Development and Application of a Wireless Sensor for Space Charge ...

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Oct 20, 2016 - in the corona area will affect the space electric field distribution of ionization zone ... corona discharge and high-voltage DC transmission line ...
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Development and Application of a Wireless Sensor for Space Charge Density Measurement in an Ultra-High-Voltage, Direct-Current Environment Encheng Xin 1 , Yong Ju 2 and Haiwen Yuan 1, * 1 2

*

School of Automation Science and Electrical Engineering, Beihang University, Beijing 100191, China; [email protected] China Electrical Power Research Institute, Beijing 100192, China; [email protected] Correspondence: [email protected]; Tel.: +86-10-8233-8697

Academic Editors: Changzhi Li, Roberto Gómez-García and José-María Muñoz-Ferreras Received: 25 May 2016; Accepted: 18 August 2016; Published: 20 October 2016

Abstract: A space charge density wireless measurement system based on the idea of distributed measurement is proposed for collecting and monitoring the space charge density in an ultra-high-voltage direct-current (UHVDC) environment. The proposed system architecture is composed of a number of wireless nodes connected with space charge density sensors and a base station. The space charge density sensor based on atmospheric ion counter method is elaborated and developed, and the ARM microprocessor and Zigbee radio frequency module are applied. The wireless network communication quality and the relationship between energy consumption and transmission distance in the complicated electromagnetic environment is tested. Based on the experimental results, the proposed measurement system demonstrates that it can adapt to the complex electromagnetic environment under the UHVDC transmission lines and can accurately measure the space charge density. Keywords: ultra high-voltage direct-current (UHVDC); space charge density; energy consumption; wireless communication; Zigbee

1. Introduction The electromagnetic environment under UHVDC transmission lines contains several inter-related electrical parameters, which are the electric field, the ion current density, the space charge density, the radio interference, the audible noise, and the magnetic field [1,2]. With the development of UHVDC transmission and transmission capacity enlargement, the generation of corona will be inevitable. Air molecules surrounding the conductor surfaces of the lines will be ionized, giving rise to positive and negative space charges, which are distributed in the space between the transmission lines and the ground [3]. Charged particles are formed by capturing charged clusters. The charged particles in the corona area will affect the space electric field distribution of ionization zone and then affect the corona discharge. The charged particles in the corona ion migration region, as space charge, will also affect the ion flow field. The effect that particulate matter, which is in the form of space charge to impact the corona discharge and space electric field, is referred to as particulate matter space charge effect. The space charge effect of charged particles is one of the important factors to affect the DC corona discharge and high-voltage DC transmission line electromagnetic environment. This means it is important for the DC power transmission project to research the interaction between space charge and the electromagnetic environment of UHVDC transmission lines [4–7]. A sensor is a key part of a space charge density measurement system. There is much literature about the measurement of the charge density in air. In [8], German researcher Ebert used two coaxial

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brass pipes as electrodes, and air was aspirated into the device by an impeller in 1901. In [9], Misakian the National of Standards theincharge density with a brass pipesfrom as electrodes, and Bureau air was aspirated into of theAmerica device bymeasured an impeller 1901. In [9], Misakian rectangular cuboid aspirator-type ion counter arranged in a cavity below the grounded electrode in from the National Bureau of Standards of America measured the charge density with a rectangular 1986. However, the device was not used at other locations in the field, or measured the charge cuboid aspirator-type ion counter arranged in a cavity below the grounded electrode in 1986. However, density under the used UHVDC lines. In [10],in Rossner Singer from Technical University of the device was not at other locations the field,and or measured the charge density under the Hamburg-Harburg also applied an aspirator-type apparatus to measure charged particles in a UHVDC lines. In [10], Rossner and Singer from Technical University of Hamburg-Harburg also diameter 1.5–10 μm inapparatus 1989. Relative error of measured results be 20% whenµm compared applied anofaspirator-type to measure charged particles in amight diameter of 1.5–10 in 1989. with an optical particle counter, which shows the difficulty of measurement of charge and particle Relative error of measured results might be 20% when compared with an optical particle counter, density. In [11], Kulon from Brunel University applied phase Doppler anemometry laser which shows the difficulty of measurement of charge and particle density. In [11], Kulon with from aBrunel 500 nm in wavelength to measure charge distribution of 0.7–1.9 particles in to a parallel plate University applied phase Doppler the anemometry with a laser 500 nmμm in wavelength measure the field in 2002. However, it may be difficult to popularise this expensive laboratory technique to charge distribution of 0.7–1.9 µm particles in a parallel plate field in 2002. However, it may be difficult measure the charge densities under outdoor transmission lines. to popularise this expensive laboratory technique to measure the charge densities under outdoor In this paper, relying on the wireless communication technologies, a space charge density transmission lines. measuring sensor is put forward with an ion counter method. The main goal of this study is to In this paper, relying on the wireless communication technologies, a space charge density develop a wireless sensor network (WSN) aiming for the reliable and flexible measurement of the measuring sensor is put forward with an ion counter method. The main goal of this study is to space charge density under UHVDC transmission lines. This paper is organized as follows. In develop a wireless sensor network (WSN) aiming for the reliable and flexible measurement of the space Section 2, the measurement system framework is described. In Section 3, the measurement principle charge density under UHVDC transmission lines. This paper is organized as follows. In Section 2, the and design of the space charge density measurement system is proposed. In Section 4, wireless measurement system framework is described. In Section 3, the measurement principle and design of network selection and communication quality evaluation is elaborated. Finally, a conclusion section the space charge density measurement system is proposed. In Section 4, wireless network selection ends this paper. and communication quality evaluation is elaborated. Finally, a conclusion section ends this paper. 2. 2. Space SpaceCharge ChargeDensity DensityMeasurement MeasurementSystem SystemFramework Framework The generation of space charge charge is is mainly mainly caused caused by by corona corona and and dose dose directional directional movement movement The generation of space between direct current conductor and earth. Due to the polarity of the wire being different, the between direct current conductor and earth. Due to the polarity of the wire being different, the formation mechanism and the movement direction of space charge are also slightly different. There formation mechanism and the movement direction of space charge are also slightly different. There are are different formed UHVDC transmission asinshown Figure 1. In this threethree different parts parts formed under under UHVDC transmission lines as lines shown Figure in 1. In this figure, the figure, the earth’s fair-weather electric field has been ignored. They are negative ion area 1, positive earth’s fair-weather electric field has been ignored. They are negative ion area 1, positive ion area 2 ion 2 and ion mixing andarea ion mixing zone 3. zone 3.

Figure 1. Space charge distribution around direct current transmission lines. Figure 1. Space charge distribution around direct current transmission lines.

The space lines is is measured The space charge charge density density under under UHVDC UHVDC transmission transmission lines measured at at different different positions positions through a number of charge density sensors. The result comes from multiple sensors which can imply through a number of charge density sensors. The result comes from multiple sensors which can the distribution of the space charge density. The framework of the space charge density measurement imply the distribution of the space charge density. The framework of the space charge density is shown in Figure 2. The is used sensor to measure thetospace charge Data collected measurement is shown inwireless Figure 2.sensor The wireless is used measure thedensity. space charge density. is sent to the remote node by Zigbee module. The remote node connected with the data centers Data collected is sent to the remote node by Zigbee module. The remote node connected withand the servers. The and value of the The space charge density displayed on the screen and in theand database. data centers servers. value of the spaceischarge density is displayed onstored the screen stored As a result, the As distribution of distribution the space charge underdensity UHVDC transmission lines can be in the database. a result, the of thedensity space charge under UHVDC transmission achieved through multiple sensors system usage. lines can be achieved through multiple sensors system usage.

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Zigbee data transmission Charge density Charge sensor density

Charge density Charge sensor density

sensor

sensor

Charge Charge Zigbee data transmission density … density Charge Charge sensor sensor density density sensor



sensor

Figure 2. Structure of the distributed measurement system. Figure Figure2.2.Structure Structureofofthe thedistributed distributedmeasurement measurementsystem. system.

3. Design of the Space Charge Density Measurement System

3.3. Design Design of of the the Space Space Charge ChargeDensity DensityMeasurement MeasurementSystem System 3.1. Space Charge Density Sensor Design and Working Principle 3.1. Charge Density Sensor Design Principle 3.1.Space Space Charge Density Sensor Designand and Working Principle The sensor realizes the measurement ofWorking space charge density using the ion counter method, and its structure design is shown in Figure 3. The sensor consists of using plates and a counter fan withmethod, a constant The and Thesensor sensorrealizes realizesthe themeasurement measurementofofspace spacecharge chargedensity density usingthe theion ion counter method, and speed. The working principle of the sensor is that charged ions in the atmosphere pass the plate itsitsstructure design is shown in Figure 3. The sensor consists of plates and a fan with a constant speed. structure design is shown in Figure 3. The sensor consists of plates and a fan with a constant electrode under the action of fan, current isions formed on thein plate ionsthe movement under The working principle of the sensor isthen that charged in the atmosphere pass plate speed. The working principle ofand the sensor is that charged ions the by atmosphere passelectrode the plate the action of the electric field force between the plate to plate. By measuring the current through the under the action of fan, and then current is formed on the plate by ions movement under the action electrode under the action of fan, and then current is formed on the plate by ions movement under plate and gas flow within a certain time, the charge contained in gas and gas volume through the ofthe theaction electric field force between the plate to plate. By measuring the current through the plate and of the electric field force between the plate to plate. By measuring the current through the plate and the charge density will be obtained. The calculation formula of space charge density is gas flow within a certain time, the charge contained in gas and gas volume through the plate and the plate and gas flow within a certain time, the charge contained in gas and gas volume through the given charge density be obtained. Thebe calculation of space charge density is given by density is platebyand the will charge density will obtained.formula The calculation formula of space charge rt tI (t ) dt  Q t0t I ( t ) dt Q 0t   = r t (1)(1) ρ= I2(rt2)W)dtW VV Q t π (RR2 2t − r ( t ) dt ( t ) dt 0   tt00 t (1) 2 V   r 2 W (t ) dt R   t0 where ρρ means means the the space space charge charge density; density; Q Q means means the charge charge arrived arrived at at plate plate in in aa particular particular time time where the period; V means the gas volume flow between plates; I means the current flows through the plate; period; volume flow between means the arrived current at flows through the plate;time R whereVρmeans meansthe thegas space charge density; Q plates; means Ithe charge plate in a particular R means the radius of the outer plate; r meansthe theradius radiusofofthe theinner innerplate; plate;and andW Wmeans meansthe theflow flow means the radius of the outer plate; r means period; V means the gas volume flow between plates; I means the current flows through the plate; R velocity in in the the device device as controlled controlled by by the fan. fan. velocity means the radius ofasthe outer plate;the r means the radius of the inner plate; and W means the flow velocity in the device as controlled by the fan.

given by

Figure Figure3. 3.Sensor Sensoroperation operationprinciple. principle. Figure 3. Sensor operation principle.

Ion movement speed is related to mobility. The mobility of charged ions means the final Ion movement speed is related to mobility. The mobility of charged ions means the final velocity velocity of the unit fieldspeed strength chargedtoion movement. The speed of is related to several factors such is related mobility. mobility charged means of the Ion unitmovement field strength charged ion movement. TheThe speed is related to severalions factors suchthe as final the asvelocity the temperature, pressure, particle size ofion themovement. ion. of the unit field strength charged The speed is related to several factors such temperature, pressure, particle size of the ion. When the applied voltage particle and the size gas of velocity between the plates are different, the number of as the temperature, pressure, the ion. charged particles which voltage will reach is also different. Theplates scopeare ofdifferent, the charged particlesof When the applied andthe theplate gas velocity between the the number which reach the plate can be expressed with critical mobility. The expression is given as charged particles which will reach the plate is also different. The scope of the charged particles which reach the plate can be expressed with critical mobility. The expression is given as

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SensorsWhen 2016, 16, 1743 4 of of 14 the applied voltage and the gas velocity between the plates are different, the number charged particles which will reach the plate is also different. The scope of the charged particles which reach the plate can be expressed with critical mobility. M  0The expression is given as kc  (2) CU Mε 0 k = (2) where M is the gas flow per unit time, ε0 is thecvacuum CU dielectric constant, C is the plate capacitor, U is the voltage between plates. For coaxial cylinder type plate, critical mobility of charged particle where M is the gas flow per unit time, ε0 is the vacuum dielectric constant, C is the plate capacitor, counter is U is the voltage between plates. For coaxial cylinder type plate, critical mobility of charged particle R counter is W  R 2  r 2 ln   (3) W R2 − r2 ln r Rr c  k c k= (3) 2UL 2UL

where LL is is the the length length of of the the plate. plate. The ion flow field formation by corona discharge contains a variety variety of ionic ionic components components [12]. [12]. Ion mobility mobility is is associated associatedwith withion ionown owncharacteristic characteristicsuch suchasas quality and volume. greater quality and volume. TheThe greater the the quality volume is,smaller the smaller themobility ion mobility The positive and negative can be quality and and volume is, the the ion is. Theis. positive and negative ions canions be divided divided two categories: (large ions) and charged molecular (smallLarge ions). into twointo categories: chargedcharged particlesparticles (large ions) and charged molecular clustersclusters (small ions). 2 cm2 /(V Large ion migration velocity is slow 102−/(V·s)), andion small ion migration velocity is and·s)), small migration velocity is quick ion migration velocity is slow ((1–2)((1–2) × 10−2×cm 2 2 quick (1–2 cm /(V·s)). (1–2 cm /(V·s)). The space toto the combination of of a series of space charge chargedensity densitymeasurement measurementdevice devicecan canbebeequivalent equivalent the combination a series resistance andand capacitance, andand thethe physical model is shown in Figure 4. 4. of resistance capacitance, physical model is shown in Figure RU CU II

I LEAK

CI

RI

CLEAK

RLEAK

RIS

RE

CE

CO

RO

Figure density measurement measurement device. device. Figure 4. 4. The The equivalent equivalent model model of of the the space space charge charge density

II is the current formed by the charged particles hit the plate, RI is the insulation resistance II is the current formed by the charged particles hit the plate, RI is the insulation resistance between the plates, CI is the equivalent capacitance between the plates, CLEAK is the stray capacitance, between the plates, CI is the equivalent capacitance between the plates, CLEAK is the stray capacitance, RLEAK is the insulation resistance of line, RIS is the leakage resistance of plate bias source module, RU is RLEAK is the insulation resistance of line, RIS is the leakage resistance of plate bias source module, RU the output resistance of the plate bias source, CU is the output capacitance of the plate bias source, RE is the output resistance of the plate bias source, CU is the output capacitance of the plate bias source, is the equivalent resistance between the charged particle counter and the earth, CE is the equivalent RE is the equivalent resistance between the charged particle counter and the earth, CE is the equivalent capacitance between the charged particle counter and the earth, RO is the input resistance of current capacitance between the charged particle counter and the earth, RO is the input resistance of current measurement circuit, CO is the input capacitance of current measurement circuit. Leakage current measurement circuit, CO is the input capacitance of current measurement circuit. Leakage current may may be produced in RI, RLEAK, RIS, RE or RO. In order to ensure the leakage current is small enough, be produced in RI , RLEAK , RIS , RE or RO . In order to ensure the leakage current is small enough, need need to make the resistance increased quickly. At the same time, to avoid the influence of stray to make the resistance increased quickly. At the same time, to avoid the influence of stray capacitance capacitance on measurements, signal transmission cables should be short. on measurements, signal transmission cables should be short. 3.2. Design 3.2. Design of of the the Sensor Sensor Sensor design design key key parameters parameters include incident velocity effective length length of of the the Sensor include the the axial axial incident velocity W, W, the the effective tube L, inner electrode radius r and external electrode radius R. In the design of the sensor, there are tube L, inner electrode radius r and external electrode radius R. In the design of the sensor, there are three main main technical technical problems problems to to be be solved. solved. First coaxial structure structure with with an an three First of of all, all, the the sensor sensor has has a a coaxial existing edge effect, which may cause distortion of the deflection electric field near the end. It will existing edge effect, which may cause distortion of the deflection electric field near the end. It will directly affect affectthe thelaw lawofof motion of ion under the action of electric field. Second, this adopts article directly motion of ion flowflow under the action of electric field. Second, this article a ventilation method to axial provide axialion incident ion flowItsvelocity. Its approximate value is aadopts ventilation method to provide incident flow velocity. approximate value is equal to the equal to the wind speed W. If the wind speed selection is not correct, ions can easily form a turbulent flow within the sensor, and cannot keep constant speed in a level. Lastly, through superposition of an external synthesis electric field, the distortion of the electric field is near the electrode end, interferes with ion movement, and causes the ion current measurement to be inaccurate.

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wind speed W. If the wind speed selection is not correct, ions can easily form a turbulent flow within the sensor, and cannot keep constant speed in a level. Lastly, through superposition of an external synthesis electric field, the distortion of the electric field is near the electrode end, interferes with ion Sensors 2016, 16, 1743 5 of 14 movement, and causes the ion current measurement to be inaccurate. In length L of thethe sensor needs to In order order to to solve solvethe theinfluence influenceofofthe thesensor sensorend endeffect, effect,the theeffective effective length L of sensor needs be reasonably selected. According to the electric field distribution characteristics, the the influence of the to be reasonably selected. According to the electric field distribution characteristics, influence of end effect in the pipe can be ignored when the effective length L >> (R − r). In order to achieve the the end effect in the pipe can be ignored when the effective length L >> (R − r). In order to achieve ion the laminar motion in the sensor, the the inner andand outer radius of the electrode andand wind speed need to be ion laminar motion in the sensor, inner outer radius of the electrode wind speed need to reasonably designed. be reasonably designed. Flows into two two types, such as as laminar laminar and and turbulent, turbulent, Flows with with the the fluid fluid viscosity viscosity may may be be divided divided into types, such which laminar to to turbulent occurs at which are are determined determinedby bythe theReynolds Reynoldsnumber numberRe. Re.The Thetransition transitionfrom from laminar turbulent occurs Re ≤ 2300. The Reynolds number is defined by at Re ≤ 2300. The Reynolds number is defined by

Re= ρVD/η VD /  Re

(4) (4)

where ρρ isisfluid fluiddensity density(kg/m (kg/m−−33),),VVisisthe the pipe pipe (m/s), (m/s), D where thevelocity velocity of of the the air air flowing flowing in in the D is is the the device device outer diameter (m), and η is the fluid viscosity (Pa·s). outer diameter (m), and η is the fluid viscosity (Pa·s). The structure of the sensor and the main parameters are shown in Figure 5. Its shell is made of The structure of the sensor and the main parameters are shown in Figure 5. Its shell is made aluminum with high surface hardness is used to avoid the accumulation of ion flow under the of aluminum with high surface hardness is used to avoid the accumulation of ion flow under the UHVDC transmission lines. Its external electrode radius R = D/2 = 12.5 mm, inner electrode radius UHVDC transmission lines. Its external electrode radius R = D/2 = 12.5 mm, inner electrode radius r = d/2 = 1.5 mm, effective length L = 148 mm, i.e., L ≈ 13 (R − r). The end electric field’s influence r = d/2 = 1.5 mm, effective length L = 148 mm, i.e., L ≈ 13 (R − r). The end electric field’s influence on measurement can be ignored. The wind speed W in tube is adjustable. When W is 1 m/s, on measurement can be ignored. The wind speed W in tube is adjustable. When W is 1 m/s, Re ≈ 167 < 2300, can meet the laminar flow conditions in tube. In addition, the voltage between plates Re ≈ 167 < 2300, can meet the laminar flow conditions in tube. In addition, the voltage between plates is 12V. Through the calculation of formula (3), the value of positive ion migration rate is 1.36 cm2/(V·s) is 12 V. Through the calculation of Formula (3), the value of positive ion migration rate is 1.36 cm2 /(V·s) and the negative ion migration rate is 1.67 cm2/(V·s), so we can know that the mentioned ion counter and the negative ion migration rate is 1.67 cm2 /(V·s), so we can know that the mentioned ion counter is designed to monitor only small ions. is designed to monitor only small ions.

Figure 5. Picture of of the the sensor. sensor. Figure 5. Picture

A diagram diagram of of the theproposed proposedwireless wirelesscircuit circuitisispresented presented Figure current collected A inin Figure 6. 6. AsAs thethe ionion current collected by by sensor is small too small to collect so conversion the I-V conversion circuit is to designed the sensor is too to collect easily, easily, so the I-V circuit is designed achieve to theachieve conversion conversion current The to voltage. The signal conditioning circuitry is the charge of current toofvoltage. signal conditioning circuitry is composed of composed the charge of amplifier, the amplifier, the band pass filter and the low pass filter for further processing. The wireless band pass filter and the low pass filter for further processing. The wireless communication system communication system selected for the sensor wireless node is the 2.4 GHz Xbee Pro radio frequency (RF) module based on IEEE 802.15.4 protocol from Digi. The wireless node’s primary function is to acquire and process the output analog signal of the wireless sensor and send the digital signal to the remote node. The IEEE 802.15.4 protocol defines 16 channels whose serial numbers are from 11 to 26 on the frequency band as shown in Figure 7. In actual work, sensor network can choose one of the

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selected for the sensor wireless node is the 2.4 GHz Xbee Pro radio frequency (RF) module based on IEEE 802.15.4 protocol from Digi. The wireless node’s primary function is to acquire and process the output analog signal of the wireless sensor and send the digital signal to the remote node. The IEEE 802.15.4 protocol defines 16 channels whose serial numbers are from 11 to 26 on the frequency band Sensors 2016, 16, 1743 6 of 14 asSensors shown in16, Figure 2016, 1743 7. In actual work, sensor network can choose one of the channels to transmission 6 of 14 signal. The RF module can operate from a 2.8 to 3.4 V supply and consume between 45 and 50 mA for between 45 and 50 mA for receiving and transmitting (Rx/Tx) operations. The typical range for the betweenand 45 and 50 mA for and transmitting x/Tx) for operations. Theistypical the receiving transmitting (R receiving /Tx ) operations. The typical(R range the module 1.5 km range line-offor sight module is 1.5 km line-of sightx with 2.0 dB dipole antenna and 90 m indoor. module is 1.5 km line-of sight with 2.0 dB dipole antenna and 90 m indoor. with 2.0 dB dipole antenna and 90 m indoor.

Figure 6. Diagram of the proposed wireless circuit. Figure6.6.Diagram Diagramofofthe theproposed proposedwireless wirelesscircuit. circuit. Figure

Figure 7. The channels distribution of Zigbee in the 2.4 GHz band. Figure 7. The channels distribution of Zigbee in the 2.4 GHz band. Figure 7. The channels distribution of Zigbee in the 2.4 GHz band.

The STM32F103VE STM32F103VE is is selected selected as as the The processor processor The the microcontroller microcontroller for for the the wireless wireless sensor. sensor. The The rich STM32F103VE is selectedInas the microcontroller for the wireless sensor. five Theuniversal processor possesses hardware resources. addition, it has a 3.3 V low voltage. It contains possesses rich hardware resources. In addition, it has a 3.3 V low voltage. It contains five universal possesses rich hardware resources. In addition, (USARTs) it has a 3.3and V low voltage. It contains five universal synchronous/asynchronous receivers three serial peripheral interfaces (SPI), synchronous/asynchronous receiverstransmitters transmitters (USARTs) and three serial peripheral interfaces synchronous/asynchronous receivers transmitters (USARTs) and three serial peripheral interfaces which which are bothare essential for communicating with the RFwith module, erasable programmable (SPI), both essential for communicating the an RFelectrically module, an electrically erasable (SPI), which are both essential for communicating with the RF module, an electrically erasable read-only memory (EERPOM) nonvolatile module, and a watchdog module. The USARTs not only programmable read-only memory (EERPOM) nonvolatile module, and a watchdog module. The programmable read-only memory (EERPOM) nonvolatile module, and a watchdog module. The interface with the RF module but also allow communication with the RS-232 port on a PC. Another USARTs not only interface with the RF module but also allow communication with the RS-232 port USARTs not only interface with the RF module but also allow communication with the RS-232 port valuable the STM32F103VE it integrates aisreal-time clock (RTC) which can provide on a PC. feature Anotherofvaluable feature of is thethat STM32F103VE that it integrates a real-time clock (RTC)a on a calendar PC. Another valuable feature of the STM32F103VE is that it integrates a real-time clock (RTC) clock function for the wireless node. Besides, three 12-bit analog-to-digital converters are which can provide a clock calendar function for the wireless node. Besides, three 12-bit which can provide a clock calendar function for the wireless node. Besides, three 12-bit embedded into the microcontroller to acquire thethe analog electric fieldto signal from theanalog sensor.electric In the analog-to-digital converters are embedded into microcontroller acquire the analog-to-digital converters are embedded into the microcontroller to acquire the analog electric signal conditioning module, a low pass filter and an impedance matching circuit are employed to field signal from the sensor. In the signal conditioning module, a low pass filter and an impedance field signal from thesignal sensor. In the signal conditioning module, a low pass filter and an impedance process the amplified so that it can be suitable for the analog digital conversion. To minimize the matching circuit are employed to process the amplified signal so that it can be suitable for the analog matching are employedcircuit, to process theprecision amplified signal so that it canwith be suitable for the analog error of conversion. the circuit signal conditioning high operational amplifier is digital To minimize thea error of the signal conditioning circuit,low a offset high voltage precision digital conversion. To minimize the error oftothe signal conditioning circuit, awireless high precision used. A measurement mode switch unit is used control the working mode of the sensor operational amplifier with low offset voltage is used. A measurement mode switch unit is used to operational or amplifier with low voltage is used. A display measurement switch unit is used (continuous intermittent). Theoffset use of a sensor liquid crystal (LCD) mode can facilitate viewing andto control the working mode of the wireless (continuous or intermittent). The use of a liquid control the working mode of the wireless sensor (continuous or intermittent). The use of a liquid recording data (LCD) for operator. The value of theand space charge density be directly obtained through crystal display can facilitate viewing recording data for can operator. The value of the space crystal display (LCD) can facilitate viewing and recording data for operator. The value of the space the LCD screen. The power of the LCD, fan control module and frequency module is controlled by charge density can be directly obtained through the LCD screen. The power of the LCD, fan control charge density can be directly obtained through the LCD screen. The power of the LCD, fan control the power unit and can be turned offpower to the management low power mode thebewireless node module andmanagement frequency module is controlled by the unit when and can turned off to modulethe andinstruction frequencyfrom module is controlled by the power management unit and can be turned off to receives the remote computer. the low power mode when the wireless node receives the instruction from the remote computer. the low power mode when the wireless node receives the instruction from the remote computer. The wireless node acquires the analog signal from the space charge density sensor using the The wireless node acquires the analog signal from the space charge density sensor using the integrated analog-digital (A/D) convertor of the microcontroller. The digital low-pass IIR filter is integrated analog-digital (A/D) convertor of the microcontroller. The digital low-pass IIR filter is employed to process the acquired data. The outcome information of the space charge density is employed to process the acquired data. The outcome information of the space charge density is calculated and then transmitted to the RF module through the serial bus. It is important to note that calculated and then transmitted to the RF module through the serial bus. It is important to note that the configuration of the wireless node is very flexible. The microcontroller can be easily the configuration of the wireless node is very flexible. The microcontroller can be easily programmed to connect any space charge density sensor whose calibration data is sent by the PC programmed to connect any space charge density sensor whose calibration data is sent by the PC and saved to the EEPROM.

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The wireless node acquires the analog signal from the space charge density sensor using the integrated analog-digital (A/D) convertor of the microcontroller. The digital low-pass IIR filter is employed to process the acquired data. The outcome information of the space charge density is calculated and then transmitted to the RF module through the serial bus. It is important to note that the configuration of the wireless node is very flexible. The microcontroller can be easily programmed to connect any space charge density sensor whose calibration data is sent by the PC and saved to Sensors 2016, 16, 1743 7 of 14 the EEPROM.

3.3.3.3. Software the Measurement System Software Design Design ofofthe Measurement System Beforedesigning designingsoftware software for the demand analysis on the needsneeds to Before the measurement measurementsystem, system, demand analysis on software the software because the the on-site operator and data have different demands for the software. to be beconsidered, considered, because on-site operator andanalyst data analyst have different demands for the The on-site are required settingfor of description of sensor and data collection, software. Theoperators on-site operators are for required setting of description of real-time sensor and real-time data as well as real-time surveillance for the node conditions. For data analysts, relevant analyses and collection, as well as real-time surveillance for the node conditions. For data analysts, relevant processing be done with stored in the after finishing measurement. Therefore,the analyses andwill processing willthebedata done with thedatabase data stored in the the database after finishing software architecture is software designedarchitecture to meet the requirements different users, as shown in Figure 8. measurement. Therefore, is designed toof meet the requirements of different users, Measuring software consists of display layer, control layer, and data layer. as shown in Figure 8. Measuring software consists of display layer, control layer, and data layer.

Figure 8. Structure chart of the measurement system software.

Figure 8. Structure chart of the measurement system software.

The display layer, locating at the upper layer, is used for displaying the sensor’s measurement The display layer, locating at the upper layer, is used for displaying the sensor’s measurement value of space charge density, location information and working conditions; control layer, locating in value of space charge density, location information and working conditions; control layer, locating the middle layer, is the core layer of the three and has much interaction with the remaining two layers, in the middle layer, is the core layer of the three and has much interaction with the remaining two supplying operations to the users so as to realize the real-time control to the space charge density; layers, supplying operations to the users so as to realize the real-time control to the space charge data layer, locating in the lower layer, realizes the real-time storage of data, in order to facilitate data density; data layer, locating in the lower layer, realizes the real-time storage of data, in order to analysis and processing. Display layer and control layer are composites of interaction surface of human facilitate data analysis processing. Display layer and control layerisare composites of interaction and machine, which isand oriented for the on-site operator, while data layer backed by database, which surface of human and machine, which is oriented for the on-site operator, while data layer is backed is oriented for data analysis personnel. by database, which is oriented for data analysis personnel. Based on the proposed structure, the software interface of the measurement system is developed Based in onFigure the proposed structure, the softwareofinterface ofthe the measurementserial system as shown 9. The software interface composites 4 parts: 1 is communication port, is developed as shown in Figure 9. The software interface composites of 4 parts: 1 is the controlling on or off of the serial port; 2 is the data display window, showing location distribution, communication serial port, controlling of the3 serial port;collection 2 is the interval, data display window, working state and abnormal informationon of or theoff sensors; is the data showing the showing location distribution, working state and abnormal information of the sensors; 3 is the data selection of data acquisition interval; 4 is the software running control part, controlling whether or not collection interval, showing the selection of data acquisition interval; 4 is the software running the software is running.

control part, controlling whether or not the software is running.

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developed as shown in Figure 9. The software interface composites of 4 parts: 1 is the communication serial port, controlling on or off of the serial port; 2 is the data display window, showing location distribution, working state and abnormal information of the sensors; 3 is the data collection interval, showing the selection of data acquisition interval; 4 is the software running Sensors 2016, 16, controlling 1743 8 of 15 control part, whether or not the software is running.

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4. Implementation of of the the Distributed Distributed Measurement Measurement System System 4. Implementation 4.1. Topology of the Wireless Sensor Network 4.1. Topology of the Wireless Sensor Network Data transmission uses the Zigbee wireless transmission mode. In according to the equipment’s Data transmission uses the Zigbee wireless transmission mode. In according to the equipment’s role, three logical device types are defined for the Zigbee specification. They are, respectively, role, three logical device types are defined for the Zigbee specification. They are, respectively, coordinator, router and end device. The coordinator is the center of the network. It is responsible for coordinator, router and end device. The coordinator is the center of the network. It is responsible for establishing and maintaining the network, one and only one in each network; the router is for relay establishing and maintaining the network, one and only one in each network; the router is for relay nodes, routing and forwarding data; and the end device, whose function is unitary, is simply to send nodes, routing and forwarding data; and the end device, whose function is unitary, is simply to send and receive information. and receive information. ZigBee supports three forms of network topology. They are star, tree and mesh topology as shown ZigBee supports three forms of network topology. They are star, tree and mesh topology as in Figure 10. The feature of star topology is that data routers between nodes only have one path; the shown in Figure 10. The feature of star topology is that data routers between nodes only have one feature of mesh topology is that when one node sends data to another node, the information transfer path; the feature of mesh topology is that when one node sends data to another node, the along the tree path up to the nearest coordinator node, and then passed on to the destination node; the information transfer along the tree path up to the nearest coordinator node, and then passed on to feature of tree topology is that it is a special kind of structure, according to the relay transmission of the destination node; the feature of tree topology is that it is a special kind of structure, according to point to point network structure, and its router can be built and maintained automatically, and has the relay transmission of point to point network structure, and its router can be built and maintained strong self-organization and self-healing function. All the nodes in the network composed of Xbee-Pro automatically, and has strong self-organization and self-healing function. All the nodes in the modules are defaulted for the peer. network composed of Xbee-Pro modules are defaulted for the peer.

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Figure Figure 10. 10. Comparison Comparison with with different different topology topology of of the the wireless wireless sensor sensornetwork. network.

The DigiMesh DigiMesh networking networking protocol protocol is is applied applied in in the the wireless wireless sensor sensor network network considering considering the the The complexityofof operating conditions and wireless transmitting distance under UHVDC complexity thethe operating conditions and wireless transmitting distance under UHVDC transmission transmission lines [13,14]. The DigiMesh networkmesh is a simplified network in are which all nodes lines [13,14]. The DigiMesh network is a simplified network inmesh which all nodes collaborative are collaborative and can be either as router or endnode.ofThe of the and can be either as router or endnode. The characteristic the characteristic network making the network expansionmaking of the the expansion of the and network is more flexible in and increases environments the robustness in transmission complicated network is more flexible increases the robustness complicated under environments under transmission lines where routers may fail due to interference or damage. The topology of the wireless sensor network is shown in Figure 10. 4.2. Communication Quality Evaluation In the course of actual use, there is a condition that the closest node from the main node to help

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lines where routers may fail due to interference or damage. The topology of the wireless sensor network is shown in Figure 10. 4.2. Communication Quality Evaluation In the course of actual use, there is a condition that the closest node from the main node to help remote nodes to transmit data. The power consumption of these nodes is bound to be a few bigger than others and it will lead to system work time shorter. In order to prolong the lifetime of the whole network, the set of differentiation must be considered. There are many basic parameters [15–19] used to determine the performance of wireless network system. Packet reception ratio (PRR) is the most intuitionistic communication quality indicators; received signal strength indicator (RSSI) can be used to determine the communication quality; link quality indicator (LQI) shows the energy and quality of the receiving data. PRR, however, needs a long time to send and receive experiments to obtain the data. RSSI and LQI are relatively easier to obtain, and can commutative conversion. The conversion relationship is RSSI = −(81 − ( LQI ∗ 91)/255)

(5)

RSSI value can be read when each data frame is received in the transceiver module of Zigbee, so RSSI data can be more easily used as link quality criteria. However, RSSI is the value difference between actual receiving signal intensity and optimal received power level. That means the higher the absolute value of RSSI is, the stronger the signal decrease is. 5. Validation and Experiment 5.1. Transmitted Power Consumption Measurement Analysis Xbee-Pro module support five different emission levels with different operation mode, they are 10 dBm, 12 dBm, 14 dBm, 16 dBm, 18 dBm respectively. To know the relationship between energy consumption and transmission power of the proposed wireless communication unit, a research on energy consumption situation with five different transmission power settings is proceeded under a double circuit experimental line on the same tower in Beijing. The complicated electromagnetic environment will have an impact on the wireless network communication quality. Therefore, the data on the relationship between power consumption and transmission power of the proposed wireless communication unit were obtained in a complicated environment as shown in Figure 11; PRR and RSSI are not fixed values but fluctuating in a certain range, which therefore cannot be replaced in the test by some instantaneous values, putting them in the category of error connecting performance. In the complicated electromagnetic environment under UHVDC transmission line, Zigbee wireless communication module can ensure the quality of good communication in 30 m with 10 dBm transmitted power, in 40 m with 12 dBm transmitted power, in 50 m with 14 dBm transmitted power, in 70 m with 16 dBm transmitted power, in 80 m with 18 dBm transmitted power respectively. PRR data from Figure 11 show high transmission power, which to some extent corresponds to good quality of communication. The data can be used as a reference of the transmission power for space charge density measurement system in a complicated electromagnetic environment.

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The complicated electromagnetic environment will have an impact on the wireless network communication quality. Therefore, the data on the relationship between power consumption and transmission power of the proposed wireless communication unit were obtained in a complicated environment as shown in Figure 11; PRR and RSSI are not fixed values but fluctuating in a certain range, which cannot be replaced in the test by some instantaneous values, putting them Sensors 2016, 16,therefore 1743 10 ofin 15 the category of error connecting performance.

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In the complicated electromagnetic environment under UHVDC transmission line, Zigbee 5.2. Complex Environment Test wireless communication module can ensure the quality of good communication in 30 m with When corona discharge occurs the UHVDC line, theinwireless signal14may be 10 dBm transmitted power, in 40 min with 12 dBm transmission transmitted power, 50 m with dBm affected. The corona hightransmitted voltage lines is notincontinuous. The testtransmitted with an ultraviolet transmitted power, indischarge 70 m withfrom 16 dBm power, 80 m with 18 dBm power imager2016, has16,been in Figure This wireless interferencepower, mainlywhich comestofrom Sensors 1743 shown 10 of 14 respectively. PRR data from Figure 12. 11 show high transmission somenegative extent transmission to line. Therefore, comparison test is taken thebetwo conditions of on-power and corresponds good qualitytheofRSSI communication. The data in can used as a reference of the and off-power for the UHVDC transmission line. There are 11 optional channels in the model off-power for the UHVDC transmission line. There are 11 optional channels in the model provided to transmission power for space charge density measurement system in a complicated electromagnetic provided to the users as Table 1 shows. RSSI on different transmission power within 100 m supplied the users as Table 1 shows. RSSI on different transmission power within 100 m supplied by RF module environment. by RF willasbe measured as shown in Figures 13 and 14. will bemodule measured shown in Figures 13 and 14. 5.2. Complex Environment Test When corona discharge occurs in the UHVDC transmission line, the wireless signal may be affected. The corona discharge from high voltage lines is not continuous. The test with an ultraviolet imager has been shown in Figure 12. This wireless interference mainly comes from negative transmission line. Therefore, the RSSI comparison test is taken in the two conditions of on-power

Figure 12. Corona discharge test.

Wireless communication unit child nodes channels were placed under the negative transmission line. Table 1. 11 frequency of the RF module. From Figure 13, the minimum value of RSSI is −35 dBm and the biggest value is −70 dBm in the Channel Number 1 2the minimum 3 4 value5of RSSI 6 is −47 7 dBm 8and the 9 biggest 10 value 11 is off-power state. Correspondingly, −78Center dBm Frequency/GHz in the on-power state 14. Compared Figures 13 and 14, when distance 10 m, 2.410in Figure 2.415 2.420 2.425 2.430 2.435 2.440 2.445 test 2.450 2.455 is2.460 the value of RSSI is −35 dBm~−42 dBm in the off-power state and −47 dBm~−60 dBm in the on-power state. Influenced by the environment, part of the transmitted power communication quality become worse with the increase of distance, gradually leading to the RSSI data quantity received being too little with distances greater than 60 m. They are not representative, so the value can be negligible. By contrast, we found that the complex electromagnetic environment will produce certain effects on the

state. Influenced by the environment, part of the transmitted power communication quality become worse with the increase of distance, gradually leading to the RSSI data quantity received being too little with distances greater than 60 m. They are not representative, so the value can be negligible. By contrast, we found that the complex electromagnetic environment will produce certain effects on the Sensors 2016, 16, 1743 11 of 15 wireless communication unit. -30

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5.3. Wireless Network Reliability Analysis WirelessCommunication communication unit child nodes were placed under the negative transmission line. FromXbee-Pro Figure 13, the minimum valueperformance of RSSI is −35 and work the biggest value is − 70 instances dBm in the module has different in dBm different environmenta. 200 of off-power state. Correspondingly, the minimum value of RSSI is − 47 dBm and the biggest value RSSI and PRR collection are done in each test. The regularity of distribution has been described by a is −78 dBmcurve in thedefined on-power in Figure 14. Compared Figures andequilibrium 14, when test distance is sigmoidal by astate lag phase, subsequent growth, and a13 final phase. It is in 10 m, the value RSSI is −35 dBm~−42 law. dBmThe in the off-power state in and −47 dBm~ conformity withofBoltzmann distribution result is as shown Figure 15. −60 dBm in the on-power state. Influenced by the environment, part of the transmitted power communication quality become worse with the increase of distance, gradually leading to the RSSI data quantity received being 1.0 too little with distances greater than 60 m. They are not representative, so the value can be negligible. By contrast, we found that the complex electromagnetic environment will produce certain effects on 0.8 the wireless communication unit. Wireless interference field in UHVDC transmission line has a strong intensity in the low frequency 0.6 stage. It decreases quickly with the increasing frequency. When the frequency is over 10 MHz, interference strength can be neglected. Generally, the wireless interference field intensity that occurred in the corona discharge in0.4UHVDC line is considered 30 MHz at most. The frequency range of this system is around 2.4 GHz, and thus corona discharge in the UHVDC transmission line does not affect the normal work of this system. 0.2 PRR Boltzmann Fit of PRR 0.0 -100

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Figure 14. RSSI of 11 channels in the on-power state.

5.3. 5.3. Wireless Wireless Communication Communication Network Network Reliability Reliability Analysis Analysis Xbee-Pro performance in different work work environmenta. environmenta. 200 Xbee-Pro module module has has different different performance in different 200 instances instances of of RSSI and PRR collection are done in each test. The regularity of distribution has been described byby a RSSI and PRR collection are done in each test. The regularity of distribution has been described sigmoidal curve a sigmoidal curvedefined definedbybya alag lagphase, phase,subsequent subsequentgrowth, growth,and andaafinal final equilibrium equilibrium phase. phase. ItIt is is in in conformity with Boltzmann distribution law. The result is as shown in Figure 15. conformity with Boltzmann distribution law. The result is as shown in Figure 15. 1.0

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From Figure 15, when the value is greater than −60 dBm, equipment in the complex work From Figure 15, when the value is greater than −60 dBm, equipment in the complex work environment can maintain stable communications, and the data receiving rate is above 90%. environment can maintain stable communications, and the data receiving rate is above 90%. 5.4. under UHVDC UHVDC Transmission Transmission Line Line 5.4. Space Space Charge Charge Density Density Measurement Measurement Test Test under To validate and thethe space charge density of 11 To2016, validate and test testthe theproposed proposedwireless wirelessmeasurement measurementsystem, system, space charge density of14 Sensors 16, 1743 12 of different points under the experimental lines was measured. The transmission line is 100 m long. 11 different points under the experimental lines was measured. The transmission line is 100 m long. The Theminimum minimumdistance distancefrom fromthe theground groundtotoconductor conductorisis7 7mmand andthe theseparation separationdistance distancebetween betweenthe the positive m. The The origin origin of ofthe themeasurement measurementresult resultcoordinate coordinatewas wasset setat positivepole poleand andthe thenegative negativepole poleisis66m. atthe thecentral centralposition positionof ofthe the positive positive pole pole and the negative negative pole. pole. The The wireless wirelesssensors sensorswere werearrayed arrayed along alongthe thedirection directionperpendicular perpendiculartotothe thetransmission transmissionlines lineswith withthe theseparation separationfrom fromthe theorigin originatat −−15 15 m, 12 m, − m, −6 −6m, m,−3 −3m, m,00m, m,33 m, m, 66 m, 9 m, m,− −12 −99 m, m, 12 12 m, m,15 15m. m.The Thearrangement arrangementdiagram diagramofofthe the measurement system is shown in Figure 16 and the photo of test base and measurement system measurement system is shown in Figure 16 and the photo of test base and measurement systemisis shown shownininFigure Figure17. 17.

Figure Figure16. 16.Arrangement Arrangementdiagram diagramofofmeasurement measurementsystem. system.

The voltage applied to the positive pole was 300 kV, 250 kV, and 200 kV. Correspondingly, the voltage applied to the negative pole was −300 kV, −250 kV and −200 kV. Other test conditions are the same as above. One of the measurement results is shown in Figure 18.

Figure 16. Arrangement diagram of measurement system.

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Figure 16. Arrangement diagram of measurement system.

Figure 17. Picture of measurement system used under UHVDC transmission line.

The voltage applied to the positive pole was 300 kV, 250 kV, and 200 kV. Correspondingly, the voltage applied to the negative pole was −300 kV, −250 kVand −200 kV. Other test conditions are the Figure 17. Picture of system used under under UHVDC UHVDC transmission line. Figure Picture of measurement measurement system used same as above. One 17. of the measurement results is shown in Figure 18. transmission line. The voltage applied to 30the positive pole was 300 kV, 250 kV, and 200 kV. Correspondingly, the 300kV voltage applied to the negative pole was −300 kV, −250 kVand −200 kV. Other test conditions are the 250kV same as above. One of the 20 measurement results is shown in Figure 18. 200kV

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As the Figure 18 shown, with the increase ofbipolar distance Distance from centerfrom (m) the transmission lines, the space As the Figure 18 shown, with the increase of distance from the transmission lines, the space charge charge density value decreases and the maximum is located at the vertical projection of the Lateral distribution test resultsis oflocated space charge density under different of voltage level. densityFigure value18. decreases and the maximum at the vertical projection the transmission transmission lines. The space charge density under UHVDC transmission lines is enhanced with the lines. The space charge density under UHVDC transmission lines is enhanced with the increase of the As voltage the Figure 18The shown, withvalue the increase of distance from the transmission lines, the space applied level. absolute of the space charge density under the negative conductor is chargethan density value decreases and conductor. the maximum is results locatedofat vertical of the larger the corresponding positive The test thethe space chargeprojection density basically transmission lines. space charge density UHVDC transmission lines is enhanced with the consistent with the The theoretical values, whichunder demonstrate that the proposed wireless measurement system can accurately measure the space charge density under transmission lines. Compared with previous studies such as the literatures [20–23], the conclusions from Figure 18, have the same change rule with them. Meanwhile, based on the wireless communication technologies and the experimental results the proposed sensor demonstrates that it can adapt to the complex electromagnetic environment under the UHVDC transmission line and can accomplish the accurate, flexible, and stable demands of the space charge density measurement. 6. Conclusions In this paper, a wireless measurement system which is used to measure the distributed space charge density under UHVDC transmission lines for fulfilling the demands of flexibility, convenience and reliability has been developed. The measurement system which is proposed comprises many wireless sensors, data centers and servers as well as a base station. A space charge density sensor based on the ion counter method is presented. An ARM microprocessor and Zigbee RF module are applied to design the wireless sensor. The use of Digimesh networking protocol makes the WSN more flexible and increases the robustness in the complex UHVDC electromagnetic environment. The implementation and test results under UHVDC transmission lines demonstrate that the proposed

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wireless measurement system can adapt to the complex electromagnetic environment and accurately collect space charge density data. Acknowledgments: This research was supported by National Natural Science Foundation of China (61273165) & State Grid Corporation of China (GYB17201400178). Author Contributions: Haiwen Yuan guides research and development of the space charge density measurement system. Encheng Xin and Haiwen Yuan conceived and designed the experiments; Encheng Xin performed the experiments; Encheng Xin, Haiwen Yuan and Yong Ju analyzed the data; Yong Ju contributed test platform and test data; Encheng Xin wrote the paper. In addition, thank Yuekui Zhang for the great effort for the preliminary work of space charge density measurement system. Conflicts of Interest: The authors declare no conflict of interest.

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