the customers served by each transformer without causing customer service interruption, a power line carrier based identifier is developed in this study to support the identification of customers served by each transformer by injecting the PLC signal at the secondary side of transformer over the low voltage distribution lines and receiving the PLC signal at the customer locations. The computer simulation of transmission characteristics of PLC signal over the distribution lines and transformers is performed to ensure the attenuation of PLC signal at different carrier frequencies is acceptable for the application. The electric circuit modules of signal coupling, power supply, data processing unit and user interface (UI) are then designed and developed. The field test of PLC identifier is then executed in Taipower distribution system so that the performance of the identifier can be verified. II. LOW VOLTAGE DISTRIBUTION SYSTEMS MODELING The framework of distribution systems is designed for the delivery of power from the utility to the customers. When the high frequency PLC signal is transmitted over the system, the characteristics of distribution components such as distribution transformers, low voltage distribution lines and customers loads characteristics will be changed due to high frequency effects. It is therefore important to determine the characteristics of all distribution components to derive the corresponding mathematical modeling of low voltage distribution networks for the simulation of PLC signal transmission. A. Distribution transformers The distribution transformer can be considered as a pure inductive element when the power delivery from the utility to the customers with 6Hz frequency. However, is transmitted over the low voltage distribution system, the transformer characteristics has to be represented as the hybrid of inductive and capacitive components when the PLC signal with much higher carrier frequency as shown in Fig /9/$ IEEE 3476 ICIEA 29

2 Vp I p Y N :1 R Z s { Figure 1. Components of distribution transformer at high frequency. With the turn ratio of distribution transformers as shown in (1), secondary side to refer to primary side, the current at the secondary side and primary side are I s and I p respectively, which are calculated in (2) and (3). For the modeling of the distribution transformer, the admittance matrix is used to represent the relationship between the node currents and node voltages in (4). The equivalent circuit model of transformers at PLC carrier frequency is therefore illustrated in Fig. 2. I s L VS γ yz = ( R+ jωl)( G+ jωc) = α + jβ (5) Zc z R+ jωl = y G+ jωc (6) C. Customer loads With various kinds of electric home appliances used by both residential and commercial customers, the problem of impedance match of customer loading will affect the attenuation of PLC signal. The impedance of each type of electric appliances should be expressed as function of frequency and included in the computer simulation for PLC signal transmission. For instance, Fig. 4 shows the impedance and phase angle of refrigerators at different carrier frequencies, which have been derived by executing the field measurement in this study. V1 I N = = 2 V 2 I (1) Impedance (Ω) Phase (θ) Is = Vp+ Vs (2) NZs Zs 1 1 I p= Y+ V V 2 p s N Z NZ s s (3) Impedance (Ω) Phase (θ) 1 1 Y I + 2 NZ p N Z s Vp Yp M V s p = = I s 1 1 Vs M Ys Vs NZs Zs Figure 2. Equivalent circuit of distribution transformer. B. Low voltage distribution lines In this paper, the electrical magnetic theory is applied to analyze the transmission characteristics of high frequency PLC carrier signal over the low voltage distribution line with mutual capacitance between cables as shown in Fig. 3. The wave length λ of PLC signal will be varied with the signal frequency. The propagation constant and characteristic impedance of the distribution line are expressed as (5) and (6) respectively[4-5], where R, L, C, G represent the resistance, inductance, capacitance and conductance of per unit length of conductors[6]. The attenuation constant α and phase constant β of PLC signal transmission are solved in (5). (4) Frequency (khz) Figure 4. Impedance and phase angle vs. frequency (refrigerators). D. Distribution systems modeling To derive the transmission characteristics of PLC signal over the distribution system, the distribution transformers, low voltage distribution lines and customer loads are represented as the two port network in Fig. 5. The transmission matrix of PLC signal which is injected at the secondary side of transformer and received at the customer site (point A) is then derived in (7)[7-8]. V s Z s I 1 V 1 I 2 V2 ZL -8-1 N Z T L1 C ground C cable C cable L2 C cable C ground Earth Figure 3. Cross sectional view of low voltage distribution lines. Figure 5. Two port network of low voltage distribution systems. TA = ZT ZL A B cos( βl) jz sin( βl) = C D Z jy sin( βl) cos( βl) T Z L AA BA = CA DA (7) 3477

3 In the distribution systems, the components such as transformer, distribution line and load are represented as two port networks[9] as shown in Fig. 6 with corresponding parameters (db) Signal attenuation (SA) Figure 6. Parameters of two port network. III. SIMULATION OF PLC SIGNAL TRANSMISSION To investigate the transmission characteristics of PLC signal over distribution system, a distribution transformer with capacity of 5kVA to serve customers at locations of A, B and C with different load composition in Fig. 7 are selected for computer simulation. The PLC signal is injected at the secondary side of the transformer and is received at different customer locations. By executing the simulation of PLC signal transmission with Matlab/Simulink, the signal attenuation (SA) at different locations has been solved as shown in Fig. 8. It is found that SA is increased with the carrier frequency for the frequency above 25kHz. The signal attenuation is solved as 2dB for the frequency from 5kHz to 14kHz, which is the frequency range to be used by the PLC chip for the identifier to be developed in this study. For the identification of the customers served by each distribution transformer, the maximum allowable SA is 6dB, which implies that the attenuation of 2dB for transmission of PLC signal over distribution systems is acceptable for this application. 2m+4m 5 kva F A 16kW 32m A 1 B C Frequency (khz) Figure 8. Transmission of PLC singal attenuation at customer sides. IV. DESIGN OF PLC BASED IDENTIFIER For the transmission of PLC signal over low voltage distribution system, the PLC signal with high frequency and low power is overlapped with the power signal with frequency of 6Hz by the signal modulator for the coding information to be transmitted as shown in Fig. 9. Each transformer is assigned a specific coding for the PLC signal at the transmitter end, which will be decoded to retrieve the coding at the receiver end (customer locations) to identify the customers served by the transformer. The identifier consists of three modules to be described as follows. Figure 9. Modulaton of PLC signal. A. PLC signal coupling circuit The PLC signal coupling circuit is to provide the communication path for the injection of PLC signal at the secondary side of distribution transformer. The module is designed by considering the impedance characteristics of low voltage distribution lines to prevent the coupling circuit from break through by high voltage power source. The band pass filter is also embedded in the module to prevent excessive distribution and attenuation of PLC signal when it is received from or transmitted to the power lines. The PLC signal generated by the PLC chip of the identifier has to be filtered and amplified before it is coupled to the distribution lines. Figure 1 shows the block diagram of PLC signal coupling module. 1m 6m 6m 14m+4m 14m+4m 32m 16kW B Figure 7. Configuration of distribution system network. Figure 1. The circuit diagram of PLC signal coupler. 3478

4 B. Power supply module To provide the power supply for the PLC based identifier to identify the connectivity of transformer and customers served for the distribution systems with voltage level from 11V to 44V, a switched type DC-DC converter is applied for the design of power supply module as shown in Fig. 11. A full wave rectifier is used with a capacitor to reduce the ripple of the DC power source after rectification. The power integrated circuit (IC) with the transformer and opto isolator are applied for voltage transformation so that constant DC voltage of 5V dc can be obtained for the wide range of AC input voltage. AC 11~44 V DC 5V power line carrier signal with different frequencies is injected at the secondary side of each transformer with the corresponding coding and received at the customer sites. TABLE I shows the strength of PLC signal for the high bit and low bit at different carrier frequencies, which has been measured at the locations of signal injection and signal receiving ends. It is found that the strength of PLC signal has been attenuated for the signal communication from the secondary side of transformer to the customers. For instance, the strength of high bit signal for carrier frequency of 6kHz has been reduced from 71.7% to 63.1%, while the strength of low bit signal has been reduced from 72.5% to 67.1%. During the testing of PLC signal in this large commercial building, the attenuation of signal strength is much less than 6dB for all frequencies, therefore the PLC based identifier can be used to identify the customers served by each transformer in a very effective manner. Power IC Opto isolator Figure 11. Power supply circuit with switched type DC-DC converter. C. Data processor unit When the PLC based identifier is operated with the transmitting mode to send the transformer coding information, each distribution transformer is assigned a specific coding with data processor module for the PLC carrier signal. When the identifier is operated with the receiving mode at the customer locations, the coding information is derived by decoding the PLC signal with the micro processor to identify the distribution transformer which serves the customer. Besides, the user interface (UI) is also embedded in the processor for the setting of operation modes and the adjustment of PLC carrier frequency according to the signal interference. Figure 12 shows the data processor module. Figure 13. The layout of distribution transformers for the test commerical buildiing. TABLE I. THE PLC SIGNAL STRENGTH(%) AT DIFFERENT CARRIER FREQUENCIES PLC carrier Signal strength (%) frequency (khz) Transformer Customer High bit Low bit High bit Low bit High bit Low bit Figure 12. The blocks of data processor unit. V. FIELD TEST OF PLC BASED INDETIFIER After completing the design and development of PLC based identifier, the field test has been conducted by Taipower engineers to verify the performance of the identifier. Figure 13 shows the layout of distribution transformers, which are located in the basement of a large commercial building. There are 4 units of 3-φ transformers, T1, T2, T3 and T5, which formulated by three 1-φ transformers to serve large commercial customers with 3φ4W, 38V power source, while T4 serves the small customers with 1φ3W 22V power source. The To study the signal noise introduced by customer loading, the strengths of PLC signal and noise level for T2 transformer have been recorded during the field test. Figure 14 shows the PLC signal strength and noise at different carrier frequencies for the high bit signal and low bit signal. For carrier frequencies below 7 khz, the signal attenuation is rather significant. Although the noise has been introduced by the customer appliances, the signal noise ratio (SNR) of PLC signal is larger than 15, which is acceptable for the identifier to measure the customers served by the transformer. To identify the customers served by T2 transformer, it is suggested that higher carrier frequencies should be considered for the identifier to measure the connectivity of customers and this transformer with better performance. 3479

5 Strength (%) (db) Frequency (khz) Signal strength Noise strength SNR Figure 14. PLC signal strength and noise at different carrier frequencys. After completing the development of PLC based identifiers, the field testing has been conducted to measure the connectivity of transformer and customers served by injecting the PLC signal at the secondary side of distribution transformer and receiving the signal at customer locations. The account numbers of customers served by Transformer T1 and T2 in Fig. 13 are retrieved from the OMS database as shown in TABLE II. According to the OMS database, there are 12 customers served by T1. After field verification by PLC based identifier, it is found that 2 customers in the OMS are actually not served by T1, while another 2 customers which are served by T1 are missing in the database. There are 3 customers listed in the OMS database are not served by T2, while another 3 customers actually served by T2 are missing in the database. By using the PLC based identifiers for the connectivity measurement, the customers served by each distribution transformer can be determined without requiring the service interruption. TABLE II. Test transformer T1 T2 CONNECTIVITY VERIFICATION OF TRANSFORMERS AND CUSTOMERS SERVED Customer account number OMS Database Field verification ** ** * * * * * SNR ** ** ** * Customers not served. ** Customers missing. VI. CONCLUSIONS In this paper, a power line carrier based transceiver has been designed and developed to provide an effective tool for the identification of customers served by each distribution transformer without requiring interruption of customer power service. The mathematical models of transformers, low voltage distribution lines and various electric home appliances at carrier frequency have been derived and included in the simulation to investigate the transmission characteristics of PLC signal over low voltage distribution systems. It is found that the signal attenuation is acceptable for the power line carrier frequency ranged from 6kHz to 14kHz, which is used by the PLC chips for the PLC signal transceiver. After completing the development of PLC based identifier, the field test has been carried out by Taipower engineers to verify the PLC signal transmission characteristics and the effectiveness of the transceiver to perform the connectivity identification of customers and distribution transformers. The strength of PLC carrier signal, the noise level introduced by the customer loadings and the PLC signal being coupled from the other transformers with different coding over the low voltage distribution lines have been measured. It is concluded that the PLC based identifier developed in this paper can determine the connectivity of distribution transformer and all of the customers served in a very effective way. By using the PLC based identifier developed in this study, the attributes of customers served by each distribution transformer in the OMS database can be updated without causing customer complaints due to service outage by the conventional identification method. When the outage has been reported by customers, the maintenance crew can identify the transformer serving the outaged customers by applying the PLC based identifier to achieve more efficient service restoration. REFERENCES [1] H. Meng, S. Chen, Y.L. Guan, C.L. Law, P.L. So, E. Gunawan and T.T. Lie, "A Transmission Line Model for High-Frequency Power Line Communication Channel", Power System Technology, 22. Proceedings. PowerCon 22, Vol. 2, Oct. 22, pp [2] T. H. Chen and J. T. Cherng, Design of a TLM application program based on an AM/FM/GIS system, IEEE Trans. on Power Systems, vol. 13, no. 3, Aug [3] Y. T. Chao, S. T. Lee, H. C. Chang and T. H. Chen, An improvement project for distribution transformer load management in Taiwan, IEEE Trans. on Power Systems, vol. 18, no. 2, pp , May 23. [4] R.C. Madge and G.K. Hatanaka, "Power line Carrier Emission from Distribution lines", IEEE Trans. on Power Delivery, Vol. 7, Oct. 1992, pp [5] Hui Xiao, Xuebin Wu, Xiaojiao Tong, Rong Lian, Zeng Xiangjun and Sheng Su, "Medium-voltage power line carrier communication system", Power System Technology, 24. PowerCon 24, Vol. 2, Nov. 24, pp [6] David K. Cheng, Fundamentals of Engineering Electromagnetics, Prentice Hall, [7] H. Meng, S. Chen, Y.L. Guan, C.L. Law, P.L. So, E. Gunawan, and T.T Lie, "Modeling of Transfer Characteristics for the broadband power line communication channel", IEEE Trans. on Power Delivery, Vol. 19, Jul. 24, pp [8] T. Banwell and S. Galli, A Novel Approach to the Modeling of the Indoor Power Line Channel Part II: Transfer Function and Its Properties., IEEE Trans. Power Del., vol. 2, no.3, pp , July

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