文獻(xiàn)翻譯 組合數(shù)字電子式電流和電壓傳感器.doc

黃河科技學(xué)院畢業(yè)設(shè)計(jì)(文獻(xiàn)翻譯) 第21頁(yè)組合數(shù)字電子式電流和電壓傳感器 作者：段雄英、鄒積巖、廖敏夫、張可衛(wèi)摘 要在電力系統(tǒng)中已開發(fā)出高性能的電流和電壓測(cè)量系統(tǒng)。該系統(tǒng)由兩部分組成：電流測(cè)量元件和電壓測(cè)量元件。洛高夫斯基線圈和電容分壓器分別用于為線電流和電壓測(cè)量。有源電子元件是電流互感器進(jìn)行在線檢測(cè)時(shí)為這些組件信號(hào)調(diào)制并提供電源的。測(cè)量信號(hào)通過(guò)光纖傳輸，光纖可以抗電磁干擾和噪聲干擾。通過(guò)精細(xì)的設(shè)計(jì)和對(duì)數(shù)字信號(hào)處理技術(shù)的應(yīng)用，整個(gè)系統(tǒng)的精度可以達(dá)到0.5%，配以高精度的供繼電保護(hù)設(shè)備可提供較大范圍的動(dòng)態(tài)檢測(cè)。關(guān)鍵詞：電子式電流傳感器（ECT），電子電壓互感器（EVT），洛高夫斯基(Rogowski)線圈，電容分壓器（CVD），光纖1 簡(jiǎn) 介電流和電壓測(cè)量在測(cè)光、保護(hù)和電力系統(tǒng)控制中起著重要作用。隨著電力系統(tǒng)的發(fā)展，傳統(tǒng)的電流互感器（CT）和電壓互感器（PT）有著不能容忍的缺點(diǎn)：絕緣結(jié)構(gòu)成本高且模式復(fù)雜，CT的飽和效應(yīng)和PT的鐵磁諧振效應(yīng)等影響。在過(guò)去的二十年中，大多興趣投入在光學(xué)電流互感器（OCT）和光學(xué)電壓互感器（OPT）的研究中。最常見的類型有利用法拉第效應(yīng)的OCT和利用普克爾斯效應(yīng)的OPT，它們都是使用光學(xué)晶體的傳感器。盡管一些成功的現(xiàn)場(chǎng)試驗(yàn)已經(jīng)完成，但在商業(yè)產(chǎn)品中OCT和OPT還不能實(shí)現(xiàn)量產(chǎn)，因?yàn)镺CT和OPT很容易受到環(huán)境溫度和機(jī)械擾動(dòng)13。近年來(lái)，麗思（Ritz）公司開發(fā)的以結(jié)合電子式電流和電壓傳感器利用洛高夫斯基（Rogowski）線圈和電容分壓器分別作為傳感器線電流和電壓測(cè)量4?；谶@一概念，一種新型電流和電壓測(cè)量數(shù)字系統(tǒng)被開發(fā)出來(lái)了。2 系統(tǒng)結(jié)構(gòu)數(shù)字電流互感器的概念已經(jīng)提出了近半個(gè)世紀(jì)，但因電子元件及常規(guī)繼電保護(hù)和測(cè)光模式的限制使其沒有得到很好的發(fā)展?？墒乾F(xiàn)在基于數(shù)字技術(shù)及信息網(wǎng)絡(luò)的繼電保護(hù)和獨(dú)占地測(cè)光系統(tǒng)操作正在被廣泛應(yīng)用于配電系統(tǒng)。各種具有特殊功能的集成芯片投入市場(chǎng)。所有這些都可能使電子CT和PT產(chǎn)品基于數(shù)字調(diào)制。合并后的電流和電壓測(cè)量系統(tǒng)框圖如圖1所示。洛高夫斯基(Rogowski)線圈是用于使電流測(cè)量和電力CT符合電力線的。洛高夫斯基(Rogowski)線圈納入有源電子元件組成了整個(gè)電流變送器。這種發(fā)射器生成信息描述性測(cè)量電流和光信號(hào)編碼。整個(gè)電流發(fā)射器和電源部分都被放到一個(gè)抗電磁干擾（EMI）的鐵盒中。空心陶瓷絕緣體用于維持系統(tǒng)的高電壓的一部分。無(wú)論是光纖承載的電流信號(hào)還是電容分壓器都為通過(guò)絕緣體的電壓傳感器使用。電容分壓器的電壓信號(hào)也在絕緣體的底部調(diào)制成光信號(hào)。接著電流信號(hào)和電壓信號(hào)通過(guò)光纖傳輸?shù)綔y(cè)光和繼電保護(hù)遠(yuǎn)程控制室內(nèi)。圖1 組合電子式電流和電壓測(cè)量系統(tǒng)3 電流和電壓傳感器在此，提出了一種洛高夫斯基（Rogowski）線圈式電流傳感器。Rogowski線圈，這是一個(gè)電感無(wú)磁環(huán)形口，已用于測(cè)量電流一段時(shí)間5。圖2所示為一個(gè)矩形截面Rogowski線圈。Rogowski線圈是基于載流導(dǎo)體的放置，線圈產(chǎn)生一個(gè)電壓E成正比，線圈交互M和電流變化的的速度為： (1)由式(1)得： (2)要獲取電流測(cè)量值，線圈的輸出電壓必須是完整的。這可以通過(guò)兩種常見的方法：（1）電子積分器的使用;（2）在使用數(shù)值積分軟件之后線圈的輸出電壓是數(shù)字化的。本文圖3所示為積分器。該積分器的輸出為: (3)其中，Vi是Rogowski線圈的輸出從目前可以得到以下等式： (4)因?yàn)闆]有鐵芯飽和，Rogowski線圈有廣泛的測(cè)量范圍。同樣的線圈可用于測(cè)量從幾安培至數(shù)百千安培的電流。此外，他們具有較高的測(cè)量精度，也就是說(shuō)可以精確至0.1%6，而且它對(duì)溫度不敏感。電容分壓器（CVD）技術(shù)已被廣泛應(yīng)用于高壓電力系統(tǒng)。影響CVD測(cè)量精度的主要因素是雜散電容和電容的溫度系數(shù)。精密電容分壓器技術(shù)用于提高測(cè)量精度。有效減小雜散電容的方法是全速淡化分頻器高度。本文中，主要是以充滿絕緣油的絕緣體來(lái)盡量減少它的高度。通過(guò)這種方法，雜散電容分壓器可以保持在較低水平，且位于高壓臂中的具有相同的溫度特性的串聯(lián)電容器可以被大大改善。分壓器終端置頂一個(gè)屏蔽電極以補(bǔ)償雜散電容的影響。將電子電壓互感器（EVT）中CVD放在室外，強(qiáng)烈的溫度變化可能影響分壓器的誤差。正如微處理器基礎(chǔ)系統(tǒng)中的傳感器一樣，在EVT中的CVD是不同于常規(guī)CVD的。因其負(fù)荷小的特點(diǎn)，故如圖4所示,特殊結(jié)構(gòu)的CVD可以消除溫度的影響。分壓器是由具有相同的電介質(zhì)、規(guī)模、價(jià)值和溫度系數(shù)的m+n個(gè)電容器Ci組成的。m個(gè)電容器串聯(lián)在系統(tǒng)中的值C1得滿足高壓臂中的要求：，低壓臂中的電容值C2 是由并聯(lián)電路得出的C2=nCi, 那么分壓比K為: (5)電容值C1由于溫度變化變?yōu)镃i +C1，這時(shí)分壓比K是: K=C1/(C1+C2) =(1/m)(Ci+C1)/(1/m)(Ci+C1)+n(Ci+C1) =1/(mn+1) (6)由式5和式6可知K= K，通過(guò)這種方式，所有具有相同參數(shù)的電容Ci都可以消除溫度對(duì)分頻比率的影響。4 信號(hào)處理如圖5所示為一個(gè)信號(hào)處理框圖?，F(xiàn)在市場(chǎng)上發(fā)售有許多特殊的A / D芯片在。有多種方式可以從一個(gè)遠(yuǎn)程發(fā)射點(diǎn)傳達(dá)測(cè)量信息。電壓頻率調(diào)制是一個(gè)純粹的數(shù)字技術(shù)，它可以提供無(wú)差錯(cuò)和無(wú)干擾的傳輸，因此，最適合在電力系統(tǒng)中使用。這項(xiàng)技術(shù)的基本概念是把測(cè)量信號(hào)轉(zhuǎn)換成PFM（脈沖頻率調(diào)制）信號(hào)，其測(cè)量信號(hào)的頻率偏差與電流或電壓的大小成線性正比。圖5 信號(hào)處理系統(tǒng)式7闡述了這種關(guān)系： (7)其中F0 是輸入直流電壓偏移設(shè)置的基本頻率，系數(shù)k是取決于實(shí)際電路。LED（發(fā)光二極管低）直接由頻率脈沖驅(qū)動(dòng)，然后信號(hào)調(diào)制成光脈沖。與LED匹配的光纖傳輸窗口具有850nm的中心波長(zhǎng)。200m多模光纖用來(lái)傳輸光信號(hào)?？刂剖医邮掌魇且粋€(gè)光電二極管，即用于重建電頻率脈沖的驅(qū)動(dòng)放大器，之后把頻率信號(hào)發(fā)送到計(jì)算機(jī)用于解調(diào)。由時(shí)間到其實(shí)是一個(gè)完整的頻率信號(hào)計(jì)數(shù)過(guò)程，并且計(jì)數(shù)值Y j i 為：Yji 是由計(jì)數(shù)器獲取的，所以： （8）式8是由到的輸入脈沖信號(hào)的積分值, 它等于由信號(hào)曲線包圍的面積值。如果輸入信號(hào)的微小變化保持在與 之間, 或者采樣時(shí)間=足夠短，其點(diǎn)的值可以是一個(gè)近似值。 （9）由式8和9，我們可以得到： （10）整合延遲效果所造成的錯(cuò)誤，可以通過(guò)軟件進(jìn)行修改。由于該系統(tǒng)采用光纖傳輸數(shù)字信號(hào)，所以它是抗電磁干擾。5 線電位編碼器電源使用光纖實(shí)現(xiàn)高低電壓間的絕緣，但在高電壓上仍有些電源問(wèn)題。有三種方法可以設(shè)計(jì)一個(gè)電流驅(qū)動(dòng)的電力供應(yīng)：電池，電流互感器，和光功率。常見的是電池的工作壽命為510年，平均修復(fù)時(shí)間為3年，動(dòng)力系統(tǒng)中是不行的。更重要的是，大尺寸的電池和其復(fù)雜的浮點(diǎn)充電電路是不可取的。一些先進(jìn)的電池成本高，如鋰或光電池成為應(yīng)用的障礙。在此提出了一種特殊設(shè)計(jì)的輔助CT是用來(lái)直接從生產(chǎn)線上獲取能量的。這種高壓CT的絕緣要求要比那些常規(guī)CT更簡(jiǎn)單，并且輸出功率低，所以它比常規(guī)CT要小的多。在電源CT設(shè)計(jì)中應(yīng)該考慮兩個(gè)關(guān)鍵問(wèn)題。首先，電力CT應(yīng)為線電流盡提供可能低的所需的電壓和電流，也就是，在該系統(tǒng)運(yùn)行中實(shí)測(cè)線電流有一個(gè)最低值。其次，在故障電流中，電源CT能吸收多余的能量成為供應(yīng)電子元器件的穩(wěn)定力量，且其本身不會(huì)被電場(chǎng)力所破壞。該電源系統(tǒng)的示意圖如圖6所示。從圖6，我們可以得到的： （11）其中，Z0-控制阻抗，Z1-負(fù)載阻抗，I-線電流， -負(fù)載電壓。為了使負(fù)載電壓是常量，那么Z0必須隨線電流I的變化而變化，也就是說(shuō)，如果我們能夠設(shè)計(jì)出隨線電流I變化的控制阻抗電路，我們可以得到一個(gè)穩(wěn)定的高能量輸出功率的電子元件。圖7說(shuō)明了電源的設(shè)計(jì)運(yùn)行原則。能量由電源CT獲得。起初，電源穩(wěn)壓電路是唯一施加在電源變壓器的次級(jí)繞組的負(fù)載。當(dāng)所有的穩(wěn)壓電源電壓是指定的值時(shí)，當(dāng)前分支電路是停止電力供應(yīng)的進(jìn)一步補(bǔ)充。通過(guò)這種設(shè)計(jì)方法，該系統(tǒng)可以正常工作在5到20的額定電流（400A）。6 系統(tǒng)性能用綜合電子式電流和電壓傳感器的單導(dǎo)線施工原型為110KV電力系統(tǒng)進(jìn)行測(cè)試，初步測(cè)試結(jié)果概述如下：電子式電流傳感器在室溫下的比率的誤差特性如圖8所示?？v軸表示比率誤差，而橫軸表示初級(jí)電流流向?qū)w。這一數(shù)字表明RMS（均方根方）的光接口輸出波形的值和初級(jí)電流作為衡量一個(gè)0.2級(jí)線圈型CT的RMS的波形的值的關(guān)系。Rogowski線圈的輸出設(shè)計(jì)為400mV相對(duì)應(yīng)的初級(jí)電流400 A 的變動(dòng)率，和積分器的參數(shù)為：R= 10k的中，C =0.1F的。由數(shù)字可以看出ECT的比率誤差范圍為1.0。如上所述，通過(guò)軟件解調(diào)信號(hào)，可以很方便地利用數(shù)字信號(hào)處理技術(shù)進(jìn)行錯(cuò)誤修改，并有望獲得更高的測(cè)量精度為0.5。測(cè)量電流的相位誤差要好于20分鐘時(shí)電流過(guò)載50Hz的20。圖9顯示電壓傳感器的測(cè)試結(jié)果。無(wú)論是高壓電容器系列和低電壓電容器聚丙烯電容器。該測(cè)試是在室溫下執(zhí)行的。這一數(shù)字表明在控制室的光纖接口輸出波形的RMS值和0.2級(jí)標(biāo)準(zhǔn)常規(guī)PT的高電壓波形的RMS值的關(guān)系。縱坐標(biāo)軸表示低電壓，而橫坐標(biāo)軸表示高電壓。該電子電壓傳感器的線性測(cè)量滿足該測(cè)量系統(tǒng)精度的0.5，當(dāng)電壓超過(guò)20的滿量程時(shí)，電壓的相位誤差要好于30分鐘的時(shí)候。7 結(jié)束語(yǔ)一個(gè)組合數(shù)字電子電流和電壓測(cè)量系統(tǒng)相比與常規(guī)的CTs和PTs具有許多優(yōu)點(diǎn)。這些可以概括如下：(1) 高測(cè)量精度Rogowski線圈和精密電容分壓器可以通過(guò)軟件很方便的修改錯(cuò)誤。且該系統(tǒng)預(yù)計(jì)將擁有0.5%的精確度。(2) 具有較寬的測(cè)量范圍和頻率范圍。因?yàn)闆]有鐵飽和 ，Rogowski線圈不直接測(cè)量電流，而不像CTs那樣，當(dāng)一個(gè)大的DC元件存在時(shí)，可以精確地測(cè)量電流。(3) 電流和電壓測(cè)量組合結(jié)構(gòu)體積小，重量輕，絕緣結(jié)構(gòu)簡(jiǎn)單。(4) 光纖高電壓具有電鍍絕緣帶。這構(gòu)成了可靠的低成本絕緣非接觸式測(cè)量系統(tǒng)和強(qiáng)大的抗電磁干擾性。(5) 系統(tǒng)的溫度特性決定于該電子元件溫度敏感性，先進(jìn)的產(chǎn)品都小于 5010-6/。(6) 目前已開發(fā)增加相容性的新的電子繼電器和測(cè)光設(shè)備。未來(lái)工作重心著手將組合數(shù)字電流和電壓測(cè)量系統(tǒng)投入實(shí)際應(yīng)用中。參考文獻(xiàn)1The Emerging Technologies Working Group. Optical current transducers for power systems: A reviewJ.IEEE Transactions on Power Delivery,1994,9(4):1778-1788.2Maffetone T D, McClelland T M.345kV Substation optical current measurement system for revenue metering and protective relayingJ.IEEE Transactions on Power Delivery,1991,6(4):1430-1436.3Christensen Lars H. Design construction and test of a passive optical prototype high voltage instrument transformer J.IEEE Trans. on PD,1995,10(3):1332-1337.4Instruction Book of Sensors M. RITZ Messwandler Hamburg, Germany, Mar.1999.5Ramboz John D. Machinable Rogowski coils, design and calibrationJ.IEEE Transactions on instrumentation and Measurement,1996,45(2):511-515.6Ljubomir Kojovic. Rogowski coils suit relay protection and measurementJ.IEEE Computer Application in Power,July1997:47-52. 來(lái)源于：上海大學(xué)學(xué)報(bào)（英文版），2002,6（1）：7984 （黃河科技學(xué)院主頁(yè) 圖書館 維普期刊 搜索關(guān)鍵字 digital electric）附：英文原文Combined Digital Electronic Current and Voltage TransducerDUAN Xiong-Ying, ZOU Ji-Yan, LIAO Min-Fu, ZHANG Ke-WeiAbstractA high-performance current and voltage measurement system has been developed in power system. The system is composed of two parts: one current measurement element and one voltage measurement element. A Rogowski coil and a capacitive voltage divider are used respectively for the line current and voltage measurements. Active electronic components are used to modulate signal,and power supply for these components is drawn from power line via an auxiliary current transformer. Measurement signal is transmitted by optical fibers, which is resistant to electromagnetic induction and noise. With careful design and the use of digital signal processing technology, the whole system can meet 0.5% accuracy for metering and provides large dynamic range coupled with good accuracy for protective relaying use.Key words：electronic current transducer (ECT), electronic voltage transducer (EVT), Rogowski coil, capacitive voltage divider(CVD), optical fiber.1 IntroductionCurrent and voltage measurements play an important role in metering, protection and control of electric power system. With the development of power system,the traditional current transformers (CT) and potential transformers (PT) show intolerant drawbacks: high cost and a complicated mode of insulation structure, the CTs saturation effect and PTs ferromagnetic resonance effect,et al.In past twenty years, considerable interest has been paid to the research on new optical current transformers (OCT) and optical potential transformers (OPT). The most common type of OCT utilizes the Farady effect, and OPTutilizes the Pockels effect. They use optical crystals as sensor. Though some successful field trials have been achieved, the large numbers of plentiful commercial products of OCTs and OPTs have not been attained, because OCT and OPT are susceptible to circumstance temperature and mechanical perturbations13. In recent years, Ritz Company developed combined electronic current and voltage transducer using Rogowski coil and a capacitive voltage divider as sensors for the line current and voltage measurements respectively4. Based on this concept, a new type of digital system for current and voltage measurement are developed.2System StructureThe concept of digital current transformer has been proposed for almost half century, but it is not well developed because of the limits of electronic components and conventional protective relaying and metering mode.Yet now protective relaying and metering system operate exclusively by digital technology and information network are being widely employed in power distribution system. All kinds of integrated chips for special function have come to markets.All of these make it possible to make electronic CT and PT products based on digital modulation.A block diagram of the combined current and voltage measurement system is given in Fig.1. A Rogowski coil used for current measurement and a power supply CT are fitted to the power line. The Rogowski coil incorporated with active electronic components composes the whole current transmitter. This transmitter generates an optical signal encoded with information describing the measured current. The whole current transmitter and the power supply part are put into an iron box for electromagnetic interferenece(MEI) immunity. A hollow ceramic insulator is used to sustain the high voltage part of the system. Both the optical fiber carrying the current signal and the capacitive voltage divider used as the voltage sensor traverse through the insulator. Voltage signal from the capacitive divider is also modulated into optical signal at the bottom part of the insulator. Then both the current signal and voltage signal are transmitted by means of optical fiber to the remote control room for metering and protective relaying.3Current and Voltage SensorIn this paper, a Rogowski coil is used as current sensor. Rogowski coil, which is a non-magnetic toroid wound like an inductor, has been used for measuring current for some time5. A Rogowski coil with rectangular cross-section is showed in Fig.2. When the Rogowski coil is placed on a current-carrying conductor, the coil generates a voltage E proportional to the coils mutual M and the rate of current change di/dt, and it is given by: (1)From Eq.1, the current is derived by means of the relationship ： (2) To obtain a measure of current, the coils output voltage must be integrated. This can be done by one of two common means: (1) by use of electronic integrator, (2) by using numerical integration in software after the coils output voltage is digitized. In this paper, an active integrator shown in Fig.3 is used. The output of the integrator is ： (3)where,Vi is the output of the Rogowski coil. From Eq. (1), Eq.(2), Eq.(3), the current can be obtained by the following equation: (4)Because there is no iron core to saturate, Rogowski coils have wide measurement range. The same coil can be used to measure currents from several amps to hundreds of kiloamps. Also they have high measurement accuracy, it is reported that they can be designed to be better accuracy than 0.1 percent, and it is insensitive to temperature6.Capacitive voltage divider (CVD) has been widely used in high voltage power system. The main factors affecting the measurement accuracy of CVD are stray capacitance and the capacitors temperature coefficient. Here precise capacitive divider technology is used to improve measurement accuracy. The effective method to minish stray capacitance is to play down the height of the divider at full steam. In this paper, the insulator is filled with oil as the main insulation to minimize its height. By this means, the stray capacitance of the divider can keep at a low level, and the identical temperature characteristic of the capacitors in series at the high voltage arm can be ameliorated greatly. A shielding electrode is also put on the top terminal of the divider to compensate the stray capacitance effect.The CVD in electronic voltage transducer (EVT) is put outdoors, great temperature change may affect the ratio error of the divider . As a sensor used in microprocessor-based system, the CVD in EVT is different from those conventional CVD. It has a small load burden, so in this paper a special structure of the CVD shown in Fig.4 is used to eliminate the effect of temperature. The divider is composed of m + n capacitors Ci which have the identical dielectric, size, value and temperature coefficient. M capacitors are connected in series to produce the C1 value required for the capacitor in the high-voltage arm, C1=Ci/m, and the capacitor value C2 in the low-voltage arm is produced by n capacitors Ci in parallel, C2=n*Ci , then the divider ratio K is (5)When the capacitor value Ci becomes Ci+Ci because of the temperature change,the divider ratio K isK=C1/(C1+C2) =(1/m)(Ci+C1)/(1/m)(Ci+C1)+n(Ci+C1) =1/(mn+1) (6)Eq.5 and Eq.6 showK=K. By this means, temperature effect on divider ratio can be eliminated provided that all capacitorsCihave the same parameters.4Signal ProcessingA block diagram of the signal processing is shown in Fig.5. Now there are many special A/D chips available in the market. Various methods can be used to convey measurement information from the transmitter to a remote point. Voltage to frequency modulation is a purely digital technique which can offer error-free and interference-free transmission, and therefor, is most attractive for use in power system. The basic concept of this technique is to convert the measurement signal into a PFM ( pulse frequency of modulated) signal whose frequency deviation is linearly proportional to the current or voltage magnitude. Eq.7 shows this relationship. (7)Where, F0 is the fundamental frequency which is set by a DC voltage offset at the input, the coefficient k is determined by the actual circuit.The frquency pulses drive the LED(low emitting diode) directly, and then the signal is modulated into optical pulse. The LED has a center wavelength of 850 nm, which matches the transmission window of the optical fiber. 200m multimode fiber is used to transmit optical signal. The receiver in the control room is a photodiode, which drives an amplifier to reconstruct the electric frequency pulses, then the frequency signal is sent into computer for demodulating.It is actually an integral process to count the frequency signal from time ti to tj, and the count value Yji is：Yji is obtained from the counter, so： （8）The left side of Eq.8 is an integration value of the input pulse signal from ti to tj, and it equals the are value surrounded by the signal curve. If the input signal keeps small change during ti to tj, or the sample time Tc=tj-ti is short enough, its value at point tj can be represented by an approximate value. （9）From Eq.8 and Eq.9, we get （10）The error caused by integration delay effect can be revised by software. Because the system uses optical fiber to transmit digital signal, it is resistant to EMI.5Line Potential Encoder Power SupplyOptical fiber uses implements insulation between high voltage and low voltage, but this also brings the problem of power supply at the high voltage. Three methods are available for designing a current driven power supply: batteries, current transformer, and optical power. Commonly the working life of the batteries is 510 years, and the mean time to be repaired is 3 years, it can not be accepted by power system. What is more, the big size of battery and its complicated floating charge circuit are undesirable.The high cost of some advanced batteries like lithium or optical batteries becomes obstacle for the application. In this paper, a special designed auxiliary CT is used to get enegy from the line directly. This CT is at the high voltage, and the insulation requirement is much simpler than those conventional CT, and the power output is low, so it has a rather smaller size than conventional CT.Two key problems should be considered during the power CT design. Firstly, the power CT should supply the required voltage and current from a line current as low as possible, that is to say, the measured line current has a minimum value at which the system will operate. Second, at fault current, the power CT can absorb the unwanted energy to supply a stable power for electronic components, and itself will not be destroyed by electric force. The schematic of the power supply system is shown in Fig.6. From Fig.6, we can obtain that (11)Where, Z0controlled impedance,Z1the load impedance, Iline current, ULload voltage.In order to keep the load voltage UL be constant, then Z0 must change as the change of line current I, that is to say, if we can design a controlled impedance circuit changing with the line current I, we can obtain a stable power energy output for the high electronic components. A diagram is shown in Fig.7 for illustrating the operating principle of the power supply design. The energy is obtained from line by power CT. At first , the power supply regulator circuit is the only load imposed on the power transformers secondary windings. When all regulated supply voltages are at their specified value, the current branch circuit is activated to stop further replenishment of power supply.By this design method, the system can work normally at 5% to 20-fold of the rate current (400 A).6System PerformanceThe prototype of a combined electronic current and voltage transducer for a single conductor construction was tested for a 110 kV power system. An overview of primary test results is described below.The ratio error characteristics of electronic current transducer at room temperature is shown in Fig.8.The ratio error is represented on the vertical axis,while the primary current flowing to the conductor is represented on the abscissa. This figure indicates the relationship between the RMS(root-mean-square)value of the output waveform of the optical interface andthe RMS value of the waveform of the primary currentas measured by a 0.2 class coil-type CT. The output of Rogowski coil is designed to be 400 mV corresponding to the 400 A rate primary current, and the parameters of integrator arer=10 k,C=0.1F. It may be seen from this figure that the ratio error of the ECT is at a range of1.0%. As discussed above,signal is demodulated by software, so it is convenient to revise error by using digital signal processing technique, and higher measurement accuracy of 0.5% is expected to obtain. The measured phase error of current is better than 20 minutes at twenty percent of full load current at 50 Hz.Fig.9 shows voltage transducer test results. Both the high voltage capacitors in series and the low voltage capacitors are polypropylene capacitors. The test was carried out at room temperature. This figure indicates the relationship between the RMS value of the output waveform of the optical interface in control room and the RMS value of the waveform of the high voltage as measured by a 0.2 class standard conventional PT. The low voltage is represented on the vertical axis, while the high voltage is represented on the abscissa. The linearity of the electronic voltage transducer was measured to meet the accuracy (0.5%) of the measurement system. The phase error of voltage is better than 30 minutes when the voltage is over than 20% percent of the full scale.7 ConclusionA combined digital electronic current and voltage measurement system has been described, which offers many advantages compared with the conventional CTs and PTs. These ca