Corona Current Data Acquisition Card Based on USB Bus in Extra High Voltage Environment

Transcription

1 2016 Sixth International Conference on Instrumentation & Measurement, Computer, Communication and Control Corona Current Data Acquisition Card Based on USB Bus in Extra High Voltage Environment Li Qi, Liu ingyi, Lv Jianxun School of Automation Science and Electrical Engineering Beihang University Beijing, China Abstract This paper introduces a corona current data acquisition card based on USB. According to the characteristics of the corona current, the hardware circuit of USB acquisition card includes signal conditioning circuit, data acquisition circuit and peripheral circuit, and the program needed for the device including PC driver and application program were designed. In order to verify the function of the corona current measurement system, the performance test and the field test of the USB acquisition card are carried out. The test prove the data integrity of the data acquisition card and the accuracy of the data acquisition. It shows that the acquisition card is steadily, accurate, and can meet the requirements of the index. Keywords corona current; data acquisition; USB; hardware circuit. I. ITRODUCTIO At present, there are many kinds of data acquisition devices, but these devices are not suitable for corona current data acquisition under UHV environment. Some self-developed data acquisition devices can be used only in the laboratory and other ideal environment. In actual transmission lines, these equipment were prone to breakdown or be interfered, which can t be reliably when carried out on actual application of the project [1-2]. On the basis of the existing research results at home and abroad, with the aid of anti-interference technology and USB transmission bus, the paper developed a low frequency band of the corona current data acquisition card. The design can measure the low-voltage side corona current signal of the UHV DC corona cage accurately and reliably, and realize the functions that fast real-time display of the waveform and the large capacity automatic storage of data. II. THE OVERALL DESIG OF THE DATA ACQUISITIO CARD The principle diagram of the corona current data acquisition card based on USB bus is shown in Fig 1. The whole system is composed of signal conditioning circuit, A/D acquisition circuit and USB interface. The corona current analog signal is processed by the signal conditioning circuit, which is converted into an electrical signal and be received by AD. The signal passes through the USB interface circuit and received by PC. Corona current Liu uanqing China Electric Power Research Institute Beijing, China yqliu02@163.com Resistance sensor PC Signal conditioning circuit Limiting protection circuit 8051 core USB transceiver C7C68013A SIE Preamplifier circuit A/D converter Anti-aliasing filter circuit GPIF USB interface Fig.1 Principle diagram of the corona current data acquisition card III. THE DESIG OF HARDWARE Amitter follower circuit A. The design of signal conditioning circuit The range of corona current is about 0~20mA [3-4], and the sensor resistance is 50 ohm, so the amplitude of the analog voltage signal is changed from 0 to 1 volt. In order to complete the conversion from analog signal to digital signal, the signal must be processed firstly [5]. (1) Corona current signal is doped with strong electromagnetic interference, which makes the signal amplitude exceeds the tolerance limit of the ADC conversion chip, so it is necessary to limit the input signal amplitude. (2) Corona current is relatively small, which needs appropriate to be amplified. Otherwise the signal amplitude is too small to recover, and even be overwhelmed by noise. (3) The signal may contain high frequency noise components. In order to avoid the phenomenon of spectrum aliasing and meet the law of yquist sampling, low-pass filter should be used in the signal acquisition. (4) In order to improve the input impedance and achieve signal isolation, the voltage follower is used as the input buffer of AD. To achieve those requirements, the components of the signal conditioning circuit include: limiting protection circuit, preamplifier circuit, anti-aliasing filter circuit, emitter follower circuit. Limiting protection circuit is mainly composed of surge protection TVS diode. When there is an instantaneous high /16 $ IEEE DOI /IMCCC

2 voltage in the circuit, the high voltage pulse can be restricted to a safe range, which can effectively avoid the high sharp pulse damage to the instrument and equipment due to the high voltage pulse. There are many schemes for the design of preamplifier circuit in engineering practice. In this paper, BURR.BROW Company's PGA202 Programmable instrumentation amplifier was chosen to design the preamplifier circuit. This design has the advantages of low drift, nonlinear, high common mode rejection ratio and wide frequency band [6]. According to the yquist sampling theorem, sampling frequency must be 2 times higher than the highest frequency of the signal, in the practical engineering usually take 3 to 5 times. Therefore, the anti-aliasing filtering unit is added to the channel in order to filter out the high frequency component which does not conform to the sampling theorem. There are two kinds of the anti-aliasing low pass filter: one is using passive components R, l and C to form passive filter; the other one is using integrated operational amplifier, and R, C to form active power filter. The former can cause the energy loss of signal, besides, the load effect of it is obvious. In addition, the use of inductive components is easy to cause electromagnetic induction. Therefore in this paper we used the MAXIM monolithic integrated filter chip MAX27 to design the two order active low pass filter [7]. The specific circuit as shown in Fig 2. Fig.3 Specific circuit B. The Design of data acquisition circuit The acquisition circuit mainly realizes two functions. One is transformed the corona current signals into numerical signals, and the other one is high speed transmission. Because the low frequency domain corona current signal frequency is below 100kHz. This design uses the 500kHZ acquisition rate, 12 bit resolution A/D converter AD7864 to realize the analog digital conversion. The C7C68013A-100 chip with GPIF function was chosen to realize high-speed data transmission. This chip integrates the USB transceiver, SIE (serial interface engine), CPU (enhanced 8051 microcontroller) and a GPIF (universal programmable interface) [9]. The schematic diagram of the interface circuit of AD7864 and C7C68013A is shown in Fig 4. DB0~DB15 are the data bus. C7C68013A uses CTL0, CTL1 and CTL2 to control AD7864 s COVERT, RD and WR pins, and through the RD0 to collect signal from AD7864 s EOC pin. Fig.2 Integrated active filter circuit Because the corona current signal is faint. In order to improve the driving ability of the circuit, the input impedance of the circuit input should be increased, and also reduce the noise. This design used high speed operational amplifier AD817 as the operational amplifier, which the gain bandwidth is up to 50MHz, and has a high driving capability, minimum output current of 50Ma [8]. The specific circuit as shown in Fig 3. Fig.4 Schematic diagram of the interface circuit C. Low-power design In addition to the traditional software and hardware design to reduce the power consumption of the acquisition card, this design also uses the time-sharing, partitioning power supply mode. According to the situation of each module of the acquisition card to take intermittent power supply, when a module is in a dormant state, the system will cut off the power 280

3 supply. When the module is to turn into the working state, the power will be supplied. Data acquisition card circuit board is shown in Fig 5. Power-on Reset Initialized State Variable TD_Init() Whether Receive The SETUP Package Delay 1S, Re-enum Fig.5 Data acquisition card circuit board IV. THE DESIG OF SOFTWARE The software design mainly includes two parts: one is the USB device firmware, which mainly to complete USB protocol processing, data exchange and other logic control function. The other one is PC applications. Experimenter can directly control the acquisition device through the computer, and view the collected data. PC application is a program that directly faces the user and is operated by the user. This design selects the VC6.0 development environment to develop the application program. This program can realizes the transmission of control command, the collection data reading, the date display and storage. The application software interface as shown in Fig 6. In this paper, we use the development framework provided by CPRESS company to design the firmware program of C7C68013A [10]. It mainly realizes the functions of initialization, data transmission, acquisition and control, programmable amplification, energy management and so on. The program flow is shown in Fig 7. Fig.7 Program flow TD_Poll() Whether Receive The SETUP Package Whether Set USE Bus Idle Flag V. PERFORMACE TESTIG Answer Device Request Whether Call The Function of TD_Suspend() Suspend Processor TD_Resume() A. The conversion rate of AD According to the working principle of the acquisition card we can know that the COVERT pin of the AD7864 chip is in charge of analog digital conversion which means this pin determines the rate of A/D conversion. That is to say the A/D conversion rate is as the same as the frequency of the AD7864 s COVERT pin. So the A/D conversion rate can be got by the measure of the COVERT pin frequency. Under laboratory conditions, the frequency of the COVERT pin is measured with an UT2102C oscilloscope. As shown in Fig 8, the A/D conversion rate is kS/s. Fig.6 Application software interface 281

4 the acquisition card is only 2.7%, which can meet the requirements of the measurement. TABLE.1 Acquisition results of different waveforms Measured signal(sinusoidal) Peak value Relative error 10V 10KHz 9.73V 2.7% 10V 1KHz 9.78V 2.2% 5V 10KHz 4.92V 1.6% 5V 1KHz 4.94V 1.2% E. Acquisition of corona current in real environment The corona current data acquisition card used in corona cage test line in Beijing City, China EPRI UHV DC test base. Fig 11 is the testing site. We used Oscilloscope and this data acquisition card to get the corona current, and compared this two result, which showed in Fig12 and Fig 13. Fig.8 Oscilloscope screenshot B. Acquisition rate Under laboratory conditions, this acquisition card was used to acquire data from 1KHz signal generated by the signal generator. Fig 9 is a waveform acquisition. In the picture, there are 512 points in the picture, which form 3.5 periodic waveforms. So there are about 142 points per cycle, which means the acquisition frequency is about 142kHz. Fig.11 corona cage test site Fig.9 Waveform acquisition screenshot C. Zero input error analysis Two input pins of the acquisition card were connected, and used an oscilloscope to obtain the acquisition result. The biggest error was selected from the results of many time measurements, which shown in Fig 10. Because the conversion range of AD is from -5V to +5V, and the maximum value in graph is 29.2mV, so zero input error is 0.29%. Fig.12 Oscilloscope data acquisition (V) kV (ms) Fig.13 data acquisition card data acquisition Fig.10 The biggest error screenshot D. Acquisition waveform comparison analysis Using this data acquisition card to collect data from different waveform which produced the DF1440 signal generator. The results are show in TABLE 1. According to the data in the table, the maximum relative measurement error of As can be seen from the figures, the oscilloscope time base by 2ms, the corona current measurement system using 500kSPS sampling rate, and the output signal waveform of the oscilloscope is consistent with the output of the corona current measurement system, which means this device can play a role in the measurement of the corona current. 282

5 VI. COCLUSIOS In this paper, a data acquisition card based on USB is designed. The set of acquisition card has the corresponding hardware circuit and software program. Through the transmission rate test and waveform comparison analysis of the corona current data acquisition card, it shows that the data acquisition capability of the data acquisition card is consistent with the theoretical analysis and has a high accuracy. Under the actual condition, the acquisition card can meet the requirement of reliability and stability, and can realize the data collection of corona current in corona cage. ACKOWLEDGMET The research was supported by Beijing atural Science Foundation ( and ), Science and technology project of China State Grid Corp (GB ) and ational atural Science Foundation of China ( ). REFERECE [1] OU Shaohua, LIU unpeng, WA Qifa. Corona Losses Measurement System of Bundle Conductor Using UHV Corona Cage [J]. High voltage engineering, (1): 36 [2] ZHAG Wenliang, U ongqing, LI Guangfan. Researches on UHVDC Technology [J]. Journal of Chinese Electrical Engineering Science, 2007, 27(22). [3] GC Sibilant, M Ijumba, A C Britten. Studies of DC conductor corona in a smallcorona cage. 2002, Africon Conference in Africa, IEEE AFRICO. 6th,2 2-4 : 661~664 [4] Sibilant G C, Ijumba M, Britten A C. Studies of DC conductor corona in a small corona cage. Proceedings of Power System Technology International Conference, 2002, :2202 ~2207 [5] CHE Shufang. Design of analog signal conditioning circuit for high precision data acquisition system [J]. Journal of Changsha Railway University, 2008, 9(1): [6] Burr-Brown Corporation. Digitally Controlled Programmable Gain ISTRUMETATIO AMPLIFIER [7] Maxim Integrated Products. 4th- and 8th- Order Continuous Time Active Filters [8] Analog Devices, Inc. High Speed, Low Power, Wide Supply Range Amplifier, AD [9] CPRESS. C7C68013 EZ-USB FX2 High-Speed USB Interface Device [10] Cypress Semiconductor Corporation. Cypress CyUsb.sys Programmer's Reference