Wireless Data Transmission from Inside Electromagnetic Fields

Journal of Microwave Power and Electromagnetic Energy, 44 (2), 2010, pp. 88-97 A Publication of the International Microwave Power Institute

Wireless Data Transmission from Inside Electromagnetic Fields José Ignacio Huertas, Roberto Barraza, Julian Mauricio Echeverry Tecnológico de Monterrey Automotive Engineering Research Center- CIMA, Eduardo Monroy Cárdenas # 2000, 50110 Toluca, México

Received: August 28, 2009 Accepted: February 15, 2010 ABSTRACT This paper describes analytical and experimental work developed to evaluate the effects of the electromagnetic fields produced by high- voltage lines (400kV) on wireless data transmission at the 900MHz band. In this work the source of the data transmission is located inside the electromagnetic field and the reception station is located at different distances from the power lines. Different atmospheric conditions are considered. KEYWORDS: Wireless data transmission, high-voltage lines, electromagnetic field. INTRODUCTION One of the most expensive activities performed by the companies in charge of the distribution of the electrical energy is the maintenance of the high voltage power transmission lines (230 and 400 kV). Routinely these companies supervise thousands of kilometers of power lines by helicopter or by land looking for faulty points under risky conditions for the involved personnel. As alternative solution several works has been done developing robotic systems to automate this labor [Nakashima et al., 1995; Wang et al., 2003; Li et al., 2004]. Sensors of vibration frequency, surface temperature and vision are used to diagnose the operative conditions of the power transmission lines, especially in the tower joining points. One of the tasks that these robotic systems will perform is the transmission of the data, collected through the diverse sensors, from the interior of the electromagnetic field generated by the power lines to near mobile stations or remote central stations. Data transmission is required to be bidirectional, to distances greater than 100 m, in real time, and with high reliability levels. Table I presents a comparison of the different available data transmission technologies. None of them have been designed to be operated under the effect of the electromagnetic fields interference generated by high voltage transmission lines. The reviewed state-of-theart technical literature does not report the effects of electromagnetic fields on wireless data transmission as a function of atmospheric conditions. This information is essential for the design of telecommunications systems operating under these circumstances. To address this need, the present document describes an analytical and experimental work developed to describe and quantify the effect of the electromagnetic noise generated around the 400kV transmission lines, as function of the atmospheric conditions, on wireless telecommunications systems when they operate from inside the electromagnetic field. 88

International Microwave Power Institute

José Huertas et al., Wireless Data Transmission from Inside Electromagnetic Fields Table I. Current available wireless data transmission technologies Application

Transmission technology

Advantages

Disadvantages Transmission Rate of frequency transmission

Transmission distance

Data transmission to remote stations (>150 Km)

Satellite telephony

Proved technology and high coverage

High operational costs, low rate of transmission

Global coverage

GSM (Global system for mobile communications)

Proved technology and available commercially

PLC (Power line communication) Physical infrastructure already installed

Allows communication through the high voltage transmission line.

Bluetooth

Wireless data transmission to nearby mobile land stations

Zig Bee

RF modem WiFi & WiMax

> 850 MHz

~ 4.8 kbps

> 850 MHz

~ 200 kbps

Coverage area of the cell companies

not available commercially

30 and 500 kHz

~ 200 kbps

> 150 Km

Available Requires a commercially high emission quick power ( ~1 w) installation and configuration, Low cost. Includes several protocols for different applications

2.4 GHz 900 MHz 868 MHz

720 kbps – 3 Mbps

1 < l < 100 m

2.4 GHz 900 MHz 868 MHz

20 – 250 kbps

10 < l < 100 m

2.4 GHz 900 MHz

9.6 -115.2 kbps

0 < l < 64 Km*

2.4 GHz 5 GHz

11 – 300 Mbps

0 < l < 32 Km*

* With high gain antennas

MATERIALS AND METHODS The following section describes how the presence of the high voltage power lines affects wireless communication. Electromagnetic Noise Electromagnetic noise generated around high voltage power lines is an undesirable disturbance, which affects wireless data transmission. It can be observed as an additive signal to the original one that can interrupt, obstruct, degrade or limit the performance of communication systems. Transmission line-generated noise is mainly due to [Maruvada, 2000]: • Electric discharges between line components • Corona effect Discharges between line components: It is only present in power lines under 70kV.

It can be originated in insulators, in metallic parts or in faulty or not properly installed equipment. This type of noise tends to dominate the frequency spectrum between 10 and 20MHz [Silva and Olsen, 2002]. Their effects can be controlled by ensuring a correct power line installation and by providing proper maintenance [Olsen and Stimson, 1988]. Corona effect: It affects power lines over 110kV, tending to dominate the frequency spectrum between 10 and 30MHz. In general, the term “Corona” describes the partial discharges generated in areas with a very strong electric field. The corona effect is characterized by visual effects, acoustic noise, electric current, energy loss, radio interference, mechanical vibrations and chemical reactions. The chemical reactions that characterize the corona effect on air produce ozone and nitrogen oxides [Goldman et al., 1985; Abdel-Salam et al., 2000].

Journal of Microwave Power and Electromagnetic Energy, 44 (2), 2010 International Microwave Power Institute

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José Huertas et al., Wireless Data Transmission from Inside Electromagnetic Fields

Three conditions must be met to start a corona discharge [Trinh et al., 1968; Veldhuizen et al., 2000]: there must be free electrons in the area of interest, the electric field intensity must be high and the non-uniformity level on the electric field must be high. The first condition is fulfilled automatically because about eight electrons per cubic centimeter of air per second are continuously produced due to the earth radioactivity [Giao et al., 1968]. Concerning the second condition, it has been determined, experimentally and by computer simulations, that corona discharges are originated at the surface of high voltage electrical conductors when the voltage gradient at the surface reaches a critical value E0 [Abdel-Salam et al., 2000; Indulkar, 2004; Pakala et al., 1968]. This magnitude depends on the polarity of the applied voltage, as well as in atmospheric pressure, temperature and humidity levels [Jianfeng et al., 2005]. The corona critic voltage gradients for electrical conductors are given by Peek’s equation [Peek, 1915]:

for different geometries [Trinh et al., 1968; Arora et al., 1992]. (3) Electromagnetic noise modeling Corona pulses in the time domain can be characterized by a double exponential [Maruvada, 2000; Fujii et al., 1993; Nayak and Thomas, 2005; Nayak and Thomas, 2003]: (4) where: ip Current amplitude (mA) K,α,β Empirical constants to be determined through the knowledge of the pulse shape t Time (ns) When equation 4 is transformed into frequency domain by means of the Fourier transform, it becomes equation 5 where ω is frequency

(1)

(2) where: r Electrical conductor radius (cm) P Atmospheric pressure (kPa) T Temperature (°C) δ Relative air density respect air density at standard conditions (25°C and 101 kPa) The last condition, which is related with the non-uniformity level of the electric field, is also obtained from empiric equations for particular electrodes shapes. For example, for parallel electrodes the corona discharges can occur if the distance between cables (a) is less than the critical distance dc =25. Similar requirements have been reported 90

(5) For typical values of K, α and β of the positive and negative semicycle, figure 1 shows the behavior of the current pulses as a function of frequency. This figure also shows that the pulses generated by the corona effect tend to dominate the spectrum at low frequencies (< 20 MHz) and its effect diminishes as frequency increases. Literature reports empirical models that allow quantifying the noise generated by high power transmission lines [Maruvada, 2000]. These models have the next characteristics: • They report values that are higher than those obtained experimentally. • Only severe weather with intense rain is considered because it is the most adverse condition for the corona effect formation.


José Huertas et al., Wireless Data Transmission from Inside Electromagnetic Fields

• Irregularities and pollution effects over the conductor surface are not considered. The most accepted model to quantify the radio interference (RI) was proposed by the BPA (Bonneville Power Administration) in 1983 ([Maruvada, 2000].

Similar analysis showed that altitude increases the radio interference level about 3dBm by each km over the sea level. Finally it is highlighted that these models cannot predict the effect of ambient conditions on the radio interference generated by the electric transmission lines.

(6)

Experimental methodology To evaluate the effect of electromagnetic field generated around high voltage power lines on wireless data transmission, for different humidity, temperature and altitude conditions, it is proposed to: a) Determine the electromagnetic field effect on the power of the signal at the receiving device: The power attenuation level of a signal emitted from inside of the electromagnetic field is measured as a function of the distance to the emitter in the parallel and perpendicular direction to the electric lines. This result is compared with the results of the same experiment performed without electromagnetic field for the same ambient conditions. Background electromagnetic noise is measured in both cases at the receiver location.

Table II describes the meaning of each one of these terms and specifies how to estimate them. The RI is the value in dB above 1 μV/m calculated for the transmission line. Here, it is proposed to extrapolate the BPA´s model for the case of electromagnetic noise generated by 400 kV lines at high frequencies (900 MHz) and different altitudes. Using the Peek´s equation to calculate the critical voltage gradient and average environment conditions at different altitudes over the sea level, the electromagnetic noise was evaluated as a function of the distance to the transmission line. Figure 2 shows that the electromagnetic noise diminishes logarithmically with the distance to the power transmission line. In addition, it shows that the electromagnetic noise diminishes as the frequency increases.

Figure 1. Frequency spectrum of the corona pulses obtained from equation 5.

Figure 2. Estimation of the radio frequency interference generated by the electric transmission lines of 400 kV as a function of the distance to the transmission line and frequency using the model proposed by the BPA through equation 6.


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José Huertas et al., Wireless Data Transmission from Inside Electromagnetic Fields Table II. Empirical model to quantify the radio interference generated by electromagnetic fields proposed by the BPA [Maruvada, 2000]. Description

Equation

Symbols

RI for the reference transmission line according to the ANSI*specification

gm RMS value of the maximum gradient (kV/cm) f Transmission frequency (MHz) A Altitude over the sea level (km) C1 Constant of reference. C2 Constant to be calculated. d Conductor diameter

RI by effect of the voltage gradient at the conductor surface RI by effect of the conductor diameter RI by effect of the frequency RI by effect of the altitude RI by change the distance to conductor

in the

* ANSI (American National Standards Institute) specifications: Measurements performed on land at a lateral distance of 15m, band width of 5 kHz and frequency of 1 MHz.

It is expected that without the presence of electromagnetic field, the scope of the waves in space will be limited by the attenuation suffered by the signal as it gets farther away from its source. Such attenuation Lp (without the presence of electromagnetic fields) is given by equation 7, where Lp in dB is the logarithm of the ratio between the received power and the emitted power (Pr /Pe ), l is the distance in km between both antennas and f is the transmission frequency in MHz. (7)

b) Determine the electromagnetic field effect on the maximum transmission distance: Maximum transmission distance is defined as the distance at which the BER (bit error rate) is higher than 10-6. BER is defined as the number of errors in the transmission divided 92

by the total amounts of bits being sent. A transmission error occurs when a transmitted bit is not properly received after one second when the bit strings are sent randomly. BER is also known as the error or error probability [Lathi, 1995]. To conduct the proposed experimental methodology, a radiofrequency spectrum analyzer, an instrument to measure distance (GPS), an ambient conditions measurement system (temperature, humidity and altitude) and a wireless data-transmitting device are needed. Due to the importance on the experiment of the wireless device Table III shows the technical specifications of the device used in this work. Results can be extrapolated to other wireless technologies like zigbee and wifi because the physical phenomena involved in the transmission process of the information carrier signal are the same. During the development of the test it must be taken into account that the antennas orientation must match their radiation pattern


José Huertas et al., Wireless Data Transmission from Inside Electromagnetic Fields Table III. Technical characteristics of the radiofrequency data transmission device used in this work [MaxStream, 2007]. Parameter

Value

Power of the emitted 1 mW – 1 W (0 – 30dBm) signal Transmission distance ~1 km –2.1 dBi antenna, (direct line of view) 64 km – high gain antenna Data transmission rate

9,600 bps, 115,200 bps

Receiver sensibility

-110 dBm @9,600 bps (BER