Quantitative effect of dust on photovoltaic supply of transmission line monitoring device and configuration correction

Aiming at the problems that the photovoltaic power supplies of transmission lines monitoring devices are liable to be discontinuous and damaged, it is found that the power loss caused by surface dust deposition cannot be ignored. Improper configuration via empirical formula may result in power shortage and long-term undercharging of the battery, finally shorten the equipment's life. What is more, artificial and field experiments were carried out to find the qualitative impact, and the attenuation coefficient related to dust deposition may ensure a more appropriate configuration. This research provides a reference for power supply design and helps to save the replacement.

Introduction: To realise more reliable operation of transmission lines, various online equipment are installed on the line to monitor faults and natural disasters [1]. Under outdoor situations, an ideal monitoring device which is usually installed on the iron towers or lines should meet continuous and long-term normal work. Therefore, a stable and reliable power supply is a necessary prerequisite. As it is hard to obtain power sources nearby, the mainstream methods to provide electrical power for the monitoring device include wind energy, solar power systems and wind-solar complementary systems. Owing to the outstanding solar conditions in northwest China, where the annual solar irradiation on the horizontal plane is up to 1610.80 kW·h/m 2 , solar applications are prevalent. The photovoltaic panels fixed by metal brackets are mounted on iron towers, and empirical engineering formulas are used to configure the panel power and the battery capacity. Under normal conditions, the average life span of photovoltaic panels is 10 to 20 years, while the design floating life of lead-acid batteries is 5 to 8 years. However, according to actual feedback, the life expectancy of this kind of power supply system cannot reach the expected life after installation [2], and problems such as ageing acceleration, insufficient power supply and power failure occur within two years or even shorter time, then discontinue the data acquisition. The replacement is costly and very inconvenient.
To prolong the service life of photovoltaic power supply and reducing maintenance frequency as possible, this Letter analyses the relation between solar performance and dust accumulated on the photovoltaic panels. Manual and field tests were designed, and the experimental data are used to correct the configuration of the solar power supply system.
Principle: Ideally, a silicon solar photovoltaic cell can be equivalent to the combination of a current source and a diode affected by illumination. The characteristic equation is as follows: where q is the charge constant, k is the Boltzmann constant, T (K) is the battery temperature, I 0 is the equivalent diode reverse saturation current, I ph is the photogenerated current and is affected by the irradiance S (W/m 2 ): where I sc is the short-circuit current, S ref is the reference irradiance, generally equals to 1000 W/m 2 and C T is the temperature coefficient, T ref usually equals to 298 K is the reference temperature. It can be seen from (2) that the photogenerated current accounts for a major part of the output current, and mainly affected by the irradiance S and the temperature T of the photovoltaic cell. Therefore, inadequate estimation of the solar intensity, temperature, rain duration and other factors at the installation site may result in improper solar panel specifications [3]. The majority of monitoring devices are balanced loads with the same daily power consumption. Suppose that the average consumption power is P (W) and the effective solar duration of the installation site is t (4 to 6 h). Taking the conversion efficiency η pv of the PV panel (usually is 60%) into consideration, the estimation of the rated power of PV arrays can be given by the empirical configuration formula as follows: where capacity safety factor K c is usually between 1.1 and 1.5. Although the empirical formula has considered the effects of sunshine hours and temperature influence, the outdoor environment is more complex. When the photovoltaic module works on the line, its output power is often lower than the rated value under the standard test condition (irradiance 1000 W/m 2 , temperature 25°C, AM1.5). Therefore, it is necessary to configure the photovoltaic power supply according to the actual situation rather than the nominal value. The particulate matter concentration in the air caused by sandy weather, haze and smog emission etc. aggravate the scattering of light to some extent. At the same time, the dust attached to the panel surface will reduce the transmittance of the photovoltaic panel thus reducing the output power. In addition, humid dust can corrode components and shortening hardware life. What is more, the long-term dust coating will cause uneven temperature distribution and produce 'hot spot effect'. Gholami et al. [4] used the glass panels to measure the dust density under different dip angles. The results show that the larger the angle tilt, the smaller the dust density is, and the dust density can reach 5.3 g/m 2 in half a year when the inclination angle is 30°. Li et al. [5] found that the photovoltaic power generation efficiency decreased by 1.5, 2.2 and 6.4% after 2, 5 and 10 days of dust covering, respectively. It can be seen that photovoltaic power loss increases with higher dust density. However, the dust situation is obviously related to the environment around the installation site. Photovoltaic panels installed in heavily polluted areas may reach the annual dust density that installed in an aircleaned and vegetation-rich area within a few weeks. Therefore, the research related to panel angle, dust composition and ashing speed are not universal for widely distributed outdoor monitoring devices. In contrast, it is of more significance to study power loss under different dust densities.
Artificial and field experiments: To explore the qualitative impact on photovoltaic output power, artificial dust coating experiments were carried out to evaluate the output power loss under certain dust density which simulated by the ash accumulated on buildings and iron towers. It is necessary to control other influencing factors to be constant when verifying the dust coating effect. The temperature in the laboratory was kept at 25°C via the air conditioner, while the simulated radiation generated by GLORIA-X500A xenon lamp is substitute for unstable natural sunlight. Since the glass of the panel is smooth, the dust is adhered to the panel surface by ultra-fine water mist. Measuring the output power after drying, then scrape the dust off and weight it by XB220A electronic balance with an accuracy of 0.001 g. The relation between dust density and output power reduction is calculated and the data are shown in Fig. 1. where dust density is ρ g/m 2 , and τ (%) is the output power loss rate of a photovoltaic panel. It can be seen that the output power loss reached about 16% when the dust density is 10 g/m 2 . Correspondingly, as shown in Fig. 2, an experimental platform for photovoltaic power supply installed on a 110 kV transmission line of three-tower and two-stage is set up in this Letter. Two sets of are fixed on the iron tower frame as loads, and two same sets photovoltaic power supply are set as a control group with dust as a variable. The device uses optical fibre to transmit data, and the working current is about 600 mA. The average power consumption of the monitoring device for 24 h is 3.2 W. According to (3), when the safety factor is 1.2 and t is 4 h, it can be calculated that the output power of photovoltaic panel should not be <38.4 W. Then the monocrystalline silicon photovoltaic panels of 40 W were selected. Its open circuit voltage is 21.6 V and short-circuit current is 3.23 A. The panel is mounted at the same height and facing south, with an installation angle of 30°.
micro-meteorological monitoring device photovoltaic panel Fig. 2 Experimental platform of photovoltaic power supply dust coating The three-month follow-up tests were conducted from 1 December 2017 to 28 February 2018, with an interval of 0.5 h from 8:00 to 17:00. In order to show the influence of dust, daily output power data returned by charge controller from 21 to 28 February 2018 are shown in Fig. 3, the red curve is the output power of the periodically cleaned panel, while the blue curve is the unclean one. It can be seen that the two panels are of the similar changing trend with time, but after 3 months of natural dust deposition, the output of the unclean panel decreases significantly. During the strong sunlight period from 12:00 to 14:00, the average power loss caused by dust is 5.3%. After the experiment, the dust density is measured to be 1.47 g/m 2 , which was slightly less than the value in artificial experiments.
Configuration correction: The device consumption, sunlight duration and photovoltaic efficiency are taken into account in the configuration (3). However, according to the data of experiments, clearly the power loss caused by surface dust deposition cannot be ignored, the photovoltaic power supply will fail to supply power as designed after long-term operation, which made the battery under low charge and reduce the overall life of the power supply. Since the iron towers or lines are too high to clean the panel surface by manpower, and the selfcleaning devices are costly. The effect of dust deposition should be considered in the process. Nevertheless, the dust coating speed varies greatly depending on the region, and it is difficult to give a fixed impact parameter. In this Letter, the output power loss ratio τ (%) caused by different densities of dust is considered as the characterisation quantity. When the dust density of panel surface reaches 6 to 10 g/m 2 within a certain period of time, the power loss will be stabilised at 14 to 17%, respectively. On the time scale, the reduction of output power can be considered as the reduction of incident radiation caused by dust, then it can be reflected in the equivalent sunlight duration t, as shown in the following equation: The value of output power loss ratio τ can be determined according to the local climate and building environment. It takes a greater value for drought, sand region and heavy industries, and a lower value for areas with rich vegetation and humid climate. The modified configuration formula can ensure continuous operation of devices under the condition of a certain quantity of dust coating. However, due to the uneven distribution of dust caused by wind, inclination and rainwater, still the hot spot effect will occur. Therefore, the cleaning work should also be done to follow the spring or autumn inspection of the power grid.
Conclusion: Aiming at the problem that the photovoltaic power supplies of transmission line monitoring devices tend to fail with high rate, the output power reduction caused by dust is taken into account in the power supply configuration formula. The reduction of incident radiation reflected in the equivalent sunlight duration so that the attenuation coefficient is converted into duration t to ensure a more appropriate configuration specification.