High voltage power line constraints for high-speed communications M. Zajc, N. Suljanović, A. Mujčić and J. F. Tasič Laboratory for digital signal, image and video processing Faculty of electrical engineering, University of Ljubljana Ljubljana, Slovenia
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Abstract - Paper presents technical limitations and constraints of high-voltage power-lines for high-speed digital PLC communications. Signal reflection and noise characteristics are recognized as major technical limitations for digital PLC system development. Results from a computer model and measurements are presented. Proposed solutions are at the end.
I. INTRODUCTION The idea of utilization of high voltage (HV) power lines for communications is present from the early developments of the power utility systems. Communications on power lines are divided into two categories: - high voltage power line communications - low and medium voltage power line communications This article focuses on the high voltage power line communications. PLC (Power Line Carrier) is a communication system that exploits the power line as a communication channel. The main advantage of the power line communications compared to other wire-line communication infrastructure is that the communication system is already available. Therefore the main costs are associated with the terminal equipment. Analog PLC (aPLC) systems are usually used for the transmission of voice, protection and low bit rate transmission. The main application of an aPLC system is a telephone connection between remote stations and a control center. For current SCADA (Supervisory Control and Data Acquisition) systems and communication with RTUs (Remote Terminal Units), aPLC system presents a satisfactory solution for the data transmission with low bit rates. Furthermore, aPLC is commonly used in protection systems of high voltage power lines. The development of digital communications in the recent years presents new challenges for PLC manufacturers. Current trends in PLC communications are based on the combination of the analog and digital modem in one system. This presents a higher degree of flexibility for the costumer. At the same time it solves the problem of relatively low reliability of the digital PLC (dPLC) for the complete substitution of the aPLC. Nowadays, dPLC covers the transmission of voice and data. On the other hand, aPLC is still used for the protection due to its reliability, robustness and low costs.
The main disadvantage of the aPLC systems is its low bit rate. A reliable dPLC would therefore present an interesting solution for higher bit rates [1]. The utilization of power lines for the communication services is generally motivated with the long distances in different countries around the globe where power lines usually present the only infrastructure. Modern communication services require higher data rates than those that are currently available. Current dPLC modems provide data rates between 28 kbit/s and 100 kbit/s. With the massive application of optical cables in some power utilities the interest for the PLC technologies has declined. Even though, PLC systems are still gaining the interest since it represents the most cost-effective solution. II. HV PLC COMMUNICATION CHANNEL Ever broader application of dPLC systems and the demand for higher bit rates require a throughout knowledge of the high-frequency characteristics of the high-voltage power line. The channel model is crucial for the development of modern digital systems. Channel models of the HV power-line channels are based on the modal analysis [2-4]. However, the derivation of these models includes simplifications, which are not crucial for the analog transmission. The power-line channel model should overcome this problem and provide also the group delay of the power-line channel which wasn’t considered before. The frequency characteristics (amplitude and phase characteristics, group delay, input impedance) as well as noise characteristics on the power line present the limitations for high speed and reliable PLC communications. High voltage power lines present a multi conductor system composed of three or more phase conductors and shielding wires. Each of these phase conductors can be used for signal transmission. Since there are several phase conductors, different coupling schemes defining how the communication equipment is connected to the power line are available. Each of these schemes is characterized with its transfer function [3, 5]. The characteristics of the PLC channel is composed of: • Power-line characteristics; • Characteristics of the coupling devices and the line trap unit (LTU); • Characteristics of the coaxial cable that connects the communication equipment with a coupling device.
PLC channel consists of (Figure 1) signal path between the transmitter and coupling device on the transmission side, power line conductors, and coupling device and receiver on the receiving side [6].
LTU
To substation
LTU
Coupling capacitor
Coupling capacitor
Coupling device Coaxial cable Communications equipment
To substation
Coupling device Coaxial cable Communications equipment
Figure 1: PLC channel of the power line. Coupling: middle phase to ground.
The PLC system is different from other communication systems in the sense that except communication equipment it requires some additional components due to the high voltage at the power line. Coupling devices are used to protect communication equipment and personnel from the high voltage but also to match impedances between coaxial cable and power line. It should be noticed that coupling devices provide a desired coupling scheme. Line trap unit is used on the phase conductors utilized for communications in order to block the signal propagation in the unwanted direction. The presence of the coupling device and LTU introduces distortions into the frequency characteristics of the PLC channel (Figure 1). PLC communications utilize frequency range from 30 to 500 kHz, which is divided into 4 kHz grid [2]. In the case of aPLC channel’s bandwidth equals 4 kHz. On the other hand, dPLC channel’s bandwidth is a multiple of 4kHz bands in order to be compatible with analog PLC systems. Frequency planning demands extra care for the interference with other channels as well as interference with local radio and navigation stations. Weather conditions significantly influence the frequency characteristics of the HV power line [6, 2]. Rain, fog, snow and ice influence the transmission of the signal over the power line. The most weather significant influence refers to ice coating on the power-line conductors. The ice on the power-line conductors can increase or decrease the PLC channel attenuation, what depends on the used frequency band. The second problem necessary to be analyzed in digital communications over HV power line relates the reflection. In analog PLC communications, the reflection doesn’t have a significant influence on voice transmission using analog modulation and low bit-rate data transmission. Reflection occurs at the locations of impedance change, like line terminals, inhomogenities and faults. The consequence of the reflection is that high-frequency power-line characteristics become rippled what can cause increase of BER in the high bit-rate data transmission. Despite the above problems, characteristics of the power line are appropriate for digital communications. Major limitations for high-speed dPLC systems present noise characteristics of the power line.
Noise sources on the power line can be divided into two categories: - High voltage power line as a noise source: corona noise, impulse noise due to switching and power-line faults. - Interference with other electronic equipment operating in the same frequency domain: radio and navigation stations, other PLC systems, etc. Noise caused by the interference is narrow-band and determined by the characteristics of the electronic equipment causing it. On the other hand, corona noise is Gaussian noise with variable variance, which is strongly weather dependant. Corona noise at a foul weather is the major source of noise on high voltage power lines. In analog PLC communications corona noise is described as white Gaussian noise. In case of digital PLC communications corona noise must be considered as the Gaussian noise with a variable variance [12]. The impulse noise due to power-line switchings, faults and lightning have a small occurrence probability. The synchronization of communication devices is strongly affected by the impulse noise. Noise in the channel together with the reflection present the main limitation factor for development of dPLC systems and substitution of aPLC systems with digital. III. RESEARCH RESULTS In this section we present research results for the 52 km long 400 kV power line with three conductors in horizontal disposition and two shielding wires, which is common design for European power systems. For such HV power line the middle phase to ground and the outer phase to the middle phase are denoted as optimal couplings [5, 3]. The high-frequency characteristic of the 400 kV power line are measured and compared with the simulated. The comparison of the simulated amplitude characteristic and group delay with the measured for the outer phase to the middle phase is given in Figure 2. The computation is based on modal analysis and the method of the matrical reflection coefficients [7, 4]. Oscillations in Figure 2 are result of reflection on the power line, which is not terminated with characteristic impedance [3, 4]. The period of oscillations is determined with the line length and signal propagation velocity. Measuring has been conducted with a network analyzer while the return path was provided with the optical link. The group delay presented in Figure 2-b encounters the delay of the optical link. Power spectral density of noise on the HV power line for fair and foul weather conditions is given in Figure 3. Noise level at a foul weather is approximately 18 dB above the noise level at a fair weather. The dominant source of noise in foul weather is the corona noise. The noise varies for 7.95 dB inside the power frequency period what additionally degrades signal to noise ratio regarding the average level [9]. The corona noise represents the Gaussian noise with the variable root mean square (RMS) [4]. The corona noise is caused by high voltage on the power line and its variation is related to the power frequency period. The measurement results confirmed that the voltage RMS normalized to the averaged value (relative corona noise)
approximately keeps the shape for different filter bandwidths (Figure 4) [9].
U RMS (t )/U RMS 2.5
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Figure 4: Relative corona noise calculated from measured samples on 400 kV power line at foul weather. Label equals to the central filter’s frequency and bandwidth, Filt500 is low pass filter with stop band frequency 500 kHz.
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Figure 2: Frequency characteristics of HV power line. Voltage: 400 kV, length 52 km, three conductors in the horizontal disposition, the outer phase to the middle phase coupling Power spectrum density of noise
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Figure 3: Measured Spectral power density of noise on 400kV power line. Foul weather above, fair weather below [9].
Figure 5: BER of the 16 QAM BITCM system including and random interleavers of length 1280 and bit rate 128 kbit/s, o- uncoded 4 QAM via PLC channel, -uncoded 4 QAM via AWGN channel, ◊- coded BITCM system via AWGN channel, *-, generator polynomials (7, 5) via PLC channel, >- , generator polynomials (15, 13) via PLC channel,
