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1、Intelligent Design of Low Voltage Side Circuit in Active Electronic Current TransformerABSTRACTWith the development of power industry and the increase of voltage level, many problems of electromagnetic current transformer had emerged, thus the concept of electronic current transformer was proposed b
2、y International Electro-technical Commission (IEC) in order to solve those questions. Thereinto, the active electronic current transformer is considered as the most feasible. However, many disadvantages such as high cost, low generality and intelligence were encountered when using Normal Integrated
3、Circuit in the low-voltage side. A new designing method of low-voltage side circuit is introduced in this paper. It takes advantage of Digital Signal Processor（DSP）s powerful data process ability to realize precise analysis and data processing. Moreover, the periphery circuit of this method is much
4、simpler than above methods. Some particular functions such as frequency measurement and harmonic analysis could also be provided by using this method. The functions of local display and remote communication could be selected in accordance with requirement. The measuring result indicates that the rat
5、ion error satisfies the national standard of 0.1 class transformer. And this method is quite general for its flexibility of the numbers of monitoring parameter in the high-voltage side and the bits of analog-to-digital convert.KEYWORDS: Electronic current transformer, DSP, Frequency measurement, Har
6、monic analysisINTRODUCTIONAs an important part of protecting and metering systems, the accuracy of current transformer is strongly relative with the security and reliability of power system.Conventional electromagnetic current transformers (CT), which are now frequently used in the power system, cou
7、ld ensure the accuracy and reliability required for metering and protection. But with the problem of difficult insulation and the effect of the saturation, the conventional current transformer is more and more unfit for the development of modern power system.Then many measurement techniques have bee
8、n developed in order to solve above disadvantages of conventional current transformers. Recently, many achievements have been acquired in the optical current measurement field due to the appearance of integrates circuit and the fiber transmission. After investigation and analysis of the present curr
9、ent measurement methods, the concept of electronic current transformer (ECT) was proposed by IEC. According to the definition of IEC, the sensor of electronic current transformers can be conventional current transformer, Rogowski coil or optical device. The new electronic current transformers could
10、be divided into active ECT and inactive ECT. And the difference is illustrated as following.If the optical device is utilized in the electronic current transformer, the signals can be acquired and transmitted in the high voltage side without power supply, thus it is the inactive electronic current t
11、ransformer. And the other name is Optical Current Transformer. But there is a long way to go before this transformer can be put into practice since the accuracy of the transformer is likely to be affected by the circumstance and material. On the other hand, the active electronic current transformer
12、cant work without power supply. And the construction of the system is shown in Fig.1.Low voltage circuitHigh voltage circuitFiber TransmissionPower supplyHigh voltage side circuitLow voltage side circuitFig.1 Construction of the systemAs shown in Fig.1, there are three parts in the ECT, which are th
13、e high voltage side circuit, optical transmission part and low voltage side circuit. For the application of the optical fiber in the signal transmission between the high voltage side circuit and the low voltage side circuit, the ECT has many advantages such as simple and reliable insulation structur
14、e, perfect Electro-magnetic anti-interference ability, no Electro-magnetic saturation problem, light weight and easy interface with other digital device, etc. After many years study in the active electronic current transformer, great progress has been made in this sort of ECT. In order to adapt the
15、analog devices and instruments, the output of low voltage system of this ECT mostly is analog voltage signals, which is counteract the development of the digital current transformer. In this paper, a Motorola DSP is used in the design of low voltage side circuit of active ECT, and then the advantage
16、 of DSPs powerful data process ability and the advanced technology of program have been taken in the low voltage of ECT in order to realize precise analysis. In addition to the function of frequency calculation and the simple structure, the output of ECT is digital signals . Because of above charact
17、eristics, this ECT will be superior to many other ECTs. And the automatic level of power system could be improved by using this method.SYSTEM DESIGNThe low-voltage side circuit is designed to deal with the optical signals that are transmitted by optical fiber from high-voltage side circuit. Besides
18、the optoelectronic converters and the control circuit, the main element of the low-voltage side circuit is a digital signal processor (DSP) unit, including data collection and separation, signal processing, display and transmission. In order to process optical signals successfully, we should know th
19、e characteristics of signals derived from the high-voltage side circuit. In this ECT, two fibers are utilized to connect the high-voltage side circuit and the low-voltage side circuit , one fiber is utilized to transmit data signals, and the other one is used to carry synchronous clock signals.Four
20、different detecting parameters are converted into digital signals in this high-voltage side circuit, they are multiplied in single fiber, and then to be transmitted to the low-voltage side circuit. The first parameter in this serial signal is the temperature of the circumstance, and then is the prot
21、ecting current signal and the measuring current signal, the decuple measurement current signal, which is used in weak signal test, is the last one. There is a time interval of 64s between two group signals , and the time interval between different parameters of the same group is about 10s.HARDWARE D
22、ESIGNWe will distinguish every signal according to above characteristics. The structure of the low-voltage side circuit is shown in Fig2. As an optical signal receiver, HFBR-2412 is introduced in this system in order to convert optical signal into electrical signal in the low-voltage side circuit.Co
23、mmunication with PCDSPControl signalOpto-electric converterHigh-voltage side circuitclockfiberclockLocal DisplaydatedateLow voltage side circuitFig2. Structure of systemA control signal is triggered as a synchronization signal of DSP in this circuit.Many internal peripherals are selected in the desi
24、gn, such modules as Serial Peripheral Interface (SPI), Asynchronous Serial Communication Interface (SCI) and General Purpose Input/ Output (GPIO). The functions of above modules are listed below:1 SPISPI is multiplied in the low voltage circuit. Data collection and separation could be achieved by th
25、is interface when the DSP is configured as a server mode. This is an outstanding character in my design. And local display could also be driven by using this Interface, but at this time the DSP is configured as a master.2 SCIThe acquired original data and the calculating results of DSP should be tra
26、nsmitted to computer via the SCI. The calibration of the ECT and the monitor of the DSP could also be realized via communication between the computer and the DSP.3 GPIOThis pin, which is used as one of the address bus, can also detect the symbol of the first parameter of the compound signals , and t
27、hen it will help SPI to separate signals successfully.SOFTWARE DESIGNData collection and process could be realized in this part, and then the results should be displayed locally or transmitted remotely which can be selected at last. Of course, many special functions such as frequency measurement, fl
28、exible multi-parameters processing and harmonic analysis, can also be provided.In software design, the interrupt of the GPIO should be detected before the data is collected in the DSP. And the result of the last time will be selected in location display or long-distance transmission before data proc
29、essing has been finished. Interrupt 1 is an external interrupt, when the interrupt 1 is opened and triggered the program will enter the interrupt 1 service routine, then the results of the FFT and the collected data will be sent to the computer; interrupt 2 is a GPIO interrupt, which makes sure that
30、 the compound data can be read by the DSP in order. After display the results of FFT, a new loop will begin. In order to improving the accuracy of the system, temperature compensation algorithm will be introduced at the end.Some functions such as frequency measurement and harmonic analysis could als
31、o be provided in this part. Because of the fixed time interval between every data of D/A convert, we can get the frequency of input signal by calculating the numbers of the data in one cycle. And the harmonic analysis is possible by using FFT methods. In general, all these intelligent characters in
32、this paper such as adjustable number of parameters or the bits of AD component could be achieved due to the application of the DSP in the low-voltage side circuit.MEASUREMENT RESULTSThe accuracy of testing system has been calibrated by virtual instrument. The measurement results of system are shown
33、in Table 1. The output of the virtual instrument is the true value, while the output of the DSP is the testing value.Table1 Measurement resultsMeasuring point(%)True value(V)Test value(V)Ratioerror1(%)Ratioerror2(%)50.06070.0605-3.236-0.330200.23620.2363-0.3290.0591001.21241.21270.3430.0261201.48971
34、.48940.330-0.0181. Original ratio error2. Ratio error after line fitting.ERROR ANALYSISIn order to improve the system accuracy, the ratio error in the small signals should be decreased. Although ratio accuracy of 0.1 class can be met at the 120% of measuring range in the system, the ratio error is s
35、till increasing, which may be caused by exceeding valid converter range of A/D chip.CONCLUSIONSA new active electronic current transformer is presented in this paper. Motorola DSP, the hard core of the system could finish the requirement of the data collection, signal separation and data processing, and many intelligent functions such as frequency measurement and harmonic analysis could also be achieved by using its strong data processing ability. The testing results show that the ratio error of above method satisfies the national standard of 0.1-class transformer.6