Development of a Sensor for Corona Current Measurement Under High-Voltage Direct-Current Transmission Lines

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 environmental 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 proposed 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 current. 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. at bt Meanwhile, other environmental factors, such as i(t)=KIp(e e ), t 0 ð3Þ humidity and temperature, also affect the breakdown intensity to some extent.11–15 However, comparing with where Ip is the corona current amplitude (mA); K, a, the air density, the effects of these factors are not and b are the constant, corona current pulse rise time, significant. and fall time depends on a and b, respectively. The positive and negative corona current time-domain expres- sions are Corona discharge pulse current features 0:01t 0:0345t To capture the corona current characteristics of the i(t)=2:335Ip(e e ) ð4Þ transient signal correctly and avoid the frequency alias- i(t)=1:33I (e0:019t e0:285t) ð5Þ ing, the overall scheme of the broadband domain cor- p ona current measurement system was formulated, The frequency domain expression of pulse current is combined with the time-frequency characteristic of the corona current pulse signal and system requirements. kI (b a) jjI(jv) = pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffip ð6Þ Corona discharge process can produce high-frequency (a2 + v2)(b2 + v2) pulse current, which contains many higher harmonic that can cause interference to radio communications The time-domain and frequency-domain characteris- and other signal. tics of positive and negative corona pulse current are Corona discharge has distinction for different elec- shown in Figure 2(a) and (b), respectively. trode polarity. In the negative corona, after the air ioni- Through the corona current, time–frequency charac- zation, electron is excluded from electrodes, and the teristic can be known that positive corona pulse current

Figure 2. Corona pulse current features: (a) time-domain characteristics and (b) frequency characteristics. 4 International Journal of Distributed Sensor Networks

Figure 3. Scheme of HVDC corona current distribution measurement system. has larger amplitude than the negative and has a longer acquisition, storage, and transmission. The corona cur- duration. Frequency domain feature reflects that pulse rent distribution measuring system structure is shown current has a wide frequency range. It also illustrates in Figure 3. the necessity to research corona effect from the perspec- The measurement system is mainly composed of tive of corona current. high-voltage (HV) local area and safe location measurement area. The HV local area, which contains different space position distribution of corona current Design of the corona current sensor sensor, is used to complete data collection and storage. In the HV local area, the high precise no-inductance The data acquisition module of UHV broadband 22–25 domain corona current measurement system is different resistor is applied as the current sensor. The resis- from the general data acquisition system. The common tor is connected in series with the bundle conductors in data acquisition system only emphasizes the validity the test lines. The optically powered electronic current and accuracy of the signal collection results, and do transformer is used to convert and transmit the current not require high requirements for the complexity of the signal. In addition, the signal is acquired by the data environment and the instantaneity of the acquisition acquisition card. Independent power supply is used to signal. Due to the wide domain, corona current data provide energy for electronic components within the acquisition system is mainly used for corona current system. The acquired data are sent by the photoelectric pulse signal acquisition under UHV environment, and conversion unit and transmitted to safe location mea- it has very high requirements for the data sampling rate surement area through outdoor optical fiber. The safe and data sampling accuracy. So, the higher perfor- location measurement area is used to complete the data mance requirements are put forward. High-speed and reception work and implement communication with high-precision data acquisition is the key technology upper computer. The collected corona current data can for corona current data acquisition system. The corona be processed and saved in computer terminal. current pulse signal has the characteristics such as short rise time, high pulse amplitude, and rich frequency composition. Therefore, higher and stricter require- Design of the block diagram of the data acquisition ments have been proposed for data storage of the system corona current pulse signal. In addition, the flexible As the corona current data acquisition system is mainly high-capacity storage and the cache settings is an used for corona current pulse signal acquisition under important consideration. extremely high-voltage direct-current (EHVDC) environment, the accuracy of the data sampling is very high, so the higher performance requirements are required. Corona current measurement system framework High-speed and high-precision data acquisition for cor- In this article, digital broadband domain corona cur- ona current signal acquisition is the key technology of rent measuring system is introduced for data data acquisition system. So, data acquisition system Xin and Yuan 5

Figure 4. Block diagram of the data acquisition system. becomes the core of the corona current measurement system. It includes data acquisition module, large- capacity data storage module, and data transmission module. Figure 4 is the block diagram of the data acquisition system. Data acquisition module realizes the corona current signal conditioning, trigger and filtering, and to achieve parameter acquisition process control and the upper machine operation; data storage module uses to realize high-speed data caching and large-capacity data storage; data transmission module is mainly to realize digital transmission of corona current signal by optical fiber. Parallel alternating sampling structure is used for Figure 5. PCB of the data acquisition system. broadband domain corona current measurement system. The input analog signal is sent to multiple channels at the same time after signal conditioning and then mul- the seamless connection between the ADC and mass tiple analog-to-digital converter (ADC) chips are used storage. The collected data are stored in the SDRAM for parallel sampling. ADC converter works according memory through the data storage module and are to their sampling clock, and the sampling rate of the transmitted through the USB interface in data trans- whole system is the sum of the ADC sampling rate, so mission module. In order to solve the problem of incon- as to realize the function of high-speed data acquisition. sistency between memory chips’ reading/writing speed In the broadband domain corona current measure- and transmission interface module clock frequency in ment system, FPGA is the main controller. The ADC the process of data transmission, read data cache also transmits data collected to the FPGA internal cache. uses the asynchronous FIFO module. The proposed FPGA can set ADC data conversion module and data circuit of data acquisition system is shown in Figure 5. output sequential circuits, so as to realize the control of the ADC and data acquisition. In the process of high-speed data acquisition system Software design of the measurement system storage, both storage capacity and storage rate are two important aspects that should be considered. The appli- Software’s design needs to consider the user’s demands. cation of large-capacity memory SDRAM can greatly The software is mainly for the operator and data ana- improve the data storage capacity. High-speed data lyst. They have different demands for the software. The caching module consists of asynchronous FIFO. High- operators are required for setting of parameters and speed sampling data from the ADC are cached through real-time data collection, as well as real-time surveil- high-speed data caching module. It can realize the strid- lance for the equipment running conditions. For data ing clock domain data processing. The use of FIFO analysts, relevant analyses and processing will be done between high-speed ADC and SDRAM can improve to the data stored in the database after finishing the 6 International Journal of Distributed Sensor Networks

electric field under transmission lines; (5) data storage box, showing the data storage paths; (6) system status display, monitoring the battery status; and (7) menu and tool bars, containing all operations for the users.

Design of the overall structure High-voltage prevention design is required to protect Figure 6. Structure chart of the measurement system the high-frequency resistance sensors. If the resistance software. sensor breaks down, collecting device will be directly accessed into the test line to be damaged. Therefore, four 400 O non-inductive resistors are used in the front measurement. Therefore, software structure is built of the equipment with parallel manner for the high- based on the demand analysis of users, as shown in frequency signal acquisition. When anyone of them Figure 6. breaks down, there will be other parallel resistances in Measuring software consists of control layer and the circuit to ensure the safety of the collecting device. data layer. Control layer, locating at the upper layer, The corona current sensor is working in a complex which is composites of interaction surface of human electromagnetic environment. The performance of the and machine, is the core layer of the two, supplying sensor will be affected by corona discharge. Therefore, operations to the users so as to realize the real-time electromagnetic shielding must be designed to restrain control to the corona current. Data layer, locating in the interference of electromagnetic field. When circuit the lower layer, which is oriented for data analysis per- is wrapped with metal shell which separated it from sonnel, realizes the real-time storage of data. electric field, there is no electric field in the shell no Based on the proposed structure, the measurement matter whether the shell is grounding. The material of software used in this system is developed as shown in the shielding box for the measurement system is alumi- Figure 7. Main interface of software consisted of seven num with 10 mm thickness. The interference will be parts: (1) power control area, used to control on and reduced and the reliability will be improved for corona off of the equipment; (2) acquisition control area, con- current measurement equipment. trolling the data acquisition work way; (3) parameter The prototype of the measuring device is shown in setting area, setting up the data acquisition parameters; Figure 8. Design and application of grading rings are (4) curve display, showing real-time distribution of used to avoid corona discharge at both ends of the

Figure 7. Software interface of the measurement system. Xin and Yuan 7

Figure 8. Measurement equipment: (a) prototype equipment and (b) general structure of the equipment. (1) Grading ring, (2) shielding case, (3) epoxy resin insulating flange, (4) PE pipe, (5) compartments, (6) sampling resistor, (7) battery, (8) optical fiber waterproof connector, (9) UHV local measurement device, and (10) shielding ground ring.

than 30 MHz. So, the sensor is able to measure the corona current whose bandwidth is within 30 MHz. The sensor impedance changes rapidly when the test frequency is above 30 MHz as a result of lumped capacitance and inductance. Fortunately, the corona current signals whose frequencies are over 30 MHz occupy only a small portion, which can be filtered beforehand in the signal processing stage.

High-voltage discharge test The corona current measurement system has been used Figure 9. The sensor impedance test result. in the China’s Electric Power Research Institute EHVDC test base. Corona current measurement equip- ments’ installation and distribution are shown in sensor. The determination of grading ring parameters Figure 10. In the corona discharge test, 800 kV voltage is given in reference. It can make the sensor surface was applied to high-voltage generator; corona dis- electric-field intensity below 20 kV/cm under 800 kV charge around the resistance sensor was observed using extremely high-voltage (EHV) environment and meet Ofil’s SuperB corona camera. Figure 11 shows the the needs of practical engineering. result of corona discharge of the resistance sensor under 800 kV EHVDC environments. It shows that the Sensor performance evaluation photon count is 0. Therefore, the sensor surface voltage gradient is not more than Eon, and the sensor meets the Impedance characteristics test engineering design requirements. Corona current measuring equipment collects corona current signal on the conductor through sampling resis- Experimental results tance sensor. The choice of sampling resistance depends on the change range of the conductor current in the test Test platform process. In order to ensure accurate measurement of To validate and test the proposed corona current mea- current signal and retain a certain measurement margin, surement system, the same-tower double-circuit HVDC a large number of experiments were tested. In the end, experimental lines were erected by the China Electric four 400 O high-power precision non-inductive resis- Power Research Institute at Beijing. The transmission tances were chosen and collected the corona current sig- line, which is shown in Figure 12, is 100 m long, and nal in the parallel form. the model of the transmission line is LGJ-95/20 with It is evident from Figure 9 that the impedance of the four bundles of 40 cm spacing. The minimum distance sensor almost keeps constant when the frequency is less from the ground to the conductor is 7 m and the 8 International Journal of Distributed Sensor Networks

Figure 10. Measuring equipment distribution: (a) corona cage and (b) testing tower.

Figure 11. Sensor high-voltage test results: (a) positive pole and (b) negative pole.

Table 1. Meteorological parameters.

Locations Altitude (m) Air pressure (kPa) Temperature (°C) Relative humidity (%)

Beijing 27 101.3 21.3 26

separation distance between the positive pole and the negative pole is 6 m. The test meteorological parameters are shown in Table 1.

Corona current measurement at test HVDC transmission line The test voltage applied to the positive pole was 0, 100, 125, 150, 175, 200, 225, 250, 275, and 300 kV. Figure 12. Actual test lines. Correspondingly, the voltage applied to the negative pole was 0, 2100, 2125, 2150, 2175, 2200, 2225, Xin and Yuan 9

same frequency band, we develop a novel multifaceted filtering approach for HVDC corona currents. Figures 14 and 15 show background signal under 0 kV and corona current signal under positive 300 kV on HVDC test lines in Beijing, respectively. By comparison of Figures 14 and 15, it can be found that relatively high narrowband signals of 3.325, 7.538, 7.987, 9.005, 9.337, 9.675, 10.351, 11.253, and 12.837 MHz exist both under 0 and + 300 kV. After long-term monitoring, these signals which have been confirmed are radio signals and broadcast signals from space; so, they can be removed using filter. However, collected corona current signals in addition to containing the narrowband interference signals still contain a lot of interference signals such as pulse interference and random noise; so, further data processing is needed to extract the effective corona current signals from the background noise. The corona Figure 13. The change of V-I characteristics of corona current. current signal after data processing is shown in Figure 16. A lot of noise and narrowband signal in original signal have been effectively filtered, and noise reduction 2250, 2275, and 2300 kV. After using bipolar voltage effect is obvious. It can promote further research for mode, the change of V-I characteristics of corona cur- HVDC corona characteristics. rent is shown in Figure 13. Corona current increases gradually and exponentially with the rise of the line Conclusion voltage grade; the positive corona current strength is slightly higher than the negative under the same voltage In this article, a broadband corona current measure- grade. ment system to measure the distributed corona current The complexity and diversity of noise in corona cur- under EHVDC transmission lines for fulfilling the rent measurements of HVDC transmission lines leads demands of extensibility, flexibility, and stability has to challenges in estimating corona performance. Using been presented. The proposed system comprises HV an offline noise suppression method for HVDC corona local area and safe location measurement area. Data current measurements is that given that the corona cur- acquisition system in HV local area is the core of the rent and background noise processes coexist within the corona current measurement system based on FPGA

Figure 14. Background signal: (a) time domain and (b) frequency domain. 10 International Journal of Distributed Sensor Networks

Figure 15. Corona current signal under positive 300 kV: (a) time domain and (b) frequency domain.

Figure 16. Corona current signal under positive 300 kV after data processing: (a) time domain and (b) frequency domain. and ARM. Based on FPGA technology, the fiber-optic demonstrate that the proposed measurement system transmission scheme for broadband corona current sig- can adapt to the complex electromagnetic environment nal has been set up to ensure the safety and the reliabil- and accurately collect the corona current data. ity of corona current measurement system. Sensor’s performance evaluation shows the sensor has flexible Declaration of conflicting interests and robust performance in complex environments under EHVDC transmission lines. The experimental The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this results under in situ HVDC transmission lines article. Xin and Yuan 11

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