Current/Voltage Measurement Scheme Using a … · 2016-01-22 · magnetic flux generated from a single current-carrying power cord is described by Ampere's circuital law as ... a

IPASJ International Journal of Computer Science (IIJCS) Web Site: http://www.ipasj.org/IIJCS/IIJCS.htm

A Publisher for Research Motivation ........ Email: [email protected] Volume 3, Issue 7, July 2015 ISSN 2321-5992

Volume 3 Issue 7 July 2015 Page 10

ABSTRACT This study develops a non-intrusive power sensor for the electricity monitoring of two-wire household appliances. The information provided by a power sensor supports home energy management systems and encourages energy-saving behavior for each household member, resulting in an increasingly efficient energy usage. The flexible non-intrusive power sensor is fabricated with a current and a voltage sensing parts on a 100 µm thick flexible polyethylene terephthalate substrate. The combination of a 30-turn coil design with two sensing electrodes in an area of 0.25×1 cm2 exhibits power sensor sensitivities of 20.6 µV/A and 63.9 µV/V for detecting 60 Hz electric current and voltage, respectively. For the practical application, the method of calibration is also proposed in the paper. Keywords: Current measurement, Voltage measurement, Non-intrusive, Calibration.

1. INTRODUCTION The reduction in natural resources and the threat to our climate by global warming are inevitable challenges. To resolve such problems, most national governments are actively developing related policies and specific programs. According to the Energy Technology Perspectives 2012, released by the International Energy Agency (IEA), the most useful approach is by increasing energy efficiency [1]. Monitoring the power consumption of appliances assists end-users in designing the best operation program to maximize energy efficiency. Deployment of demand-response (DR) electricity monitoring systems to manage electricity usage in residential areas is a cost-effective way of improving energy efficiency [2]. Previous studies have demonstrated the performance of electrical energy monitoring and control systems in homes, and all of these scenarios emphasize energy information feedback [3]-[5]. As a result, the behavior of customers changes with respect to energy saving. In DR implementation, a key component is the power sensor, which is combined with a pair of voltage and current sensors. For devices that monitor household electricity, power sensors should be affordable, minute, and easily installed. Power sensors can be separated into two parts. In the current-sensing field [6]-[18], shunt resistors require contact with conducting paths and demonstrate large power losses in high-current applications [7], [8]. Rogowski coils must encircle the signal line separated from the neutral line in a two-wire appliance and produce electromotive forces proportional to the time derivative of the current in the conductor [9], [10]. Current transformers are similar to the Rogowski coils, but with the disadvantages of magnetic hysteresis and high current saturation [11], [12]. Hall sensors have large thermal drifts, offset voltages, and supply current requirements [13]. Magnetoresistance sensors exhibit large thermal drift and high nonlinearity [14]. Fiber-optic current sensors are expensive, extremely large, and are barely adequate for high-current applications [15], [16]. All of these sensors are not suitable for use in household appliances, whose root mean square (RMS) current ranges from 0 A to 15 A in two-wire power cords. Recently, the use of a novel piezoelectric microelectromechanical system (MEMS) current sensor has been proposed to sense the electric current in two-wire power cords noninvasively [17], [18]. Sensing signal is caused by cantilever beam and is thus influenced by vapor. In the voltage-sensing field [19]-[27], potential transformers require contact with conducting paths and exhibit magnetic hysteresis and saturation [19], [20]. Kelvin probes [21] and electric field mills [22], [23], which are principles of vibrating capacitors, should be combined with actuators, so they are expensive and large. Electro-optic field meters are based on the Pockels or Kerr effect, and their structures are complex and expensive [24], [25]. Thus, they are suitable for intensity measurement in high electric fields, but not for electric (voltage) measurement in homes. Moreover, new micromachined electric-field mill voltage sensors reported for DC and AC field measurement, may require a complex packaging process [26], [27]. To avoid these issues and to enhance electricity monitoring systems, this study develops a low-cost, flexible, and reliable power sensor that fits closely with the power cords of household appliances. The rest of this paper is organized as follows. Section 2 describes the power sensor sensing mechanism. Section 3 presents the fabricating process. Section 4 presents the experimental results and discussions. Section 5 draws the conclusions.

Current/Voltage Measurement Scheme Using a Flexible Coil/Electrode Power Sensor to Monitor the Power of Two-Wired Household Appliances

Shih-Hsien Cheng , Sheng-Fuu Lin

Institute of Electrical Control Engineering, National Chiao Tung University, 1001 University Road, Hsinchu, Taiwan 30010,

ROC.




2. POWER SENSING MECHANISM The power sensor proposed in this work is constituted of an inductive coil and metal electrodes (Fig. 1). The sensing principle is illustrated in two parts: current and voltage.

Figure 1 Scheme of the flexible non-intrusive power sensor.

Principle of current sensing According to Faraday’s law of induction, closely attaching the coil current sensor to the power cord at the center of two wires will yield the maximum output voltage signal generated by the electric current flowing through the power cord. The magnetic flux generated from a single current-carrying power cord is described by Ampere's circuital law as follows [28]:

πrωtIμB ac

2cos0

(1)

Where B is the magnetic flux density, Iac is the testing current inside the cord, ω is the angular frequency of current, and r is the radius from the center of the conductor to the sensor. The induced voltage is proportional to the magnetic flux change caused by the time-variant current flowing through the coil and can be described as follows:

N

ncoil

N

n

n

dtdBA

dtdΦV

11 (2)

V is the induced voltage of the coil, N is the total number of coil turns, Φn is the integral of the vertical component of magnetic flux passing through the nth turn area, and Acoil is the effective area which varies with each turn. In this study, the sensing coil is designed in 10 mm × 5 mm area with 30 µm in line-width, 30 µm in spacing that included 30 turns. The distribution of magnetic flux density is simulated by Ansoft Maxwell (Fig. 2). It shows that the magnetic flux that passed through the current sensor on the center of two wires is twice that of the current that passed through one wire.

Figure 2 Distribution of the magnetic flux density around a 1.25mm2 cross-section two-wire power cord with 1A current.




Principle of voltage sensing The voltage sensor consists of two electrodes, which pinch a power cord to sense the electric field as shown in Fig. 1. Given a sinusoidal electric field in the cord, the current is induced in the electrodes as follows [26]:

dtdEεAi electrode

(3)

Where ε is the (absolute) permittivity. E, which is proportional to the voltage on the line, is the component of the electric field that is normal to the electrodes, and Aelectrode is the effective area of the electrode. An electrode close to the live line makes induced current ilive and an electrode close to the natural line makes induced current inatural. When the ground (earth) is connected, a sensing signal is acquired from ilive, and the reference is ground. If no ground is connected, a sensing signal is acquired from the differential of ilive and inatural, where the reference is caused by inatural [29]. In this study, no ground is connected to the structure, and the sensing area of each electrode is designed in 0.25×1 cm2. The distribution of electric field is simulated by Ansoft Maxwell (Fig. 3).

Figure 3 Distribution of the electric field around a 1.25 mm2 cross-section, two-wire power cord with 120V voltage.

In practice, the current sensor is also affected by the electric fields. When the power sensor is implemented as depicted in Fig. 4, the directions of the currents produced by the magnetic and electric fields oppose each other. Hence, Eq. (2) must be modified as follows:

.RdtdE'εA

dtdBAV coil

N

ncoil

1 (4)

Where Acoil' is the effective metal area of the coil that is normal to the electric field, and R is load resistance.

Figure 4 The effect of electric and magnetic fields on the coil.




3. FABRICATING PROCESS The flexible current sensor is fabricated using the MEMS process [30]. First, the polyethylene terephthalate (PET) substrate is cut into a 4 in wide circle. It is then glued onto a 4 in silicon handling wafer. The MEMS procedure is explained as follows: Step 1:The wafer is sputtered with Ti/Cu layer (50 nm/100 nm) as an adhesion/seed layer. Step 2:Using photolithography, we pattern the inductive coil area of the current sensor and the electrode area of the voltage sensor. We then fabricate the 2 µm thick Cu coil and capacitive electrodes by electroplating. Steps 3:We remove the AZ4620 photoresist and the Ti/Cu seed/adhesion layer using acetone and buffered HF and Cu etchant, respectively. The Cu surface is covered with a thin Ni/Au layer to enhance its resistance to oxidation and corrosion. Step 4:A 2 µm thick SU-8 layer covers the metal patterns. We then generate a hole as an opening for air-bridging connection by photolithography. Step 5:We sputter another Ti/Cu adhesion/seed layer and coat it with a photo-patterned AZ4620 photoresist mold to electroplate the Cu bridging connection. Step 6:Cu is electroplated. Steps 7:We remove the AZ4620 photoresist and Ti/Cu layers using acetone and buffered HF and Cu etchant, respectively. Step 8:PET substrate is detached from the Si handling wafer. We generate the sensor tag by dividing the entire PET into sensor pieces. Fig. 5 and 6 display the process flow of fabrication and the produced prototypes.

Figure 5 Manufacturing Process flow.

Figure 6 Prototypes of flexible, non-intrusive power sensors.

4. EXPERIMENT RESULTS The parameters provided in the section of power sensing mechanism, namely, Iac, ω, r, N, and Acoil, are set at 1 A, 60 Hz, 1.8 mm, 30 turns, and 50 mm2, respectively. Based on these settings and Eq. (2), the output voltage of the current sensor is in µV. Eq. (2) also indicates that the high-frequency signals are amplified by the differential function. In Eq. (3), the voltage sensor is similar to the current sensor, thus demonstrating that the differential function amplifies high-frequency




signals. To demonstrate that filters should be designed for practical application, a comparison between the signal of voltage sensors with utility power, with and without an integrator, is shown in Fig. 7. As presented in Fig. 8, this study utilizes a two-stage amplifier and an integrator. For the current sensor, the first operational amplifier (op amp) provides a 40 dB gain, the second op amp functions as an integrator, and the third op amp provides a 26 dB gain. For the voltage sensor, the function of the filter is similar, but the two gains are 0 and 26 dB, respectively. Eq. (5) defines a low-pass filter (LPF) with multiple-feedback architecture and its transfer functions. In this study, the LPF cut-off frequency (wc) was set to 1 kHz for the highest harmonic considered as the 15th harmonic. The op amp OPA376 operates on a single supply and is used to implement the sensor nodes easily. The total harmonic distortion and the 0.0007% signal-to-noise ratio ensure that signal linearity remains unaffected. Power is supplied by the low-dropout regulator TPS76330, the output of which is 3.0 V. The 1.2 V reference voltage set by REF3312 satisfies dipolar input signals because the op amp only has a single supplier, as depicted in Fig. 9.

227211319

2

18

2721272119

18

27

)(1)(

sRRCCwsR

RRRRCw

RR

sTcc

LPF

(5)

(a) (b)

Figure 7 Signal of voltage sensor with utility power. Outputs (a) without and (b) with an integrator.

(a)




(b)

Figure 8 Schematic diagram of a two-stage amplifier and an integrator. (a) for current sensors and (b) for voltage sensors.

Figure 9 Single power supply and reference voltage circuits.

Fig. 10 shows the testing system, which includes a current/voltage source, an oscilloscope, a two-wire power cord, a flexible power sensor, and sensing circuits. First, the source is supplied with current from 1 A to 10 A, and second, the source is changed to be voltage from 30 V to 240 V. Using the oscilloscope, we measured the experimental data at the output of the amp prior to analysis. With the designed circuits (66 and 26 dB gains), the sensitivities of the current and the voltage sensors are 20.6 µV/A and 63.9 µV/V, respectively. All fitting curves of the testing current versus output of the current sensor, the testing voltage versus output of the current sensor, and the testing voltage versus output of the voltage sensor are linear. Fitting indices, R-square, are 0.9999, 0.9999 and 0.9993, respectively (Fig. 11). Fig. 1l also shows that induced voltage on the current sensor (at the output of amplifier) with 120 V equal to the effect by 0.55 A.




Figure 10 A measurement platform with a standard current/voltage source.

(a)

(b)

Figure 11 (a) Testing current versus output of the current sensor; and (b) testing voltage versus output of the current sensor and the voltage sensor.




Hence, the power measurement rules could be worked up with a standard current/voltage source as follows: Rule 1:On the voltage sensor, the output value per volt is obtained with a standard voltage source (V1V). Rule 2:On the current sensor, the output value per volt is derived from a standard voltage source (I1V). Rule 3:On the current sensor, the output value per ampere is obtained through a standard current source (I1A). Rule 4:The values measured from the current and voltage sensors are determined under utility power and an appliance load. Rule 5:Voltage (Vreal) is equal to the measured value (Vmeasured) divided by V1V. Rule 6:Current (Ireal) is determined by subtracting measured value (Imeasured) from the value of Vreal, multiplying it by I1V, and then dividing it by I1A. At the beginning, Rules (1) to (3) are followed with a standard current/voltage source to obtain three measurement system parameters, namely, V1V, I1V, and I1A. During application, Rules (4) to (6) are followed to determine the RMS values of current and voltage. Fig. 12 presents the experimental platform for measurement under utility power, and Table 1 verifies the current measurement. The experimental results are linear; therefore, the coil current and electrode voltage sensors can be used as sensing components in a power monitoring system.

Figure 12 A platform for current and voltage measurement with utility power.

Table 1: Verification of current measurement rules.

Test conditions Equivalent current [A]

Estimative current [A]

Error [%]

120.1Ω/122.6V 1.02 1.04 1.96 60.1Ω/123V 2.05 2.00 -2.44 39.9Ω/122.2V 3.06 3.02 -1.31 30.1Ω/122.9V 4.08 3.98 -2.45 24.1Ω/122.1V 5.07 5.06 -0.20

＊ Note: Error is defined as (Estimative current- Equivalent current)/ Equivalent current*100 %. We also examined the case without a standard power source, in which a socket-type meter is used as a reference for calibration as indicated in Fig. 13. The calibrated socket-type meter and the uncalibrated meter, which is called a clamped meter, are coupled with a flexible coil/electrode power sensor. These components communicate with the Zigbee network. The socket-type meter acts as a coordinator, whereas the clamped meter is an end device. During calibration mode, the socket-type meter is added and the Zigbee network is activated. When the household appliance is turned off, the values of the voltage and current sensors, namely, Vs_off and Is_off, are obtained. The reference values of voltage and current, which are designated as Vr_off and Ir_off, are transferred from the coordinator to the end device. When the appliance is active, the values of the voltage and current sensors (Vs_on and Is_on), as well as their reference values (Vr_on and Ir_on), are determined. Based on the values of Vs_off, Is_off, Vs_on, Is_on, Vr_off, Ir_off, Vr_on, and Ir_on, we develop three equations [Eq. (6)–(8)]. By solving these equations, we derive three measurement system parameters, namely, V1V, I1V, and I1A.




Voffroffs VVV 1__ (6)

AoffrAoffroffs IIVVI 1_1__ (7)

AonrAonrons IIVVI 1_1__ (8)

At the measurement mode, the socket-type meter is removed and the Zigbee network is deactivated, Rules (4) to (6) are applied. The RMS values of the measured current and voltage are thus calculated.

Figure 13 The calibration for a clamped meter with a zigbee network.

To ensure the practicality of the flexible coil/electrode power sensor, two calibration methods are proposed in the paper.

5. CONCLUSIONS A flexible power sensor with inductive coil and capacitance electrodes is demonstrated to have the potential for application to low-cost, reliable, and pervasive DR electricity monitoring systems for residential power management. The efficacy of the physical model of the power sensor design has been tested and verified. Simultaneously reducing the metal width and increasing the coil number enhanced the sensitivity of the current sensing part of the model. Similarly, the sensitivity of the voltage sensing part of the model can be improved by increasing the electrodes area. Future studies can focus on new algorithms that can compensate for power sensor installation variations which is an inherent disadvantage for non-intrusive sensing technology. Another direction is the application for three-phase, three-wire power system measurements. It has the potential to conserve a greater amount of energy.

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AUTHOR

Shih-Hsien Cheng was born in Chiayi, Taiwan, R.O.C. in 1975. He received his B.S. degree in Control Engineering from Feng Chia University, Taichung, Taiwan in 1998 and his M.S. degree in Electrical Engineering from National Sun Yat-sen University, Kaohsiung, Taiwan in 2000. He is currently a Ph.D. candidate at National Chiao Tung University, Hsinchu, Taiwan and is also a research staff with the Green Energy & Environment Research Laboratories, Industrial Technology Research Institute, Chutung, Taiwan. His current research interests include power electronics, power measurement, neural networks, fuzzy theory, and wireless sensor networks.

Sheng-Fuu Lin was born in Tainan, Taiwan, R.O.C. in 1954. He received his B.S. and M.S. degrees in Mathematics from National Taiwan Normal University in 1976 and 1979, respectively, his M.S. degree in Computer Science from the University of Maryland, College Park, in 1985, and his Ph.D. degree in Electrical Engineering from the University of Illinois, Champaign, in 1988. Since 1988, he has been with the faculty of the Department of Electrical Engineering at National Chiao Tung University, Hsinchu, Taiwan, where he is currently a professor. His research interests include image processing, image recognition, fuzzy theory, neural networks, automatic target recognition, intelligent system design, and computer vision.

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Current/Voltage Measurement Scheme Using a … · 2016-01-22 · magnetic flux generated from a single current-carrying power cord is described by Ampere's circuital law as ... a