Frequency Characteristics Measurement of Overhead High-Voltage ...

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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 22, NO. 4, OCTOBER 2007

Frequency Characteristics Measurement of Overhead High-Voltage Power-Line in Low Radio-Frequency Range M. Zajc, Member, IEEE, N. Suljanovic´, Member, IEEE, A. Mujˇcic´, Member, IEEE, and J. F. Tasiˇc, Member, IEEE

Abstract—This paper presents measurement results of frequency characteristics of the high-voltage power line in low radio-frequency range: amplitude and phase characteristic, group delay, and input impedance. Measurement results for the 400-kV overhead power-line under operation are presented together with the developed measurement methodology. The methodology provides a method to measure amplitude and phase characteristics, group delay and the return loss of the HV power line for a geographically distributed system utilizing an optical link as a return path. The measurement results confirm very low attenuation with a linear phase, but with oscillations present in all characteristics caused by reflection of the signal on the power line. The complete knowledge of frequency characteristics of the HV power line and noise description is crucial for the design of digital power-line carrier (PLC) systems. Index Terms—Digital communications, high-voltage power line, power-line communications, transfer function measurement.

I. INTRODUCTION

H

IGH-VOLTAGE (HV) power lines are, together with electric power transmission, utilized for communications. Such communication systems are known as power-line carrier (PLC) communication system. Communications via power lines are divided into three categories: high-voltage, medium voltage, and low-voltage power line communications. This paper focuses on the measurement of high-voltage power line communication characteristics in the low radio-frequency range [1]. The motivation for the work is in the worldwide effort to develop a reliable high-voltage high-speed digital PLC system. Power lines are primarily designed for energy transfer and their communication characteristics are worse compared to other communication media [1]–[8]. Noise and reflection are recognized as the two major factors degrading HV power-line communication characteristics. Noise and reflection are the greatest obstacles for further development of digital HV PLC Manuscript received June 21, 2006; revised August 16, 2006. This work was supported in part by the Slovene Ministry of the Economy and in part by the Ministry of Higher Education, Science and Technology, Slovenia, under Scientific Program P2-0246. Paper no. TPWRD-00336-2006. M. Zajc and J. F. Tasiˇc are with the Faculty of Electrical Engineering, University of Ljubljana, Ljubljana, SI-1000, Slovenia (e-mail: [email protected]; [email protected]). N. Suljanovic´ and A. Mujˇcic´ are with the University of Tuzla, Faculty of Electrical Engineering, Bosnia and Herzegovina (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPWRD.2007.905369

communications. This paper together with paper [8] describes the communication characteristics of HV power lines. The transfer function together with the noise description represents an origin for computation of PLC communication channel performance in the manner defined in [9]. In the past, attention has mainly been paid to amplitude distortion in the PLC communication channel since this factor has been the primary information in narrowband analog PLC communications [5]–[7], [10]–[12]. For development of a reliable high-speed digital system, a complete understanding of the frequency characteristics of high-voltage power lines in the range devoted for PLC communications is needed while these characteristics are not widely available in the literature. With the availability of other communications systems, interest in the PLC system has declined. Analog PLC systems have remained in some applications as a part of power-line protection systems or as a redundant communication path. Nevertheless, PLC communications have recently gained interest since they represent an efficient, reliable, and cost-effective solution. In the past, HV PLC communications were based on analog systems which are still in use today. Developments in the digital communications domain have brought forth new challenges to PLC systems manufacturers. Application of digital communication techniques makes digital PLC systems more concurrent due to their higher data transmission rates and compatibility with other systems which is not the case with analog PLC systems. The digital PLC system design is founded more on communication characteristics of HV power lines than analog, making new requirements for accurate determination of HV power-line frequency characteristics [9]. Selection of appropriate communication techniques such as modulation, channel coding, and equalization are directly related to these characteristics. In general, a power line as a communication channel is characterized with a relatively high noise level compared to other communication media since noise is time and weather dependent. Moreover, a power line itself is a noise source. The major source of noise is the corona noise. The noise level caused by the corona strongly varies in the power-frequency period and is dominant during rainy and snowy weather. Our research results on corona noise measurements and modeling are available in [8]. The second important property of the power-line channel is that a power line is not terminated with a characteristic impedance and as a result reflected waves occur on the power line. The presence of reflection introduces oscillations in the

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ZAJC et al.: FREQUENCY CHARACTERISTICS MEASUREMENT OF OVERHEAD HIGH-VOLTAGE POWER-LINE

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Fig. 1. Principal elements of a power line communication channel.

frequency characteristics (amplitude and phase characteristics, group delay, input impedance) of the PLC channel, and consequently introduces a smaller signal-to-noise ratio (SNR) and a higher bit error rate (BER) in digital systems. Our research objective was to systematically measure high-voltage power line communication characteristics under different weather conditions with the power line under operation. Our measurement results of phase characteristic and group delay of the PLC channel contribute to future developments of reliable digital communications systems via HV power lines. In this paper, we present a measurement methodology of HV frequency power-line characteristics in the low radio-frequency range and measurement results obtained systematically on an actual 400 kV power line under operation in the frequency range devoted to HV PLC communications [2]–[4]. Measurement results are given for the middle-phase-to-outer-phase coupling since such coupling equipment was provided for the tested power line. II. HV POWER-LINE COMMUNICATION SYSTEM A power-line communication system consists of four major components (Fig. 1): power line, coupling device with a coupling capacitor, line trap unit (LTU) with inductor and PLC communication equipment. Since the primary role of the power line is electric power transfer, the communication medium is under high voltage. This power-frequency high voltage is blocked with a coupling capacitor providing a safe operational environment for the communication equipment. The main task of an inductor and LTU is to prevent propagation of a signal in the opposite direction (Fig. 1). An HV overhead power line is a multiconductor line where one or more conductors are used for communication purposes. As a result, there are several coupling schemes utilizing phase conductors and ground, each with specific frequency characteristics [7], [13]. The selection of an appropriate coupling scheme is defined by two factors: transfer function and costs. The propagation characteristic of signals for each coupling scheme is, in general, determined with a modal analysis of a multiconductor line [7], [13]. If we consider a power line with three phase conductors in a horizontal disposition, the coupling scheme with the lowest attenuation is one with two outer phases with a return inner phase. This solution is, however, the most expensive since it requires coupling and LTU equipment for all

Fig. 2. A 400-kV overhead power-line pole with a horizontal conductor disposition.

three phases. A more cost-effective solution utilizes one or two phases. The inner-to-outer phase coupling scheme (Fig. 1) and inner-phase-to-ground coupling scheme are denoted as optimal coupling schemes [6], [7], [13]. III. MEASUREMENT CONDITIONS Measurements were conducted on a 400 kV overhead powerline under operation. The power line had three phase conductors with a horizontal disposition and two shielding wires. Measurements were conducted systematically in fair and foul weather conditions. A. Power Line Data Power line data are given in Fig. 2 and Table I. We assumed 6 m sag and the ground resistance per unit length 50 m (swampy ground). Since the phase at both sides was connected to a HV transformer with large inductivity, the nonoperating phase was considered to be open-circuited.1 B. Equipment Data Measurement of the HV frequency characteristics in low radio-frequency range is limited to the operational frequency range of the already mounted devices. Therefore, it is necessary to estimate the frequency characteristics of the coupling devices and LTUs for the correct interpretation of measurement results. Data gathered from the 400 kV power-line design project is as follows. • The coupling devices available on the power-line terminals provide middle-phase-to-outer-phase coupling. • The operational range of the LTU is from 168 to 304 kHz, (the coupling used with the minimal impedance 600 is a phase-to-phase coupling and the expected input line impedance is 600 ). • The coaxial cable length is 500 m. The cable attenuation varies linearly from 0.2 to 0.35 dB/100 m in the frequency 1From the project documentation with the permission of ELES power utility (designed by IBE)

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TABLE I SELECTED ELECTRICAL AND GEOMETRICAL PARAMETERS OF THE POWER LINE

Fig. 3. Frequency characteristics of the coupling devices used: (a) amplitude characteristic and (b) group delay.

range from 150 to 350 kHz. The group delay of the coaxial cable is approximately constant and equals 5 ns/m. • The capacity of the coupling capacitor is 4400 pF. The coupling device cut-off frequency is 50 kHz (a high-pass filter). The frequency characteristics of the installed coupling devices with LTU for middle phase-to-outer phase coupling are shown in Fig. 3. Due to the frequency characteristics of the mounted LTU, the computation of the HV power-line amplitude characteristic and group delay is measured in the frequency range from 150 to 350 kHz. The input impedance of the power line under operation can not be directly measured due to the high voltage on the power line under operation. Therefore, the input impedance is, instead, measured at the end of the coaxial cable to assure impedance match between the coaxial cable and the PLC communication equipment. C. Return Path Used in Measurements Some modern power systems have optical cables present in phase conductors or in the shielding wires of power lines. The presence of the optical cable in the measured power-line offered an opportunity to utilize it as a return path. The influence of the return path characteristic was considered so as to eliminate its impact on the measured characteristics. The optical cable introduces attenuation that can be considered constant in the frequency range devoted to the PLC communications (up to 500 kHz). An optical transmitter provides amplification to compensate attenuation introduced by the optical cable. There

is also an additional delay present due to signal propagation. It can be considered constant and equal to (1) and are the length of the optical cable and vewhere locity of signal propagation in the optical cable. The velocity de, pends on the refractive index of the fiber core . For the velocity of the signal propagation in the optical cable is (2) D. Weather Conditions Since HV power-line frequency characteristics are very weather dependent, systematic long-term measurements were conducted under fair and foul weather conditions through all the four seasons. The results presented in Section V for three weather groups, namely warm, very warm, and snow where • warm weather corresponds to measurements with the air temperature around 20 C; • very warm weather corresponds to measurements with the air temperature around 30 C; • snow corresponds to foul weather with intensive snowing with air temperatures below 0 C. IV. MEASUREMENT METHODOLOGY This section defines the methodology used for the HV powerline frequency characteristics measurement: • amplitude, phase, and group delay characteristics;


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Fig. 5. Return-loss measurement. Fig. 4. Measurement of the amplitude, phase, and group delay characteristics.

• input impedance and return loss characteristics. The measurement principles accompanied with measuring schemes were developed and tested for the power line presented in the previous section. All the measured characteristics correspond to the power-line communication channel presented in Fig. 1. Since HV power lines can be hundreds of kilometers long, the return path is, in general, not easily provided. The communication channel frequency characteristics measurement methodology used utilizes a network analyzer [14] with an optical return path between the two line ends (Fig. 4).

A. Measurement of the Frequency Characteristics of HV Power Line With a Return Path Our measurements of the PLC channel characteristics (amplitude, phase characteristic and group delay) are presented in Fig. 4. Factors having the greatest impact on the accuracy of measurements are • source impedance and load impedance mismatch; • noise coming from the power line. Source impedance and load impedance mismatch are covered in the following section. The influence of noise on the measurement accuracy can be reduced by increasing the source amplitude, narrowing the line filter at the input and averaging measurements. The role of the line filter at the input of the network analyzer is to track the source frequency and minimize the noise. A wider bandpass of the line filter ensures faster measurements but a higher noise floor. Since the line attenuates the signal and the noise level at the receiving end is relatively high, the source signal is amplified. The introduced amplification is compensated in the final results (Fig. 4). A custom-designed optical transmitter is used to convert the electrical signal extracted from the HV power line into an optical signal which is further transmitted to the first power line end via optical cable (Fig. 4). At the first line end, the optical signal is reconverted into an electrical signal and transmitted to the input of the network analyzer. Attenuation and delay introduced in the optical system are compensated in the measurement results.

Fig. 6. Return-loss bridge.

B. Measurement of the Power-Line Input Impedance and Return Loss Knowledge of the power-line input impedance is significant in the sense of the impedance match with the input impedance of the coaxial cable linking from the switchgear with the communication center. The impedance mismatch introduces an additional attenuation in the communication channel and it increases the signal reflection. Measurement of the power-line input impedance is not possible directly on the power line due to the high voltage on the power line under operation. As a result we measured the input impedance at the coaxial cable and the line-filter connection point (Fig. 4). The return loss is a measure of reflection and is related to the ratio of two impedances: line characteristic impedance and line termination impedance. In practice, the HV power line is not terminated with a characteristic impedance and reflection is generated. This is especially evident on short power lines and manifested in a rippled amplitude characteristic [13]. The coupling device is designed to match the nominal impedance of the line and the coaxial cable. In the aspect of communications, it is interesting to measure reflection at the coaxial cable and the line filter connection point (Fig. 4). The return loss RL is defined as a logarithm of the ratio of and incident wave magnitudes, that is a logreflected arithm of reflection coefficient magnitude

(3) While the reflection coefficient magnitude varies in a range from one to zero, return loss varies in the range from zero to infinity.

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Fig. 7. Measured amplitude characteristic of the PLC channel for different frequency bands.

The measurement of the return loss as a measure of reflection is presented in Fig. 5. The measurement methodology utilized a network analyzer and a return-loss bridge (Fig. 6) [15], [16]. is the input impedance of the coaxial cable Impedance is a referwith a nominal value of 75 . Impedance ence impedance of 75 . The return loss is calculated from

dB

Nonlinearities in the phase characteristic are more easily observed in the group delay characteristic since it corresponds to a differentiated phase response. In Figs. 7 and 8, we observe oscillations in the amplitude characteristic and the group delay. Oscillations are a consequence of the reflection phenomenon at power-line terminals. The period between two neighboring maximum and minimum is found from [5], [6], [13]

(4)

and are voltages measured at switch opened and where is constant. closed. In both cases, the voltage V. MEASUREMENT RESULTS It is known that the HV power-line characteristics vary significantly with the weather conditions [2]–[7]. Thus, important information accompanying measuring results is data about the weather conditions during measurement. With the conditions presented in Section III we systematically measured the frequency characteristics of the 400 kV power line in different weather conditions in order to capture their influence. A. Amplitude Characteristic and Group Delay Accurate measurements are obtained in the frequency range from 150 kHz to 350 kHz, allowed by the installed coupling devices. The measured amplitude characteristic and group delay of the PLC channel in this frequency range are given in Figs. 7 and 8, respectively.

(5) If wave velocity is approximately 300 000 km/s for a line km, the period of oscillations equals 1.5 kHz. Reflected waves on the power line manifest themselves as echoes in the time domain. The echo delay is defined with the line length and velocity of signal propagation. Intensity of echoes is determined with the reflection coefficient and line attenuation. B. Return Loss and Input Impedance Measurements Measurements of the input impedance and the return loss at the coaxial cable terminal are given in Fig. 9 and 10, respectively. The return loss is a measure of reflection and is equal to the reflection coefficient. Fig. 9 can be also interpreted in the sense of the reflection coefficient. Sudden changes (glitches) in the characteristic appear due to interference with other PLC systems operating at these frequencies on neighboring power lines. Accurate return loss and impedance measurements could not be obtained in foul weather due to large attenuation of a return-


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Fig. 8. Group delay characteristic of the PLC channel for different frequency bands.

loss bridge at a high noise level. The measurements presented correspond to fair (warm and dry) weather. The measured impedance (Fig. 9) is expressed in units where one unit equals 75 . This impedance is measured on the coaxial cable and represents the input impedance of the PLC channel put in parallel with the 75 input impedance of the filter used to terminate the cable. Since the characteristic impedance of the used coaxial cable is 75 , deviation of the measured input impedance from 75 corresponds to the measured return loss. The measured return loss greater than 10 dB represents a satisfactory impedance match.

VI. DISCUSSION ON MEASUREMENT RESULTS Communication characteristics of HV power lines found in the available literature are obtained from numerical simulations and are mostly relevant for analog PLC systems. The reason is found in the fact that measurements of such characteristics are difficult since the system under consideration is geographically distributed while access to HV power lines under operation is usually restricted. From the measured characteristics, it is obvious that measuring power-line frequency characteristics below 160 kHz and above 350 kHz is not possible due to LTU characteristics (Section III). Measurement in the wider frequency range requires broadband coupling devices and LTU-s while replacement of the existing equipment at the measured power line was not possible.

It is evident that a HV power line is characterized by relatively low attenuation. Analysis of the attenuation is correlated to allowed signal power at the transmitting end limited by EMC problems as well as the power and character of noise. Therefore, digital communication performance analysis is complex and related to determination of BER performance for such systems starting with the amplitude characteristic and noise description for required SNR [9]. The measurements proved the existence of oscillations in the amplitude characteristic and group delay caused by reflection. The period of oscillations is determined by the line length and signal propagation velocity. Oscillations are expressed on short power lines causing increased BER. This problem is treated by application of appropriate equalizers and echo cancellers in digital PLC modem design. VII. CONCLUSION In the paper we propose a measuring methodology for HV power-line frequency characteristics in a low radio-frequency range and present systematic long-term measurement results of a 400 kV power line obtained in different weather conditions. Utilization of an optical link with a specially designed optical transmitter and receiver as a return path enabled measurements of phase characteristics and group delay. Measurements were conducted on an in-service 400 kV power line reflecting the actual state in application of the PLC systems. The coupling scheme and the measurement frequency range were determined by already installed coupling devices.

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Fig. 9. Input impedance for different frequency bands: upper real part and lower imaginary part.

Fig. 10. Return loss for different frequency bands.

The measurements confirmed the existence of oscillations in amplitude characteristic and group delay caused by signal reflection at the line terminals. The reflection phenomenon manifests itself in the time domain as echoes. The delay is defined with the line length. Intensity of echoes is determined with the line termination at both ends and the line attenuation. Our research efforts are aimed at forwarding an appropriate measurement methodology for HV power lines since design of high-speed digital PLC systems requires a complete understanding of these characteristics. Measurements results in this

paper together with the paper [8] represent a complete description of a HV power line as a communication media. ACKNOWLEDGMENT The authors are grateful to ELES d.o.o., Fotona d.d., and Iskra Sistemi d.d. REFERENCES [1] IEEE Guide for Power-Line Carrier Applications, IEEE Std. 643-2004, 2005.


Nermin Suljanovic´ (M’06) received the B.Sc. and M.Sc. degrees in electrical engineering from the Faculty of Electrical Engineering, University of Tuzla, Bosnia and Herzegovina, in 1997 and 2000, and the Ph.D. degree from the University of Ljubljana, Ljubljana, Slovenia, in 2004. Currently, he is an Assistant Professor with the University of Tuzla, Bosnia and Herzegovina. His research interest is power-line communications and communication channel modeling.

[2] G. Ericsson, “Classification of power systems communications needs and requirements: Experiences from case studies at the Swedish national grid,” IEEE Trans. Power Del., vol. 17, no. 2, pp. 345–347, Apr. 2002. [3] CIGRÉ Study Committee 35, “Report on digital power line carrier,” 2000. [4] “Planning of (single-sideband) power line carrier systems,” Int. Electrotech. Comm., Tech. Rep. IEC/TR 60663, 1980. [5] Y. P. Shkarin, “High-frequency channels via overhead power lines,” (in Russian) Appendix to Energetik, 2001, pt. 1 and 2. [6] G. V. Mikutski and Y. P. Shkarin, “High-frequency channels on overhead power lines,” (in Russian) Energoatomizdat, 1986. [7] G. V. Mikutski, “High frequency channels for power system protection and automation,” (in Russian) Energetik, 1976. [8] N. Suljanovic´ , A. Mujˇcic´ , M. Zajc, and J. F. Tasic, “Computation of high-frequency and time characteristics of corona noise on HV power line,” IEEE Trans. Power Del., vol. 20, no. 1, pp. 71–79, Jan. 2005. [9] A. Mujˇcic´ , N. Suljanovic´ , M. Zajc, and J. F. Tasic, “Error probability of MQAM signals in PLC channel,” in Proc. IEEE Electrotechnical and Computer Science Conf. ERK, 2003, pp. 71–74. [10] V. H. Ishkin and Y. P. Shkarin, “Computation of parameters of highfrequency channels via overhead power lines,” in Elect. Power Inst. (in Russian). Moscow, Russia: Elect. Power Inst., 1999. [11] G. V. Mikutski, “Computation of attenuation in high-frequency channels,” (in Russian) Electrichestvo, no. 9, pp. 51–53, 1964. [12] L. M. Wedepohl, “Electrical characteristics of polyphase transmission systems with special reference to boundary-value calculations at power-line frequencies,” Proc. Inst. Elect. Eng., vol. 112, pp. 2103–2112, Nov. 1965. [13] N. Suljanovic´ , A. Mujˇcic´ , M. Zajc, and J. Tasiˇc, “Approximate computation of high-frequency characteristics for power line with horizontal disposition and middle-phase to ground coupling,” Elect. Power Syst. Res., vol. 69, pp. 17–24, Apr. 2004. [14] Hewlett Packard HP 3589A User’s Guide, HP 3589, 1991. [15] IEEE Standard for WideBand (Greater Than 1 Decade) Transformers, IEEE Std. 111-2000, 2000. [16] IEEE Recommended Practice for Testing Electronic Transformers and Inductors, IEEE Std. 389-1990, 1990. Matej Zajc (S’98–M’00) received the B.Sc. degree from the Faculty of Electrical Engineering, University of Ljubljana, Ljubljana, Slovenia, in 1995, the M.Sc. degree from the University of Westminster, Westminster, U.K., in 1996, and the Ph.D. degree from the University of Ljubljana in 1999. He is an Assistant Professor at the University of Ljubljana. His research interests include modern communication systems, advanced digital signal processing, and parallel computing.

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Aljo Mujˇcic´ (M’06) received the B.Sc. degree from the Faculty of Electrical Engineering, University of Belgrade, Belgrade, Yugoslavia, and the M.Sc degree from the Faculty of Electrical Engineering, University of Tuzla, Bosnia and Herzegovina, in 1992 and 1999, and the Ph.D. degree from the University of Ljubljana, Ljubljana, Slovenia, in 2004. Currently, he is an Assistant Professor with the University of Tuzla. His research interests include power-line communications, modulation, and channel coding techniques.

Jurij F. Tasiˇc (M’86) received the B.Sc., M.Sc., and Ph.D. degrees in electrical engineering from the University of Ljubljana, Ljubljana, Slovenia, in 1971, 1973, and 1977, respectively. From 1971 to 1992, he was a Researcher with the Jozef Stefan Institute. From 1992 to 1993, he was a Visiting Researcher and Lecturer at the University of Westminster, Westminster, U.K. Since 1994, he has been a Professor with the University of Ljubljana. His research interests include advanced algorithms in digital signal processing and modern communication systems.