sensors Article

Influence and Correction from the Human Body on the Measurement of a Power-Frequency Electric Field Sensor Dongping Xiao *, Huaitong Liu, Qiang Zhou, Yutong Xie and Qichao Ma State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Chongqing 400044, China; [email protected] (H.L.); [email protected] (Q.Z.); [email protected] (Y.X.); [email protected] (Q.M.) * Correspondence: [email protected]; Tel.: +86-131-0896-3250 Academic Editor: Debbie G. Senesky Received: 14 March 2016; Accepted: 3 June 2016; Published: 10 June 2016

Abstract: According to the operating specifications of existing electric field measuring instruments, measuring technicians must be located far from the instruments to eliminate the influence of the human body occupancy on a spatial electric field. Nevertheless, in order to develop a portable safety protection instrument with an effective electric field warning function for working staff in a high-voltage environment, it is necessary to study the influence of an approaching human body on the measurement of an electric field and to correct the measurement results. A single-shaft electric field measuring instrument called the Type LP-2000, which was developed by our research team, is used as the research object in this study. First, we explain the principle of electric field measurement and describe the capacitance effect produced by the human body. Through a theoretical analysis, we show that the measured electric field value decreases as a human body approaches. Their relationship is linearly proportional. Then, the ratio is identified as a correction coefficient to correct for the influence of human body proximity. The conclusion drawn from the theoretical analysis is proved via simulation. The correction coefficient kb = 1.8010 is obtained on the basis of the linear fitting of simulated data. Finally, a physical experiment is performed. When no human is present, we compare the results from the Type LP-2000 measured with Narda EFA-300 and the simulated value to verify the accuracy of the Type LP-2000. For the case of an approaching human body, the correction coefficient kb * = 1.9094 is obtained by comparing the data measured with the Type LP-2000 to the simulated value. The correction coefficient obtained from the experiment (i.e., kb *) is highly consistent with that obtained from the simulation (i.e., kb ). Two experimental programs are set; under these programs, the excitation voltages and distance measuring points are regulated to produce different electric field intensities. Using kb = 1.9094, the corrected measurement of electric field intensity can accurately reflect the original environmental electric field intensity, and the maximal error is less than 6% in all the data comparisons. These results verify the effectiveness of our proposed method. Keywords: power-frequency electric field; portable measurement; human body influence; correction coefficient; linear fitting

1. Introduction Strong electric fields exist around high-voltage electrical equipment. Existing research shows that strong power-frequency electric fields have potential harmful effects on human health and safety [1]. Therefore, relevant institutions and international organizations have developed standards or have recommended limitations regarding power-frequency electric fields for the general public and power system practitioners, as shown in Table 1 [2–6]. Here, the spatial electric field in the absence of any object is considered. Sensors 2016, 16, 859; doi:10.3390/s16060859

www.mdpi.com/journal/sensors

Sensors 2016, 16, 859

2 of 14

Table 1. Exposure limits regarding power-frequency electric fields developed by relevant institutions and international organizations. Name

Publish Time

E (kV/m)

Frequency (Hz)

Occupational Exposure

Public Exposure

ICNIRP 1

2010

50 60

10 8.3

5 4.2

IEEE 2

2002

50

20

5

NRPB 3

2004

50 60

10 8.3

5 4.2

2004

50 60

10 8.3

—

1999

50 60

—

5 4.2

EU

4

1. ICNIRP——International Commission on Non-Ionizing Radiation Protection. 2. IEEE——Institute of Electrical and Electronics Engineers. 3. NRPB——National Resources Planning Board. 4. EU——European Union.

However, power system practitioners are likely to be exposed to strong electric fields in a short amount of time because of their handling of high-voltage electrical equipment. Excessively strong electric field intensities result in discomfort and panic for practitioners, and such effects are harmful to personal safety and can lead to disoperation [7–9]. If practitioners can be equipped with a portable safety protection instrument that can measure the electric field intensity of a working area in real time and issue a safety precaution, then the safety and health of practitioners in the workplace can be ensured. Our group has been working toward this objective. Several power-frequency electric field measuring instruments are available for commercial purposes [10]; they include the Narda EF series of Germany, the PMM of Italy, and the CA of France. In addition, many research teams have studied and developed electric field measuring instruments for special purposes [11–13]. Most current electric field measuring instruments require bracing the probe with an insulated bracket; they also require measuring technicians to be far away from the probe to avoid the influence of human body proximity on measurements [14]. However, the electric field warning devices developed by our team need to be carried by working practitioners, who thus become inevitably involved and exert a potential effect on measurements. Electric field intensities measured in real time are used as the main index of safety precaution. Meanwhile, the original environmental electric field intensity subjected to exposure limits and the influence of the human body during measurement are interesting topics that are worth exploring. The influence of the human body on electric field measurement has attracted increasing attention in recent decades. In [15], the existence of a human body was proposed to make the spatial electric field change in a certain range. In [16], the changes in the electric field distribution on the different parts of the human body when the body was isolated from the ground were demonstrated. In [17], the proportional relation between undistorted electric field and distorted electric field at certain points on the surface of a human body model was presented. However, these studies only showed that human body occupancy leads to the distortion of spatial electric fields; they did not focus on the influence of human body proximity on electric field measurement and the ways to correct measured results. In the present work, we build an equivalent circuit that explains the principle of electric field measurement on the basis of a single-shaft electric field measuring instrument called a Type LP-2000 [8], which was independently developed by our research team. Then, simulation via Ansoft/Maxwell is conducted. The influence of the human body on the electric field measuring sensor of a plate capacitor is discussed theoretically, followed by a proposal of a correction coefficient for the influence of the human body on electric field measurements. In the laboratory, the electric field generated by different excitation voltages in the absence of a human body is measured with a Type LP-2000 and Narda

Sensors 2016, 16, 859

3 of 14

EFA-300. The numerical comparison and error analysis of the measured data and simulated data verify a Sensors the 2016,correct 16, 859 measurement of the Type LP-2000. Thereafter, electric fields are measured with 3 of 13 portable Type LP-2000 that is fixed on an individual’s arm under two sets of experimental programs. The corrected electric field values obtained usingthe thereal-time real-timemeasured measured values values multiplied by corrected electric field values obtained bybyusing by the the correction coefficient are compared with the original electric field in the simulation environment; the correction coefficient are compared with the original electric field in the simulation environment; the errors errorsare aresubsequently subsequentlyanalyzed. analyzed. 2.2. Equivalent Equivalent Circuit Circuit Analysis Analysis of of the the Influence Influence of of the the Human Human Body Body on on Electric Field Measurement Electric Field Measurement 2.1. In the Absence of a Human Body 2.1. In the Absence of a Human Body The core of Type LP-2000 comprises a couple of metal polar plates that can be seen as a capacitor. The core of Type LP-2000 comprises a couple of metal polar plates that can be seen as a capacitor. Inductive charges with the same frequency appear on the metal plates when the measuring sensor is Inductive charges with the same frequency appear on the metal plates when the measuring sensor is put into the alternating electric field. Then, the induced voltage can be measured. The relation between put into the alternating electric field. Then, the induced voltage can be measured. The relation the induced voltage and the electric field intensity at the measuring point is linearly proportional when between the induced voltage and the electric field intensity at the measuring point is linearly the physical dimension of the planar plate capacitor is sufficiently small [18]. proportional when the physical dimension of the planar plate capacitor is sufficiently small [18]. .

.

UU E C C“= D DE

(1) (1)

.

.

where distance between two polar plates andand U C and are the induced voltage and electric whereDDisisthe the distance between two polar plates E are the induced voltage and U C Eand field intensity in phasor form respectively. electric field intensity in phasor form respectively. The the measuring measuring system system for forobtaining obtainingthe theinduced inducedvoltage voltageis isshown shown The equivalent equivalent circuit circuit of of the in in Figure 1. Figure 1.

+ 1 jωC

1 jωCM

•

Ri

Ui

•

UC

Figure 1. Equivalent circuit of the measuring system. Figure 1. Equivalent circuit of the measuring system. •

In Figure 1, the induced voltage is equivalent to a voltage source . U C with angular frequency ω, In Figure 1, the induced voltage is equivalent to a voltage source UC with angular frequency ω, C M is the capacitance of the measuring capacitor, C is the inherent capacitance of the plate capacitor, C is the inherent capacitance of the plate capacitor, CM is the capacitance of the measuring capacitor, Ri is • . the input resistance of measuring the measuring circuit, isoutput the output voltage. Therefore, Ri isinput the resistance of the circuit, andand Ui isUthe voltage. Therefore, i . • jωRiipC (CM +C 11`+ jωR Cq) .• M ` U ii C = UU C “ jωR jωRiiCC .

(2) (2) •

By circuit, thethe RMS value Uirms of U obtained. The RMS crms Byfurther furtherprocessing processingthe the circuit, RMS value Uirms ofi can be obtained. Thevalue RMS U value U i becan of UC is: • b Ucrms of U C is: 1 ` ω2 Ri 2 pC ` C M q 2 UCrms “ Uirms (3) 1 + ω2ωR Ri 2 (i C C + C M )2 (3) U C rms = U i rms .

ωRi C

By combined Equations (1) and (3), we obtain the RMS value Erms of the electric field, that is,

Erms =

1 + ω2 Ri 2 (C + C M ) 2 DωRi C

U i rms

(4)

Sensors 2016, 16, 859

4 of 14

By combined Equations (1) and (3), we obtain the RMS value Erms of the electric field, that is, b Sensors 2016, 16, 859

Erms “

1 ` ω2 Ri 2 pC ` C M q 2 DωRi C

Uirms

4 of(4) 13

In In the the measurement measurement circuit, circuit, the the measuring measuring capacitance capacitance is is at at the the nF-level nF-level and and is is much much larger larger than than the inherent capacitance of the sensor, i.e., C M >> C; thus, Equation (4) can be simplified to the inherent capacitance of the sensor, i.e., C >> C; thus, Equation (4) can be simplified to M

E

rms Erms

2 1b+1ω R2 iR2C2 CM 22 ` ω i M U i rms =“ DDωR ωRi Ci C Uirms

(5) (5)

“ KUirms

= KU i rms where K K is is aa ratio ratio coefficient coefficient written written as as where

b 2 22 11 + `ωω2 2RR2iC CMM

KK“=

i

DωR DωR iCC

(6) (6)

i

In Equation Equation (5), (5), ω, ω,RRi,i ,C, C,CCMM, and , andDDare areall allfixed fixedvalues; values;thus, thus,KKisisaaconstant. constant.When Whenthe theparameters parameters In of the the measuring measuring circuit circuit are are all all fixed, fixed, the the electric electric field field intensity intensity in in the the location location of of the the parallel-plate parallel-plate of capacitive sensor is proportional to the output voltage of the sensor. The electric field intensity Erms of capacitive sensor is proportional to the output voltage of the sensor. The electric field intensity Erms the observation point can be obtained by measuring U . of the observation point can be obtained by measuringirms Uirms. 2.2. In In the the Case Case of of an an Approaching Approaching Human Human Body Body 2.2. In practical practical measurements, measurements, when when an an individual individual approaches approaches the the sensors, sensors, the the measured measured electric electric In field intensity changes because the human body possesses complex tissues. The relative dielectric field intensity changes because the human body possesses complex tissues. The relative dielectric 5 6 constant of of living is 5–10,6and constant living tissue tissue in in aapower—frequency power—frequencyenvironment environmentisisabout about1010–10 , andthe theconductivity conductivity about 0.1 S/m [19]. The human body can be regarded as a conductor; therefore, charges gather on its is about 0.1 S/m [19]. The human body can be regarded as a conductor; therefore, charges gather on surface as as it remains in the electric field, andand such accumulation affects the the spatial distribution of the its surface it remains in the electric field, such accumulation affects spatial distribution of original electric fieldfield [20].[20]. the original electric The relationship relationship between between the the human human body body and and the the measuring measuring sensor sensor of of the the plate plate capacitor capacitor is is The shown in in Figure Figure 2. 2. shown

Body Polar A

PolarB

d1 C1

D C C2

Figure 2. Relationship between the human body and the polar plates of the capacitor. Figure 2. Relationship between the human body and the polar plates of the capacitor.

In Figure 2, PolarA and PolarB are the two polar plates of the capacitor, and D is the distance In Figure 2, plates, PolarAwith and the Polar thewith twoepoxy polar plates of the capacitor, and D the distance B are between the two gap filled resin. The distance between theishuman body between the two plates, with the gap filled with epoxy resin. The distance between the human body and PolarB is d1, with an air medium in the middle. The body with PolarA and body with PolarB form and Polar is d , with an air medium in the middle. The body with Polar and body with Polar B 1 B form the equivalent capacitors, C1 and C2, respectively. The equivalent circuitAof the measuring system in the case equivalent capacitors, Chuman The equivalent circuit of the measuring system in 1 and Cbody 2 , respectively. the of an approaching is shown in Figure 3. the case of an approaching bodylarger is shown According to Figure 3, human CM is much thaninC Figure and C1;3.therefore,

Erms

1 + ω 2 Ri 2 (C M +C2 ) 2 * = U i rms CC DωRi ( 1 ) C1 + C = K bU i*rms

(7)

Sensors 2016, 16, 859

5 of 13

Kb = Sensors 2016, 16, 859

1 + ω 2 Ri 2 (C M +C2 ) 2 CC DωRi ( 1 ) C1 + C

(8) 5 of 14

+ 1 jωC1 1 jωC

1 jωC2

1 jωCM

Ri U i*

•

UC

Figure 3. Equivalent circuit of the measuring system in the case of an approaching human body. Figure 3. Equivalent circuit of the measuring system in the case of an approaching human body.

Equation (7) shows that when the measuring parameters are fixed, Kb is a constant, and the According to Figure 3, Cremains larger than to C and C1 ; therefore, M is much output voltage of the sensor proportional the electric field at the measuring point. By comparing Equations (5) and (7), we b observe that CM + C2 > CM and (C1C)/(C1 + C) < C1, Kb > K. 2 1 `ω 2 R i 2 p C M 2q Moreover, we note that Ui*rms < UEirms if E“ rms is constant. In` Cother the measured induced voltage Ui˚words, rms rms C1 C (7) DωR p q i C1 `C field measuring sensor. Without correction, decreases when a human body approaches the electric ˚ “ Kb Usmaller the measured electric field intensity becomes than the physical truth. i rms In sum, the correction coefficient kb can be defined as where Kb is also a ratio coefficient written as b k = Kb b 2 pC `C q2 1 ` ω2 RiK 2 M

(9)

(8) Kb “ C1 C Thus, the spatial electric field intensity in DωR the absence i p C1 `C qof a human body can be obtained by using the electric field intensity measured with the portable Type LP-2000 in real time multiplied by the Equation (7) shows correction coefficient kb.that when the measuring parameters are fixed, Kb is a constant, and the output voltage of the sensor remains proportional to the electric field at the measuring point. By comparing Equations (5) and (7), we observe that CM + C2 > CM and (C1 C)/(C1 + C) < C1 , Kb > K. 3. Simulation Analysis Moreover, we note that Ui *rms < Uirms if Erms is constant. In other words, the measured induced voltage decreases whenModel a human body approaches the electric field measuring sensor. Without correction, the 3.1. Simulation Setting measured electric field intensity becomes smaller than the physical truth. The simulation model is set up with the software Ansoft/Maxwell, and most of the simulation In sum, the correction coefficient kb can be defined as parameters are set on the basis of the physical experiment condition shown in Section 4. Different electric field intensities are producedKwhen different levels of sinusoidal voltage with kb “ b (9) power-frequency are applied to the power transmission line. In the simulation model, the power K transmission line is a copper conductor with a diameter of 14 mm and a distance of 122 cm from the Thus, the spatial electric field intensity in the absence of a human body can be obtained by using ground. Observation point P is at the same level as the conductor and at 53 cm from the conductor in the electric field intensity measured with the portable Type LP-2000 in real time multiplied by the the horizontal distance. correction coefficient kb . The simulation model of the plate capacitor is shown in Figure 4. The two polar plates are made of copper, and Analysis measure 50 mm in length, 36 mm in width, and 1 mm in distance. The filling medium 3. Simulation is epoxy resin. The center of the sensor is coincident with the observation point P. In addition, theSetting condition of the bottom boundary is set as grounding, and the boundary 3.1. Simulation Model conditions of the other five surfaces are set as the balloon boundary with the electrical potential set The simulation model is set up with the software Ansoft/Maxwell, and most of the simulation to zero at infinity during the simulating. parameters are set on the basis of the physical experiment condition shown in Section 4. Different electric field intensities are produced when different levels of sinusoidal voltage with power-frequency are applied to the power transmission line. In the simulation model, the power transmission line is a copper conductor with a diameter of 14 mm and a distance of 122 cm from the ground. Observation point P is at the same level as the conductor and at 53 cm from the conductor in the horizontal distance.

Sensors 2016, 16, 859

6 of 14

The simulation model of the plate capacitor is shown in Figure 4. The two polar plates are made of copper, and measure 50 mm in length, 36 mm in width, and 1 mm in distance. The filling medium is epoxy resin. The center of the sensor is coincident with the observation point P. Sensors 2016, 16, 859

6 of 13

Copper

Sensors 2016, 16, 859

6 of 13

Copper

Copper Epoxy Copper Figure 4. Simulation model of measuring sensor. Figure 4. Simulation model of measuring sensor.

3.2. Simulation Results and Analysis in the Absence of a HumanEpoxy Body In addition, the condition of the bottom boundary is set as grounding, and the boundary Figureof5 the shows thefive simulated the electric field spatial distribution surrounding the conditions other surfacesresults are setofas the balloon boundary with the electrical potential observation point Pduring before and placing the ofmeasuring sensor under the same excitation Figure 4.after Simulation model measuring sensor. set to zero at infinity the simulating. voltage conditions. 3.2.Simulation SimulationResults Resultsand andAnalysis Analysisininthe theAbsence AbsenceofofaaHuman HumanBody Body 3.2. Figure55shows showsthe thesimulated simulated results electric spatial distribution surrounding Figure results of of thethe electric fieldfield spatial distribution surrounding the the observation P before after placing measuring sensorunder underthe thesame same excitation excitation observation pointpoint P before andand after placing thethe measuring sensor voltageconditions. conditions. voltage

P

P

P

P (a)

(b)

Figure 5. Electric field spatial distribution surrounding observation point P (no influence of a human body). (a) Before placing the sensor; (b) after placing the sensor.

Figure 5 shows that point P is evenly (a) the original electric field surrounding the observation (b) distributed. After the placement of the sensor, the electric field at the four corners of the sensor Figure 5.5.Electric spatial surrounding observation point PP(no influence ofofaahuman become significantly distorted and enhanced, whereas the surface electric outside of the polar Figure Electricfield field spatialdistribution distribution surrounding observation point (nofield influence human body). (a) Before placing the sensor; (b) after placing the sensor. plates is slightly enhanced [21,22]. The field the at observation point P is still even, whereas the body). (a) Before placing the sensor; (b)electric after placing sensor. electric field intensity decreases relative to the original value because of the shielding effect of the Figure 5 shows that the original electric field surrounding the observation point P is evenly polar plates. Figure 5 shows that the original electric field surrounding the observation point P is evenly distributed. the placement ofresults the sensor, the electric field field at the 0four of theErms sensor Table 2After shows simulated of the original electric and corners electric field after distributed. After thethe placement of the sensor, the electric field at the fourEcorners of the sensor become become significantly distorted and enhanced, whereas the surface electric field outside of the polar placing the measuring sensor at point whereas P when different levers of excitation voltage arepolar generated. significantly distorted and enhanced, the surface electric field outside of the plates is plates is slightly enhanced [21,22]. The electric field at observation point P is still even, whereas the slightly enhanced [21,22]. The electric field at observation point P is still even, whereas the electric electric field2.intensity decreases relativevalues to the original valueand because of thethe shielding effect of the Table Comparison of electric point because P before after placing measuring sensor field intensity decreases relative to field the originalatvalue of the shielding effect of the polar plates. polar plates. (driven by different voltages). Table 2 shows the simulated results of the original electric field E0 and electric field Erms after Table 2 shows the simulated results of the original electric field E0 and electric field Erms after placing measuring at point P12when different levers voltage are22generated. Usthe (kV) 8 sensor 10 14 16 of excitation 18 20 24 placing the measuring sensor at point P when different levers of excitation voltage are generated. E0 (kV/m) 2.5491 3.1865 3.8237 4.4610 5.0984 5.7357 6.3732 7.0103 7.6476 Erms (kV/m) 0.7852 0.9816 1.1779 1.3742 1.5705 1.7668 1.9631 2.1594 2.3557 Table 2. Comparison of electric field values at point P before and after placing the measuring sensor (driven by different voltages).

The data in Table 2 are linearly fitted, as shown in Figure 6.

Us (kV)

8

10

12

14

16

18

20

22

24

Sensors 2016, 16, 859

7 of 14

Table 2. Comparison of electric field values at point P before and after placing the measuring sensor (driven by different voltages). U s (kV)

8

10

12

14

16

18

20

22

24

E0 (kV/m) Erms (kV/m)

2.5491 0.7852

3.1865 0.9816

3.8237 1.1779

4.4610 1.3742

5.0984 1.5705

5.7357 1.7668

6.3732 1.9631

7.0103 2.1594

7.6476 2.3557

The data in Table 2 are linearly fitted, as shown in Figure 6.

Sensors 2016, 16, 859

7 of 13

8 7 6

E0 (kV/m)

Sensors 2016, 16, 859

7 of 13 5 8

4

7

3

E0 (kV/m)

6

20.65 0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

Erms (kV/m) 4

Figure 6. Relation curve of the electric field values at point P before and after placing the 3 Figure 6. Relation curve of the electric field values at point P before and after placing the measuring sensor.

measuring sensor.

2 0.6

0.8

1

1.2

E

1.4

1.6

(kV/m)

1.8

2

2.2

2.4

rms Figure 6 shows that a linear relation exists between Erms and E0. Their relation can be expressed Figure 6. Relation curve of the electric field values at point P before and after placing the mathematically Figure 6 showsasthat a linear relation exists between Erms and E0 . Their relation can be expressed

measuring sensor.

mathematically as

E0 = k0 Erms

Figure 6 shows that a linear relation exists E0 “between k0 ErmsErms and E0. Their relation can be expressed coefficient between E0 and Erms. Here, k0 = 3.2464. wheremathematically k0 is the correction as

where k0 is the correction coefficient between E0 and Erms . Here, k0 = 3.2464. E0 = k0 Erms 3.3. Simulation Result and Analysis in the Case of an Approaching Human Body

(10)

(10)

(10)

3.3. Simulation Analysis in the between Case of Ean Approaching correction coefficient 0 and Erms. Here, k0Human = 3.2464.Body whereResult k0 is theand

We assume that the height of the approaching individual is 176 cm. Given that the individual’s shoes are thickness of the theininsulating is assumed to be 2 mm during thethe simulation We assume that the the height of approaching individual is 176 cm. Given that individual’s 3.3. isolated, Simulation Result and Analysis the Case of materials an Approaching Human Body so that the person does not directly come into contact with the ground. The measuring sensor issimulation fixed shoes are isolated, the thickness of the insulating materials is assumed to be 2 mm during the We assume that the height of the approaching individual is 176 cm. Given that the individual’s on the the person’s arm, and its center come is located point P, with similar to ground. that shown in measuring Figure 5. so that person does not directly intoatcontact the The sensor is fixed shoes are isolated, the thickness of the insulating materials is assumed to be 2 mm during the simulation Figure 7a shows the electric field distribution surrounding the human body and conductor. so that the person not directly come into the ground. The shown measuring is fixed on the person’s arm, anddoes its center is located atcontact pointwith P, similar to that insensor Figure 5. Figureon7bthe shows thearm, partially detail at surrounding thetomeasuring sensor. person’s and itsenlarged center is located point P, similar that shown in Figure 5.

Figure 7a shows the electric field distribution surrounding the human body and conductor. Figure 7a shows the electric field distribution surrounding the human body and conductor. Figure 7b Figure shows7bthe partially enlarged detail surrounding the measuring sensor. shows the partially enlarged detail surrounding the measuring sensor.

(a) (a)

(b)(b)

FigureFigure 7. Electric fieldfield distribution in in the case human body. Surrounding human 7. Electric distribution the caseofofan anapproaching approaching human body. (a) (a) Surrounding human

Figure 7. Electric field distribution in the case of ansensor. approaching human body. (a) Surrounding human bodyconductor; and conductor; (b) surrounding measuringsensor. body and (b) surrounding measuring body and conductor; (b) surrounding measuring sensor. 3. Comparison of measured electricfield fieldvalues values at and after the the approach of a of a Table Table 3. Comparison of measured electric at point pointPPbefore before and after approach human body (driven by different voltages). human body (driven by different voltages). Us (kV)

8

10

12

14

8 0.7852 100.9816 12 14 Us (kV) Erms (kV/m) 1.1779 1.3742 Erms (kV/m) E*rms (kV/m)0.7852 0.4360 0.9816 0.5450 1.1779 0.6540 1.3742 0.7630 E*rms (kV/m) 0.4360 0.5450 0.6540 0.7630

16 16 1.5705 1.5705 0.8720

18 18 1.7668 1.7668 0.9810

0.8720

0.9810

20 22 24 20 2.159422 2.3557 24 1.9631 1.9631 1.1990 2.15941.30812.3557 1.0900

1.0900

1.1990

1.3081

Sensors 2016, 16, 859

8 of 14

Table 3. Comparison of measured electric field values at point P before and after the approach of a human body (driven by different voltages). U s (kV)

8

10

12

14

16

18

20

22

24

Erms (kV/m) E*rms (kV/m)

0.7852 0.4360

0.9816 0.5450

1.1779 0.6540

1.3742 0.7630

1.5705 0.8720

1.7668 0.9810

1.9631 1.0900

2.1594 1.1990

2.3557 1.3081

Sensors 2016, 16, 859

8 of 13

As shown in Figure 7, the electric field spatial distribution outside the sensor is distorted further in the of an approaching humanfield body, whereas the electric field polar pates is Ascase shown in Figure 7, the electric spatial distribution outside thebetween sensor isthe distorted further evenly distributed. in the case of an approaching human body, whereas the electric field between the polar pates is Table 3 shows the simulation results of the electric field before and after the approach of a evenly distributed. human body (i.e., Ethe and E*rms results respectively) at pointfield P when different levels of excitation rmssimulation Table 3 shows of the electric before and after the approach of a voltage human are generated. body (i.e., Erms and E*rms respectively) at point P when different levels of excitation voltage are generated. The data data in in Table Table 33 are are linearly The linearly fitted, fitted, as as shown shown in in Figure Figure 8. 8. 2.4 2.2

Erms (kV/m)

2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4

0.5

0.6

0.7

0.8

0.9

1

E*rms (kV/m)

1.1

1.2

1.3

1.4

Figure 8. Relation curve of the electric field intensity at P before and after the approach of a human body. Figure 8. Relation curve of the electric field intensity at P before and after the approach of a human body.

Figure 8 shows that after the approach of the human body, the electric field intensity at the Figure point 8 shows that after approach oftothe human the electric field intensity the observation decreases but the is proportional that beforebody, the approach of the human body.atThis observation point decreases but is proportional to that before the approach of the human body. This situation is explained in the aspect of the principle in Section 2. The linear relation between Erms and situation is explained in the aspect of the principle in Section 2. The linear relation between Erms and E* rms can be expressed as E*rms can be expressed as ˚* = kb E Erms (11) EErms (11) rms “ rms where kbb is is the correction coefficient it it is coefficient regarding regardingthe theinfluence influenceofofthe thehuman humanbody bodyononmeasurement; measurement; the same asas that inin Equation (9). Here, kb k=b =1.8010. is the same that Equation (9). Here, 1.8010. Considering the dual influence human body, wewe cancan correct the Considering influence of ofthe themeasuring measuringsensor sensorand and human body, correct electric field field intensity measured with thewith portable Type LP-2000 on thetwice basis on of Equations the electric intensity measured the portable Typetwice LP-2000 the basis (10) of and (11) to(10) reflect the original environmental electric field. electric field. Equations andaccurately (11) to reflect accurately the original environmental 4. Experimental ExperimentalVerification Verification 4. 4.1. Experimental Result and Analysis in the Absence of a Human Body 4.1. Experimental Result and Analysis in the Absence of a Human Body The experimental platform composed of a voltage regulator, step-up transformer, conductor The experimental platform composed of a voltage regulator, step-up transformer, conductor and and insulator is shown in Figure 9. Narda EFA-300 and a portable Type LP-2000 are used as the insulator is shown in Figure 9. Narda EFA-300 and a portable Type LP-2000 are used as the measurement devices. The technical characteristics of the equipment are shown in Table 4. measurement devices. The technical characteristics of the equipment are shown in Table 4.

4. Experimental Verification 4.1. Experimental Result and Analysis in the Absence of a Human Body The experimental platform composed of a voltage regulator, step-up transformer, conductor and 14 in Figure 9. Narda EFA-300 and a portable Type LP-2000 are used as9 ofthe measurement devices. The technical characteristics of the equipment are shown in Table 4. Sensors 2016,is 16,shown 859 insulator

(a)

(b)

Figure 9. Experimental measurement (in the absence of a human body). (a) Measuring worksite of Figure 9. Experimental measurement (in the absence of a human body). (a) Measuring worksite of Narda EFA-300; (b) measuring worksite of Type LP-2000. Narda EFA-300; (b) measuring worksite of Type LP-2000. Table 4. Technical characteristics of the experimental equipment. Name

Technical Characteristics

voltage regulator

input: 220 V with power frequency adjustable range: 0–250 V rated capacity: 10 kVA

transformer

rated capacity: 10 kVA ratio: 200

EFA-300

measurable frequency range: 5 Hz–32 kHz measurable electric field range: 0.1 V/m–200 kV/m

LP-2000

measurable frequency range: 5 Hz–1 kHz measurable electric field range: 20 V/m–200 kV/m

In the absence of a human body, the comparisons are conducted among the measured result ELP-2000 of Type LP-2000, the measured result EEFA-300 of Narda EFA-300, and the original electric field E0 obtained via stimulation. The measuring errors are defined as e1 “

|ELP-2000 ´ E0 | ˆ 100% E0

(12a)

e2 “

|EEFA-300 ´ E0 | ˆ 100% E0

(12b)

The statistical data and error analyses are shown in Table 5. Table 5. Measurement data statistics of stimulation and experiment and error analysis (in the absence of a human body). U s (kV)

8

10

12

14

16

18

20

22

24

E0 (kV/m) ELP-2000 (kV/m) EEFA-300 (kV/m) e1 (%) e2 (%)

2.5491 2.3921 2.6234 6.16 8.82

3.1865 3.1431 3.3840 1.36 7.12

3.8237 3.8580 3.8467 0.89 0.29

4.4610 4.4237 4.3699 0.84 1.22

5.0984 5.0997 5.1124 0.02 0.25

5.7357 5.7912 5.7983 0.97 0.12

6.3732 6.5125 6.4842 2.19 0.44

7.0103 6.9905 7.4047 0.28 5.59

7.6476 7.6198 7.8630 0.36 3.19

Figure 10 shows the fitting curve based on the data in Table 5.

ELP-2000 (kV/m) EEFA-300 (kV/m) e1 (%) e2 (%)

2.3921 2.6234 6.16 8.82

3.1431 3.3840 1.36 7.12

3.8580 3.8467 0.89 0.29

4.4237 4.3699 0.84 1.22

5.0997 5.1124 0.02 0.25

5.7912 5.7983 0.97 0.12

6.5125 6.4842 2.19 0.44

6.9905 7.4047 0.28 5.59

7.6198 7.8630 0.36 3.19

Sensors 2016, 16, 859

10 of 14

Figure 10 shows the fitting curve based on the data in Table 5.

(a)

(b)

Figure 10. 10. Relation Figure Relation curves curves related related to to the the excitation excitation voltage voltage variation variation (in (in the the absence absence of of aa human human body). body). (a) Curves of electric field intensity; (b) curves of error. (a) Curves of electric field intensity; (b) curves of error.

Given the interference produced by other electric equipment in the laboratory, certain errors Given the interference produced by other electric equipment in the laboratory, certain errors emerge between the measured electric field intensity and the stimulation results. However, the value emerge between the measured electric field intensity and the stimulation results. However, the value measured by Type LP-2000, in general, is relatively close to the stimulation value and the value measured by Narda EFA-300. Therefore, the accuracy of Type LP-2000 is verified. 4.2. Experimental Results and Analysis in the Case of an Approaching Human Body

Sensors 2016, 16, 859

10 of 13

4.2.1. Regulating the Excitation Voltages of Electric Field measured by Type LP-2000, in general, is relatively close to the stimulation value and the value Figureby 11Narda showsEFA-300. that the Type LP-2000 is fixed on the arm ofisthe individual at a horizontal measured Therefore, thesensor accuracy of Type LP-2000 verified. distance of 53 cm from the conductor. Different electric field intensities can be generated by regulating 4.2. Experimental Resultsofand in the Case of an Approaching Humandata BodyELP-2000 of Type LP-2000 the excitation voltages theAnalysis conductor. The uncorrected measurement and the original electric field E0 in the stimulation are listed in Table 6. 4.2.1.Figure Regulating the Excitation Voltages of Electric electric Field field intensity in the case of an approaching 12 shows that the uncorrected measured human body less than the Type original electric fieldisintensity. Furthermore, an approximately linear Figure 11isshows that the LP-2000 sensor fixed on the arm of the individual at a horizontal relation exists between them. The linear coefficient can be obtained by linear fitting, i.e., k * = 1.9094, b distance of 53 cm from the conductor. Different electric field intensities can be generated by which is close kb = 1.8010 as obtained in conductor. the stimulation. The error results from the electromagnetic regulating thetoexcitation voltages of the The uncorrected measurement data ELP-2000 of interference in the experimental environment. Type LP-2000 and the original electric field E0 in the stimulation are listed in Table 6.

LP-2000

Figure 11. Measuring worksite of Type LP-2000 (in the case of an approaching human body). Figure 11. Measuring worksite of Type LP-2000 (in the case of an approaching human body). Table 6. Uncorrected data statistics with different excitation voltages (in the case of an approaching human body). Us (kV) E0 (kV/m)

8 2.5491

10 3.1865

12 3.8237

14 4.4610

16 5.0984

18 5.7357

20 6.3732

22 7.0103

24 7.6476

Sensors 2016, 16, 859

11 of 14

Figure 11. Measuring worksite of Type LP-2000 (in the case of an approaching human body). Table 6. Uncorrected data statistics with different excitation voltages (in the case of an approaching

Table 6. Uncorrected data statistics with different excitation voltages (in the case of an approaching human body). human body). U s (kV)

8

10

12

14

16

Us (kV) 8 10 12 14 16 E0 (kV/m) 2.54912.5491 3.18653.8237 3.8237 4.4610 4.4610 5.0984 5.0984 E0 (kV/m) 3.1865 E (kV/m) 1.4361 1.7331 2.1793 2.3066 2.3745 ELP-2000 LP-2000 (kV/m) 1.4361 1.7331 2.1793 2.3066 2.3745

18

18 5.7357 5.7357 2.9281 2.9281

20

22

24

20 22 24 6.3732 6.3732 7.0103 7.01037.6476 7.6476 3.2647 3.4245 3.5080 3.2647 3.4245 3.5080

Figure showsthe thefitting fittingcurve curvebased based on on the data Figure 1212 shows data in in Table Table6.6. 8 measurement linear fitting

7

E0 (kV/m)

6 5 4 3 2 1

1.5

2

2.5 3 ELP-2000 (kV/m)

3.5

4

Figure 12.12.Relation curve data (in (inthe thecase caseofofanan Figure Relation curveofofstimulation stimulationand and uncorrected uncorrected experimental experimental data approaching human body). approaching human body).

The measured result is corrected to obtain the corresponding E*LP-2000 by using the correction coefficient kb * = 1.9094. The comparison between the corrected measured data E*LP-2000 and the original electric field E0 is shown in Table 7. Table 7. Corrected data statistics with different excitation voltages (in the case of an approaching human body). U s (kV)

8

10

12

14

16

18

20

22

24

E0 (kV/m) E*LP-2000 (kV/m) error (%)

2.5491 2.4452 4.08

3.1865 3.3538 5.25

3.8237 3.7979 0.67

4.4610 4.3544 2.39

5.0984 4.8430 5.01

5.7357 5.8499 1.99

6.3732 6.3979 0.34

7.0103 6.8109 2.84

7.6476 7.2634 5.02

Table 7 shows that the measured result of Type LP-2000 after correction accurately reflects the actual electric field intensity in the environment and that the maximal error is less than 6%. 4.2.2. Regulating the Distances between the Measuring Point and the Conductor As most of the experimental conditions are kept identical to those in the previous experiment and the excitation voltage of the conductor is set to 12 kV, different electric field intensities can be obtained at different measuring points by regulating the distances between the measuring point and the conductor, as shown in Figure 13. The corrected measured data E*LP-2000 are obtained by using the correction coefficient kb * = 1.9094. The original electric field E0 in the simulation and the corresponding error values are listed in Table 8.

As most of the experimental conditions are kept identical to those in the previous experiment and the excitation voltage of the conductor is set to 12 kV, different electric field intensities can be obtained at different measuring points by regulating the distances between the measuring point and the conductor, as shown in Figure 13. The corrected measured data E*LP-2000 are obtained by using the Sensors 2016, coefficient 16, 859 12 of 14 correction kb* = 1.9094. The original electric field E0 in the simulation and the corresponding error values are listed in Table 8.

dL

LP-2000

Figure 13. Electric field measurements in different distances (in the case of an approaching human body). Figure 13. Electric field measurements in different distances (in the case of an approaching human body). Table 8. Corrected data statistics with different measurement distances (in the case of an approaching human body). Table 8. Corrected data statistics with different measurement distances (in the case of an approaching 53 63 73 83 93 103 human body).dL (cm)

E0 (kV) dLLP-2000 (cm)(kV) E* Eerror 0 (kV)(%) E*LP-2000 (kV) error (%)

3.8237 53 3.8582 0.8970 3.8237

3.2423 63 3.3455 3.1675 3.2423

2.7417 73 2.7956 1.9441 2.7417

2.372 83 2.3144 2.4452 2.372

3.8582 0.8970

3.3455 3.1675

2.7956 1.9441

2.3144 2.4452

2.0744 1.8264 93 1.9733 1.7836103 4.8882 2.0744 2.3763 1.8264 1.9733 4.8882

1.7836 2.3763

Table 8 also shows that the corrected electric field intensity measured by Type LP-2000 is consistent with the original electric field intensity in the environment and that the maximal error is less than 5%. 5. Conclusions The influence of the human body on electric field measurement was investigated by using the Type LP-2000 single-shaft electric field measuring sensor developed by our research team. The results obtained from the principal model, simulation, and physical experiment showed that as an individual approached the sensor, the measured electric field intensity became less than the original environmental electric field intensity in the absence of a human body; however, both of them were proportional. The original environmental electric field intensity was obtained by using the electric field intensity measured with the portable Type LP-2000 in real time and multiplying its value by the defined correction coefficient. The correction coefficient kb obtained in the simulation was 1.8010, and the correction coefficient kb * obtained in the experiment was 1.9094; the two values can be considered approximately equal. Two experimental programs were established; under these programs, the excitation voltages and the distance measuring points were regulated to produce different electric field intensities. Using kb * = 1.9094, the corrected measured electric field intensity accurately reflected the original environmental electric field intensity, and the maximal error was less than 6% in all the data comparisons. These results verify the effectiveness of our proposed correction method. Additionally, the single-shaft sensor proposed in this study may be more suitable for the electric field generated by transmission lines than for that generated in substation. Therefore, the 3D measurement sensor will be explored in our future research. Acknowledgments: This work was supported by the National Natural Science Foundation of China (NSFC 51407016), and the Fundamental Research Funds for the Central Universities (106112015CDJXY150008).

Sensors 2016, 16, 859

13 of 14

Author Contributions: Dongping Xiao and Huaitong Liu conceived and designed the study. Qiang Zhou and Qichao Ma performed the experiments. Dongping Xiao and Huaitong Liu wrote the paper. Yutong Xie and Qichao Ma reviewed and edited the manuscript. All authors read and approved the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

References 1.

2. 3. 4. 5.

6. 7. 8. 9. 10.

11. 12.

13. 14. 15. 16. 17. 18. 19.

20.

Ztoupis, I.N.; Gonos, I.F.; Stathopulos, I.A. Measurement and calculation of power frequency electric fields generated by high voltage overhead power lines. In Proceedings of the 2014 International Conference on High Voltage Engineering and Application (ICHVE), Poznan, Poland, 8–11 September 2014; pp. 1–4. Ziegel, B.G.; Repacholi, M.; Mckinlay, A. International commission on non-ionizing radiation protection. Prog. Biophys. Mol. Biol. 2007, 92, 1–3. General. In American Conference of Governmental Industrial Hygienists; Applied Industrial Hygiene: Cininnati, OH, USA, 1987. IEEE. Standard for Safety Levels with Respect to Human Exposure to Electromagnetic Fields, 0–3 kHz; IEEE C95.6: New York, NY, USA, 2002. Decat, G.; Deckx, L.; Meynen, G. Magnetic Fields of Induction Heaters in the Framework of Directive 2004/40/EC of the European Parliament and of the Council. Int. J. Occup. Saf. Ergon. Jose 2006, 12, 169–176. [CrossRef] [PubMed] Hui, J.; Guan, Z.; Liu, Y. Values and Rationales of Limits of Power Frequency Electric and Magnetic Fields in Various Countries. High Volt. Eng. 2006, 32, 51–54. Beriain, A.; Gutierrez, I.; Solar, H.; Berenguer, R. 0.5 v and 0.43 pj/bit capacitive sensor interface for passive wireless sensor systems. Sensors 2015, 15, 21554–21566. [CrossRef] [PubMed] Ze-Wen, H.U.; Wei, H.E.; Yao, D.G.; Wang, J.G.; Wen, J.; Luo, L.I. Research of high-voltage power frequency electric field warning instrument. Electr. Meas. Instrum. 2009, 9, 45–48. Kletzing, C.A.; Kurth, W.S.; Acuna, M.; Macdowall, R.J.; Torbert, R.B.; Averkamp, T. The electric and magnetic field instrument suite and integrated science (emfisis) on rbsp. Space Sci. Rev. 2013, 179, 127–181. [CrossRef] Van, R.E.; Croft, R.; Juutilainen, J.; Lagroye, I.; Miyakoshi, J.; Saunders, R. Effects of radiofrequency electromagnetic fields on the human nervous system. J. Toxicol. Environ. Health B Crit. Rev. 2009, 12, 572–597. Rong, Z.; Bo, W.; Ben, N.; Zhanqing, Y. Development and application of integrated optical sensors for intense e-field measurement. Sensors 2012, 12, 11406–11434. Vaillancourt, G.H.; Carignan, S.; Jean, C. Experience with the detection of faulty composite insulators on high-voltage power lines by the electric field measurement method. IEEE Trans. Power Deliv. 1998, 13, 661–666. [CrossRef] Lin, Y.Y.; Zhao, Y.Y.; Li, P.L.; Lin, Y.Y.; Zhao, Y.Y.; Li, P.L. Study on measurement errors of the ball type sensor in the power frequency electric field. Adv. Mater. Res. 2014, 1022, 415–418. [CrossRef] Zhou, Q.; He, W.; Li, S.; Hou, X. Research and experiments on a unipolar capacitive voltage sensor. Sensors 2015, 15, 20678–20697. [CrossRef] [PubMed] Xia, X.; Yan, X.; Zexing, D. Thermodynamic modeling and analysis of an optical electric-field sensor. Sensors 2015, 15, 7125–7135. [CrossRef] [PubMed] Meng-Ting, Y.U.; Wang, J.G.; Jian, L.I. The experimental study on the human body influence on measurement of high voltage power frequency electric field. Electr. Meas. Instrum. 2013, 6, 24–28. Abdel-Salam, M.; Mohamed, A.H. Transmission-line electric field induction in humans using charge simulation method. IEEE Trans. Biomed. Eng. 1995, 42, 1105–1109. [CrossRef] [PubMed] Liu, Y.; Zou, J.; Zhang, B.; Yuan, J.; Zhen, W.U.; Guangming, Y.E. Analysis of influence of 50 Hz electric fields generated by the high voltage power lines on human body. High Volt. Eng. 2004, 30, 35–37. Abd-Allah, M.A. Magnetic field-induced currents in human body in the proximity of power lines. In Proceedings of the IEEE Power Engineering Society General Meeting, Toronto, ON, Canada, 13–17 July 2003. Chiba, A.; Isaka, K.; Yokoi, Y. Application of finite element method to analysis of induced current densities inside human model exposed to 60 Hz electric field. IEEE Trans. Power Appar. Syst. 1984, 103, 1895–2102. [CrossRef]

Sensors 2016, 16, 859

21. 22.

14 of 14

Safigianni, A.S.; Tsompanidou, C.G. Electric- and Magnetic-Field Measurements in an Outdoor Electric Power Substation. IEEE Trans. Power Deliv. 2009, 24, 38–42. [CrossRef] Yuanzhe, X.; Cheng, G.; Yanxin, L.; Yunfen, C.; Bihua, Z. Calculation and Experimental Validation of 3-D Parallel Plate Sensor for Transient Electric Field Measurement. In Proceedings of the 2007 International Symposium on Microwave, Antenna, Propagation and EMC Technologies for Wireless Communications, Hangzhou, China, 16–17 August 2007; pp. 1267–1271. © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).