文献翻译组合数字电子式电流和电压传感器

组合数字电子式电流和电压传感器 作者：段雄英、邹积岩、廖敏夫、张可卫摘 要在电力系统中已开发出高性能的电流和电压测量系统。该系统由两部分组成：电流测量元件和电压测量元件。洛高夫斯基线圈和电容分压器分别用于为线电流和电压测量。有源电子元件是电流互感器进行在线检测时为这些组件信号调制并提供电源的。测量信号通过光纤传输，光纤可以抗电磁干扰和噪声干扰。通过精细的设计和对数字信号处理技术的应用，整个系统的精度可以达到0.5%，配以高精度的供继电保护设备可提供较大范围的动态检测。关键词：电子式电流传感器（ECT），电子电压互感器（EVT），洛高夫斯基(Rogowski)线圈，电容分压器（CVD），光纤1 简 介电流和电压测量在测光、保护和电力系统控制中起着重要作用。随着电力系统的发展，传统的电流互感器（CT）和电压互感器（PT）有着不能容忍的缺点：绝缘结构成本高且模式复杂，CT的饱和效应和PT的铁磁谐振效应等影响。在过去的二十年中，大多兴趣投入在光学电流互感器（OCT）和光学电压互感器（OPT）的研究中。最常见的类型有利用法拉第效应的OCT和利用普克尔斯效应的OPT，它们都是使用光学晶体的传感器。尽管一些成功的现场试验已经完成，但在商业产品中OCT和OPT还不能实现量产，因为OCT和OPT很容易受到环境温度和机械扰动13。近年来，丽思（Ritz）公司开发的以结合电子式电流和电压传感器利用洛高夫斯基（Rogowski）线圈和电容分压器分别作为传感器线电流和电压测量4。基于这一概念，一种新型电流和电压测量数字系统被开发出来了。2 系统结构数字电流互感器的概念已经提出了近半个世纪，但因电子元件及常规继电保护和测光模式的限制使其没有得到很好的发展。可是现在基于数字技术及信息网络的继电保护和独占地测光系统操作正在被广泛应用于配电系统。各种具有特殊功能的集成芯片投入市场。所有这些都可能使电子CT和PT产品基于数字调制。合并后的电流和电压测量系统框图如图1所示。洛高夫斯基(Rogowski)线圈是用于使电流测量和电力CT符合电力线的。洛高夫斯基(Rogowski)线圈纳入有源电子元件组成了整个电流变送器。这种发射器生成信息描述性测量电流和光信号编码。整个电流发射器和电源部分都被放到一个抗电磁干扰（EMI）的铁盒中。空心陶瓷绝缘体用于维持系统的高电压的一部分。无论是光纤承载的电流信号还是电容分压器都为通过绝缘体的电压传感器使用。电容分压器的电压信号也在绝缘体的底部调制成光信号。接着电流信号和电压信号通过光纤传输到测光和继电保护远程控制室内。图1 组合电子式电流和电压测量系统3 电流和电压传感器在此，提出了一种洛高夫斯基（Rogowski）线圈式电流传感器。Rogowski线圈，这是一个电感无磁环形口，已用于测量电流一段时间5。图2所示为一个矩形截面Rogowski线圈。Rogowski线圈是基于载流导体的放置，线圈产生一个电压E成正比，线圈交互M和电流变化的的速度为： (1)由式(1)得： (2)要获取电流测量值，线圈的输出电压必须是完整的。这可以通过两种常见的方法：（1）电子积分器的使用;（2）在使用数值积分软件之后线圈的输出电压是数字化的。本文图3所示为积分器。该积分器的输出为: (3)其中，Vi是Rogowski线圈的输出从目前可以得到以下等式： (4)因为没有铁芯饱和，Rogowski线圈有广泛的测量范围。同样的线圈可用于测量从几安培至数百千安培的电流。此外，他们具有较高的测量精度，也就是说可以精确至0.1%6，而且它对温度不敏感。电容分压器（CVD）技术已被广泛应用于高压电力系统。影响CVD测量精度的主要因素是杂散电容和电容的温度系数。精密电容分压器技术用于提高测量精度。有效减小杂散电容的方法是全速淡化分频器高度。本文中，主要是以充满绝缘油的绝缘体来尽量减少它的高度。通过这种方法，杂散电容分压器可以保持在较低水平，且位于高压臂中的具有相同的温度特性的串联电容器可以被大大改善。分压器终端置顶一个屏蔽电极以补偿杂散电容的影响。将电子电压互感器（EVT）中CVD放在室外，强烈的温度变化可能影响分压器的误差。正如微处理器基础系统中的传感器一样，在EVT中的CVD是不同于常规CVD的。因其负荷小的特点，故如图4所示,特殊结构的CVD可以消除温度的影响。分压器是由具有相同的电介质、规模、价值和温度系数的m+n个电容器Ci组成的。m个电容器串联在系统中的值C1得满足高压臂中的要求：，低压臂中的电容值C2 是由并联电路得出的C2=nCi, 那么分压比K为: (5)电容值C1由于温度变化变为Ci +C1，这时分压比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，通过这种方式，所有具有相同参数的电容Ci都可以消除温度对分频比率的影响。4 信号处理如图5所示为一个信号处理框图。现在市场上发售有许多特殊的A / D芯片在。有多种方式可以从一个远程发射点传达测量信息。电压频率调制是一个纯粹的数字技术，它可以提供无差错和无干扰的传输，因此，最适合在电力系统中使用。这项技术的基本概念是把测量信号转换成PFM（脉冲频率调制）信号，其测量信号的频率偏差与电流或电压的大小成线性正比。图5 信号处理系统式7阐述了这种关系： (7)其中F0 是输入直流电压偏移设置的基本频率，系数k是取决于实际电路。LED（发光二极管低）直接由频率脉冲驱动，然后信号调制成光脉冲。与LED匹配的光纤传输窗口具有850nm的中心波长。200m多模光纤用来传输光信号。控制室接收器是一个光电二极管，即用于重建电频率脉冲的驱动放大器，之后把频率信号发送到计算机用于解调。由时间到其实是一个完整的频率信号计数过程，并且计数值Y j i 为：Yji 是由计数器获取的，所以： （8）式8是由到的输入脉冲信号的积分值, 它等于由信号曲线包围的面积值。如果输入信号的微小变化保持在与 之间, 或者采样时间=足够短，其点的值可以是一个近似值。 （9）由式8和9，我们可以得到： （10）整合延迟效果所造成的错误，可以通过软件进行修改。由于该系统采用光纤传输数字信号，所以它是抗电磁干扰。5 线电位编码器电源使用光纤实现高低电压间的绝缘，但在高电压上仍有些电源问题。有三种方法可以设计一个电流驱动的电力供应：电池，电流互感器，和光功率。常见的是电池的工作寿命为510年，平均修复时间为3年，动力系统中是不行的。更重要的是，大尺寸的电池和其复杂的浮点充电电路是不可取的。一些先进的电池成本高，如锂或光电池成为应用的障碍。在此提出了一种特殊设计的辅助CT是用来直接从生产线上获取能量的。这种高压CT的绝缘要求要比那些常规CT更简单，并且输出功率低，所以它比常规CT要小的多。在电源CT设计中应该考虑两个关键问题。首先，电力CT应为线电流尽提供可能低的所需的电压和电流，也就是，在该系统运行中实测线电流有一个最低值。其次，在故障电流中，电源CT能吸收多余的能量成为供应电子元器件的稳定力量，且其本身不会被电场力所破坏。该电源系统的示意图如图6所示。从图6，我们可以得到的： （11）其中，Z0-控制阻抗，Z1-负载阻抗，I-线电流， -负载电压。为了使负载电压是常量，那么Z0必须随线电流I的变化而变化，也就是说，如果我们能够设计出随线电流I变化的控制阻抗电路，我们可以得到一个稳定的高能量输出功率的电子元件。图7说明了电源的设计运行原则。能量由电源CT获得。起初，电源稳压电路是唯一施加在电源变压器的次级绕组的负载。当所有的稳压电源电压是指定的值时，当前分支电路是停止电力供应的进一步补充。通过这种设计方法，该系统可以正常工作在5到20的额定电流（400A）。6 系统性能用综合电子式电流和电压传感器的单导线施工原型为110KV电力系统进行测试，初步测试结果概述如下：电子式电流传感器在室温下的比率的误差特性如图8所示。纵轴表示比率误差，而横轴表示初级电流流向导体。这一数字表明RMS（均方根方）的光接口输出波形的值和初级电流作为衡量一个0.2级线圈型CT的RMS的波形的值的关系。Rogowski线圈的输出设计为400mV相对应的初级电流400 A 的变动率，和积分器的参数为：R= 10k的中，C =0.1F的。由数字可以看出ECT的比率误差范围为1.0。如上所述，通过软件解调信号，可以很方便地利用数字信号处理技术进行错误修改，并有望获得更高的测量精度为0.5。测量电流的相位误差要好于20分钟时电流过载50Hz的20。图9显示电压传感器的测试结果。无论是高压电容器系列和低电压电容器聚丙烯电容器。该测试是在室温下执行的。这一数字表明在控制室的光纤接口输出波形的RMS值和0.2级标准常规PT的高电压波形的RMS值的关系。纵坐标轴表示低电压，而横坐标轴表示高电压。该电子电压传感器的线性测量满足该测量系统精度的0.5，当电压超过20的满量程时，电压的相位误差要好于30分钟的时候。7 结束语一个组合数字电子电流和电压测量系统相比与常规的CTs和PTs具有许多优点。这些可以概括如下：(1) 高测量精度Rogowski线圈和精密电容分压器可以通过软件很方便的修改错误。且该系统预计将拥有0.5%的精确度。(2) 具有较宽的测量范围和频率范围。因为没有铁饱和 ，Rogowski线圈不直接测量电流，而不像CTs那样，当一个大的DC元件存在时，可以精确地测量电流。(3) 电流和电压测量组合结构体积小，重量轻，绝缘结构简单。(4) 光纤高电压具有电镀绝缘带。这构成了可靠的低成本绝缘非接触式测量系统和强大的抗电磁干扰性。(5) 系统的温度特性决定于该电子元件温度敏感性，先进的产品都小于 5010-6/。(6) 目前已开发增加相容性的新的电子继电器和测光设备。未来工作重心着手将组合数字电流和电压测量系统投入实际应用中。参考文献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. 来源于：上海大学学报（英文版），2002,6（1）：7984 （黄河科技学院主页 图书馆 维普期刊 搜索关键字 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 can be summarized as follow.(1) High measure