Research Article International Journal of Distributed Sensor Networks 2016, Vol. 12(8) Development of a sensor for corona Ó The Author(s) 2016 DOI: 10.1177/1550147716664243 current measurement under high- ijdsn.sagepub.com voltage direct-current transmission lines Encheng Xin and Haiwen Yuan Abstract A sensor-based distributed measurement system is designed for collecting and monitoring the corona current under the extremely high-voltage direct-current transmission lines. It is highly significant for researching the corona characteristics of extremely high-voltage direct-current transmission lines to reduce the corona effect, operating loss, and environmen- tal impact. A new shared memory strategy has been studied based on field-programmable gate array and ARM. Based on field-programmable gate array technology, the fiber-optic transmission scheme for wide-frequency corona current high- speed signal has been set up to ensure the safety and the reliability of corona current measurement system. The pro- posed system has been used in China’s state grid high-voltage direct-current test base and the in situ power transmission projects. Based on the experimental results, the proposed measurement system demonstrates that it can adapt to the complex electromagnetic environment under the transmission lines and can accomplish the accurate, flexible, and stable demands of the electric-field measurement. Keywords Sensor, corona current, high voltage direct current, data acquisition Date received: 12 March 2016; accepted: 20 June 2016 Academic Editor: Yonghe Liu Introduction because of the weak signal and wide bandwidth of the corona current, the value of corona current is difficult The electromagnetic environment (such as audible noise, to be measured accurately. The traditional corona cur- radio interference, and total electric field) caused by cor- rent measurement system bandwidth is low (usually ona discharge not only restricts the conductor selection 2 MHz or less), and measuring performance cannot and structural design of transmission line parameters meet the voltage level and gradually increased research but also often brings environmental issues. With the needs. Only a few papers have reported work on the improvement of the energy saving and environmental measurement of the corona current by far. In Greason protection requirements of the transmission line, only et al.1 and Kucerovsky et al.,2 the authors used an opti- relying on measurement and analysis of corona effect, cally decoupled data acquisition module to measure the such as audible noise, radio interference, and corona loss, has not met the engineering requirements to study the electromagnetic environment of transmission lines. School of Automation Science and Electrical Engineering, Beihang We need to continue in-depth study of the problem of University, Beijing, China corona discharge, such as the relationship between cor- Corresponding author: ona current and corona effect aspects. Encheng Xin, School of Automation Science and Electrical Engineering, The basic mechanism of the corona discharge can be Beihang University, Beijing 100191, China. obtained by measuring the corona current. However, Email: [email protected] Creative Commons CC-BY: This article is distributed under the terms of the Creative Commons Attribution 3.0 License (http://www.creativecommons.org/licenses/by/3.0/) which permits any use, reproduction and distribution of the work without further permission provided the original work is attributed as specified on the SAGE and Open Access pages (http://www.uk.sagepub.com/aboutus/ openaccess.htm). 2 International Journal of Distributed Sensor Networks corona current generated in a pin-plane gap, but its maximum sampling rate is 16 kHz and can only cover a bandwidth of 8 kHz. In Liu et al.,3 a corona loss monitoring system is proposed. It used an optical power supply electronic current transformer to acquire the corona current signals and transmitted the signals by optical fibers. This method can measure current in ultra-high-voltage (UHV) environment safely. But, the measuring bandwidth of the system is still limited to 7.5 kHz. In Wang and Zhang,4 a commercial low- Figure 1. The equivalent circuit model of corona discharge. inductance resistance is used to measure the corona cur- rent. After improving the sampling rate of the data acquisition system, the signal acquisition width can reach Theoretical overview up to 4.5 MHz. However, besides the sampling rate, the Principle of sensor measurement distributed parameters of the resistance sensor also affect the integrity of the corona current to a great extent, If there is corona phenomenon generating in transmis- which is less well considered in the present methods. sion line, it will have corona current in it. Corona cur- Moreover, relevant studies on the corona current mea- rent will cause power loss. In general, corona current is surement described in Mihailovic et al.,5 Bull et al.,6 small and unit length line corona loss is also less severe, Zhang et al.,7 and Kucerovsky et al.8 are mostly carried but the transmission line is so long that corona loss can- out under a test voltage that is less than 500 kV. The cor- not be neglected. Corona-caused power loss can be cal- ona current data available mostly have been measured in culated by the corona current, and corona-equivalent the experimental environment, and the data are generally circuit is essentially equivalent to a nonlinear resistor– not verified in high-voltage direct-current (HVDC) trans- capacitor (RC) circuit as shown in Figure 1. mission lines. With the development of the UHV trans- In Figure 1, Von is the corona onset voltage, C1 is mission technology, the anti-breakdown requirement of the geometric capacitance of the conductor configura- the measurement system becomes much more rigorous tion, C2 is the additional nonlinear capacitance due to under the UHV environment, and it is necessary to find corona, C3 is the capacitance representing the charge a new method to measure the UHV corona current. loss to the air, and G is the nonlinear conductance rep- The object of this article is to describe the develop- resenting corona loss. The measuring equipment is ment of a new sampling resistance sensor for corona placed between the power generation and the transmis- current measurement in a wide band under UHV envi- sion conductor. When the voltage of high-voltage DC ronment which the impedance could keep invariable generator is below the corona onset voltage Von, when the frequencies vary between the ranges of 0 and C2 = C3 = G = 0. Only capacitance charging current 10 MHz. To start with, a new structure, which is spe- with the wire-to-ground and a small amount of leakage cially designed according to the UHV dielectric break- current exist. When the voltage of high-voltage DC down and wide-band measuring requirements of the generator is above the corona onset voltage Von, the sensor, is introduced. Then, the synchronous dynamic nonlinear conductance G will increase rapidly, while C2 random-access memory (SDRAM) controller and the and C3 increase very slowly, and subsequently, large related first input first output (FIFO) settings accord- corona current appears with increasing voltage. The ingly have been introduced during the development corona current on the conductor is equal to the current of the wide-frequency corona current measurement sys- through the resistor minus the leakage current of insu- tem, and the large-capacity cache problems involved in lators. The quantity of leakage current is negligible the wide-frequency corona current data acquisition sys- compared with the corona current. Therefore, only cor- tem have effectively been solved based on Field- ona current is measured through the high-frequency Programmable Gate Array (FPGA) and advanced sensor and its data are recorded. RISC machines (ARM) technologies. Finally, the high- voltage discharge and frequency characteristic tests prove the UHV dielectric breakdown performance and Dielectric breakdown performance the wide-band measuring characteristics of the designed The surface electric-field intensity of the sensor work- sensor, respectively. The test results in an actual ultra- ing under the UHV environment should be lower than high-voltage direct-current (UHVDC) transmission line the dielectric breakdown intensity of air. According to demonstrate that the sensor can accurately measure the the Peek equation,9,10 the breakdown intensity of the corona current under the UHV environment. air around the conductor can be given by Xin and Yuan 3 K positive ions will be gathered on electrode surface. Eon =30md 1 + pffiffiffiffiffi ð1Þ rd When the electric field continues to strengthen, positive ions will be sucked into the electrode to form a pulse where Eon and K are the empirical constants depending current and flow to both sides of the wire. With nega- on the nature of the applied voltage; r is the radius of tive charge continues to spread into the surrounding the conductor (cm); m is the roughness factor of the space, electrode surface electric-field intensity increases, conductor, which is usually between 0.75 and 0.85; and and the next corona ionization process will happen and d is the atmospheric correction coefficient given by next pulse current. A series of pulse group will appear for the circulation. When normal polarity corona 0:386P d = ð2Þ occurs, negative ion or electron is sucked into the elec- 273 + t trode, and the same repetitive pulsed corona current where t is the atmosphere temperature (K) and P is the will be formed. atmospheric pressure (mmHg). With corona discharge,16–18 a series of impulse cur- When t is 298 K, the breakdown intensity Eon is rent will inject transmission lines. Pulse current is 30 kV/cm. So, the sensor designed surface electric-field double-exponential distribution with time.18–21 It can intensity should be lower than 30 kV/cm when the vol- be represented by the following formula tage reaches up to 800 kV on the transmission lines.