| With the development of electric power industry and smart grid, it is of great importance to supervise the condition of high-voltage transmission lines in real time, and a large amount of monitor equipment needs to be employed in the vicinity of transmission lines. However, traditional power supply is hardly available for these pieces of monitor equipment since most of transmission lines are located remotely, thus a special kind of power supply needs to be devised for them. A current transformer (CT) feeding power supply, which generates electricity by exploiting the current induced in its secondary winding from the primary transmission line, is able to provide maximal power to monitor equipment compard with other kinds of power sources in literature. Nevertheless, considering that the current of transmission line varies in an extremely wide range, a CT feeding power supply may be unable to provide adequate power in the case of small primary current, while its output voltage may increase considerablely and even get out of control as large primary current occurs. This paper aims to alleviate these problems and apply the solutions found out to the practical design of a CT feeding power suuply.Firstly, a resonant capacitor is paralleled at the secondary side of CT, which significantly increases the output power of a CT feeding power supply in the condition of small primary current. Furthermore, the maximal power a CT can generate from a specified small value of primary current is worked out as well as the secondary voltage and the paralleled capacitance corresponding to the maximal power point. These calculation results contribute to the determination of parameters in practical power supply design such that it realizes maximal power output and meets the requirement of monitor equipment when the transmission line current is equal to the specified value.Secondly, a bypass switch is paralleled with the seconday winding of CT so as to short circuit it for some time in each current period, thus the power transfer from CT to the power supply is interrupted and the output voltage rising is suppressed in the case of large primary current. With the purpose of simplifying the hardware design of CT feeding power supply, a special rectificer circuit which integrates the bypass switch into the rectifier bridge is adopted. A control scheme suitable for this rectifier is proposed, and the circuit operational principle is analysed in detail. In addition, several other topologies and control methonds of rectifiers used in CT feeding power supply are discussed in this paper.Finally, to deal with the problem that the output power of CT feeding power supply is insufficient for the monitor equipment when its primay current is lower than the above-mentioned specific value, batteries are employed as a supplement. In order to minimize the dependence on the batteries, the CT feeding power supply still needs to output maximal power in this circumstance. Buck-Boost or Flyback topology is selected as the optimal final stage DC-DC converter for the power supply since its duty cycle is almost invariable with the primary current when it outputs maximal power, thus simplifying the design of DC-DC converter. |
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