Current Sensing System for Protection of High Power ... - IEEE Xplore

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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 41, NO. 10, OCTOBER 2013

Current Sensing System for Protection of High Power Frequency Converters ˇ Ceslovas Šimkeviˇcius, Nerija Žurauskien˙e, Member, IEEE, Saulius Baleviˇcius, Senior Member, IEEE, Voitech Stankeviˇc, Skirmantas Keršulis, and Algirdas Baškys

Abstract— The design and test results of a current-sensing system used for the protection of frequency converters, which are used for controlling the speeds of ac induction motors, are presented. The proposed pulse current-sensing system consists of a magnetic field sensor, which is made of thin polycrystalline La-Sr-Mn-O manganite film exhibiting the colossal magnetoresistance (CMR) effect, and a microcontroller with a 12-bit A/D converter that is used for recording the fault current signal. The magnetic field generated by the high power current cable is fed using a flux concentrator to the manganite magnetic field sensor (CMR-resistor). The design of the contactless current sensor, consisting of two manganite resistors, is developed and investigated. The system is tested in the ambient temperature range of 0–45 °C. The possibilities of using this method in other pulsed power systems are discussed. Index Terms— AC induction motors, colossal magnetoresistance, current sensor, magnetic field sensor, microcontroller, pulsed power systems, thin manganite films.

I. I NTRODUCTION

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URRENT sensors and current-sensing systems based on these sensors are used extensively in industrial applications, such as automotive equipment, motor speed controls, and power systems. Various promising designs of such current control elements in pulsed power systems, such as transportable pulse power generators, relay protectors, switched mode power supplies [1]–[4], and others are suggested. One of the reasons for the development of such current-sensing systems is their use for the protection of frequency converters that are used to convert the standard one or three phase ac voltage into variable-frequency, variable-amplitude threephase ac voltage for the supply of ac induction motors. These are used to control the speed of such motors [5]. Such motors often become overloaded, thus the current passing through them can increase significantly and damage

Manuscript received November 30, 2012; revised March 19, 2013 and April 30, 2013; accepted May 23, 2013. Date of publication June 18, 2013; date of current version October 7, 2013. Research conducted in the scope of the European Pulsed Power Laboratories known as the EPPL. This work was supported in part by the Agency for Science, Innovation and Technology, Lithuania, the High-Tech Development Programme Projects MAGEPS under Grant 31V-27, and KEITIKLIS under Grant 31V-37. ˘ Šimkeviˇcius, N. Žurauskienˇe, S. Baleviˇcius, V. Stankeviˇc, and C. A. Baškys are with Semiconductor Physics Institute, Center for Physical Sciences and Technology, Vilnius LT-01108, Lithuania, and also with Vilnius Gediminas Technical University, Vilnius LT-03227, Lithuania (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]). S. Keršulis is with Semiconductor Physics Institute, Center for Physical Sciences and Technology, Vilnius LT-01108, Lithuania (e-mail: [email protected]). Digital Object Identifier 10.1109/TPS.2013.2266199

the output stage of the frequency converter as well as the motor itself. Thus, a current-sensing system has to be incorporated into the frequency converter. It is used to monitor the current and to generate a current fault signal when the current exceeds a limiting value. Such current sensors have to guarantee galvanic insulation between the circuits in which the current is being measured and the circuit in which the sensor’s signal is being conditioned and processed. The most widely used sensors are based on the Hall effect with concentrators for the amplification of the magnetic field [6]–[8]. Contactless current sensors based on magneto-resistive elements are of more simple construction, usually without concentrator. However, these require the keeping of a small distance between the sensor’s circuit and the object being measured [4]–[6], [7]. Contactless current sensors can also be based on magnetoresistive elements using manganite thin films. The colossal magnetoresistance (CMR) phenomenon found in manganite perovskites produces very large magnetoresistance (MR) values in a temperature range close to the Currie temperature (TC ), the temperature of the ferromagnetic ordering of manganese spins [9]. Sensors based on such manganite perovskites are widely investigated, but most of these have exhibited large sensitivity only in temperature ranges considerably lower than room temperature. They are also sensitive to temperature variation. The experimental results obtained in [10], however, showed that polycrystalline manganite films have sufficient MR values along a wide range of temperature, including room temperature. This paper describes the design of a current-sensing system consisting of a contactless current sensor based on CMR manganite thin film and a microcontroller with a 12-bit A/D converter, which is used for converting the signal of the current sensor to a digital output and for further processing. This current-sensitive system can be used for the measurement of the current in general and for power management in particular. For example, it can be used to protect the frequency converters operating at room temperatures. The peculiarities of the proposed system are discussed. II. D ESIGN OF THE C ONTACTLESS C URRENT S ENSOR The design of the contactless current sensor based on manganite CMR-resistors sensitive to magnetic fields is shown

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ˇ ŠIMKEVICIUS et al.: CURRENT SENSING SYSTEM

in Fig. 1. The current sensor consists of a magnetic field concentrator and two magnetic field sensors (CMR-resistors). The current to be measured flows through the wire which is wound around the concentrator. The number of windings n depends on the measurement conditions. The concentrator increases the field available to the magnetoresistor in proportion to the current strength, the number of windings n and is inversely proportional to the width of the gap. One of the CMR-resistors (active sensor S1) is placed in the gap of the concentrator and is used as the active element, the other (passive sensor S2) is used for the compensation of the thermal resistance drift of the first manganite CMR-resistor. During experiments, a stabilized dc power supply SM 120-50 (Delta Elektronika) is used as the measured current source. The active elements of this current-sensing system can be made of epitaxial or polycrystalline manganite thin films [11]. The main requirements for the magnetoresistive manganite films used in these contactless sensors are the linearity of their changes of resistance because of changes in the magnetic flux (magnetic induction), small hysteresis during the reversal of the direction of the magnetic field and small temperature coefficients of their resistance. Epitaxial and polycrystalline La1−x Srx MnO3 (LSMO) and La1−x Cax MnO3 (LCMO) thin films with Sr or Ca content x = 0.17–0.33 are investigated to pick the best candidate for use in the contactless current sensor. The films are grown using a metal–organic chemical vapor deposition technology onto a dielectric lucalox substrate (polycrystalline films) and onto SrTiO3 (epitaxial ones). The thickness of the films is ∼400 nm. The electrical contacts of the fabricated CMR-resistors, spaced by a 50-μm gap, are made by thermal deposition of Ag, using a Cr sublayer. The dimensions of the resistor chips are as low as 1.0 × 0.5 × 0.3 mm with a sensitive area of ∼0.025 mm2 . Two 0.1-mm diameter Cu isolated bifilarly twisted wires are soldered to the chip. The chip is encapsulated with special inert silicone. Our investigations have shown that the polycrystalline manganite La0.78 Sr0.22 MnO3 films best satisfy all the above mentioned requirements for contactless current sensors. Such films exhibited nontextured single-phase polycrystalline structures with clusters consisting of small crystallites and having average dimensions of 300 nm. These films have resistance changes in their magnetic fields of about 4%/T (the resistance of a manganite film decreases when in a magnetic field), sufficient linearity of their changes of resistance because of changes in the magnetic flux, negligible hysteresis at temperatures higher than temperature TC and small temperature sensitivity when close to room temperature. To eliminate the influence of such temperature sensitivity, two manganite resistors with identical resistances versus temperature dependencies are used [12]. These resistors (S1 and S2) are connected into an unbalanced Wheatstone bridge [6]. It is obvious that the second (compensating) resistor needs to be positioned where its change of resistance due to the magnetic field generated by the current being measured would be negligible. This requirement can be realized by the appropriate placement of the second resistor and employing for this purpose the anisotropic behavior of the MR in these films.

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Fig. 1. Design of contactless current sensor: S1 is active CMR-resistor, S2 is passive CMR-resistor.

Fig. 2. Dependence of MR of polycrystalline LSMO film on the magnetic induction at 27 °C temperature. The magnetic field is (1) parallel to the film plane and perpendicular to the direction of current, and (2) perpendicular to film plane.

Fig. 2 shows the dependence of the MR of a 400-nm thick LSMO polycrystalline film on the magnetic field at different orientations in respect to the film plane at 27 °C temperature. One can see that up to 400 mT, the sensitivity of the sensor to magnetic induction is poor when the magnetic field is perpendicular to the film plane. But when the magnetic field is parallel to the plane and is perpendicular to the direction of current, the MR is about 0.8% at 200 mT. It should to be noted that as the resistance of these CMR-resistors is ∼3 k in a zero magnetic field, it is easy to measure its changes when in this field. Thus, the first CMR-resistor (active sensor S1) is placed in the gap hence its plane is parallel to the direction of the magnetic field and the direction of the electric current in the sensor is perpendicular to the direction of the magnetic field (Fig. 1). The second CMR-resistor (passive sensor S2) used for compensation of the temperature drift is placed outside the concentrator at a sufficient distance from the gap (where magnetic induction is more than ten times lower than in the gap), hence the plane of the manganite film is perpendicular to the direction of the magnetic field. Therefore, the resistance change produced by the passive resistor upon magnetic field is negligible. This sensor has similar temperature dependence of its resistivity as the first CMR-resistor placed in the gap.

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Fig. 3.


Block diagram of the current-sensing system. Fig. 5. Relation between the measured current and the output signal of the microcontroller calculated according to (1).

produced by the electromagnetic disturbances. After reading the signals U1 and U2 , the microcontroller calculates the value of the current and sends this data to the controller of the frequency converter. The equation for the calculation of the current value is expressed by the following expression: I = (U2 − U1 )/(a0 + a 1 U2 + a 2 U22 ) + b 0 + b 1U2 + b 2 U22

Fig. 4. Current sensor’s signal (U2 − U1 ) at temperature (1) 12 °C and (2) 27 °C. Supply voltage 3.5 V, number of turns n = 10.

For the provision of identical thermal conditions for both sensors, they are glued to a holder made from material having high thermal conductivity. III. D ESIGN OF C URRENT S ENSING S YSTEM We used a microcontroller in the contactless current-sensing system for signal conditioning and for the compensation of the zero and sensitivity drifts due to the temperature of the manganite CMR-resistors. Because of there not being exact identical temperature dependences of the resistances in both CMR-resistors, a certain zero signal temperature shift is observed. In addition, as it is mentioned above, the CMRresistors have a high temperature drift of their sensitivity to magnetic field (magnetoresistance). As this is a disadvantage when measuring the magnetic field, thus it has to be compensated for. A block diagram of this current-sensing system is shown in Fig. 3. The voltage drops U1 across CMR-resistor S1 and U2 across CMR-resistor S2 are supplied to the two inputs of the microcontroller (a PIC18F2458 from Microchip), which has a 12 bit A/D converter with a full-scale measurement range of 2.5 V. The signals U1 and U2 did not exceed 1.8 V because the resistance of resistors R1 and R2 is approximately equal to the resistance of the sensors S1 and S2 (CMR-resistors). Capacitors C1 and C2 are used for filtering the noise signals

(1)

where a0 , a1 , a2 , b0 , b1, and b2 are the coefficients. These coefficients are obtained in advance by calibrating the sensors (CMR-resistors). This calibration is performed in the following way: The dependences of the signals U1 and U2 (output data) versus current are measured at various temperatures in the temperature range of 0–45 °C in steps of 5 °C. The typical output characteristics (U2 –U1 ) at two temperatures are shown in Fig. 4. One can see that as the sensitivity of the sensor and the zero signal depend on the temperature, thus further data processing using a microcontroller is required. The coefficients a0 , a1 , and a2 , which adjust the sensors’ signals to the maximal value of the current at the various temperatures, are obtained from the (U2 –U1 ) versus current dependence. The coefficients b0 , b1 , and b2 , which adjust the temperature drift of the sensors’ zero signals, are obtained from the U2 versus temperature dependence. The values of these coefficients are stored in the memory of the microcontroller. After the current-sensing system is calibrated, the further measurement of the current by a user is simple enough: the microcontroller calculates the current values by using (1) and sends them to the control unit. IV. R ESULTS AND D ISCUSSION The parameters of the current sensor are as follows: length of the median line of concentrator rc = 63 mm, the relative permeability of the concentrator material μc ≈ 8000, the width of the gap δ = 1.3 mm. For these parameters, the magnetic field in the gap at current I = 10 A and number of turns n = 10 is ∼190 mT. As it can be seen from Fig. 4, the signal change is ∼7.73 mV at temperature 12 °C and 5.53 mV at 27 °C, while the zero

ˇ ŠIMKEVICIUS et al.: CURRENT SENSING SYSTEM

Fig. 6. Block diagram of the frequency converter with the current-sensing system.

signal temperature drift is ∼0.62 mV in this temperature range, i.e., about 8% of the sensor’s signal at 12 °C. It is observed that there is sufficient nonlinearity of the characteristics in the 0–5 A range, but this has a small influence on the sensor’s operation as the actual values of the current that is to be controlled are more than 10 A. The typical output signal of the microcontroller [results calculated according to (1)] versus the measured current at different temperatures is shown in Fig. 5. The current-sensing system is calibrated in advance and the appropriate coefficients are entered into the microcontroller. One can see that the absolute current measurement error does not exceed ±0.45 A at both temperatures. This error can be reduced by using more terms in the polynomial expression (1). However, the digital processing capabilities and memory resources of the microcontroller being used are not sufficient to realize such improvements. The relative error in the temperature range of 0–45 °C and the current range of up to 100 A is < 2.5%. Additional investigations have shown that during transient time when current slowly changes from its minimal to its maximal value, some heating of flux concentrator (closed core) occurs and, therefore, the thermal conditions for both resistors become dissimilar (nonidentical). This can increase the measurement error up to 5.5%. This error can be reduced by increasing the heat dissipation in the part of the flux concentrator where the turns of the current leads are mounted. The response speed of these CMR manganite sensors is sufficiently high (rise time < 0.05 ms) as it was shown in [11]. Thus, it can be used to measure fast current changes. A microcontroller with a 20-MHz clock frequency is sufficiently fast to compute the current value during ∼0.1 ms. This processing time is sufficient fast, allowing this currentsensing system to be used in the overload controllers of most frequency converters. The voltage step linked to one bit of

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the A/D converter of this microcontroller is ∼0.61 mV. It corresponds to a current value of ∼1 A (at supply voltage 3.5 V and number of turns n = 10). The proposed current-sensing system is tested in the current overload protection systems of frequency converters that are developed at the Semiconductor Physics Institute of the Center for Physical Sciences and Technology (Vilnius, Lithuania) [13]. A block diagram of the frequency converter used with this current-sensing system is shown in Fig. 6. The frequency converter consisted of a rectifier that produces the dc voltage and delivers it through the dc bus to the inverter. The inverter converts the dc voltage to a variable-frequency, variableamplitude, three-phase ac voltage for the supply of the ac induction motor. The control of the frequency converter is provided by a control unit based on the digital signal processor dsPIC30F6010. The current-sensing system, based on the microcontroller PIC18F2458, processes the data obtained from the current sensor and sends it to the control unit. The current sensor measures the dc bus current. Thus this system protects not only the motor against overload, but also protects the rectifier in the case of a failure of the inverter’s transistors as well. This system is tested in an ambient temperature range of 0–45 °C and is able to protect the frequency converters at threshold current values of higher than 10 A. It has to be noted that the proposed current measurement system could be used also for other applications. For example, this system could be applied for the measurement of current and control in pulsed power generators in place of Rogowski coils, which need signal integration [1]. This would be especially useful in cases where the measured current pulse has a wide frequency band [11], which makes integration a difficult procedure. It is also possible to increase the highest frequency limit of the measured signal by using a higherspeed microcontroller. For measuring higher pulsed currents (few kA), a concentrator with a single-turn winding may be used. V. C ONCLUSION This programmable pulse current-sensing system, consisting of a contactless current sensor and a microcontroller, was proposed for the protection of frequency converters against pulsed fault currents. It was shown, that in this case, sensors made from polycrystalline La-Sr-Mn-O manganite films can be used as the magnetic field sensing devices. To minimize the signal of the zero temperature drift, the sensing system used two CMR-resistors incorporated into an unbalanced Wheatstone bridge. The displacement of the CMR-resistors in flux concentrator, used for the amplification of the magnetic field generated by the high current, allowed the achievement of a maximal difference of the effect of the magnetic field on these resistors. For signal conditioning and for the compensation of the influence of the temperature drift, a microcontroller with 12-bit A/D converter was used. The investigations of this current-sensing system showed that it was possible to record the current changes during 0.1 ms with ±2.5% measurement error. The system was tested in an ambient temperature range from 0 to 45 °C and was able to protect frequency converters,

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used for control of the speed of ac induction motors, at threshold current values higher than 10 A.

Nerija Žurauskien˙e (M’10) received the Ph.D. degree in physics from the Semiconductor Physics Institute, Vilnius, Lithuania, in 1996. She is currently a Senior Research Scientist with the Semiconductor Physics Institute, Center for Physical Sciences and Technology, Vilnius, and a Professor with Vilnius Gediminas Technical University, Vilnius. He is the co-author of more than 100 scientific papers. Her current research interests include investigation of the influence of short high power electric and magnetic pulses on low dimensional solid-state materials, and investigation of optical properties of semiconductor quantum dots. Dr. Žurauskien˙e received the Lithuanian National Award in Science in 2000.

R EFERENCES [1] B. M. Novac, I. R. Smith, P. Senior, M. Parker, and G. Louverdis, “Transportable high-energy power generator,” Rev. Sci. Instrum., vol. 81, pp. 054706-1–054706-5, May 2010. [2] L. A. Kojovic, “Application of Rogowski coils for advanced power systems solutions,” in Proc. 18th Int. Conf. Exhibit. Electr. Distrib., Jul. 2005, pp. 1–4. [3] J. Pelegrí, D. Ramírez, E. Sanchis, A. E. Navarro, and S. Casans, “Giant magnetoresistive sensor in conductance control of switching regulators,” IEEE Trans. Magn., vol. 36, no. 5, pp. 3578–4580, Sep. 2000. [4] P. Ripka, “Electric current sensors: A review,” Meas. Sci. Technol., vol. 21, p. 112001, Sep. 2010. [5] V. Bleizgys, A. Baskys, and T. Lipinskis, “Induction motor voltage amplitude control technique based on the motor efficiency observation,” Electron. Electr. Eng., vol. 109, no. 3, pp. 89–92, Mar. 2011. [6] C. Reig, M.-D. Cubells-Beltran, and D. Munoz, “Magnetic field sensors based on giant magnetoresistance (GMR) technology: Applications in electrical current sensing,” Sensors, vol. 9, pp. 7919–7942, Oct. 2009. [7] P. Ripka, “Current sensors using magnetic materials,” J. Optoelectron. Adv. Mater., vol. 6, pp. 587–592, Jun. 2004. [8] R. Popovic and W. Heidenreich, “Magnetogalvanic sensors,” in Sensors. A Comprehensive Survey, vol. 5. New York, NY, USA: VCH, 1989, pp. 76–81. [9] M. Ziese, “Extrinsic magnetotransport phenomena in ferromagnetic oxides,” Rep. Prog. Phys., vol. 65, pp. 143–249, Jan. 2002. [10] N. Žurauskiene, S. Balevicius, P. Cimmperman, V. Stankevic, S. Keršulis, J. Novickij, A. Abrutis, and V. Plaušinaitiene, “Colossal magnetoresistive properties of La0.83 Sr0.17 MnO3 thin films grown by MOCVD on lucalox substrate,” J. Low. Temp. Phys., vol. 159, nos. 1–2, pp. 64–67, Apr. 2010. [11] N. Žurauskiene, S. Balevicius, V. Stankevic, S. Keršulis, M. Schneider, O. Liebfried, V. Plaušinaitiene, and A. Abrutis, “B-scalar sensor using CMR effect in thin polycrystalline manganite films,” IEEE Trans. Plasma Sci., vol. 396, no. 1, pp. 411–416, Jan. 2011. [12] C. Israel, S. Kar-Narayan, and N. D. Mathur, “Eliminating the temperature dependence of the response of magnetoelectric magnetic-field sensors,” IEEE Sensors J., vol. 10, no. 5, pp. 914–917, May 2010. [13] A. Baskys, V. Bleizgys, and V. Gobis, “The impact of output voltage modulation strategies on power losses in inverter,” Electron. Electr. Eng., vol. 94, no. 6, pp. 47–50, Jun. 2009.

ˇ Ceslovas Šimkeviˇcius received the Ph.D. degree in technological sciences from Polytechnical Institute, Kaunas, Lithuania, in 1986. He is currently a Senior Research Associate with the Semiconductor Physics Institute, Center for Physical Sciences and Technology, Vilnius, Lithuania, and an Associate Professor with Vilnius Gediminas Technical University, Vilnius. He is the co-author of more than 30 scientific papers. His current research interests include design of various sensors, semiconductor pressure sensors technology, and research and development of magnetic field sensors.

Saulius Baleviˇcius (SM’01) received the Ph.D. degree in physics from Vilnius University, Vilnius, Lithuania, in 1980, and the Habilitation Doctor degree in physics from the Semiconductor Physics Institute, Vilnius, in 2002. He is currently the Head of the Department of Material Sciences and Electrical Engineering, Semiconductor Physics Institute, Center for Physical Sciences and Technology, and a Professor with Vilnius Gediminas Technical University, Vilnius. He is the author or co-author of more than 100 scientific papers and holds 24 inventions. His current research interests include the influence of high-power electric, magnetic, and light and shock wave pulses on solid-state materials. Dr. Baleviˇcius received the Lithuanian National Award in Science in 2003.

Voitech Stankeviˇc received the Ph.D. degree in physics from the Crystallography Institute, Moscow, Russia, in 1986. He is currently a Senior Research Associate with the Semiconductor Physics Institute, Center for Physical Sciences and Technology, Vilnius, Lithuania, and an Associate Professor with Vilnius Gediminas Technical University, Vilnius. He is the co-author of more than 40 scientific papers. His current research interests include material engineering, semiconductor pressure sensors technology, design of various converters, manganites technology and research, and development of magnetic field sensors.

Skirmantas Keršulis received the Ph.D. degree in physics from Vilnius Gediminas Technical University, Vilnius, Lithuania, in 2010. He is currently a Researcher with the Semiconductor Physics Institute, Center for Physical Sciences and Technology, Vilnius. He is the co-author of ten scientific papers. His current research interests include investigation of short high power electric and magnetic pulses influence on solid-state materials.

Algirdas Baškys received the Ph.D. degree in technological sciences from the Electronics Institute, Minsk, Belarus, in 1983. He is currently a Senior Research Associate with the Semiconductor Physics Institute, Center for Physical Sciences and Technology, Vilnius, Lithuania, and a Professor with Vilnius Gediminas Technical University, Vilnius. He is the co-author of more than 100 scientific papers. His current research interests include power electronics, control methods and controllers for automatic control, and high current density analytic models of BJT.